Добірка наукової літератури з теми "Limiting thermal flow"

The paper presents the specification of research methods commonly encountered in the studies of heat transfer processes in minichannels. In particular the following methods have been emphasized: electrochemical limiting current method as well as the thermal balance method. In thermal balance method the mean heat transfer coefficient is determined by the set of experimental thermal measurements of the investigated heat exchanger. In turn, limiting current method is based on heat and mass transfer analogy. The discussed research methods have been implemented on two specially designed and constructed test facilities with compact minichannel heat exchangers, which have been presented and described in details. In order to validate the designed setup, the preliminary experimental measurements of two minichannel heat exchangers with hydraulic diameter of 2 mm and rectangular cross sections during single-phase liquid flows have been carried out. In further perspective it is planned to extend the experimental studies of minichannel heat exchangers and to compare the results obtained by both methods described.

Often used model of turbulent convection from localized sources of buoyancy and (or) momentum acting for a short time – isolated thermals. In such a model, the perturbation region (thermal) is approximately represented as a “bubble” or a vortex ring of variable volume and mass that rises (or descends depending on the perturbation sign). The volume of thermals is gradually increasing due to the capture of adjacent volumes of the environment (“entrainment”). The dynamics of a thermal is described by a nonlinear system of ordinary differential equations – the equations of balance of mass, momentum and buoyancy. In the present work, the nonlinear integral model of turbulent thermals is generalized to the case of the presence of horizontal components of its motion relative to the medium (for example, the emergence of a thermal in a shear flow). Compared to traditional models, the possibility of the presence in the thermal of volume heat and momentum sources is additionally taken into account. The problem is solved in quadratures. One of the possible applications is the artificial stimulation by local sources of impulse of downward movements in the atmosphere in order to influence convective clouds. The solution depends on nine parameters – stratification, vertical shear of the background current, intensities of the above-mentioned volume sources, initial conditions for the thermal radius, its buoyancy, and the three components of the thermal velocity. Different limiting cases are analyzed. Attention is paid to the nonlinear effect of the interaction of the horizontal and vertical components of the thermal motion, since each of the components affects the intensity of entrainment, i.e. on the growth rate of thermal dimensions and, consequently, on its mobility. Intensification of entrainment due to the interaction of a thermal with a transverse flow can lead to a significant decrease in its mobility. From this, in turn, depends on the degree of horizontal transfer of a thermal by a background current. Some limiting cases were previously analyzed in the author’s cited below. This study was supported by Program 56 of the Fundamental Research of the Presidium of the Russian Academy of Sciences.

In this work, an analytical study on the limiting thermal transport characteristics for the flow of a non-Newtonian fluid namely, power law liquid, between asymmetrically heated parallel plates is presented. The transport equations for mass, momentum, and energy are analytically solved to obtain closed-form expressions for the temperature distributions in the flow field and the limiting Nusselt number in terms of relevant parameters. The effects of frictional heating due to viscous dissipation on the heat transfer characteristics are studied. Results are presented in terms of degree of asymmetric heating, Brinkman number, and power law index. The most important finding from the present work is the nontrivial interplay between degree of asymmetric heating and the rheological behavior of the fluid in dictating the thermal transport characteristics of heat.

This paper studies the thermal flow concentration near an interface crack in a layered medium. Solution method for the thermal flow intensity factor is established. Both the Griffith crack and the penny-shaped crack are studied. Limiting cases of the current problem include (1) the solution of crack problem associated with classical Fourier heat conduction, (2) the solution of an interface crack in an infinite layered medium, and (3) the solution of a crack in a homogeneous medium.

Density variations in fluid flows can arise due to acoustic or thermal fluctuations, compositional changes during mixing of fluids with different molar masses, or phase inhomogeneities. In particular, thermal and compositional (with miscible fluids) density variations have many similarities, such as in how the flow interacts with a shock wave. Two limiting cases have been of particular interest: ( a) the single-fluid non-Oberbeck–Boussinesq low–Mach number approximation for flows with temperature variations, which describes vertical convection, and ( b) the incompressible limit of mixing between miscible fluids with different molar masses, which describes the Rayleigh–Taylor instability. The equations describing these cases are remarkably similar, with some differences in the molecular transport terms. In all cases, strong inertial effects lead to significant asymmetries of mixing, turbulence, and the shape of mixing layers. In addition, density variations require special attention in turbulence models to avoid viscous contamination of the large scales.

