An experimental investigation of the distribution function for the near free molecular parallel plate heat transfer problem

Measurements of the molecular velocity distribution function between differentially heated parallel plates were obtained for Knudsen numbers in the transition flow range. The gas used was argon and the plate temperature ratio was 1.7. The specific distribution in terms of one velocity component, obtained by measuring the Doppler broadening of the 4609.6A AII spectral line emitted as a result of excitation of the gas by an electron beam. Various plate spacing and pressure combinations were used to obtain Knudsen numbers between 0.07 and 3. The eight moment linearized model of Gross and Ziering provided the best overall representation of the date, with the four moment linearized model of Gross and Ziering and the biomodal solution of Lees predicting a substantially greater degree of asymetry in the distribution function than was observed at low Knudsen numbers. The good match between line data and the eight moment method indicates that the range of validity of linearized solutions may be greater than would be expected based solely on the linearizing condition.