Multi-objective topology optimization of end plates of proton exchange membrane fuel cell stacks

End plates of the proton exchange membrane fuel cell (PEMFC) need to be well designed because their strength and rigidity directly affect the clamping pressure distribution and thus affect the performance and lifetime of fuel cell stacks. In this paper, a multi-objective topology optimization model of the end plates in a PEMFC stack with nonlinear contact boundary conditions was established to obtain an optimized structural design. It was found that the design improved with topology optimization is not only light but also meets manufacturability requirements. This provides good guidance for the design of a high-performance end plate.     

Improvement of heat transfer model for adsorptive hydrogen storage system

Hydrogen adsorption on high surface area activated carbon is an effective solution of hydrogen storage. Improvement is necessary for the heat transfer model of adsorptive hydrogen storage system. Distributed and lumped parameter models are implemented by the Comsol software and Matlab/Simulink software respectively. The evolution of pressure and temperature during charge and discharge processes is investigated. We adopted following measures for a further improvement on the model: (1) Wall temperature is improved by varying heat transfer coefficient; (2) A more realistic geometry with insert tube improves near inlet temperature; (3) Lumped parameter model is improved by considering thermal conductivity; (4) Distributed and lumped parameter models are well validated by experiments; (5) Heat transfer is modeled under conditions of air cooling and water cooling. The water cooling condition is better than air cooling condition in decreasing the temperature of the storage tank and improving the storage capacity.     

Shape optimization of thermal forced convection fields

This paper presents a numerical analysis method for solving two shape optimization problems, namely, determining temperature distributions in subdomains and maximizing the thermal dissipation on sub‐boundaries of steady‐state thermal convective fields. The square‐error integral between the actual temperature distributions and the prescribed temperature distributions in the prescribed subdomains is employed as the objective function for determining the temperature distribution. The shape gradients for these shape determination problems were derived theoretically using the adjoint variable method, the Lagrange multiplier method and the formulae of the material derivative. Reshaping was accomplished using a traction method that was proposed as a solution to the domain optimization problems. A new numerical procedure is proposed that applies the finite element method to the shape determination problems. The validity of this method was confirmed by 2D numerical analysis. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(5): 313–328, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20202     

Design and multi-objective optimization of heat exchangers for refrigerators

In this study, a detailed thermodynamic model for a refrigerator based on an irreversible Carnot cycle is developed with the focus on forced-air heat-exchangers. A multi-objective optimization procedure is implemented to find optimal design values for design variables. Minimizations of energy consumption and material cost were the two objectives considered. Since these objectives are conflicting, no single design will satisfy both simultaneously. The result of this research is a set of multiple optimum solutions, which are called ‘Pareto optimal solutions’. Air and refrigerant side correlations were combined with an elemental approach to model the heat exchangers. This paper presents a detailed design model development. A limited validation is presented with experimental test-data obtained from a typical household refrigerator. Detailed simulation models are typically complex and computationally demanding. An optimization algorithm requires several evaluations of such models. Response surface based metamodels for objective functions were used to save computational effort. A genetic-algorithm based optimization tool is used for multi-criteria optimization.     

Laminar combined magnetoconvection in a wavy enclosure with the effect of heat conducting cylinder

In the current work, a numerical study of the flow characteristics on combined magnetoconvection in a lid-driven square enclosure, differentially heated, is carried out. This problem is solved by using finite element method of the partial differential equations, which are the heat transfer and stream function in Cartesian coordinates. The tests are performed for different solid–fluid thermal conductivity ratio, cylinder location and Richardson number while the Prandtl number, Reynolds number, magnetic and Joule heating parameters are kept constant. One geometrical configuration is used namely two undulations. The outcome obtained shows that the heat conducting inner square cylinder affects the flow and the heat transfer rate in the enclosure. The trend of the local heat transfer is found to follow a wavy pattern. Results are presented in terms of streamlines, isotherms, average Nusselt number at the heated wavy wall, average temperature of the fluid in the enclosure and dimensionless temperature at the cylinder center for different combinations of the governing parameters.     

A technique for uncertainty analysis for inverse heat conduction problems

A technique is presented for the uncertainty analysis of the linear Inverse Heat Conduction Problem (IHCP) of estimating heat flux from interior temperature measurements. The selected IHCP algorithm is described. The uncertainty in thermal properties and temperature measurements is considered. A propagation of variance equation is used for the uncertainty analysis. An example calculation is presented. Parameter importance factors are defined and computed for the example problem; the volumetric heat capacity is the dominant parameter and an explanation is offered. Thoughts are presented on extending the analysis to include the non-linear problem of temperature dependent properties.     

The synthesis of cost optimal heat exchanger networks with unconstrained topology

The optimization of heat exchanger network (HEN) synthesis still remains an open problem because of the complexity of the space comprising all the possible solutions, and most of the proposed methods introduce simplifying assumptions that mainly affect the topological features of the candidate solutions considered and thus artificially limit the boundaries of the search space.This work is devoted to the pursuit of cost-optimal HENs with unconstrained topology, exploiting the advantages deriving from two graph representations of a HEN. One representation is used by an evolutionary algorithm to manage HEN topology and the other is used by a NLP algorithm to manage heat load distribution among the exchangers. The proposed two-level hybrid optimization method is applied to four test cases taken from the literature about HEN synthesis, among which the well-known Aromatics Plant problem.     

