Acoustic Enhancement of the Rate of Heat Transfer Over a Flat Plate-An Experimental Investigation

Abstract

Increasing the rate of heat transfer can improve product quality and lower energy cost for many energy systems. Pulsating fluid flow has been used to increase the rate of heat transfer in some situations. Specifically, sound waves below the audible limit, termed infrasound, have been used to increase the rate of heat transfer from small-diameter wire 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 the 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 rectangular channel by two loudspeakers driven sinusoidally by a function generator at the resonant frequency of the system. The top of the channel was formed by a copper plate maintained at a constant temperature. Thermocouples placed along the centerline of the channel measured the temperature of the air inside the channel and heat flux gages mounted on the inside surface of the copper plate were used to measure the local 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 varied by an inlet damper. The use of infrasound increased the rate of heat transfer by approximately an order of magnitude when compared to natural convection. Infrasonic enhancement of the rate of heat transfer over a two-dimensional region in forced convection was more effective in the laminar flow regime, for Reynolds numbers based on the hydraulic diameter between zero and 10,000. Typically for laminar flow, infrasound increased the rate of heat transfer up to five times the rate of heat transfer without infrasound. For turbulent air flow, however, the increase of the rate of heat transfer was almost negligible. The effect of infrasound on the rate of heat transfer was shown to depend on the air velocity inside the channel, the hydraulic diameter of the channel, and the sound pressure level inside the channel. The temperature of the copper plate over the limited range tested did not significantly affect the heat transfer coefficient. The speakers used were limited to a maximum sound pressure level of 121 dB, while infrasonic generators are capable of producing sound pressure levels over 170 dB.