• CN:11-2187/TH
  • ISSN:0577-6686

›› 2011, Vol. 47 ›› Issue (14): 140-145.

• 论文 • 上一篇    下一篇

基于温焓分析的最小熵增法在流体换热中的应用

宋卫东;赵力   

  1. 天津大学热能与制冷工程系
  • 发布日期:2011-07-20

Application of Minimum Entropy Method in Fluid Heat Transfer on the Basis of Temperature and Enthalpy Analysis

SONG Weidong;ZHAO Li   

  1. Department of Thermal Energy and Refrigeration, Tianjin University
  • Published:2011-07-20

摘要: 为寻求最小熵增工况,避免最大熵增的出现,提出基于温焓分析的最小熵增法。基于温焓关系推导出极值熵增工况需满足的表达式。拟合出换热流体的温焓关系。在一定对数平均温度的约束下,用C语言编程分别计算二氧化碳跨临界循环冷凝段和R407C热泵循环冷凝段的换热工况。结果表明,换热器两端流体的换热温差在对数平均温差附近时,或者冷流体侧较接近等温,热流体出口较接近冷流体进口温度时熵增最小,换热流体两侧都接近等温时熵增最大;传热窄点处单位换热量熵增较小并且在较小范围内波动时系统单位换热量熵增会出现较大范围的波动。一般情形下,传热窄点处单位换热量熵增较小时,系统单位换热量熵增也较小。这种方法能有效的预测可用能损失最小时的运行工况,为换热器标准工况的制定提供较为可靠的理论方法。

关键词: 换热流体, 可用能损失, 温度匹配, 温焓分析, 最小熵增法

Abstract: In order to obtain the working condition of minimum heat transfer entropy increase and to avoid the maximum entropy increase, the method of minimum entropy increase based on temperature and enthalpy analysis is proposed. Firstly, with the extreme values of the entropy increase as benchmark, the expression is deduced on the basis of the relationship between temperature and enthalpy. Then, the relationships between temperature and enthalpy of the two heat transfer fluids are fitted. With a constant logarithmic mean temperature difference, the C language program is compiled to compute the heat transfer conditions of condensations of the carbon dioxide transcritical cycle and R407C heat pump cycle. The results indicate that the minimum heat transfer entropy increase will be obtained when the heat transfer temperature differences approach to the logarithmic mean temperature difference at both ends of the heat exchanger, or the cold fluid side is nearly isothermal and the outlet temperature of hot fluid is close to the inlet temperature of cold fluid. While the maximum heat transfer entropy increase will be obtained when the heat transfer fluids on both sides are isothermal; when the entropy increase with unit heat transfer rate at the pinch point is small and shows small-scale fluctuations, the system entropy increase with unit heat transfer rate will present large range fluctuations. In general, the smaller the entropy increase with unit heat transfer rate at the pinch point is, the smaller the system entropy increase with unit heat transfer rate will be. This method is not only used to forecast the working condition of minimum energy loss, but also provides a reliable theoretical way for establishing the standard design conditions of heat exchangers.

Key words: Exergy loss, Heat transfer fluids Temperature matching, Minimum entropy increase, Temperature and enthalpy analysis

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