流体力学与 及传热课件Heating and Cooling of Fluid in Forced Convection outside Tubes.pptVIP

4.4.5 Heating and Cooling of Fluid in Forced Convection outside Tubes ;The heat-transfer coefficient varies over the body. ; The average heat-transfer coefficient is given in the empirical relationships . ;Fluids flowing normal to a single tube ;Dimensional analysis gives ;4.4.6 Natural Convection ; Density differences in the fluid arising from the heating process provide the buoyancy force required to move the fluid. ;At the bottom of the plate, the temperature gradient is steep, as shown by the full line marked “Z = 10 mm” in Fig.; At distances above the bottom of the plate, the gradient becomes less steep, as shown by the full curve marked “Z = 240 mm” of Fig. ;At a height of about 600 mm from the bottom of the plate, the temperature–distance curves approach an asymptotic condition and do not change with further increase in height.; The natural convection currents surrounding a hot, horizontal pipe are more complicated than those adjacent to a vertical heated plate, but the mechanism of the process is similar. ; Natural convection in liquids follows the same pattern, because liquids are also less dense hot than cold. The buoyancy of heated liquid layers near a hot surface generates convection currents just as in gases. ;On the assumption that h depends upon pipe diameter, specific heat, thermal conductivity, viscosity, coefficient of thermal expansion, acceleration of gravity, density, and temperature difference, dimensional analysis gives ;For single horizontal cylinders, the heat-transfer coefficient can be correlated by an equation containing three dimensionless groups, the Nusselt number, the Prandtl number, and the Grashof number, or specifically ;the fluid properties μf, ρf , and kf are evaluated at the mean film temperature [Eq. (4.4-37)]. Radiation is not accounted for in this equation. ;In Fig.4.15 is shown a relationship, based on Eq. (4.4-41), which satisfactorily correlates experimental data for heat transfer from a single horizontal cylind