PRESSURE-BASED PREDICTION OF SPRAY COOLING HEAT TRANSFER

Abstract

One of the main challenges of spray cooling technology is the prediction of local and average heat flux on the heater surface. It has been suggested that spray cooling heat transfer depends on the local spray mass flux. However, in this work it is hypothesized and demonstrated that local single-phase and boiling heat transfer can be predicted within ±25% of the measured values from the local normal pressure produced by the spray. In the single-phase study, hollow cone, full cone, and flat fan sprays operated at three standoff distances, five spray pressures, and two nozzle orientations were used to identify the relation between impingement pressure and heat transfer coefficient. PF-5060, PAO-2, and PSF-3 were used as test fluids, resulting in Prandtl number variation between 12-76. A 7×7 mm2 micro-heater array consisting of 96 platinum resistance heaters operated at constant temperature was used to measure the local heat flux. A separate test rig was used to make impingement pressure measurements for the same geometry and spray pressure. The heat flux data were then compared with the corresponding impingement pressure data to develop a pressure-based correlation for single-phase spray cooling heat transfer. Hollow cone and full cone PF-5060 sprays at three subcooling levels were used for the two-phase heat transfer study. The conventional wisdom is that the temperature at which critical heat flux (CHF) is observed changes with the droplet impact velocity, droplet number density, and droplet size. However, the present measurements indicate that although the magnitude of CHF is strongly dependent on the spray characteristics, the temperature at which CHF occurs lies within a very narrow band (about ±5°C) for smooth flat surfaces. This was also observed from local measurements at various radial distances using hollow cone and full cone spray nozzles where the local mass flux varies dramatically. This observation along with liquid properties and subcooling were used to develop a correlation to predict local CHF for PF-5060 sprays. The single-phase and CHF correlations were combined to predict local spray cooling curve within ±25% of the measured valued over the sprays impingement zones.