The document discusses the phenomenon of rewetting, where a liquid comes into contact with a hot surface. Initially, a vapor layer forms preventing contact between the liquid and hot surface. As the surface cools, the vapor layer collapses and the liquid wets the surface, increasing heat transfer. The study analyzes rewetting during emergency cooling following loss of coolant accidents and quenching applications. A test facility uses an impinging liquid jet on a heated metal foil to observe rewetting front propagation and surface temperature changes, evaluating how parameters like initial temperature and nozzle spacing affect rewetting velocity and heat flux.

1. Title: Rewetting of Hot Surfaces
Abstract: When liquid is brought in contact with the hot surface, it cannot wet the hot surface
immediately. A vapour blanket is formed on the solid surface that prevents the contact between
the hot surface and liquid. Consequently, the heat transfer from hot surface reduces due to poor
conduction of heat through the vapor layer. If the process is allowed to continue, the hot surface
will reach a temperature at which the vapour film collapses and liquid wets the hot surface. This
process is termed as rewetting, this means re-establishment of hot surface-liquid contact. The
heat removal rate increases significantly during this process.
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 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.