DIRECT SIMULATION OF AXIALLY-SYMMETRICAL EFFUSIVE FLOW FROM A POROUS WALL INTO A VACUUM

Axially symmetric effusion flows from a circular wall made of porous m aterial such as sintered metal were studied based upon the DSMC method . The effusion flow in the simulation is rapidly accelerated to the so nic speed in the vicinity of the surface of porous wall and thereafter it expands like a supersonic free jet. Virtual stagnation pressure of this expansion flow is much lower than the pressure of the stationary gas inside the wall. Present results show that the distance from the porous surface to the sonic line (or sonic surface) tends to approach a finite value, say x(s)/D approximate to 0.11. Sonic conditions for t he present axially symmetric effusion flows are same as the sonic cond ition for the one-dimensional evaporation problem. Virtual sonic orifi ce of the equivalent expansion flow was located at x/D= -0.2, inside o f the porous wall. Present simulation was carried out under the suspic ion that thermal motion of gas molecules would partially change into a back flow in an effusion flow from a porous wall. No substantial back flow is, however, seen in the present simulation for the hard sphere molecule. Mass flux transferred backward is only (about) 0.2% of the t otal effusion flow for the monatomic gas.