(大空间自然对流)naturalconvection.ppt

Example 3 直径为0.1mm的电阻丝被置于空气流中,与来流方向垂直。来流温度为20℃,电热丝温度为40℃,加热功率为17.8W/m。若略去其它损失，试确定此时的流速。 [Solution] Determine the air properties From Appendix 5, we have Assume that From Table 6-5，we have Example 4 截面为1m×1m的空调管道，外表面温度为20℃,管道安装在室内,室温为30℃,计算每米长管道得热量。 [Solution] Determine the air properties From Appendix 5, we have h of the top surface From (6-40), we have h of the bottom surface h of the two side surface From Table (6-10), we have Total heat transfer 作业 英文版：7-14中文版：6-21，6-34， 6-37，6-48 注意： 特征长度：管外径d。 特征流速：最小截面处平均流速。 定性温度： 进出口流体的平均温度； PrW - 管束的平均壁温。 应用范围： 管排数≥16， 影响对流换热表面传热系数h的因素。 (2)实验关联式 P261表6-7，6-8 茹卡乌斯卡斯公式 6-5 Correlations of Natural Convection natural convection in enclosures 有限空间自然对流 buoyancy 浮升力 Grashof number 格拉晓夫数 natural convection in an infinite space 大空间自然对流 natural convection in an infinite space (大空间自然对流) natural convection in enclosures (有限空间自然对流) Characteristics Due to density changes Driving force - buoyancy 浮升力 Low velocity Temperature and velocity distribution u(x,y) y g x turbulent and laminar flow RT P RT P = T = r r For an ideal gas Thus Governing equations the x-direction momentum equation density difference Grashof number 格拉晓夫数 Grashof number in natural convection is analogous to the Reynolds number in forced convection. Grashof number indicates the ratio of the buoyancy force to the viscous force. Higher Gr number means increased natural convection flow. (6-34) Correlations for natural convection in an infinite space Constant wall temperature reference temperature: characteristic dimension: for vertical plate and cylinder: height for horizontal cylinder : external diameter temperature difference of Gr: (6-37) For vertical cylinder: heated plate facing upward cooled plate facing downward heated plate facing downward cooled plate facing upward (6-39) (6-40) reference temperature: characteristic dimension: Constant heat flux Use the correlations for