Yuzhen Lin

Experiments were conducted on a full-scale premixed arrayed micro-tube hydrogen combustor to explore the effect of the stratification ratio (SR) on combustion instability. Four typical flame modes were observed. Attached main-stage flames were most frequently observed over SR = 0.8–1.5. At an air mass flow rate of 220 g/s and a total equivalence ratio of 0.300, the pressure amplitude exhibited a W-like local-extrema pattern for pilot equivalence ratios of 0.4 and 0.5. Under the same air mass flow rate and total equivalence ratio, the dominant frequency changed from approximately 480 Hz at SR < 0.8 to 420–450 Hz at SR > 0.8. These spectral changes are reported as observations rather than evidence of stage-wise acoustic-mode dominance. Further results indicate that high-amplitude oscillations concentrate in high-SR regions, and increasing pilot-stage equivalence ratio reduces pressure amplitude at SR > 1.6. The pressure amplitude rises significantly with global equivalence ratio, which lowers the critical SR for high-amplitude oscillations.

This study investigates the regulatory mechanism of vortex generators (VGs) concerning the penetration and mixing characteristics of sub/supercritical kerosene transverse jets within a dual-variable combustor. Combining shadowgraph imaging experiments and numerical simulations under inflow conditions simulating combustor entry (Ta = 500 K, pa = 0.3 MPa, Va = 50 m/s), the effects of VG height (Hv = 5–10 mm) and expansion angle (β = 30°–60°) on the evolution of multiphase jet behavior (constant fuel flow rate ṁf = 1.6 g/s) were systematically studied. Experimental results demonstrate that the introduction of VGs universally enhances jet penetration depth. Supercritical jets (Tinj = 670 K) demonstrate enhanced mixing efficiency in comparison to liquid jets (Tinj = 425 K), necessitating approximately 70% less axial distance to attain equivalent mixing levels. Numerical simulation analysis reveals that the principal flow mechanisms for improving penetration involve the induction of streamwise vortices and the creation of low-speed wake regions by VGs. These phenomena modify the local momentum ratio and augment the transport of normal momentum. Furthermore, an optimal range exists for VG geometric parameters (Hv = 7.5 mm, β = 36°–45°), achieving a balance between flow control benefits and turbulent losses. This research provides a substantial theoretical basis and evidence-based optimal design guidelines for the development of high-efficiency fuel injection systems in dual-variable combustors.

The centrally-staged annular combustor is widely used in aeroengines, but it faces prominent challenges in ignition process. This paper focuses on an experimental study of spray ignition performances with a centrally-staged annular model combustor under atmospheric conditions. The aviation kerosene, No. 3 jet fuel (RP-3), is used as fuel. High-speed camera is used to capture the flame propagation during the ignition process. This study obtains the operating map, light-round time, flame propagation paths, flame propagation patterns, and ignition probability of the annular combustor within the pressure drop range of 0.5%–3% and fuel–air ratio range of 0.013–0.032 (when converted to the equivalence ratio, it is 0.19–0.47). The fuel–air ratio at the ignition boundary of the combustor decreases at first and then increases with the rise of pressure drop, which is greater than 0.02 in all operation conditions and reaches the highest value of 0.03 at 0.5% pressure drop. While the fuel–air ratio at the blow out boundary rapidly decreases from 0.016 and then remains nearly constant around 0.014 regardless of the increase in pressure drop. The light-round time of the annular combustor shortens as the fuel–air ratio increases or the pressure drop increases. At low fuel–air ratios, fuel concentration primarily limits flame propagation speed. Higher air velocity makes flame kernels more likely to extinguish suddenly, hindering ignition. In contrast, at high fuel–air ratios, the impact of increased fuel concentration on flame propagation is limited, while higher flow velocity can enhance the process, thus shortening the ignition time. The circumferential propagation of flame shows asymmetric in the annular combustor. The flame propagation speed is faster in the direction of the swirling flow rotation, but remains constant in the other direction. The flame propagation paths are also different in the two directions. Three different flame propagation patterns are observed as “archlike-axial,” “entrainment-rotation,” and “sweep-transverse.” The fuel–air ratio and pressure drop are the main parameters controlling the flame propagation pattern. The results of ignition probability at different positions indicate that ignition between two adjacent burners has a higher ignition probability, while ignition from the downstream position increases the probability a little.