The theory of plasma ablation by laser irradiation from cylindrical and spherical solid targets is considered when thermal conduction is dominant and absorption is local at the critical density. Analytic solutions for both inhibited and uninhibited heat fluxes are developed, but only investigated in detail when flux limiting does not introduce a step discontinuity. In most cases it is found that only a restricted region of flow is steady, and must be terminated by a rarefaction wave. The transition from quasi-planar to strongly divergent flow is shown to depend on a characteristic parameter, which represents the ratio of the thermal conduction length to the target radius.

The paper tests the changes in the pH value of the anolyte and catholyte. The 3-D multi-phase 3-D multi-current conductivity values analyze the electricity generation process and energy utilization of the microbial fuel cell (AMFC) and provide a theory for improving the AMFC following the performance. The test results show that with the operation of AMFC, the pH value of the anolyte and the 3-D multi-flow conductivity show a downward trend, the pH value of the catholyte and the 3-D multi-flow conductivity show an upward trend, and the ratio of the pH value of the catholyte the pH value of the anolyte is about 0.30-0.50 higher, and the average 3-D multi-current conductivity of the anolyte and catholyte does not change much. When AMFC operates stably, the internal ohmic resistance is 29.69 ?, the limiting current is 2.69 mA, the maximum output power is about 0.8 mW, and the corre?sponding internal resistance is about 95.72 ?. The mass transmission of potassium ferricyanide is the limiting factor of limiting current. Numerical analysis of 3-D multi-phase flow found that other microorganisms consume 91.1% of the glucose in AMFC anolyte, and only 8.9% of the glucose is used for power generation. The 88.5% of the energy of the glucose used for power generation is converted into other forms of energy, only 11.5% of the energy is converted into electricity.

The utilization rate of ink liquid in the chamber is critical for the thermal bubble inkjet head. The difficult problem faced by the thermal bubble inkjet printing is how to maximize the use of ink in the chamber and increase the printing frequency. In this paper, by adding a flow restrictor and two narrow channels into the chamber, the H-shape flow-limiting structure is formed. At 1.8 μs, the speed of bubble expansion reaches the maximum, and after passing through the narrow channel, the maximum reverse flow rate of ink decreased by 25%. When the vapor bubble disappeared, the ink fills the nozzle slowly. At 20 μs, after passing through the narrow channel, the maximum flow rate of the ink increases by 39%. The inkjet printing frequency is 40 kHz, and the volume of the ink droplet is about 13.1 pL. The structure improves the frequency of thermal bubble inkjet printing and can maximize the use of liquid in the chamber, providing a reference for cell printing, 3D printing, bioprinting, and other fields.

The increasing global demand for energy necessitates devoted attention to the formulation and exploration of mechanisms of thermal heat exchangers to explore and save heat energy. Thus, innovative thermal transport fluids require to boost thermal conductivity and heat flow features to upsurge convection heat rate, and nanofluids have been effectively employed as standard heat transfer fluids. With such intention, herein, we formulated and developed the constitutive flow laws by utilizing the Rossland diffusion approximation and Stephen’s law along with the MHD effect. The mathematical formulation is based on boundary layer theory pioneered by Prandtl. Governing nonlinear partial differential flow equations are changed to ODEs via the implementation of the similarity variables. A well-known computational algorithm BVPh2 has been utilized for the solution of the nonlinear system of ODEs. The consequence of innumerable physical parameters on flow field, thermal distribution, and solutal field, such as magnetic field, Lewis number, velocity parameter, Prandtl number, drag force, Nusselt number, and Sherwood number, is plotted via graphs. Finally, numerical consequences are compared with the homotopic solution as a limiting case, and an exceptional agreement is found.

