Acoustical measurements of vibrational relaxation in moist N2 at elevated temperatures

Abstract

Sound absorption measurements were conducted by the resonant tube technique to study vibrational relaxation in moist N2 at 301°, 343°, and 387 °K. Analysis of the data yields the following results: (1) The improved measurement accuracy at the higher temperatures establishes vibrational–vibrational (V–V) energy transfer as the operative relaxation path for the de-excitation of N2* by H2O beyond the uncertainty of experimental error. (2) The best-fitted temperature dependence of the V–V rate constant is k30 = 5.41×106 exp(−25.3/T1/3 )(atm⋅s)−1, an expression which is consistent with seven independent sets of both acoustical and nonacoustical data. (3) The measured humidity and temperature dependence of the relaxation frequency of N2 in air differs substantially from that specified by the recent ANSI Standard S1.26/ASA23–1978. The N2 contribution causes error, up to a few dB/1000 ft, in the total sound absorption coefficient predicted by both the ANSI Standard and ARP866A.