不同偏流角下潮流能水轮机水动力特性与熵产率分析

采用CFD与熵产理论相结合的手段,分析了不同偏流角对潮流能水轮机水动力特性的影响,发现偏流使无导管水轮机和导管水轮机输出功率及轴向推力均不同程度地降低,偏流20°时,两者功率最大下降率分别达到30.6%、16.8%,转子轴向推力最大下降率分别达到30.2%、19.8%。由于导管的聚流效应,不同偏流角下导管水轮机相比无导管水轮机仍保持较高的输出功率。在处于低叶尖速比时,直接耗散熵产损失对水轮机能量获取影响较大,此范围内总熵产损失较低;在高叶尖速比时,湍流耗散损失对水轮机能量获取影响较大,此范围内总熵产率随叶尖速比的增加而快速上升,导致水轮机输出率随叶尖速比的增加而不断下降。

Entropy Analyses of Droplet Combustion in Convective Environment with Small Reynolds Number

This paper analyzes the entropy generation rate of simple pure droplet combustion in a tempera-ture-elevated air convective environment based on the solutions of flow, and heat and mass transfer between the two phases. The flow-field calculations are carried out by solving the respective conservation equations for each phase, accounting for the droplet deformation with the axisymmetric model. The effects of the temperature, velocity and oxygen fraction of the free stream air on the total entropy generation rate in the process of the droplet combustion are investigated. Special attention is given to analyze the quantitative effects of droplet deformation. The results re-veal that the entropy generation rate due to chemical reaction occupies a large fraction of the total entropy generated, as a result of the large areas covered by the flame. Although, the magnitude of the entropy generation rate per volume due to heat transfer and combined mass and heat transfer has a magnitude of one order greater than that due to chemical reaction, they cover a very limited area, leading to a small fraction of the total entropy generated. The en-tropy generation rate due to mass transfer is negligible. High temperature and high velocity of the free stream are advantageous to increase the exergy efficiency in the range of small Reynolds number (<1) from the viewpoint of the second-law analysis over the droplet lifetime. The effect of droplet deformation on the total entropy generation is the modest.

Analyses of thermodynamic performance for the endoreversible Otto cycle with the concepts of entropy generation and entransy

In this paper, the endoreversible Otto cycle is analyzed with the entropy generation minimization and the entransy theory. The output power and the heat-work conversion efficiency are taken as the optimization objectives, and the relationships of the output power, the heat-work conversion efficiency, the entropy generation rate, the entropy generation numbers, the entransy loss rate, the entransy loss coefficient, the entransy dissipation rate and the entransy variation rate associated with work are discussed. The applicability of the entropy generation minimization and the entransy theory to the analyses is also analyzed. It is found that smaller entropy generation rate does not always lead to larger output power, while smaller entropy generation numbers do not always lead to larger heat-work conversion efficiency, either. In our calculations, both larger entransy loss rate and larger entransy variation rate associated with work correspond to larger output power, while larger entransy loss coefficient results in larger heat-work conversion efficiency. It is also found that the concept of entransy dissipation is not always suitable for the analyses because it was developed for heat transfer.