Experimental study on R-410A subcooled flow boiling heat transfer and bubble behavior inside horizontal annuli

The present research is concerned with the experimental work on R-410A subcooled flow boiling in horizontal annuli. Firstly, boiling curves and boiling heat transfer coefficients are used to explore effects of input heat flux (q), liquid mass flux (G), inlet refrigerant subcooling (ΔTsub), saturation temperature (Tsat) and annular gap (δ). The results indicate the obvious temperature drop occurs during onset of nucleate boiling (ONB) and increasing G and ΔTsub enlarges the temperature undershoot. With the increasing q, the heat transfer coefficient is raised apparently. Higher ΔTsub and larger δ both lead to lower boiling heat transfer coefficients, but varying G and Tsat has small influence on the boiling heat transfer. Secondly, the visualization of bubble behavior discloses increasing q improves bubble size, formation, and departure. With the increase in G, ΔTsub, Tsat and δ, the bubble growth is suppressed, leading to the reduction in bubble detachment diameter (dp). Moreover, the bubble detachment frequency (fb) and mean density of active nucleation site (Nac) are reduced with either the increase in ΔTsub and δ or the decrease in Tsat. Finally, empirical correlations are proposed for dp, fb, Nac, and total heat flux, which yields satisfactory agreement with our measured data.