Heat Transfer BOILING HEAT TRANSFERBulkBoilingAs system temperature increases or system pressure drops, the bulk fluid can reach saturationconditions. At this point, the bubbles entering the coolant channel will not collapse. The bubbleswill tend to join together and form bigger steam bubbles. This phenomenon is referred to as bulkboiling. Bulk boiling can provide adequate heat transfer provided that the steam bubbles arecarried away from the heat transfer surface and the surface is continually wetted with liquidwater. When this cannot occur film boiling results.FilmBoilingWhen the pressure of a system drops or the flow decreases, the bubbles cannot escape as quicklyfrom the heat transfer surface. Likewise, if the temperature of the heat transfer surface isincreased, more bubbles are created. As the temperature continues to increase, more bubbles areformed than can be efficiently carried away. The bubbles grow and group together, coveringsmall areas of the heat transfer surface with a film of steam. This is known as partial filmboiling. Since steam has a lower convective heat transfer coefficient than water, the steampatches on the heat transfer surface act to insulate the surface making heat transfer more difficult.As the area of the heat transfer surface covered with steam increases, the temperature of thesurface increases dramatically, while the heat flux from the surface decreases. This unstablesituation continues until the affected surface is covered by a stable blanket of steam, preventingcontact between the heat transfer surface and the liquid in the center of the flow channel. Thecondition after the stable steam blanket has formed is referred to as film boiling.The process of going from nucleate boiling to film boiling is graphically represented in Figure13. The figure illustrates the effect of boiling on the relationship between the heat flux and thetemperature difference between the heat transfer surface and the fluid passing it.Rev. 0 Page 41 HT-02
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