Global sensitivity analysis of two-stage thermoelectric refrigeration system based on response variance

In this paper, a mathematical model of two-stage thermoelectric refrigeration system is established considering the influence of external heat transfer and its performance is analysed based on finite-time thermodynamics and Newton's heat transfer law. Taking the cooling capacity and coefficient of performance of the two-stage thermoelectric refrigeration system as separate objective functions, the general relationship between cooling capacity, coefficient of performance, and working design parameters of the system is determined. The influence of the fluctuation of the input design parameters on the output performance parameters is studied using a global sensitivity analysis based on the variance response. The main and total Global sensitivity indices of input parameters that affect the output performance are calculated, and the related sensitivity ranking are obtained. The results can be used to guide the performance analysis and parameter optimization of two-stage thermoelectric refrigeration system in application.     

Optimization of microchannel-based cooling systems

Abstract

In this article, microchannel systems for cooling applications such as in the thermal management of electronic equipment are investigated and optimized. Numerical simulations are carried out to study the conjugate heat transfer and flow behavior. The numerical model has been validated by comparing with analytical results. Two sets of design variables are evaluated: (Set I) Incoming flow rate and the number of channels and (Set II) Incoming flow rate and heat flux input. Response surfaces are used to represent the thermal and fluid behavior in the microchannel systems. Based on the polynomial response surface (PRS) modeling results, a multi-objective optimization problem is formulated to reduce both pumping power and thermal resistance. Two major practical concerns, hot-spot temperature and pressure difference, serve as optimization constraints. With varying weights on the two conflicting objectives, Pareto frontiers are obtained. It is also shown that an optimal configuration exists under pressure and temperature constraints. This study provides a feasible design domain and optimal solutions for microchannel-based cooling systems. The optimization process can also be applied to different applications of similar thermal systems.     

Troubleshooting Shell-and-Tube Heat Exchangers

A newly developed methodology for the design of multistream plate-fin heat exchangers in the optimization of heat exchanger networks is described in detail. The designed heat exchanger consists of several block sections with intermediate entry and exit points along its length, and these sections are determined by the composite curves in the Pinch technology. The requirements of heat transfer and pressure drop are fulfilled through proper surface selection. The methodology was applied to an industrial case study, and the detailed design parameters are given. The example confirms the validity of the suggested methodology.     

某4缸乘用车柴油机冷却水套的设计优化

介绍了长城汽车研发的一款高性能"2.0VGT"柴油发动机冷却水套的设计优化过程,该发动机在进行可靠性试验过程中发现缸盖第1缸鼻梁区存在裂纹,导致冷却水渗入燃烧室发生失火。经过CFD模拟分析,采用优化垫片水孔结构,调整水孔上水量,得到优化后方案整体压力场,局部速度、对流传热系数、温度场的分布等均比优化前有明显改善。特别是优化后方案提高了缸盖第1缸鼻梁区(排气道与喷油器之间水路)冷却水的流速,避免了鼻梁区由于热负荷较大而导致的结构断裂问题。     

Performance Analysis and Design Optimization of Gapped Pin-Fin in a Cooling Channel

Performance analysis and optimization of a circular pin-fin with inside gaps in a rectangular cooling channel were performed at Reynolds number, 10,000, using three-dimensional Reynolds-averaged Navier–Stokes equations and a multi-objective genetic algorithm. The low-Reynolds-number version of the shear stress transport model was used as turbulence closure. A parametric study was also performed to identify the geometrical effects of the pin-fin on heat transfer and pressure drop. The straight and reference gapped pin-fins yielded better performances than those of the circular pin-fin without the gap in terms of both heat transfer and pressure drop. The objective of the optimization was to maximize the heat transfer and minimize the pressure loss, simultaneously. The area-averaged Nusselt number and pressure loss coefficient were considered as objective functions, and three design variables related to the geometry of the gapped pin-fin were chosen for the optimization. Twenty-seven design points were generated using Latin hypercube sampling in the design space, and response surface approximation models were constructed for the objective functions. The optimization results were analyzed using five representative solutions on the Pareto-optimal front. The objective functions were found to be significantly affected by variation in the design variables, especially, the width of front gap and the rear gap angle.     

