Analytical modeling of microwave absorption in a flowing gas

A two-dimensional model is developed for the absorption of microwave energy in a flowing gas. The flowfield treated is a helium flow through a sphere-cylinder combination. The diameter of the sphere is used as a parameter and ranges from the same diameter as the cylinder up to four times the size of the cylinder, the latter geometry corresponding to the experimental configuration. The plasma discharge is formed at the center of the sphere except for the straight cylinder case where it forms at the end of the cylinder through which the microwaves are introduced. The computational model solves the coupled Havier-Stokes and Maxwell equations. Because the higher TMpi2 mode is used, the Maxwell equations are solved in time-dependent form rather than as an harmonic in time. The predictions are in good quantitative agreement with available experimental data The size and location of the plasma are properly predicted, as its relative movement with changes in Reynolds rmmber. The peak temperatures also agree well with experimental measurements. The one experimental observation that is not properly predicted is the minimum threshold power needed to hold the plasma. Hie computations predict a higher threshold. Reasons for this discrepancy are discussed. The calculations also show that the experiments straddle the Froude number range for which the dowrward-f lowing helium contains a recirculation region. Recirculation is noted at low velocities, but not at the higher ones.