The influence of vapor bubble sliding on forced convection boiling heat transfer

This paper describes experimental efforts aimed at examining the effect of vapor bubble sliding on forced convection boiling heat transfer. Flow boili ng experiments using FC-87 were conducted for vertical upflow and downflow configurations. Both slightly subcooled single-phase and saturated annular flow boiling were considered. Significantly higher heat transfer rates were measured for vertical upflow than for downflow with the same wall superhea t and slightly subcooled single-phase inlet conditions. This increase in he at transfer is directly attributable to sliding vapor bubbles, which remain attached to the wall during upflow and lift off the wall during downflow D ifferences in the measured upflow and downflow heat transfer rates are not as significant for annular flow boiling, which is due in part to the simila r vapor bubble dynamics which have been observed for upflow and downflow. H eat transfer experiments in single-phase subcooled upflow with air bubble i njection at the heating surface suggest that sliding bubbles enhance the bu lk liquid turbulence at the wall, which contributes significantly to the ma croscale heat transfer: It is concluded from this work that vapor bubble sl iding heat transport can be a significant heat transfer mechanism, and shou ld be considered in the development of mechanistic flow boiling heat transf er models.