Multi‐objective design optimization for mini‐channel cooling battery thermal management system in an electric vehicle

Lithium‐ion battery packs have been generally used as the power source for electric vehicles. Heat generated during discharge and limited space in the battery pack may bring safety issues and negative effect on the battery pack. Battery thermal management system is indispensable since it can effectively moderate the temperature rise by using a simple system, thereby improving the safety of battery packs. However, the comprehensive investigation on the optimal design of battery thermal management system with liquid cooling is still rare. This article develops a comprehensive methodology to design an efficient mini‐channel cooling system, which comprises thermodynamics, fluid dynamics, and structural analysis. The developed methodology mainly contains four steps: the design of the mini‐channel cooling system and computational fluid dynamics analysis, the design of experiments and selection of surrogate models, formulation of optimization model, and multi‐objective optimization for selection of the optimum scheme for mini‐channel cooling battery thermal management system. The findings in the study display that the temperature difference decreases from 8.0878 to 7.6267 K by 5.70%, the standard temperature deviation decreases from 2.1346 to 2.1172 K by 0.82%, and the pressure drop decreases from 302.14 to 167.60 Pa by 44.53%. The developed methodology could be extended for industrial battery pack design process to enhance cooling effect thermal performance and decrease power consumption.     

车用内燃机冷却系统动态传热模型

提出了一个基于集总参数法的车用内燃机冷却系统动态传热模型。考虑了内燃机燃烧室、散热器和水泵的传热和冷却系统的工作,建立了机体、散热器、水泵与冷却介质之间的热耦合计算公式。对一台单缸柴油机冷却系的稳态及动态温度进行了计算,结果证实该模型可用于内燃机冷却系统的动态传热特性研究。     

Optimization of the direct Carnot cycle

A model for the study and optimization of two heat reservoirs thermal machines is presented. The mathematical model basically consists of the First and Second Laws of Thermodynamics applied to the cycle and entire system, and the heat transfer equations at the source and sink. The internal and external irreversibilities of the cycle are considered by taking into account the entropy generation terms. Several constraints imposed to the system composed by the engine and the two heat reservoirs (namely, engine efficiency, or power output, or heat flux received by the engine, each of them together with imposed internal entropy generation and total number of heat transfer units of the machine heat exchangers) allow us to find the optimum operational conditions, as well as the limited variation ranges for the system parameters. Emphasis is put on coupling between various possible objective functions, namely thermal cost, useful effect, first law efficiency and whole system dissipation. It is for the first time to our knowledge when it has been proved that if one of the possible objective functions is fixed (as a parameter with imposed value), the optima of the other three always correspond to each other for the corresponding stationary state system, with a given optimum heat conductance allocation (one degree of freedom). Other interesting results are also reported in this paper. Some sensitivity studies were developed, too, with respect to various parameters of the model (engine performance, internal entropy generation, total number of heat transfer units).     

某中速柴油机冷却液流动及流固耦合传热计算分析

为获得直观的流场和温度场数据,对某16缸中速柴油机建立了完整的冷却水三维模型,采用FLUENT流体计算软件对其进行了绝热CFD计算分析,得到了冷却水流速、压力等数据。考虑到各缸冷却水套为并列排布方式,各缸之间相对独立,建立了单缸的缸盖-缸套-冷却水耦合传热模型,对其进行了缸盖-缸套-冷却水耦合传热仿真,获得了各部件比较精确直观的温度场分布。结果表明:仿真结果与实测数据吻合较好,从而为柴油机冷却水套的优化设计提供了依据。     

六缸柴油机冷却系统流动与传热的数值模拟研究

冷却水的流动与传热直接影响柴油机的冷却效率、高温零件的热负荷、整机的热量分配和能量利用。在冷却系统传热计算时，利用流固耦合的方法，较为准确地确定了缸体水套的传热边界条件。采用CFD商用软件STAR—CD对直列六缸柴油机的冷却系统的流动与传热进行三维数值模拟，给出了整机冷却水套内冷却液的流场、换热系数及压力场分布，为柴油机冷却水腔的优化设计提供了理论依据。     

Thermoeconomic optimization for a finned‐tube evaporator configuration of a roof‐top bus air‐conditioning system

