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.

Algorithm for capturing primary relaxation processes in excitable gases by two-frequency acoustic measurements

“…Those two values can be synthesized from the measurements of sound absorption and sound speed at two frequencies alone, especially without detecting the gas density. Moreover, since generally only a single significant relaxation process (which corresponds to only one significant peak in the sound absorption curve) is observed in most excitable gases [1][2][3][4][7][8][9][10][11][12][13][14][15][16][17][18], our proposed algorithm can be used to precisely reconstruct the sound absorption and dispersion curves for most cases.…”

“…In the study of acoustic relaxation processes in gases, it is known that the position of the acoustic absorption maximum (or speed dispersion step) along the abscissa (log f /P) is determined by the relaxation time and the height of the absorption maximum by the relaxation strength [1,7,8]. Thus, the acoustic experimental method to investigate molecular relaxation phenomena in excitable gases is generally operated by measuring the frequency of absorption maximum and its height [9][10][11][12][13]. As a result of that, capturing those two characteristic values, i.e.…”

“…We could provide the following suggestions of selecting the two measurement frequencies: firstly, for most polyatomic gases (consisting of more than two atoms) at atmosphere pressure and room temperature, the relaxation frequencies of primary relaxation processes are in the range 1 kHz-1 MHz [1][2][3][4][7][8][9][10][11][12][13][14][15][16][17][18]. Thus, we can choose the two measurement frequencies ( f 2 and f 1 ) within 10-200 kHz to cover those possible primary processes, while for the other main processes out of the range 1 kHz-1 MHz, the algorithm's coverage ability can be easily expanded just by adding a few frequencies.…”

A prototype acoustic gas sensor based on attenuation

“…[1][2][3][4][5][6][7][8][9][10] In his book, Lambert 11 describes various acoustic experimental techniques to measure molecular relaxation.…”