CRITICAL HEAT FLUX IN A LONG, RECTANGULAR CHANNEL SUBJECTED TO ONE-SIDED HETING-II. ANALYSIS OF CRITICAL HEAT FLUX DATA

摘要

A theoretical model to predict critical heat flux in ling, rectangular channels is presented. The theoretical development is complemented by an extensive flow visualization analysis presented in Part I of this study. The observation of a periodic distribution of increasingly larger vapor patches along the surface just prior to CHF is idealized as a sinusoidal interface with amplitude and wavelength increasing in the flow direction. A separated flow model provides phase velocities and an average vapor thickness which are utilized by an instability analysis to predict the critical interfacial wavelength. An energy balance assumes the transfer of heat from the surface to the fluid occurs only a the troughs of the interface, called wetting fronts, and that the surface is insulated below the vapor patches. The lift-off of the most upstream wetting front, which occurs when the pressure difference serving to maintain interfacial contact with the surface is overcome by the vapor momentum emanating from the wetting front, is the trigger mechanism which precipitates CHF. The ratio of wetting front length to vapor wavelength obtained from the flow visualization represents a key contribution to the model. CHF predictions are accurate to within a mean absolute error of 10.0/100 for data obtained at near-sated conditions for velocities of 0.25-10.0 ms~-1.