Effect of acoustic cavitation on boiling heat transfer

https://doi.org/10.1016/S0894-1777(02)00201-7Get rights and content

Abstract

Boiling heat transfer on a horizontal circular copper tube in an acoustical field is investigated experimentally and the relation between the liquid cavitation, the boiling and the micro bubble radii are analyzed theoretically. The results show that cavitation bubbles have an important influence on the nucleation, growth and collapse of vapor embryo within cavities on the heat transfer surface and that the enhancement of boiling heat transfer by acoustic cavitation mainly depends on whether the vapor embryo is activated by the cavitation bubbles to initiate boiling.

Introduction

Boiling heat transfer is widely utilized in processes related to energy resources, power, aviation and space flight due to its high heat transfer capability. However, the increasing heat fluxes in sophisticated equipment require more efficient and reliable cooling methods [1]. We must fully understand boiling to efficiently control the boiling heat transfer process. Although much experimental data is available for heat fluxes and wall superheats with various experimental expressions relating nucleation and growth of bubbles, effective methods have not yet been found to control boiling heat transfer [2].

The process of rupturing a liquid by decreasing the pressure at roughly constant liquid temperature is often called cavitation and is a physical phenomenon in liquids. Acoustic cavitation refers to the formation of small bubbles and their subsequent growth and collapse within a liquid due to acoustic excitation. Wong and Chon [3] measured the effect of ultrasonic vibrations on boiling heat transfer and showed that the effect became negligible in fully developed nucleate boiling regime. Recently, Zhou and Liu [4] systematically investigated the effect of sound source intensity, distance, vibration location and fluid subcooling on boiling heat transfer on a horizontal circular copper tube and found that the boiling heat transfer could be enhanced or reduced with different sound source intensities. However, they gave only a mechanistic explanation for the effects. Subsequently, Zhou and Liu [5] examined the effects of acoustical field parameters and nanometer size particles on the boiling heat transfer of acetone and confirmed that the heat transfer could be enhanced or reduced by the generation of cavitation bubbles and the addition of the particles.

The major aim of the present research is to describe the mechanism for the effect of acoustic cavitation on boiling heat transfer.

Section snippets

Experimental apparatus and methods

The experimental apparatus and instrumentation are shown schematically in Fig. 1. The test chamber consisted of a cubical vessel made of stainless steel with inside dimensions of 200mm×200mm×230 mm. To reduce heat loss, five sides of the vessel except the up side were covered with insulations. Two viewing windows were installed on opposite sides of the vessel for easy observation and take pictures of the test section. A horizontal copper tube with an electrical heating element inside passed

Mechanism of liquid cavitation and boiling

Cavitation in liquids may be caused due to vortices, ultrasonic excitation or pulsed heating lasers [6]. When the ultrasonic waves excite the liquid molecules, the negative pressure peak −Pm of the sound wave may essentially counteract the liquid static pressure Pl to form a very lower pressure region if the amplitude of the alternating sound pressure Pm is larger than the liquid static pressure. The liquid will expand to form a cavitation bubble when the negative pressure PlPm is large enough

Theoretical study of liquid cavitation

Micro gas bubbles are a prerequisite to liquid cavitation. The bubbles can be tiny air or vapor bubbles attached to solid impurities, dust or surfaces and cracks or they can be tiny dissolved gas regions resulting from decreased liquid pressure due to asymmetrical molecular arrangement. Analysis of the bubble equilibrium radius and its stability has shown that the liquid pressure was closely related with the incipient embryo radius R0 in the liquid. Larger incipient embryo radii imply lower

Conclusions

The relation among the micro bubble radii, the liquid cavitation and the boiling was analyzed theoretically and boiling heat transfer on a horizontal circular copper tube was experimentally investigated to clarify the acoustic cavitation effect and its mechanism. The most important findings of this study are

  • (1)

    Cavitation and boiling processes both involve the disruption of the liquid pressure, but in different way. Cavitation is accompanied by the production of cavitation bubbles, while boiling

Acknowledgements

This project was financially supported by the National Key Basic Research Science Foundation of China (grant no. G2000026305).

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