The boiling crisis of water under exponentially escalating heat inputs in subcooled flow boiling at atmospheric pressure

We investigated the boiling crisis of water under exponentially escalating heat inputs using a specially-designed experimental apparatus featuring high-speed infrared thermography to measure the time-dependent temperature and heat flux distributions on the boiling surface, and high-speed video diagnostics to image the bubble growth process. We conducted flow boiling experiments with water at atmospheric pressure on a plate-type heater installed in a 3 x 1 cm(2) cross section channel. We tested various values of flow velocity (corresponding to Reynolds numbers from 0 to 35,000), water subcooling (from 10 to 75 degrees C) and rate of power rise (corresponding to exponential power escalation periods, tau, from 1.5 to 500 ms). At long periods critical heat flux (CHF) is independent of the power escalation period and the boiling processes appear to be physically similar to steady state. As the period decreases, the CHF values tend to increase following an asymptotic 1/root tau trend. The mechanism that determines this trend depends on subcooling. For high subcooling, CHF monotonically increases as the period decreases, and the DNB occurs through a fully developed nucleate boiling process. By contrast, for low subcooling, CHF first increases as observed for high subcoolings, then decreases, and finally increases again as the period decreases. We observe that such non-monotonic transition is due to a change in the boiling crisis mechanism. Specifically, for very short periods and low subcooling, the boiling crisis happens during the growth of the very first generation of bubbles, which never detach from the heated surface, shortly after the onset of nucleate boiling. In such cases, fully-developed boiling is not achieved. (C) 2020 Elsevier Ltd. All rights reserved.