Comparison of simulations using quantum and continuous models for chemistry and energy exchange

Abstract

777/5 paper reports on a preliminary study to introduce quantised vibrational energy exchange and reaction probability models into a DSMC method. Results from the quantised models are compared with those based on continuous assumptions. The models, now implemented in the Sandia Icarus code, are the continuous Bird/TCE (which has neither preferential disposal nor sensitivity of reaction rate to internal energy) , the continuous Maximum ~Entropy (which has both preferential disposal and rate sensitivity), and the quantised Semi-Quantum Maximum Entropy or SQME model. The latter approach was developed in order to introduce a state specific capability that allows more accurate modelling of the preferential reaction probability and the nonequilibrium disposal of energy post collision exhibited by reactive collisions. This is a more realistic representation of chemical behaviour not fully catered for in the continuous methods. The conditions in the LENS high enthalpy experiment were used as the basis for the case study. The LENS experiment was chosen because it provides a well characterised uncomplicated reaction set. The study has shown that there are indeed differences in the chemical behaviour observed when using the continuous and quantised approaches. This would be expected due to the close coupling between the vibrational quantum levels and the probability of reaction in the quantised method. * Senior Member of the AIAA ** Member AIAA This paper is declared a work of the US government and is not subject to copyright Introduction The chemical models used in particle simulation codes must represent accurately a wide range of chemical behaviour. For some reactions there is dependence of chemical reaction probability on the internal energy of the reactant molecules and a similarly non-Boltzmann distribution of internal energy after collision. The role of vibrational energy is particularly important since it is known to enhance or depress the chemical rate of certain reactions compared with what may l>e expected on purely equilibrium statistical grounds. When considering reacting systems, the products of a collision may be involved subsequently in a chemical reaction and this means that for both reactive and non-reactive collisions modelling of the rate and amount of vibrational energy exchanged is very important. In many cases there are sufficient non-reacting collisions to ensure that the reactant molecules may be well represented by sampling from equilibrium populations. However, there are also many physically occurring situations where this is not the case. Reacting rarefied gases, rapidly changing flow fields at high temperature, chemistry on or near hot surfaces and chemistry that occurs due to absorption of radiation at low temperature (such as in the atmosphere) are all examples where equilibrium based methods would give misleading results. Figure 1 shows an example of the non-equilibrium disposal of energy after a chemical reaction. Molecular beam data can be used to show the effects of a reactive collision on the internal energies of the product molecules and the figure shows the results of a molecular beam experiment studying products of the direct exchange reaction HI + Cl -> HC1 +1, reproduced from reference 1. This is a fairly common displacement type reaction with low activation energy. It can be seen that the distribution of energies in the vibrational Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc.