Boundary Layer Energy Transport in Plasma Devices

Abstract

The purpose of this research was to develop a model of boundary-layer energy transport in electric launchers, and perform a numerical simulation to investigate the influence of turbulence, thermal radiation and ablation on energy flux to plasma-facing surfaces. The model combines boundary-layer conservation equations with a k-omega turbulence model and multi-group radiation transport, and uses plasma models for fluid properties such as viscosity, thermal conductivity and specific heat capacity. The resulting TURBFIRE computer code is the most comprehensive simulation to date of boundary-layer turbulence and radiation transport in electric launcher plasmas.

TURBFIRE was run for cases with and without ablation. Temperature and velocity profiles are presented for all code runs, as are values of heat flux to the wall. The results indicate that both radiation transport and turbulence are important mechanisms of energy transport in the boundary layer, and therefore that both should be modeled in future simulations. Additionally, heat flux to the wall via both conduction and radiation was found to be significant for all cases run. Other authors have theorized that conduction could be neglected, but the current results show that this is not the case near the wall.

This research is also novel for its advances in computational fluid dynamics (CFD). The energy equation was written in terms of internal energy and discretized in a manner more implicit than in typical CFD codes. These changes were necessary to enable the code to accurately calculate heat capacity, which changes greatly with temperature for even weakly-ionized plasmas. Additionally, zero-gradient boundary conditions were used at the free stream for the turbulent kinetic energy and its dissipation rate (k and omega). Experimentally determined freestream values of k and omega are typically used in CFD codes, but these data are not available for most plasma devices.