3D numerical optimization of a heat sink base for electronics cooling

In this paper, the possible optimal thickness of a heat sink base has been explored numerically with different convective heat transfer boundary conditions in a dimensionless three dimensional heat transfer model. From the numerical results, relations among different heat transfer mechanisms (natural or forced, air or liquid), different area ratios of a heat sink to a heating source, and the lowest thermal resistance have been obtained and discussed. Also a simple correlation for these three parameters from data fitting is given for guiding a heat sink design.     

A numerical and experimental investigation of flow maldistribution in a micro-channel heat sink

A study of flow mal-distribution in U-type micro-channel configuration is presented. Numerical simulations indicate that flow deceleration and associated pressure recovery in the inlet header lead to flow separation and recirculation which cause oscillations in channel-wise mass flow distribution. Increase in flow resistance by decrease in channel depth, width or number of channels or increase in channel length, results in a more uniform distribution. Mal-distribution increases at high flow rate or low viscosity due to the dominance of inertial phenomena. Experiments performed on a 25-channel setup illustrate that small manufacturing variations in channel dimensions introduce random fluctuations in flow distribution.     

A 2D transient numerical model combining heat/mass transport effects in a tubular solid oxide fuel cell

The purpose of this study is to present a 2D transient numerical model to predict the dynamic behavior of a tubular SOFC. In this model, the transient conservation equations (momentum, species and energy equations) are solved numerically and electrical and electrochemical outputs are calculated with an equivalent electrical circuit for the cell. The developed model determines the cell electrical and thermal responses to the variation of load current. Also it predicts the local EMF, state variables (pressure, temperature and species concentration) and cell performance for different cell load currents. Using this comprehensive model the dynamic behavior of Tubular SOFC is studied. First an initial steady state operating condition is set for the SOFC model and then the time response of the fuel cell to changes of some interested input parameters (like electrical load) is analyzed. The simulation starts when the cell is at the steady state in a specific output load. When the load step change takes place, the solution continues to reach to the new steady state condition. Then the cell transient behavior is analyzed. The results show that when the load current is stepped up, the output voltage decreases to a new steady state voltage in about 67 min.     

A numerical model for transport in flat heat pipes considering wick microstructure effects

A transient, three-dimensional model for thermal transport in heat pipes and vapor chambers is developed. The Navier–Stokes equations along with the energy equation are solved numerically for the liquid and vapor flows. A porous medium formulation is used for the wick region. Evaporation and condensation at the liquid–vapor interface are modeled using kinetic theory. The influence of the wick microstructure on evaporation and condensation mass fluxes at the liquid–vapor interface is accounted for by integrating a microstructure-level evaporation model (micromodel) with the device-level model (macromodel). Meniscus curvature at every location along the wick is calculated as a result of this coupling. The model accounts for the change in interfacial area in the wick pore, thin-film evaporation, and Marangoni convection effects during phase change at the liquid–vapor interface. The coupled model is used to predict the performance of a heat pipe with a screen-mesh wick, and the implications of the coupling employed are discussed.     

Performance investigation on a novel 3D wave flow channel design for PEMFC

Flow field structure can largely determine the output performance of Polymer electrolyte membrane fuel cell. Excellent channel configuration accelerates electrochemical reactions in the catalytic layer, effectively avoiding flooding on the cathode side. In present study, a three-dimensional, multi-phase model of PEMFC with a 3D wave flow channel is established. CFD method is applied to optimize the geometry constructions of three-dimensional wave flow channels. The results reveal that 3D wave flow channel is overall better than straight channel in promoting reactant gases transport, removing liquid water accumulated in microporous layer and avoiding thermal stress concentration in the membrane. Moreover, results show the optimal flow channel minimum depth and wave length of the 3D wave flow channel are 0.45 mm and 2 mm, respectively. Due to the periodic geometric characteristics of the wave channel, the convective mass transfer is introduced, improving gas flow rate in through-plane direction. Furthermore, when the cell output voltage is 0.4 V, the current density in the novel channel is 23.8% higher than that of conventional channel.     

