Numerical investigation of critical heat flux during subcooled flow boiling in a vertical rectangular Mini-channel

The critical heat flux (CHF) of flow boiling in a mini-channel is vital for equipment safety, but the boiling crisis triggering mechanism has been proved unsatisfactory. In this study, the VOSET method is adopted to investigate CHF of subcooled flow boiling in a vertical rectangular mini-channel by considering the conjugated heat transfer and using a reasonable nucleation site density model. Hundred of bubbles are accurately captured, and the evolutions of flow pattern, dry patch, and wall superheating are reproduced. Based on those, the relationship between boiling crisis and dry patch is demonstrated. When the heat flux is lower, some middle-sized isolated bubbles adhere to the wall surface with a dry patch below. This dry patch can be rewetted quickly. Hence, the wall superheating increases slightly along the flow direction and is within control. With the increasing heat flux, the dry patch expands because some big bubbles merge into the elongated bubble, leading to the local heat transfer deterioration, but the local wall superheating rise is still limited due to heat conduction inside the solid wall and the rewetting effect. However, the continuous coalescence of elongated bubbles midstream and downstream causes the appearance of an almost permanent dry patch with a high local wall superheating of 183.04 K near the outlet of the mini-channel when the heat flux rises to 500 kW/m2, which has exceeded the CHF of 375 kW/m2 obtained in this study. This triggering mechanism of the flow boiling crisis is new and different from the traditional ones. Besides, the departure from nucleate boiling (DNB) is found at 325 kW/m2, beyond which the wall superheating increases rapidly with heat flux.