Heat transfer of bubbly flows in microchannels at varied aspect ratios and hydraulic diameters

•Subcooled flow boiling sequences in microchannels at high aspect ratios are explored.•Effects of aspect ratio, hydraulic diameter and subcooled degree on thermal- fluid characteristics are studied.•Increasing aspect ratio and inlet subcooled temperature realize better cooling performance.•A decrease in hydraulic diameter can augment thermal/frictional outcomes.•Channel with an aspect ratio of 5.0 achieve coefficient of performance up to 116277.8.