Thermal Transpiration Based Propulsion

Abstract

Thermal transpiration based propulsion is studied. Thermal transpiration describes flowing of the gas through a narrow channel with an imposed temperature gradient. As gas flows from the cold to hot side in the chamber, a pressure gradient is created across the channel induced by the temperature gradient. Between the two sides of the chamber an aerogel substance, which functions as an excellent insulator, is used as a thermal transpiration membrane and allows gas diffuse to the hot chamber. The induced pressure gradient is the driving factor in the propulsion of air, or any gas, into the chamber and through the porous membrane. The use of a porous substance such as aerogel as the transpiration membrane and a pressure gradient served as the two requirements in order to successfully achieve thermal transpiration. The gas diffusion through the aerogel transpiration membrane indicates that the average pore size of the aerogel must be comparable with the free path of the molecules. This concept can be taken further if the outlet chamber served as a combustion reactor. The flowing gas is motivated by the heat produced from the combustion process. Along with the exceptionally low thermal conductivity of the aerogel, the gas flow permits the propulsion device to be self-sustaining. The implications of providing a self-sustaining heat source signify that no external electrical heating is required. The effectiveness of this device can be measured as a function of the porous size of the membrane and the temperature difference applied to the system and pressure gradient created.