一次表面整体回热器传热和流动性能数值研究新方法

本文建立了一种带壁厚多层全通道新模型,用于流动交错角θ为0°和60°的一次表面回热器(PSR),在相同传热量的情况下进行传热和流动的数值分析和比较。结果发现在保持相同壁面温度条件下,流动交错角为60°和0°模型沿程的压降△P,阻力系数f,以及对流换热系数h的相应比值基本保持不变。以此比值作为修正系数,在θ=0°全通道经验计算结果的基础上,对交错角为60°的PSR进行整体的传热和流动性能预测分析。     

Optimization of target speeds of high-speed railway trains for traction energy saving and transport efficiency improvement

In the pursuit of higher operation speed at the passenger train services in China, the impacts of high-speed operation on energy consumption and transport efficiency are however not clearly identified. This research attempts to analyze the traction energy cost and transport operation time per 10,000 passenger-kilometers of high-speed railway (HSR) trains with a range of target speeds on certain HSR lines in China through a simulation approach. Having considered the effect of inter-stop transport distances, traction characteristics of HSR trains and gradients, curvatures, etc. of the rail lines, this study has deduced that the target speed of a HSR train for an inter-stop transport distance shorter than 100 km should be below 190 km/h from the perspectives of traction energy saving and transport efficiency improvement. Moreover, the study results also indicate that, unlike the actual HSR operation, the target speed should be dynamically adjusted according to the transport distances between stops if the transport capacity of the rail line is not extensively used. The exact target speed for each inter-stop transport distance shorter than 100 km should be further determined according to the traction characteristics of the train and the track geometry of the rail line.     

3D unsteady numerical analysis of conjugate heat transport and turbulent/laminar flows in LEC growth of GaAs crystals

We present the conjugated 3D unsteady numerical analysis of industrial-scale LEC GaAs crystal growth, including the calculation of heat transfer in the crystal and crucibles, melt convection, and the encapsulant flow. The analysis of unsteady turbulent melt convection is performed in terms of the large eddy simulation approach. A special procedure was introduced into the calculations to predict the geometry of the crystallization front. The results of the 3D unsteady calculations are compared to the results obtained in terms of the conventional steady-state Reynolds averaged approach with respect to the calculation of the geometry of the crystallization front. The effect of convective heat transfer in the encapsulant is specially studied using the 3D unsteady analysis. To investigate details of dynamic interaction between two immiscible liquids having a plane interface, preliminary computational tests were performed in a model setup.     

Analytical and numerical investigation of the characteristics of a soil heat exchanger for ventilation systems

A mathematical model is developed for calculating the temperature of the soil and air in a soil heat exchanger for ventilation systems. The model is based on the representation of temperature in the form of the Fourier integral. For high-frequency components with characteristic times of the order of 24 h an exact analytical solution is used. Calculation of low-frequency components with characteristic times of the order of a year is based on simulation of a tube by a linear heat source. The degree of decrease in the efficiency of the heat exchanger with decrease in the spacing between its tubes is evaluated. The dependences of the thermal power of the system on the length and diameter of the tubes, depth of their burial and air flow rate are calculated. An analytical expression is obtained for the optimum length of the tube. The evolution of the thermal power of the system during its operation for 10 years only in the winter period is calculated. The results of calculations are compared with experimental data. The procedure developed does not require cumbersome calculations and can be used for working out design recommendations.     

A novel effective approach for solving nonlinear heat transfer equations

In this paper, a novel analytic technique, namely the Laplace transform new homotopy perturbation method (LTNHPM), is applied for solving the nonlinear differential equations arising in the field of heat transfer. This approach is a new modification to the homotopy perturbation method based on the Laplace transform. Unlike the previous approach implemented by the present authors for these problems, the present method does not consider the initial approximation as a power series. The nonlinear convective–radiative cooling equation and nonlinear equation of conduction heat transfer with the variable physical properties are chosen as illustrative examples. The exact solution has been found for the first case and for the others; results with remarkable accuracy have been achieved which verify the efficiency as well as accuracy of the presented approach. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20411     

