Dissertations / Theses on the topic 'Heat exchangers'

In some industrial air-cooled heat exchangers, such as those in the generating set industry, the flow must turn through 90° after exiting the heat exchanger. In such arrangements, the plenum depths are typically very shallow. Furthermore., the axial fan often operates in the mixed-flow region of the fan characteristic, due to the restrictive nature of the system. These two factors lead to a reduction in the thermal performance of the system. The purpose of this study was to investigate the effect on thermal performance of inclining the heat exchanger relative to the axial fan. It was also important to compare this with simply increasing the plenum depth without inclining the heat exchanger, since inclination itself may increase the mean plenum depth. This was achieved through an isothermal experimental investigation, complemented with a numerical study using CFD. The results showed that as the heat exchanger was inclined, the low velocity core at the centre of the heat exchanger tended to move to one side. The opposite side had increased flow through the heat exchanger due to the inclination. For a mixed-flow fan operating point typical of some industries, it was found that inclination has a negligible effect on the performance of the system, when compared with a baseline case. Increasing the plenum depth also had no significant effect. At the axial fan operating point investigated, it was found that an angle of approximately 30° inclination gave the best performance. Increasing the plenum depth was found to improve the performance more than inclination. The best performing case was the non-inclined case with a plenum depth of 0.65 fan diameters. This gave an increase in flow of2.8% over the baseline case, and a corresponding 1.1 % increase in thermal performance.

Disertační práce je zaměřena na kovové a polymerní výměníky tepla. Hlavním předmětem zkoumání je optimalizace teplosměnných ploch za účelem zvýšení účinnosti výměníku tepla. Tyto cíle byly dosaženy experimentálně a numericky pomocí modelování v ANSYS. Na základě dosažených výsledků byla rozpracována technologie křížového navíjení polymerních výměníků z dutých vláken. Experimentální zařízení původně určené pro navíjení tlakových nádrží bylo modifikované pro automatizovanou výrobu polymerních výměníků z dutých vláken, ježto může být použita při jejich masové výrobě. Tato práce se také zabývala výměníky tepla pro klimatizační systémy. Byly zkoumány možnosti využití polymerních výměníků z dutých vláken v těchto systémech. Mimo jiné byla provedena studie vlivu cyklického tepelného zatížení standardního kovového žebrovaného tepelného výměníku.

Thesis (Ph. D.)--University of Missouri-Columbia, 2007.
The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on March 13, 2008) Vita. Includes bibliographical references.

Thesis (Mechanical Engineer and M.S. in Mechanical Engineering)--Naval Postgraduate School, June 2003.
Thesis advisor(s): Ashok Gopinath. Includes bibliographical references (p. 73-74). Also available online.

This experimental study explores the heat transfer from heated bare and finned tubular surfaces to particulates in packed bed cross flow. The results from this experiment will be used to help select the type of particulates that will be used. Additionally, these results will assist in estimating heat transfer in prototype and commercial particle to fluid heat exchangers (PFHX). This research is part of larger effort in the use of particulates in concentrating solar power technology. These solid particles are heated by concentrated sunlight to very high temperatures at which they are a suitable heat source for various thermal power and thermochemical cycles. Furthermore, one of the advantages of this concept is the ability to store thermal energy in the solid particles at relatively low cost. However, an important feature of any Particle Heat Receiver (PHR) system is the PFHX, which is the interface between the solar energy system and the thermal power or chemical system. In order to create this system material data is needed for the design and optimization of this PFHX. The paper focuses on the heat transfer properties of particulates to solid surfaces under plug flow conditions. The particulates will be evaluated for three grain sizes of sand and two grain sizes of proppants. These two materials will be tested at one, five and ten millimeters per second in order to see how the various flow rates, which will be required for different loads, will affect the heat transfer coefficient. Finally the heat transfer coefficient will also be evaluated for both finned and non-finned heat exchangers to see the effect that changes in the surface geometry and surface area have on the heat transfer coefficient. The heat transfer coefficient will help determine the appropriate material that will be used in the PHR system.

The main goal of this thesis is to find out if a liquefied natural gas multistream heat exchanger numerical model is achievable. This should include several features usually neglected in nowadays available heat exchanger models, such as flow maldistribution, changes in fluid properties and heat exchanger dynamic behaviour. In order to accomplish that objective a simpler case is modelled. Efforts are put in achieving numerical stability.A counter flow natural gas and mixed refrigerant heat exchanger is modelled. Some important characteristics of the obtained model are: (1) it allows a dynamic study of the heat exchanger, (2) mass flow rate is a consequence of inlet and outlet pressure difference, (3) fluid properties change is taken into account, (4) it presents a time step control function and (5) fluid movement is not neglected. Some interesting numerical behaviours included in heat exchangers models design that have been observed during the course of this thesis are discussed. For instance, the comparison of the effects of choosing one heat transfers correlation or another.Dynamic response of the modelled heat exchanger during start up and during an abrupt change in mixed refrigerant inlet temperature are shown and discussed.

COMISSÃO NACIONAL DE ENERGIA NUCLEAR
Da segunda lei da termodinâmica, os conceitos de disponibilidade e irreversibilidade são aplicados à trocadores de calor. Equações relacionando a irreversibilidade com a efetividade de trocador de calor são desenvolvidas e aplicadas à vários casos. Relações entre o decréscimo de irreversibilidade e custos operacionais são também estabelecidos. Embora seja comumente dito que o trocador de calor em contra-correntes é menos irreversível que o trocador de calor em correntes paralelas, o presente trabalho mostra que isto não é sempre verdadeiro. Um critério muito simples, baseado na mínima perda de disponibilidade, foi estabelecido. Este critério indica, para os dois tipos de trocadores de calor, qual deles tem a irreversibilidade mínima. Foi também provado que a irreversibilidade do trocador de calor tem um ponto de máximo dado por E igual a Cmax/(Cmin mais Cmax), onde e é a efetividade do trocador de calor. Este valor de e é um parâmetro importante e não depende do tipo de trocador de calor considerado.
From the second Law of Thermodynamics, the concepts of availability and irrervesibility are applied to heat exchangers. Equations relating the irreversibility to the heat-exchanger effectiveness are developed and applied to several cases. Relations between the decrease of irreversibility and operational costs are also established. Although it is commonly Said that the counterflow heat-exchanger is less irrerversible than the parallel flow heat-exchanger, the present work shows that this is not always true. A very simple criterion, based on the minimum loss of availability, was established. This criterion indicares, for the two types of heat exchangers, which one has the minimum irrerversibilty. It has been also proved that the heat-exchanger irreversibility has a point of maximum given by E iqual Cmax/ (Cmin more Cmax), where e is the heat-exchanger effectiveness. This value of e is na important parameter and it does not depend on the type of heat-exchanger under consideration.

A detailed investigation of novel air-coupled absorbers for use in a diesel engine exhaust-driven ammonia-water absorption system operating in extreme ambient conditions was conducted. Electrically driven vapor-compression systems are under scrutiny due to the environmental impact of synthetic refrigerants and the exacerbation of electric utility loads during peak demand periods. One alternative to vapor-compression systems is the absorption heat pump that uses environmentally benign working fluids and can be driven by a variety of heat sources, including waste heat and solar thermal processes. Direct air coupling of the absorber and condenser instead of indirect hydronic coupling can reduce absorption system size, complexity, and inefficiency, but materials compatibility issues with ammonia-water and the poor heat transfer properties of air present challenges. Heat and mass transfer modeling was used to predict the performance of round-tube corrugated-fin and compact tube-array absorbers designed for a 2.64-kW absorption chiller operated in high ambient temperature (51.7°C) conditions. A single-pressure ammonia-water test facility was constructed and used in conjunction with a temperature- and humidity-controlled air-handling unit to evaluate the absorbers at design and off-design operating conditions. Absorber performance was recorded over a range of air temperatures (35-54.4°C), air flow rates (0.38-0.74 m3 s-1), inlet solution temperatures (92-102°C), concentrated solution flow rates (0.006-0.010 kg s-1), and concentrated solution concentrations (38-46%). At design conditions, round-tube corrugated-fin absorbers of 394 and 551 Fins Per Meter (FPM) demonstrated comparable performance (Q394-FPM,exp = 4.521±0.271 kW; Q551-FPM,exp = 4.680±0.260 kW), and measured heat transfer rates were 0.7-1.9% AAD higher than those predicted through modeling. The measured heat transfer rate in the prototype tube-array absorber was significantly lower than the values predicted at design conditions (Qprot,exp = 2.22±0.24 kW; Qprot,mod = 4.33 kW). Maldistribution of the two-phase flow in the tube array is the probable cause of the disparity between the prototype absorber data and model predictions. Results from this investigation can be used to guide the development of air-coupled heat and mass exchangers for compact absorption heat pumps.

