流体力学与传课件Heat Transfer to Fluids with Phase Change.ppt

Also, even when the flow remains laminar throughout, coefficients measured experimentally are about 20 percent larger than those calculated from the equation. This attributed to the effect of ripples on the surface of the falling film. In general, the coefficient of a film condensing on a horizontal tube is considerably larger than that on a vertical tube under similar conditions unless the tubes are very short or there are many horizontal tubes in the stack. Vertical tubes are preferred when the condensate must be appreciably subcooled below its condensation temperature. effect of Noncondensible gases When a multicomponent mixture contains a noncondensing gas, the rate of condensation is seriously reduced. As in the condensation of a mixture of condensable vapors, the condensing molecules must diffuse through a film of noncondensing gas which does not move toward the condensate surface. As condensation proceeds, the relative amount of this inert gas in the vapor phase increases significantly. The presence of even small amounts of noncondensing gas in a condensing vapor seriously reduces the rate of condensation. Problem A vapor may condense on a cold surface in two ways: ( )and( ) the average coefficient for dropwise condensation is ( ) than that for film-type condensation. In general, the coefficient of a film condensing on a horizontal tube is ( ) than that on a vertical tube under similar conditions It is difficulty to benefit practically from a dropwise condensation because this type of condensation is ( ), and increase in the coefficient of dropwise condensation ( ) the overall coefficient. the presence of even small amounts of noncondensing gas in a condensing vapor ( ) the rate of condensation. In general, the coefficient of a film condensing on a single horizontal tube is