Non-Uniform Onset of Nucleate Flow Boiling of R-134a Inside a Glass Minichannel

Abstract

The present work is an experimental investigation of the incipient boiling of R134a inside a circular glass minichannel mounted horizontally and equipped with a series of transparent indium tin oxide heaters. The effects of heat flux input levels and refrigerant mass fluxes on the onset nucleate boiling process and on the saturated boiling heat transfer rate are quantitatively explored. The flow pattern visualizations, carried on by means of a high-speed camera, show that the nucleation process is oddly non-uniform: the first vapor bubbles are always generated on the upper side of the tube and lead to a first wall temperature drop. A further increase in the heat flux values results in an increased wall superheat until bubble nucleation also originates on the lower side of the tube, causing a second wall temperature drop. Finally, at higher heat input levels, the boiling process becomes uniformly distributed on the inner tube surface. This phenomenon occurred also after a 180° rotation of the glass tube, and, after a critical analysis of the potential origins, it remains presently unexplained. An evaluation of heat transfer coefficients for low vapor quality regimes is finally presented.

Keywords:

• flow boiling
• onset nucleate boiling
• flow pattern
• HFC134a refrigerant

Acknowledgments

The work was financed by Italian Ministry of Universities through project PRIN 2009 “Experimental and Numerical Analysis of Two-Phase Phenomena in Microchannel Flows for Ground and Space Applications.” The authors would like to acknowledge Dr. Stefano Dall'Olio for the experimental set-up in Dalmine and Dr. Stefano Zinna and Eng. Antonello Cattide for their help and discussions.

Notes

Color versions of one or more figures in this article can be found online at www.tandfonline.com/ueht