Intermittent spray cooling — Solution to optimize spray cooling

An optimal spray cooling system would deliver just the right amount of coolant to remove the required heat flux and simultaneously avoid both, a dry out scenario and a thick film (of coolant) deposition on the surface. In most systems, the heat flux varies temporally and the object to be cooled needs to be maintained within a particular temperature range. Promoting phase change helps in reducing coolant requirement. One of the ways to meet all the three requirements is by intermittent spray cooling (ISC), in which the spray mechanism is activated only when the temperature starts rising above a set limit. A commercially available, low flow rate, nozzle was used along with a micro-solenoid valve to implement intermittent spray cooling. A thermal test chip (with integrated heaters to simulate heat source and diodes which act as temperature sensors) was used as target surface to be cooled. De-ionized water was used as coolant and flow rate was within the range of 0.25-0.5 ml/sec. Both steady state (continuous sprays) and intermittent spray cooling experiments were conducted. The main objective of this work is to study the effect of different parameters (heat flux, flow rate, and set-point temperature) on the fluctuation (amplitude) of surface temperature, heat transfer coefficient, valve frequency and on-off periods. Transient temperature fluctuations, transient heat transfer coefficients and frequency of the process were recorded for nozzle pressures of 2, 4 and 6 bar for heat fluxes of 11, 22, and 33 Watts/cm² at 5, 10 and 15 °C above the steady state temperature. This paper attempts to understand the various physical factors which affect and dominate the intermittent spray cooling process. An important conclusion is that the temperature fluctuations are minimized when the surface temperature is at sufficient superheat, due to the cushioning effect provided by the evaporation/boiling of the liquid film present on the surface during spray off period.