Theoretical Evaluation of a 10‐Watt Cooling Power Thermoacoustic Refrigerator

This article deals with the design, simulation, and analysis of a 10‐watt capacity thermoacoustic refrigerator using short‐stack boundary layer approximation assumptions and dimensional normalization technique. The variation of stack diameter with average gas pressure and cooling power is studied. The theoretical evaluation of quarter‐wavelength and one‐fifth‐wavelength resonator using helium gas is discussed for an operating frequency ranging from 300–500 Hz in the steps of 50 Hz. For the optimized stack, a 38.5% improvement in the stack performance for a 10‐watt cooling power quarter‐wavelength hemispherical‐ended resonator operating at 350 Hz frequency resulted in a 13.5% improvement in the coefficient of performance (COP) compared to published results. The resonator design is tested with DeltaEC software with 2% and 3% drive ratios using helium, hydrogen, neon, air, and carbon dioxide as working gases and the results are discussed. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(7): 577–591, 2014; Published online 3 October 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21094     

Effects of variation in working fluids and operating conditions on the performance of a thermoacoustic refrigerator

This study reports on a numerical investigation of the effects of variation in working fluids and operating conditions on the performance of a thermoacoustic refrigerator. The performance of a thermoacoustic refrigerator is evaluated based on the cooling power, coefficient of performance (COP), and the entropy generation rate within the device. The effect of the variation of the working fluid is observed by changing the Prandtl number (Pr) between 0.7 and 0.28. The operating conditions investigated are drive ratio (DR), stack plate spacing (y₀), and mean pressure (p_m). The present research shows that lowering the Pr of the working fluid does not improve the performance of a thermoacoustic refrigerator for all of the selected operating conditions. COP increases 78% by reducing the Pr from 0.7 to 0.28 at y₀ = 3.33δ_k, at atmospheric pressure and a DR of 1.7%. While the COP decreases by reducing the Pr from 0.7 to 0.28 at y₀ = 1.0δ_k, at atmospheric pressure, and a DR of 1.7%. The results are compared with the available experimental data and found good agreement.     

Design and optimization of a heat driven thermoacoustic refrigerator

The present paper deals with the design and optimization of a heat driven thermoacoustic refrigerator. A simplified model is developed which enables to pinpoint and examine the most important physical characteristics of a compact traveling wave thermoacoustic refrigerator driven by a traveling wave thermoacoustic engine. The model can explain the so-called traveling standing wave effect in thermoacoustics very well. The position, length and hydraulic radius of the refrigerator are optimized for the maximum total COP. The prime mover efficiency, refrigerator COP and dimensionless dissipation and their impacts on total COP are investigated and discussed. The results indicate that a COP of 28.7% at T_RF,cold = 273 K is achievable.     

Optimum packing factor of the stack in a standing‐wave thermoacoustic prime mover

A bench consisting of a pulse tube refrigerator driven by a standing‐wave thermoacoustic prime mover has been set up to study the relationship among stack, regenerator and working fluids. The stack of the thermoacoustic prime mover is packed with dense‐mesh wire screens because of their low cost and easy manufacture. The effect of the packing factor in the stack on onset temperature, refrigeration temperature and input power is explored. The optimum packing factor of 1.15 pieces per millimeter has been found experimentally, which supplies an empirical value to satisfy a compromise for enhancing thermoacoustic effect, decreasing heat conduction and fluid‐friction losses along the stack. The pulse tube cooler driven by the thermoacoustic prime mover is able to obtain refrigeration temperatures as low as 138 and 196K with helium and nitrogen, respectively. Copyright © 2002 John Wiley & Sons, Ltd.     

丝网热声板叠的最佳填充率

自行研制了热声驱动脉管制冷机实验台,着重研究了热声机械中热声转换的关键部件丝网板叠的填充率对热声驱动脉管制冷机起振温度,制冷温度和加热功率等的影响,并通过实验发现了丝网板叠的最佳填充率,以氮和氮作工质,分别获得了196K和138K的无负荷制冷制度,达到国际先进水平,为热声机械的实用化奠定了基础。     

基于人工神经网络的3 kW质子交换膜燃料电池电堆一致性优化

电堆工作过程中各个单体电压的一致性是提高其性能的关键,在3 kW电堆台架实验数据的基础上,利用人工神经网络（ANN）模型,对空气流量、氢气压力、过量空气系数等气体供给参数进行优化,并对优化前后电堆的功率特性、一致性特性进行对比分析。结果表明：当氢气入口压力为0.128 MPa、空气入口流量为11 g/s、过量空气系数为6.4时,电堆电压差异系数CV及功率参数可达到最佳。     

A semi-empirical model for steady-state simulation of household refrigerators

A semi-empirical modeling approach for simulating the steady-state behavior of vapor-compression refrigeration systems is presented in this study. The model consists of first-principles algebraic equations adjusted with experimental data obtained from the refrigeration system under analysis. Measurements of the relevant variables were taken at several positions along the refrigeration loop, supplying data for the calibration of the computer model. The predicted values of refrigeration capacity and power consumption when compared with experimental data were within ±10% error bands. The model was also employed to assess the effect of various system parameters on the refrigerator performance. Although a top-mount frost-free refrigerator was chosen for this study, the modeling methodology can be easily extended to other types of refrigeration systems, from domestic refrigerators to heat pumps.     

