Jiangbo Peng

The complexity of high-temperature non-equilibrium flow leads to significant limitations of existing computational fluid dynamics models. To further refine these models, there is a pressing need to acquire ro-vibrational temperature data in non-equilibrium flows, which serves to characterize the molecular population distribution. To address the limited single-pulse energy of broadband Stokes sources for high-repetition-rate nanosecond coherent anti-Stokes Raman spectroscopy (CARS), we developed a 10 kHz ns-CARS system using a burst-mode-pumped non-collinear optical parametric oscillator (NOPO) for N 2 ro-vibrational thermometry in non-equilibrium flows. The NOPO enables tunable, broadband, pulse-burst laser output, delivering a maximum single-pulse energy of up to 22.61 mJ with the laser bandwidth of up to 382 cm − 1 . We employed the unstable-resonator spatially enhanced detection CARS technology, which could reduce the interference from strong vibrations in the real-world engineering environment on CARS signal measurements. The sub-frame burst imaging mode of a high signal-to-noise-ratio (SNR), low-frame-rate EM-CCD camera was utilized to meet the demands of 10 kHz CARS signal acquisition. High-repetition-rate, single-shot, highSNR CARS spectral measurements were realized and highly time-resolved ro-vibrational temperatures of N 2 were obtained in an arc heater.

Accurate measurement of aerothermodynamic states in pulsed supersonic wind tunnels is essential for evaluating hypersonic vehicle environments. This study investigates quantitative, non-intrusive thermometry based on toluene two-color planar laser-induced fluorescence (PLIF) in the FL-63 pulsed combined high-enthalpy wind tunnel operated in Ludwieg-tube mode at Ma 4.0. A static calibration system consisting of a tube furnace and a 266 nm Nd:YAG laser was first established to measure toluene fluorescence spectra from room temperature to 473 K. By coupling the measured spectra with the transmittances of the BP280 and WG295 filters, a temperature inversion model based on the dual-band intensity ratio was obtained (R² = 0.982). The model was then applied to flow-visualization and thermometry experiments over a 20-degree wedge, with and without a transition band. The extrapolated freestream static temperature agrees well with theoretical and numerical values, and the measurements clearly resolve the shock-induced local temperature rise associated with the transition-band configuration. Furthermore, a time-series evolution analysis of the absolute fluorescence intensities and their corresponding ratio in the shock wave and freestream regions revealed a step-like signal decay law corresponding to the step-like pressure drops characteristic of the Ludwieg tube. Based on this physical law, the flow field evolution was precisely demarcated into four stages, identifying a structurally stable and effective test window lasting approximately 570 ms. The results demonstrate that toluene two-color PLIF can provide simultaneous flow-structure visualization, quantitative 2D thermometry, and test-window assessment in pulsed supersonic facilities.

Optical diagnostics are essential for capturing thermochemical dynamics in plasma flows during thermal protection system (TPS) material evaluation. We developed a simultaneous planar laser-induced fluorescence (PLIF) and optical emission spectroscopy (OES) system to address TPS thermal response assessment requirements. The PLIF system generated 226 nm laser at 500 Hz with 0.28 mJ pulse energy. This was achieved using a laser-pumped dye laser and frequency mixing unit. The configuration provided an effective excitation source for nitric oxide (NO) in arc jet flows. By optimizing light collection, imaging, and optical coupling configurations, the OES system enabled 500 Hz spectral measurements with one-dimensional spatial resolution. We successfully employed this system in an arc-heated wind tunnel to characterize spatiotemporally resolved properties of high-enthalpy flows over carbon/carbon (C/C) thermal protection material. The experiment captured NO-PLIF images and spatially resolved optical emission spectra of NO, cyanogen (CN), and molecular nitrogen ions (N + 2 ) from the C/C test sample surface under 20 MJ/kg enthalpy conditions. Rotational temperature profiles along streamwise and spanwise directions were derived through theoretical spectral fitting. Combined PLIF/OES data and flow pressure measurements revealed the temporal evolution of NO distribution, temperature, and enthalpy conditions during thermal response testing.

This paper presents the flow field reconstruction and prediction of powder fuel transport systems based on representative feature extraction from scattering images using deep learning techniques. A laboratory-built powder fuel supply system was used to conduct scattering spectroscopy experiments on boron-based fuel under various flow rate conditions. Based on the acquired scattering images, a prediction and reconstruction method was developed using a deep network framework composed of a Stacked Autoencoder (SAE), a Backpropagation Neural Network (BP), and a Long Short-Term Memory (LSTM) model. The proposed framework enables accurate classification and prediction of the dynamic evolution of flow structures based on learned representations from scattering images. Experimental results show that the feature vectors extracted by the SAE form clearly separable clusters in the latent space, leading to high classification accuracy under varying flow conditions. In the prediction task, the feature vectors predicted by the LSTM exhibit strong agreement with ground truth, with average mean square error, mean absolute error, and r-square values of 0.0027, 0.0398, and 0.9897, respectively. Furthermore, the reconstructed images offer a visual representation of the changing flow field, validating the model’s effectiveness in structure-level recovery. These results suggest that the proposed method provides reliable support for future real-time prediction of powder fuel mass flow rates based on optical sensing and imaging techniques.

