基于(火积)耗散的换热器热阻分析

讨论了一维导热、典型对流换热过程和换热器中热阻的概念,提出了基于煨耗散的换热器热阻和换热器热阻因子的定义.建立了基于这一热阻的换热器分析方法,讨论了传热单元数和热容量流比对换热器热阻的影响.     

(火积)——描述物体传递热量能力的物理量

从导热过程与导电过程的比拟出发，引入了与电容器的能量相对应的新的物理量Eh=QvhT／2．它具有“能量”的性质，它描述了一物体所具有的热量传递的总能力．由于它是热容量与温度乘积之半，因此把此物理量称之为[火积]．热量传递是一个不可逆过程，在传递过程中部分[火积]将被耗散，其数值可由[火积]耗散函数的体积分求得．在建立了[火积]平衡方程的基础上定义了[火积]传递的效率，从而可讨论传热过程的优化．在变分分析的基础上，提出了导热过程优化的[火积]耗散极值原理：对于具有一定的约束条件并给定热流边界条件时，当[火积]耗散最小，则导热过程最优（温差最小）；在给定温度边界条件时，[火积]耗散最大，则导热过程最优（热流最大）．基于[火积]的耗散这个物理量定义了多维导热问题中的当量热阻，从而可把导热优化的[火积]耗散极值原理归结为导热优化的最小热阻原理．最后，以体点散热问题为例，计算了使导热性能最好的导热系数的最佳分布，并对优化前后的导热性能作了比较．     

基于[火积]耗散的换热器热阻分析

讨论了一维导热、典型对流换热过程和换热器中热阻的概念,提出了基于[火积]耗散的换热器热阻和换热器热阻因子的定义．建立了基于这一热阻的换热器分析方法,讨论了传热单元数和热容量流比对换热器热阻的影响．     

耗散极值原理的电热模拟实验研究

建立了具有32×16节点电阻的电热模拟装置.每个节点的4个电阻可以通过并联低阻值电阻模拟在基材中填充高导热材料.实验表明,使导热性能最优的高导热材料分布与数值模拟结果相一致,从而进一步验证了导热优化的(火积)耗散极值(最小热阻)原理.实验还证明在一般情况下导热过程的优化不宜采用最小熵产原理,因为后者属于热力学优化原理,适用于热功转换过程的优化.     

提高客车空调机组经济性的(火用)优化方法

为了提高客车空调机组的经济性,本文以kk-30型空调机组为例,从(?)分析入手,采用综合考虑损耗的办法,通过(?)优化目标分析,可以求得在一定几何结构、尺寸等条件下最经济的优化参数。所得结果可以作为机组改进的依据,所用方法可供设计、使用工作参考。     

基于(火用)方法的空调冷热源系统优化决策

结合方法和热经济学原理建立了空调冷热源系统的分析模型,阐述了产品成本是评价冷热源系统方案优劣的关键因素.采用多目标模糊决策方法,综合考虑了单位产品成本、初投资、技术先进性、安装面积、运行安全可靠性、维护管理难易程度和环保性等多种影响因素,基于方法的冷热源优化决策算法.通过应用实例,说明了此新优化决策算法相比工程中传统决策方法,可靠性更高,使各决策方案间优劣区分度更加明显。     

换热器的(火积)理论强化传热评价准则研究

对于现有的(火积)理论只考虑了换热器换热量或传热温差的影响,而没有考虑压降和泵功的问题,分析和推导得出两种新的(火积)评价指标ET/EP2/3和ET/EP,并命名为EPEC(entransy performance evaluation criterion)指标,分别表征了单位压降和单位泵功下的换热量,这对换热器传热强化性能评价工作具有重要的参考价值。同时,对18°和27°平面螺旋折流板换热器和折面螺旋折流板换热器的实验结果分别进行了基于热力学第一定律、热力学第二定律以及新的(火积)理论评价指标的分析,研究表明:ET/EP2/3和ET/EP与单位压降和单位泵功下的换热量随有效度的变化趋势基本一致,验证了新评价指标的有效性和可靠性。不同评价指标均从不同方面反映了换热器的性能,同时也说明了不同评价指标之间的差异和针对不同换热器不同工况统一标准的必要性。     

基于火积的乏风催化装置换热器网络优化

以自主设计组装的乏风瓦斯预热直流催化装置预热系统为研究对象，基于热力学定律与火积分析理论，建立系统换热器网络热导系数模型，研究特定参数范围内最高运行温度、循环温差变化对换热器网络热导系数的影响。结果表明：当总热导系数大于某值时，随着最高运行温度的升高和循环温差的降低，总热导系数大幅下降；当总热导系数小于该值时，参数变化对其影响不再明显。鉴于此提出阈值优化概念，以一级预热系统为例，对比原设计值，采用阈值优化值作为设计参数时系统换热器网络对应总热导系数下降20.2%，热容流下降23.9%；对比以最小总热导系数为目标的优化，阈值优化能结合系统实际运行需求，以增加12.5%总热导系数换取热容流下降39.1%。     

