基于(火积)耗散率最小的“盘点”冷却流道构形优化

基于构形理论, 以(火积)耗散率最小为优化目标, 对冷却流道的“盘点”传热问题进行构形优化, 得到冷却流道的圆盘构造体最优构形. 结果表明: 对于扇形单元体, 在其泵功率给定的条件下, 存在最佳展弦比使得扇形单元体无量纲当量热阻取得最小值; 对于一级树状圆盘, 在其总泵功率给定的条件下, 存在一级与单元级最佳流道宽度比和扇形单元体最佳无量纲半径使 得一级树状圆盘无量纲当量热阻取得最小值, 且一级与单元级最佳流道宽度比仅与单元体分支数有关. 当中心圆盘半径等于0时, 一级树状圆盘最终退化成辐射状圆盘, 此时一级树状圆盘半径为临界半径. 当一级树状圆盘半径大于临界半径时, 需对圆盘冷却流道采用树状布置, 反之则采用辐射状布置. 存在最佳单元体分支数使得无量纲当量热阻取得最小值, 这与高导热材料通道的“盘点”导热构形优化结果有明显区别. (火积)耗散率最小和最大温差最小的一级树状冷却流 道圆盘构造体最优构形是不同的. 与最大温差最小的冷却流道圆盘构造体相比, (火积)耗散率最小的冷却流道圆盘构造体当量热阻得到极大降低, 其整体传热性能得到明显提高. 因此, (火积)耗散极值原理与对流构形优化相结合, 有助于进一步揭示(火积)耗散极值原理在传热优化方面的优越性. 关键词： 构形理论 (火积)耗散率 冷却流道 广义热力学优化     

基于(火积)耗散率最小的复杂肋片对流换热构形优化

基于构形理论, 以基于(火积)耗散率定义的当量热阻最小为优化目标对复杂肋片进行构形优化, 得到同时考虑肋片导热和对流换热(火积)耗散性能的肋片最优构形, 并比较不同形状和不同优化目标下的肋片最优构形. 结果表明: 存在最佳单元级直肋、中部空腔以及肋片末梢高度和长度比使得复杂肋片当量热阻取得三重最小值. 当量热阻最小的复杂肋片最优构形与T-Y形肋片最优构形相比, 复杂肋片结构使得肋片整体传热性能大大提高. 当肋片传热为二维传热且根部较宽时, 肋片根部温度越不均匀, 当量热阻最小和最大热阻最小的复杂肋片最优构形差别越大. 在保证热安全性的前提下, 工程上对肋片进行优化设计时可选择当量热阻最小的肋片构形设计方案以降低其平均传热温差、提高整体传热性能. 本文从传热优化角度为复杂肋片的优化设计提供了参考.     

Analysis of elliptical thermal cloak based on entropy generation and entransy dissipation approach

In this work, we designed the elliptical thermal cloak based on the transformation thermotics. The local entropy generation rate distribution and entransy dissipation rate distribution were obtained, and the total entropy generation and entransy dissipation of different types of elliptical cloaks were evaluated. We used entropy generation approach and entransy dissipation approach to evaluate the performance of the thermal cloak, and heat dissipation analysis was carried out for models with different parameters. Finally, the optimized elliptical thermal cloak with minimum entropy generation and minimum entransy dissipation is found, and some suggestions on optimizing the structure of elliptical thermal cloak were given.     

对流换热过程的热力学优化与传热优化

为了进一步明确对流换热过程中热力学优化与传热优化之间的差异,本文分别利用熵产最小原理、(火积)耗散极值原理针对两种边界条件下的对流换热问题进行分析,讨论熵产,(火积)耗散与有用能损失以及对流换热能力之间的关系.结果表明:熵产最小意味着系统的有用能损失最小,但并不反映系统的对流换热能力的强弱;而(火积)耗散取极值意味着系统的对流换热能力最强,但与系统的有用能损失不存在对应关系.因此,对于将降低有用能损失作为优化目标的换热问题应采用熵产最小原理进行分析;而对于需要将提高换热能力作为优化目标的对流换热问题应采用(火积)耗散极值原理进行分析.     

Constructal Equivalent Thermal Resistance Minimization for Tau-Shaped Fin

With the aid of constructal theory and entransy theory, a Tau-shaped fin (TAUSF) is investigated in this paper, and the widths of the bend end and elemental fins are assumed to be different. The construct of the TAUSF is optimized by the minimum equivalent thermal resistance (ETR) obtained by entransy dissipation rate. The constraints of total enveloping volume and fin material volume are considered. The results show that in the specified range of width ratio, the twice minimum ETR of the TAUSF can be yielded by an optimal width ratio and an optimal length ratio. In addition, comparing the optimal performance of the TAUSF with the counterpart of a T-shaped fin, the former sacrifices a small amount of heat transfer performance and its stiffness increases due to its structure with the bend end. The optimal structure of the TAUSF yielded from ETR minimization is conspicuously different with the counterpart yielded from maximum thermal resistance minimization. Comparing the thermal performances of the two optimal constructs, the ETR of the former optimal construct is declined by 10.58%, whereas the maximum thermal resistance is augmented by 5.22%. The former optimal construct can lead to the uniformity of temperature gradient and the reduction in thermal stress, and can guide the engineering designs of practical fins.     

