Vibrational relaxation in methyl hydrocarbons at high temperatures: Propane, isobutene, isobutane, neopentane, and toluene

Abstract

Vibrational relaxation has been seen in shock waves in propane, isobutene, isobutane, neopentane, and toluene dilute in krypton with the laser-schlieren technique. These experiments cover about 600-2200 K and post-shock pressures from 5 to 29 Torr. The process cannot be resolved in any for T<600 K, or in any for large molecule fraction. All the ultrasonic measurements of relaxation in these at room temperature show characteristic times in the 1-5 ns atm range, corresponding to fewer than five collisions, whereas the relaxation times in the shock waves range from 20 to 200 ns atm, with a clearly defined negative or "inverted" temperature dependence. It would seem the observed slowdown of relaxation with increasing T is simply a consequence of the much increased energy transfer required at high temperature in such large polyatomics when this is combined with a collision efficiency, here interpreted as <DeltaE>down, already so large it cannot much increase. The simple method for the extraction of a <DeltaE>down from relaxation data offered here by consideration of the energy relaxation equation for Evib=0 appears to be original and should prove quite useful in connecting thermal relaxation data to values obtained from spectroscopy and master-equation analyses. Here it is found that the derived <DeltaE>down extrapolate well to room temperature ultrasonic measurements, showing a slight increase with temperature.

Collisional energy transfer in polyatomic molecules at high temperatures: Master equation analysis of vibrational relaxation of shock-heated alkanes

“…The rotational quantum numbers were generated from the thermal distribution specified by the initial rotational temperature T r , and the rotational energies were randomly distributed as angular momenta. Since relaxation of rotational states generally occurs on a much shorter time scale than vibration, it was assumed that the rotational distribution of the molecule instantly relaxed and maintained the thermal distribution during the observation period of the shock experiments [14,28]. Therefore, the rotational temperature, T r , was set to be equal to the bath gas temperature, T.…”

“…Kiefer and co-workers [13,14,[26][27][28] reported a series of vibrational relaxation studies of shock-heated hydrocarbons in krypton baths using LS densitometry. They observed fast vibrational relaxation times of a few tens to hundreds of ns atm [13,14,26], which displayed 'inverted' temperature dependences as a consequence of the large amounts of energy that must be transferred to reach vibrational equilibrium at high temperatures. These relaxation data enable investigation of the collisional energy transfer process at high temperatures that are relevant to combustion modeling.…”

“…This work presents the classical trajectory calculations of the energy transfer in collisions between selected alkanes (ethane, propane, isobutane, and neopentane) and Kr at high temperatures. The calculations were performed to elucidate the validity of the predicted energy transfer parameters by conducting master equation analyses of the vibrational relaxation times and comparing the results with the available experimental data [14,28]. Some comparisons were also made between the computed energy transfer parameters and those obtained from thermal decomposition rate constants [28,29].…”

“…A variety of spectroscopic methods, such as infrared fluorescence [6][7][8], ultraviolet absorption [9,10], kinetically controlled selective ionization [11,12], and laser schlieren (LS) densitometry [13,14], have been E-mail address: a.matsugi@aist.go.jp used to directly or indirectly measure the energy transfer rates of polyatomic molecules in collision with bath gases. These studies revealed that the energy transfer rate depends on the initial energy, bath temperature, and type of third-body collider.…”

Propene concentration sensing for combustion gases using quantum-cascade laser absorption near 11 μm

“…Because of such a disturbance in the profile at early times, it could be entertained that the effect is attributable to vibrational non-equilibrium arising from the passing shock wave. However, for large molecules such as propene, equilibration times are 10-50 ns, which are typical for hydrocarbons [30,31]. Such timescales are four orders of magnitude shorter than the initial 500 μs period, and therefore render vibrational non-equilibrium to not be a problem.…”