The Effect of Dissolving Salts in Water Sprays Used for Quenching a Hot Surface: Part 1—Boiling of Single Droplets

We conducted experiments on the effect of dissolving three different salts sodium chloride (NaCl), sodium sulfate $Na2SO4$ and magnesium sulfate $MgSO4$ in water droplets boiling on a hot stainless steel surface. Substrate temperatures were varied from 90°C to 220°C. We photographed droplets as they evaporated, and recorded their evaporation time. At surface temperatures that were too low to initiate nucleate boiling all three salts were found to reduce droplet evaporation rates because they lower the vapor pressure of water. In the nucleate boiling regime, low concentrations (<0.1 mol/l) of $Na2SO4$ and $MgSO4$ enhanced heat transfer because they prevented coalescence of vapor bubbles and produced foaming in the droplet, significantly reducing droplet lifetimes. Increasing the salt concentration further did not produce a corresponding increase in droplet boiling rate. Dissolved salts prevent bubble coalescence because they increase surface tension and stabilize the liquid film separating bubbles, and because electric charge that accumulates on the surfaces of bubbles produces a repulsive force, preventing them from approaching each other. $Na2SO4$ and $MgSO4,$ which have high ionic strengths, produced a large amount of foaming in droplets and increased their boiling rate significantly. NaCl, which has low ionic strength, had little effect on droplet boiling.