Investigation Of Microgrooved Surface Spray Cooling

Phase change technology is a science that is continually finding new applications, from metallurgical processes to semiconductor cooling. The primary heat transfer techniques associated with phase change heat transfer are pool boiling, flow boiling, and spray cooling. Of these techniques, spray cooling presents the most appealing heat transfer technique, because of its capability of providing high heat fluxes in excess of 1,000 W/cm2 for water, and its potential for very high heat transfer with low temperature difference.Many previous studies have emphasized heat flux as a function of spray parameters and test conditions. To date, enhanced spray cooling investigations have been limited to surface structure studies. These studies concluded that structured surfaces can strongly enhance the heat transfer effects because of the capillary force. A spray cooling study using enhanced surfaces with only open microgrooves may display more stable heat flux enhancement along with more uniform liquid distribution. In the pool cooling with binary mixture working liquid, the lower boiling temperature component is considered to evaporate to be nucleation sites, resulting in higher critical heat flux (CHF). A spray cooling on microgrooved surface, using binary mixtures as the working liquid, spray cooling may further be enhanced with much higher CHF.In the present study, a group of open microgrooved surfaces have been investigated to determine the effects of enhanced surface structure on heat flux. The microgrooves were directly machined on the top surface of copper heater blocks with an effective crosssectional area of 2.0 cm×2.0 cm. For baseline comparison purposes, measurements were obtained for a flat surface. Thermal performance data was obtained with different flow rates, different surface orientation and different spray pressure. The study shows that microgrooved surfaces promote heat flux enhancement for all spray cooling conditions. The study also shows flow rates greatly enhance the heat transfer. Study results indicate that the effects of surface orientation, spray pressure and spray orientation differ for spray cooling of a microgrooved surface and spray cooling of a flat surface.The binary mixtures of distilled water and ethanol were investigated to determine the effects of binary mixtures on the heat transfer of spray cooling on microgrooved surfaces. Results indicate that the effects of binary mixture on spray cooling are different from its effect on pool cooling. The study also shows that the molar ratio of binary mixtures strongly affects the spray cooling CHF, and that there is an optimal ratio value, corresponding to the highest CHF.Futhermore, the VOF (the volume of fluid) method has been used to simulate the effects of droplet-solid surface/thin liquid film interaction on heat transfer. The study shows that the We number and contact angle strongly influence the droplet spread behavior on solid surfaces, resulting in different heat transfer coefficients. The study also shows that the droplet-thin liquid film interaction can enhance heat transfer; and that this effect is strongly affected by the thin film dynamics.