The Pillow-Plate Heat Exchangers (PPHEs) are rated as innovative type of heat transfer equipment. The present paper gives the information about the research made in Paderborn University aimed to investigate the heat transfer and pressure drop in the channels of the small-scale PPHE.

Refrigerant charge minimization is one of the most important targets for heating and air conditioning applications when using natural refrigerants like hydrocarbons or ammonia to cope with the new environmental challenges. Some applications of minichannels for charge minimization in heat pumps are presented and discussed in this thesis. The design of an innovative condenser, an evaporator and an internal heat exchanger is presented. These devices are shell-and-tube heat exchangers using 2 mm i.d. minichannels and realized for the use with propane. Computational procedures based on empirical correlations available in the literature and a simplified model of the heat transfer and pressure drop processes have been developed and used for the design. Experimental performance data of the heat exchangers when using R22 and propane is reported and compared against the predictions given by the computational procedures. The shell-and-tube minichannel heat exchangers have been installed in a 100 kW heat pump using propane as the refrigerant. The unit has been designed for laboratory tests and the minichannel shell-and-tube heat exchangers have been installed in the facility together with a conventional plate condenser and an evaporator. Different configurations have been tested in order to quantify the advantages of operating the heat pump using the low charge heat exchangers, with regard to both energy performance and propane charge. In particular, the experimental performance when using the minichannel condenser is compared to the one obtained when using the plate condenser, and the influence of the internal heat exchanger on the performance of the equipment is measured and discussed. Experimental data about the efficiency with propane of the semihermetic compressor installed in the heat pump is also reported. Besides empirical correlations to predict the global thermal performance, a more complete understanding of the two-phase flow and heat transfer in minichannels is needed for the design and optimization of heat exchangers. Some CFD simulations are presented in this thesis using the innovative Volume Of Fluid (VOF) method, which is able to directly compute multiphase flows without using any empirical closure law to model the interaction between the phases. In order to assess the capability of the method to compute the motion of the gas-liquid interface, which is crucial for two-phase flow and heat transfer, simulations of the adiabatic churn flow regime of air-water mixture at different pipe diameters and liquid and gas superficial velocities have been initially performed. A comparison of the numerical results with experimental visualizations is reported and a simplified theoretical model of the wave levitation process has been developed and used to explain the numerical results. The VOF simulations were then extended to the study of condensation of R134a inside a minichannel with 1 mm internal diameter. Computational results about the evolution of the vapour-liquid interface and the heat transfer coefficient along the channel are reported.
La riduzione della carica di refrigerante nelle applicazioni di condizionamento e riscaldamento è uno dei vincoli di progetto principali quando vengono utilizzati, per motivi di carattere ambientale, refrigeranti naturali come idrocarburi ed ammoniaca. Alcune applicazioni dei minicanali per la minimizzazione della carica nelle pompe di calore vengono presentate e discusse nella presente tesi. Viene presentato il progetto di un condensatore, un evaporatore ed uno scambiatore di calore rigenerativo innovativi. Questi componenti sono degli scambiatori di calore a fascio tubiero utilizzanti minicali del diametro di 2 mm e progettati per l’uso con propano. Delle procedure di calcolo basate su di correlazioni disponibili in letteratura ed un modello semplificato del processo di scambio termico sono state utilizzate per il progetto. Le prestazioni sperimentali degli scambiatori con R22 e propano vengono riportate e confrontate con le stime fornite dalle procedure di calcolo. Gli scambiatori di calore sono stati installati in una pompa di calore della capacità termica di 100 kW utilizzante propano come fluido frigorigeno. Nell’impianto della pompa di calore, destinata a test di laboratorio, sono stati installati anche un condensatore ed un evaporatore a piastre convenzionali. In questo modo è stato possibile confrontare diverse configurazioni al fine di quantificare in via sperimentale i vantaggi apportati dall’utilizzo degli scambiatori a minicanali, in termini sia di prestazioni energetiche, sia di carica di propano richiesta. In particolare, le prestazioni delle configurazioni utilizzanti il condensatore a minicanali vengono confrontate con quelle delle configurazioni utilizzanti lo scambiatore a piastre, e l’influenza sulle prestazioni energetiche dello scambiatore rigenerativo viene misurata e discussa. Vengono inoltre riportati dati sperimentali relativi all’efficienza con propano del compressore semiermetico installato nella pompa di calore. Oltre a correlazioni empiriche in grado di stimare le prestazioni termiche globali, il progetto e l’ottimizzazione di scambiatori di calore richiede una più approfondita conoscenza del deflusso e dello scambio termico all’interno di minicanali. Vengono presentate in questa tesi delle simulazioni di termofluidodinamica computazionale tramite l’innovativo metodo VOF (Volume Of Fluid) in grado di simulare direttamente deflussi multifase senza la necessità di utilizzare correlazioni empiriche per la modellazione dell’interazione tra le fasi. Al fine di validare l’efficacia di questo metodo nel calcolare il moto dell’interfaccia gas-liquido, il quale è un aspetto cruciale nello scambio termico bifase, sono state in un primo momento eseguite delle simulazioni del regime di deflusso ”churn flow” per una miscela aria-acqua nel caso di un tubo liscio verticale adiabatico, a differenti valori di diametro del tubo e di velocità superficiale delle due fasi. I risultati sono stati confrontati con visualizzazioni sperimentali ed un modello teorico semplificato del processo di levitazione delle onde è stato sviluppato ed utilizzato per commentare i risultati numerici. Le simulazioni con il metodo VOF sono state in un secondo momento estese allo studio della condensazione di R134a all’interno di un minicanale del diametro di 1 mm. Vengono riportati risultati computazionali relativi all’evoluzione dell’interfaccia vapore-liquido e dei coefficienti di scambio termico lungo il minicanale.

The sales of Ground Source Heat Pumps in Sweden and many other countries are having a rapid growth in the last decade. Today, there are approximately 360 000 systems installed in Sweden, with a growing rate of about 30 000 installations per year. The most common way to exchange heat with the bedrock in ground source heat pump applications is circulating a secondary fluid through a Borehole Heat Exchanger (BHE), a closed loop in a vertical borehole. The fluid transports the heat from the ground to a certain heating and/or cooling application. A fluid with one degree higher or lower temperature coming out from the borehole may represent a 2-3% change in the COP of a heat pump system. It is therefore of great relevance to design cost effective and easy to install borehole heat exchangers. U-pipe BHEs consisting of two equal cylindrical pipes connected together at the borehole bottom have dominated the market for several years in spite of their relatively poor thermal performance and, still, there exist many uncertainties about how to optimize them. Although more efficient BHEs have been discussed for many years, the introduction of new designs has been practically lacking. However, the interest for innovation within this field is increasing nowadays and more effective methods for injecting or extracting heat into/from the ground (better BHEs) with smaller temperature differences between the heat secondary fluid and the surrounding bedrock must be suggested for introduction into the market.

This report presents the analysis of several groundwater filled borehole heat exchangers, including standard and alternative U-pipe configurations (e.g. with spacers, grooves), as well as two coaxial designs. The study embraces measurements of borehole deviation, ground water flow, undisturbed ground temperature profile, secondary fluid and groundwater temperature variations in time, theoretical analyses with a FEM software, Distributed Thermal Response Test (DTRT), and pressure drop. Significant attention is devoted to distributed temperature measurements using optic fiber cables along the BHEs during heat extraction and heat injection from and to the ground.