A comparison of different entransy flow definitions and entropy generation in thermal radiation optimization

In thermal radiation, taking heat flow as an extensive quantity and defining the potential as temperature T or the black body emissive power U will lead to two different definitions of radiation entransy flow and the corresponding principles for thermal radiation optimization. The two definitions of radiation entransy flow and the corresponding optimization prin ciples are compared in this paper. When the total heat flow is given, the optimization objectives of the extremum entransy dissipation principles （EEDPs） developed based on potentials T and U correspond to the minimum equivalent temperature difference and the minimum equivalent blackbody emissive power difference respectively. The physical meaning of the definition based on potential U is clearer than that based on potential T, but the latter one can be used for the coupled heat transfer optimization problem while the former one cannot. The extremum entropy generation principle （EEGP） for thermal radiation is also derived, which includes the minimum entropy generation principle for thermal radiation. When the radiation heat flow is prescribed, the EEGP reveals that the minimum entropy generation leads to the minimum equivalent thermodynamic potential difference, which is not the expected objective in heat transfer. Therefore, the minimum entropy generation is not always appropriate for thermal radiation optimization. Finally, three thermal radiation optimization examples are discussed, and the results show that the difference in optimization objective between the EEDPs and the EEGP leads to the difference between the optimization results. The EEDP based on potential T is more useful in practical application since its optimization objective is usually consistent with the expected one.     

(火积)理论在热功转换过程中的应用探讨

分析和讨论了(火积)理论在热功转换过程的应用及其局限性.对Carnot循环的分析表明,Carnot循环中系统的(火积)是平衡的,但(火积)和熵之间不存在dG=T2dS这样的联系.对于一般热力学过程,分析表明,在热量传递到内可逆循环中间接对外做功时,现有的(火积)理论可用于系统的分析.讨论了热功转换过程分析中(火积)理论与熵理论的不同.分析表明,两个理论的分析角度及优化输出功的前提条件是不同的.熵产从可用能损失的角度分析热功转换过程,而(火积)理论则从热量势能消耗的角度.当输入系统的可用能给定或者输入系统的热量及热量进、出系统的热力学力给定时,熵产最小化对应于输出功最大;对于(火积)理论,则当输入系统的热量及热量进、出系统的温度给定时,最大(火积)损失对应于最大输出功.同时,它们各自均有局限性.当相应的前提条件不满足时,最大(火积)损失或最小熵产可能不与最大输出功相对应.     

辐射(火积)耗散与空间辐射器温度场均匀化的关系

(火积)理论是针对传热优化发展起来的,并获得了越来越多的应用。本文基于辐射(?)原理,对空间辐射器散热过程中的散热量分布、发射率分布和散热面积分布问题进行了分析。对于以上三类优化分布问题,理论分析和数值计算均表明,辐射器最小的辐射(?)耗散和辐射热阻均对应于散热表面均匀的温度场。因此,辐射(?)原理可用于空间辐射器的温度场均匀化设计。     

微、纳米尺度下圆盘(火积)耗散率最小构形优化

基于构形理论, 以(火积)耗散率最小为优化目标, 在微、纳米尺度下对圆盘导热问题进行构形优化, 得到尺寸效应影响下的无量纲当量热阻最小的圆盘构造体最优构形. 结果表明: 在微、纳米尺度下, 尺寸效应影响下的圆盘构造体最优构形与无尺寸效应影响时的圆盘构造体最优构形有明显区别. 存在最佳无量纲高导热材料通道长度使无量纲当量热阻取得最小值; 随着扇形单元体数目的增大, 最小无量纲当量热阻先减小后增大, 存在最佳的扇形单元体数目使得无量纲当量热阻取得双重最小值, 这与常规尺度下圆盘构造体相应的性能特性明显不同. (火积)耗散率最小的圆盘构造体(火积)耗散率比最大温差最小的构造体(火积)耗散率降低了7.31%, 也即圆盘构造体的平均传热温差降低了7.31%. 微、纳米尺度下基于(火积)耗散率最小的圆盘构造体最优构形能够降低圆盘构造体的平均传热温差, 同时有助于提高其整体传热性能. 本文工作有助于进一步拓展(火积)耗散极值原理的应用范围. 关键词： 构形理论 (火积)耗散率最小 微、纳米尺度 广义热力学优化     

