Dissertations / Theses on the topic 'Two-phase closed thermosyphon'

An experimental study was conducted to investigate the performance of stationary two-phase closed thermosyphons with two-fluid mixtures. Three mixtures were used as the working fluids: (1) water-ethanol, (2) water-ethyleneglycol and (3) water-glycerol. The results are presented to show the effects of ratio of two fluid component, mean operating temperature, different fluid mixtures, heat flux and inclination of the thermosyphon with respect to the direction of the gravitational force. An analytical model is presented to predict the variation of the overall heat transfer coefficients with mole fraction for the thermosyphon with binary mixtures. A good agreement between the prediction by the model and the experimental results was obtained.

Interest in the use of heat pipe technology for heat recovery and energy saving in a vast range of engineering applications has been on the rise in recent years. Heat pipes are playing a more important role in many industrial applications, especially in increasing energy savings in commercial applications and improving the thermal performance of heat exchangers. Computational techniques play an important role in solving complex flow problems for a large number of engineering applications due to their universality, flexibility, accuracy and efficiency. However, up to now, computational studies on heat pipes are still at an early stage due to the complexity of multiphase flow characteristics and heat and mass transfer phase changes. Therefore, the main objective of this study is to develop a CFD modelling that includes the complex physical phenomena of both the heat transfer processes of evaporation and condensation and the mass transfer process of phase change during the pool boiling and film condensation. In this thesis, two novel numerical models were developed in ANSYS FLUENT. In the first, a two-dimensional CFD model was developed to visualise the two-phase flow and the evaporation, condensation and heat transfer phenomena during the operation of a wickless heat pipe, that otherwise could not be visualised by empirical or experimental work. An in-house code was developed using user-defined functions (UDFs) to enhance the ability of FLUENT to simulate the phase change occurring inside the heat pipe. Three different fluids, water, R134a and R404a, were selected as the working fluids of the investigated wickless heat pipe. The cooling system of the condenser section was simulated separately as a three-dimensional CFD model of a parallel-flow double pipe heat exchanger to model the heat transfer across the condenser section's heat exchanger and predict the heat transfer coefficients. The overall effective thermal resistance along with the temperature profile along the wickless heat pipe have been investigated. An experimental apparatus was built to carry out a thermal performance investigation on a typical wickless heat pipe for the purpose of validating the CFD simulation. A theoretical model based on empirical correlations was developed to predict the heat transfer thermal resistances in the evaporator and the condenser section. The second model was developed to combine the two-dimensional CFD simulation of the wickless heat pipe and the three-dimensional CFD simulation of the condenser section's heat exchanger to simulate the two-phase flow phenomena of boiling and condensation and the cooling system of the condenser section through a comprehensive three-dimensional CFD model of a wickless heat pipe. Two fluids, water and R134a, were selected as the working fluids of the investigated wickless heat pipe. This model was validated using a transparent glass wickless heat pipe to visualise the phenomena of pool boiling and comparing the results with the three-dimensional CFD flow visualisation. This study demonstrated that the proposed CFD models of a wickless heat pipe can successfully reproduce the complex physical phenomena of both the heat transfer process of evaporation and condensation and the mass transfer process of phase change during the pool boiling that takes place in the evaporator section and the filmwise condensation that takes place in the condenser section. The CFD simulation was successful in modelling and visualising the multiphase flow characteristics for water, R134a and R404a, emphasising the difference in pool boiling behaviour between these working fluids. The CFD simulation results were compared with experimental measurements, with good agreement obtained between predicted temperature profiles and experimental temperature data.

