Direct simulation Monte Carlo modeling of condensation in supersonic plume expansions of small polyatomic systems

The condensation phenomenon in free expansion plumes that has been observed in both space and laboratory measurements during the last several decades has a number of important aerospace applications. For example, spaceborne optical systems may be sensitive to optical contamination of their local environment by gases or condensate particles produced by the operation of attitude control system (ACS) jets. To simulate microscopic nucleation, which concerns the formation of clusters and collisions between clusters and monomers, a kinetic approach that may be applied to the direct simulation Monte Carlo (DSMC) method, is developed. The thesis research extends the previous condensation modeling performed in our research group in two important ways. First, we extend our study from the Lennard-Jones gas of argon to the study of non-Lennard Jones gases such as water. Second, we have initiated the modeling of heterogeneous condensation.;To model homogeneous condensation of small, polar molecules, such as water, accurate microscopic cluster models need to be developed. The molecular dynamics (MD) method is used to develop a model of water cluster sizes, cluster-monomer collisions, and sticking probabilities necessary for the study of water homogeneous condensation in a plume expanding to low pressure, space conditions. The MD results are integrated into DSMC simulations of homogeneous condensation in free expansion water plumes and the simulated Rayleigh scattering intensities are compared with the Arnold Engineering Development Center (AEDC) experimental data.;The simulation results show that the nucleation rate is a key factor for accurately modeling condensation phenomenon. We use MD simulations of a free expansion to explore the microscopic mechanisms of water dimer formation and develop collision models required by DSMC. The bimolecular dimer cluster formation mechanism was found to be the main mechanism in expanding flows to vacuum. MD simulations between two water molecules were performed to predict the bimolecular dimer formation probability. The probabilities and post-collisional velocity and energy distributions were then integrated into DSMC simulations of a free expansion of an orifice condensation plume with different chamber stagnation temperatures and pressures. The terminal dimer mole fraction, similar to experiment, was found to decrease with chamber stagnation temperatures and increase linearly with chamber stagnation pressures, which is consistent with a bimolecular nucleation mechanism.;A new heterogeneous condensation model, kinetic-based model of N2 molecules condensing on CO2 nuclei, was developed using MD techniques and was implemented in the DSMC simulation of an expanding heterogeneous condensation flow of a 5% CO2 and 95% N2 mixture experimentally studied at AEDC. Another nitrogen flow for the same expansion conditions was observed to not produce any clusters. It was found that incorporation of the heterogeneous condensation process of N2 molecules condensing on CO2 nuclei causes the average cluster size to increase from 10 (the homogeneous condensation result) to about 2,000. The predicted Rayleigh scattering intensity from the simulation results was found to agree well the AEDC experimental data.;In addition to the condensation studies, the thesis also includes the DSMC modeling of radiation from a side jet, a chemically reacting, three-dimensional plume-atmospheric interaction at high altitudes. Highly nonequilibrium radiation transitions from electronically excited states generated by collisions of atomic oxygen with the reaction control system thruster plumes is modeled for different altitudes and vehicle velocity conditions. The radiation rate is compared for two types of overlay methods and with direct simulation in DSMC. This research enabled the development of the SMILE software in terms of majorant collision frequency schemes, elastic/inelastic collisions and chemical reaction models. It is however outside the scope of the majority of this doctoral research and so is included as Appendix A.