Investigation of the scatter in reported pool boiling CHF measurements including analysis of heat flux and measurement uncertainty evaluation methodology

This study investigates the effect of experimental setup design factors on pool boiling CHF, compares spatial temperature gradient calculation methods and analyzes the uncertainty of heat flux and surface superheat. Reported CHF values on smooth copper surfaces, measured for saturated pool boiling of water at atmospheric pressure on flat horizontal samples, are highly scattered, which cannot be explained solely by the measurement uncertainty or the randomness of the boiling process. CHF data for 54 experiments from 47 publications is analyzed using regression analysis and ANOVA to determine which experimental setup design factors influence the CHF value. Methods for estimating the axial temperature gradient in a heating stem are compared using the Monte Carlo method and analytical nonlinear gradients. Heat flux values calculated using temperature measurements in a cylindrical copper heating stem together with either constant or temperature-dependent thermal conductivity and various temperature gradient calculation methods are compared. Overall heat flux and surface superheat measurement uncertainties are analyzed and the impact of contributing uncertainties including that of the thermal conductivity, temperature measurement and distance between thermocouples is reported.