基于能量耗散原理的红砂岩崩解机制研究

为了研究红砂岩的崩解机制,寻找阻滞其崩解的方法,通过分析红砂岩崩解过程中传递的不同能量类型及其定量计算方法,基于能量耗散原理建立红砂岩崩解的能量耗散模型。结合工程实例,运用该模型分析红砂岩在崩解过程中的能量变化规律。研究结果表明:在岩石自然环境中的崩解过程中,随着岩石粒径变小,其新增表面积越来越大,其新增表面能随时间呈线性增大,但是,在这个过程中,岩石对吸收能量的利用率下降,说明红砂岩在不同崩解阶段对吸收热量的利用率不同;红砂岩崩解能量利用率随崩解呈指数衰减。建议工程上在治理红砂岩问题时可采用预先崩解的方法,然后采用压实破碎,包边封闭的方法尽量减小红砂岩与外界环境接触的面积,切断软岩崩解所需的能量来源,以延迟其崩解。     

(火用)损失的机理研究

将非平衡态热力学理论应用于分析,导出了损率与推动力的一般关系,阐明了由不可逆过程引起损失的机理.结果表明,一种形式的推动力不仅引起该种形式的损失,而且由于不同形式的推动力之间的耦合而引起其它形式的损失;并进一步指出,工程中计算损失时,耦合项一般可以忽略.     

Constructal optimization on T-shaped cavity based on entransy dissipation minimization

The entransy dissipation extremum principle provides new warranty and criterion for optimization of heat transfer. For a heat transfer model of a rectangular solid wall with an open T-shaped cavity, a dimensionless equivalent thermal resistance based on entransy dissipation is taken as optimization objective, and constructal optimization for the model is carried out when the system volume, the cavity volume and the volume of rectangle occupied by T-shaped cavity are fixed. Numerical results indicate that the optimal geometry construct of cavity can be schemed out based on entransy dissipation extremum principle. The formulation of dimensionless global （maximum） thermal resistance presented in a literature is modified; some new rules which are different from those reported in the literature are obtained based on the minimization of the modified objective. Comparisons of the numerical results show that the optimal system constructs deduced respectively from the two thermal resistance objectives are very different. The optimization by taking equivalent thermal resistance minimization as objective can more effectively reduce mean temperature difference of heat transfer than the optimization by taking maximum thermal resistance minimization as objective, so that the performance of heat transfer for the total system can be improved. The more freedom the cavity has, the better the total system performance is. The correlations of the equivalent thermal resistance and the maximum thermal resistance of the system and three geometric degrees of freedom are found by using function fitting.     

The entransy dissipation minimization principle under given heat duty and heat transfer area conditions

Under given heat duty and heat transfer area conditions, the equipartition of the entransy dissipation (EoED) principle, the equipartition of the temperature difference (EoTD) principle, and the equipartition of the heat flux (EoHF) principle are applied to the optimization design of a heat exchanger with a variable heat transfer coefficient. The results show that the difference between the results obtained using the EoED and EoTD principles is very small, far smaller than that between the results obtained using the EoED and EoHF principles. The correct entransy dissipation minimization principle is chosen to optimize the parameters in the hot and cold fluids in a two-fluid heat exchanger, under given heat duty and heat transfer area conditions. The results indicate that the proper choice of the two alternative fluids has an important role in the successful application of the entransy dissipation minimization principle. The fluid that could improve the total heat transfer coefficient should be chosen, or the fluid that makes the temperature profiles of the hot and cold fluids parallel and decreases the temperature difference between the hot and cold fluids after optimization simultaneously, could be the proper one.     

Least dissipation principle of heat transport potential capacity and its application in heat conduction optimization

The heat conduction following the Fourier law widely exists in nature and engineering. Usually, the thermal resistance is applied to evaluating the perform-ance of the heat conduction, i.e. the less resistance corre-sponds to the better performance. Therefore, the heat conduction is often enhanced by means of using high conductivity materials or reducing the thermal contact resistance. The more general performance criterion is the heat duty for the given temperature difference DT, or the temp…     

Least dissipation principle of heat transport potential capacity and its application in heat conduction optimization

In the viewpoint of heat transfer,heat transport potential capacity and its dissipation are defined based on the essence of heat transport phenomenon,Rspectively,their physical menings are the overall heat transfer capability and the dissipation rate of the heat transfer capacity.Then the least dissipation principle of heat transport potential cpacity is presented to enhance the heat conduction efficiency in the heat conduction optimization .The principle is, for a conduction process with the constant integral of the thermal conductivity over the region ,the optimal distribution of thermal conductivity,which corresponds to the highest heat conduction efficiency ,is characterized by the least dissipation of heat transport potential capacity .Finally the principle is applied to some cases in heat conduction optimization.     

