Chapter22-总结.doc

A SUMMARY for Chapter 2 Steady State Heat Conduction 2.1通过单层壁的对流+导热+对流复合传热（一维稳态传热问题） Heat transfer process through a single wall 热力学第一定律： Heat flow-rate into CV by cond. conv. - Heat flow-rate out CV by cond. conv. + Heat generated from other forms of energy within CV = The rate of change of thermal energy within CV Plane wall Cylindrical wall 热 平 衡 方 程 微分方程 边界条件 ； ； 固体壁内 温度分布 壁内导热 热 流 量 边界对流 热 流 量 ， , 传热过程 方 程 ∵ ∴ ∵ ∴ Note: for Cylindrical wall, , 2.2通过多层壁的对流+导热+对流复合传热（稳态） 对于平板： 对于圆筒：， 对于球壳：， 多层壁复合传热的电路模型： 2.3保温层问题 传热方程： 或 无保温层时 保温层第一临界半径：；：↗，Q↘) 保温层第二临界半径：（， 由 得 或 其中 2.4翅片强化传热问题 参数定义：A翅片横截面积，P换热截面周长，L翅片长度没有翅片时传热壁面的总面积 单个翅片的传热表面积（对于直翅片） 单个翅片根部（z=0处）的横截面积（对于直翅片） k翅片导热系数，翅片热系数（1） （2.3.13） 或 （2）翅片散热量 The fin of general form Straight fin 实际散热量 最大散热量 翅片热效率 无翅片换热表面散热量 具有m个翅片的 两侧均有翅片的单层壁的传热方程 其中， 可视为每一侧总的有效换热面积 单层壁两侧加翅片后的传热强化比 EXAMPLE: Heat transfer enhancement by straight rectangular fins Water and air are separated by a mild steel plane wall. It is proposed to increase the heat transfer rate between these fluids by adding straight rectangular fins of 1.27mm thickness and 25.4mm length, spaced 12.7mm apart. The air-side and the water-side heat transfer coefficients may be assumed constant with values of 13 and 299 W/m2·K, respectively. What percent increase in heat transfer can be realized if fins are placed on (a) the water side? (b) the airside? (c) the both sides? Solution: For the airside: =0.933 (a) When fins are added to the airside alone: For the water side: = =27.78 (300%) (b) When fins are added to the waterside alone =1.03 (3%) (c) When fins are added to the both sides: =4.46 (346%) Conclusion: To be efficient, the fin should be added to the side with smaller heat transfer coefficient. Chapter 2 Heat Transfer by Conduction: Summary 2006-09-20 4 — —