Experimental investigation of inlet subcooling effects on flow boiling in an open microchannel under hydrostatic inlet-head control

Open microchannels are a promising configuration for flow boiling because the top gap facilitates vapor evacuation and liquid replenishment, thereby improving thermal performance and moderating flow instabilities. However, their behavior under different inlet subcooling conditions remains insufficiently understood under hydrostatically controlled inlet-head operation, particularly when water is used as the working fluid. Deionized water flow boiling was experimentally investigated in an open microchannel heatsink featuring a 100 μm top gap and microstructures with an aspect ratio of 15 and a length of 50 mm. Experiments were conducted at four prescribed hydrostatic heads (from 45 to 85 cm) at three different inlet temperatures: 60, 75 and 90 ºC. Quantitative thermohydraulic measurements were complemented by high-speed flow visualization. Raising inlet temperature from 60 to 90 ºC reduced the heat flux at the onset of nucleate boiling (ONB) by 73–79%, whereas the critical heat flux (CHF) decreased by only 19–34%. Consequently, the two-phase operating window widened and was largest at 90 ºC. The lowest subcooling condition yielded the highest heat transfer performance, with maximum heat transfer coefficients of 12,847–23,686 W/m 2 ·K under surge-like vapor motion, but also the sharpest pre-CHF deterioration. This wider operating window incurred hydraulic and dynamic penalties: pressure drop increased by up to 335% from ONB to CHF and, at 102 W/cm 2 under the highest hydrostatic-head condition, pressure drop oscillation severity increased by 169%. Increasing hydrostatic head from 45 to 85 cm more than doubled the ONB flow rate, whereas lower subcooling accelerated flow rate depletion and fluctuations toward CHF.