Experimental study of condensation heat transfer of R134a inside the micro-fin tubes at high mass flux

•The annular and intermittent flow regimes all exist inside the micro-fin tube under the experimental conditions.•The liquid film changes from rotational flow to coexistence of rotational flow with horizontal flow with decreasing vapor quality at a constant mass flux.•The ratio of liquid film thickness to fin height can represent the flow mechanism of liquid film.•The proposed correlation shows a better prediction effect on the experimental data at a high mass flux.