Speaker
Description
Background of the Study
Recent studies in non-equilibrium thermochemical CFD suggest that boundary conditions involving the ejection of reactive ablation gases from the wall can increase chemical heating near the wall at moderate mass fluxes, potentially leading to an increase in heat flux [1]. Furthermore, under high-speed conditions such as hyper-orbital reentry and entry into the atmospheres of outer planets, shock layer radiation becomes prominent in addition to convective heating [2]; therefore, the ablation gas reaction zone may contribute to wall heating as a near-wall gas phase layer accompanied by emission, absorption, and re-radiation. Therefore, in future low-density TPS designs, it is necessary to clarify whether the ejection of ablation gas simply shields the wall surface or increases wall heating through interaction with the high-temperature main flow.
However, in actual ablation tests, the composition, mass flux, and timing of pyrolytic gas generation are simultaneously determined as a result of the material’s internal response; thus, it is difficult to independently isolate the effects of the reaction, luminescence, and radiation fields generated in the gas phase [3]. Furthermore, while arc jet tests are excellent for material evaluation, the main flow pressure and dissociation state differ from those in the actual environment, making it difficult to simultaneously reproduce the gas-phase reaction field that becomes a problem during high-speed reentry at speeds of the order of 10 km/s [3].
Therefore, this study aims to establish a framework for evaluating the effects of ablation gas outflow on the near-wall gas phase by combining thermochemical non-equilibrium CFD with optical measurements, using a known gas outflow isolated from material response.
Methodology
In this study, we focus on a porous carbon leading edge placed in a hypersonic, high-enthalpy flow and simulate ablation gas release by applying a controlled gas outflow from the wall surface. By isolating the outflow conditions from the internal material response, we systematically investigate the effects of gas species, outflow intensity, and outflow timing on the near-wall gas phase.
The experiments were conducted using the MX6 free-piston expansion wave tube at Tokyo Metropolitan University [4]. A high-porosity porous carbon material was placed on the front surface of the test specimen, and gas was introduced from an external supply system via a plenum inside the model. By synchronizing wind tunnel operation with the gas injection system, the start time of gas injection relative to the arrival time of the main flow is controlled. The effects of the supply system pressure, the pressure inside the model, and the main flow conditions are verified, and the luminescence field near the wall is simultaneously acquired. The obtained luminescence distribution is compared with the injection layer structure and the distribution of major chemical species predicted by non-equilibrium thermochemical CFD and used to interpret changes in the near-wall gas phase.
In the numerical analysis, we employed non-equilibrium thermochemical CFD based on the compressible Navier–Stokes equations, introducing outlet boundary conditions that specify the mass flux and gas composition in the direction normal to the wall. In the future, we plan to evaluate the relationship between reactive zone formation and changes in the radiation field in stages by extending the study to reactive model gases and more detailed optical measurements.
Results
An evaluation of the supply system response in a stationary field confirmed that the rise in discharge pressure is strongly influenced by the gas species and the piping and plenum systems. In particular, while the rise in supply pressure and internal model pressure was relatively rapid for lighter gases, gases with higher molecular weights required a longer time to establish pressure. These results indicate that, in order to establish blowout within the short test duration of an expansion wave tube, it is necessary to consider the valve opening lead time, piping volume, and the influence of the internal plenum of the model for each gas species.
In the expansion wave tube tests, conditions with different blowout start times were compared. Under conditions where a shock wave velocity close to the non-blowout threshold could be maintained, there was a tendency for the blowout pressure not to stabilize sufficiently within the test duration. On the other hand, under conditions where blowout overlapped with the test duration, changes appeared in the light emission distribution near the wall, while an effect on the mainstream shock wave velocity was also confirmed. This indicates that, in short-duration, high-enthalpy flow, simultaneously satisfying the conditions for blowout establishment and non-interference with the mainstream is a key constraint in this problem.
Furthermore, under certain blowout conditions, a tendency was observed for the light emission intensity near the porous wall to decrease compared to non-blowout conditions. In thermochemical non-equilibrium CFD simulations as well, a tendency for the oxygen atom distribution near the wall to change due to gas ejection was observed, suggesting a qualitative consistency with the changes in the luminescence field observed experimentally. These results suggest that the near-wall gas layer formed by controlled gas ejection alters the distribution of chemical species and the luminescence field in high-enthalpy flow.
Conclusion
In this study, focusing on the ablation gas blowout problem in low-density TPS, we established a numerical and experimental framework for evaluating near-wall reactions and luminescence fields using a known gas blowout decoupled from the internal material response. We confirmed the response of the outflow supply system, its influence on the main flow in the expansion wave tube, and changes in the luminescence field near the wall. By combining these findings with an interpretation of the near-wall gas layer structure obtained via thermochemical non-equilibrium CFD, we demonstrated the potential for outflow to alter the near-wall gas phase in a high-enthalpy flow.This method is characterized by its ability to independently address the effects of gas composition, mass flux, and ejection timing—factors that are difficult to separate in plasma wind tunnel tests using actual ablators.
In the future, we will improve the compatibility between the blowout conditions and the main flow conditions, and by expanding the use of reactive simulated ablation gas and optical measurements, we will evaluate the formation of the heat-generating zone, chemical reactions, and changes in the luminescence and radiation fields in stages. Through this, we aim to clarify the competition between the blowout shielding effect and the increase in heating mediated by chemical reactions and radiation in low-density TPS.
References
[1] Maxime Lalande, Nicolas Dellinger, Ysolde Prévereaud, and Nathalie Bartoli, “Assessment of Air-Pyrolysis Gas Reactions on the Wall Heat Flux During Atmospheric Re-Entry,” International Journal of Heat and Mass Transfer, Volume 256, Article 127984, 2026. DOI: 10.1016/j.ijheatmasstransfer.2025.127984.
[2] Aaron M. Brandis, Christopher O. Johnston, Brett A. Cruden, and Dinesh K. Prabhu, “Equilibrium Radiative Heating from 9.5 to 15.5 km/s for Earth Atmospheric Entry,” Journal of Thermophysics and Heat Transfer, Volume 31, Issue 1, pages 178–192, 2017. DOI: 10.2514/1.T4878.
[3] Bernd Helber, Alessandro Turchi, James B. Scoggins, Annick Hubin, and Thierry E. Magin, “Experimental Investigation of Ablation and Pyrolysis Processes of Carbon-Phenolic Ablators in Atmospheric Entry Plasmas,” International Journal of Heat and Mass Transfer, Volume 100, pages 810–824, 2016. DOI: 10.1016/j.ijheatmasstransfer.2016.04.072.
[4] Yosuke Kurosaka and Kohei Shimamura, “Microwave Radar Diagnostics of Piston Motion in a Free-Piston-Driven Expansion Tube,” Shock Waves, Volume 34, pages 465–474, 2024. DOI: 10.1007/s00193-024-01194-1.
Summary
This study investigates simulated ablation-gas blowing for low-density thermal protection system studies using a porous carbon leading-edge model in a free-piston expansion tube. A controlled gas injection system was synchronized with the short-duration high-enthalpy flow, and near-wall optical emission was measured to evaluate changes in the gas phase caused by blowing. The results show that the establishment of blowing depends strongly on the gas species and supply-system response, and that certain blowing conditions modify the near-wall emission field while also affecting the main-flow shock velocity. These findings provide a basis for future studies of reactive simulated ablation gases and radiation–chemistry coupling near low-density ablators.