QC 20100517
EFFSYS2
Efficient Use of Energy Wells for Heat Pumps

As an alternative and renewable energy source, the shallow geothermal energy evolving as one of the most popular energy source due to its easy accessibility and availability worldwide, and the ground source heat pump (GSHP) systems are the most frequent applications for extracting the energy from the shallow subsurface. As the heat extraction capacity of the GSHP system applications arises, the design of the borehole heat exchangers (BHE), which is the connected part of the system in the ground, become more important. The backfilling materials of BHEs, particularly, the grout material must provide a suitable thermal contact between the ground and the heat carrier fluid in the high density polyethylene (HDPE) pipes and ensure durability to the induced thermal stresses due to the heat loading. In addition, for the heating purposes of buildings, BHEs immerged in groundwater may be operated below the freezing point of water with anti-freeze mixture in the pipe, leading to freezing-induced ice pressure which may damage the grout.In order to propose a proper grouting for BHEs, the thermo-hydro-mechanical behavior of the grout and its interferences with the adjacent ground conditions must be evaluated in the near field, and the thermal interactions of each BHE in a multi-BHEs field in the long-term operations must also be considered at a further field.Primarily, we have evaluated the performance of various grouting materials, through thermal, hydraulic and mechanical laboratory characterizations. In particular, we have proposed a homemade grout material, with the addition of graphite powder to improve the thermal properties of grout material. In parallel, the characteristics of two different widely used commercial grouting materials (i.e. calcite-based and silica-sand based materials) have been also investigated. In the subsequent study, the heat flow rate per meter of a BHE and the borehole resistance of borehole heat exchangers are assessed experimentally in a 1×1×1 m3 sandbox under, successively, dry sand and fully water-saturated sand conditions. During the operations, the monitored temperatures in the sandbox are in good agreement with analytical predictions. This study demonstrated that the homemade admixture prepared with 5 % natural flake graphite can be considered as an appropriate grout for BHEs regarding to its rheological and thermo-physical properties. Thermally-enhanced grouting can be of significant interest in a high thermal conductivity ground (such as saturated sand) because it minimizes the thermal resistance of the BHE.After characterizing and testing the efficiency of various grout materials, the thermal stresses occurred in BHEs due to heat injection or extraction has been investigated with the analytical solution of hollow cylinder model that is adapted for time-dependent heat loading, the geometry of a BHE, and the thermo-mechanical properties of surrounding ground conditions. Firstly, the hollow cylinder model has been solved for the considered boundary conditions in 2D plane stress. Secondly, the temperature differences at the inner and outer circles of the cylinder is evaluated with the heat line source models for continuous and discontinuous loadings to observe the impact of the heat loading schedule. The developed analytical solution for thermal stress investigation is validated with numerical models. It is demonstrated that the analytical solutions agree well with numerical results for two types of BHE configurations (co-axial and single U-shaped pipes). Furthermore, the calculated maximum stresses are compared with the tensile strength of grout materials obtained from Brazilian tests. It is predicted that thermal contraction of the grout, partially constrained by the surrounding rock, generates tensile stresses that may lead to cracking in the BHE. According to the results, the stiffness of rock has primary role on the developed tensile stresses, and the relationship between the thermal conductivity of the ground and of the grout induces a proportional impact on the magnitude of thermal stresses.Another major concern is the freeze-resistance of the grout materials, when the system is operated for heating purposes. Firstly, we conducted an experimental setup in a small-scale sandbox to understand the behavior of the grout material by evaluating the permeability change during freeze-thaw cycles of a BHE. According to the results, the permeability of grout materials did not change after 10 freeze-thaw cycles due to the thermal transfer with the adjacent soil partially reducing the impact of freezing in the grout material. Therefore, in order to test the freeze-resistance of a BHE, we have investigated the freezing impact of pore water pressure and thermal stress with analytical models and experimental setups on BHEs. For the theoretical approach, an analytical solution has been developed by using the hollow cylinder model that accounts for both the HDPE pipe and the grout material. Firstly, the freezing pore water pressure is adapted to the generalized Hooke’s law equations in 2D plane stress, and secondly the model is solved for the considered boundary conditions. In order to validate the developed model, the experimental setup is conducted in agreement with the geometry of the considered analytical model and the BHE probes are prepared with three different grout materials having large difference in the thermal and hydraulic characteristics (i.e. silica-sand based, calcite based and the homemade enhanced thermally with natural flake graphite powder). According to the experiments for 50 h of freezing operation, the calcite based grout and the homemade grout, having lower permeability and relatively higher porosity, are fractured. In contrast, the silica-sand based grout having higher permeability did not exhibit any damage. Compared with the theoretically obtained results, the observations from the experiments are consistent with the calculated stress results. The effective tangential stress induced by the freezing pore water pressure causes the crack development and agrees with the crack patterns. As a conclusion, the porosity and the permeability play a significant role on the grout failure.In a multi-BHEs field, the thermal interaction between each BHE may have a significant influence on the near-field investigation results in long-term operations. Therefore, in order to complete the near-field investigation, a far-field long-term operation study is required. However, existing analytical solutions for thermal analysis of ground source heat pump (GSHP) systems evaluate temperature change in the carrier-fluid and the surrounding ground in the production period of a single BHE only if a continuous heat load is assigned. In this study, we modified the Green’s function, which is the solution of heat conduction/advection/dispersion equation in porous media, for discontinuous heat extraction by analytically convoluting rectangular function or pulses in time domain both for single and multi-BHEs field. The adapted analytical models for discontinuous heat extraction are verified with numerical finite element code. The comparison results agree well with numerical results both for conduction and advection dominated heat transfer systems, and analytical solutions provide significantly shorter runtime compared to numerical simulations (approx. 1500 times shorter). Furthermore, we investigated the sustainability and recovery aspects of GSHP systems by using proposed analytical models under different hydro-geological conditions. According to the engineering guideline VDI 4640, a linear relationship between thermal conductivity of the ground and the sustainable heat extraction rate is demonstrated for multi-BHEs. In addition, we developed an MATLAB interface for users in which the analytical model can be used easily and more efficiently.In addition, in order to extend the case studies for a ground including several layers, we proposed a finite line source model for BHEs that takes into account conduction/advection/dispersion mechanism in multilayer porous media. Firstly, the anisotropy is added to the moving finite line source model, and we used an existing composite model approach for conductive multilayer ground. The comparison with the numerical model results demonstrates the suitability of the approach. The proposed model can provide a faster solution than classical numerical approaches and help to optimize the heat extraction rate in multilayer media. However, further investigations are required to validate the model with the field measurements.
Doctorat en Sciences de l'ingénieur et technologie
info:eu-repo/semantics/nonPublished

Selective Laser Melting (SLM), a layer-based Solid Freeform Fabrication (SFF) process, was used to fabricate micro cross-flow heat exchangers from 316L stainless steel, bronze (Cu 90%, Sn 10%) and Inconel 718 powder. Their mechanical and thermal properties were determined using solid blocks of SLM material prior to the fabrication of the micro cross flow heat exchangers. Initially the process parameters for the fabrication of high density (>97%) parts for the different materials were defined. The mechanical and thermal properties of SLM parts were then measured. The tensile test results exhibited yield strength values superior to the parent metals, but also showed low tensile strength and ductility as a result of the inherent residual porosity (2-4%). Results obtained from the thermal conductivity of the stainless steel material system were in good agreement with the bulk material values. The heat transfer performance of the heat exchangers with either micro channels or lattice structures as heat exchange surfaces was investigated experimentally and the results were evaluated in terms of geometry and materials. The performance of the micro heat exchangers was found to be dependent not only on the choice of material but also on the heat exchanger media geometry.

