The effect of dissolving salts or gases in water sprayed on a hot surface

grantor: University of TorontoA study on the effect of dissolved salts and gas on droplet boiling and spray cooling was conducted. The effect of dissolved salts and gas on droplet boiling was observed on a hot stainless steel surface at temperatures ranging from 100°C to 300°C. Dissolving small amount of salts in water does not change the properties of water significantly; it may, however, have a great effect on droplet boiling. Dissolved salts reduced heat transfer in the evaporation regime since they lower the vapour pressure of water, while dissolved CO2 enhanced the evaporation rate slightly. In the nucleate boiling regime, dissolved salts (Na2CO3, Na2SO 4, MgSO4) prevented bubble coalescence in the liquid and promoted foaming, reducing droplet evaporation time significantly. Heat transfer was enhanced by increasing salt concentration until it reached the "transition concentration". Above this limit, dissolved salts started to reduce droplet evaporation rate since they lower the vapor pressure of water. Dissolved NaHCO3 reduced droplet lifetimes even more than Na2CO 3, because it decomposed when heated to produce CO2, further augmenting bubble formation. It was also found that when a droplet impinged on a surface above the Leidenfrost temperature, dissolved CO2 suppressed heterogeneous nucleation because CO2 escaped from below the drop and prevented it touching substrate. Experiments on spray cooling of a hot surface were performed with water sprays containing dissolved salts (NaCl, Na2SO4, MgSO 4). Dissolving salts in water significantly enhanced heat transfer in the transition and nucleate boiling regimes during spray cooling. The increase in heat transfer was observed at all mass fluxes and impact velocities as well as concentrations in the range of experiments conducted. The critical heat flux was found to increase with increasing molar concentration of dissolved salts in water until the concentration reached the "transition concentration". Dissolving salts in water over the transition concentrations reduced spray cooling due to the reduction of vapor pressure. Precipitated salt particles on the hot surface increased surface roughness, improving heat transfer in transition boiling. Furthermore, dissolved salts increased the Leidenfrost temperature in spray cooling by preventing bubble coalescence and producing particles which increase surface roughness.Ph.D