Charge,mass and heat transfer interactions in solid oxide fuel cells operated with different fuel gases—A sensitivity analysis

The interaction between charge, heat and mass transfer occurring in SOFCs is investigated applying a finite-volume-based SOFC model. The strong interactions are the consequence of the high degree of integration of different processes (chemical/electrochemical reactions, diffusion, heat and mass transfer) within SOFCs. The understanding of these interactions is a key for the future development and application of SOFCs. The investigation was conducted by means of a sensitivity analysis for two different fuel gases, where one gas features a considerable amount of methane inducing steam reforming reactions as additional disturbance factor in the energy and mass balance system of SOFCs. In order to isolate the impact of the varied model parameters and the according changes in the interactions of charge, mass and heat transfer from side effects, the sensitivity analysis was conducted at constant fuel utilization. It was found that the impact of different fuel gases on the operational conditions of SOFCs dominates geometrical and material-induced phenomena. The power output was most affected by the fuel, followed by the values for the activation polarization activation energy that reflects the employed electrode catalysts activity.     

拓扑优化理论在活塞结构设计中的应用

建立了某高速发动机活塞的变密度法拓扑优化数学模型,以活塞销座和推力面之间的区域为优化区域,在保证裙部变形稳定性的前提下,利用HyperW orks软件对该区域的活塞结构进行了拓扑优化。材料密度相同时,优化后的活塞质量仅为原来活塞质量的70%左右。对优化前后活塞裙部型线的稳定性进行了比较和分析,优化后活塞的纵截面型线基本上未发生改变,而横截面型线是一条近似水平线,对应的活塞横截面接近正圆,表明拓扑优化能明显改善活塞裙部对变形的适应性。对优化模型进行了有限元静力学分析,结果表明优化前后活塞的应力和整体变形趋势符合实际情况,验证了模型的合理性。     

Performance optimization of a solar driven heat engine with finite-rate heat transfer

An optimal performance analysis for an equivalent Carnot-like cycle heat engine of a parabolic-trough direct-steam-generation solar driven Rankine cycle power plant at maximum power and maximum power density conditions is performed. Simultaneous radiation-convection and only radiation heat transfer mechanisms from solar concentrating collector, which is the high temperature thermal reservoir, are considered separately. Heat rejection to the low temperature thermal reservoir is assumed to be convection dominated. Irreversibilities are taken into account through the finite-rate heat transfer between the fixed temperature thermal reservoirs and the internally reversible heat engine. Comparisons proved that the performance of a solar driven Carnot-like heat engine at maximum power density conditions, which receives thermal energy by either radiation-convection or only radiation heat transfer mechanism and rejects its unavailable portion to surroundings by convective heat transfer through heat exchangers, has the characteristics of (1) a solar driven Carnot heat engine at maximum power conditions, having radiation heat transfer at high and convective heat transfer at low temperature heat exchangers respectively, as the allocation parameter takes small values, and of (2) a Carnot heat engine at maximum power density conditions, having convective heat transfer at both heat exchangers, as the allocation parameter takes large values. Comprehensive discussions on the effect of heat transfer mechanisms are provided.     

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。     

Thermodynamic analysis of hydrogen tank filling. Effects of heat losses and filling rate optimization

A thermodynamic analysis of the refueling of a gaseous fuel tank and a thermal analysis of heat losses through tank walls is presented. The objective of the thermodynamic analysis is to compare the temperature and pressure evolutions coming from different equations of state and from thermodynamic tables. This comparison is performed with nitrogen and hydrogen and the compression is assumed adiabatic. It is shown that the ideal-gas assumption results in under-prediction of the tank temperature and pressure for hydrogen but in over-prediction for nitrogen. An approximate analytical expression of the Redlich–Kwong equation of state is given which is in very good agreement with thermodynamic tables. To handle heat losses, different approaches are used and compared. First, a global thermal conductance is introduced which allows deriving analytical expressions. Then, a thermal nodal modeling of tank walls is proposed to take into account thermal capacity effects. Finally a 1D semi-infinite modeling of the tank walls is presented. Finally, this model is used to optimize mass flow rate in order to limit the temperature rise during the filling process.     

Shape optimization of steady‐state heat‐conduction fields considering temperature dependency of thermal conductivity coefficient

This paper presents a numerical analysis method for shape optimization of domains with steady‐state heat‐conduction fields considering the temperature dependence of the thermal conductivity coefficient. In this paper, we formulate two shape optimization problems, namely, maximization of thermal dissipation on heat transfer boundaries and minimization of heat‐conduction fields. The shape gradient functions for these shape optimization problems are derived theoretically using the Lagrange multiplier method and formulae of the material derivative. Reshaping is accomplished using the traction method proposed as a solution to the shape optimization problems. The proposed method is validated from the results of two‐dimensional numerical analysis. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20374     

Three-dimensional simulation of heat and mass transport phenomena in planar SOFCs

High temperature Solid Oxide Fuel Cells (SOFCs) represent a promising and efficient technology for electrochemical conversion of chemical energy of a fuel into electrical energy. The future development of such technology depends on the availability of detailed and efficient multi-dimensional modeling tools. In this paper, a new three-dimensional finite element algorithm, based on a detailed mathematical model for fuel cells and on the fully explicit Artificial Compressibility (AC) Characteristic Based Split (CBS) scheme, is employed for the effective and efficient modeling of heat and mass transport phenomena coupled with electrochemical reactions in SOFC. The thermal field in the fuel cell is analyzed and the influence of the operating temperature on the fuel cell overall performance is investigated. The three-dimensional results obtained in this work are also compared to the results carried out by employing the two-dimensional version of the present scheme. The results are validated against experimental data available in the literature.     

Dendritic fins optimization for a coaxial two-stream heat exchanger

This paper documents the fundamental relation between the maximization of global performance and the maleable (morphing) architecture of a flow system with global constraints. The example is the coaxial two-stream heat exchanger with flow through a porous bed in the annular space. It is shown that the constraints force the design toward heat exchangers with finite axial length, where additional improvements are derived from installing high-conductivity fins across the porous bed. The maximization of global performance is achieved through the optimization of the configuration of plate fins. Configurations with radial fins are optimized analytically and numerically. Configurations with branched fins are optimized numerically. It is shown that the best configuration (radial vs. branched) depends on the size of the heat exchanger cross-section. When the size is small, the best is the radial pattern. When the size exceeds a certain threshold, the best configuration is the optimized branched tree of fins.     

Shape optimization of inclined ribs as heat transfer augmentation device

This work presents numerical optimization techniques for the design of a rectangular channel with inclined ribs toenhance turbulent heat transfer.The response surface method with Reynolds-averaged Navier-Stokes analysis isused for optimization.Shear stress transport turbulence model is used as a turbulence closure.Computational re-sults for local heat transfer rate show a reasonable agreement with the experimental data.Width-to-rib height ratioand attack angle of the rib are chosen as design variables.The objective function is defined as a linear combina-tion of heat-transfer and friction-loss related terms with the weighting factor.Full-factorial experimental designmethod is used to determine the data points.Optimum shapes of the channel have been obtained in a range of theweighting factor.     

Simulation of temperature and moisture changes during storage of woody biomass owing to weather variability

The objective of this study is to describe moisture content and temperature profile in the woody biomass pile by a two dimensional mathematical model. Woody biomass in the form of wood chips and bundles was stored for a period of one year. Heat and moisture transfer model for drying processes was solved by finite element method using MATLAB programming. The simulation was performed using the recorded climate conditions during the experiment and constant drying air conditions. The temperature change inside the bundles shows the same trend and effect with ambient air temperature, however, in case of wood chips shows lesser effect at various ambient air temperature. Uniformly declined moisture content was observed inside the covered wood chips pile during the storage period. The proposed two dimensional model is in close agreement with experimental data to describe the moisture and temperature profile of the pile wood chips and bundles. However, as the wood chips pile height increases more than 3 m temperature development inside the pile could be rapid and the effect of chemical reaction in the wood chips pile has to be included for better accuracy of prediction.     

槽式太阳能光热发电油盐换热装置设计优化

简要介绍油盐换热装置的工作原理,熔盐、导热油介质的特性。分析在设计槽式太阳能光热发电的关键设备油盐换热装置时需要考虑的问题,并详细描述油盐换热设备的设计方案。     

Effect of carbon-fiber brushes on conductive heat transfer in phase change materials

Brushes made of carbon fibers are used to improve the thermal conductivities of phase change materials packed around heat transfer tubes. The transient thermal responses measured in brush/n-octadecane composites essentially improve as the volume fraction of the fibers and the brush diameter increase. However, there is a critical diameter above which further improvement is not expected due to thermal resistance between the fibers and the tube surface. A two-dimensional heat transfer model describing anisotropic heat flow in the composite is numerically solved. The calculated transient temperatures agree well with the experimental. A simple model is also developed to predict the heat exchange rate between the composite and the heat transfer fluid. The values of the correction factors are identified on the basis of the results for the anisotropic model.     

Magnetohydrodynamics stability of a rotating flow with heat transfer

We study the magnetohydrodynamic stability of an axisymmetric rotating flow in a cylindrical enclosure filled with a liquid metal (Pr = 0.015), having an aspect ratio equal to 2, and subjected to a vertical temperature gradient and an axial magnetic field. The finite volume method is used in order to solve the equations of continuity, momentum, energy, and electric potential. Without magnetic field, the critical Reynolds number is a decreasing function of the Richardson number owing to the destabilizing contribution of natural convection. In the presence of a vertical magnetic field, the flow stability is preserved for higher values of the Reynolds number. The stability diagram which is established shows the dependence of the critical Reynolds number with the increase of the Hartmann number, Ha, for various values of the Richardson number. This study confirms the possibility of stabilization of a liquid metal flow in mixed convection by application of an axial magnetic field.