基于恒截面流道矩形单元体的积耗散率最小传质构形优化

以质量积耗散率最小为优化目标,对恒截面高渗透率通道的矩形单元体传质构形问题进行了分析和优化,得到结构体内平均传质效果最好的结构外形。结果表明:对于单元体和各级构造体,其平均传质压差均为最大传质压差的2/3。因为高渗透率材料中质流率密度符合线性分布,所以基于积耗散率最小与最大压差最小的最优构形完全一致。所得最优构形同时使得传质能力和传质安全性最好。     

Sensible and latent thermal energy storage with constructal fins

Energy storage is one of the most important components of renewable energy systems. Among different methods, thermal energy storage (TES) in forms of sensible or latent has been the subject of many studies in the past decades. The main difficulty in optimal design of storage tanks is associated with low thermal conductivity of the storing (solid or phase change) material. In fact, the distribution of thermal energy from a source to the body of storing material poses a volume to point problem which is the subject of constructal theory. Therefore, the objective of the present paper is to investigate the transient behavior of a rectangular thermal energy storage tank equipped with fin configurations optimized for heat conduction based on constructal theory. Results of numerical simulations reveal that the more complex configurations perform better in sensible TES systems; almost as well as what is expected based on analytical steady state solutions. However, because of the convection currents in the melting process of a PCM tank, the final full charging time of the latent systems are approximately the same.     

Three-dimensional multi-scale plate assembly for maximum heat transfer rate density

This paper extends the design concept for generating multi-scale structures in forced convection for a finite-size flow system to a three-dimensional heat-generating plate with the objectives of maximising heat transfer rate density, or the heat transfer rate per unit volume. The heat-generating plates, arranged in a stack form channels in which the fluids are forced through by an applied pressure difference. The first stage of this work consists of numerical simulation of the flow and heat transfer in a large number of flow configurations, to determine the optimum plate spacing, and the maximum heat transfer rate density. In the subsequent stages, shorter plates are inserted in the centers at adjacent (longer) plates in the entranced region were the boundary layer are thin and there is a core of unused fluid. The heat transfer density is further increased by progressively inserting another set of even shorter plates between the plates and then optimizing the whole structure. The resulting structure is an optimized multi-scale and multi channel structure with horizontal equidistant heated plates of decreasing lengths scales. Further more the effects of plate thickness and dimensionless pressure drop number on the multi-scale structure was investigated. The numerical results are found to be in good agreement with predicted analytical results.     

Optimal design of tree-shaped water distribution network using constructal approach: T-shaped and Y-shaped architectures optimization and comparison

This paper presents an original approach for the optimization of drinking water distribution networks. A multi-scale design of point-to-points networks using the Lagrangian function coupled with the constructal approach is performed. Two tree-shaped networks (T- and Y-) are optimized and compared by taking diameters of pipes and bifurcation angles in the network as selected design variables. The objective is to minimize the total head losses (factor of energy consumption) subject to the overall water residence time (factor of water quality). Y-shaped structures are found better than T-shaped structures.     

最大热阻最小化的环形高导热通道三维体点导热最优构形的解析解法

基于构形理论,采用解析解法,以最大温差最小为优化目标,对基于环形高导热通道和圆柱形单元体的三维“体-点”导热模型进行构形优化,得到无量纲最大热阻最小的三维圆柱体最优构形.结果表明:增大高、低导热材料导热系数比、高导热材料占比和单元体数目均有助于提高圆柱形构造体的导热性能,但当单元体数目较大时,圆柱形构造体的导热性能改善...     

海底数据中心散热优化系统的设计与仿真研究

随着电子信息存储设备高能耗密度化的发展趋势,海底数据中心通常需要耗费大量的能源,且其仍存在运行效率较低以及设备能耗较高等问题。研究并设计了一种海底数据中心散热优化装置及其检测系统。首先建立相关模型并分析计算电能利用效率PUE(power usage effectiveness)值,客观地评价海底数据中心散热状况,接着参考工程实例对比不同散热结构并进行仿真实验验证,最后结合散热检测软件平台所监测到的相关数据,印证了散热优化系统能够有效提高海底数据中心的散热效率。     

H形对流传热脉管的熵产率最小构形优化

针对H形对流传热脉管,以脉管区域面积和通道总表面积为约束,在线热流密度恒定时,基于熵产生最小化进行了构形优化,给出了无量纲总熵产量和无量纲单位传热负荷的总熵产率与脉管组合级数、无量纲质量流量和无量纲泵功率的完整解析关系式及优化算例.结果表明:给定通道总表面积与文献中给定通道总体积约束的优化结果是不同的;无量纲单位传热负...     