This thesis describes an investigation into the acoustic phenomenon in in-line tubular heat exchangers subjected to cross flow. The flow through such a heat exchanger can result in the production of very high noise levels, which occur as a result of the excitation of an acoustic standing wave in the cavity between the tube rows. This acoustic vibration can occur in large and small heat exchangers alike, resulting in drastically impared performance and working life. The phenomenon associated with such vibration is poorly understood and considerable anomalies still exist in published literature. There are several theories which attempt to describe this mechanism, however, none of these can satisfactorily account for its complex nature. The objective of the present work was to carry out an investigation to assess this acoustic phenomenon. The initial stages of this work produced an experimental rig to allow the phenomenon to be fully investigated, this included the examination of the effect of row depth and bank geometry on the acoustic resonance. The next step was to determine the role played by acoustic damping. This included making measurements of the damping under flow conditions and establishing its dependancy on velocity. A method of increasing the acoustic damping of a given bank was developed and incorporated in a tube bank. The results obtained from these experiments revealed that the acoustic system behaved in a manner which was consistent with that of a self excited system. Finally an appropriate mathematical model of the system was developed. The model,, which considers an acoustic feedback effect, was found to give quite an accurate representation of the system, and has the ability to account for all the observations made in this investiagation. This, together with the experimental results, enabled a series of guidelines to be presented as a basis for the design of such a tube bank.

Thesis (MEng)--Stellenbosch University, 2000.
ENGLISH ABSTRACT: The main objective of this study to determine the air-side heat transfer and pressure drop performance characteristics of commercially available round and elliptical fined tubes. A . computer code to compare the performance of industrial forced and induced draught aircooled heat exchangers (henceforth referred to as f\CHE's) commonly found in the petrochemical industry is also developed. The comparison is extended to include both round and elliptical finned tubes. From the tests conducted, it is found that there is generally an increase in the heat transfer parameter with a decrease in the fin pitch. The decrease in the fin pitch however also results in an increase in the pressure drop across the tube bundle. The performance of the tubes is compared with round reference tubes having circular or plate fins for which performance correlations are available in the literature. The fan power required by an induced draught aircooled heat exchanger is found to be higher than that required by a forced draught heat exchanger rejecting the same amount of heat.
AFRIKAANSE OPSOMMING: Die hoofdoel van hierdie studie is om die werksverrigting van industriële geforseerde trek -- en geinduseerde trek lugverkoelde warmteruilers (LVWR) soos algemeen aangetref in die petrochemiese nywerheid te vergelyk. Warmteruiier bundels word gewoonlik opgebou uit ronde vinbuise. Die werksverrigting van laasgenoemde word vergelyk met die werksverrigting van warmteruiier bundels wat gebruik maak van elliptiese vinbuise. Die werksverrigting van 'n reeks kommersieël verkrygbare ronde en elliptiese vinbuise word deur middel van toetsing bepaal. In die algemeen word 'n toename in die warmteoordrag-parameter waargeneem met In afname in die vinsteek. 'n Toename in die vinsteek gaan egter gepaard met" 'n toename in die drukverlies oor die bundel. Die werksverrigting van hierdie buise word vergelyk met bestaande korrelasies vir die werksverrigting van ronde verwysingsbuise wat gebruik maak van ronde of plaatvinne. Daar is bevind dat die drywing wat benodig word deur 'n waaier vir 'n geinduseerde trek lugverkoelde warmteruiler, heelwat hoër is as in die geval van 'n geforseerde trek lugverkoelde warmteruiler, vir dieselfde hoeveelheid hitte verwydering

Sweden has been one of the first countries in the world to use ground-source heat pumps (GSHP) to supply heating and cooling to its buildings. Today, it is the leading country in Europe and new installations tend to have larger capacities. The use of deeper borehole heat exchangers (BHE) is an opportunity to extract larger amounts of heat over small land areas. However, there are only few case studies on BHE deeper than 300 m, hence such systems may be optimized. This study focuses on a deep GSHP system recently installed in central Stockholm, composed of four 510 m deep BHE. The objectives were to get the system ready for monitoring and to analyze the first sets of data recorded. First, a review of all the sensors already installed, of the data needed, and of the different ways to extract it, has been led. Practically, an acquisition system has been set up and connected to new and existing sensors such as thermometers and flow-meters. Theoretically, a method to derive the thermodynamic cycles of the different heat pumps has been determined. It led to the determination of COPs for several days during late spring 2017. The system globally showed reasonable efficiency, with an overall performance factor (equivalent to SPF2) of 3.42 including the circulation pumps of the ground loop. However, it could certainly be improved in several ways, for example by avoiding short cycles or by finding an optimum flow in the secondary ground loop. Furthermore, these results should be juxtaposed with those that will be obtained during winter, when the heating demand will be the highest.
Sverige var ett av de första länderna i världen som använt bergvärmepumpar (GSHP) för att täcka värme- och kylbehoven i byggnader. Sverige är, idag, det ledande landet i Europa och nya bergvärmeanläggningar tenderar att vara större, åtminstone kapacitetsmässigt. Användningen av djupa borrhålsvärmeväxlare (BHEs) ger möjligheten att extrahera en större mängd värme i areabegränsade egendomar. Det finns dock bara få studiefall om anläggningar med borrhål djupare än 300 m och de anläggningarna skulle därför kunna optimeras. Den här studien fokuserar på ett GSHP system med djupa borrhål som nyligen installerades i centrala Stockholm och som består av fyra 510 m djupa borrhål, bland annat. Målen var att förberedda prestandauppföljningssystemet och analysera de första insamlade mätningarna. Första steget var att samla information om de sensorerna som redan var installerade, bestämma vilka mätvärde var nödvändiga och hur skulle de kunna mättas. Praktiskt har en datainsamlingsenhet iordningställts och anslutits till befintliga och nya sensorer såsom temperaturgivare och flödesmätare. Teoretiskt har en metod frambringats för att bestämma den termodynamiska cykeln av varje värmepump. Det möjliggjorde beräkningen av COP:n under vissa dagar under våren 2017. Globalt visade systemet rimlig prestanda med en prestandafaktor (likvärdig SPF2) på 3.42, inklusive cirkulationspumpar i bergvärmeskretsen. Det skulle dock kunna förbättras på olika sätt, t.ex. genom att undvika kort-cykling av kompressorer eller gnom att hitta ett optimalt köldbärarflöde i bergvärmeskretsen. Resultaten som det här arbetet kom fram till borde dessutom jämföras med en liknande analys under vintertid, då värmebehovet är högst.

