Hydroxyl Formation Mechanisms and Models in High-Altitude Hypersonic Flows

Abstract

The formation of vibrationally hot OH isexamined for a raree ed e ow about a sphere at 80 and 100 km using the direct simulation Monte Carlo method. Four main processes are considered leading to OH production in the e ow, which include water dissociation, exchange reaction between water and atomic oxygen, and exchange reactions between hydrogen (H and H 2) and oxygen (O 2 and O). The principal mechanism of OH production at 100 km is shown to be the H+O 2 !OH+O reaction, with the maximum OH temperature greater than 5000 K. Water dissociation is found to be the most important source of OH at 80 km, with the maximum vibrational temperature of approximately 2500 K. The molecular dynamics results incorporated into the direct simulation Monte Carlo method areused to simulate waterdissociation by N 2 and arecompared to theresultsobtained by theconventional total collision energy model. The OH spatial distribution along the stagnation line predicted by the molecular dynamics approach is signie cantly different than that obtained with the total collisional energy model. I. Introduction T HE modeling of ultraviolet emissions from hypersonic bow shocks provides an indication of the e owe eld gas properties and compliments the usual hypersonic e ight surface diagnostics of temperature and pressure. Moreover, the prediction of ultraviolet radiation from hypersonic vehicles is an important engineering application relevant to the prediction of passive optical signatures. Despite the strong operational and scientie c reasons for measuring opticalemissionsfromhypersonicvehicles,suchdataarerare.Spectral radiation from high-energy raree ed e ows provides a sensitive metric for the evaluation of energy exchange and chemical reaction models. The bow shock ultraviolet e ight experiments 1 (BSUV) obtained spectra of OH( A 2 6 C i X 2 5) UV radiation between altitudes of 100 and 80 km. These experiments indicate that the OH( A) species formed in raree ed, high-energy e ows have internal energies much greater than that of the bulk e ow. Because there are so few collisions in these e ows, the spectral data may be used to test the ability of theory to model near-nascent distributions of chemically produced radiating species and energy exchange. The UV spectra of OH(A) are well dee ned and originate from a single excited electronic state. Under raree ed conditions, the OH( A) UV spectra are sensitive to the chemical processes involved in OH formation. UV radiation from the OH ( A) system has been studied extensively in the combustion and atmospheric sciences communities. 2 Earlier work 3 demonstrated thesensitivity oftheOHspectrato variationinthevibrationaltemperature.Itwasshownthattheratioofthe peakheightsat2800 ƒ