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 established experimental method of investigating the CHF behaviour of prototype fluids (usually high pressure water) by employing low cost testing with modelling fluids. In most cases the modelling fluids are refrigerants at low pressures and saturation temperatures just above room temperature. 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 limitations include larger flashing rates at higher flow rates, and greater conversions from enthalpy into kinetic and gravitational energy in refrigerants, resulting in larger quality gradient and dissimilarity in quality distribution in equivalent systems between water and refrigerants. The friction heat at high flows can make a more significant contribution to the overall heat generation in refrigerants. Finally critical flow ('choking') ends to occur in refrigerants at lower flow than in an equivalent water system. (author)