Investigation of coupled systems consisting of fluid movers and heat-exchange devices

ABSTRACT

A heat exchanger and the fluid mover that delivers a working fluid to the exchanger inlet may experience profound interactions, which argues against treating them as separate entities. On the other hand, the design practice commonly assumes that the fluid delivered to the heat exchanger inlet is specifiable without consideration of any possible influence of the exchanger. The magnitude of the flow rate arriving at the exchanger inlet is generally based on the pressure rise—flow rate (P-Q) curve supplied by the manufacturer of the fan and coupled with the assumption that that flow is uniformly distributed across the exchanger inlet. It was found that the complexity of the fluid flow delivered by the rotating fan gives rise to a large fluid resistance within the pin-fin array, such that the delivered air flow rate was only about 37% of that for the P-Q case. On the other hand, the corresponding reduction in the rate of heat transfer was, at most, 27%.     

Enhancement of convective heat transfer in an air-cooled heat exchanger using interdigitated impeller blades

The enhancement of convective heat transfer through a finned heat sink using interdigitated impeller blades is presented. The experimentally investigated heat sink is a subcomponent of an unconventional heat exchanger with an integrated fan, designed to meet the challenges of thermal management in compact electronic systems. The close integration of impeller blades with heat transfer surfaces results in a decreased thermal resistance per unit pumping power. The performance of the parallel plate air-cooled heat sink was experimentally characterized and empirically modeled in terms of nondimensional parameters. Dimensionless heat fluxes as high as 48 were measured, which was shown to be about twice the heat transfer rate of a traditional heat sink design using pressure-driven air flow at the same mass flow rate.     

The effect of global cross flows on the flow field and local heat transfer performance of miniature centrifugal fans

Centrifugal fans are often integrated into thermal management solutions for a range of applications. Consequently, centrifugal fan designs can be subjected to varying environmental conditions, many of which can alter fan performance characteristics and ultimately influence the heat transfer performance of the cooling solution. Global cross flows are a commonly encountered practical operating condition, particularly in the cooling of electronics. Air-cooled electronic enclosures often incorporate miniature centrifugal fans to maintain reliable component operating temperatures at a local level, while larger system level fans are used to simultaneously control the ambient temperature within the enclosure. This type of operating condition has been investigated by introducing a uniform crossing air flow above a centrifugal fan inlet. Two scaled miniature centrifugal fan designs were selected to fundamentally assess the influence on local velocity field and heat transfer performance. This was achieved experimentally using Particle Image Velocimetry, and a combined infrared and heated-thin-foil technique developed for the accurate measurement of local heat transfer coefficients. the introduction of a crossing air flow above the fan inlet indirectly reduced both the local and global thermal performance of the centrifugal fan, and the resultant distorted inflow shifted the surface heat transfer distribution at the fan outlet from an axisymmetric to asymmetric profile. However, strategic positioning of components relative to a centrifugal fan can maintain the average component heat transfer coefficient at a similar level to a case without any cross flow. Results also indicate issues associated with the implementation of miniature centrifugal fan designs into crossing air flow environments, with reductions in thermal performance of over 30% observed.     

相变蓄热供热装置热性能试验研究与系统经济性分析

为分析相变蓄热装置在充热和放热过程中的热性能,设计并搭建一套相变蓄热供热装置中试实验系统,研究主要运行参数对相变蓄热装置热性能的影响;在此基础上,结合项目案例,对相变蓄热供热系统经济性进行分析。结果表明：相变材料(Phase Change Material, PCM)凝固过程中的传热主要受相变介质内部导热控制;而在其熔化过程中自然对流对传热起重要控制作用;蓄热装置充热速率快于放热速率。提高传热流体流量有助于增强PCM中的热传递,缩短充/放热时间,但蓄热装置内PCM温度分布均匀性有所降低;为降低系统能耗,提高储放热效率,优先选用小流量进行充/放热。该相变蓄热供热项目的动态投资回收期为3.55年,具有良好的经济性。研究结果可对相变蓄热供热系统的设计及应用推广提供参考依据。     