This paper presents a methodology of a design optimization technique that can be useful in assessing the best configuration of a finned‐tube evaporator, using a thermoeconomic approach. The assessment has been carried out on a direct expansion finned‐tube evaporator of a vapor compression cycle for a roof‐top bus air‐conditioning (AC) system at a specified cooling capacity. The methodology has been conducted by studying the effect of some operational and geometrical design parameters for the evaporator on the entire cycle exergy destruction or irreversibility, AC system coefficient of performance (COP), and total annual cost. The heat exchangers for the bus AC system are featured by a very compact frontal area due to the stringent space limitations and structure standard for the system installation. Therefore, the current study also takes in its account the effect of the variation of the design parameters on the evaporator frontal area. The irreversibility due to heat transfer across the stream‐to‐stream temperature difference and due to frictional pressure drops is calculated as a function of the design parameters. A cost function is introduced, defined as the sum of two contributions, the investment expense of the evaporator material and the system compressor, and the operational expense of AC system that is usually driven by an auxiliary engine or coupled with the main bus engine. The optimal trade‐off between investment and operating cost is, therefore, investigated. A numerical example is discussed, in which a comparison between the commercial evaporator design and optimal design configuration has been presented in terms of the system COP and evaporator material cost. The results show that a significant improvement can be obtained for the optimal evaporator design compared with that of the commercial finned‐tube evaporator that is designed based on the conventional values of the design parameters. Copyright © 2007 John Wiley & Sons, Ltd.     

基于传热系数反求法的船用柴油机组合活塞热分析

建立了柴油机组合活塞的2D有限元模型,并利用传热系数反求法对该模型进行热分析。在仿真分析中,将活塞头部外表面传热系数定义为设计参数,实测温度与仿真温度的差值定义为目标函数。通过仿真计算得到了活塞头部外表面传热系数的分布,并利用反求法得到的传热系数对有限元模型进行热分析,仿真结果与实测温度吻合;传热系数反求法收敛迅速,可以有效用于活塞及其他内燃机零部件的温度分布和热负荷分析。     

利用汽车发动机余热的溴化锂吸收式制冷研究

根据现有汽车空调的制冷系统和发动机冷却水及排气系统的结构特点,结合溴化锂吸收式制冷系统的工作原理,提出将汽车排气管和发动机冷却水箱进行结构改造作为溴化锂吸收式制冷系统的发生器,代替传统的汽车空调的制冷和采暖系统及发动机冷却系统。并对该溴化锂制冷系统进行了热力计算和传热面积的计算,计算结果表明,溴化锂制冷系统充分利用了废气余热和冷却水余热,减少了汽车油耗,并且改造后的排气热交换器和冷却水箱传热面积小,结构简单紧凑。     

An analysis of beta type Stirling engine with rhombic drive mechanism

Stirling engine system is one of the options for electrifying a remote community not serviceable by the grid, which can operate on energy input in the form of heat. Major hurdle for the wide-spread usage of rhombic drive beta type Stirling engine is complexity of the drive and requirement of tight tolerances for its proper functioning. However, if the operating and geometrical constraints of the system are accounted for, different feasible design options can be identified. In the present work, various aspects that need to be considered at different decision making stages of the design and development of a Stirling engine are addressed. The proposed design methodology can generate and evaluate a range of possible design alternatives which can speed up the decision making process and also provide a clear understanding of the system design considerations. The present work is mainly about the design methodology for beta type Stirling engine and the optimization of phase angle, considering the effect of overlapping volume between compression and expansion spaces. It is also noticed that variation of compression space volume with phase angle remains sinusoidal for any phase difference. The aim of the present work is to find a feasible solution which should lead to a design of a single cylinder, beta type Stirling engine of 1.5 kW_e capacity for rural electrification.     

Performance of a combined organic Rankine cycle and vapor compression cycle for heat activated cooling

Heat activated cooling has the potential of utilizing thermal sources that currently go unused such as engine exhaust heat or industrial waste heat. Using these heat sources can provide enhanced energy utilization and reduced fuel usage in applications where cooling is needed. The concept developed here uses waste heat from stationary and mobile engine cycles to generate cooling for structures and vehicles. It combines an organic Rankine cycle (ORC) with a conventional vapor compression cycle. A nominal 5 kW cooling capacity prototype system was developed based on this concept and tested under laboratory conditions. In order to maintain high system performance while reducing size and weight for portable applications, microchannel based heat transfer components and scroll based expansion and compression were used. Although the system was tested off of its design point, it performed well achieving 4.4 kW of cooling at a measured heat activated COP of 0.48. Both the conversion and 2nd law efficiencies were close to the model results, proving it to be an attractive technology. The measured isentropic efficiency of the scroll expander reached 84%, when the pressure ratio was close to the scroll intrinsic expansion ratio. The reduced cooling capacity was attributed to off design operation.     