Thesis (MScIng)--University of Stellenbosch, 2004.
ENGLISH ABSTRACT: As the demand for less expensive energy is increasing world-wide, energy conservation is becoming a more-and-more important economic consideration. In light of this, means to recover energy from waste fluid streams is also becoming more-and-more important. An efficient and cost effective means of conserving energy is to recover heat from a low temperature waste fluid stream and use this heat to preheat another process stream. Heat pipe heat exchangers (HPHEs) are devices capable of cost effectively salvaging wasted energy in this way. HPHEs are liquid-coupled indirect transfer type heat exchangers except that the HPHE employs heat pipes or thermosyphons as the major heat transfer mechanism from the high temperature to the low-temperature fluid. The primary advantage of using a HPHE is that it does not require an external pump to circulate the coupling fluid. The hot and cold streams can also be completely isolated preventing cross-contamination of the fluids. In addition, the HPHE has no moving parts. In this thesis, the development of a range of air-to-air HPHEs is investigated. Such an investigation involved the theoretical modelling of HPHEs such that a demonstration unit could be designed, installed in a practical industrial application and then evaluated by considering various financial aspects such as initial costs, running costs and energy savings. To develop the HPHE theoretical model, inside heat transfer coefficients for the evaporator and condenser sections of thermosyphons were investigated with R134a and Butane as two separate working fluids. The experiments on the thermosyphons were undertaken at vertical and at an inclination angle of 45° to the horizontal. Different diameters were considered and evaporator to condenser length ratios kept constant. The results showed that R134a provided for larger heat transfer rates than the Butane operated thermosyphons for similar temperature differences despite the fact that the latent heat of vaporization for Butane is higher than that of R134a. As an example, a R134a charged thermosyphon yielded heat transfer rates in the region of 1160 W whilst the same thermosyphon charged with Butane yielded heat transfer rates in the region of 730 W at 23 °C . Results also showed that higher heat transfer rates were possible when the thermosyphons operated at 45°. Typically, for a thermosyphon with a diameter of 31.9 mm and an evaporator to condenser length ratio of 0.24, an increase in the heat transfer rate of 24 % could be achieved. Theoretical inside heat transfer coefficients were also formulated which were found to correlate reasonably well with most proposed correlations. However, an understanding of the detailed two-phase flow and heat transfer behaviour of the working fluid inside thermosyphons is difficult to model. Correlations proposing this behaviour were formulated and include the use of R134a and Butane as the working fluids. The correlations were formulated from thermosyphons of diameters of 14.99 mm, 17.272 mm, 22.225 mm and 31.9 mm. The evaporator to condenser length ratio for the 31.9 mm diameter thermosyphon was 0.24 whilst the other thermosyphons had ratios of 1. The heat fluxes ranged from 1800-43500 W/m2. The following theoretical inside heat transfer coefficients were proposed for vertical and inclined operations (READ CORRECT FORMULA IN FULL TEXT ABSTRACT) φ = 90° ei h = 3.4516x105Ja−0.855Ku1.344 φ = 45° ei h = 1.4796x105Ja−0.993Ku1.3 φ = 90° l l l ci l l v h x k g 1/ 3 2.05 2 4.61561 109Re 0.364 ν ρ ρ ρ − ⎡ ⎡ ⎛ ⎞⎤ ⎤ = ⎢ ⎢ ⎜ ⎟⎥ ⎥ ⎢ ⎢ ⎜ − ⎟⎥ ⎥ ⎣ ⎣ ⎝ ⎠⎦ ⎦ φ = 45° l l l ci l l v h x k g 1/ 3 1.916 2 3.7233 10 5Re 0.136 ν ρ ρ ρ − ⎡ ⎡ ⎛ ⎞⎤ ⎤ = ⎢ ⎢ ⎜ ⎟⎥ ⎥ ⎢ ⎢ ⎜ − ⎟⎥ ⎥ ⎣ ⎣ ⎝ ⎠⎦ ⎦ The theoretically modelled demonstration HPHE was installed into an existing air drier system. Heat recoveries of approximately 8.8 kW could be recovered for the hot waste stream with a hot air mass flow rate of 0.55 kg/s at an inlet temperature of 51.64 °C and outlet temperature of 35.9 °C in an environment of 20 °C. Based on this recovery, energy savings of 32.18 % could be achieved and a payback period for the HPHE was calculated in the region of 3.3 years. It is recommended that not withstanding the accuracies of roughly 25 % achieved by the theoretically predicted correlations to that of the experimental work, performance parameters such as the liquid fill charge ratios, the evaporator to condenser length ratios and the orientation angles should be further investigated.
AFRIKAANSE OPSOMMING: As gevolg van die groeiende aanvraag na goedkoper energie, word die behoud van energie ‘n al hoe belangriker ekonomiese oorweging. Dus word die maniere om energie te herwin van afval-vloeierstrome al hoe meer intensief ondersoek. Een effektiewe manier om energie te herwin, is om die lae-temperatuur-afval-vloeierstroom (wat sou verlore gaan) se hitte te gebruik om ‘n ander vloeierstroom mee te verhit. Hier dien dit dan as voorverhitting van die ander, kouer, vloeierstroom. Hittepyp hitteruilers (HPHR’s) is laekoste toestelle wat gebruik kan word vir hierdie doel. ‘n HPHR is ‘n vloeistof-gekoppelde indirekte-oordrag hitteruiler, behalwe vir die feit dat dié hitteruiler gebruik maak van hittepype (of hittebuise) wat die grootste deel van sy hitteoordragsmeganisme uitmaak. Die primêre voordele van ‘n HPHR is dat dit geen bewegende dele het nie, die koue- en warmstrome totaal geïsoleer bly van mekaar en geen eksterne pomp benodig word om die werkvloeier mee te sirkuleer nie. In hierdie tesis word ‘n ondersoek gedoen oor die ontwikkeling van ‘n bestek van lug-totlug HPHR’s. Hierdie ondersoek het die teoretiese modellering van so ‘n HPHR geverg, sodat ‘n demonstrasie eenheid ontwerp kon word. Hierdie demonstrasie eenheid is geïnstalleer in ‘n praktiese industriële toepassing waar dit geïvalueer is deur na aspekte soos finansiële voordele en energie-besparings te kyk. Om die teoretiese HPHR model te kon ontwikkel, moes daar gekyk word na die binnehitteoordragskoëffisiënte van die verdamper- en kondensordeursneë, asook R134a en Butaan as onderskeie werksvloeiers. Die eksperimente met die hittebuise is gedoen in die vertikale en 45° (gemeet vanaf die horisontaal) posisies. Verskillende diameters is ook ondersoek, maar met die verdamper- en kondensor-lengteverhouding wat konstant gehou is. Die resultate wys dat R134a as werksvloeier in die hittebuise voorsiening maak vir groter hitteoordragstempo’s in vergelyking met Butaan as werksvloeier by min of meer dieselfde temperatuur verskil – dít ten spyte van die feit dat Butaan ‘n hoër latente-hittetydens- verdampings eienskap het. As voorbeeld gee ‘n R134a-gelaaide hittebuis ‘n hitteoordragstempo van omtrent 1160 W terwyl dieselfde hittebuis wat met Butaan gelaai is, slegs ongeveer 730 W lewer by 23 °C. Die resultate wys ook duidelik dat hoër hitteoordragstempo’s verkry word indien die hittebuis bedryf word teen ‘n hoek van 45°. ‘n Tipiese toename in hitteoordragstempo is ongeveer 24 % vir ‘n hittebuis met ‘n diameter van 31.9 mm en ‘n verdamper- tot kondensor-lengteverhouding van 0.24. Teoretiese binne-hitteoordragskoëffisiënte is ook geformuleer. Dié waardes stem redelik goed ooreen met die meeste voorgestelde korrelasies. Nieteenstaande die feit dat gedetailleerde twee-fase-vloei en die hitteoordragsgedrag van die werksvloeier binne hittebuise nog nie goed deur die wetenskaplike wêreld verstaan word nie. Korrelasies wat hierdie gedrag voorstel is geformuleer en sluit weereens die gebruik van R134a en Butaan as werksvloeiers in. Die korrelasies is geformuleer vanaf hittebuise met diameters van onderskeidelik 14.99 mm, 17.272 mm, 22.225 mm en 31.9 mm. Die verdamper- tot kondensor-lengteverhoudings vir die 31.9 mm deursnit hittebuis was 0.24 terwyl die ander hittebuise ‘n verhouding van 1 gehad het. Die hitte-vloede het gewissel van 1800-45300 W/m2. Die volgende teoretiese geformuleerde binne-hitteoordragskoëffisiënte word voorgestel vir beide vertikale sowel as nie-vertikale toepassing (LEES KORREKTE FORMULE IN VOLTEKS OPSOMMING) φ = 90° ei h = 3.4516x105Ja−0.855Ku1.344 φ = 45° ei h = 1.4796x105Ja−0.993Ku1.3 φ = 90° l l l ci l l v h x k g 1/ 3 2.05 2 4.61561 109Re 0.364 ν ρ ρ ρ − ⎡ ⎡ ⎛ ⎞⎤ ⎤ = ⎢ ⎢ ⎜ ⎟⎥ ⎥ ⎢ ⎢ ⎜ − ⎟⎥ ⎥ ⎣ ⎣ ⎝ ⎠⎦ ⎦ φ = 45° l l l ci l l v h x k g 1/ 3 1.916 2 3.7233 10 5Re 0.136 ν ρ ρ ρ − ⎡ ⎡ ⎛ ⎞⎤ ⎤ = ⎢ ⎢ ⎜ ⎟⎥ ⎥ ⎢ ⎢ ⎜ − ⎟⎥ ⎥ ⎣ ⎣ ⎝ ⎠⎦ ⎦ Die wiskundig-gemodelleerde demostrasie HPHR is geïnstalleer binne ‘n bestaande lugdroër-sisteem. Drywing van om en by 8.8 kW kon herwin word vanaf die warm-afvalvloeierstroom met ‘n massa vloei van 0.55 kg/s teen ‘n inlaattemperatuur van 51.64 °C en ‘n uitlaattemperatuur van 35.9 °C binne ‘n omgewing van 20 °C. Na aanleiding van hierdie herwinning, kan energiebesparings van tot 32.18 % verkry word. Die HPHR se installasiekoste kan binne ‘n berekende tydperk van ongeveer 3.3 jaar gedelg word deur hierdie besparing. Verdamper- tot kondensator-lengteverhouding, vloeistofvulverhouding en die oriëntasiehoek vereis verdere ondersoek, aangesien daar slegs ‘n akkuraatheid van 25 % verkry is tussen teoretiese voorspellings en praktiese metings.

