Study of Mass Transfer by Condensation in Humid Air for Life Support Systems

The development of life support artificial environments for crews and plants for long duration space flights is a challenge today. The concept of bio-regenerative life support system (BLSS) imitates a simplified natural environment. Experimental or theoretical modeling of BLSS requires a thorough understanding of natural activities from micro to macro scales, like with plant culture for food production, or CO2/O2 conversion … Moreover, life support in space relies on both the amount of food and atmospheric O2 produced by plants in a confined space. It is well known that, the enhancement of the gas exchange with leaves and the growth of plants are dependent on the organoleptic and the surrounding thermo-physical factors. Insufficient air movement around plants and/or condensation on plant leaves generally limit their growth by decreasing photosynthetic and transpiration rates. Thus, the optimization of a BLSS will require controlling the airflow and coupled gas/liquid transfer at the plant surfaces. Hence, we have developed an experimental setup at 1-g to study the hydrodynamics and the condensation mass flux on specific geometries in controlled environmental conditions. An air-conditioned closed circuit wind tunnel was used to generate a flow of controlled temperature, hygrometry and hydrodynamics. Condensation of humid air on a small size horizontal flat plate was investigated. The temperature of the active surface is controlled and maintained below the dewpoint to induce condensation. The experiments were performed at ambient temperature, with a relative humidity between 35-70% and for a velocity range of 1.0-3.0 m.s -1 . The results lead to the evaluation of the local mass transfer coefficients in various conditions. The goal of this study is to develop a theoretical model to predict condensation mass flux at interfaces to be used in a CFD approach related to a closed environment for space applications: planetary flights, lunar-Mars bases.