基于元体能量平衡法的垂直U型埋管换热特性的研究

基于元体能量平衡法建立了垂直U型埋管的传热模型,模型考虑了流体温度的沿程变化,并通过引入热干扰角与等效传热间距反映两管脚间的热干扰问题,使之更符合实际的传热情况。通过所建模型对U型埋管的换热特性进行了数值模拟,结果表明:增加土壤与回填物导热系数、管脚间距、管内流体流量及减小管脚热干扰角与进口流体温度(供热工况下)均可以增强埋管的换热效果,其中土壤导热性影响最为显著。但回填物导热系数不可无限制增大,其大小还要考虑对增强管脚间热干扰的影响及其与管脚间距的相互关联性。同时,流量的增加要考虑对流动阻力增加的限制,可以采用变流量设计来进行调节与优化。此外,为了充分发挥地源热泵的能效,实际设计应该考虑埋管、热泵及负荷三者间的相互匹配性。     

Heat transfer in a twin-screw multiphase pump: Thermal modeling and one application in the petroleum industry

This paper presents a model of the heat transfer processes in the casing and rotors of a twin-screw multiphase pump. The model was developed to study the influence of temperature rise in the subsea multiphase pumping system-500 (SMPS), being developed by Petrobras, that operates with a twin-screw multiphase pump. The model is divided in three parts: heat transfer in the casing, in the rotor and energy balance of fluid. For the rotor, a helicoidal coordinate system is used to calculate the heat transfer. Axial symmetry is considered so it is possible to construct a two-dimensional model. The casing is modeled using an eccentric cylindrical coordinate system. In this case, the temperature gradient in axial direction is neglected and a two-dimensional calculation is carried out. The finite volume method is used to solve the transformed partial differential equations. With the two heat transfer models implemented, the fluid temperature is calculated using a simple energy balance that takes into account electric power, transferred heat and fluid internal energy. The implemented model was used to simulate thermal behavior of casing and rotors during loss of prime events faced by SMPS-500. Experimental data collected in pump trials are used as initial input parameters and the model calculates temperature evolution during the loss of prime events.     

Performance prediction of proton exchange membrane fuel cell engine thermal management system using 1D and 3D integrating numerical simulation

The cooling system of proton exchange membrane fuel cell (PEMFC) engine was simulated by 1D and 3D collaborative simulation method. Firstly, the resistance characteristics of the flow channel are obtained by simulating the airside flow model. A three-dimensional simulation model including dual fans and radiator is also established to simulate the airflow distribution. The one-dimensional simulation model of 30 kW PEMFC engine cooling system that are mainly composed of a thermostat, water pump, and fan and radiator model is established. Secondly, the heat dissipation performance of the cooling system is calculated by using the coupled simulation model. It is found that the simulation results of the amount of heat transferred are in good agreement with the experimental data by compromising, which proves that the model is reasonable. Finally, the thermal performance of the extreme operating conditions of the PEMFC system is simulated by means of a simulation model. By monitoring the flow of the pump and the fan speed, we can maintain the stack internal heat balances, so that the stack efficient and stable operation. The results demonstrate that the 3D simulation can get the distribution of fluid flow more accurately, while the simulation time of 1D thermal system is short and can guide the matching of heat transfer parts quickly.     

新型高效紧凑式换热器仿真及热力分析

为了提高新型高效紧凑式换热器设计的功能性,并使其满足热力学性能需求,对绕管的结构参数及桥接布管方式进行设计。采用一种新型的变径变线桥接方式,在体积有限的情况下实现密集的管束布置形式;对该新型换热器设计进行全尺寸流域建模及CFD数值模拟;并将三维建模结果与一维程序计算结果对比,进行可靠性验证。计算结果表明：三维计算的各项热力学性能结果与一维计算仅有较小偏差,总传热系数相对误差仅为3.74%,总传热量相对误差仅为1.04%,验证了该三维计算模型具有较好的准确性;结合温度云图证明了换热区域基本集中在绕管段,为简化复杂换热器的计算提供了思路;该新型高效紧凑式换热器设计实现了管侧双股流可独立运行且同层间不存在无效换热区,整体换热平顺进行,壳侧流阻较小,换热能力保持较好;在工况范围内整机换热体积功率达到4.67 MW。     

Effect of Thermal Conductivity on Cooling of Square Heat Source Array under Natural Convection in a Vertical Channel

