Water flow boiling in large aspect ratio microchannels was experimentally investigated in a rectangular microchannel 300 mu m deep and 6 mm wide; hence, the hydraulic diameter was 571 mu m and the aspect ratio was 20. The tests used mass fluxes of G = 261-961 kg/(m(2) s) and heat fluxes of q '' = 631-987 kW/m(2) with the combined effects on the flow boiling phenomena characterized by the Boiling number at an inlet temperature of 65 degrees C. The results show the flow patterns and the heat transfer and pressure drop characteristics during flow boiling in the large aspect ratio microchannel. Sweeping flow with relatively high heat transfer rates was observed while the strengthening effect of the bubble confinement on the heat transfer did not occur during the subcooled flow boiling in the large aspect ratio microchannel. Nucleate boiling dominated the heat transfer with the regular pressure drop fluctuations detected during the sweeping flow and the churn flow. (C) 2017 Elsevier Ltd. All rights reserved.