Studies on Sonic Drying of Agricultural Products (I)

Effect of Sound on Heat Transfer from a Single Copper Sphere

Abstract

The authors intended the application of low frequency sound waves which scarcely generates any heat durring the drying of agricultural products. In this paper, we tried a few experiments on the effects of position of the sphere in a standing sound field, sound pressure and frequency on heat transfer from a single copper sphere. A summary of the results is shown below.
1) The effect of position of the sphere in a standing sound field on heat transfer from a copper sphere was evident. No relationship between sound pressure and Nu number was found at antinode. However, Nu number increased from about 145dB at node. That is, the heat transfer from a copper sphere increased when it was located at node which had maximum particle velocity.
2) Nu number increased with sound pressure (that is to say, particle velocity).
3) Aeration affected sound pressure level from which Nu number began to increase. Nu number increased from about 145dB when air velocity was zero. On the other hand, it was 148dB when aeration was performed.
4) The lower the frequency, the larger was the Nu number.
5) The following relation between Nu number and (Re)_a number within the limits of this experiment was obtained.
Nu=h.D/K=K(Re)_a0.19
where 6.6<K<8.6 for 0<Re<900
6) Electric power required for speaker driver was given by a exponential function of sound pressure. A resonant frequency of the tube did not show a special increase in Nu number as compared with the other frequencies, but the electric power required was less than the others.