Parameter optimization for a PEMFC model with a hybrid genetic algorithm

Many steady‐state models of polymer electrolyte membrane fuel cells (PEMFC) have been developed and published in recent years. However, models which are easy to be solved and feasible for engineering applications are few. Moreover, rarely the methods for parameter optimization of PEMFC stack models were discussed. In this paper, an electrochemical‐based fuel cell model suitable for engineering optimization is presented. Parameters of this PEMFC model are determined and optimized by means of a niche hybrid genetic algorithm (HGA) by using stack output‐voltage, stack demand current, anode pressure and cathode pressure as input–output data. This genetic algorithm is a modified method for global optimization. It provides a new architecture of hybrid algorithms, which organically merges the niche techniques and Nelder–Mead's simplex method into genetic algorithms (GAs). Calculation results of this PEMFC model with optimized parameters agreed with experimental data well and show that this model can be used for the study on the PEMFC steady‐state performance, is broader in applicability than the earlier steady‐state models. HGA is an effective and reliable technique for optimizing the model parameters of PEMFC stack. Copyright © 2005 John Wiley & Sons, Ltd.     

Experimental study of a fuel cell power train for road transport application

The development of fuel cell electric vehicles requires the on-board integration of fuel cell systems and electric energy storage devices, with an appropriate energy management system. The optimization of performance and efficiency needs an experimental analysis of the power train, which has to be effected in both stationary and transient conditions (including standard driving cycles).In this paper experimental results concerning the performance of a fuel cell power train are reported and discussed. In particular characterization results for a small sized fuel cell system (FCS), based on a 2.5 kW PEM stack, alone and coupled to an electric propulsion chain of 3.7 kW are presented and discussed. The control unit of the FCS allowed the main stack operative parameters (stoichiometric ratio, hydrogen and air pressure, temperature) to be varied and regulated in order to obtain optimized polarization and efficiency curves. Experimental runs effected on the power train during standard driving cycles have allowed the performance and efficiency of the individual components (fuel cell stack and auxiliaries, dc–dc converter, traction batteries, electric engine) to be evaluated, evidencing the role of output current and voltage of the dc–dc converter in directing the energy flows within the propulsion system.     

Parameter optimization for tubular solid oxide fuel cell stack based on the dynamic model and an improved genetic algorithm

The tubular solid oxide fuel cell (SOFC) stack has important parameters that need to be identified and optimized for the control of high performance. In this paper, a simple SOFC electrochemical model which its parameters need to be optimized is introduced to implement stack control for high output power. A dynamic SOFC model is built based on three sub-models to provide a large numbers simulated data and different condition for optimization. Unlike the traditional parameter optimization method--simple genetic algorithm (SGA), an improved genetic algorithm (IGA) is introduced. The proposed method shows more accuracy and validity by comparing the different results using SGA and IGA methods, the simulated data, and experimental data. The models and IGA method are adapted to control processes.     

Cost numerical optimization of the triple‐pressure steam‐reheat gas‐reheat gas‐recuperated combined power cycle that uses steam for cooling the first GT

