An experimental study of flash evaporation from liquid pools

Abstract

The objective of this study is, based on experiments, to improve the understanding of flash evaporation from pools, and to develop empirical correlations of the total quantity of vapor released and the rate of vapor generation (both in non-dimensional form) as a function of the primary nondimensional parameters which govern the process: the Jakob number (Ja p), Prandtl number (Pr), a dimensionless hydrostatic head (Δp/H), and the salt concentration (C). The work was done in the range of parameters characteristic to desalination plants and to open-cycle ocean-thermal energy conversion. Experiments were performed with fresh water and with saline water (3.5% NaCl concentration), for initial temperatures from 25°C – 80°C, flash-down temperature differences from 0.5°C – 10°C, and pool depths of 6.5″ (165 mm), 12″ (305 mm) and 18″ (457 mm). The prediction of flashed mass as a function of the nondimensional parameters is accurate to within 7.5%. A new time scale τ = (σ/Δp) 2/α 1, (where σ = surface tension, α 1 = thermal diffusivity of the liquid, and Δp = pressure difference between the liquid and the vapor space) was employed. The expressions for the rate of flashed steam using this time scale show a scatter of ± 68%. The asymptotic value of the flashed mass increases with Jakob number, increases slightly with pool depth, and decreases with increasing liquid Prandtl number. The comparisons show very good agreement with the experimental values, but underpredict the data obtained from other experiments.