对流换热习题解答

9-2水以/sk8.0gqm的流量在内径mm25d的管内流动，管子内表面温度90wt℃，进口水的温度20'ft℃，试求水被加热到40℃时所需管长。解：该换热属于管内强制对流换热。定性温度：℃3040202121'fffttt查取物性参数：KmW618.0f，/sm10805.026f，42.5Prf，37.995mkg平均流速：smdqum64.17.995025.014.38.04422计算已定准则：4631009.510805.0102564.1Reudf选取实验关联式：6.26342.51009.5023.0PrRe023.04.08.044.08.0fffNu计算表面传热系数：KmW/2.65161025618.06.26323dNuff修正系数：1.19.3145.80111.011.0wftcKmW/82.71671.1*2.65161025618.06.26323dNuff计算管子长度，根据热平衡原理有：fwffpmttdlttcq')(97.160025.014.382.71672041748.0'mttdttcqlfwffpm9-5外径76mm的暖气管，横掠穿过室内，管表面温度为100℃，室内温度为20℃，计算管壁自由流动换热表面传热系数及单位管长散热量。解：确定换热类型：自然对流换热计算定性温度：60201002121wfmttt℃查取物性参数：KmW029.0m/sm1097.1826f696.0Prf计算已定准则：6263232310875.21097.1827360076.08081.9mTtlgtlgGr计算公式：93.19696.010875.253.0Pr53.04/164/1GrNu计算表面传热系数：KmW/6.7076.0029.079.662lNu散热量：mW09.145201001076.014.36.71fwwttdlttA9-890℃的饱和蒸汽以0.125kg/s的凝结率，在高为1m，直径为25mm，温度为50℃的一些竖管的外表面上凝结。试问这个凝汽器需要多少根管子？解：7050902121wsmttt℃KmW668.0mkg/m.s101.4066l3mkg8.977lkgkJ7.2282srW5.285337125.02282700rqm41324122)5090(10004061.0668.08.977228270081.913.1)(13.1wslllttHgrKmW/50302根）(1.1840025.014.350305.285337tAn9-10电加热器管子直径16mm，长4m，加热功率3kW，试求在标准大气压下沸腾时加热器管的表面温度及换热表面传热系数。解：该换热属于沸腾对流换热234.149284016.014.33000mWdlAq℃.44.263510132534.1492856.056.0215.07.015.07.0mWpq)(swttq℃66.10510044.263534.14928swtqt