Predictable Modelling of Heat Transfer Coefficient between Spraying Water and a Hot Surface above the Leidenfrost Temperature.

Abstract

In order to evaluate the cooling intensity of water spray impacting on a hot metallic surface above the Leidenfrost temperature, the formulation of heat transfer coefficient in the forced convection boiling region has been made as a function of the droplet size, the impinging velocity and the number density of droplets whose parameters are independent of each other. So far, many works on the mist/spray cooling process have been made, in particular, from an experimental point of view. However, the general procedure capable of evaluating heat transfer rate between a hot metallic surface and water spray has not been established yet, because there are a large number of parameters affecting the spray cooling process. Then, we have experimentally derived a new formula consisting of the above three parameters to be dominant for heat transfer rate in the spray cooling process. The stainless steel surface heated to about 900°C has been cooled by water spray of ∼20°C and the time history of the surface temperature has been measured. We have selected some kinds of full cone nozzles whose characteristics such as the average droplet diameter, the velocity and the distribution of water flux have been different from each other, and performed the cooling tests using them. Finally, the formula capable of giving best-fit to the experimental results has been proposed. The effect of the spraying characteristics on the heat transfer rate has been discussed from an experimental point of view.