Computational Fluid Dynamics (CFD) has become one of the main instruments for the prediction of many commercial and research oriented fluid flow and heat transfer problems. While single phase flow analysis through CFD has gained grounds within the commercial industry, multiphase flow analysis is still the subject of further research and development. Heat Pipes and thermosyphones are no exception to this. However, the involvement of more than one fluid phase within these devices has made their analysis through CFD more challenging and computationally more demanding to perform. In this thesis, computational fluid dynamics is used as a modelling tool in order to predict the thermal hydraulic behaviour of multiphase environment within thermosyphones and heat pipes. Eulerian two-fluid method is used to solve the conservation equations for mass, momentum and energy, for each phase along with the inclusion of interfacial heat and mass transfer terms. Numerical predictions are obtained for the steady-state and transient operation of stationary thermosyphon, while rotating heat pipes operation is also simulated using axially and radially rotating heat pipe models. Apart from using the commercially available CFD code for the analysis of thermosyphones related simulation, numerical work is performed regarding the coupling of momentum equations based on Eulerian two-fluid modelling scheme. OPENFOAM open source code is used and modified to include the Partial Elimination Algorithm (PEA) for the coupling of interfacial exchange terms, including interfacial mass transfer term, in the momentum equations of both phases. Results obtained from above discussed studies provide good agreement with corresponding experimental and analytical observations.