DF4D型机车制动系统总风管路爆裂故障分析

沈阳机务段配属DF4D型机车6 7台。在运用过程中,共发生两起总风管路爆裂事故,造成空气压缩机、总风管、干燥器、油水分离器、预热系统及车体不同程度的损坏,危及行车安全和人身安全。1　故障原因分析机车正常运行过程中,总风压力受70 4型调压器控制,维持在75 0～90 0kPa范围内,     

A numerical investigation on multi-phase transport phenomena in a proton exchange membrane fuel cell stack

In this study, the simulation of a fuel cell stack is performed by applying a general numerical model with VOF method that has been successfully applied to single PEMFC model to investigate the fluid dynamics, mass transport, flooding phenomenon and the effects of liquid water on the stack performance. The performance of three single cells in series connection in the fuel cell stack is examined according to the presence of liquid water in different single cells. The distributions of fluid flow, species concentration and the current density are presented to illustrate the effects of liquid water on the performance of each single cell. The numerical results locate that the low distributions of species in the flooding cell certainly degrade the performance of this cell. Moreover, it can be seen that the performance of the flooding cell will significantly affect the whole stack performance since the values of average current density must be identical in all single cells.     

A 3-D numerical investigation on the heaving performance of a wave floater

A 3-D mathematical model of regular wave flume is conducted to investigate the heaving performance of a cylindrical floater. The results show that the transverse velocities near the floater distribute symmetrically at the wave crest and trough times, its symmetry axis is the longitudinal straight line across the floater center, and 4 extreme regions distribute near the floater. With the increase of relative width of wave flume, the trapped wave energy and the relative response amplitude increase firstly. Then, they decreases and appear to be stable for the relative width of wave flume higher than 10. The trapped wave energy and the relative response amplitude increase with the increase of incident wave height, respectively. With the increase of wave period, the trapped wave energy decreases, however, the relative response amplitude increases.     

A novel thermoelectric refrigeration system employing heat pipes and a phase change material: an experimental investigation

This paper presents results of tests carried out to investigate the potential application of heat pipes and phase change materials for thermoelectric refrigeration. The work involved the design and construction of a thermoelectric refrigeration prototype. The performance of the thermoelectric refrigeration system was investigated for two different configurations. The first configuration employed a conventional heat sink system (bonded fin heat sink) on the cold side of the thermoelectric cells. The other configuration used an encapsulated phase change material in place of the conventional heat sink unit. Both configurations used heat pipe embedded fins as the heat sink on the hot side. Replacement of the conventional heat sink system with an encapsulated phase change material was found to improve the performance of the thermoelectric refrigeration system. In addition, it provided a storage capability that would be particularly useful for handling peak loads and overcoming losses during door openings and power-off periods. Results showed that the heat sink units employing heat pipe embedded fins were well suited to this application. Results also showed the importance of using a heat pipe system between the cold junction of the thermoelectric cells and the cold heat sink in order to prevent reverse heat flow in the event of power failure.     

A numerical investigation on the heat and fluid flow of various nanofluids on a stretching sheet

Following the necessity of investigating fluid flow and heat transfer in the stretching sheet problem and effect of nanofluids on them, performance of various nanofluids were investigated in the present study. Three base fluids (deionized water, ethylene glycol, and engine oil) in combination with 18 nanoparticles (metals and their oxides) were investigated. While experimental methods are preferable, a mathematical model was developed and solved by applying differential quadrature method due to lack of such experimental data. With the results obtained in the real dimensions, the error caused by the cancellation of the viscosity effect due to the dimensionless variables was omitted. Effects of magnetic field and volume fraction of nanoparticle on the fluid flow and heat transfer characteristics were investigated. Highest heat transfer rate as well as small amounts of shear stress was obtained for deionized water–Al and deionized water–Mg nanofluids. Increasing volume fraction of nanoparticle was observed to increase both heat transfer and shear stress rates, while presence of a magnetic field caused an increase in shear stress and decrease in heat transfer rate.     

