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首页> 外文期刊>International Journal of Heat and Mass Transfer >Entropy generation in condensation in the presence of high concentrations of noncondensable gases
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Entropy generation in condensation in the presence of high concentrations of noncondensable gases

机译：在高浓度不凝性气体存在下的凝结中产生熵

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摘要

The physical mechanisms of entropy generation in a condenser with high fractions of noncondensable gases are examined using scaling and boundary layer techniques, with the aim of defining a criterion for minimum entropy generation rate that is useful in engineering analyses. This process is particularly relevant in humidification-dehumidification desalination systems, where minimizing entropy generation per unit water produced is critical to maximizing system performance. The process is modeled by a consideration of the vapor/gas boundary layer alone, as it is the dominant thermal resistance and, consequently, the largest source of entropy production in many practical condensers with high fractions of noncondensable gases. Most previous studies of condensation have been restricted to a constant wall temperature, but it is shown here that for high concentrations of noncondensable gases, a varying wall temperature greatly reduces total entropy generation rate. Further, it is found that the diffusion of the condensing vapor through the vaporoncondensable mixture boundary layer is the larger and often dominant mechanism of entropy production in such a condenser. As a result, when seeking to design a unit of desired heat transfer and condensation rates for minimum entropy generation, minimizing the variance in the driving force associated with diffusion yields a closer approximation to the minimum overall entropy generation rate than does equipartition of temperature difference.
机译：使用比例缩放和边界层技术检查了具有高比例的不可凝气体的冷凝器中熵产生的物理机制，目的是确定在工程分析中有用的最小熵产生率的标准。此过程在加湿-除湿淡化系统中特别重要，在该系统中，使每单位水产生的熵最小化对于最大化系统性能至关重要。该过程是通过仅考虑蒸气/气体边界层来建模的，因为它是主要的热阻，因此是许多具有高比例的不可冷凝气体的实际冷凝器中最大的熵产生源。以前的大多数凝结研究都局限于恒定的壁温，但此处表明，对于高浓度的不可凝气体，变化的壁温会大大降低总熵产生率。此外，发现在这种冷凝器中，冷凝蒸气通过蒸气/不可冷凝混合物边界层的扩散是熵产生的较大且通常是主要的机理。结果，当寻求设计用于最小熵产生的期望的热传递和冷凝率的单位时，与扩散相关的驱动力的变化最小化产生的最小总熵产生率比等分的温度差更接近。

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