Spray Cooling With Ammonia on Microstructured Surfaces: Performance Enhancement and Hysteresis Effect

Experiments were performed to investigate spray cooling on microstructured surfaces. Surface modification techniques were utilized to obtain microscale indentations and protrusions on the heater surfaces. A smooth surface was also tested to have baseline data for comparison. Tests were conducted in a closed loop system with ammonia using RTI's vapor atomized spray nozzles. Thick film resistors, simulating heat source, were mounted onto 1x2 cm(2) heaters, and heat fluxes up to 500 W/cm(2) (well below critical heat flux limit) were removed. Two nozzles each spraying 1 cm(2) of the heater area used 96 ml/cm(2) min (9.7 gal/in.(2) h) liquid and 13.8 ml/cm(2) s (11.3 ft(3)/in.(2) h) vapor flow rate with only 48 kPa (7 psi) pressure drop. Comparison of cooling curves in the form of surface superheat (Delta T-sat=T-surf-T-sat) versus heat flux in the heating-up and cooling-down modes (for increasing and decreasing heat flux conditions) demonstrated substantial performance enhancement for both microstructured surfaces over smooth surface. At 500 W/cm(2), the increases in the heat transfer coefficient for microstructured surfaces with protrusions and indentations were 112% and 49% over smooth surface, respectively. Moreover, results showed that smooth surface gives nearly identical cooling curves in the heating-up and cooling-down modes, while microstructured surfaces experience a hysteresis phenomenon depending on the surface roughness level and yields lower surface superheat in the cooling-down mode, compared with the heating-up mode, at a given heat flux. Microstructured surface with protrusions was further tested using two approaches to gain better understanding on hysteresis. Data indicated that microstructured surface helps retain the established three-phase contact lines, the regions where solid, liquid, and vapor phases meet, resulting in consistent cooling curve and hysteresis effect at varying heat flux conditions (as low as 25 W/cm(2) for the present work). Data also confirmed a direct connection between hysteresis and thermal history of the heater.