流体力学与 及传热课件4.5.2 Heat Transfer to Boiling Liquids.pptVIP

4.5.2 Heat Transfer to Boiling Liquids ; The boiling liquid is usually contained in a vessel with a heating surface of tubes or vertical or horizontal plates which supply the heat for boiling. ; The heating surfaces can be heated electrically or by a hot or condensing fluid on the other side of the heated surface .;Pool boiling of saturated liquid ;Bubbles of vapor are generated at the heating surface, rise through the mass of liquid, and disengage from the surface of the liquid. ;Boiling is a complex phenomenon. Suppose we consider a small heated horizontal tube in a vessel containing water boiling at 373.2 K. ;The heat flux is q/A ; Δt = Tw - t , where Tw is the tube wall temperature; ; In the first segment, at low temperature drops, the line AB is straight and has a slope of 1.25. This is consistent with the correlation for natural convection;In the first section, the mechanism is that of heat transfer to a liquid in natural convection, and the variation of h with T agrees with that given by equation below. ; Bubbles form on the surface of the heater, are released from it, rise to the surface of the liquid, and are disengaged into the vapor space; ;2.nucleate boiling; The slope of line BC depends upon the specific experiment; it usually lies between 3 and 4. The second segment terminates at a definite point of maximum flux, which is point C;During nucleate boiling, the bubbles occupy but a small portion of the heating surface at a time, and most of the surface is in direct contact with liquid. ;The bubbles are generated at localized active sites, usually small pits or scratches on the heating surface. ;As the temperature drop is raised, more sites become active, improving the agitation of the liquid and increasing the heat flux and the heat-transfer coefficient.;The temperature drop corresponding to point C is called the critical temperature drop, and the flux at point C is the peak flux.;3. transition boiling;In this region, corresponding to segment