Design of airborne multi-output power supply unit (MOPS) is restricted by space, weight and predefined geometry of air flow path. The unit is cooled by ram air and hence, exposed to the extreme external thermal environment that changes typically from +55°C to -40°C, from ground to cruising altitude within a few minutes. Hence the design should meet the thermal requirements of the electronics inside the packaging adequately, for both the positive and negative extremities of the temperature, so that device limiting temperatures are not exceeded. At the same time, it must accommodate the necessary circuitry. Details of the thermal and mechanical design and performance of the MOPS unit at various altitudes, hot spot location, flow requirements and optimal heat sink design are presented in this paper.

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.14.06 «Технічна теплофізика та промислова теплоенергетика». – Національний технічний університет України «Київський політехнічний інститут імені Ігоря Сікорського», Міністерство освіти і науки України, Київ, 2018. Робота присвячена підвищенню енергетичної ефективності та спрощенню інтеграції сонячних систем на основі комбінованих сонячних колекторів у фасади і дахи будівель за рахунок використання як елемента теплопоглинальної поверхні алюмінієвих канавчатих теплових труб. Встановлено, що на ефективність роботи комбінованого сонячного колектора з алюмінієвими канавчатими тепловими трубами у режимі термосифона істотно впливають теплотехнічні характеристики теплових труб, які своєю чергою залежать від таких параметрів: діаметр парового простору, теплофізичні властивості робочої рідини, довжини зон нагріву і конденсації, а також загальна довжина алюмінієвих канавчатих теплових труб. Підвищення теплопередавальної здатності алюмінієвих канавчатих теплових труб можна досягти завдяки забезпеченню подачі гарантованої кількості теплоносія в зону нагріву та вибору оптимальних конструктивних параметрів теплових труб при відповідних режимах роботи. Аналіз розрахунків та експериментальних даних показав, що комбінований сонячний колектор з алюмінієвими канавчатими тепловими трубами дає змогу підвищити ефективність отримання електричної енергії до 18 % за рахунок охолодження фотоелектричних перетворювачів, при цьому максимальна електрична потужність комбінованого сонячного колектора становила 135 Вт/м2. Крім електроенергії, одночасно можна отримати до 457 Вт тепла з 1 м2 теплопоглинальної поверхні за температури вихідного теплоносія 25 ºС і густини сонячного потоку 900 Вт/м2. На основі теоретичного аналізу виявлено найбільш оптимальні режими експлуатації комбінованого сонячного колектора – режим функціонування за значень 30–50 ºС температурного перепаду між теплопоглинальною поверхнею та навколишнім середовищем. Нова конструкція комбінованого сонячного колектора має більш ефективну роботу порівняно з роздільними тепловими сонячними колекторами та фотоелектричними батареями за низьких температур на теплопоглинальній поверхні (нижче 50 ºС) і зазвичай за більш високих значень сонячного потоку (більше 600 Вт/м2).
Thesis for the Candidate degree in Technical Science on the specialty 05.14.06 «Technical thermophysics and industrial thermal power engineering». – National Technical University of Ukraine "Igor Sikorsky Kyiv Polytechnic Institute", Ministry of Education and Science of Ukraine, Kyiv, 2018. The work is devoted to increasing energy efficiency and simplification of integration of solar systems on the basis of the photovoltaic thermal (PV/T) collector in the facades and roofs of buildings due to use as an element of the absorbing surface of aluminum grooved heat pipes (AGHP). It is established that the efficiency of the operation of the PV/T collector with AGHP in the thermosyphon mode is significantly influenced by the thermal characteristics of the HP, which in turn depends on the following parameters: the diameter of the steam space, the thermophysical properties of the working fluid, the lengths of the heating and condensation zones, and the total length of the AGHP. Increasing the thermal conductivity of AGHP can be achieved by providing a guaranteed amount of coolant to the heating zone and selecting the optimal design parameters of the HP at the appropriate operating modes. A new approach to the implementation of PVT collectors on the basis of AGHPs is proposed. In this case, AGHPs perform a complex role – at the same time it is a highly efficient thermal conductor and a system of cooling solar cells. The design of an aluminum heat pipe with a grooved capillary structure for