A proper observation and characterization of wall nucleation phenomena in a forced convective boiling system
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Experimental information regarding fundamental bubble parameters such as bubble departure diameter and bubble departure frequency is crucial for the development and validation of mechanistic models to treat the wall nucleation phenomena in a forced convective boiling system. However, a thorough review of the literature reveals that there are several unidentified issues in the previous experimental works, issues which have never been explicitly addressed despite their possibly critical impact on the experimental results. Also, the experimental efforts are still being made without properly addressing the impact of those issues. Thus, in an effort to reveal such issues and to verify the errors created by not addressing them, we performed a series of systematic experimental investigations to evaluate the impacts of these issues and to subsequently make recommendations. The explored issues include (i) the effect of different measurement views on the observation of the nucleating bubbles; (ii) the effect of applying different visualization recording speeds; and (iii) the effect of the number of experimental observations (i.e., sample size) on the statistics of measured bubble parameters. Then, based on the findings, more appropriate ways of observing and characterizing the fundamental bubble parameters were discussed. Additionally, an image analysis method was developed to efficiently and accurately extract the quantitative information of fundamental bubble parameters from the experimental images of boiling bubbles. The performance of this method was tested in subcooled boiling flow circumstances, and it seems promising as a means of analyzing the vast number of images required to satisfy the high statistical demands to characterize wall nucleation features. The experimental findings and discussions in the present study will provide valuable guidance towards the proper investigation of the wall nucleation phenomena in a subcooled flow boiling system or, more generally, in a forced convective boiling system. 2014 Elsevier Ltd. All rights reserved.
