Berechnung der Vergrößerung der Austauschflächen und Ventilationsleistungen beim Übergang von Wasser- auf Luftkühlung

Through use of a simple equation for the economic optimization of heat exchangers used to reject heat to the environment, a functional relationship is developed for heat transfer area and required system ventilator power. Thermal resistance has been neglected between the heat exchanger and heat rejecting fluid. It is found that for a system in which the heat absorbing fluid is air, both the required heat transfer area and ventilator power are 76 times greater than in a system in which the heat absorbing fluid is water.     

Comparisons of the heat transfer and pressure drop of the microchannel and minichannel heat exchangers

The present study investigated the comparisons of the heat transfer and pressure drop of the microchannel and minichannel heat exchangers, both numerically and experimentally. The results obtained from this study indicated that the heat transfer rate obtained from microchannel heat exchanger was higher than those obtained from the minichannel heat exchangers; however, the pressure drops obtained from the microchannel heat exchanger were also higher than those obtained from the minichannel heat exchangers. As a result, the microchannel heat exchanger should be selected for the systems where high heat transfer rates are needed. In addition, at the same average velocity of water in the channels used in this study, the effectiveness obtained from the microchannel heat exchanger was 1.2–1.53 times of that obtained from the minichannel heat exchanger. Furthermore, the results obtained from the experiments were in good agreement with those obtained from the design theory and the numerical analyses.     

Predict the temperature distribution in gas-to-gas heat pipe heat exchanger

A theoretical model has been developed to investigate the thermal performance of a continuous finned circular tubing of an air-to-air thermosyphon-based heat pipe heat exchanger. The model has been used to determine the heat transfer capacity, which expresses the thermal performance of heat pipe heat exchanger. The model predicts the temperature distribution in the flow direction for both evaporator and condenser sections and also the saturation temperature of the heat pipes. The approach used for the present study considers row-by-row heat-transfer in evaporator and condenser sections of the heat pipe heat exchanger.     

Effect of inertial particles with different specific heat capacities on heat transfer in particle-laden turbulent flow

The effect of inertial particles with different specific heat on heat transfer in particle-laden turbulent channel flows is studied using the direct numerical simulation(DNS) and the Lagrangian particle tracking method. The simulation uses a two-way coupling model to consider the momentum and thermal interactions between the particles and turbulence. The study shows that the temperature fields display differences between the particle-laden flow with different specific heat particles and the particle-free flow,indicating that the particle specific heat is an important factor that affects the heat transfer process in a particle-laden flow. It is found that the heat transfer capacity of the particle-laden flow gradually increases with the increase of the particle specific heat. This is due to the positive contribution of the particle increase to the heat transfer. In addition,the Nusselt number of a particle-laden flow is compared with that of a particle-free flow.It is found that particles with a large specific heat strengthen heat transfer of turbulent flow, while those with small specific heat weaken heat transfer of turbulent flow.     

Experimental and theoretical research on capillary limit of micro heat pipe with compound structure of sintered wick on trapezium-grooved substrate

Since heat flux increases sharply yet cooling space in microelectronic and chemical products gradually decreases, a micro heat pipe has been an ideal device for heat transfer for high heat-flux products, and its performance depends largely on its capillary limit. This study proposed an integrated utilization of the advantages of lower backflow resistance to working fluid in trapezium-grooved-wick micro heat pipes and greater capillary force in sintered-wick micro heat pipes; first the factors that are crucial to both types’ heat transfer performances were analyzed, and then mathematical modeling was built for capillary limit of a micro heat pipe with the compound structure of sintered wick on trapezium-grooved substrate, and finally heat transfer limits for micro heat pipes with a trapezium-grooved wick, a sintered wick and with a compound structure were tested through experiments. Both the theoretical analysis and experimental results show that for a micro heat pipe with proposed compound structure, its capillary limit is superior to that of a micro heat pipe with a simplex sintered wick or trapezium-grooved wick.     

Performance analysis of wick-assisted heat pipe solar collector and comparison with experimental results

The performance of heat pipe solar collector is investigated theoretically and experimentally. The system employs wick-assisted heat pipe for the heat transfer from the absorber (evaporator) to a heat exchanger (condenser). The heat pipe is made with a copper tube and the evaporator section is finned with aluminium plate. Theoretical model predicts the outlet water from heat exchanger, heat pipe temperature and also the thermal efficiency of solar collector. The results are compared with experimental data.     

A fitting, simple and versatile window program (HSHPTM) design using lumped parameters and one-dimensional thermal resistance models

A heat sink-heat pipes thermal module (HSHPTM) applies U-type or L-type heat pipes to transferring the total heat capacity from the heat source to the based plate and fins successfully, and then dissipates heat flow into the surrounding air. This article utilizes Visual Basic commercial software to develop a window program named HSHPTM V1.0 for proper design of the heat sink-heat pipes thermal module. The computing core of the HSHPTM program employs the theoretical thermal resistance analytical approach with iterative convergence stated in this study to obtain a numerical solution. The results show that this calculated error comparison with experimental results is within ±5 %. The embedded U-type heat pipes carry 46 and 63 percentages of the total dissipated heat capacity for one-pair and two-pairs embedded U-type heat sink-heat pipes thermal modules, and dissipate 87.2 % heat flow for six embedded L-type heat pipes, respectively. There has a benefit for HSHPTM V1.0 of rapidly and capably calculating the thermal performance of a heat sink-heat pipes thermal module installed with a processor horizontally by inputting simple and lumped parameters.     

Theoretical investigation on thermal performance of heat pipe flat plate solar collector with cross flow heat exchanger

Based on the heat transfer characteristics of absorber plate and the heat transfer effectiveness-number of heat transfer unit method of heat exchanger, a new theoretical method of analyzing the thermal performance of heat pipe flat plate solar collector with cross flow heat exchanger has been put forward and validated by comparisons with the experimental and numerical results in pre-existing literature. The proposed theoretical method can be used to analyze and discuss the influence of relevant parameters on the thermal performance of heat pipe flat plate solar collector.     

A review on transient test techniques for obtaining heat transfer design data of compact heat exchanger surfaces

Accurate and reliable dimensionless heat transfer characteristic is very essential for the analysis of heat exchangers. It is also required for the rating and sizing problems of heat exchangers. One of the important experimental methods used to determine the heat transfer coefficient between the heat transfer surface of the heat exchanger and the flowing fluid is transient test techniques. The transient test techniques are usually employed to establish Colburn factor versus Reynolds number characteristics of a high NTU heat exchanger surfaces like compact or matrix heat exchangers. In those situations, a single-blow test, where only one fluid is used, is employed to conduct the transient test. The transient technique may have the fluid inlet temperature having a step change, periodic or an arbitrary rise/drop. In this paper, various transient test techniques that are used for the determination of heat transfer characteristics of high NTU heat exchanger surfaces are discussed.     

Thermographic validation of a novel,laminate body,analytical heat conduction model

The two-region fin model captures the heat spreading behaviour in multilayered composite bodies (i.e., laminates), heated only over a small part of their domains (finite heat source), where there is an inner layer that has a substantial capacity for heat conduction parallel to the heat exchange surface (convection cooling). This resulting heat conduction behaviour improves the overall heat transfer process when compared to heat conduction in homogeneous bodies. Long-term heat storage using supercooling salt hydrate phase change materials, stovetop cookware, and electronics cooling applications could all benefit from this kind of heat-spreading in laminates. Experiments using laminate films reclaimed from post-consumer Tetra Brik cartons were conducted with thin rectangular and circular heaters to confirm the laminate body, steady-state, heat conduction behaviour predicted by the two-region fin model. Medium to high accuracy experimental validation of the two-region fin model was achieved in Cartesian and cylindrical coordinates for forced external convection and natural convection, the latter for Cartesian only. These were conducted using constant heat flux finite heat source temperature profiles that were measured by infrared thermography. This validation is also deemed valid for constant temperature heat sources.     

Das Anlaufverhalten eines Wärmespeichers mit stangenförmigen Speicherkapseln,die ein Metall mit niedrigem Schmelzpunkt enthalten

In the starting period of heat consuming apparatus a very high heat flow rate is usually needed. To start such systems by heat accumulator metals are suitable as accumulating materials, although they are considered only rarely because of their relatively high costs. The latent heat accumulator, which delivers heat at constant temperature, is an adequate means in those cases. In the present paper the heat charging characteristics of a new type of latent heat accumulator consisting of capsule bars containing Wood's metal are investigated. The heat discharge characteristics can be estimated by considering them with inversed signs. The characteristics are compared with a heat accumulator which has a straight water channel.     

Analysis on heat transfer correlations of supercritical CO<Subscript>2</Subscript> cooled in horizontal circular tubes

The objective of the present study is to analyze the heat transfer correlations of supercritical CO₂ cooled in horizontal circular tubes. In the paper, heat transfer correlations are first reviewed and compared with the experimental data at different heat fluxes. The results show that most of the previous correlations agree well with the experimental data under lower heat flux, but fail to predict the heat transfer coefficient well when the heat flux is as high as 33 kW/m². The study of buoyancy effect on convective heat transfer shows that buoyancy effect significantly affects the heat transfer with the increase of heat flux, and both free and forced convections operate in the turbulence flow during supercritical CO₂ cooling process. The influencing factors on heat transfer coefficient are summarized and the new correlation can be developed with the four dimensionless numbers.     

Convective heat transfer from molten salt droplets in a direct contact heat exchanger

This paper presents a new predictive model of droplet flow and heat transfer from molten salt droplets in a direct contact heat exchanger. The process is designed to recover heat from molten CuCl in a thermochemical copper–chlorine (Cu–Cl) cycle of hydrogen production. This heat recovery occurs through the physical interaction between high temperature CuCl droplets and air. Convective heat transfer between droplets and air is analyzed in a counter-current spray flow heat exchanger. Numerical results for the variations of temperature, velocity and heat transfer rate are presented for two cases of CuCl flow. The optimal dimensions of the heat exchanger are found to be a diameter of 0.13 m, with a height of 0.6 and 0.8 m, for 1 and 0.5 mm droplet diameters, respectively. Additional results are presented and discussed for the heat transfer effectiveness and droplet solidification during heat recovery from the molten CuCl droplets.     

Boiling heat transfer enhancement of water on tubes in compact in-line bundles

In desalinization devices and some heat exchangers making use of low-quality heat energy, both wall temperatures and wall heat fluxes of the heated tubes are generally quite low; hence they cannot cause boiling in flooded tube-bundle evaporators with common large tube spacing. However, when the tube spacing is very small, the incipient boiling in restricted spaces can generate and results in higher heat transfer than that of pool boiling at the same heat flux. This study investigated experimentally the effects of tube spacing, positions of tubes and test pressures on the boiling heat transfer of water in restricted spaces of the compact in-line bundles consisting of smooth horizontal tubes. The experimental results show that tube spacing and tube position have significant effects on the boiling heat transfer in a compact tube bundle. There is an optimum tube spacing that provides the largest heat transfer coefficient at the same heat flux.     

Alternative approach in thermal analysis of plate heat exchanger

This paper presents alternative approach in heat transfer analysis of plate heat exchangers. In order to obtain heat transfer rate and effectiveness values of plate heat exchanger, neural network (NN) approach was used. Experimentally, system used in plate heat exchanger for heating and cooling applications was designed and constructed. Experimental data were used for training and testing network. The training and validation were performed with good accuracy. The correlation coefficient obtained when unknown data were applied to the networks was 0.9994 for heat transfer rate and 0.9976 for effectiveness, which is very satisfactory. Using the weights obtained from the trained network, a new formulation is presented for determination of heat transfer rate and effectiveness. This formulation can provide simplicity in thermal analysis of plate heat exchanger. The presented procedure can also help to heat exchanger designer and manufacturer.     

Research on heat transfer of two-phase dispersed flow in narrow annular channel

Narrow channel heat transfer technique is a new developing heat transfer technique in recent years. As the temperature of droplet, steam and wall are decided by forced convection heat transfer between the steam and the wall, between the droplet and the wall, between the steam and the droplet and radiation heat transfer, which makes heat transfer mechanism of dispersed flow be difficultly interpretative. Dispersed flow in narrow annular channel is analyzed in the paper, investigating the influence of all kinds of heat transfer processes on dispersed flow, building annular channel dispersed flow model using thermodynamic non-equilibrium model. Calculation results show heat transfer is mainly controlled by heat transfer process between steam and wall. When temperature is low, radiation can be ignored on heat transfer coefficient calculation. The calculation of model can provide a reference for engineering application of steam generator, refrigeration system and so on.     

L- and U-shaped heat pipes thermal modules with twin fans for cooling of electronic system under variable heat source areas

This study utilizes a versatile superposition method with thermal resistance network analysis to design and experiment on a thermal module with embedded six L-shaped or two U-shaped heat pipes and plate fins under different fan speeds and heat source areas. This type of heat pipes-heat sink module successively transfer heat capacity from a heat source to the heat pipes, the heat sink and their surroundings, and are suitable for cooling electronic systems via forced convection mechanism. The thermal resistances contain all major components from the thermal interface through the heat pipes and fins. Thermal performance testing shows that the lowest thermal resistances of the representative L- and U-shaped heat pipes-heat sink thermal modules are respectively 0.25 and 0.17 °C/W under twin fans of 3,000 RPM and 30 × 30 mm² heat sources. The result of this work is a useful thermal management method to facilitate rapid analysis.     

Numerical modeling of compact high temperature heat exchanger and chemical decomposer for hydrogen production

The present study addresses fluid flow and heat transfer in a high temperature compact heat exchanger which will be used as a chemical decomposer in a hydrogen production plant. The heat exchanger is manufactured using fused ceramic layers that allow creation of channels with dimensions below 1 mm. The main purpose of this study is to increase the thermal performance of the heat exchanger, which can help to increase the sulfuric acid decomposition rate. Effects of various channel geometries of the heat exchanger on the pressure drop are studied as well. A three-dimensional computational model is developed for the investigation of fluid flow and heat transfer in the heat exchanger. Several different geometries of the heat exchanger channels, such as straight channels, ribbed ground channels, hexagonal channels, and diamond-shaped channels are examined. Based on the results, methods on how to improve the design of the heat exchanger are recommended.     

Thermal performance of heat pipe with suspended nano-particles

Nanofluids are employed as the working medium for a conventional cylindrical heat pipe. A cylindrical copper heat pipe of 19.5?mm outer diameter and 400?mm length was fabricated and tested with two different working fluids. The working fluids used in this study are DI-water and Nano-particles suspension (mixture of copper nano particle and DI-water). The overall heat transfer coefficient of the heat pipe was calculated based on the lumped thermal resistance network and compared with the heat transfer coefficient of base fluid filled heat pipe. There is a quantitative improvement in the heat transfer coefficient using nano-particles suspension as the working medium. A heat transfer correlation was also developed based on multiple regression least square method and the results were compared with that obtained by the experiment.     

Relaxation equation of heat conduction and generation — an analytical solution by Laplace transforms method

Heat and Mass Transfer - The relaxation equation of heat conduction and generation permits the relaxation of heat flux (a finite speed of heat propagation) as well as the relaxation of heat source...