PVT collectors has been developed. An n-pentane is chosen as the optimum coolant for a two-phase system. The developed samples of heat pipes can provide the operation of the PVT collector in the thermal mode from 0 oC to 120 oC. In this case, the temperature range of its operation is from −40 °C to +230 °C. The analysis of calculations and experimental data showed that the PV/T collector with AGHP allows to increase the efficiency of obtaining electric energy up to 18 % due to the cooling of the PV, while the maximum electric power PV/T collector was 135 W/m2. In addition to electricity, simultaneously, it is possible to get up to 457 W of heat from 1 m2 of heat-absorbing surface, at a temperature of the output coolant 25 oС and a density of solar flux of 900 W/m2. On the basis of theoretical analysis, the most optimal modes of operation of the PV/T collector were identified – the most optimal one is the mode of PV/T collector functioning at values of 30–50 oC of the temperature difference between the absorbent surface and the environment. The new PV/T collector design has a more efficient performance compared to separate thermal solar collectors and photoelectric batteries at low temperatures on an absorbent surface (below 50 oC), and usually at higher solar flux values (over 600 W/m2). The first developed programs and methods of research of PVT collectors in artificial and natural light developed an engineering methodology for calculating the thermal characteristics of PVT collector with AGHPs during their operation in a thermosyphon mode. The recommendations for the production of PVT collectors and their use in solar power systems are given. The results of the work in the future can be used at the enterprises of LLC «Effectprof» (Kyiv), PC Sumy SPO M.V. Frunze (Sumy), PE Scientific-Implementation Firm "Thermal Technologies" (Kiev), which are engaged in the development, manufacture and implementation of heat-exchange equipment and energy-efficient systems. For further implementation, it is necessary to carry out works on designing and manufacturing an industrial design of PVT collector or facade PVT collector and to conduct tests in the field.
Диссертация на соискание ученой степени кандидата технических наук по специальности 05.14.06 «Техническая теплофизика и промышленная теплоэнергетика». – Национальный технический университет Украины «Киевский политехнический институт имени Игоря Сикорского», Министерство образования и науки Украины, Киев, 2018. Работа посвящена повышению энергетической эффективности и упрощению интеграции солнечных систем на основе комбинированных солнечных коллекторов в фасады и крыши зданий за счет использования в качестве элемента теплопоглощающей поверхности алюминиевых канавчатых тепловых труб. Установлено, что на эффективность работы комбинированного солнечного коллектора с алюминиевыми канавчатыми тепловыми трубами в режиме термосифона существенно влияют теплотехнические характеристики тепловых труб, в свою очередь зависят от следующих параметров: диаметр парового пространства, теплофизические свойства рабочей жидкости, длины зон нагрева и конденсации, а также общая длина алюминиевых канавчатых тепловых труб. Повышение теплопередающих способности алюминиевых канавчатых тепловых труб можно достичь благодаря обеспечению подачи гарантированного количества теплоносителя в зону нагрева и выбора оптимальных конструктивных параметров тепловых труб при соответствующих режимах работы. Анализ расчетов и экспериментальных данных показал, что комбинированный солнечный коллектор с алюминиевыми канавчатыми тепловыми трубами позволяет повысить эффективность получения электрической энергии до 18% за счет охлаждения фотоэлектрических преобразователей, при этом максимальная электрическая мощность комбинированного солнечного коллектора составляла 135 Вт/м2. Кроме электроэнергии, одновременно можно получить до 457 Вт тепла с 1 м2 теплопоглощающей поверхности при температуре исходного теплоносителя 25 °С и плотности солнечного потока 900 Вт/м2. На основе теоретического анализа выявлены наиболее оптимальные режимы эксплуатации комбинированного солнечного коллектора – режим функционирования при значениях 30–50 °С температурного перепада между теплопоглощающей поверхностью и окружающей средой. Новая конструкция комбинированного солнечного коллектора имеет более эффективную работу по сравнению с раздельными тепловыми солнечными коллекторами и фотоэлектрическими батареями при низких температурах на теплопоглощающей поверхности (ниже 50 °С) и обычно при более высоких значениях солнечного потока (более 600 Вт/м2).

