化工原理英文教材chapter12.ppt

Chapter12 Heat transfer to fluids without phase change Regimes of heat transfer in fluids A fluid being heated or cooled may be flowing in different flow patterns. Also, the fluid may be flowing in forced or natural convection. At ordinary velocities the heat generated from fluid friction is negligible in comparison with the heat transferred between the fluids. Because the situations of flow at the entrance to a tube differs from those well downstream from the entrance, the velocity field and associated temperature field may depend on the distance from the tube entrance The properties of the fluid-viscosity, thermal conductivity, specific heat, and density are important parameters in heat transfer. Each of these, especially viscosity, is temperature-dependent. Heat transfer by forced convection in turbulent flow Perhaps the most important situation in heat transfer is the heat flow in a stream of fluid in turbulent flow. Since the rate of heat transfer is greater in turbulent flow than in laminar flow, most equipment is operated in the turbulent range. A dimensional analysis of the heat flow to a fluid in turbulent flow through a straight pipe yields dimensionless relations. (12-27) The three groups in Eq(12-27) are recognized as the Nusselt(Nu), Reynolds(Re), and Prandtl (Pr) numbers respectively. The Nusselt number for heat transfer from a fluid to a pipe or from a pipe to a fluid equals the film coefficient multiplied by d/k The film coefficient h is the average value over the length of the pipe Prandtl number Pr is the ratio of the diffusivity of momentum μ/ρ to the thermal diffusivity k/ρcp The Prandtl number of a gas is usually close to 1（0.69 for air, 1.06 for steam). The Prandtl number of gases is almost independent of temperature because the viscosity and thermal conductivity both increase with temperature at about the same rate. Empirical equation For heat