宏观克劳修斯熵与微观玻耳兹曼熵的统一─兼谈宏观克劳修斯熵公式的微观诠释及本质

论证了宏观克劳修斯熵与微观玻耳兹曼熵的统一性．对宏观克劳修斯熵公式的微观诠释及本质进行了探讨．     

Numerical and entropy studies of hydrogen-fuelled micro-combustors with different geometric shaped ribs

Micro-combustor design plays an important role in determining the performances of Micro Thermo-Photovoltaic (MTPV) systems. In this work, 3D numerical simulations are conducted on a hydrogen-fuelled micro-combustor with two ribs to achieve a more uniform but higher wall temperature. The effects of: 1) the shape of the ribs, 2) the axial location, 3) the height, 4) the inlet velocity and 5) the equivalence ratio are evaluated. The numerical model is built with a standard k-ε turbulence model and EDC chemical reaction model (eddy dissipation concept). These models are validated before being applied to study 3 different ribs with a cross-sectional view of: 1) rectangular, 2) Ո-shaped and 3) Ս-shaped (defined basing on the bottom rib). It is found that the combustor with 2 ribs performs generally better than that of a single-rib one under the same flow conditions. The optimum design is found to be the Ս-shaped ribs, since the mean temperature of the outer wall is increased by 25.4 K in comparison with other designs. In addition, the mean temperature is observed to increase with increased inlet velocity. However, it decreases slightly with increased rib height. Further analysis is conducted on entropy production due to chemical reactions and heat transfer processes. It is found that the chemical reaction and the conduction heat transfer contribute 70% and 15% of the total entropy generation respectively. Furthermore, the thermodynamic 2^nd-law efficiency remains in the range of 46%–51%, as the equivalence ratio varies from 0.8 to 1.2. This study provides physical insights on the optimum design of a hydrogen-fuelled micro-combustor.     

未确知测度理论在采空区危险性评价中的应用

采空区灾害是矿山频繁高发的灾害之一。将未确知测度理论用于评价采空区的危险性,考虑到空区稳定性影响因素数值的易取性和显著性,经综合分析,选取空区岩体结构、地质构造、围岩抗压强度、水文因素、采空区布置形状、矿体倾角、高跨比、采空区体积、埋深、最大暴露面积、暴露时间、采动扰动情况、相邻空区情况等13项因素作为空区危险性的判别指标。根据空区危险性等级和评价指标的分级标准,结合采空区失稳机理和相关研究成果,建立评价指标的单指标测度函数,采用信息熵理论计算指标权重和综合测度,依照置信度识别准则进行等级判定。对某金属矿12组空区危险性进行评判。结果表明,该法的判别结论与采用模糊综合评判法和物元分析方法得出的结果完全相符,这充分说明该方法是可靠实用的,可以在工程实践中推广应用。     

Viscous dissipation effect on entropy generation in curved square microchannels

The viscous dissipation effect on the thermodynamic performance of the curved square microchannels in laminar flow is numerically investigated. The classical Navier-Stokes equations are adopted; aniline and ethylene glycol are selected as the working fluids. The results show that the heat transfer entropy generation number and frictional entropy generation number augment relatively under viscous dissipation effect for the case of fluid heated, and the opposite results can be found for the case of fluid cooled. The heat transfer entropy generation number increases with Reynolds number at large Reynolds number region under viscous dissipation effect when ethylene glycol is heated. The total entropy generation number extremum exists for aniline, and the extremum happens earlier when aniline is heated than when aniline is cooled. The smaller the curvature radius is, the earlier the extremum appears. The extremum does not occur for ethylene glycol due to the predomination of frictional entropy generation in the total entropy generation.     

Local entropy generation analysis on passive high-concentration DMFCs (direct methanol fuel cell) with different cell structures

In this paper, the local entropy generation analysis has been conducted based on a two-dimensional, two-phase, non-isothermal DMFC (direct methanol fuel cell) model, the entropy generation contributed by the chemical reactions, heat transfer, mass diffusion, and viscous dissipation is investigated. Then, the performance of fuel cells with different methanol barrier layers and electrolyte membranes have been studied based on the local entropy generation analysis. Results indicate that the entropy generation during cell operation is mainly caused by the irreversible electrochemical reactions, and that the entropy generated by mass diffusion and viscous dissipation can be considered negligible. The entropy generated by heat transfer is about two magnitudes less than the entropy generated by the electrochemical reactions in the passive DMFCs operating near room temperature. The overall entropy generation rate in a DMFC can be decreased by increasing the thickness of the methanol barrier layer and decreasing the thickness of the electrolyte membrane.     

