Effect of Heating Surface Geometry on the Droplets Evaporation under Leidenfrost Conditions

Abstract

Physical and geometric factors are generally regarded as the main cause of evaporation characteristics of the Leidenfrost droplets levitating above the hot surface. It is well-known and generally accepted that similar research is conducted under different conditions and on individual measurement set-ups. This is one of the potential reasons for the differences in the results of thermal fluxes and computational models in scientific papers. This paper discusses the influence of the heating surface geometry on the heat transfer coefficient h during water drops evaporation under film boiling regime. The variable geometry parameters are the curvature radius of the heating bowl of R = 64 and 254 mm. Individually compiled test stands made it possible to measure the instantaneous drop mass for each R radius and to determine the coefficient h. The methodology was validated by calculating the relative error. It changes with the curvature radius and the droplet size, and for droplet mass from about 2 g to 0.3 g does not exceed ±10%. The heat transfer coefficient h is about 15% higher for a drop located on a surface with a larger radius of curvature. Moreover, the method that was devised allows us to estimate the h value for asymmetric droplet shapes. The advantage of the adopted method of measuring the drop mass over time is the possibility of analyzing heat transfer processes in any drop shape range, even in the case of asymmetric ones. Previous research methods were mainly based on determining the mass of the drop by calculating its volume.