Measurement of contribution of microlayer evaporation applying the microlayer volume change during nucleate pool boiling for water and ethanol

Significance 

Recent studies have confirmed that a liquid film of micrometer thickness normally forms beneath boiling bubbles during the bubbles’ growth process. Consequently, this film, basically termed as microlayer, is very significant in matters relating to boiling and heat transfer due to its intensive evaporation during growth of boiling bubbles. Preceding studies have also highlighted that the vigorous evaporation of the microlayer plays an important role in boiling heat transfer, due to the high thermal conductance of the micron-order thickness of liquid film. As a result, the microlayers’ structures have been measured by many researchers using varying techniques. Interestingly, this topic still attracts attention when studying boiling heat transfer. Nonetheless, the experimental analysis of microlayer evaporation contribution has been limited due to difficulties associated with measurement of evaporation.

In this view, a team of Tianjin University Dr. Yoshio Utaka, Dr. Kang Hu , Dr. Zhihao Chen in collaboration with Dr. Takayuki Morokuma at Yokohama National University demonstrated measurement of contribution of microlayer evaporation applying the microlayer volume change during nucleate pool boiling for water and ethanol. In particular, they used the laser interferometric method to assess the microlayer structure during the nucleate pool boiling for ethanol. Their work is currently published in the research journal, International Journal of Heat and Mass Transfer.

For the experimental work, the laser light with different wavelengths were adopted as the light sources for interferometric observation, and a high-speed camera for capturing images that were crucial during analysis. The microlayer evaporation rate was analyzed based on the change in microlayer volume, which was determined by observing variation of microlayer thickness in one bubble cycle through the images captured by the high-speed camera. All in all, the thickness distribution of the microlayer and its variation were derived on the basis of recorded images of interference fringes.

The authors observed a bended shape (crest-like structure) of the initial microlayer, during nucleate pool boiling for ethanol. In addition, they noted that the microlayer was depleted faster for ethanol than where water was used. This observation was attributed to the fact that ethanol has lower latent heat of vaporization than water. Furthermore, the scholars found out that the microlayer evaporation contributed significantly to the total evaporation, approximately 39% for ethanol and 14–44% for water based on the conditions pertinent to their study.

In summary, the study demonstrated the simultaneous measurement of microlayer thickness and bubble volume during nucleate pool boiling. Generally, the contribution of microlayer evaporation was analyzed on the basis of variations in the two-dimensional shapes of microlayers, and several observations made. Altogether, their work confirmed that the microlayer evaporation contributes significantly to the total evaporation, and the contribution ratio they recorded coincided with those obtained in other similar studies.

About the author

Zhihao Chen is an Associate Professor in School of Mechanical Engineering, Tianjin Univerisity, China. He was graduated from Yokohama National University (YNU) and obtained his PhD Degree in 2011. Then, he continued his research in the same group of Yokohama National University as an Assistant Professor. After that, he came back to Tianjin University and been an Associate Professor from 2015.

His research interests focused on the phase change heat transfer, including boiling and condensation, especially for the microscopic mechanisms of boiling and condensation heat transfer.

His recent achievements include the high speed measurement of microlayer during nucleate boiling by using laser interferometric method, the new technique proposed for enhancing critical heat flux of boiling, the discovery of spontaneous drop movement induced by surface tension gradient during Marangoni condensation.

About the author

Yoshio Utaka is a professor of Mechanical Engineering at Tianjin University, China and professor emeritus of Yokohama National University, visiting professors of Tamagawa University, Kanto-Gakuin University, Open University of Japan and a research fellow at The University of Tokyo, Japan. He received his degrees of M.Eng and D.Eng from The University of Tokyo.

His major research is concerning the heat transfer with phase change (condensation, boiling, solidification and melting) and heat and mass transfer in PEFC, and has resulted in 160 journal papers and 70 international conference papers. JSME (Japan Society of Mechanical Engineers) Medal for Outstanding Paper, academic awards from Heat Transfer Society of Japan (HTSJ) and Japan Society of Refrigerating and Air Conditioning Engineers were received. He is a member of The Engineering Academy of Japan, fellow of JSME, honorary member of JSME and HTSJ and former president of Heat Transfer Society of Japan.

Reference

Yoshio Utaka, Kang Hu, Zhihao Chen, Takayuki Morokuma. Measurement of contribution of microlayer evaporation applying the microlayer volume change during nucleate pool boiling for water and ethanol. International Journal of Heat and Mass Transfer, volume 125 (2018) page 243–247.

Go To International Journal of Heat and Mass Transfer

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