Multiple concentric spirals for the flow field of a proton exchange membrane fuel cell

The present analysis considers a three-dimensional non-isothermal model in a single phase of a PEM fuel cell with a flow field path in the shape of 1, 2, 3, 4, 6, and 8 concentric spirals. The current density contours, the water content and the entropy generated in all zones of the fuel cell are predicted. The analysis of the three-dimensional model includes the gas flow channels in the six geometric shapes mentioned above, the current collectors, gas diffusion layers, catalyst layers on both sides of the model, anode and cathode, and a proton exchange membrane in between. The energy equation, mass conservation, and transport of species equations are solved, including source terms that take into account the electrochemical effects occurring inside the cell. Also, the entropy generation equation is added to the governing equations of the model. The results allow a comparison to help to decide which of the 6 analyzed configurations improve the performance of the fuel cell, increasing the current density produced, reducing the pressure drop and producing the most uniform current density. The entropy generation analysis reveals the effects that cause the most significant losses (irreversibilities) in the cell. The Bejan number and the Π number are used to compare the irreversibilities produced by the matter flow and by the heat transfer for each one of the six models.     

Performance optimisation of laminar fully developed flow through square ducts with rounded corners

A study on combined first and second law based optimisation of thermal-hydraulic performance of laminar fully developed flow through square ducts with rounded corners has been presented in this paper. The objective functions have been considered according to suggestions of Webb and Bergles [7]. Four specific geometric constraints have been imposed on the shape of the ducts and these ducts have also been subjected to three different thermal and (or) hydraulic constraints. Two different thermal boundary conditions have been considered and the correlations for friction factor and Nusselt numbers have been adopted from the study of Ray and Misra [21]. The results obtained from the present study clearly show that the optimal duct geometry strongly depends on geometric and thermal-hydraulic constraints, as well as, the objective functions and hence, no general comment can be made with respect to the superiority of a particular geometry of the ducts. Nevertheless, the present study also shows that although entropy generation minimisation may be considered to be an important tool, one requires being careful in using it for thermal-hydraulic optimisation since it may lead to contradictory results for some of the performance evaluation criteria.     

Thermodynamic performance analysis of a molten carbonate fuel cell at very high current densities

This study is basically composed of two sections. In the first section, a CFD analysis is used to provide a better insight to molten carbonate fuel cell operation and performance characteristics at very high current densities. Therefore, a mathematical model is developed by employing mass and momentum conservation, electrochemical reaction mechanisms and electric charges. The model results are then compared with the available data for an MCFC unit, and a good agreement is observed. In addition, the model is applied to predict the unit cell behaviour at various operating pressures, temperatures, and cathode gas stoichiometric ratios. In the second section, a thermodynamic model is utilized to examine energy efficiency, exergy efficiency and entropy generation of the MCFC. At low current densities, no considerable difference in output voltage and power is observed; however, for greater values of current densities, the difference is not negligible. If the molten carbonate fuel cell is to operate at current densities smaller than 2500 A m⁻², there is no point to pressurize the system. If the fuel cell operates at pressures greater than atmospheric pressure, the unit cell cost could be minimized. In addition, various partial pressure ratios at the cathode side demonstrated nearly the same effect on the performance of the fuel cell. With a 60 K change in operating temperature, almost 10% improvement in energy and exergy efficiencies is obtained. Both efficiencies initially increase at lower current densities and then reach their maximum values and ultimately decrease with the increase of current density. By elevating the pressure, both energy and exergy efficiencies of the cell enhance. In addition, higher operating pressure and temperature decrease the unit cell entropy generation.     

熵域流形空间中的目标尺度感知策略

针对场景中不同目标的尺度感知问题,提出一种基于熵度量的场景目标尺度感知计算模型。通过对熵域空间中流形分布的统计特性分析,描述图像流形空间中的目标分布规律,在四叉树金字塔描述的基础上,依据尺度感知评价函数,形成了多目标尺度感知计算过程。实验结果说明,四叉树空间金字塔近似人类视觉感知的层次结构,基于熵域流形空间的目标描述可以有效实现目标尺度感知。     

Evaluation of Black-Start Schemes Employing Entropy Weight-Based Decision-Making Theory

Optimization of schemes for black-start is one of the most important factors determining the speed of power system restoration after either a global blackout or a local outage. In this paper a novel method employing the entropy weight-based decision-making theory is proposed for this application. By using the method for the decision making in black start, the subjective weights acquired from knowledge and experience of experts could be used in conjunction with the objective weights (entropy weighs) deduced from some closely related indices. Obviously, the developed method can greatly improve the performance in power system restoration by overcoming the shortcomings of conventional methods that only use either the subjective or the objective weights. Furthermore, the proposed method is applicable in power systems with different levels of black-start experience for assisting the optimal black-start decision making. It is shown that the proposed method is suitable for power systems with different levels of black-start experience.