基于火积理论的多级套管式相变蓄热系统优化

本文基于最小火积耗散热阻原理,在考虑相变材料导热热阻以及非稳态传热过程的基础上,对多级套管式相变蓄热系统的融化温度进行了数值优化,获得了最优融化温度分布。在此基础上,研究了相变材料导热系数和传热管长度对最优融化温度、火积耗散热阻和平均蓄热速率的影响。研究结果表明,与现有理论优化方法相比,本文提出的数值优化方法具有更好的适用性;优化后多级套管式相变蓄热系统可有效提高相变蓄热系统的平均蓄热速率,降低火积耗散热阻;随着相变材料导热系数增大和传热管长度增加,多级套管式相变蓄热系统最优融化温度的温差愈加明显,其强化传热性能呈上升趋势。     

A three dimensional exact equation for the turbulent dissipation rate of Generalised Newtonian Fluids

The flow of non-Newtonian fluids is of interest in many biological and industrial applications, including nanofluids. Most of the papers of the literature on turbulent non-Newtonian fluids focused the attention on viscoelastic fluids. In order to make accurate and low cost prediction of turbulent inelastic non-Newtonian fluids, a RANS Generalised Newtonian Fluid (GNF) turbulence model, based on the exact equations for the turbulent variables, is required. In a previous paper of the same authors the exact equations for the turbulent kinetic energy and the dissipation rate have been derived in a two-dimensional (2D) domain, through the introduction of an apparent viscosity equation. The aim of the present paper is to extend the approach to a three-dimensional (3D) domain, giving the full mathematical demonstration of the exact equations.     

Two new differential equations of turbulent dissipation rate and apparent viscosity for non-newtonian fluids

A new equation for the dissipation rate of turbulent kinetic energy is derived exactly in conservative form for a Generalized Newtonian Fluid (GNF). The transport equations for mass, momentum, and turbulent kinetic energy are written along to the transport equation for the shear rate. A new transport equation for the apparent viscosity is derived assuming the viscosity as dependent only on the shear rate. The assumption is of incompressible two-dimensional GNF flow.     

The forced air cooling heat dissipation performance of different battery pack bottom duct

Due to the requirement of the battery for the thermal management system, based on the coupling relationship between the velocity field and the thermal flow field of the field synergy principle, the flow paths of the forced air cooling system for different battery packs were analyzed. First, the thermodynamic parameters of the battery were collected through experiments and verified by simulation. Secondly, based on the collected thermodynamic parameters of the battery, the heat generation model of the battery, the heat conduction model of the gas, and the coupled heat dissipation model of the battery and air were established. Determine the boundary conditions, calculation methods and evaluation indicators required for simulation; Finally, based on four different driving conditions, the forced air cooling performance of the double “U” shape duct and double “1” type duct is simulated. Through the analysis of the results, the dual “U” air ducts have a more heat dissipation effect on the battery pack than the double “1” shape duct. The results conform to the definition of the field synergy principle for the coupling relationship between the velocity field and the heat flow field. Then research provide references for the design of battery packs and matching of cooling systems.     

Thermodynamic optimization alternatives: minimization of physical size subject to fixed power

This paper explores alternatives to the usual thermodynamic optimization formulation, where the thermodynamic performance of a system is improved subject to physical size constraints (e.g., entropy generation minimization). The alternative is to minimize the physical size subject to specified thermodynamic performance. It is shown that the alternate optimization leads to the same physical design—the same configuration—as the usual optimization. This is demonstrated by means of three examples: power plant model with two heat exchangers, power plant model driven by a hot stream of single‐phase fluid, and refrigerator model with irreversibility due to imperfect insulation. Copyright © 1999 John Wiley & Sons, Ltd.     

