Chemical Scavenger Probes in Nonequilibrium Gasdynamics

IRECT, local measurements of atom, free radical, excited molecule, and/or ion concentrations are required in the experimental study of nonequilibriu m flow fields and for calibrating high enthalpy test facilities In attempts to simulate the conditions of hypervelocity flight it is necessary to know whether the test gas composition (e g , population of excited states) is not, in some sense, singular, particularly when an electrical discharge is used to heat the gas Although gas-sampling techniques have been successfully applied to the study of local stable species concentrations both in subsonic and supersonic steady flows,13 rapid heterogeneous and homogeneous leactions in the sampling system have precluded their direct use for unstable species We wish to point out here that this difficulty can frequently be eliminated by introducing a scavenger gas immediately inside the probe The scavengei rapidly and quantitatively reacts with the unstable species in the sampled gas to form one or more stable pioducts, which can then be analyzed downstream by any one of a numbei of conventional techniques The authors have successfully applied this principle in sampling nonequilibiium supersonic sti earns of active nitiogen for both atoms and excited molecules Details of the experimental technique, and the implications of this work to our understanding of the chemistiy of active nitrogen will be found in a forthcoming paper 4 Here we confine our attention to some