BOILING HEAT TRANSFER Heat TransferBOILING HEAT TRANSFERThe formation of steam bubbles along a heat transfer surface has asignificant effect on the overall heat transfer rate.EO 1.15 DESCRIBE the process that occurs in the followingregions of the boiling heat transfer curve:a. Nucleate boilingb. Partial film boilingc. Film boilingd. Departure from nucleate boiling (DNB)e. Critical heat fluxBoilingIn a nuclear facility, convective heat transfer is used to remove heat from a heat transfer surface.The liquid used for cooling is usually in a compressed state, (that is, a subcooled fluid) atpressures higher than the normal saturation pressure for the given temperature. Under certainconditions, some type of boiling (usually nucleate boiling) can take place. It is advisable,therefore, to study the process of boiling as it applies to the nuclear field when discussingconvection heat transfer.More than one type of boiling can take place within a nuclear facility, especially if there is arapid loss of coolant pressure. A discussion of the boiling processes, specifically local and bulkboiling, will help the student understand these processes and provide a clearer picture of whybulk boiling (specifically film boiling) is to be avoided in nuclear facility operations.NucleateBoilingThe most common type of local boiling encountered in nuclear facilities is nucleate boiling. Innucleate boiling, steam bubbles form at the heat transfer surface and then break away and arecarried into the main stream of the fluid. Such movement enhances heat transfer because the heatgenerated at the surface is carried directly into the fluid stream. Once in the main fluid stream,the bubbles collapse because the bulk temperature of the fluid is not as high as the heat transfersurface temperature where the bubbles were created. This heat transfer process is sometimesdesirable because the energy created at the heat transfer surface is quickly and efficiently"carried" away.HT-02 Page 40 Rev. 0
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