A system for three-dimensional computed tomography of chemiluminescence was developed to measure flames over a large field angle. Nine gradient-index rods, with a 9 × 1 endoscope and only one camera are used. Its large field of view, simplicity, and low cost make it attractive for inner flow field diagnostics. To study the bokeh effect caused by the imaging system on reconstruction solutions, fluorescent beads were used to determine the blurring function. Experiments using a steady diffusion flame were conducted to validate the system. Three models, namely the clear-imaging, out-of-focus imaging, and deconvolution models, were utilized. Taking the bokeh effect into account, the results suggest that based on run-times the deconvolution model provides the best reconstruction accuracy without increasing computational time.

Precise prediction of aerothermal loads is significantly limited by the unclear interactions between the thermal protection system surface and the surrounding high-enthalpy gas. To address this, we propose an optical diagnostic method based on optical emission spectroscopy and laser absorption spectroscopy to investigate the gas–surface interactions within the boundary layer. Experiments are conducted in an air plasma flow produced by the 1.2 MW inductively coupled plasma wind tunnel at the China Academy of Aerospace Aerodynamics with an enthalpy of 20 MJ/kg and a heating time of 100 s. The cylindrical samples made of pure silicon carbide are tested, and quartz samples with the same exposed geometry are tested in parallel as a reference material. The optical emission spectroscopy system has four spectrometers to cover the wide wavelength range of 200–1100 nm, providing qualitative, spatially, and spectrally resolved measurements of the multi-species radiative emission adjacent to the sample surface. Laser absorption spectroscopy is deployed at different axial locations to quantify the number density and translational temperature of OI (3s5S) with a 500 Hz scanning rate and 200 kHz acquisition rate. Additionally, the surface temperature of each sample is detected by an infrared pyrometer. Scanning electron microscopy and energy dispersive spectrometry are performed before and after the plasma heating. Our measurement results provide valuable information on surface reaction pathways and catalytic recombination effects on atomic oxygen number density distributions. Finally, these self-consistent results show that the proposed method is reliable to deeply investigate gas–surface interactions within boundary layer in harsh aerothermal environment.

Quantitative measurements of chemiluminescence emissions as a result of CH*, OH*, C₂*, and CO₂* were conducted in CH₄/air premixed flames at different equivalence ratios (θ = 0.7–1.33), and numerical results based on one-dimensional flame simulations were compared to these data. A wavelength-dependent and optical-path-corrected emission calibration method was applied to quantify the emission of each excited species. The numerically simulated emission intensities of OH* and CH* are within the same order of magnitude as the experimental values for the same flame conditions. When self-absorption and cooling water thermosteresis are taken into account, the simulated OH* values are 1.0–1.4 times greater than the experimental results, while the CH* values are approximately 2.3–4.1 times greater. These findings suggest that more work is required to refine the temperature dependence factors for the CH* formation rate coefficients at high temperatures.

Internal temperature monitoring of high-speed propulsion systems is highly important for engine performance evaluation and lifetime prediction. As a passive optical measurement method without the need for an external light source and without flow field interference, the emission spectrum measurement technique has good application prospects for harsh measurement environments. As the main combustion product, high-temperature water vapor shows a strong emission intensity that is highly suitable for temperature measurement applications. We propose use of the band integral ratio to remove the high resolution measurement requirements for the spectrum acquisition system. In addition, the temperatures of methane-oxygen flames with different equivalent ratios are measured successfully under the condition that the influence of self-absorption on the measurements is considered.