Experimentally investigating effects of gap size and injection flowrate on heat transfer and boiling characteristics for the downward facing wall heating

The majority of this paper is to investigate the heat transfer and boiling characteristics for the downward facing heating via the experiments. The sensitivity studies of different gap distance and injection flowrate are also considered. These characteristics include temperature histories for all the cases studied, snapshots of bubble transport and heat transfer characteristics, relationship of Nusselt (Nu) number and temperature difference, and boiling curves. The detailed descriptions of boiling curves to investigate the parametric effects are seldom revealed in the previous researches. It can be clearly revealed in the temperature histories and boiling curves that there are several different patterns of heat transfer and boiling characteristics, namely single-phase flow, bubble nucleate boiling, and film boiling regimes. In the single-phase flow regime, the heat transfer is elevated as the injection flowrate increases or the gap distance is reduced. However, the gap effect is significant only for the higher injection flowrate. Based on the measured data, the heat transfer in the nucleate boiling for the downward facing heating can be enhanced by increasing the upward injection flowrate, which is different from the normal upward facing heating. The similar enhancement effect of injection flowrate is also revealed in the stable film boiling for the downward facing heating. •Boiling heat transfer for downward facing heating is studied experimentally.•Effects of gap distance and injection flowrate are considered.•Main measured data include temperature history, boiling curve and Nu vs ΔTin.•Single-phase heat transfer is enhanced by increasing injection flowrate.•Gap effect on boiling heat transfer is significant for higher injection flowrate.