Experimental study of sound emission in subcooled pool boiling on a small heating surface

In this study, sound emission in subcooled pool boiling with a small plate heater was investigated experimentally. Acoustic signals were recorded using a hydrophone and analyzed using sound pressure level (SPL), fast Fourier transform, and discrete wavelet transform (DWT). The results show that the SPL combined with an analysis of the sound signal by DWT can be used to identify boiling modes and heat transfer intensification. In contrast to those for a large plate heater or long wire, the plot of the SPL against wall superheat in nucleate boiling region exhibits an 'N” shape. Visualization results indicate that this difference mainly originates from the effects of heater size on bubble growth and departure. Compared with the nucleate boiling, in the region of microbubble emission boiling (MEB), the SPL exhibits a step increase of 10-20 dB, and continues to increase with the further increase of the heat flux. A comparison of the bubble frequency and features of the acoustic signals shows that the loud boiling sound in the MEB could be mainly attributed to the vapor film collapse and vapor volume oscillations, which also significantly affect the heat transfer process.