Thermal transpirational flow in the transitional flow regime

Pressure and mass flow rate due to thermal transpiration in the transitional flow regime is experimentally investigated for a wide range of Knudsen numbers, from 0.06 to 2.9, using a nanoporous membrane. A temperature gradient is created by heating one side of the membrane, and cooling the other side with a heat sink. The largest pressure difference and mass flow rates are found for the smallest pore size, 25 nm. It is found that a pressure difference and mass flow rate can still be observed for the largest pore size studied, 1.2 μm. This demonstrates that pressures due to thermal transpiration may be significant for certain MEMS devices with temperature gradients and sub-micron gaps. Using a fit for the temperature difference across the membrane, the experimental and theoretical results are in agreement.