Heat transfer enhancement at solid-liquid and solid-gas interfaces by near-surface coolant agitation

Heat transfer enhancement from glass ceramic plate to water or air by agitation of the coolant near the heated surface was investigated. The experimental arrangement consisted of a ceramic plate of dimensions of 47×35×0.26 mm³, attached to a brass vessel with inner dimensions of 34×30×4 mm³. The ceramic plate was provided with a 5×5 mm² size printed heater on the opposite side from the vessel. The temperature field across the plate was recorded by an infra-red camera. The agitation of the coolant in the vessel (air or water) was performed by a vibrating piezoelectric beam of dimensions of 26.5×12×0.6 mm³, fixed at 1 mm distance from the heated plate, or alternatively by a magnetic rod of diameter of 2.2 mm and length of 15 mm. The vibration of the beam with amplitude of some tenths of mm peak to peak at frequencies of 200 to 400 Hz caused the sinking of the peak temperature of the heater from about 90°C to 45°C in case of water as the coolant and from about 110°C to 100°C in the air. The magnetic rod, rotating in water at speed of some rounds per second lowered the heater´s peak temperature from 85°C to 50 to 60°C. The ambient temperature in all experiments was 22 to 25°C and the heating power 1-2 W. The power needed for agitation was about 50 mW in case of piezoelectric vibrator and about 1 W in case of the rotating agitator drived by a fan. Using numerical simulation by ANSYS, it was demonstrated, that the temperature distribution across the plate with heater can be satisfactorily simulated using a two-dimensional (2-D) model with appropriately enhanced heat conductivity of the plate and heat transfer coefficient from the plate. For the experimental arrangement used the equivalent heat conductivity of the ceramic plate in case of agitated liquid cooling was up to 150 W/m·K and heat transfer rate up to 300 W/m ²·K