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dR021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0.0.20.40.60.80.1.20.4.60.81.21.4.61.82.0u (m /s) R (m)图 2.15 下集管圆形射流壁面射流速度径向变化曲线0.0.20.40.60.80.11234567891012x0-3 (W /(m2K ) R (m)图 2.16 下集管圆形射流换热系数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 Reference Values panel. Please note that is a constant value that should be representative of the problem. Its unit quantity is the heat-transfer- coefficient.