Annular jet micro-mixing (MM) nozzles, due to the presence of central recirculation zones (CRZs), outer recirculation zones among the nozzles (ORZs), and flame interaction zones (FIZs), have greater potential than the circular nozzles in maintaining flame stability when facing the frequent changes in operating conditions of gas turbines. This study focused on the annular jet MM nozzles using syngas. At the different equivalence ratio (Φ) and air velocity at the nozzle inlet (uair), this study introduced a dimensionless parameter C (the relative contribution ratio of the CRZ) to quantify the contributions of dual recirculation zones to flame anchoring through simulations. Then the effects of the FIZ on flame anchoring and the pollutant emissions of the nozzles were investigated through experiments using hydroxyl planar laser-induced fluorescence. The results show that as Φ or uair increases, C rises by 70% and 152%, respectively. At uair = 101.0 m/s, C is approximately 50%, indicating the role of the CRZ in flame anchoring becomes increasingly important, even comparable to that of the ORZ. The flame interaction enables flame anchoring in the ORZ under all conditions. As Φ or uair increases, the relative area of the FIZ increases by 57% and 131%, respectively. The enhanced flame interaction also promotes flame anchoring in the CRZ. When Φ exceeds 0.438 or uair surpasses 67.4 m/s, carbon monoxide emission drops below 5 μL/L (@15% O2). Nitric oxide emission maintains below 8 μL/L (@15% O2) in all conditions.

This study addresses the complexities of measuring the soot volume fraction in large combustion chambers of aeroengines, where achieving uniform saturation excitation and mitigating intense background light interference are necessary for accurately determining two-dimensional distributions—a critical aspect for optimizing engine design. We present an advanced laser-induced incandescence (LII) method specifically developed for aeroengine afterburners. This method incorporates a total-reflection-based beam-homogenizing cavity to ensure uniform laser energy distribution across the measurement area. By calculating the threshold laser fluence for two-dimensional saturation excitation, our approach minimizes laser absorption and path integral effects, achieving uniform saturation excitation. A 1000 mm focusing mirror is used to maintain a consistent light-sheet thickness. Our LII system was calibrated in situ, showing 95% consistency and enabling the use of single-band LII signals to capture precise two-dimensional soot volume fraction distributions. To counteract intense background interference, filtering algorithms were applied to the LII images, resulting in a high signal-to-noise ratio of 78.3 dB. Our analysis provides crucial insights into how the overall and axial distributions of soot volume fraction in recirculation areas correlate with global equivalence ratios and incoming air flow rates, offering critical data for enhancing combustion efficiency and reducing emissions.

The planar laser induced fluorescence (PLIF)/Mie scattering technique has been established as an effective method for measuring the Sauter mean diameter (SMD) distribution in dense fuel sprays. This technique typically utilized a 266 nm laser. However, the strong absorption of laser wavelengths ranging from 266 nm to 274 nm in dense fuel sprays affected the detection of Mie scattering signals. Therefore, it was proposed to use a 266 nm laser for fluorescence excitation and a 532 nm laser for Mie scattering radiation in dense fuel sprays. In this study, a look-up table was created by correlating the PLIF/Mie ratio with the SMD using the phase Doppler particle analyzer, reducing the SMD_error from 27% to 13%. Furthermore, the proposed method was successfully applied to investigate the atomization field of room temperature and atmospheric pressure in a dual-stage axial swirl combustion chamber. By comparing the time-averaged PLIF images and time-averaged Mie images, the vapor and liquid phases of the fuel in the spray were successfully imaged and identified. The results indicated that, under the same fuel flow, increasing the air mass flow led to a gradual increase in the spray cone angle inside the dual-stage axial swirl combustor. Additionally, at a constant fuel flow, increasing the air mass flow resulted in a more uniform droplet size distribution, thereby enhancing the atomization effect. The presented technique provides a reliable and accurate tool for studying fuel spray behavior under various operating conditions. This knowledge can contribute to the design of more efficient combustion systems and the optimization of fuel injection strategies.

The minimum ignition equivalence ratio of the strut stabilizer is an important parameter in the design of integrated afterburners. The ignition location significantly affects the ignition equivalence ratio and flame propagation, and therefore, it should be deeply studied. The ignition equivalence ratio and flame propagation at different axial ignition locations downstream of the strut stabilizer are studied in this paper. When the ignition distance is approximately the bluff body trailing edge width, a lower ignition equivalence ratio is required for ignition, and the flame propagates faster through the entire combustion chamber. For different ignition locations, the generated flame kernel at different locations all first propagates to the shear layer. Subsequently, the unilateral flame rapidly extends, ultimately igniting the entire combustion chamber. The flame propagation trajectory depends on the ignition location controlled by the non-reacting flow field and the distribution of kerosene concentration. The flame propagation trajectory mainly includes three paths: (1) the flame kernel is directly downstream the shear layer when the ignition location is close to the tail edge of the stabilizer, (2) the flame propagates upstream into the shear layer in a U-shape when the ignition location is far from the stabilizer but still in the recirculation zone, and (3) the flame propagates upstream into the recirculation zone and shear layer in a U-shape when the ignition location is outside the recirculation zone. In addition, the time for flame propagation to the shear layer is directly related to the ignition performance when the ignition location is within the recirculation zone. If the flame reaches the shear layer in a longer time, there will be more energy loss during the flame propagation process, and the ignition performance will deteriorate. The speed of the flame-trailing edge extension is directly related to the ignition fuel-air ratio, and the downstream extension of the flame is mainly affected by the turbulence velocity in the shear layer.

Diluted micromix combustion technology is an effective way to realize stable H₂ flames with low NO_x emission, and the turbulent flame structures of a micromix burner are experimentally revealed in this work with corrected OH-PLIF images.