Electron Heating in Hypersonic Flows: A New Thermodynamically Consistent Model

Abstract: Accurate prediction of electron temperature ($T_{\rm e}$) in hypersonic non-equilibrium flows is critical, yet hampered by inadequate models for electron heating from vibrationally excited nitrogen. Prior models often relied on ad-hoc scaling or flawed applications of detailed balance that failed to ensure the convergence of electron and vibrational temperatures ($T_{\rm v}$) at thermal equilibrium. This paper introduces a novel, thermodynamically consistent electron heating model derived rigorously from the principle of detailed balance. By assuming a Boltzmann vibrational distribution and employing an effective activation energy, our approach yields a simple heating-to-cooling ratio of $\exp(\theta_{\rm v}/T_{\rm e}-\theta_{\rm v}/T_{\rm v})$, where $\theta_{\rm v}$ is the characteristic vibrational temperature of nitrogen. This formulation guarantees that $T_{\rm e}$ correctly converges to $T_{\rm v}$ at equilibrium. A key advantage is that our model can utilize total cooling rates determined from swarm experiments, leading to higher accuracy at the low electron temperatures typical of hypersonic flight. This results in simulations that better agree with flight-test data, predicting an electron temperature up to six times lower than previous models. These more reliable predictions can significantly enhance the fidelity of plasma modeling for hypersonic technologies, including radio-frequency blackout mitigation, MHD aerocapture, electron transpiration cooling, and electromagnetic shielding.