HEAT FLUX CORRELATION FOR SPRAY COOLING IN THE NUCLEATE BOILING REGIME

Abstract

In this work an experimental study of spray cooling using a spray in which drop sizes and velocities are uniform (monodisperse spray) impinging on a flat and heated surface is reported. The objective of the work was to formulate an empirical model describing the heat flux (HF) for the nucleate boiling regime and for the critical heat flux (CHF). Monodispersed water droplets with a known diameter and velocity, produced by a droplet generator, were directed toward a heated surface and the heat transfer was registered using a data acquisition system. The resulting high heat flux was investigated as function of the droplets’ diameter and velocity, mass flow rate, ambient pressure, subcooling degree of liquid, wall superheat, and surface roughness. The resulting matrix of variables investigated in the experiments included mass flux rate (340 < m˙″ < kg/m2 s), subcooling degree (25 < Tsub < 78°C), ambient pressure (1 < P < 1.8 bar), and surface roughness (79 < Rt < 5 μm). Generalized correlations were developed for the dimensionless HF and CHF as function of the dimensionless mass flow rate, wall superheat, subcooling degree, surface roughness, and ambient pressure, along with the Jacob number. The form of the correlation for the HF ishaving a confidence level greater than 95%; the differences between predicted and experimental HF were less than ±19%. The corresponding equation for the CHF has the formhaving a confidence level of 95% with a maximum error in the experimental data of ±15%.