The present study is concerned with a cooling system for multichip modules (MCM) in telecommunication systems. Due to the higher device operation speed and higher packaging density, the resulting heat flux is known to be 1 to 2 W/cm$\sp2$, which is one or two orders of magnitude higher than those of conventional systems, and beyond the capacity of high performance air cooling systems. A cooling system which can deal with a high heat flux of up to 4 W/cm$\sp2$ is the one that employs two-phase closed thermosyphons, (i.e., wickless heat pipes) and the present study presents the results of experimental and simulation study on a cooling system using two-phase closed thermosynphons for the cooling of MCM.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2011.
ENGLISH ABSTRACT: The feasibility of a closed loop thermosyphon for the Reactor Cavity Cooling System of the Pebble Bed Modular Reactor has been the subject of many research projects. Difficulties identified by previous studies include the hypothetical inaccuracies of heat transfer coefficient correlations available in literature. The aim of the research presented here is to develop inside-pipe heat transfer correlations that are specific to the current design of the RCCS. In order to achieve this, a literature review is performed which identifies reactors which employ closed loop thermosyphons and natural circulation. The literature review also explains the general one-dimensional two-fluid conservation equations that form the basis for numerical modelling of natural circulation loops. The literature review lastly discusses available heat transfer coefficient correlations with the aim of identifying over which ranges and under which circumstances these correlations are considered accurate. The review includes correlations commonly used in natural circulation modelling in the nuclear industry in aims of identifying correlations applicable to the modelling of the proposed RCCS. One of the objectives of this project is to design and build a one-third-height-scale model of the RCCS. Shortcomings of previous experimental models were assessed and, as far as possible, compensated for in the design of the model. Copper piping is used, eliminating material and surface property uncertainties. Several sight glasses are incorporated in the model, allowing for the visual identification of two-phase flow regimes. An orifice plate is used allowing for bidirectional flow measurement. The orifice plate, thermocouples and pipe-in-pipe heat exchangers are calibrated in-situ to minimize experimental error and aid repeatability. Twelve experiments are performed with data logging occurring every ten seconds. The results presented here are limited to selected single and two-phase flow operating mode results. Error analyses and repeatability of experimental measurements for single and two-phase operating modes as well as cooling water mass flow rates are performed, to show repeatability of experimental results. These results are used to mathematically determine the experimental inside-pipe heat transfer coefficients for both the evaporator and condenser sections. Trends in the heat transfer coefficient profiles are identified and the general behaviour of the profiles is thoroughly explained. The RCCS is modelled as a one-dimensional system. Correlations for the friction factor, heat transfer coefficient, void fraction and two-phase frictional multiplier are identified. The theoretical heat transfer coefficients are calculated using the mathematical model and correlations identified in the literature review. Fluid parameters are evaluated using experimentally determined temperatures and mass flow rates. The resulting heat transfer coefficient profiles are compared to experimentally determined profiles, to confirm the hypothesis that existing correlations do not accurately predict the inside-pipe heat transfer coefficients. The experimentally determined coefficients are correlated to 99% confidence intervals. These generated correlations, along with identified and established twophase heat transfer coefficient correlations, are used in a mathematical model to generate theoretical coefficient profiles. These are compared to the experimentally determined coefficients to show prediction accuracy.
AFRIKAANSE OPSOMMING: Die haalbaarheid van ‘n natuurlike sirkulasie geslote lus vir die Reaktor Holte Verkoeling Stelsel (RHVS) van die Korrelbed Modulêre Kern-Reaktor (KMKR) is die onderwerp van talle navorsings projekte. Probleme geïdentifiseer in vorige studies sluit in die hipotetiese onakkuraatheid van hitte-oordrag koëffisiënt korrelasies beskikbaar in literatuur. Die doel van die navorsing aangebied is om binne-pyp hitte-oordrag koëffisiënt korrelasies te ontwikkel spesifiek vir die huidige ontwerp van die RHVS. Ten einde dit te bereik, word ‘n literatuurstudie uitgevoer wat kern-reaktors identifiseer wat gebruik maak van natuurlike sirkulasie lusse. Die literatuurstudie verduidelik ook die algemene een-dimensionele twee-vloeistof behoud vergelykings wat die basis vorm vir numeriese modellering van natuurlike sirkulasie lusse. Die literatuurstudie bespreek laastens beskikbare hitte-oordrag koëffisiënt korrelasies met die doel om te identifiseer vir welke massavloei tempo waardes en onder watter omstandighede hierdie korrelasies as korrek beskou is. Die ontleding sluit korrelasies in wat algemeen gebruik word in die modellering van natuurlike sirkulasie in die kern industrie met die hoop om korrelasies vir gebruik in die modellering van die voorgestelde RHVS te identifiseer. Een van die doelwitte van die projek is om ‘n een-derde-hoogte-skaal model van die RHVS te ontwerp en te bou. Tekortkominge van vorige eksperimentele modelle is geidentifiseer en, so ver as moonlik, voor vergoed in die ontwerp van die model. Koper pype word gebruik wat die onsekerhede van materiaal en opperkvlak eindomme voorkom. Verkseie deursigtige polikarbonaat segmente is ingesluit wat visuele identifikasie van twee-fase vloei regimes toelaat. ‘n Opening plaat word gebruik om voorwaartse en terugwaartse vloeimeting toe te laat. Die opening plaat, termokoppels en hitte uitruilers is gekalibreer in plek om eksperimentele foute te verminder en om herhaalbaarheid te verseker. Twaalf eksperimente word uitgevoer en data word elke tien sekondes aangeteken. Die resultate wat hier aangebied word, is beperk tot geselekteerde enkel- en tweefase vloei meganismes van werking. Fout ontleding en herhaalbaarheid van eksperimentele metings, om die herhaalbaarheid van eksperimentele resultate te toon. Hierdie is gebruik om wiskundig te bepaal wat die eksperimentele binne-pyp hitte-oordrag koëffisiënte is vir beide die verdamper en kondenseerder afdelings. Tendense in die hitte-oordrag koëffisiënt profiele word geïdentifiseer en die algemene gedrag van die profiles is deeglik verduidelik. Die RHVS is gemodelleer as 'n een-dimensionele stelsel. Korrelasies vir die wrywing faktor, hitte-oordrag koëffisiënte, leegte-breuk en twee-fase wrywings vermenigvuldiger word geïdentifiseer. Die teoretiese hitte-oordrag koëffisiënte word bereken deur middle van die wiskundige model en korrelasies wat in literatuur geidentifiseer is. Vloeistof parameters is geëvalueer met eksperimenteel bepaalde temperature en massa-vloei tempos. Die gevolglike hitte-oordrag koëffisiënt profiles is vergelyk met eksperimentele profiele om die hipotese dat die bestaande korrelasies nie die binne-pyp hitte-oordrag koëffisiënte akkuraat voorspel nie, te bevestig. Die eksperimenteel bepaalde koëffisiënte is gekorreleer en die gegenereerde korrelasies, saam met geïdentifiseerde twee-fase hitte-oordrag koëffisiënt korrelasies, word gebruik in 'n wiskundige model om teoretiese koëffisiënt profiele te genereer. Dit word dan vergelyk met die eksperimenteel bepaalde hitteoordrag koëffisiënte om die akkuraatheid van voorspelling te toon. Tekortkominge in die teoretiese en eksperimentele model word geïdentifiseer en aanbevelings gemaak om hulle aan te spreek in die toekoms.

