The improvement of heat transfer using ultrasound

Abstract

The power distributions of three different ultrasound sources were characterised by

the use of the calorimetric method. A 40 kHz and a 512 kHz bath, as well as a 1 MHz

therapeutic ultrasound kit, were mapped using degassed water and a mixture of 1 %

Decon solution. At low frequencies the method generated good results. In the middle

frequency range (512 kHz) with a low liquid volume and a low power a wider variation in results led to a higher standard deviation. High frequencies (1 MHz) generated only a small temperature change, making the calculation of power unreliable. The 40 kHz bath was used for the convectional heat transfer experiments. The "lump capacity" method determined the improvement of ultrasound on heat transfer. Potatoes, steel and aluminium cylinders with different diameters and lengths were used.

The potato experiments used fresh and precooked potatoes and showed an improvement in the heat transfer of approximately ten percent. This finding was also true even when the final temperatures of the experiment were changed. Under these higher temperature conditions the steel results did not fit the experimental limitations (Biot number >1) and therefore are not considered. The results using aluminium were difficult to reproduce and an investigation was undertaken to identify factors which could produce any improvement. For example an attempt was made to vary the distance between the ultrasonic source and the samples. This investigation illustrated that cavitation zones have a primary role in heat transfer. Also a stirrer was used to vary the external flow over the sample and this proved to be of crucial importance in improving reproducibility.

Date of Award	2005
Original language	English
Awarding Institution	Coventry University
Supervisor	Timothy Mason (Supervisor)