Hysteresis in Spray Cooling of Micro-Structured Surfaces

Spray cooling experiments were performed in a closed loop with ammonia using RTI’s vapor atomized spray nozzles. Thick film resistors, simulating heat source, were mounted onto 1cm × 2cm heater surfaces and heat fluxes up to 500 W/cm² (well below critical heat flux (CHF) limit) were removed. Two nozzles each spraying 1 cm² of heater area utilized 96 ml/cm²-min (9.7 gal/in²-hr) liquid and 13.8 ml/cm²-s (11.3 ft³/in²-hr) vapor flow rate with only 48 kPa (7 psi) pressure drop. A smooth surface and two types of micro-structured surfaces with indentations and protrusions were used as test surfaces. Comparison of cooling curves in the form of surface superheat (ΔT_sat = T_surf − T_sat) vs. heat flux in the heating-up and cooling-down modes (for increasing and decreasing heat flux conditions) demonstrated substantial performance enhancement for both micro-structured surfaces over a smooth surface. Moreover, results showed that smooth surface gives nearly identical cooling curves while micro-structured surfaces experience a hysteresis phenomenon depending on the surface roughness level and yields lower surface superheat in the cooling-down mode, compared to the heating-up mode, at a given heat flux. Micro-structured surface with protrusions was tested using two approaches to gain better understanding on hysteresis. Data mainly indicated that micro-structured surface helps retain established three-phase contact line, the region 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 furthermore confirmed a direct connection between hysteresis and thermal history of the heater.