Flow boiling heat transfer in mini channel with serrated fins: Experimental investigation and development of new correlation

•The flow foiling heat transfer was experimentally studied in two mini channels with serrated fins.•The influences of channel diameter, heat flux, mass flux and saturation were identified.•Flow patterns during the experiment were observed and analysed.•A new correlation to predict the heat transfer coefficient was proposed. Extensive studies have been conducted on flow boiling heat transfer in mini or micro channels with square and circular configurations. Few investigations, however, have focused on a channel configuration featuring serrated fins. Therefore, the present study experimentally investigated the flow boiling heat transfer characteristics of refrigerant R134a in two mini channels with serrated fins. The equivalent diameters are 1.28 mm with 11 channels and 1.59 mm with 9 channels. Quantitative experiments were conducted under a mass flux of 60–308 kg/(m2.s), heat flux of 6–48 kW/m2, saturation pressure of 0.27–0.45 MPa and vapour quality of 0–1. Flow patterns during flow boiling in the channel were also captured and analysed. Results indicated that in the low vapour quality region, the local heat transfer coefficient increases with both heat flux and saturation pressure. However, the influence of mass flux is related to heat flux. The flow pattern in this region starts with dispersed bubbly flow and develops to vigorous bubbly flow. Slug flow and annular flow are then observed along the flow direction of the refrigerant. In contrast, in the high vapour quality region, the influence of heat flux and saturation pressure seem to vanish and higher refrigerant mass flux corresponds to a greater heat transfer coefficient. The annular flow continues in this region with intermittent dry-out, which leads to a sharp decrease in the heat transfer coefficient. As the existing empirical correlation for calculating flow boiling heat transfer coefficient fails to give a reasonable prediction for the present experimental data, a new correlation was proposed with a mean absolute relation deviation of 15.0% for all 1429 data points. The present experimental data and new proposed correlation can provide meaningful guidance for the design of a mini channel two phase heat exchanger with serrated fins.