换热器性能分析新方法

鉴于以加热或冷却为目的的热量传递过程,其不可逆性应以NFDA1的耗散率来度量,为此可以用换热器中的NFDA1耗散率定义换热器的当量热阻,它既包含换热器中的传热热阻,还包含了由非逆流形式和非平衡流引起的附加热阻. 换热器当量热阻的倒数称之为换热器的当量热导. 通过NFDA1耗散定义的换热器当量热阻建立了传热不可逆性与有效度的联系,并导得了换热器有效度与当量热导（热阻）和热容量流比的统一函数关系式,它适用于不同流程布置的换热器. 因此,有效度-热导（热阻）方法能更方便于不同类型换热器性能的分析和比 关键词： 换热器 热阻 耗散 熵产     

矩形肋片热沉(火积)耗散率最小与最大热阻最小构形优化的比较研究

针对矩形肋片热沉, 分别以最大热阻最小化和基于(火积)耗散定义的当量热阻最小化为优化目标, 采用二维传热模型并结合有限元数值仿真对其进行构形优化, 比较了两种目标下的热沉最优构形, 并分析了全局参数(综合了对流换热系数、肋片占据的总面积及其热导率的函数)和材料占比对两种目标(最大热阻、当量热阻)及其对应最优构形的影响. 结果表明: 热沉外形固定时, 两种目标下均不存在最优的肋片厚度; 热沉外形自由变化时, 两种目标下的最优构形存在一定的差异. 此外, 全局参数对两种目标下的最优构形均没有影响, 而材料占比对两种目标下的最优构形均有较大影响. 提高全局参数和材料占比均可以减小最大热阻最小值和当量热阻最小值, 但对两种目标的减小程度不同. 总体上, 调节热沉结构参数使当量热阻最小, 可以同时获得很好的局部极限性能; 而调节热沉结构参数使最大热阻最小, 获得的整体平均散热性能却较差. 因此, 对本文热沉模型进行优化时, 以当量热阻最小化为优化目标更合理.     

对流换热中的准(火积)耗散函数

通过分析对流换热过程中微元控制体内的(火积)流动平衡,得到类似于热传导过程中(火积)耗散函数表达形式的对流换热准(火积)耗散函数,表明对流换热过程中的(火积)耗散由热传导和对流形成的耗散共同构成。并通过分析对流换热过程的特点,对准(火积)耗散函数中对流项的影响进行了修正,使用圆管内热充分发展流动与无限宽平行通道内的对流换热过程检验,控制体内准(火积)耗散函数的计算结果与边界(火积)流的结果一致,表明文中提出的准(火积)耗散函数是可靠的。     

高热流密度下矩形微小通道对流换热的模拟与优化

本文采用FLUENT软件对高热流密度下微小通道的换热效果进行数值模拟,以热源平均温度、热源表面温差和泵功作为判据,对微小通道的结构尺寸进行优化。通过优化结果发现,在文中所讨论的尺寸范围内,最佳水力直径为0.615 mm、深宽比为3.25,拟合出各无量纲参数之间影响传热的关系式,并应用模拟结果对(火积)耗散原理进行验证。     

NATURAL CONVECTION FROM A VERTICAL RECTANGULAR FIN

An experimental investigation of steady-state natural convection from vertical rectangular mild steel and aluminum fins was conducted using laser holographic interferometry. Infinite-fringe interferograms demonstrate the effect of fin base heating. Depending on the fin material and base temperature, the local heat transfer coefficients vary along the fin with maximum values at positions about 22-48% of the fin height measured from the base. Temperature measurements along the fin show good agreement with the classical one-dimensional corwective and adiabalic tip solutions. Hence, in the thermal design of vertical aluminum fins of low Biot numbers, the classical one-dimensional fin solutions may be used together with an average heat transfer coefficient obtained from established correlations for natural convection from an isothermal flat plate.     

Output power analyses of an endoreversible Carnot heat engine with irreversible heat transfer processes based on generalized heat transfer law

In this paper, an endoreversible Carnot heat engine with irreversible heat transfer processes is analyzed based on generalized heat transfer law. The applicability of the entropy generation minimization, exergy analyses method, and entransy theory to the analyses is discussed. Three numerical cases are presented. It is shown that the results obtained from the entransy theory are different from those from the entropy generation minimization, which is equivalent to the exergy analyses method. For the first case in which the application preconditions of the entropy generation minimization and entransy loss maximization are satisfied, both smaller entropy generation rate and larger entransy loss rate lead to larger output power. For the second and third cases in which the preconditions are not satisfied, the entropy generation minimization does not lead to the maximum output power, while larger entransy loss rate still leads to larger output power in the third case. For the discussed cases, the concept of entransy dissipation is not applicable for the analyses of output power.The problems in the negative comments on the entransy theory are pointed out and discussed. The related researchers are advised to focus on some new specific application cases to show if the entransy theory is the same as some other theories.     