Abstract

This article presents experimental and numerical investigation on natural convection air-cooling of discrete square heat source array in a vertical channel. Conjugate heat transfer for three-dimensional laminar developing flows over an array of square heat sources representing integrated circuit components for electronic cooling has been studied. Experiments are conducted using three-substrate board materials viz. FR4, Bakelite, and copper clad board having thermal conductivities of 0.3, 1.4, and 8.8?W/m K to study the effects of substrate thermal conductivity on fluid flow and heat transfer. A finite element-based software is used to solve the coupling between heat transfer in solids and fluid region. Incompressible flow over discrete square heat sources is modeled using Navier–Stokes equations under Boussinesq approximation. Air-cooling of circuit boards populated with heat sources is modeled and simulated to present heat transport in combination with the fluid flow resulting from the natural air circulation at constant heat fluxes of 1,000, 2,000, and 3,000?W/m². Multilayer copper clad board of thermal conductivity of 40.5?W/m K have been studied numerically. The results show that single sided copper clad board is the preferred candidate. Experiments indicate a deviation of under 5% with simulations.     

Heat and Mass Transport in Proton Exchange Membrane Fuel Cells—A Review

This review brings out those aspects of the development of proton exchange membrane (PEM) fuel cells over the last two to three decades that are of interest to the heat and mass transfer community. Because the heat transport and mass transport in proton exchange membrane fuel cells are very important from the efficiency point of view, an emphasis is given here to these transports and their influence on operating cell parameters. The works are classified as models with either isothermal or non-isothermal conditions of various assumed dimensionality and with either single-phase or two-phase flow. Along with modeling, a few experimental studies available are also reported here. Researchers in the area of PEM fuel cells are involved in activities such as development of new and low-cost materials, modeling the relevant physical processes, and electrochemical experimentation. These collective efforts may lead to making this technology viable to meet world needs for clean and cheap energy. This review brings out the fact that computational fluid dynamics (CFD) has become an inevitable tool in fuel cell analysis, as the detailed interactions between the flow structure geometry, fluid dynamics, multiphase flow, heat transfer, mass transfer, and electrochemical reaction can be modeled simultaneously, given the present state of the art in CFD. Through the predictive capability of CFD, it will be possible for fuel cell designers to better optimize the design and operating parameters of fuel cells before testing them in laboratory.     

Simplified Analysis of Radiation Heat Exchange in Boiler Superheaters

This article describes the determination of the radiation heat transfer coefficient in radiant platen superheaters and on convective heating surfaces. A new simple formula for determination of the heat transfer coefficient is derived on the basis of a diffusivity model of radiation heat exchange. The radiation heat transfer coefficients are determined on the tube surface in a convective evaporator, in a second stage convective heat superheater, and in a platen superheater of a pulverized coal-fired boiler. The calculations were carried out applying the method presented in this article, the Central Institute for Boilers and Turbines method, and formulas resulting from the analysis of heat exchange in an enclosure containing a gas of a constant temperature. In order to assess the accuracy of the achieved results, the flow of flue gas and the heat exchange were modeled using a commercial computational fluid dynamics program.     

Thermal management of methanol reforming reactors for the portable production of hydrogen

Three-dimensional numerical simulations were performed to address the thermal management issues associated with the design of a methanol reforming microchannel reactor for the portable production of hydrogen. The design of the reactor was fundamentally related to the direct coupling of reforming and combustion reactions by performing them on opposite sides of dividing walls in a parallel flow configuration. Effective autothermal operation was achieved through a combination of microchannel reactor technology with heat exchange in a direction perpendicular to the reacting fluid flow. Computational fluid dynamics simulations and thermodynamic analysis were carried out to investigate the effect of various design parameters on the characteristics of the generation, consumption, and exchange of thermal energy within the system. The results indicated that the ability to control temperature and temperature uniformity is of great importance to the performance of the system. The degree of temperature uniformity favorably affects the autothermal operation of the reactor. Temperature uniformity of the reactor can be improved by controlling the rate of heat transfer through a variety of factors such as wall thermal conductivity, fluid velocities, and dimensions. High wall thermal conductivity would be greatly beneficial to the performance of the system and the temperature uniformity of the reactor.     

