A computational approach capable of modeling homogeneous condensation in nonequilibrium environments is presented. The approach is based on the direct simulation Monte Carlo (DSMC) method, extended as appropriate to include the most important processes of cluster nucleation and evolution at the microscopic level. The approach uses a recombination-reaction energy-dependent mechanism of the DSMC method for the characterization of dimer formation, and the RRK model for the cluster evaporation. Three-step testing and validation of the model is conducted by (i) comparison of clusterization rates in an equilibrium heat bath with theoretical predictions for argon and water vapor and adjustment of the model parameters, (ii) comparison of the nonequilibrium argon cluster size distributions with experimental data, and (iii) comparison of the nonequilibrium water cluster size distributions with experimental measurements. Reasonable agreement was observed for all three parts of the validation.
We address direct simulation Monte Carlo (DSMC) implementation of phenomenological models of the rotational relaxation process suitable for an arbitrary gas mixture composed of atomic and quantized diatomic species. The macroscopic relaxation process is parametrized by a constant or temperature-dependent collision number Zr such as that of Parker [Phys. Fluids 2, 449 (1959)]. The energy redistribution properties predicted by such a model at the collision level are compared with a recent quasiclassical state-to-state model. Modified forms of the constant collision number, and thus constant relaxation probability, serial quantized Borgnakke–Larsen algorithm [Phys. Fluids A 5, 2278 (1993)] and the null collision SICS-D algorithm [Phys. Fluids A 4, 1782 (1992)] are shown to be equivalent. The generalization to an energy-dependent relaxation probability [Phys. Fluids 6, 4042 (1994)] leads to a systematic bias toward delayed relaxation, due to approximations inherent in the analytical formulation. The error induced in the predicted relaxation behavior as a function of temperature is approximately equivalent in magnitude to a previously proposed, but unrelated, correction factor [Phys. Fluids 6, 2191 (1994)], and also to the variation in the temperature-dependent Parker collision number over a wide range of conditions. Comparisons between DSMC and state-to-state calculations of the rotational distribution function in a relaxing bath quantify the microscopic limitations of the phenomenological model. Finally, a direct comparison of DSMC results with experimental shock layer measurements demonstrates that the energy-dependent relaxation model has a negligible advantage over the constant probability model when the collision number is chosen judiciously.
Collisional quenching of the v′=0 level of the A 2Σ+ state of the OH molecule has been studied for a variety of collision partners. The pressure dependence of time-resolved, laser-induced fluorescence furnishes the quenching cross sections σQ. OH radicals are produced in a microwave discharge or by photolysis of HNO3 at 193 nm, always in sufficient Ar bath to produce a thermal rotational population at 300K in the laser-excited A 2Σ+ state. For Kr and Xe, the σQ are 8 and 27 Å 2, respectively; comparison with a prior study suggests a decrease in σQ with increasing rotational level and/or increasing temperature. σQ (O2)=18 Å 2,and σQ (H2O)=80 Å 2; cross sections were measured for selected freons and butanes also important in tropospheric laser measurements of OH.
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