碩士
淡江大學
機械與機電工程學系碩士班
95
In this study, a two-phase closed thermosyphon was fabricated with annular array grooved wicks by copper. The thermosyphon consisted of a 82mm long cooper tube having an inside diameter of 19.4mm and outside diameter of 25.4mm. Rectangular grooves with 0.4mm width and 2.4mm deep were cut by WEDM inside the tube to provide device wick capillary. The tube was sealed with other components by copper-silver welding. Thermal performance of the thermosyphon was evaluated experimentally in a fan-heat sink CPU test apparatus with a heating area of 31mm×31mm. the clamping pressure between the thermosyphon and the heat sink was maintained at 13.5 kgf during the test. The influence of working fluid filling ratio and heat load are studied. D.I. water was used as the working fluid and three different filling ratio,10.5%, 18.2% and 29.7% were investigated in the research. After evaluation, the thermosyphon with 18.2% fill rate showed the best performance compared to the other samples. It had a evaporator temperature at 80℃, corresponding to a thermal resistance of 0.35℃/watt at an actual input power of 140W.

碩士
大同大學
機械工程研究所
91
This article is mainly concentrated on the properties of heat transfer mechanism of the latent heat rapidly released by two-phase closed thermosyphon. This device will be applied to the conventional aluminum heat sink device so as to improve the efficiency of heat conductivity and meet the efficient heat-sinking demand of the high temperature electronic component in the future. This research mainly aims at three critical points. The first is filling charge ratio of working fluid. When the filling charge ratio is remarkably low, it will cause the drought phenomenon on the evaporated part. If the fulfillment ratio is too high, it will decrease the boiling phenomena. Seeing from the data, the filling charge ratio of 10% will assure of the best efficiency. The second is wind blowing intensity and fan location. Suitable consideration of fan location will make air pass through the long and thin fin pads with the excellent air circulation to improve the overall efficiency of heat sink. Knowing from the present results, it is found that the wind blowing direction from bottom upside will create better efficiency. Also, the value of wind intensity will affect the efficiency of heat conductivity. The third is additive nanometer material. The evaporated part is added with the nanometer materials of high heat conductivity to improve the heat absorption rate. In the present study, the effect due to the nanofluid is not pronounced.

碩士
中國文化大學
機械工程學系數位機電碩士班
107
The purpose of this study is to study and analyze the two-phase thermosyphon heat pipe ice storage tank. The two-phase thermosyphon heat pipe ice storage tank proposed by the research institute has three different operation modes: general mode, ice storage mode and ice melting mode. In this paper, the heat exchange characteristics of two-phase thermosyphon heat pipes in different modes are analyzed experimentally. In the research on the performance of ice storage mode, the working fluids used in the heat pipe of this study are R-134a and R-410a. The internal working fluid of the heat pipe absorbs the heat of the energy storage material to generate boiling, and the gaseous working fluid flows upward to the double set due to buoyancy. In the tube heat exchanger, after the low-temperature refrigerant generated by the vapor compression cycle exchanges heat with the working fluid in the double casing, the working fluid inside the heat pipe condenses, and the condensate acts by gravity along the heat pipe wall. During the process of flowing down to the bottom of the heat pipe, the heat of the energy storage material is absorbed into a film Evaporation, and the heat is returned to the heat storage end of the ice storage end to complete the cycle. In this paper, the ice storage tank running performance test is carried out by experimentally studying the working fluid filling amount of 42%, 50% and 58%. By changing the brine temperature provided by the constant temperature water tank, the cold storage performance of each working fluid filling amount is analyzed and discussed. Analysis of heat exchange characteristics of phase-heat siphon heat pipes.

碩士
國立臺灣大學
機械工程學研究所
107
In recent years, due to the dramatic changes in the climate, the amount of electricity used in summer air conditioners has risen sharply. If the peaks can be used for power transfer, using off-peak power for energy storage can help improve the power supply situation in Taiwan. In this study, the superior conduction characteristics of the two-phase thermosyphon heat pipe are used for energy transfer and storage, and can be operated without the supply of external electric energy. Film evaporation and film condensation of the working fluid inside the thermosiphon (Film) Condensation) The high heat transfer capacity mechanism for heat exchange can effectively improve the performance of the ice storage tank. This study proposes a new two-phase thermosyphon heat pipe ice storage tank design, analyzing the ice storage performance of different ice storage temperatures, exploring the parameters of the influence through theoretical models, and compares the heat transfer with the ice storage tank experimental test. The error rate is approximately 3.5% to 6%. Then, based on the theoretical model of the thermoresistive capacitance, the improvement strategy of the thermosyphon heat pipe ice storage tank is analyzed, and the direction of improving the ice storage performance in the future is proposed.

