平均换热系数公式及设置

一 平均换热系数公式 平均换热系数的计算公式为 其中R 为某一径向坐标 R为该dRR R R R 0 2 2 1 坐标处的局部换热系数 下集管圆形射流换热特性的影响因素包括 表面射流出口速度v0 射流的出口直径D 射流 出口距钢板表面的距离H 等 下集管射流的换热系数 随射流出口距钢板表面的距离H 的增大而 减小 其他因素对换热特性的影响与上集管射流相同 下面还有设置平均平均换热系数 0 000 020 040 060 080 10 0 0 0 2 0 4 0 6 0 8 1 0 1 2 1 4 1 6 1 8 2 0 u m s R m 图图 2 15 下集管圆形射流壁面射流速度下集管圆形射流壁面射流速度 径向变化曲线径向变化曲线 0 000 020 040 060 080 10 0 1 2 3 4 5 6 7 8 9 10 11 12 x10 3 W m2 K R m 图图 2 16 下集管圆形射流换热系数下集管圆形射流换热系数 径向分布曲线径向分布曲线 平均换热系数 局部换热系数 二 模型设置问题如图 Wall Func Heat Tran Coef 定义定义 Wall Func Heat Tran Coef is defined by the equation 30 4 44 where is the specific heat is the turbulence kinetic energy at point and is defined in Equation 12 10 5 The law of the wall implemented in FLUENT has the following composite form 12 10 5 where is computed by using the formula given by Jayatilleke 165 12 10 6 and turbulent kinetic energy at point P density of fluid specific heat of fluid wall heat flux temperature at the cell adjacent to wall temperature at the wall molecular Prandtl number turbulent Prandtl number 0 85 at the wall Van Driest constant 26 mean velocity magnitude at Note that for the pressure based solver the terms and will be included in Equation 12 10 5 only for compressible flow calculations The non dimensional thermal sublayer thickness in Equation 12 10 5 is computed as the value at which the linear law and the logarithmic law intersect given the molecular Prandtl number of the fluid being modeled The procedure of applying the law of the wall for temperature is as follows Once the physical properties of the fluid being modeled are specified its molecular Prandtl number is computed Then given the molecular Prandtl number the thermal sublayer thickness is computed from the intersection of the linear and logarithmic profiles and stored During the iteration depending on the value at the near wall cell either the linear or the logarithmic profile in Equation 12 10 5 is applied to compute the wall temperature or heat flux depending on the type of the thermal boundary conditions The function for given by equation Equation 12 10 6 is relevant for the smooth walls For the rough walls however this function is modified as follows 12 10 7 where is the wall function constant modified for the rough walls defined by To find a description of the roughness function you may refer to Equation 7 13 3 in Section 7 13 1 Surface Heat Transfer Coef in the Wall Fluxes category as defined in FLUENT is given by the equation 30 4 33 where is the combined convective and radiative heat flux is the wall temperature and is the reference temperature defined in the ReferenceReference ValuesValues panel Please note that i