Rewetting of hot surfaces

Abstract

Present dissertation reports the theoretical and experimental investigations pertaining to the thermo-hydraulic phenomenon during rewetting of hot surface. The objective of the present study is to analyze the phenomena of rewetting during emergency cooling following the loss of coolant accident and quenching during metallurgical applications. Variational integral method has been employed to analyze a variety of rewetting problems. This includes the basic two region rewetting model that assumes a constant heat transfer coefficient in the wet region and adiabatic condition in the dry region ahead of wet front. Subsequently the analysis has been extended to include the effect of precursory cooling, variation in heat transfer in multiple step functions and exponential functions and variation in property. Based on the analysis, closed form solution is obtained for the temperature field and rewetting velocity. A test facility is developed to analyze the phenomena of rewetting by impinging liquid jet on a hot vertical surface. Here, a vertical thin stainless steel foil (0.15 mm thickness) is electrically heated to obtain the required initial temperature. During coolant impingement on the hot surface, the formation and propagation of wet front have been studied through visual observation. The thermal imaging technique by using an infrared camera (A655sc, FLIR System) is used to record the transient temperature. The temperature data was used to evaluate the rewetting velocity and surface heat flux distribution on the hot surface. The effect of various parameters, namely, initial surface temperature, Reynolds number and, nozzle to plate spacing, on the rewetting velocity and heat flux distribution are discussed in the present analysis. Key words: quenching, rewetting, precursory cooling, three-region, sputtering, property variation, variational integral method, liquid jet impingement, heat flux, rewetting velocity, thermal imaging technique, Peclet number.