John P. Kizito

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 RTIs vapor atomized spray nozzles. Thick film resistors, simulating heat source, were mounted onto 1 ×2 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 (ΔT 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.

Jet Impingement Heat Transfer Using Air-Laden Nanoparticles With Encapsulated Phase Change Materials

Nanoparticles made of polymer encapsulated phase change materials (PCM) are added in air to enhance the heat transfer performance of air jet impingement flows applied to cooling processes. Encapsulation prevents agglomeration of the PCM (paraffin) nanoparticles when they are in the liquid phase. The sizes of the particles are chosen to be small enough so that they maintain near velocity equilibrium with the air stream. Small solid paraffin particles can absorb a significant amount of energy rapidly from a heat source by changing phase from solid to liquid. Nanoparticle volume fraction is found to play an important role in determining the overall pressure drop and heat transfer of the jet impingement process. Specifically, air jets laden with 2.5% particulate volume fraction were shown to improve the average heat transfer coefficient by 58 times in the air flow speed range of 4.6 to 15.2m/s when compared to that of pure air alone. In addition, the structural integrity of the encapsulating shells was demonstrated to be excellent by the repeated use of the nanoparticles in closed loop testing. [DOI: 10.1115/1.4023563]

Microscale surface modifications for heat transfer enhancement.

In this experimental study, two surface modification techniques were investigated for their effect on heat transfer enhancement. One of the methods employed the particle (grit) blasting to create microscale indentations, while the other used plasma spray coating to create microscale protrusions on Al 6061 (aluminum alloy 6061) samples. The test surfaces were characterized using scanning electron microscopy (SEM) and confocal scanning laser microscopy. Because of the surface modifications, the actual surface area was increased up to 2.8× compared to the projected base area, and the arithmetic mean roughness value (Ra) was determined to vary from 0.3 μm for the reference smooth surface to 19.5 μm for the modified surfaces. Selected samples with modified surfaces along with the reference smooth surface were then evaluated for their heat transfer performance in spray cooling tests. The cooling system had vapor-atomizing nozzles and used anhydrous ammonia as the coolant in order to achieve heat fluxes up to 500 W/cm(2) representing a thermal management setting for high power systems. Experimental results showed that the microscale surface modifications enhanced heat transfer coefficients up to 76% at 500 W/cm(2) compared to the smooth surface and demonstrated the benefits of these practical surface modification techniques to enhance two-phase heat transfer process.

Spray cooling with ammonia on micro-structured surfaces

Experiments were performed to investigate spray cooling enhancement on micro-structured surfaces. Surface modification techniques were utilized to obtain micro-scale 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 RTFs vapor atomized spray nozzles. Thick film resistors, simulating heat source, were mounted onto 1 cm times 2 cm heaters 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 used 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. Results for micro-structured surfaces with protrusions and indentations offered significant performance enhancement of 115% and 52% increase in heat transfer coefficient over smooth surface respectively.

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 (ΔTsat = Tsurf − Tsat ) 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.Copyright