碩士
國立臺灣大學
機械工程學研究所
100
The rmosyphons are also called gravity-supported heat pipes. They are highly efficient heat transfer elements which become increasingly applied in terrestrial heat transport and heat recovery system. Their performance is limit by various heat transport limitations. When thermosyphons work at high temperature , the maximum operating heat transfer rates for two-phase closed thermosyphons due to entrainment limitation or boiling limitation. And these limitations will affect the heat transfer performance. Experiments with three copper water thermosyphons, which are 130mm、200mm、300mm long respectively and 7mm outer diameter have been carried out. According to the error analysis, the error between the boiling limitation measured by experiment and the prediction by theory is 8.7 percentage, and the average error of entrainment limitation between experiment and theory is 6.7 percentage. We can take the result of this experiment as standard when we use the other size of thermosyphons to measure the operating limit limits in the future, and predict by the programs.

博士
國立臺灣大學
機械工程學研究所
91
It’s widespread to use heat pipes in many occasions, i.e., aeronautics and astronautics technology, electronics industry and national defense technology. The heat pipe was utilized to solve the problems involving heat transfer due to its superior heat transfer characteristics. The purpose of this thesis is to study the thermal performance of the energy storage system which utilizes two-phase closed thermosyphon as heat transfer mechanism and use phase change material as energy storage material. According to the conclusions of this thesis, the stored thermal heat per unit volume of the system, which using paraffin wax 116 as energy storage material, is 53.8% more than using water as. The results show that the storage system employing alcohol as working fluid in the loop thermosyphon provides better performance; the system gives optimum heat charge and discharge performance under 70% fill ratio, regardless whether the working fluid is water or alcohol. The system displays optimum charge efficiency of 62% and optimum discharge efficiency of 71% with alcohol as working fluid and using phase change material as energy storage material. Furthermore, the experimental results of the system undergoing simultaneous heat charge and heat discharge show that, the system is verified to regulate the energy automatically.

碩士
國立清華大學
工程與系統科學系
104
A two-phase closed loop thermosyphon (TPCLT) is a high-efficiency two-phase heat transfer device, which is composed of evaporator, condenser, vapor head, vapor line, liquid head and liquid line. The basic concept is the phase change of a working fluid is used to transport heat from evaporator to condenser through natural circulation without any external driving forces. The device is capable to transport heat for a long distance, so it is proper to apply on the building integrated solar thermal systems. This study presents the possibility of the TPCLT apply on the solar thermal storage system. This experiment contains two different types of TPCLT, the small one and the large one. In small TPCLT, experimental results show that when heating power is 360W, the non-symmetric TPCLT’s efficiency is 70.88%, the top-U-symmetric TPCLT’s efficiency is 74.6% and the both-U-symmetric TPCLT’s efficiency is 76.1%. When the porous material is put in the top-U-symmetric TPCLT, the efficiency increases to 81%. In large TPCLT, experimental results show that the storage efficiency is 69% in filling ratio 70% with heating power 630W. The storage efficiency is 63% in filling ratio 60% with heating power 630W. When the system is in filling ratio 70%, the lowest start-up heating power is 270W. When the system is in filling ratio 60%, the lowest start-up heating power is 180W.

