流体力学与传热课件Het Transfer and Its Applications.ppt

Figure shows a pipe with two layers of insulation around it, that is, a total of three concentric hollow cylinders. The temperature drop is T1 - T2 across material A The heat-transfer rate q will be the same for each layer. Writing an equation similar to Eq. (4.2-15) for each concentric cylinder. Using the same method of combining the equations as was done for the flat walls in series problem The reason for causing natural convection is the result of buoyancy force generated by ( ) Heat transfer coefficient depends on ( ),( ) The rate of heat transfer by a radiation is proportional to ( ) of absolute temperature A mild steel tube with outside radius of r1 covered with two layers of insulated materials having different thickness(bBbA), the distribution of temperature in both insulated materials are shown as in figure, the conductivity of material A is ( ) that of material B. 温度分布 B A Less than B) can not judge C) larger than D) the same as For a steady-state heat transfer through a series of cylindrical layers, the heat-transfer rate q will be ( ) for each layer and the heat flux q/A will be ( ) for each layer. For a steady-state heat transfer through a series of flat layers, the heat-transfer rate q will be ( ) for each layer and the heat flux q/A will be ( ) for each layer. A) the same B) different C) analogous D) uncertain For steady-state heat transfer by conduction, the thermal resistance will Gases have the smallest thermal conductivities, with values as low as 0.007 W/m.oC. For air at 0oC, k is 0.024 W/m.oC. The thermal conductivity decreases with increasing molecular weight or with decreasing temperature because of changes in the molecular velocity and energy distribution. 2. Liquids Since an adequate molecular theory of liquids is not available, most corr