![]() ![]() ![]() For water at atmospheric pressure, this corresponds to a heat flux of around 4♱04 W/m2 and an excess temperature of around 120 ☌. FILM BOILING To sustain film boiling, the excess temperature and applied heat flux must be greater than the Leidenfrost point. The region is referred to as transition boiling because there are continuous local oscillations between nucleate and film boiling regimes. This is because a thin layer of low thermal conductivity vapor begins to insulate the solid surface from the liquid, reducing the heat transfer coefficient between the fluid and solid surface. TR A N S I T I O N B O I L I N G To generate excess temperatures above 30 ☌ the heat flux absorbed must actually decrease below the critical heat flux. The same surface heat flux can result in different excess temperatures depending on the boiling regime (Ref. Convection Nucleate Film Transition Critical heat flux Leidenfrost point Onset of boiling Figure 1: Boiling curve for water at atmospheric pressure (101,325 Pa). K), much higher than any heat transfer coefficient that occurs due to convection alone.This results in a heat ©2013 COMSOL 1 | B O I L I N G WA T E R Solved with COMSOL Multiphysics 4.3b transfer coefficient of more than 104 W/(m2 At this point liquid can still rapidly replace the vapor produced, in effect continuously wetting the surface. Nucleate boiling is most efficient at the critical heat flux (see Figure 1). This means that even though the applied heat flux is very high, the excess temperature remains low. Because the detaching bubbles are quickly replaced by liquid, the effective thermal conductivity of the fluid layer close to surface is high. The bubbles begin to rise due to buoyancy forces and may merge with other isolated bubbles to form jets or columns. In this regime nucleation sites, which form due to imperfections in the solid surface, separate and expand due to mass transfer from the liquid phase to the vapor phase. The heat flux required to sustain this surface temperature is between 104 W/m2 and 106 W/m2. NUCLEATE BOILING For water at atmospheric pressure, nucleate boiling occurs for an excess temperature between 5 ☌ and 30 ☌. This model shows how to solve a boiling flow problem with the Laminar Two-Phase Flow, Phase Field user interface. The excess temperature is defined as the difference between the temperature of the solid surface and the saturation temperature of the liquid. The boiling regime depends on the excess temperature and the magnitude of the applied heat flux to the surface. There are three distinct regimes that characterize boiling induced by a heated surface: nucleate, transition, and film. It is possible to accomplish this in many ways two of the most common ways are by applying an external heat flux to a solid surface in contact with a liquid or by reducing the pressure in the surrounding environment. Introduction Boiling flow is an example of phase transition of the first kind and can be initiated by raising the temperature of a liquid above its saturation temperature. Solved with COMSOL Multiphysics 4.3b Boiling Water Note: This model requires the CFD Module and the Heat Transfer Module. ![]()
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