Thermal analysis and management of proton exchange membrane fuel cell stacks for automotive vehicle

The thermal management of a proton exchange membrane fuel cell (PEMFC) is crucial for fuel cell vehicles. This paper presents a new simulation model for the water-cooled PEMFC stacks for automotive vehicles and cooling systems. The cooling system model considers both the cooling of the stack and cooling of the compressed air through the intercooler. Theoretical analysis was carried out to calculate the heat dissipation requirements for the cooling system. The case study results show that more than 99.0% of heat dissipation requirement is for thermal management of the PEMFC stack; more than 98.5% of cooling water will be distributed to the stack cooling loop. It is also demonstrated that controlling cooling water flow rate and stack inlet cooling water temperature could effectively satisfy thermal management constraints. These thermal management constraints are differences in stack inlet and outlet cooling water temperature, stack temperature, fan power consumption, and pump power consumption.     

Combined local microchannel-scale CFD modeling and global chip scale network modeling for electronics cooling design

Microchannel cold plates enjoy increasing interest in liquid cooling of high-performance computing systems. Fast and reliable design tools are required to comply with the fluid mechanics and thermal specifications of such complex devices. In this paper, a methodology accounting for the local as well as the device length scales of the involved physics is introduced and applied to determine the performance of a microchannel cooler. A unit cell of the heat transfer microchannel system is modeled and implemented in conjugate CFD simulations. The fluidic and thermal characteristics of three different cold plate mesh designs are evaluated. Periodic boundary conditions and an iteration procedure are used to reach developed flow and thermal conditions. Subsequently, two network-like models are introduced to predict the overall pressure drop and thermal resistance of the device based on the results of the unit cell evaluations. Finally, the performance figures from the model predictions are compared to experimental data. We illustrate the cooling potential for different channel mesh porosities and compare it to the required pumping power. The agreement between simulations and experiments is within 2%. It was found that for a typical flow rate of 250 ml/min, the thermal resistance of the finest microchannel network examined is reduced by 7% and the heat transfer coefficient is increased by 25% compared to the coarsest channel network. On the other hand, an increase in pressure drop by 100% in the case of densest channel network was found.     

Thermal intensification of heat transfer characteristics on the plate‐fin heat sink with piezoelectric fan

This study applied the computational fluid dynamic (CFD) code, ANSYS Fluent for simulating the effect a piezoelectric fan installed inside the rectangular channel by numerical simulation method for transient flow field and investigating the influence of each parameter. To remove the disorganized form of energy from the electronic components, the reversible piezoelectric effect is employed to energize the piezoelectric fan. To observe the variation of fan characteristics and to predict the convective heat transfer coefficient, CFD code ANSYS Fluent 15.0 is used. The numerical simulation parameters included are Nusselt number, number of fins (n = 12 and 14), and counter‐shift (inward and outward‐phase), and distance between the upper portion of the fan tip to the front part of the low thermal reservoir. Numerical analysis was carried out to evaluate the effect of thermal flow fields on the heat sink and piezoelectric fan employed in a flow domain. the results showed that by varying the height from channel bottom to the center of piezoelectric fan improves the performance of the piezoelectric fan, piezoelectric fan swinging in a transient phenomena and also simultaneously influences fluid flow behavior on the heat source surface, the fan vibration at counter‐phase has a better rate of heat transfer than vibration in in‐phase.     

Heat transfer enhancement analysis of a cylindrical surface by a piezoelectric fan

Piezoelectric fans consist of a thin flexible blade attached to a vibrating piezoelectric patch, and provide an effective means of enhancing the heat transfer in low convective regions. In this study, the characteristics of the three-dimensional heat and fluid flow fields generated by the vibrating fan are examined by performing numerical simulations and experimental measurements. In performing the simulations, the fluid domain is discretized using a dynamic meshing scheme to take account of the time-varying shape and position of the vibrating blade. The results show that two counter-rotating screw-type flow structures on either side of the blade appeared on either edge of the blade, and a pair of asymmetric vortex is formed around the fan tip. The experiment is conducted with a total of eighteen T-type thermocouples attaching to the cylindrical surface to measure the variation of temperature. The experimental and numerical results indicate that the piezoelectric fan improves the heat transfer coefficient by 1.2–2.4 times. Moreover, the augmentation of local heat transfer coefficient can be achieved by 2.85 times.     