Visualization of transient natural convection heat transfer from a vertical rectangular fin

Experiments on thermal visualization of transient natural convection from short vertical rectangular fins were conducted using the technique of laser holographic interferometry. A sequence of infinite-fringe interferograms recorded for the heating regime of an aluminum fin demonstrates the effect of fin base heating on local convection coefficients and reveals alternating and oscillatory buoyancydriven flows similar to those over the top surface of heated horizontal plates. The effect of fin base heating results in greater uniformity of the local heat transfer coefficient along the fin. It also significantly reduces the steady-state heat transfer coefficients of short vertical fins compared to their transient values. Hence, the use of steady-state solutions for the design of short vertical fins operating in transient conditions may not introduce as much error as was previously thought.     

Hydromagnetic Carreau Nanoliquid in Frames of Dissipation and Activation Energy

The novel characteristics of magnetic field and entropy generation in mixed convective flow of Carreau fluid towards a stretched surface are investigated.Buongiornio nanoliquid model consists of thermophoresis and Brownian movement aspects is opted for analysis.Energy expression is modeled subject to thermal radiation and viscous dissipation phenomenon.Concentration by zero mass flux condition is implemented.Consideration of chemical reaction and activation energy characterizes the mass transfer mechanism.Total entropy generation rate and Bejan number is formulated.The utilization of transformation variables reduces the PDEs into non-linear ODEs.The obtained nonlinear complex problems are computed numerically through Shooting scheme.The impact of involved variables like local Weissenberg number,magnetic parameter,thermal radiation parameter,Brownian motion parameter,thermophoresis parameter,buoyancy ratio parameter,mixed convection parameter,Prandtl parameter,Eckert number,Schmidt number,non-dimensional activation energy parameter,chemical reaction parameter,Brinkman number,dimensionless concentration ratio variable,diffusive variable and dimensionless temperature ratio variable on velocity,temperature,nanoparticles concentration,entropy generation,Bejan number,surface drag force and heat transfer rate are examined through graphs and tables.     

Constructal Optimization of Rectangular Microchannel Heat Sink with Porous Medium for Entropy Generation Minimization

A model of rectangular microchannel heat sink (MCHS) with porous medium (PM) is developed. Aspect ratio of heat sink (HS) cell and length-width ratio of HS are optimized by numerical simulation method for entropy generation minimization (EGM) according to constructal theory. The effects of inlet Reynolds number (Re) of coolant, heat flux on bottom, porosity and volume proportion of PM on dimensionless entropy generation rate (DEGR) are analyzed. From the results, there are optimal aspect ratios to minimize DEGR. Given the initial condition, DEGR is 33.10% lower than its initial value after the aspect ratio is optimized. With the increase of Re, the optimal aspect ratio declines, and the minimum DEGR drops as well. DEGR gets larger and the optimal aspect ratio remains constant with the increasing of heat flux on bottom. For the different volume proportion of PM, the optimal aspect ratios are diverse, but the minimum DEGR almost stays unchanged. The twice minimized DEGR, which results from aspect ratio and length-width ratio optimized simultaneously, is 10.70% lower than the once minimized DEGR. For a rectangular bottom, a lower DEGR can be reached by choosing the proper direction of fluid flow.     

(火积)耗散理论在换热器设计中的应用

本文首先说明(火积)耗散理论避免了最小熵产原理和傅里叶定律间的矛盾,显示了其在处理导热问题上的优越性.然后利用热力学(火积)和熵的关系,推出了换热器中由流体阻力引起的(火积)耗散表达式.     

Analyses of the endoreversible Carnot cycle with entropy theory and entransy theory

The endoreversible Carnot cycle is analyzed based on the concepts of entropy generation, entropy generation number, entransy loss, and entransy loss coefficient. The relationships of the cycle output power and heat-work conversion efficiency with these parameters are discussed. For the numerical examples discussed, the preconditions of the application for these concepts are derived. When the inlet temperatures and heat capacity flow rates of hot streams and environment temperature are prescribed, the results show that the concepts of entropy generation and entransy loss are applicable. However, in the presence of various inlet temperatures of streams, larger entransy loss rate still leads to larger output power, while smaller entropy generation rate does not. When the heat capacity flow rates of hot streams are various, neither larger entransy loss rate nor smaller entropy generation rate always leads to larger output power. Larger entransy loss coefficient always leads to larger heat-work conversion efficiency for the cases discussed, while smaller entropy generation number does not always.