Optimal design analysis of a tubular heat exchanger network with extended surfaces using multi-objective constructal optimization

The present work aims to investigate the influence of extended surfaces (fins) on the multi-objective optimization of a tubular heat exchanger network (THEN). An increase in the heat transfer area using various extended surfaces (fins) to enhance the performance of the heat exchanger was used while considering the effectiveness and total heat transfer area as two objective functions. In addition to the simulation of simple fins, a new set of fins, called constructal fins, was designed based on the constructal theory. Tubular heat exchanger network design parameters were chosen as optimization variables, and optimization results were achieved in such a way as to enhance the effectiveness and decrease the total heat transfer area. The results show the importance of constructal fins in improving the objective functions of heat exchangers. For instance, the simple fins case enhances the effectiveness by up to 5.3% compared to that without fins (usual heat exchanger) while using constructal fins, in addition to the 7% increment of effectiveness, reduces the total heat transfer area by 9.47%. In order to optimize the heat exchanger, the heat transfer rate and cold fluid temperature must increase, and at the same time, the hot exiting fluid temperature should decrease at the same constant total heat transfer area, which is higher in the constructal fins case. Finally, optimized design variables were studied for different cases, and the effects of various fins were reported.     

复杂热源下不可逆诺维科夫热机最大功率输出

考虑实际热机工作下的旁通热漏和内部耗散等不可逆因素,建立了包括连续均匀分布、三角形分布、二次分布和帕累托分布等四种不同的统计概率分布高温热源温度下的广义不可逆诺维科夫热机模型,导出了热机最大输出功率及相应的热效率和熵产率随高温热源温度、内部不可逆性等因素变化的关系式。结果表明：热漏和内部耗散分别对热机性能有着不同的影响,热漏使统计热源温度分布下最大功率输出对应的热效率减小,同时也增大了熵产率,但对热机的最大功率输出无影响;内部耗散不可逆性使热机的最大输出功率及相应热效率均明显减小,但使熵产率先增大后减小;熵产率随高温热源温度的标准差增大而减小。研究结果对太阳能发电厂性能提升具有一定理论指导意义。     

Influence of scalar dissipation on flame success in turbulent sprays with spark ignition

A Direct Numerical Simulation (DNS) study is performed to determine a quantitative indicator of imminent global extinction in spray flames ignited by a spark. The cases under consideration have Group Combustion numbers sufficiently small that each droplet has an individual flame form around it, which subsequently merge. The structure of the flames is examined, including identification of non-premixed behaviour in the core of the flame and premixed flame fronts except in the presence of droplets, which cause strong non-premixed behaviour. The reaction progress variable c is studied and its dissipation rate is identified as being a key indicator of whether a flame will globally extinguish after being ignited by the spark. Specifically, immediately after the spark is deactivated, the volume containing the end of the flame front and hot products is studied in detail with respect to c. For successful flames, it is observed that regions of zero dissipation of c   were predominantly restricted to the highest reaction progress variable (c>0.98)$(c>0.98)$, with zero probability within the range 0.95<c<0.98$0.95<c<0.98$ and low probability within 0.9<c<0.95$0.9<c<0.95$. In contrast, cases which subsequently extinguished had substantial probability of zero dissipation for 0.95<c<0.98$0.95<c<0.98$. This region was a secondary structure separate from the main flame kernel that was unable to evaporate sufficient liquid to create a self-sustaining flame and therefore contributed to the subsequent quenching of the flame. In the successfully-burning case under consideration, this region was part of the main flame structure. The low reaction rate contributed to a thickened flame structure near the hot core, which reduced the heat transfer to the flame front and prevented effective evaporation and preheating of the fluid ahead of the flame front. Calculation of the conditional probability of c   for its dissipation rate being zero could provide a quantitative measure to determine whether a flame is likely to extinguish within a relatively short timeframe. This is equivalent to detecting that, for every value of 0.9<c<1$0.9<c<1$, there are volumes of significant size where the value of c   is uniform. Note that a successful flame must have a volume of substantial size with c=1$c=1$. From a practical perspective, if each individual flame kernel is monitored, then extinction is imminent if secondary structures of incomplete reactions are present when the spark ceases adding energy.     

