Numerical Evaluation of Inelastic Mechanisms in a Nitrogen Arcjet

Abstract

Introduction T HE electrons in a low-power arcjet thruster are thermally excited through strong interactions with the electric Ž eld via ohmic heating andmay be out of equilibriumdue to low density and small thrusterdimensions.Numericalevaluationof suchnonequilibrium states is not only important for accurate prediction of thruster performance but also of interest from the viewpoint of  uid dynamics. For a propellant composed of molecular species, the rate of electron energy loss due to inelastic collisions with molecules can be of many orders of magnitude larger than that due to elastic collisions.One common approach taking into account these effects is to multiply the collision frequencies for electron–molecule encounters by the inelastic loss factor ±, but there exist considerable uncertaintiesin determiningthe valueof ±, whichwas assumed to be constant in Refs. 1 and 2. In fact, inelasticmechanismsare expected to be strongly nonlinearwith respect to electron energy distribution as pointed out in Ref. 3. This Note attempts to evaluate the degree of the electron– molecule inelastic effects for a nitrogen arcjet by consistently accounting for each energy transfer mechanism, not assuming a constant ±. This is essentially the same approach as the work of Megli et al. in the sense that the ± value can be evaluated from the solution obtained, but emphasis is placed on the detailed description of the different mechanisms involved and on the determination of the relative importance among them.