ACOUSTIC ENHANCEMENT OF THE RATE OF HEAT-TRANSFER OVER A FLAT-PLATE -AN EXPERIMENTAL INVESTIGATION

Increasing the rate of hear transfer can improve product quality and l ower energy cost for many energy systems. Pulsating fluid flow has bee n used to increase the rate of heat transfer in some situations. Speci fically, sound waves below the audible limit, termed infrasound, have been used to increase the rate of heat transfer from small-diameter wi re rods. This study examined the effects of infrasound on the rate of heat transfer from a flat plate. A standing sound wave is formed in th e neck of a Helmholtz resonator and may be enhanced by producing sound waves at the resonant frequency at or near the neck of the resonator. In this study, a standing wave of infrasound was produced in a rectan gular channel by two loudspeakers driven sinusoidally by a function ge nerator at the resonant frequency of the system. The top of the channe l was formed by a copper plate maintained at a constant temperature. T hermocouples placed along the centerline of the channel measured the t emperature of the air inside the channel and hear flux gases mounted o n the inside surface of the copper plate were used to measure the loca l rate of heat transfer from the plate to the air inside the channel. Air flow inside the channel was produced by a centrifugal blower and v aried by an inlet damper. The use of infrasound increased the rate of heat transfer by approximately art order of magnitude when compared to natural convection. Infrasonic enhancement of the rate of heat transf er over a two-dimensional region in forced convection was more effecti ve in the laminar flow regime, for Reynolds numbers based on the hydra ulic diameter between zero and 10,000. Typically for laminar flow, inf rasound increased the rate of heat transfer up to five times the rate of heat transfer without infrasound. For turbulent airflow, however, t he increase of the rate of heat transfer was almost negligible. The ef fect of infrasound on the rate of heat transfer was shown to depend on the air velocity inside the channel, the hydraulic diameter of the ch annel, and the sound pressure level inside the channel. The temperatur e of the copper plate over the limited range tested did not significan tly affect the heat transfer coefficient, The speakers used were limit ed to a maximum sound pressure level of 121 dB, while infrasonic gener ators are capable of producing sound pressure levels over 170 dB.