Abstract This paper aims at experimental investigations of the life limiting mechanisms of regeneratively cooled rocket combustion chambers, especially the so called doghouse effect. In this paper the set up of a cyclic thermo-mechanical fatigue experiment and its results are shown. This experiment has an actively cooled fatigue specimen that is mounted downstream of a subscale GOX-GCH$$_{\text {4}}$$ combustion chamber with rectangular cross section. The specimen is loaded cyclically and inspected after each cycle. The effects of roughness, the use of thermal barrier coatings, the length of the hot gas phase, the oxygen/fuel ratio and the hot gas pressure are shown. In a second experiment the flow in a generic high aspect ratio cooling duct is measured with the Particle Image Velocimetry (PIV) to characterize the basic flow. The main focus of the analysis is on the different recording and processing parameters of the PIV method. Based on this analysis a laser pulse interval and the window size for auto correlation is chosen. Also the repeatability of the measurements is demonstrated. These results are the starting point for future measurements on the roughness effect on heat transfer and pressure loss in a high aspect ratio cooling duct.

AbstractWith the increase of stringent emission standards and higher road transportation cycles in the last few decades, the importance of transport and fuel efficiency plays a major role. The aerodynamic forces on trucks have a huge impact of the overall fuel consumption rate. For a 40 tonnes semi-trailer truck at 85 km/h on a flat highway, around 40% of the provided engine power is needed to overcome the air resistance (Hucho in Aerodynamik des Automobils. Vieweg + Teubner, Wiesbaden, [1]). An efficient way to reduce the aerodynamic drag of trucks is to build a platoon of trucks. To assess the potential of a truck platoon due to slipstream effect, computational fluid dynamic (CFD) simulations were conducted. The simulations were performed for a platoon with three trucks for different constant velocities at different inter-vehicle distances. The results are summarised in a normalised drag coefficient and fuel reduction map. As a limiting factor of platooning, the thermal management aspect must be considered, because the slipstream reduces the air mass flow through the engine compartment. This aspect of reduced air mass flow through the engine compartment was analysed as well.

AbstractGradual solar energetic-particle (SEP) events are “big proton events” and are usually much more “gradual” in their decay than in their onset. As their intensities increase, particles streaming away from the shock amplify Alfvén waves that scatter subsequent particles, increasing their acceleration, eventually limiting ion flow at the “streaming limit.” Waves generated by higher-speed protons running ahead can also throttle the flow of lower-energy ions, flattening spectra and altering abundances in the biggest SEP events. Thus, we find that the A/Q-dependence of scattering causes element-abundance patterns varying in space and time, which define source-plasma temperatures T, since the pattern of Q values of the ions depends upon temperature. Differences in T explain much of the variation of element abundances in gradual SEP events. In nearly 70% of gradual events, SEPs are shock-accelerated from ambient coronal plasma of ~0.8–1.6 MK, while 24% of the events involve material with T ≈ 2–4 MK re-accelerated from residual impulsive-suprathermal ions with pre-enhanced abundances. This source-plasma temperature can occasionally vary with solar longitude across the face of a shock. Non-thermal variations in ion abundances in gradual SEP events reaccelerated from the 2–4 MK impulsive source plasma are reduced, relative to those in the original impulsive SEPs, probably because the accelerating shock waves sample a pool of ions from multiple jet sources. Late in gradual events, SEPs become magnetically trapped in a reservoir behind the CME where spectra are uniform in space and decrease adiabatically in time as the magnetic bottle containing them slowly expands. Finally, we find variations of the He/O abundance ratio in the source plasma of different events.

