Improved non-equilibrium film method for the design of high-temperature-glide, mini- and microchannel condensers

High-temperature-glide (i.e., large difference in dew and bubble point temperature) zeotropic mixtures such as ammonia/water have the potential to improve efficiency as new working fluids in advanced energy cycles for heating, cooling and power. Furthermore, the high heat capacity of ammonia/water mixtures makes them particularly attractive for use in compact mini- and microchannel devices. The non-isothermal condensation process of zeotropic mixtures leads to coupled heat and mass transfer resistances in each phase, which are not accounted for by single-component in-tube condensation modeling and correlation techniques. Previous attempts to design zeotropic condensers have relied on use of non-equilibrium film theory or mixture resistance correction factors. The film theory models have been developed with many simplifying assumptions including annular flow, negligible condensate and vapor sensible heat loads, and/or laminar condensate film, while the correction factor approaches do not directly consider mass transfer resistances. In the present study of high-temperature-glide mixtures in small channels, these assumptions are relaxed, and a new design method for mini- and microchannel zeotropic condensers is introduced. The approach is validated with experiments conducted for a range of tube diameters (0.98 < D < 2.16 mm), mass fluxes (50 < G < 200 kg m^-2 s^-1) and mass fractions of ammonia (0.80 < x_bulk < 0.96). The results can be used in the development of compact, highly efficient heat and mass transfer devices.