Experimental study on influence of oscillatory heat flux on heat transfer of R-134a time-periodic subcooled flow boiling in annular duct

Abstract

The present study pertains to the experimental study on the R-134a subcooled flow boiling stemming from oscillatory heat flux in narrow annuli oriented horizontally with focus on heat transfer characteristics. The test section used in the experiment is composed of a Pyrex glass tube, which is completely designed to measure the flow boiling heat transfer coefficient (FBHTC). In this experiment, the annular gap of the horizontal duct is in the range of 1–5 mm with governing parameters used for practical applications in air conditioning and refrigerating devices. Particularly, the focus is on the effects of heat flux oscillation mean value (q‾), amplitude (Δq) and period (tp) on the heat transfer features of subcooled flow boiling. The experimental results show that the obvious temperature undershoot takes place during onset of nucleate boiling (ONB) for R-134a oscillating subcooled flow boiling. Increasing either Δq or tp has relatively little effect on the boiling curve and FBHTC at a given heat flux, while the annular gap plays an important role. Moreover, increasing either the amplitude or period of incident heat flux enhances the wall temperature and FBHTC; while an increase in mean heat flux tends to enhance the wall temperature, but has negligible effect on FBHTC for the flow boiling. Then, a flow pattern map is provided to describe the boundary that distinguishes the different boiling modes of the oscillating heat flux to the subcooled R-134a flow. Finally, a parametric analysis is carried out to further understand the effects of Δq and tp on the heating wall temperature.