Feasibility of macroscopic parameters for NS to DSMC solver switching in micronozzle simulations

Abstract

Enhancing the design and performance of micronozzles could lead to novel applications and advancements in propulsion systems, making the exploration of micronozzles crucial for the future. This paper critically examines the feasibility of utilizing macroscopic property-based Kn as indicator for defining the breakdown region during the transition from the NS solver to the DSMC solver in micronozzle simulations. The aim is to specify a parameter that can be calculated from both NS and DSMC simulations, making it suitable for implementation in hybrid simulations that dynamically switch between the two solvers. The results show that the density-based Kn accurately represents the continuum breakdown, and it exhibits an earlier breakdown compared to pressure and temperature-based Kn values. The study also analyzes the rarefaction effects and introduces the rarefaction parameter (RP), quantifying the increase in Kn for a unit change in the non-dimensionalized distance. The findings demonstrate that at very low exit pressures, the rarefaction effects increase rapidly as the flow moves towards the nozzle exit, leading to a transition from the continuum to the rarefied regime. The hybrid NS-DSMC simulations show good agreement with experimental data, validating the proposed approach. Additionally, the research examines the effect of back pressure on the RP and identifies the transition regime based on the slope of the RP curve. Therefore, the manuscript provides detailed insights into novel elements, such as the quantification of rarefaction within the nozzle using the RP, the classification of the nozzle into different regimes (continuum, slip, and transition), the definition of an easily obtainable parameter for switching between NS and DSMC methods, and an examination of the contributions of the shear stress term and heat addition term to non-equilibrium conditions.