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.

Effect of nanoparticles concentration on the characteristics of nanofluid sprays for cooling applications

“…[1][2][3]. As for the various liquid cooling techniques explored by the researchers within the last two decades, spray cooling is among the most popular, given the high heat transfer coefficients that can be achieved (of the order of 10 4 -10 5 W/m 2 K or higher - [4]). Nevertheless, the efficient implementation of this strategy must cope with the increasingly demanding heat loads that are dissipated, so that continuous efforts have been put to further enhance the heat transfer processes.…”

Enhancement of Heat Transfer with Pool and Spray Impingement Boiling on Microporous and Nanowire Surface Coatings

“…They found that the CHF increased by 50% relative to the uncoated surface. Bostanci et al [15] performed spray cooling experiments with ammonia on microstructured surfaces with indentations and protrusions with heat fluxes up to 500 W/cm 2 . They observed an enhancement of 49%−112% in the heat transfer coefficient with respect to a smooth surface.…”

Experimental evaluation of spray cooling of R-134a on plain and enhanced surfaces

“…Moreover, from a design point of view, the heating-up mode is generally more conservative as it gives rise to lower heat transfer coefficients than the cooling-down mode, especially when micro-structured surfaces are used (Bostanci et al, 2009b).…”

“…Bostanci et al (2009a) carried out experiments on spray cooling of simulated power inverter modules with microprotrusion and micro-indentation enhanced surfaces using an antifreeze solution with normal boiling and freezing points of 90 C and À25 C, respectively. Heat fluxes of the order of 400 W cm À2 were achieved for wall superheating degrees of 14 C. Bostanci et al (2009b) evaluated boiling hysteresis phenomena in spray cooling of enhanced surfaces similar to those presented in Bostanci et al (2009a) using ammonia (NH 3 ) as the working fluid. For a heat flux of 500 W cm À2 using the micro-protrusion surface, the heat transfer coefficient was increased by as much as 112% with respect to a plain smooth surface.…”