Pseudo two-dimensional heat and mass transfer study on randomly packed membrane evaporator

Regarded as the next-generation heat rejection technology applied to extravehicular activity (EVA) spacesuits, membrane evaporator is capable to handle high-flux metabolic heat load of astronauts attributed to the high evaporation latent heat of water. Packed with numerous fine hollow fiber membranes, the membrane evaporator is difficult to fabricate in uniform configuration. Aiming at the random packing configuration, this paper develops a pseudo two-dimensional heat and mass transfer model. The axial dimension is characterized by conservation equations, while the pseudo radial dimension is characterized by probability density function applying Voronoi tessellation method. The proposed model reaches a good agreement with the experiment which is conducted on a prototype membrane evaporator densely and randomly packed with 3800 hollow fiber membranes. Through the numerical simulation, the local heat and mass transfer characteristics are analyzed in corporation with the discussion on the distribution features of temperature, pressure and velocity fields. It is found that there exists an optimal packing density balancing the mass transfer driving force and mass transfer area. Another key finding is that in low-back-pressure scenarios like EVA spacesuit cooling the random packing exerts slight impact on the membrane evaporation performance due to the incomparable permeate-side convective mass transfer resistance to the membrane-side diffusion resistance. In high-back-pressure scenarios like electronics cooling in high-altitude and high-speed aircrafts, however, the channeling in the random packing will lead to the performance degradation particularly at low packing densities as the permeate-side convective mass transfer resistance accounts for an increasing proportion of the overall mass transfer resistance.