Subcooled forced convection boiling flow measured using high-resolution techniques at the COSMOS-L facility and accompanying CFD simulation employing an interface-tracking scheme

Abstract

This paper describes state-of-the-art experimental measurement techniques and computational fluid dynamics methods applied to subcooled forced convection boiling flow at 2-3 bar, both featuring high spatial- and time-resolution measurements. The objectives are: (i) to provide the measured boiling flow data, e.g. average and RMS profiles of axial and radial velocities and bubble size distributions, and (ii) to evaluate the capabilities of state-of-the-art numerical simulation techniques to model the observed phenomena, by comparison between measured data and numerical prediction. In the experiment, the bubble velocities in the axial and radial directions were measured using laser Doppler anemometry, and the bubble size was estimated by a shadowgraphy method. The simulation results, incorporating an interface capturing technique, showed good agreement with measurement in terms of the wall temperature and axial velocity, but underestimation of the radial velocity, and overestimation of bubble sizes. The paper shows that the discrepancies between measurement and simulation may be traced to: (i) the uncertainties in the nucleation site density model; (ii) too coarse a grid being adopted, which is not able to resolve the thermal boundary layer around the bubbles; and (iii) spurious numerical bubble coalescence, a feature not seen in the experiment. The mechanism of bubble lift-off from the hot surface, without any sliding motion, as observed in the experiment, is discussed in detail, based on the simulation results, and the condensation on the bubble cap as the bubble is ‘sucked’ into the bulk flow is also examined.