Characterization of Benzene Laser-Induced Nonthermal Equilibrium via Nitric Oxide Laser Induced Fluorescence Temperature Measurements

Abstract

A benzene Laser Induced Non-thermal Equilibrium (LINE) technique was characterized for utilization in N₂ supersonic and hypersonic gas flow fields. Characterization of a LINE technique, toward tuning of the rate, magnitude, and geometry of a Collisional Energy Transfer (CET) perturbation to or from the bath is desirable because CET rate has been shown to be coupled with macroscopic flow field properties like turbulent fluctuations in velocity. Therefore, the creation of a tunable LINE technique will allow for testing of the coupling of Non-Thermal Equilibrium (NTE) and turbulence in canonical flow fields in order to create models that can help design more efficient supersonic and hypersonic vehicles. To characterize benzene LINE in this dissertation the relaxation of highly vibrationally excited benzene, generated by pulsed UV laser excitation, was studied using the transient rotational-translational temperature rise of the N₂ bath, which was measured by proxy using two-line Laser Induced Fluorescence (LIF) of seeded NO. The resulting experimentally measured time-dependent N₂ temperature rises were modeled with MultiWell based simulations of CET from benzene Vibration to N₂ Rotation-Translation (V-RT).

In flow fields above room temperature we find that the average energy transferred in benzene deactivating collisions depends linearly on the internal energy of the excited benzene molecules and depends approximately linearly on the N₂ bath temperature between 300 K and 600 K. The results are consistent with experimental studies and classical trajectory calculations of CET in similar systems. In low temperature flow fields (140 K-300 K) the CET rate was found to have an inverse temperature dependence which may indicate the turning on of a new CET pathway at low temperatures. Since very little work on the relaxation of highly vibrationally excited molecules at temperatures below ~250 K has been done in the past, more experiments and simulations are needed to determine the mechanism of the increase in CET rate at low temperature.

A benzene LINE technique has now been characterized over a diverse range of temperatures. It may now be utilized to generate perturbations in supersonic and hypersonic flow fields.