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RAINBOW SCHLIEREN-BASED WHOLE FIELD DETERMINATION OF TEMPERATURE AND HEAT TRANSFER RATES AROUND A VAPOR BUBBLE IN ISOLATED NUCLEATE POOL BOILING

Tajinder Singh
Indian Institute of Technology-Bombay

Surya Narayan L
Indian Institute of Technology-Bombay

Atul Srivastava
Department of Mechanical Engineering, Indian Institute of Technology Bombay, Powai – 400076, Mumbai, India

DOI: 10.1615/IHMTC-2017.1010
pages 721-727

Abstract

The experiments reported in this study are concerned with the non-intrusive investigation of the phenomena of the formation of an oscillating vapor bubble under sub-cooled pool boiling conditions. One of the refractive index-based imaging techniques, namely the rainbow schlieren deflectometry has been employed as the diagnostic tool. The rainbow schlieren images obtained have first been qualitatively interpreted in order to understand various subprocesses associated with the formation of the vapor bubble. The whole field two dimensional temperature distributions have been reconstructed from the obtained rainbow schlieren images using Abel inversion. The qualitative analysis of the obtained schlieren images and the reconstructed temperature fields revealed that the bubble oscillations observed under the experimental conditions are primarily driven by the evaporating microlayer region and the condensing apex region of the vapor bubble that is exposed to the surrounding sub-cooled conditions. It was also seen that the oscillating vapor bubble continuously disrupts the boundary layer formed in the vicinity of the heated surface, thereby enhancing the heat transfer rates. Bubble dynamic study of the high speed image sequence recorded involved the determination of the equivalent bubble diameters. The evaporative heat transfer signature of the bubble formation revealed that the amplitude of oscillations grew in magnitude with an increase in the degree of sub-cooling. Fourier analysis of the oscillatory apex position of vapor bubble revealed that the frequency of the observed oscillation is almost independent of the degree of sub-cooling.

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