Entropy generation extremum and entransy dissipation extremum for heat exchanger optimization

The applicability of the extremum principles of entropy generation and entransy dissipation is studied for heat exchanger optimization. The extremum principle of entransy dissipation gives better optimization results when heat exchanger is only for the purpose of heating and cooling, while the extremum principle of entropy generation is better for the heat exchanger optimization when it works in the Brayton cycle. The two optimization principles are approximately equivalent when the temperature drops of the streams in a heat exchanger are small. Supported by Major State Basic Research Development Program of China (Grant No. 2007CB206901)     

Optimization for entransy dissipation minimization in heat exchanger

A common of two-fluid flow heat exchanger, in which the heat transfer between high- and low-temperature sides obeys Newton＇s law [q∝△（T）], is studied in this paper. By taking entransy dissipation minimization as optimization objective, the optimum parameter distributions in the heat exchanger are derived by using optimal control theory under the condition of fixed heat load. The condition corresponding to the minimum entransy dissipation is that corresponding to a constant heat flux density. Three kinds of heat exchangers, including parallel flow, condensing flow and counter-flow, are considered, and the results show that only the counter-flow heat exchanger can realize the entransy dissipation minimization in the heat transfer process. The obtained results for entransy dissipation minimization are also compared with those obtained for entropy generation minimization by numerical examples.     

Constructal entransy dissipation rate minimization for heat conduction based on a tapered element

Based on constructal theory,the structure of a tapered element and high-conductivity link is optimized by taking the minimization of the entransy dissipation rate as the optimization objective.The results show that the mean temperature difference of the heat transfer cannot always decrease when the internal complexity of the control-volume increases.There exists an optimal constructal order leading to the minimum mean temperature difference for heat transfer.The thermal current density in high-conductivity links with variable shapes does not linearly depend on the length.Therefore,the optimized constructs based on the minimization of the entransy dissipation rate are different from those based on the minimization of the maximum temperature difference.Compared with the construct based on the minimization of the maximum temperature difference,the construct based on the minimization of the entransy dissipation rate can reduce the mean temperature difference,and improve the heat transfer performance significantly.Because entransy describes the heat transfer ability more suitably,various constructal problems in heat conduction may be addressed more effectively using this basis.     

Application of entransy theory in the heat transfer optimization of flat-plate solar collectors

The flat-plate solar collector is an important component in solar-thermal systems, and its heat transfer optimization is of great significance in terms of the efficiency of energy utilization. However, most existing flat-plate collectors adopt metallic absorber plates with uniform thickness, which often works against energy conservation. In this paper, to achieve the optimal heat transfer performance, we optimized the thickness distribution of the absorber with the constraint of fixed total material volume employing entransy theory. We first established the correspondence between the collector efficiency and the loss of entransy, and then proposed the constrained extreme-value problem and deduced the optimization criterion, namely a uniform temperature gradient, employing a variational method. Finally, on the basis of the optimization criterion, we carried out numerical simulations, with the results showing remarkable optimization effects. When irradiation, the ambient temperature and the wind speed are 800 W/m², 300 K and 3 m/s, respectively, the collector efficiency is enhanced by 8.8% through optimization, which is equivalent to a copper saving of 30%. We also applied the thickness distribution optimized for wind speed of 3 m/s in heat transfer analysis with different wind speed conditions, and the collector efficiency was remarkably better than that for an absorber with uniform thickness.     

风冷式热泵机组结构优化设计及散热性能研究

基于ANSYS ICEM软件对风冷式热泵机组进行物理建模和网格划分,在常规工况下利用Fluent软件对运行机组进行温度场及速度场仿真模拟,分析其散热过程。针对夏季高温,风冷式热泵机组散热效率低下的问题,设计一种辅助散热系统并进行结构优化,通过水帘预冷与雾化喷淋装置强化换热过程,提高散热效率、减少散热死区,并通过实验测试不同辅助装置对机组散热性能的影响。研究结果表明,增加辅助散热系统可有效提高机组散热效率。相同的室外空气状态下,在机组的散热性能和蒸发效率方面,雾化喷淋装置相比水帘预冷装置更具优势。