Safe heating-up of a full scale SOFC system using 3D multiphysics modelling optimisation

Heating-up strategies of full scale solid oxide fuel cell (SOFC) systems still affect the safe operation of the system and incorporation of the technology into the global energy sector. To ensure rapid start-up times whilst retaining the structural reliability of the SOFC system components, requires a safe heating-up operation. To master a controlled heating-up stage, detailed understanding of the component interaction and multiphysics within a fuel cell system is required. State of the art dynamic fuel cell system modelling comprises sub-models of the assembly, or is based on empirical nature. However, invaluable information of the multiphysics inside the system is lost. Therefore, it is of paramount importance to understand and improve the knowledge of the detailed processes, occurring within the interacting components. The effect of integrating different electrical heater cartridges at different locations has been thoroughly investigated to optimise the heating-up of the system. The study utilises a previously developed and experimentally validated full scale three dimensional planar type SOFC system model to mitigate experimental costs and shed light on the details, occurring within the system. A comparison to a simplified variant of the model has been added to shed light on its effect on the results.     

A numerical study of algebraic flux models for heat and mass transport simulation in complex flows

A numerical study of heat and mass transport based on the Reynolds-Averaged Navier–Stokes and scalar transport equations is presented to establish the predictive capabilities of algebraic flux models compared to the standard eddy-diffusivity representation. The analysis of scalar transport in simple-shear flows is initially performed to provide a basic validation of numerical techniques and scalar flux closures. The evaluation of algebraic models is carried out by examining the flow and scalar transport phenomena over a wavy wall and comparing the results to reliable direct numerical simulations. Despite the similarities with the standard gradient-diffusion hypothesis and the questionable validity of local-equilibrium conditions, the results show that algebraic models provide an efficient way to improve heat and mass transport predictions in complex flows with respect to the standard eddy-diffusivity model. The impact of abandoning the isotropic eddy-diffusivity in favor of a tensorial representation is found particularly significant in the analysis of scalar dispersion from a point source over the wavy wall, where lateral transport comes into play. While it is found that algebraic closures also represent a reasonable approximation for the spanwise scalar flux, the lateral spread of scalar concentration is considerably under-estimated by the standard gradient-diffusion model.     

Acetaldehyde/ethyl-alcohol reaction for a medium-level heat transport system

An acetaldehyde/ethyl-alcohol reversible hydrogenation and dehydrogenation reaction is proposed for a medium-level heat transport system. Under atmospheric pressure the dehydrogenation converts heat at a temperature of 350–400°C to chemical energy. At another location chemical energy may be converted back, by the hydrogenation, to thermal energy at 200–250°C For both reactions, the same silica-supported copper catalysts, particularly of ion-exchange type, have high activities and selectivities. A small amount of ethyl acetate is produced. However, the reaction producing ethyl acetate is also exothermic, so that the acetate only slightly decreases the amount of heat to be transported with this system.     

Theoretical investigation of the performance of a novel loop heat pipe solar water heating system for use in Beijing,China

A novel loop heat pipe (LHP) solar water heating system for typical apartment buildings in Beijing was designed to enable effective collection of solar heat, distance transport, and efficient conversion of solar heat into hot water. Taking consideration of the heat balances occurring in various parts of the loop, such as the solar absorber, heat pipe loop, heat exchanger and storage tank, a computer model was developed to investigate the thermal performance of the system. With the specified system structure, the efficiency of the solar system was found to be a function of its operational characteristics - working temperature of the loop heat pipe, water flow rate across the heat exchanger, and external parameters, including ambient temperature, temperature of water across the exchanger and solar radiation. The relationship between the efficiency of the system and these parameters was established, analysed and discussed in detail. The study suggested that the loop heat pipe should be operated at around 72 °C and the water across the heat exchanger should be maintained at 5.1 l/min. Any variation in system structure, i.e., glazing cover and height difference between the absorber and heat exchanger, would lead to different system performance. The glazing covers could be made using either borosilicate or polycarbonate, but borosilicate is to be preferred as it performs better and achieves higher efficiency at higher temperature operation. The height difference between the absorber and heat exchanger in the design was 1.9 m which is an adequate distance causing no constraint to heat pipe heat transfer. These simulation results were validated with the primary testing results.