In hospitals, where the air must be changed at least 15 times per hour, humidity control is as important as controlling the space temperature. The high air change rate applied in hospitals instantly implies that there would be energy-saving potential in using a heat recovery system between the air being exhausted from the conditioned space and the fresh air being supplied in to replace it. In addition, the fact that in very humid climates the moisture removal requirements often necessitates overcooling air and then reheating it implies that there may be a further opportunity for heat exchange as a conservation of energy measure. Research has been undertaken on heat pipe heat exchangers (HPHXs) for coolness recovery in tropical climates to explore the potential for energy savings in HVAC systems through using HPHXs. In this work, the proprietary simulation software package, TRNSYS, has been utilised to model both the building characteristics and the HVAC system of an operating theatre suite in a Kuala Lumpur hosp ital. The model has been "driven" by the hour-by-hour climatic data for a Typical Meteorological Year (TMY) for that location, producing detailed hour-by-hour predictions of temperature and relative humidity variation within a selected week of the year, and also overall energy usage for the existing HVAC system for the complete year. To simulate accurately the influence of adding one or more HPHXs to the existing system, it was essential to know the performance characteristics of a HPHX under the situation it would experience in such a system. These operating performance characteristics in tropical climate conditions have been determined by an extensive series of laboratory measurements of the performance of an actual HPJ-I)( under the range of moist air states that it would be required to operate under in service. This experimental stage needed the specification, design and construction of a HPHX, an associated conventional chilled water coil, a fan and duct system, and a full range of properly calibrated sensors. The experimental rig also allowed the examination of the effect, if any, that tilting the HPHX would have on its effectiveness. From the experiments on inclining the HPHX, the effect of the inclination angle on the performance of the HPHX has been shown to be negligible for the range of air conditions examined and the degree of tilt investigated. Based on the results from these experiments, custom-written modules have been added to the TRNSYS package to represent the behaviour of a heat pipe heat exchanger (HPHX). The energy advantages to be gained through modifying the existing HVAC system to incorporate either one or two HPHX units have been predicted through using this extended TRNSYS model, with appropriate allowance being made for the additional fan energy penalty incurred as a result of the increased pressure drops introduced by the presence of the HPHXs. Based on this investigation, the likely energy savings as a basis for assessing economic feasibility of the HPHX in tropical cIimates have been identified. Energy savings, and the resulting pay-off period for retrofitting the HPHXs, were seen to be sensitive to the coefficient of performance (COP) of the HVAC system's chiller from the simulations. At the existing HVAC plant's claimed average COP of 4.0 the pay-off period would be 4.5 years, decreasing to 3.9 years if the average COP was 3.2. These pay-off periods could be further decreased if the application of HPHXs was incorporated in the initial HVAC system design, rather than as a retrofit. This indicates the importance of fully integrating the design process right from the outset of the system design if HPHXs are to be incorporated into a HVAC system so as to give the maximum possible energy saving benefits.

Understanding the mechanisms that serve to increase heat transfer provides valuable knowledge to minimize the size and maximize the performance of compact heat exchangers. This document presents a detailed experimental heat transfer study of six scaled up louvered fin geometries that are typical of those found in modern louvered fin compact heat exchangers. Heat transfer measurements were performed over a range of Reynolds numbers and with two different boundary conditions. A fully heated boundary condition allowed the effects of the thermal field to be observed while an adiabatic boundary condition allowed the effects of the flow field to be observed. The results indicated that the complex thermal and flow field patterns that developed within the louvered fin geometries strongly affected the heat transfer of individual louvers. In the entrance region of the louvered array, the effects of the flow field were dominant while in the fully developed region of the louvered arrays, the effects of the thermal field were dominant. A companion two-dimensional CFD study indicated that the heat transfer trends of the louvers resulting from both the thermal and flow fields were well predicted. Based on heat transfer performance, it was determined that the theta = 27°, Fp/Lp = 1.52 geometry performed the best at Re = 230 and Re = 370, while the theta = 39°, Fp/Lp = 0.91 geometry performed best at Re = 1016.
Master of Science

Three applications of two-phase flow and heat transfer in plate-fin heat exchangers have been studied. A dephlegmator is a heat exchanger in which reflux condensation of a vapour mixture occurs, and plate-fln versions have importance in cryogenic gas separation processes. Numerical calculations for different binary mixtures show that the number of transfer units can be expressed as a simple function of the inlet vapour state and flow rate, heat load, and channel geometry. The calculations also show that the vapour and liquid exit compositions tend to limiting values as the number of transfer units increase. These limiting values correspond to liquid leaving the exchanger in equilibrium with the vapour entering. The effect of surface tension on liquid running down the rectangular passages of plate-fin exchangers is to draw it to the corners leaving less liquid on the walls and possible dry-out. A comparison of two CFD calculations with and without surface tension shows that effect can be significant. For a liquid with a surface tension only 1% that of water, about 50% more mass flows down the corner region of a square duct of side 0.944 mm. This transfer of liquid to the corner takes place in the first millimetre of flow downwards. Experimental measurements of pressure drop were taken for air and air-water flow through eight different plate-fin pads. The two-phase pressure drops for the serrated fin were two to five times larger than those for the plain fin. However, the effect of increasing the water flow rate at a fixed air flow rate was to increase the pressure drop by up to 75% in both cases. Over 200 two-phase pressure drops measurements were made, nearly all of the data were predicted to within 20% by the Lockhart and Martinelli (1949) correlation with C factor of 12.

Mini-/microchannel components have received attention over the past few decades owing to their compactness and superior thermal performance. Microchannel heat sinks are typically manufactured through traditional manufacturing practices (milling and sawing, electrodischarge machining, and water jet cutting) by changing their components to work in microscale environments or microfabrication techniques (etching and lost wax molding), which have emerged from the semiconductor industry. An extrusion process is used to produce multiport minichannel-based heat exchangers (HXs). However, geometric manufacturing limitations can be considered as drawbacks for all of these techniques. For example, a complex out-of-plane geometry is extremely difficult to fabricate, if not impossible. Such imposed design constraints can be eliminated using additive manufacturing (AM), generally known as three-dimensional (3D) printing. AM is a new and growing technique that has received attention in recent years. The inherent design freedom that it provides to the designer can result in sophisticated geometries that are impossible to produce by traditional technologies and all for the redesign and optimization of existing models. The work presented in this thesis aims to investigate the thermal performance of enhanced minichannel HXs manufactured via metal 3D printing both numerically and experimentally. Rectangular winglet vortex generators (VGs) have been chosen as the thermal enhancement method embedded inside the flat tube. COMSOL Multiphysics, a commercial software package using a finite element method (FEM), has been used as a numerical tool. The influence of the geometric VG parameters on the heat transfer and flow friction characteristics was studied by solving a 3D conjugate heat transfer and laminar flow. The ranges of studied parameters utilized in simulation section were obtained from our previous interaction with various AM technologies including direct metal laser sintering (DMLS) and electron-beam melting (EBM). For the simulation setup, distilled water was chosen as the working fluid with temperaturedependent thermal properties. The minichannel HX was assumed to be made of AlSi10Mg with a hydraulic diameter of 2.86 mm. The minichannel was heated by a constant heat flux of 5 Wcm−2 , and the Reynolds number was varied from 230 to 950. A sensitivity analysis showed that the angle of attack, VG height, VG length, and longitudinal pitch have notable effects on the heat transfer and flow friction characteristics. In contrast, the VG thickness and the distance from the sidewalls do not have a significant influence on the HX performance over the studied range. On the basis of the simulation results, four different prototypes including a smooth channel as a reference were manufactured with AlSi10Mg via DMLS technology owing to the better surface roughness and greater design uniformity. A test rig was developed to test the prototypes. Owing to the experimental facility and working fluid (distilled water), the experiment was categorized as either a simultaneously developing flow or a hydrodynamically developed but thermally developing flow. The Reynolds number ranged from 175 to 1370, and the HX was tested with two different heat fluxes of 1.5 kWm−2 and 3 kWm−2 . The experimental results for the smooth channel were compared to widely accepted correlations in the literature. It was found that 79% of the experimental data were within a range of ±10% of the values from existing correlations developed for the thermal entry length. However, a formula developed for the simultaneously developing flow overpredicted the Nusselt number. Furthermore, the results for the enhanced channels showed that embedding VGs can considerably boost the thermal performance up to three times within the parameters of the printed parts. Finally, the thermal performance of the 3D-printed channel showed that AM is a promising solution for the development of minichannel HXs. The generation of 3D vortices caused by the presence of VGs ii can notably boost the thermal performance, thereby reducing the HX size for a given heat duty.