Heat transfer in a pulsating heat pipe with open end

Heat transfer in the evaporator and condenser sections of a pulsating heat pipe (PHP) with open end is modeled by analyzing thin film evaporation and condensation. The heat transfer solutions are applied to the thermal model of the pulsating heat pipe and a parametric study was performed. The results show that the heat transfer in a PHP is mainly due to the exchange of sensible heat. The frequency and amplitude of the oscillation is almost unaffected by surface tension after steady oscillation has been established. The amplitude of oscillation decreases with decreasing diameter. The amplitude of oscillation also decreases when the wall temperature of the heating section is decreased, but the frequency of oscillation is almost unchanged.     

Thermal analysis of a 2-D heat recovery system using porous media including lattice Boltzmann simulation of fluid flow

The present work deals with the fluid flow simulation and thermal analysis of a two-dimensional heat recovery system using porous media. A basic high-temperature flow system is considered in which a high-temperature non-radiating gas flows through a random porous matrix. The porous medium, in addition to its convective heat exchange with the gas, may absorb, emit and scatter thermal radiation. It is desirable to have large amount of radiative heat flux from the porous segment in the upstream direction (towards the thermal system). The lattice Boltzmann method (LBM) is used to simulate fluid flow in the porous medium. The gas and solid phases are considered in non-local thermal equilibrium, and separate energy equations are applied to these phases. Convection, conduction and radiation heat transfers take place simultaneously in solid phase, but in the gas flow, heat transfer occurs by conduction and convection. In order to analyze the thermal characteristics of the heat recovery system, volume-averaged velocities through the porous matrix obtained by LBM are used in the gas energy equation and then the coupled energy equations for gas and porous medium are numerically solved using finite difference method. For computing of radiative heat flux in the porous medium, discrete ordinates method is used to solve the radiative transfer equation. Finally the effect of various parameters on the performance of porous heat recovery system is studied.     

Coupled Conduction and Intermittent Convective Heat Transfer from a Buried Pipe

An accurate estimate of the heat transfer from a buried pipe to the surrounding ground is essential for the design of the ground-loop portion of a ground-source heat pump. Exact analytical solutions to this problem are complicated by the fact that heat pump systems rarely operate continuously. Complete numerical simulations of system designs can be carried out, but these are unwieldy and difficult to justify for initial scoping calculations, or for preliminary performance estimates. The purpose of this article is to provide insight into the heat transfer mechanisms and to describe the development of simple algebraic correlations that can be used to approximate the intermittent overall heat transfer between a fluid flowing in an isolated buried pipe and the surrounding ground. The correlations described in this article were drawn from results of a numerical finite-difference analysis of a fluid flowing intermittently in a single round pipe and exchanging heat with the surrounding ground. It is found that the cycle average heat transfer is always lower for the intermittent case than for the continuous case, but that the average over just the active part of the cycle is always higher for any intermittent case than for the continuous case. The effect of the ground thermal diffusivity is largest when the heat transfer coefficient is large, and decreases with decreasing heat transfer coefficient. The range of heat transfer coefficients where isothermal wall conditions are approached is illustrated. Correlations for the operating average and cycle average total heat transfer are presented as functions of the thermal diffusivity, intermittence factor, and heat transfer coefficient.     

Experimental Investigation of Heat Transfer Characteristics and Cooling Performance of Mini‐ and Microheat Exchangers

This paper presents an experimental investigation of forced convection heat transfer in two heat sinks for electronic system cooling and investigated the comparisons of the thermal behavior of the mini‐ and microchannel heat exchangers. The hydraulic dimension of one of the heat sinks is 2 mm while that of the other is . Deionized water was used as the working fluid for studies conducted in both the heat exchangers. The effect of heat flux and volumetric flow rate (in laminar flow regime) on temperature and heat transfer coefficient is studied. Irrespective of the average heat transfer coefficient and the total thermal resistance, advantages and limitations of each device are analyzed and discussed in the light of experimental results. Furthermore, the results obtained from the experiments were in good agreement with those obtained from the design theory analyses.