Visual Study of Flow Pattern Evolution of Flow Boiling in a Microtube

Abstract

High-speed photography was applied to visualize the flow pattern evolution of flow boiling of liquid nitrogen in an upward quartz microtube coated with a layer of transparent indium tin oxide film as the heater. The inner diameter of the employed tube was about 1.33 mm. The characteristic of nucleation site activation, which was the beginning of the flow pattern evolution, was studied visually. In the case of low heat flux and small mass flux, only a single nucleation site was activated. The departed bubbles grew to the size of the inner diameter of the tube soon and the transition from the bubbly flow to slug and annular flow occurred earlier compared to the conventional normal-sized tubes. In the case of high mass flux and high subcooling, multiple nucleation sites were activated. The interaction between adjacent nucleation sites was considered. Slug and annular flow were found to suppress the downstream nucleation site. Bubble coalescence was one of the key factors for the flow pattern evolution, and it was found that the bubble coalescence would bring about great disturbance to the adjacent nucleation site. The characteristics of bubble condensation in the subcooled liquid nitrogen were also demonstrated. Flow pattern evolution beyond the boiling crisis was also investigated. Post-dryout regimes such as inverted bubbly, inverted slug, and inverted annular flow were observed in the microtube. Flow reversal and liquid entrainment, which were relevant to flow instability in the flow pattern evolution, were demonstrated clearly.

Acknowledgments

This research is supported by the National Natural Science Foundation of China under contract 50776057 and the NSFC-JSPS co-operative project under contract 50911140104.

Xin Fu is a Ph.D. student at Shanghai Jiao Tong University. He received his M.S. degree from Shanghai Jiao Tong University, China, in 2008. He is currently working on the visualization study of two-phase flow in microchannels at cryogenic temperature range.

Peng Zhang is a professor of mechanical engineering at Shanghai Jiao Tong University, China. He received his Ph.D. degree in 1999. His research interests are energy storage, transportation, upgrading, and related applications, thermal insulation, and heat transfer at low temperatures. He has co-authored more than 100 referred journal and conference papers. He is a member of the Academic Panel of the Chinese Association of Refrigeration and Cryogenic Committee. He serves as a board member of The Open Superconductors Journal and The Open Renewable Energy Journal. He was recognized with a CEC-ICMC Meritorious Student Paper Award (1999), National Excellent Ph.D. Dissertation Award (2002), and Young Investigator Award of the Chinese Association of Refrigeration (2007), and was honored as a JSPS Postdoctoral Research Fellow (2002) and Danfoss Honoring Professor (2005).

Caojin Huang is an M.S. student at Shanghai Jiao Tong University, China. He received his bachelor's degree in power engineering from Zhengzhou University, Henan, in 2007. He is currently working on the maldistribution of compact heat exchangers at cryogenic temperature range.

Ruzhu Wang is a professor of mechanical engineering at Shanghai Jiao Tong University, China. He graduated from Shanghai Jiao Tong University in 1984 and 1987 with his bachelor's and master's degrees, respectively. He received his Ph.D. from Shanghai Jiao Tong University in 1990 in the field of refrigeration and cryogenics. He has been the director of Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, since 1993. He has published about 300 journal papers; about 180 of them are in international journals. He has written five books on refrigeration technologies. His major contributions are adsorption refrigeration, heat transfer to superfluid helium, heat pumps, CCHPs, and solar energy systems. He was elected as CheungKong Chaired Professor in 2000 by the Ministry of Education of China. He is an associate editor of Energy—The International Journal, Solar Energy, Chinese Journal of Solar Energy (Acta Solaries Sinica), and Chinese Journal of Refrigeration. He is also an editorial board member of International Journal of Refrigeration, Applied Thermal Engineering, and Energy Conversion and Management. He was recognized as one of the top 100 professors in China Higher Education.