PHOTOGRAPHIC STUDY OF BUBBLE DEPARTURE DIAMETER IN SATURATED POOL BOILING TO ELECTROLYTE SOLUTIONS

Bubble departure diameters during saturated pool boiling to pure water and three different electrolyte solutions including NaCI KNO3 and KCI aqueous solutions are experimentally measured. Variable heat fluxes up to 90 kW/m(2) and different salt concentrations from 10.6 to 69.6 kg/m(3) are applied in order to investigate their effects on the bubble size during pool boiling around the horizontal rod heater. Visual observations demonstrate that larger vapor bubbles generate on the heat transfer surface at higher salt concentrations and lower heat fluxes in all of the solutions tested while in distilled water bubbles become slightly larger with increasing heat flux. Furthermore, the effects of different important physical properties like surface tension, viscosity, and density of the solutions on the bubble departure diameter are also discussed. NaCI solutions have surface tension higher than the other electrolyte solutions. Furthermore, the addition of NaCI to distilled water slightly increases the viscosity of the solution whereas other salts have no measurable effect on the viscosity. Therefore, larger bubbles are expected to appear on the heat transfer surface during the boiling of NaCI solutions, which is in agreement with the experimental results.