Intermittent spray cooling: Prediction and design

For intermittent spray cooling, two extra parameters must be defined compared to continuous spray cooling — the duty cycle, which specifies the percentage of time the spray is active, and the cycle frequency, which is the time between successive spray events. Together with the mass flux of drops at the substrate to be cooled, these three parameters determine what the steady state average temperature of the substrate will be for a given input heat flux, and the magnitude of substrate temperature fluctuations between the on and off phases of the spray. The novelty of the present study is twofold. On the one hand a computational framework, built on several previous studies investigating spray cooling in the nucleate and film boiling regimes, is introduced to predict the outcome of a given intermittent spray cooling scenario. On the other hand, the novelty lies in the methodology to specify suitable values of these three parameters to achieve a given substrate steady state temperature, varying with lower and upper limits during the active and inactive periods of the spray. Thus both predication and design of intermittent spray cooling systems is addressed in this study. Specifically, an analytic procedure is derived for choosing these three parameters for a given cooling problem. The cooling problem is defined by knowing the input heat flux, the thermal properties of the substrate, and the size and velocity distributions of the spray drops immediately before impact. This analytical procedure is then validated using existing experimental data. Furthermore, the performance of an intermittent spray cooling system operated at off-design conditions will be evaluated.