Abstract
This study considers spray cooling starting at surface temperatures of about 1200 °C and finishing at the Leidenfrost temperature. Cooling is in the film boiling regime. The paper uses experimental techniques for the study of which spray parameters are necessary for good prediction of spray cooling intensity. The research is based on experiments with water and air-mist nozzles. The following spray parameters were measured together with a heat transfer coefficient: water flowrate, water impingement density, impact pressure, droplet size and velocity. Derived parameters as droplet kinetic energy, droplet momentum and droplet Reynolds number are used in the tested correlations as well. Ten combinations of spray parameters used for correlation functions for the heat transfer coefficient (HTC) are studied and discussed. Correlation functions for prediction of HTC are presented and it is shown which spray parameters are necessary for reliable computation of HTC. The best results were obtained when the parameters impact pressure and water impingement density were used together. It was proven that the correlations based only on water impingement density, which are the most frequent in literature, can not provide reliable results.
Highlights
Design and control of spray cooling systems require knowledge of cooling intensity
This paper is dedicated to the study of which spray parameters are necessary for good prediction of the heat transfer coefficient
The results published are valid for high temperature areas with film boiling on the cooled surface
Summary
Design and control of spray cooling systems require knowledge of cooling intensity. This information can be obtained either by measurement or by computation from the spray parameters. As the droplet size and velocity is not influenced only by air pressure and water flow, and for instance by the nozzle type or spraying height, Hernandez-Bocanegra, et al, based on their experimental work, ranked factors by the importance of their influence on the heat flux. It was shown that spray properties (water flow rate, air pressure, water temperature, droplet size and velocity, water or air contamination) participate in the heat transfer but that it is important to consider the impinged surface. Parameters such as surface roughness, thermal properties of surface material, surface contamination [21] (for instance by oxides) are important for film boiling. Ambient conditions that can change heat transfer or fluid flow (ambient air pressure, ambient temperature or air flow)
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