流体力学与传热（伍钦）08chapter4-6.pptVIP

Agitation Agitation increases the coefficient by increasing the velocity of the liquid across the surface, which helps to sweep away bubbles. Maximum flux and critical temperature drop The maximum flux (q/A)max depends mainly on the nature of the boiling liquid and on the pressure, and somewhat on the type of heating surface. The maximum flux for water is several times that for most organic liquids because of the differences in the heat of vaporization and other physical properties. The corresponding critical temperature drop also varies with pressure, from large values at low pressures to very small ones near the critical pressure. At atmospheric pressure the critical temperature drop for water is usually between 30 and 50°C The peak flux at the critical temperature drop is large. For water it is in the range of 350 to 1250 kW/m2 depending on the purity of water, the pressure, and the type and condition of the heating surface. For organic liquids, the peak flux lies in the range of 125 to 400 kW/m2 These limits apply to boiling under atmospheric pressure on a horizontal tube or a flat horizontal surface. problems The heat flux and the heat-transfer coefficient both ( ) as the temperature drop is raised in transition boiling. Boiling in natural convection, the heat-transfer coefficient is proportional to ( ) power of temperature difference. For film boiling, heat is transferred by ( ) and (at very high temperature drops) by( ) Film boiling is not usually desired in commercial equipment because the heat transfer coefficient is ( ). Heat-transfer apparatus should be so designed and operated that the temperature drop is ( ) than the critical temperature drop. 4.5.2 Heat Transfer to Boiling Liquids 1. Mechanisms of boiling Heat transfer to a boiling liquid is very important in evaporation and distillation as well as in other kinds of chemical and biolog