Acoustic enhancement of the rate of heat transfer over a flat plate

Increasing the rate of heat transfer can improve product quality and lower energy cost of many industrial processes. Pulsating flow produced by sound waves have been used to increase the rate of heat transfer in some situations. Infrasound, sound waves below the audible limit, has been used to increase the rate of heat transfer from small diameter, wire rods. The Aluminum Company of America, ALCOA, has proposed to use infrasound to increase the rate of heat transfer in aluminum ingot furnaces. This study examined the effects of infrasound on the rate of heat transfer from a two-dimensional region. A standing wave of infrasound was produced in the rectangular neck of a Helmholtz resonator by two loudspeakers. One side of the neck was formed by a copper plate maintained at a constant temperature. Thermocouples imbedded in the back of the copper plate were used to measure the plate temperature. Heat flux gages mounted on the inside surface of the copper plate measured the rate of heat transfer from the plate to the air provided by a blower connected to the neck of the Helmholtz resonator. The rate of heat transfer from a flat plate was increased by infrasound. Infrasound produced the largest increase in the rate of heat transfer when compared to natural convection. Infrasonic effects decreased with increasing air velocity when compared to forced convection. The effect of infrasound on the rate of heat transfer was shown to be a function of the air velocity inside the neck, the hydraulic diameter and the sound pressure level inside the neck. Infrasonic enhancement of the rate of heat transfer inside aluminum ingot furnaces does not appear practical at this time since the improvement is negligible at high fluid velocities.