Comparative study of intermittent spray cooling in single and two phase regimes

Abstract

Spray cooling systems have diverse applications like cooling steel castings, cryo freezing in food industries, cooling of high power electronics, lasers and avionics systems. An intermittent spray cooling (ISC) process is an advanced process where the spray is pulsed at a certain frequency and with a certain open period. This versatile spray process can be adjusted using the mass flow rate, frequency and duty cycle (percentage of open time in one cycle) to match the required cooling rate on the target. One major advantage of ISC is that it utilizes the coolant effectively and therefore the energy efficiencies are higher than conventional sprays. The objective of the present work is to compare the ISC process in the single-phase regime at low heat flux removal rates with that in the two-phase regime at high heat flux removal rates. The experiments were conducted using a commercial nozzle (TG SS 0.3 Spraying systems Co) connected to a solenoid valve. The coolant fluid was water and the top surface of an insulated copper block served as the cooling target. The solenoid valve was controlled by closed-loop feedback control, based on the measured surface temperature of the copper block. The experiments were conducted at different heat flux regimes – 11, 22, 33 (low heat flux, single phase regime), 55, 88 (transitional regime), 108, 125 and 149 W/cm2 (high heat flux, two phase regime). The experiments were also conducted for various spray pressures – 2, 4 and 6 bar. The transient heat transfer coefficient was estimated using an inverse heat conduction problem through a sequential function estimation method. Thus it was possible to estimate the heat transfer during the on and off periods of the spray. At lower heat fluxes the duty cycle was between 20 and 70% and at moderate heat fluxes the duty cycle increased to a range of 60–90% and at high heat fluxes duty cycles were in the range of 40–85%. A higher duty cycle led to a lower frequency of the intermittent cycles. At medium and high heat fluxes when the surface temperature was near the boiling point of the coolant, there was significant phase change and this led to significant coolant savings compared to continuous spray cooling. In the two phase regime the heat transfer coefficient was high even during the spray off periods and resulted in utilization of the available energy to the maximum extent. Also at high heat fluxes the temperature fluctuations reduced drastically.