Analysis of coupled heat and mass transfer during condensation of high temperature glide zeotropic mixtures in small channels

Condensing zeotropic mixtures have the potential to improve the thermodynamic performance of power generating, cooling, and heating systems. Further improvements can be realized by developing equipment using mini- And microchannels. However, it has been shown that the concentration gradients arising from the changing composition of the vapor and liquid phases during condensation introduce additional mass transfer resistances, degrading the overall heat transfer. Furthermore, the coupled heat and mass transfer in mixtures at this scale is not well understood. Results from experiments on condensation of ammonia/water mixtures at varying concentration (80-100% NH3), mass flux (G = 50 - 200 kg m-2 s-1), and tube diameter (D = 0.98-2.16 mm) are reported here. Other researchers have reported an apparent heat transfer coefficient for zeotropic mixtures, which fails to explicitly account for mass transfer. The present work quantifies the liquid film heat transfer coefficient by accounting for mass transfer through a film model approach, which allows the relative significance of the vapor and liquid heat and mass transfer resistances to be quantified.