流体力学与热课件Heating and Cooling of Fluid in Forced Convection outside Tubes.ppt

4.4.5 Heating and Cooling of Fluid in Forced Convection outside Tubes When heat transfer occurs during immersed flow, the flux is dependent on the geometry of the body, the position on the body (front, side, back, etc.), the proximity of other bodies, the flow rate, and the fluid properties. The heat-transfer coefficient varies over the body. In the flow of fluids across a cylindrical shape, boundary layer separation occurs. No sharp distinction is found between laminar and turbulent flow, and a common correlation can be used for both low and high Reynolds numbers. The average heat-transfer coefficient is given in the empirical relationships . Fluids flowing normal to a single tube The variables affecting the coefficient of heat transfer to a fluid in forced convection outside a tube are do, cp, μ, and k, respectively, of the fluid; and G, the mass velocity of the fluid approaching the tube. Dimensional analysis gives For any one gas for which the Prandtl number is nearly independent of temperature, the Nusselt number is a function only of the Reynolds number. Experimental data for air are plotted in this way. For heating and cooling liquids flowing normal to single cylinders the following equation is used: This equation can also be used for gases from Re=1 to Re = 104, but it gives lower values of the Nusselt number than Figure at higher Reynolds numbers. (4.4-38) 4.4.6 Natural Convection Natural convection heat transfer occurs when a solid surface is in contact with a gas or liquid which is at a different temperature from the surface. 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. A hot, vertical plate in contact with the air. The temperature of the air in contact with the plate will be that of the surface of the plate, and a temperature gradient will