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

Abstract

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 1×2 cm2 heaters, and heat fluxes up to 500 W/cm2 (well below critical heat flux limit) were removed. Two nozzles each spraying 1 cm2 of the heater area used 96 ml/cm2 min(9.7 gal/in.2 h) liquid and 13.8 ml/cm2 s(11.3 ft3/in.2 h) vapor flow rate with only 48 kPa (7 psi) pressure drop. Comparison of cooling curves in the form of surface superheat (ΔTsat=Tsurf−Tsat) 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/cm2, 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/cm2 for the present work). Data also confirmed a direct connection between hysteresis and thermal history of the heater.