Experimental and numerical study on membrane evaporator applied to extravehicular activity spacesuit cooling

Membrane evaporator is expected to replace the conventional sublimator that has been in use since the Apollo program and emerge as the next-generation cooling technology for extravehicular activity (EVA) spacesuit. In the membrane evaporator, the hydrophobic micro-porous membranes allow the water vapor evaporated from the chilled water to transport through towards the low-pressure space, realizing high heat rejection capacity attributed to the considerable latent heat. The membrane evaporator has been experimentally studied for many years and even launched to International Space Station (ISS) for on-orbit test recently. However, there is still a lack of thorough theoretical studies capable of predicting the membrane evaporation performance precisely in a wide range of operating conditions. The existing models have the over-prediction problem universally. Based on the model validation by an experimental study on a hollow fiber membrane evaporator, this paper proposes a segmented model capable of capturing the local heat and mass details along the flow direction. Numerical simulation reveals the coupled relationship among heat transfer, mass transfer and flow friction. The extremely high flow velocity appearing in the permeate side results in a considerable pressure drop, which reduces the mass transfer driving force and therefore inhibits the membrane evaporation in turn. Through analyzing the local heat and mass transfer characteristics, the effects of the operating conditions, structural parameters and flow configurations are discussed in detail. It is particularly worth mentioning that highly dense packing of membranes may not be beneficial to the mass transfer. Instead, there exists an optimal membrane packing density. Finally, the critical icing conditions are determined to find a safe boundary for practical operation.