2012/2013
The exploitation of geothermal heat by ground source heat pumps is presently growing throughout Europe and the world. In Italy, at the end of 2010, borehole heat exchangers covered most of the 30% of the total energy used for space conditioning, showing an increase of 50%compared to 2005. The forecasts for 2015 suggest a further increase in the direct uses of the geothermal heat exceeding 50% compared to 2010 and a corresponding increase in the geothermal energy consumption. The possibility to design plants with higher efficiency and lower costs of installation and operation is required, to support the growth of the ground source heat pump systems and the consequent diffusion of the exploitation of the geothermal resources. Research and better knowledge of the processes involved in the heat transfer between the borehole heat exchanger and the surrounding ground is crucial to predict the behavior of the plant-geothermal source interaction in any possible operational condition. The knowledge of the hydrogeological characteristics of the specific site where the plant has to be installed is also essential to prevent over- or under-sizing of the heat exchanger(s) due to a rough design. Over the years, several analytical solutions have been proposed to calculate the temperature distribution around a borehole heat exchanger during operation. The infinite line source analytical model considers an infinite linear heat source which exchanges heat with the surrounding ground by conduction only. Other models, based on the infinite linear heat source, have been later developed, considering also the contribution to the conductive heat transfer due to groundwater flow. The presence of flowing water around a borehole heat exchanger implies forced convection, resulting in an increased efficiency of the heat transfer between the ground and the borehole heat exchanger. Studying this process may suggest new ways to improve the efficiency and to reduce the cost of ground source heat pump systems. In this thesis, the contribution of groundwater flow in the heat transfer process between borehole heat exchangers and surrounding ground has been investigated, in order to increase the theoretical knowledge as well as to improve the existing design tools. Two-dimensional models have been considered, taking into account the actual cylindrical geometry of the borehole. The groundwater flow has been modeled as steady, horizontal and with variable flow rates, in order to encompass most of the real ground source heat pump applications. Gravitational effects, i.e. the effects of a possible natural convection, have been neglected. The results suggest that in the considered range of Darcy number, the calculation of the heat transfer efficiency is not affected if Darcynian model is used to describe the velocity field, although the viscous effects, and consequently the formation of the hydraulic boundary layer, are neglected. Calculations made using numerical simulations are compared with an analytical solution which takes into account forced convection due to groundwater flow and based on the linear heat source model. The regions of space and time where this analytical solution is affected by the effects of the line source assumption, in both cases of single- and multiple-borehole(s) systems, have been defined. The potential of the thermal response test analysis as a tool to predict the spacing between boreholes when groundwater flow occurs has been investigated, defining and studying the Influence Length as function of groundwater flow rate. The results suggest that even relatively low flow rates allow to reduce significantly the spacing between boreholes in the perpendicular direction with respect to groundwater flow. The distance from the borehole where the temperature disturbance becomes not-significant (Influence Length) is roughly predictable by thermal response test analysis. The study of the Influence Length may be a useful tool in the design of dissipative multiple-boreholes systems, as well as in areas with a high density of single-borehole plants, to reduce the spacing avoiding thermal interferences. Moreover, an expeditious, graphical method to estimate the hydraulic conductivity of the ground by thermal response test analysis has been proposed. An example of application of the methodology is presented, taking into account experimental data as well as plausible hydrological and petrological assumptions when the data are unavailable. The obtained result is in agreement with the hydraulic conductivity range reported in literature for the type of substrate considered in the example. In order to verify this method, further inv1estigations and developments are required. In fact, the graphs used in the procedure presented in this work are referred to specific borehole conditions (borehole filled by groundwater) and are based on two-dimensional models (i.e. end-effects and natural convection are neglected). Besides, the assumptions required to compensate the unavailable data imply that the method cannot be considered verified. Finally, further studies are suggested in order to improve and develop the proposed methods.
Negli ultimi anni, l’utilizzo del calore geotermico tramite pompe di calore accoppiate al terreno sta aumentando significativamente in tutta Europa e in generale nel mondo. In Italia, alla fine del 2010, le sonde geotermiche coprivano più del 30% dell’energia totale utilizzata per riscaldamento e raffrescamento degli edifici, mostrando un aumento del 50% rispetto al 2005. Le previsioni per il 2015 suggeriscono un ulteriore aumento degli utilizzi diretti del calore geotermico maggiore del 50% rispetto al 2010 e un analogo incremento del consumo di energia geotermica in generale. Con l’aumento della diffusione di questa tecnologia, e quindi un maggior sfruttamento di tale risorsa, aumenta anche la necessità di progettare impianti con la massima efficienza possibile e con bassi costi di installazione ed esercizio. La comprensione dei processi coinvolti nel trasferimento di calore tra sonda geotermica e terreno circostante è fondamentale per prevedere il comportamento degli impianti. Anche la conoscenza delle caratteristiche idrogeologiche del sito specifico nel quale l’impianto deve essere installato è essenziale al fine di evitare un’errata progettazione che può causare sovra- o sotto-dimensionamento della sonda. Nel corso degli anni, diverse soluzioni analitiche sono state proposte per calcolare la distribuzione di temperatura attorno alla sonda geotermica durante il suo utilizzo. Il modello analitico della sorgente di calore lineare e infinita considera lo scambio di calore che avviene per sola conduzione attorno ad una sorgente di raggio infinitesimo e di lunghezza infinita. Altri modelli successivi a questo e anch’essi basati sulla sorgente di calore lineare ed infinita, tengono conto anche del contributo convettivo dovuto al flusso dell’acqua di falda. La presenza di un flusso di acqua attorno ad una sonda geotermica, infatti, comporta convezione forzata e, di conseguenza, un aumento dello scambio di calore tra sonda e terreno. Per questo motivo, lo studio degli effetti di tale processo è un fattore chiave per riuscire a migliorare l’efficienza degli scambiatori di calore accoppiati al terreno. Questa tesi presenta lo studio del contributo del flusso delle acque di falda sul processodi scambio termico tra sonde geotermiche e terreno circostante, al fine di incrementare la conoscenza teorica e migliorare gli strumenti di progettazione già esistenti. Per raggiungere questo scopo ci si è serviti di modelli numerici bi-dimensionali che tengono conto della reale geometria cilindrica della sonda. Il fusso delle acque di falda è stato assunto come stazionale e orizzontale. Al fine di includere la maggior parte delle applicazioni geotermiche reali, un vasto range di portate è stato preso in considerazione. Gli effetti gravitativi, e quindi i possibili effetti di convezione naturale, sono stati invece trascurati. Sono stati confrontati i risultati del calcolo del trasferimento di calore ottenuti utilizzando rispettivamente l’equazione di Darcy e l’equazione di Darcy-Brinkman per descrivere il campo di velocità dell’acqua di falda attorno alla sonda. Le conclusioni raggiunte suggeriscono che utilizzando il modello di Darcy, il risultato risulta comunque sufficientemente accurato per i numeri di Darcy considerati, nonostante gli effetti viscosi, e quindi la formazione dello strato-limite fluidodinamico, vengano trascurati. I risultati delle simulazioni numeriche sono stati comparati con un modello analitico che prevede convezione forzata, dovuta al flusso di falda, attorno ad una sorgente di calore lineare ed infinita. Sono quindi state definite le regioni dello spazio e del tempo dove tale soluzione analitica è soggetta agli effetti della linearit`a della sorgente, sia nel caso di sonda singola, sia nel caso di campo-sonde. Sono inoltre state studiate le potenzialità dell’analisi del test di risposta termica come strumento per prevedere la spaziatura tra le sonde in funzione della portata del flusso dell’acqua di falda. I risultati suggeriscono che portate relativamente modeste, permettono una riduzione significativa della spazitura tra le sonde in direzione perpendicolare rispetto a quella di scorrimento dell’acqua di falda. Sfruttando l’analisi del test di risposta termica, è possibile stimare approssimativamente la distanza dalla sonda alla quale il disturbo di temperatura diventa trascurabile (distanza di influenza). Lo studio di questa distanza di influenza pu`o essere un utile strumento per la progettazione di sistemi dissipativi composti da sonde multiple, così come nelle aree con un’alta densità di impianti a sonda singola, al fine di ridurre la spaziatura tra le sonde, evitando allo stesso tempo l’insorgere di interferenze termiche tra sonde adiacenti. Inoltre è stato proposto un metodo grafico e speditivo per la stima della conducibilità idraulica del substrato tramite l’analisi del test di risposta termica. È stato presentato un esempio dell’applicazione di questa metodologia utilizzando sia dati sperimentali sia assunzioni plausibili di carattere idrologico e petrologico, quando non è stato possibile avvalersi di dati sperimentali. I risultati ottenuti sono in accordo con i valori di conducibilità idraulica proposti in letteratura per il tipo di substrato dell’esempio. Per poter verificare l’affidabilità di questo metodo, ulteriori studi e sviluppi sono sono necessari. Infatti, i grafici utilizzati nella procedura presentata in questa tesi, si riferiscono a specifiche condizioni della sonda (acqua di falda come materiale di riempimento) e sono inoltre basati su modelli bi-dimensionali (trascurando quindi gli effetti di fine-pozzo e il contributo della convezione naturale). Infine vengono forniti suggerimenti riguardo ulteriori studi che consentirebbero di migliorare e sviluppare ulteriormente le metodologie proposte.
XXVI Ciclo
1985

