Effect of gravity and surfactant on spray cooling of hot surfaces

The effect of gravity and surfactant on spray cooling of hot surfaces was investigated. The objectives were to understand (1) the effect of reducing gravity on the boiling of a single droplet impacting a hot surface, (2) the effect of adding a surfactant to a single droplet impacting a hot surface and (3) the effect of adding a surfactant and changing surface orientation on spray cooling effectiveness.;The boiling of single droplets of water and n-heptane on a hot stainless steel surface was observed in a low gravity environment, obtained on board a platform in free fall. Droplet impact was photographed using a single-shot flash-photographic method. Surface temperature variation during droplet impact was recorded using a fast-response thermocouple. Droplets could not be maintained in stable film boiling in low gravity: the pressure of vapor under droplets pushed them away from the surface. Nucleate boiling of droplets was not affected by a reduction in gravity.;The effect of adding a surfactant (Sodium Dodecyl Sulfate) to droplets impacting and boiling on a hot stainless steel surface was studied. Droplet impact and evaporation was recorded using both video and 35 mm cameras. Addition of the surfactant significantly reduced droplet evaporation time in the evaporation and nucleate boiling regimes. For surface temperatures below those required to initiate nucleate boiling, the principal effect of the surfactant was to reduce liquid-solid contact angle, increasing the surface area wetted by the drop. At higher surface temperatures the surfactant promoted vapor bubble nucleation and foaming in the liquid, greatly enhancing heat transfer.;Experiments were done on the cooling of a hot surface by water sprays in which the surfactant (Sodium Dodecyl Sulfate) was also dissolved. Adding the surfactant significantly enhanced nucleate boiling and reduced the surface superheat required to initiate boiling during spray cooling, confirming results of single droplets. Heat transfer in film boiling was slightly reduced; whereas transfer in transition boiling was enhanced, but the magnitude of enhancement was dependent upon spray mass flux. Changing surface orientation had no measurable effect on spray cooling heat transfer. Photographs of the surface showed that depending on surface temperature, spray impact on the surface could be classified into three different states, essential for the design of spray cooling equipment.