A Comparative Analysis of the Spray Cooling Performance Characteristics of Alumina Nanofluids

Nanofluids are promising as spray coolants in various thermal management systems where they can enhance the surface heat transfer rate. Generally, cooling performance of spray coolants is experimentally evaluated on a flat or inclined surface with the nozzle arranged in a vertical orientation.
In this study, experiments were extended to the case of a hemispherical, curved surface, where the cooling performance was compared between a pure water spray and an Alumina nanofluid spray. A port-fuel injector was utilised to spray a jet of varying Aluminium oxide nanoparticle suspensions, at 3 bar pressure, on to a smooth, curved, heated surface, mimicking the geometry of a bowled combustion chamber. The temperature of the surface was varied in the range of 25 - 180 °C. A Phase Doppler Anemometer was used to measure droplet diameter, axial and radial velocity distributions at various locations with and without spray impingement. High-speed video observations showed that the spray cone angle was reduced by 7° with the addition of nanoparticles compared to the pure water spray. In the heated piston cases, the heat transfer effect was only enhanced in the case of the higher nanoparticle mass concentration, resulting in a 15 °C reduction in surface temperature after approximately 35 seconds, when compared to the water case with the same injection duration and frequency. The transition to single-phase cooling occurred approximately 10 seconds earlier in the pure water case for
the location directly under the impinging spray jet. The deposition of a rough, solid film of Aluminium oxide on the piston surface at the point of spray impact was observed.