The cryogenic industry has experienced a continuous growth in the last decades, partially sustained by the worldwide development of Liquefaction of Natural Gas (LNG) projects. LNG technology provides an economically feasible way of transporting natural gas over long distances, and currently accounts for nearly 30% of the international trade of this resource. The economic feasibility of these projects, in terms of both capital and operating costs, is to a large extent controlled by the performance of the main cryogenic two-phase flow heat exchanger. This industrial scenario provides then the motivation for a detailed study of the heat exchanger from a design perspective. On the one hand, it is widely accepted that a highly detailed analysis is required at a micro scale to properly take account of the two phase heat transfer process. On the other hand, a process-level description corresponds to larger time and space scales. In general, determining the proper methodology for considering these scales and their interaction remains a challenging problem. For this reason, current techniques focus in only one particular scale. The main objective of this project is then to develop a multiscale model applicable for two-phase flow heat exchangers. In this context, a three-scale framework is postulated. This thesis was divided into macro, meso (medium) and micro scale analysis. First, a macroscopic analysis provides a broad description in terms of overall heat transfer and pressure drop, using simple models without taking into account the details of physical phenomena at lower scales. Second, at mesoscale level, flow in parallel channels is considered following a homogenization approach, thus including the effects of flow maldistribution and partial mixing. Third, the microscopic description conceives a phenomenological representation of boiling flows, following multifluid formulations, for two specific flow patterns: annular-mist and post-dryout regimes. Finally, a multiscale design algorithm is proposed.

Fouling is a chronic problem in many heat transfer systems and results in the need for frequent heat exchanger (HEX) cleaning. In the dairy industry, the associated operating cost and environmental impact are substantial. Antifouling coatings are one mitigation option. In this work, the fouling behaviour of fluoropolymer, polypropylene and stainless steel heat transfer surfaces in processing raw milk and whey protein solution are studied. Methodologies to assess the economics of antifouling coatings are developed and applied. Two experimental apparatuses were designed and constructed to study fouling at surface temperatures around 90 °C. A microfluidic system with a 650 x 2000 µm flow channel enables fouling studies to be carried out by recirculating 2 l of raw milk. The apparatus operates in the laminar flow regime and the capability to probe the local composition of delicate fouling deposit $\textit{in-situ}$ with histological techniques employing confocal laser scanning microscopy. A larger bench-scale apparatus with a 10 x 42 mm flow channel was built to recirculate 17 l of solution in the turbulent flow regime which is more representative of conditions in an industrial plate HEX. Experimental results demonstrate that fluoropolymer coatings can reduce fouling masses from raw milk and whey protein solution by up to 50 %. Surface properties affect the structure and composition of the deposit. At the interface with apolar surfaces raw milk fouling layers are high in protein, whereas a strongly attached mineral-rich layer is present at the interface with steel. Whey protein deposits generated on apolar surfaces are more spongy and have a lower thermal conductivity and/or density than deposits on steel. The attraction of denatured protein towards apolar surfaces and the formation of a calcium phosphate layer on steel at later stages of fouling are explained with arguments based on the interfacial free energy of these materials in water. The financial attractiveness of coatings is considered for HEX subject to linearly and asymptotically increasing fouling resistance and using a spatially resolved fouling model. An explicit solution to the cleaning-scheduling problem is presented for the case of equal heat capacity flow rates in a counter-current HEX. Scenarios where the use of coatings may be attractive or where there is no financial benefit in cleaning a fouled exchanger are identified. Finally, experimental data are used to estimate the economic potential of fluoropolymer coated HEXs in the ultra-high-temperature treatment of milk. In the considered case, the value of a fluoropolymer coating inferred from the reduction in fouling is estimated to be around 2000 US$/m².

Thesis (M.S. in mechanical engineering)--Washington State University, August 2009.
Title from PDF title page (viewed on Sept. 21, 2009). "School of Engineering and Computer Science." Includes bibliographical references (p. 82-85).

The mathematical equations describing transient heat transfer between the fluid flowing through a fixed bed and a moving bed of packing were formulated. The resistance to heat transfer within the packing due to its finite thermal conductivity was taken into account. An approximate integral method was applied to obtain an analytical solution to transient response of the bed packing. Results for two cases of fixed and moving bed were obtained. The validity of the approximate method was checked against the more exact method employed by Handley and Heggs who obtained the results for a fixed bed of packing with a step change in fluid inlet temperature. It was concluded that the approximate method gives results that agree well with the more exact methods. The method considered here provides a quick determination of the packing mean temperature in order to obtain the effectiveness. The other peculiarity of this method is that the effect of packing thermal conductivity can be examined very quickly since the solution is in analytical form. The analysis of the results revealed that as the thermal conductivity of the packing decreases the difference between its surface and mean temperature increases. A series of charts showing the comparison between the packing surface and mean temperatures for different thermal conductivities are presented. The approximate method was a moving bed of packing. It was packing thermal conductivity is series of charts representing versus dimensionless length conductivities are presented. then applied to the case of concluded that the effect of more severe than expected. A the moving bed effectiveness for different thermal
Applied Science, Faculty of
Mechanical Engineering, Department of
Graduate