Analysis and Prediction of the Thermal Performance of Piezoelectrically Actuated Fans

An experimentally validated numerical model is developed to analyze the operation of a piezoelectrically actuated cantilever vibrating close to a heated surface. The vibrating cantilever acts as a fan and provides localized cooling. The numerical results for the flow field and heat transfer show satisfactory agreement with experiments. The numerical model is used to develop fan curves for the piezoelectric fans, using a methodology similar to that used in constructing pump or fan curves for conventional fans. A simplified model based on stagnation region heat transfer in impingement flows is also proposed to estimate the heat transfer from a piezoelectric fan. The velocities obtained from the piezoelectric fan curves generated are used in this impingement heat transfer model, and the predictions are found to agree with measured stagnation zone Nusselt numbers with an average deviation of 17%.     

Optimal design of transverse ribs in tubes for thermal performance enhancement

We conducted an optimization using the second-order response surface method to determine the transverse rib geometry required to achieve the highest cooling performance in a circular channel. The best rib geometry was based on three design variables; rib height, rib width, and rib pitch. The turbulent heat transfer coefficients and friction losses were first calculated and then used to determine the thermal performance. We constructed the response surfaces of the three design variables as functions of the average Nusselt number ratio, friction loss, and thermal performance. These functions led to the optimum design point at the highest heat transfer rate in the special case of an actual turbine cooling passage with a constant friction loss.     

Enhancement in Evaporative Effectiveness of an Evaporative Tubular Heat Dissipator Using Experimental Design Approach

An experimental investigation of evaporative effectiveness and mass transfer coefficient on a bundle of tubes of an evaporative tubular heat dissipator is presented. Based on the experiments, correlations of evaporative effectiveness and mass transfer coefficient are derived using multiple regression analysis. A statistical model is developed to correlate the operating variables using design of experiment approach by selecting central composite design of a response surface methodology. Results shown in this article indicate that as the cooling film flow rate increases, evaporative effectiveness and mass transfer coefficient increases provided that the air flow rate is constant which is flowing from underneath the tubes of the evaporative tubular heat dissipator. Derived correlations are helpful in improvement of the design of heat transfer devices and many other engineering applications. Consideration of relative humidity of upstreaming air as one of the operating variables leads to the contribution to heat and mass transfer study of evaporative tubular heat dissipators in the present investigation.     

An experimental and theoretical investigation of refrigerant charge on a secondary loop air‐conditioning heat pump system in electric vehicles

Automotive air‐conditioning heat pump systems are particular interest worldwide in energy conservation and emission reduction for electric vehicles, hybrid electric vehicles, and fuel cell electric vehicles. Refrigerant charge amount is a key factor for the air‐conditioning heat pump system optimization affecting the condensing pressure and subcooling in both heating and cooling modes. In this paper, the influence of the refrigerant charge on system performances was investigated using the experiment method on a secondary loop air‐conditioning heat pump system. The typical heat transfer and flow parameters were recorded, and both cooling and heating performances of the system were investigated and illustrated by pressure‐enthalpy diagrams. The critical refrigerant charges were determined in both heating and cooling modes. Three typical void fraction correlation models were also applied for the refrigerant charge determination modeling as a system off‐design method. Results show that the Hughmark void fraction correlation method has the best prediction of the critical refrigerant charge in both cooling and heating modes.     

车用内燃机整机热平衡动态传热模型

提出了一个基于集总参数法的发动机动态热平衡模型，考虑了发动机燃烧室的传热、发动机主要部件的传热、冷却系统和润滑系统的工作。建立了燃烧气体与发动机部件、各部件之间、部件与冷却液、部件与润滑油、部件与周围空气之间的热耦合计算公式，对一台单缸柴油机的热平衡及主要部件的温度进行了计算．结果证实该模型可用于发动机热平衡的研究。