Constructal optimisation of conjugate triangular cooling channels with internal heat generation

This paper presents the development of the three-dimensional flow architecture of conjugate cooling channels in forced convection with internal heat generation within a solid. Two types of cross-section channel geometries were used. The first involved equilateral triangles with three equal legs in length and all three internal angles of 60°. The second was isosceles right triangles with two legs of equal length and internal angles of 90°, 45° and 45°. Both the equilateral triangle and isosceles right triangle are special case of triangle that can easily and uniformly be packed and arranged to form a larger constructs. The configurations were optimised in such a way that the peak temperature of the heat generating solid was minimised subject to the constraint of a fixed global volume of the solid material. The cooling fluid was driven through the channels by the pressure difference across the channel. The degrees of freedom of the channels were aspect ratio, hydraulic diameter and channel to channel spacing ratio. The shape of the channel was allowed to morph to determine the best configuration that gives the lowest thermal resistance. A gradient-based optimisation algorithm was applied in order to search for the best optimal geometric configurations that improve thermal performance by minimising thermal resistance for a wide range of dimensionless pressure difference. The effects of porosities, applied pressure and heat generation rate on the optimal aspect ratio and channel to channel spacing are reported. It was found that there are unique optimal design variables for a given pressure difference. The numerical results that were obtained were in agreement with the theoretical formulation using scale analysis and method of intersection of asymptotes. Results obtained show that the effects of applied dimensionless pressure drop on minimum thermal resistance were consistent with those obtained in the open literature.     

Convective‐radiative fins with simultaneous variation of thermal conductivity,heat transfer coefficient,and surface emissivity with temperature

This paper is a numerical study of thermal performance of a convective‐radiative fin with simultaneous variation of thermal conductivity, heat transfer coefficient, and surface emissivity with temperature. The convective heat transfer is assumed to be a power function of the local temperature between the fin and the ambient which allows simulation of different convection mechanisms such as natural convection (laminar and turbulent), boiling, etc. The thermal conductivity and the surface emissivity are treated as linear functions of the local temperature between the fin and the ambient which provide a satisfactory representation of the thermal property variations of most fin materials. The thermal performance is governed by seven parameters, namely, convection–conduction parameter Nc, radiation–conduction parameter Nr, thermal conductivity parameter A, emissivity parameter B, the exponent n associated with convective heat transfer coefficient, and the two temperature ratios, θ_a and θ_s, that characterize the temperatures of convection and radiation sinks. The effect of these parameters on the temperature distribution and fin heat transfer rate are illustrated and the results interpreted in physical terms. Compared with the constant properties model, the fin heat transfer rate can be underestimated or overestimated considerably depending on the values of the governing parameters. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20408     

Preferential diffusion effects on flame characteristics in H2/CO syngas diffusion flames diluted with CO2

Numerical study is conducted to clarify preferential diffusion effects of H₂ and H on flame characteristics in synthetic diffusion flames of the compositions of 80% H₂/20% CO and 20% H₂/80% CO as representatively H₂-enriched and CO-enriched H₂/CO flames. Impacts of CO₂ addition to the flames are also examined through the variation of added CO₂ mole fraction from 0 to 0.5. A comparison was made by employing a mixture-averaged diffusivity and the suppression of the diffusivities of H and H₂. It is found that preferential diffusion effects on maximum flame temperature cannot be explained by the well-known behavior between maximum flame temperature and scalar dissipation rate but by chemical processes. The concrete evidence is also presented through the examination of the behavior of maximum H mole fraction and the behavior of importantly-contributing reaction steps to overall heat release rate.     

包含多变过程的内可逆Otto循环有限时间热力学分析

应用有限时间热力学理论分析了包含多变过程的内可逆Otto循环,由数值计算给出了考虑传热损失时循环输出功与压缩比、效率与压缩比以及输出功与效率的特性关系,分析了多变指数和传热损失对循环性能的影响,通过分析可知多变指数和传热对Otto循环性能有较大影响。计算所得的结果对实际Otto热机的设计和改进有一定的指导意义。     

沼气池产气率随气温地温变化规律的研究

为了准确掌握户用沼气池在不同模式条件下的产气能力,探讨其产气率率环境温度的关系,以便科学地评价其经济效益,更好地指导农户管理、使用沼气池,采用观察、记载统计方法,得到“四位一体”模式、“三结合”模式和露地池的产气量变化规律.