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影响对流换热的因素有哪些?

问答题
2021-12-27 15:53
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正确答案

对流换热是一个比较复杂的能量质量交换过程,故影响对流换热程度强弱的因素也较多,主要的有:
1.流体流动状态凡是靠外力推动流体流动的称为受迫运动;由于流体各部分温度不同引起的自然对流称自由运动。受迫运动比自由运动的流体对流换热系数大;处于紊流状态的流体比处于层流状态的流体换热系数大。这是因为流体流速高时,扰动强烈而使换热加强。
2.流体有无相变发生液体气化时或气体凝结时的对流换热系数要比无相变时大得多。如水在沸腾过程中,由于汽泡不断在壁面产生、扩大、脱离,冷流体不断地冲向壁面,使流体剧烈扰动,换热强度大幅度提高。
3.流体的物理性质流体种类不同,物性不同,对流换热系数也有较大差别。一般导热系数大的流体,其对流换热系数也大;比热容和密度大的流体,其对流换热系数较大;流体的粘度大时,其对流换热系数较小。
4.流体相对受热面管子的流动方向流体横向冲刷管子比纵向冲刷管子时对流换热系数大。
5.受热面的结构特性管径、节距、排列方式对换热系数均有影响。管径小,节距小,对流换热系数大;管子采用叉排比顺排的对流换热系数大。因为这些措施都使流体扰动强烈,而使换热加强。


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