In the previous chapter, we dealt with the distribution of solutes between gas, liquid and solid phases in the soil at equilibrium; and with the rates of redistribution between these phases within soil pores. In this chapter, we consider movement of the order of 1 –1000 mm from one volume of soil to another. Such movements occur largely by diffusion and mass flow of the soil solution or soil air, and by mass movement of the body of the soil. Major movements that involve the balance and amount of solutes in the whole soil profile, including plant uptake and drainage losses, are treated in chapter 11. The process of diffusion results from the random thermal motion of ions, atoms or molecules. Consider a long column of unit cross-section orientated along the x axis, and containing a mixture of components in a single phase at constant temperature and external pressure. If the concentration of an uncharged component is greater at section A than at section B, then on average more of its molecules will move from A to B than from B to A. The net amount crossing a unit section in unit time, which is the flux, is given by the empirical relation known as Pick’s first law: . . . F = − D dC/dx (4.1) . . . where F is the flux, and dC/dx is the concentration gradient across the section. The minus sign arises because movement is from high to low concentration in the direction of increasing x. The diffusion coefficient, D, is thus defined by the equation as a coefficient between two quantities, F and dC/dx, which can be measured experimentally. It is not necessarily a constant. The diffusion coefficient of the molecules in a phase is directly proportional to their absolute mobility, u, which is the limiting velocity they attain under unit force. Terms D and u are related by the Nernst-Einstein equation: . . . D = ukT (4.2) . . . where k is the Boltzmann constant and T is the temperature on the Kelvin scale. The Nernst-Einstein equation is derived as follows (Atkins 1986, p. 675).

Continuous flow polymerase chain reaction (CFPCR) devices are compact reactors suitable for microfabrication and the rapid amplification of target DNAs. For a given reactor design, the amplification time can be reduced simply by increasing the flow velocity through the isothermal zones of the device; for flow velocities near the design value, the PCR cocktail reaches thermal equilibrium at each zone quickly, so that near ideal temperature profiles can be obtained. However, at high flow velocities there are penalties of an increased pressure drop and a reduced residence time in each temperature zone for the DNA/reagent mixture, potentially affecting amplification efficiency. This study was carried out to evaluate the thermal and biochemical effects of high flow velocities in a spiral, 20 cycle CFPCR device. Finite element analysis (FEA) was used to determine the steady-state temperature distribution along the micro-channel and the temperature of the DNA/reagent mixture in each temperature zone as a function of linear velocity. The critical transition was between the denaturation (95°C) and renaturation (55°C-68°C) zones; above 6 mm/s the fluid in a passively-cooled channel could not be reduced to the desired temperature and the duration of the temperature transition between zones increased with increased velocity. The amplification performance of the CFPCR as a function of linear velocity was assessed using 500 and 997 base pair (bp) fragments from λ-DNA. Amplifications at velocities ranging from 1 mm/s to 20 mm/s were investigated. Alternative design of PCR was investigated. Shuttle PCR has a single straight channel and a DNA plug, driven by electrokinetic flow, will move forward and backward in the microchannel to achieve the repetitive thermal cycles. Thermal performance, independent insulated temperature blocks, and molecular and thermal diffusion were evaluated.

Abstract Connacher Oil and Gas has deployed Flow Control Devices (FCDs)on an infill well liner as part of a Steam Assisted Gravity Drainage (SAGD) exploitation strategy. Infill wells are horizontal wells drilled in between offsetting SAGD well pairs in order to access bypassed pay and accelerate recovery. These wells can have huge variability in productivity, based on several factors: variable initial temperature due to variable steam chamber development and initial mobility variable injectivity from day one limiting steam circulation and stimulation significant hot spots during production that limit drawdown of the well and oil productivity FCDs have shown great value in several SAGD schemes and are becoming common throughout SAGD applications to manage similar challenges in SAGD pairs, but their application in infill wells is less prevalent and presents a novel challenge to design and evaluate performance. This case study will examine the theory, operation, and early field results of this field trial. Density-based FCDs designed for thermal operations were selected to minimize the impact of viscous fluids commonly encountered early in cold infill well production. The design also limited steam outflow during the stimulation phase, where steam is injected in order to initiate production of the well. Distributed Temperature Sensing (DTS) data, pressures and rates are utilized to analyze the impact of the FCDs towards conformance of the well in the early life. The value of FCDs has led to further piloting of this technology in a second group of nine infill wells, where further value is to be extracted using slimmer wellbores.

