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.
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