Optimization is an important method for improving the efficiency and power of the combined cycle. In this paper, the triple‐pressure steam‐reheat gas‐reheat gas‐recuperated combined cycle that uses steam for cooling the first gas turbine (the regular steam‐cooled cycle) was optimized relative to its operating parameters. The optimized cycle generates more power and consumes more fuel than the regular steam‐cooled cycle. An objective function of the net additional revenue (the saving of the optimization process) was defined in terms of the revenue of the additional generated power and the costs of replacing the heat recovery steam generator (HRSG) and the costs of the additional operation and maintenance, installation, and fuel. Constraints were set on many operating parameters such as air compression ratio, the minimum temperature difference for pinch points (δT_ppm), the dryness fraction at steam turbine outlet, and stack temperature. The net additional revenue and cycle efficiency were optimized at 11 different maximum values of turbine inlet temperature (TIT) using two different methods: the direct search and the variable metric. The optima were found at the boundaries of many constraints such as the maximum values of air compression ratio, turbine outlet temperature (TOT), and the minimum value of stack temperature. The performance of the optimized cycles was compared with that for the regular steam‐cooled cycle. The results indicate that the optimized cycles are 1.7–1.8 percentage points higher in efficiency and 4.4–7.1% higher in total specific work than the regular steam‐cooled cycle when all cycles are compared at the same values of TIT and δT_ppm. Optimizing the net additional revenue could result in an annual saving of 21 million U.S. dollars for a 439 MW power plant. Increasing the maximum TOT to 1000°C and replacing the stainless steel recuperator heat exchanger of the optimized cycle with a super‐alloys‐recuperated heat exchanger could result in an additional efficiency increase of 1.1 percentage point and a specific work increase of 4.8–7.1%. The optimized cycles were about 3.3 percentage points higher in efficiency than the most efficient commercially available H‐system combined cycle when compared at the same value of TIT. Copyright © 2008 John Wiley & Sons, Ltd.     

Experimental optimization of the fabrication parameters for anode-supported micro-tubular solid oxide fuel cells

A systematic optimization of several parameters significant in the fabrication of anode-supported micro-tubular solid oxide fuel cell via extrusion and dip coating is presented in this study. Co-sintering temperature of anode-support and electrolyte, the vehicle type and solid powder content used in electrolyte dip-coating slurry, electrolyte submersion time, cathode sintering temperature, powder ratio in the cathode functional layer, submersion time for the cathode functional layer and, submersion time and coating number of the anode functional layer are studied in this respect and optimized in the given order according to the performance tests and microstructural analyses. The performance of the micro-tubular cell is significantly improved to 0.49 Wcm⁻² at 800 °C after the optimizations, while that of the base cell is only 0.136 Wcm⁻². 12-cell micro-tubular stack is also constructed with the optimized cells and the stack is tested. Each cell in the stack is found to show very close performance to the single-cell performance and the stack with a maximum power of ~26 W at an operating temperature of 800 °C is therefore evaluated to be successful.     

Influence of acoustic pressure amplifier dimensions on the performance of a standing-wave thermoacoustic system

A standing-wave thermoacoustic engine, employing an acoustic pressure amplifier (APA), is simulated with linear thermoacoustics to study the influence of APA’s dimensions on performance of the thermoacoustic system. Variations of operating parameters, including pressure ratio, acoustic power, hot end temperature of stack etc., versus length and diameter of APA are presented and discussed based on an analysis of pressure and velocity distribution in APA. Simulation results indicate that a largest amplification effect of both pressure ratio and acoustic power output is achieved at a critical length for the occurrence of pressure node and velocity antinode in APA, close to but less than one fourth of the wavelength. The distribution characteristics of pressure and velocity in APA are similar to a standing-wave acoustic field, which is the reason for the amplification effect. From the viewpoint of energy, the amplification effect results from the changed distribution of acoustic energy and acoustic power loss in the thermoacoustic system by APA. Experiments have been carried out to validate the simulation, and experimental data are presented.     

Transient temperature profile inside thermoacoustic refrigerators

The linear theory used to calculate the thermal quantities inside the stack in the classical thermoacoustic refrigerators always overestimates those measured. The causes of these discrepancies have to be found in the complex processes of thermal exchanges. The analytical study of the transient response should provide an interpretation of these complex processes. This present paper provides such analytical modelling. This modelling remains within the framework of the classical linear theory. It includes the effects of the thermoacoustic heat flux carried along the stack, the conductive heat flux returning in the solid walls of the stack and through the fluid inside the stack, the transverse heat conduction in the stack and the heat leakages through the duct walls, the heat generated by viscous losses in the stack, the heat generated by vorticity at the ends of the stack, and the heat transfer through both ends of the stack. A modal analytical solution for the temperature profile is proposed, assuming the usual approximations in such thermal problems to avoid intricate calculations and expressions. The theoretical transient response of a thermoacoustic refrigerator is compared with experimental data. A good qualitative agreement is obtained between analytical and experimental results after fitting empirical coefficients.     

