LIMITATIONS OF THE FLUID-TO-FLUID SCALING TECHNIQUE FOR CRITICAL HEAT-FLUX IN FLOW BOILING

CHF (critical heat flux) fluid-to-fluid scaling has become an establis hed experimental method of investigating the CHF behaviour of prototyp e fluids (usually high pressure water) by employing low cost testing w ith modelling fluids. In most cases the modelling fluids are refrigera nts at low pressures and saturation temperatures just above room tempe rature. Because of the low heat of vaporization for refrigerants, the power requirements are only 6-8% of that in water. This paper examines the various limitations of CHF fluid-to-fluid scaling. These limitati ons include larger flashing rates at higher flow rates, and greater co nversions from enthalpy into kinetic and gravitational energy in refri gerants, resulting in larger quality gradient and dissimilarity in qua lity distribution in equivalent systems between water and refrigerants , The friction heat at high flows can make a more significant contribu tion to the overall heat generation in refrigerants. Finally critical flow ('choking') tends to occur in refrigerants at lower flow than in an equivalent water system.