Two-phase convective heat transfer in miniature pipes under normal and microgravity conditions

Detailed numerical simulations have been performed to study the effect of flow orientation with respect to gravity on two-phase flow heat transfer (without phase change) in small diameter pipes. The Nusselt number distribution shows that the bubbly, slug, and slugtrain regimes transport as couch as three to four times more heat from the tube wall to the bulk flow than pure water flow. The flow blockage effect of the inclusions results in a circulating liquid flow superimposed on the mean flow. For upflow, the breakup into bubbles/slugs occurs earlier and at a higher frequency. The average Nusselt numbers are not significantly affected by the flow orientation with respect to gravity. A mechanistic heat transfer model based on frequency and length scale of inclusions is also presented.