Heat transfer enhancement with laminar liquid-gas slug flows

This paper investigates a two-phase non-boiling slug flow regime for the purposes of enhancing heat transfer in microchannel heat sinks or compact heat exchangers. The primary focus is upon understanding the mechanisms leading to enhanced heat transfer and also the effect of utilizing different Prandtl number fluids. Experiments were conducted using Infrared thermography and results presented in terms of Nusselt number versus inverse Graetz parameter. These results spanned both the thermal entrance and fully developed flow regions. Nusselt numbers enhancements were observed throughout when data was reduced to account for void fraction. The maximum heat transfer rates observed were up to an order of magnitude greater than those in equivalent single phase flows. However, the extent of enhancement observed was strongly dependent on ratio of slug length to channel dimension with shorter liquid slugs providing optimum performance. It was also highlighted that the thermal entrance region for the slug flow regime analyzed was independent of flow Reynolds number and instead was characterized by the liquid slug length alone. This was verified through Nusselt number measurements over inverse Graetz number ranges from 10^-4 to 100 and slug length to channel diameter ratio from 1 to 32. The results obtained also highlight some interesting variations between the transitions from entrance to fully developed flow when using different Prandtl number fluids. Low Prandtl number fluids show a strong oscillation in heat transfer rates resulting from an internal circulation within liquid slugs however, these diminished significantly when the high Prandtl number fluids were employed. Overall, this study highlights the mechanisms which offer significantly heat transfer enhancements in heat exchange devices employing two-phase gas-liquid flows without boiling.