Objective  Hypersonic vehicle travels at a very high speed in atmosphere. Due to the high flight velocity, hypersonic vehicle has to endure very high heating rates on surface, ablative material is widely used in the design of thermal protection system (TPS). During the ablation process, gaseous ablator species are injected into the flow field, these gaseous species can involve inflow air in the chemical reaction, which changes flow field species distribution and temperature distribution, thus changing the target infrared radiation characteristics of hypersonic vehicle. Infrared radiation characteristics are the foundation of aircraft detection, identification and interception. Therefore, it is necessary to study the effect of thermal protection material ablation on aircraft target infrared radiation characteristics. For this purpose, this paper focuses on strategic warhead blunt body configuration with carbon-based thermal protection material. Numerical simulation of flow field and its infrared radiation is conducted, ablation effects on infrared radiation of flow over reentry body are discussed.

  Methods  Numerical simulation of flow field is conducted by solving three-dimensional thermal-chemical non-equilibrium Navier-Stokes equations. To simulate the surface ablation effect, surface velocity boundary condition, surface mass balance condition and surface energy balance condition are introduced into the computation process of flow field simulation. Oxidation, catalytic reaction and sublimation reaction of surface ablation material are also taken into account. To simulate the chemical reactions in the flow field, chemical reactions model of high temperature air with gaseous ablator species is used. Based on spectral band radiation model and by solving high temperature gas radiation transport equation, numerical simulation of flow field infrared radiation is conducted, the radiation mechanism of NO, CO, CO₂, CN, N₂, O, N is considered.

  Results and discussion  Numerical simulation results at typical condition agree well with experiments and numerical simulation results in literature (Fig.1-3), the computation model and method are validated. The main ablation product on surface is CO, infrared radiation in the waveband of 0.8-8 μm of flow field mainly comes from the radiation of high temperature CO, NO, CO₂ and CN (Fig.4). Ablation effect can increase flow field infrared radiation intensity, this phenomenon is more significant in 3-8 μm waveband than 1-3 μm waveband (Fig.5). Radiation from 3-8 μm waveband mainly comes from CO and NO, mass fraction of these species and flow field temperature increases as flight altitude decreases and flight velocity increases. Due to this, the radiation of 3-8 μm waveband increases as the flight altitude decreases and flight velocity increases (Fig.7-8). Radiation from 1-3 μm waveband in flow field around vehicle body increases as flight velocity increases, the radiation from 1-3 μm waveband in wake flow shows nonmonotonic variation due to the change of flow structure (Fig.7-8).

  Conclusions  In this paper, the ablation effects on infrared radiation of flow over reentry body covered with carbon-based thermal protection material is studied. By solving high temperature gas dynamics equations and radiation transfer equations, numerical simulation of thermal protection material ablation flow field and its infrared radiation is conducted. The distribution and changing rules of infrared radiation in different wavebands from ablation flow are analyzed. The study shows that ablation effect has significant influence on infrared radiation of reentry flow, which makes integral radiation intensity of wake flow increase more than one order of magnitude compared with non-ablation case in 3-8 μm waveband; The infrared radiation of ablation flow mainly comes from CO, NO and CO_2, and the ablation effect has less effect on the radiation in 1-3 μm waveband; The infrared radiation intensity of ablation flow increases with the decrease of reentry height at the same flight velocity, which weakens with the decreasing reentry velocity at the same flight height in 3-8 μm waveband.