This paper presents an analytical investigation of the thermally developing and periodically fully-developed flow in a parallel-plate channel comprised of superhydrophobic walls. The superhydrophobic walls considered in this paper exhibit alternating micro-ribs and cavities positioned perpendicular to the flow direction and the transport scenario analyzed is that of constant wall heat flux through the rib surfaces with negligible thermal transport through the vapor cavity interface. Axial conduction is neglected in the analysis and the problem is one of Graetz flow with apparent slip-flow and periodicity of constant heating. Closed form solutions for the local Nusselt number and wall temperature are presented and are in the form of infinite series expansions. Previously it has been shown that significant reductions in the overall frictional pressure drop can be expected relative to the classical smooth channel laminar flow. The present results reveal that the overall thermal transport is markedly influenced by the relative cavity region (cavity fraction), the relative rib/cavity module width, and the flow Peclet number. The following conclusions can be made regarding thermal transport for a constant heat flux channel exhibiting the superhydrophobic surfaces considered: 1) Increases in the cavity fraction lead to decreases in the average Nusselt number; 2) Increasing the relative rib/cavity module length yields a decrease in the average Nusselt number; and 3) as the Peclet number increases the average Nusselt number increases. For all parameters explored, the limiting upper bound on the fully-developed average Nusselt number corresponds to the limiting case scenario of classical laminar flow through a smooth-walled channel with constant heat flux.

Abstract The simultaneous propagation of thermal and evaporation waves through a one-dimensional, water-wet porous media subjected to through-flow air drying is investigated through a set of theoretical, numerical, and experimental studies. The injection of dry air at a constant temperature, equal to the uniform initial temperature of the medium, is considered. Theory is developed for the limiting cases of i) a fast evaporation wave velocity relative to the characteristic thermal wave velocity, and ii) a fast thermal wave velocity relative to evaporation wave velocity. Mass and energy conservation equations are transformed to coordinates moving with the waves and the general energy transfer and drying behavior is described. Numerical simulations and experiments were performed for the two limiting cases plus one in which the evaporation wave velocity was approximately equal to the speed of the thermal wave. The numerical simulations and experimental data verified the basic features of the theoretical description. Theoretical, numerical and experimental results show that dryout times and temperature distributions in the one-dimensional porous medium were strongly dependent on the relative speeds of the thermal and evaporation waves. Contrary to conventional thought, the temperatures found in the porous matrix are not directly related to wet-bulb values. Under the condition where the evaporation wave velocity is equal to the speed of the thermal wave, a theoretical singularity suggests a very large temperature drop at the front; simulations and data for velocities approximately equal show the temperature at the front continuing to decrease with time to an extrapolated limit of freezing water.

Most current fuel schedules for accelerating or decelerating gas turbines use a limiting non-dimensional fuel flow expressed as a function of some engine parameter such as compressor pressure ratio. Usually the same schedule is used for, for example, accelerating ‘cold’ and ‘hot’ engines. This results in differing acceleration rates and differing usage of surge margins. The paper describes two methods of compensating the fuel schedules to account for the engine’s immediately preceding temperature history. One method uses the temperature response of the aerofoil of blades in the H.P. Compressor. The second method, which appears to be an improvement, uses a ‘delayed’ H.P. Shaft speed signal.

Abstract Turbulent Flow in closely spaced staggered tube bundles is numerically investigated using a finite-volume approach in general curvilinear coordinates. Attention if focused on the hydrodynamic and thermal effects of the longitudinal displacement of alternate tube rows. The computations used both standard and 2-layer k–ϵ turbulence models in conjunction with a streamwise periodic finite volume formulation. The computations are in excellent agreement with experimental data for the limiting case of flow and heat transfer in undisplaced tube banks. Furthermore, the results indicate increases in both pressure drop and heat transfer with an increase in displacement. The results of this study may serve as an aid in the design of shell and tube cross flow heat exchangers.

