A Heat Flux Correlation for Spray Cooling in the Nucleate Boiling Regime

In this work an experimental study of spray cooling using monodispersed droplet sprays impinging on a flat and heated surface is reported. The aim of the work was to formulate an empirical model describing the heat flux (HF) for the nucleate boiling regime. 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 and surface roughness. The resulting matrix of variables investigated in the experiments included; mass flux rate (340 < ṁ″ < 750 kg/m2s), subcooling degree (25 < Tsub < 78 °C), ambient pressure (1 < P < 1.8 bar), and surface roughness (79 < Rt < 5 μm). A generalized correlation was developed for the dimensionless HF as function of the dimensionless mass flow rate, temperature, surface roughness and pressure, along with the Jacob number. The form of the correlation is q ˙ ″ ρ V h f g = 0.245 ( Ja ) 1.038 ( Δ T sub Δ T sat ) 0.491 ( ρ σ m ˙ μ 3 ) 0.133 ( R t D ) 0.0213 ( P P 0 ) 0.291 having a confidence level greater than 95%, the differences between predicted and experimental HF were less than ±19%.