博士
國立清華大學
工程與系統科學系
105
The objected of this paper is propose using capillary pumped loop (CPL) and two-phase closed loop thermosyphon (TPCLT) applied to a solar water heater. Experimental investigation of performance of CPL and TPCLT in different condition. Due to those devices are high-efficiency heat transfer device capable of transporting thermal energy over long distances without the need for other mechanical forces, such as pumps. This makes CPL and TPCLT particularly suitable for applications involving solar water heaters and apply on the building integrated solar water heater. First, we presents an experimental examination of a railing-type collector solar water heater, employing a CPL as a heat transfer device. The structure and characteristics of the CPL solar water heater are also outlined. We conducted various experiments to investigate the start-up behavior and thermal storage efficiency under various filling ratios with different heat loads and tilt angles. Experiment results revealed that 70% is the ideal filling ratio for a CPL and that the heat load presents critical limitations with regard to stable operations. The highest thermal storage efficiency obtained in this study was 77%. We also determined that when a railing-type collector has a non-zero tilt angle, loose limitations pertaining to heat load can be relaxed without sacrificing stable operations. The first experiment results show that the fill ratio of the working fluid has a critical impact on the performance and start-up time of TPCLTs. Our objective in this work was to increase the performance of TPCLT solar water heaters by using an evaporator with a porous wick structure (PWS), especially in low heating power. Our results demonstrate that employing a PWS within the evaporator can enhance efficiency by 12.7% and decrease start-up time by 26.5% under low heating power. The above experiment setup all use small scaler solar water heater and operating in laboratory environment. Therefore, we designed the larger scaler TPCLT solar water heater and operating in laboratory environment and outdoor environments. In laboratory environment, the results shows that the efficiency is 69% in filling ratio 70% with heating power 630W. The efficiency is 63% in filling ratio 60% with heating power 630W. When the system is in filling ratio 70%, the lowest start-up heating power is 270W. When the system is in filling ratio 60%, the lowest start-up heating power is 180W. In outdoor environment, the water temperature can reach 42 OC.

碩士
國立臺灣大學
機械工程學研究所
93
With the development of electronic industry, electronic components keep going for high performance and small volume. Since the heat in unit area is getting higher and higher, the problem of cooling appears. In early days, people solve these problems with fins and fans, but this method won’t work for the electronic components of next generation. So we have to solve the problem of cooling the electronic components of next generation in other ways. In the research, we studied the performance of Two Phase Close Loop Thermosyphon Cooling System under different parameter with water as working fluid. There are evaporator, condenser, and adiabatic section in the system. The working fluid in the evaporator absorbs the heat from the electronic components and changes its phase to vapor by the mechanism of evaporation or boiling. Then the vapor goes through adiabatic section to the condenser to release the heat and condense. There are three enhanced surfaces-grooved surface, etched surface, and sintered surface with vertical condenser for the system. We studied the performance of these three surfaces under different filling ratio and power. Then compared the performance of condenser with Chang’s[29]. In the research, it was found that the total thermal resistance, evaporator thermal resistance, and condenser thermal resistance are all going down with the increasing power. It says that the system is suitable for cooling the electronic components of next generation. And we also found that because of thin film evaporation, the system will have the best performance at 15% filling ratio with sintered surface.

碩士
國立臺灣大學
機械工程學研究所
94
The increase that the electronic product makes performance and processing speeds constant with the progress of science and technology, in order to accord with the demand of industry, but electronic product can produce a large amount of heat energy too in while in high-speed computation, so how make heat energy valid to is it produce to come loose under the area in limited heat dissipation, becoming a ring which can''t be ignored in the electronic product design at present, the most frequently used way is one slice of associations of fan fin now, because the heat energy of the electronic product is higher and higher in density, and the heat dissipation performance of slice of fan fin has already been close to the bottleneck, so research Two Phase Close Thermosyphon Cooling System as the heat dissipation to try, research this heat dissipation mould group behavior difference, heat dissipation of performance under different experiment parameter by way of experiment, find out this heat dissipation optimization, mould of group design, two phase flow closed thermosyphon heat dissipation system can divide into person who evaporate, condensation department, convection department three major part mainly, its operation principle is to utilize the liquid job fluid in the evaporating department to produce the looks change in order to absorb a large amount of heat energy, turn into job fluid of gaseous state will rise to condensation department and is it condense hot fast release among the external world to produce ancient piece of jade. The experiment parameter of this experiment has surface structure of filling rate, the heating rate of the simulation heat source, evaporating department baseplate of the heat dissipation system, study every experiment heat dissipation performance of parameter display in order to improve direction of system this as future, found this system has good heat dissipation performance to the high heating rate by the experimental result, because electronic product higher and higher in density to heat energy at present once, this experiment heat dissipation system has very fine heat dissipation to behave to apply to electronic cooling.