Temperature and velocity fields in high-temperature lead coolant flows in a circular clearance for controlled oxygen impurity content in a flow were experimentally studied at the Nizhny Novgorod State Technical University by R.E. Alekseev (NNSTU). Temperature and velocity fields were simultaneously studied in “cold” and “hot” parts of the circuit in the following operating conditions: the lead temperature is t = 400–550 °C, the thermodynamic activity of oxygen is a = 10−5–100; the Peclet number is Pe = 500–7000, the coolant flow velocity is w = 0.1–1.5 m/s, and the average heat flux is q = 50–160 kW/m2. It has been found that the oxygen impurity content and characteristics of protective oxide coatings affect temperature and velocity fields in round and circular channels. This is due to the fact that oxygen in a coolant and oxide coatings on the surfaces limiting a liquid metal flow influence characteristics of the wall boundary region. The heat transfer process that occurs when HLMC transversely flows around heat exchange pipes is investigated now at the NNSTU. The experimental facility is a combination of two high-temperature liquid-metal stands, i.e., FT-2 with the lead coolant and FT-1 with the lead-bismuth coolant combined with an experimental section. The temperature of a heat-exchange surface is measured by thermocouples of diameter 1 mm mounted in walls of heat-exchange pipes. Velocity and temperature fields in a high-temperature HLMC flow are measured by special sensors placed in the flow cross section between rows of heat-exchange pipes. Heat transfer characteristics and temperature and velocity fields in a high-temperature lead coolant flow are studied in the following operating conditions: the lead temperature is t = 450–500 °C, the thermodynamic activity of oxygen is a = 10−5–100, and the coolant flow rate through the experimental setup is Q = 3–6 m3/h, which corresponds to coolant flow velocities of V = 0.4–0.8 m/s. Integrated experimental studies of characteristics of the heat transfer that occurs when the lead coolant transversely or obliquely flows around pipes have been carried out for the first time and the dependences Nu = f(Pe) for controlled content of thermodynamically active oxygen impurity and sediments of impurities have been obtained. It is assumed that the obtained experimental data on distribution of velocity and temperature fields in a HLMC flow will permit to study heat transfer processes and to use them for developing program codes for engineering calculations of heat exchange surfaces (steam generators) with a HLMC flow around them.

Heat pipes have immense potential in the future of thermal management in electronic devices. As a passive device, they rely solely upon capillary forces to recirculate the coolant from the condenser to the evaporator via a wicking structure. In intermediate temperature heat pipes the limiting factor for heat removal is the capillary limit, which indicates the maximum recirculation rate that the capillary forces can induce. This capillary limit must be increased to allow heat pipes to remain a viable option for heat management within electronic devices. The aim of this work is to characterize and optimize the capillary limit of micropillared thermal wicks for heat pipe application in micro-electronics cooling. Towards this goal, an analytical model, and a novel thermo-hydraulic experimental setup was developed. The analytical model of the micropillared array wicking structure provides a theoretical basis from which the pillar geometry and arrangement can be optimized. A capillary limit model was used to determine the geometric relationship between the pillar arrays and the maximum capillary flow rate through the wick. This model considers the effects of gravity and mass transfer due to evaporation. Finally, the thermo-hydraulic characterization setup, designed to minimize environmental losses, was used to experimentally determine the capillary limits of different silicon based micropillared wick samples. The heater and wicking structure were enclosed in a temperature and humidity controlled vacuum chamber. The results obtained from this setup were used to validate the analytical model shown in this paper.

It is now widely recognized that three-dimensional (3D) system integration is a key enabling technology to achieve the processing speeds and performance needs of future microprocessor integrated circuits (ICs). To provide modular thermal management in 3D stacked ICs, interlayer microfluidic cooling scheme is adopted and analyzed in this study. The effects of cooling scheme and essential geometry variations on the routing completion and congestion of electrical interconnect are quantitatively analyzed. Also, the thermal and hydraulic performance of several two-phase refrigerants is discussed in comparison with single-phase cooling. The results show that refrigerants in two-phase flow are thermally preferred due to the higher heat transfer coefficients, and relatively constant fluid temperature throughout the microchannel. However, the large internal pressure and pressure drop act as significant limiting factors in realizing the merits of two-phase cooling. It is also concluded that integration of high performance hot-spot thermal management is a key to addressing a challenge of mass flow rate mal-distribution.