不可逆半导体固态热离子制冷器性能分析和优化

建立了考虑外部有限速率传热过程和热源间热漏的不可逆半导体固态热离子制冷器模型,基于非平衡热力学和有限时间热力学理论导出了热离子制冷器的制冷率和制冷系数的表达式;对比分析了不可逆热离子制冷器与可逆热离子制冷器的发射电流密度特性、电极温度特性以及制冷系数特性;研究了不可逆系统的制冷率与制冷系数最优性能,得到了制冷率和制冷系数的最优运行区间;通过数值计算,详细讨论了外部传热以及内部导热、热源间热漏损失、热源温度、外加电压、半导体材料势垒等设计参数对热离子装置性能的影响。在总传热面积一定的条件下,进一步优化了高、低温侧换热器的面积分配以获得最佳的制冷率和制冷系数特性。结果表明,由于存在内部和外部的不可逆性,热离子装置的发射电流密度及制冷系数都会明显降低;不可逆半导体固态热离子制冷器的制冷率与制冷系数特性呈扭叶型;合理地选外加电压、势垒等参数,可以使制冷器设计于最大制冷率或最大制冷系数的状态。     

Temperature distribution along stack in an acoustic-resonance tube: discussions on advanced linearized theory

Verification of the applicability of the linearized thermoacoustic theory proposed in 1988 by Swift, J. Acoust. Soc. Am., 84 , (4), 1145–1180, to an acoustic- resonance tube refrigerator and discussions on the advanced linearized theory were conducted through a comparison with the measured temperature distribution along the stack in a simulated acoustic-resonance tube. The measured temperature in all cases of various stack configurations such as stack plate spacing and length showed an almost linear distribution along the stack, while Swift's linearized theory gave a curved distribution with relatively large deviation from experimental data. Eddy diffusivity and/or steady streaming effects excited by sound waves were taken into account in the linearized model, and the effects of these terms on the prediction of temperature distribution were examined. The agreement between theory and experiment was markedly improved by the introduction of steady streaming. This provided a guideline for the construction of an advanced linearized theory. © 1999 Scripta Technica, Heat Trans Jpn Res, 27(8): 551–567, 1998     

Optimum allocation of heat transfer surface area for cooling load and COP optimization of a thermoelectric refrigerator

The theory of finite time thermodynamics is applied to analyze and optimize the performance of a thermoelectric refrigerator, which is composed of multi-elements. For the fixed total heat transfer surface area of two heat exchangers, the ratio of the heat transfer surface area of the high temperature side heat exchanger to the total heat transfer surface area of the heat exchangers is optimized for maximizing the cooling load and the coefficient of performance of the thermoelectric refrigerator. The effects of various parameters on the optimum performance are analyzed. The results may provide guides for the analysis and optimization of practical thermoelectric refrigerators.     

Numerical comparison of thermoacoustic couples with modified stack plate edges

It has been demonstrated that the bulk of time-averaged heat transfer between the oscillating fluid and a thermoacoustic couple is concentrated towards the edges of the stack plate. Previous numerical studies which have considered thermoacoustic couples of finite thickness have used a rectangular form for the plate edge. In practice however, current manufacturing practices allow for a variety of stack edges which may improve the efficiency of heat transfer and/or reduce entropic losses. In this numerical study, the performance of a thermoacoustic couple is investigated at selected drive ratios and using a variety of stack plate edge profiles. Results indicate that stack profiles with enlarged and blunter shapes improve the rate of heat transfer at low drive ratios but retard the rate of heat transfer at higher drive ratios due to increased residence time of the fluid in contact with the stack plate. The improvement in COP through minimisation of acoustic streaming on the inside face of the stack, and increased effective cooling power by greater retention of stack thickness at the plate extremities, leads to recommendation of the Rounded edge shape profile for thermoacoustic stack plates in practical devices.     

翅片式热管散热器自然对流换热特性分析与多目标结构优化

采用数值计算方法对一种应用于半导体制冷片热端散热的翅片式热管散热器进行模拟,探究自然对流条件下不同翅片参数对散热器换热特性的影响。结合多目标遗传算法(NSGA-Ⅱ),以影响散热器散热的两个主要参数——翅片表面传热系数和肋面效率为优化目标,对散热器整体做出综合优化,并对优化结果进行K均值聚类分析,提出了翅片端优化原则。结果表明,肋面效率对散热器性能的影响有限,提高表面传热系数可显著降低散热器总热阻;与未优化方案相比,所选优化方案可使基板热端面温度下降3.5K,散热器热阻降低18.22%。     

NUMERICAL STUDY OF THERMOACOUSTIC HEAT EXCHANGERS IN THE THIN PLATE LIMIT

The velocity and temperature fields in an idealized thermoacoustic refrigerator are analyzed computationally. The numerical model simulates the unsteady mass, momentum, and energy equations in the thin-plate, low-Mach-number limits. Two-dimensional unsteady calculations of the flow field in the neighborhood of the stack and heat exchangers are performed using a vorticity-based scheme for stratified flow. The computations are applied to analyze the effects of heat-exchanger length and position on the performance of the device. The results indicate that the cooling load peaks at a well-defined heat-exchanger length, stack gap, and distance between the heat exchangers and the stack plates.