Comparing the Chemical Models by Fan-Shen and by Bird through a 2D DSMC Code

The present work is the logical continuation of a paper by Zuppardi and Romano where the models by Bird and by Fan and Shen, evaluating the occurrence of a chemical reaction in a collision between two molecules, were compared in a Direct Simulation Monte Carlo (DSMC) process. The main difference between the models is in handling molecular vibration; the Bird model considers the vibrational energy only as a contribution to the collision energy, the Fan-Shen model considers a physical link between vibrational excitation and dissociation/exchange reactions. In order to make comparable the two models, a parameter similar to the steric factor by Bird, was introduced into the DSMC implementation of the Fan-Shen model. A “simple” DSMC code, computing just the occurrence of a reaction in the collision between two molecules, was written for that purpose. Computer tests considered three dissociation and two exchange reactions in air. The results verified that the direct influence of vibration increases the reactivity level. In order to get over the limitations of the code and therefore to provide a more precise analysis, in the present work both models have been implemented in a 2D DSMC code, allowing the simulation of the flow field on a flat plate and therefore making possible an evaluation of the influence of a different chemical composition (due to the different chemical models) on the aerodynamic parameters both along the surface and in the flow field. The present tests consider the dissociation reactions of oxygen and nitrogen. According to what already found, the reactivity level of the Fan-Shen model verifies to be greater than the one of the Bird model. No significant influence of the different gas composition is found on the aerodynamic parameters along the flat plate (heat flux, pressure and shear stress). On the contrary, a significant influence is found on the flow field parameters (Mach number, rotational and vibrational temperatures).