It is well established that hydrogen has become one of the most promising fuels for achieving zero-carbon emissions. However, hydrogen combustion still faces many challenges, particularly the flashback issue and high NOx emissions, which necessitate the exploration of hydrogen combustion technologies and pollution control measures. This paper proposes a micromix diffusion combustion scheme with a disturbance vortex generator (DVG). The flow characteristics of the micromix element were analyzed, including the velocity distribution, penetration depth, jet vortex length, and vortex structure. It was found that DVG would create an air streamwise vortex within the main airflow, thus accelerating the axial velocity decay and facilitating the penetration of hydrogen. The hydrogen jet vortex rolls up the mainstream air, forming a coupled vortex system, which further enhances mixing. Sensitivity analysis was carried out to study the influences of the key parameters of the micromix element. The hydrogen injection hole diameter and the DVG height were found to be the most influential factors. The interactions between vortex, flame, and NOx were investigated to obtain the mechanism of NOx formation. By conducting a comparative analysis of the effects of the hydrogen orifice diameter and the vortex generator height, it has been observed that augmenting the length of the jet vortex can significantly promote the mixing of hydrogen with the mainstream air, playing an essential role in reducing emissions. This study establishes a solid foundation for future research endeavors in the development of micromix diffusion combustors.

The escalating requirements for thermal management in advanced high-performance aviation engines necessitate the injection of aviation fuels under near- and supercritical conditions. The jet morphology both exterior and interior to the nozzle of aviation kerosene jets, injected from near-critical and supercritical states into a quasi-quiescent, near-atmospheric environment, was experimentally investigated, focusing on the effects of in-nozzle phase transitions. The in-nozzle flow behavior was characterized using shadowgraph imaging and pressure profile measurements, while downstream jet behavior was captured using both shadowgraph imaging and planar Mie scattering techniques. It is found that without observable in-nozzle phase transitions (OIPTs), the near-nozzle jet structure exhibits low sensitivity to injection temperature. However, under OIPT conditions, a reduction in injection temperature precipitates notable increases in jet expansion angle, jet diameter, and Mach disk diameter. Additionally, it initially provokes a downstream displacement of the Mach disk, subsequently reversing its direction upstream. Analysis using the surrogate fuel thermodynamic phase diagram indicates that without OIPT, jets injected from the supercritical and vapor states disintegrate similarly due to stages of isentropic expansion, compression, and isobaric mixing, leading to fuel condensation. Under OIPT conditions, crossing the Widom line or saturation line leads to an increase in the specific heat ratio of the fuel along the injection path and changes the near-nozzle shock structure from barrel-shaped to bowl-shaped, potentially triggering a compression shock wave in the ambient air. These findings are useful for the design and modification of supercritical fuel nozzles, fuel supply systems, and combustion chambers.

The ignition and flame propagation process within the centrally staged annular combustor is considerably intricate, particularly under low pressure conditions. Experiments with kerosene as fuel were conducted under varying pressures ranging from 30 to 90 kPa. A high-speed camera was employed to capture images of the ignition process. The experiments illustrate that the fuel–air ratio at the ignition boundary initially decreases and then increases with increasing pressure drop at different inlet pressures. The ratio increases as the pressure decreases at a constant pressure drop, exhibiting a more pronounced effect, particularly at lower pressures. The flame propagation time of annular combustion is shortened by the increase in the fuel–air ratio. Moreover, an increase in pressure drop enhances flame propagation speed and reduces flame propagation time. Under identical working conditions and parameters, lower inlet pressures result in longer flame propagation time. Additionally, asymmetry is observed in circumferential flame propagation within the annular combustor. Since the swirl flow direction exhibits faster propagation speeds, the ratio of propagation speeds remains nearly constant across different directions. Furthermore, distinct flame propagation paths are identified in various directions. Three different flame propagation patterns were observed, including “archlike-axial,” “entrainment-rotation,” and “sweep-transverse.” Fuel–air ratio and pressure drop serve as primary parameters governing flame propagation patterns. The flame propagation pattern exhibits similarities to that of atmospheric conditions, with the exception of the inhibition observed in the entrainment-rotation pattern. Notably, compared to the ignition between two adjacent burners, ignition in the middle of a certain burner shows a higher probability of successful ignition.

Wavelength modulation spectroscopy (WMS) is recognized as a good noise immunity method in tunable diode laser absorption spectroscopy (TDLAS). It can obtain high signal-to-noise ratio harmonic intensity signals for precise gas parameter measurement, like temperature and concentration. However, the TDLAS-based velocity is sensitive to the Doppler shift in the laser frequency caused by the high-speed flow, which usually requires complex wavelength calibration and precise signal alignment. This work proposed a novel method for the precise velocity measurement directly by extracting the time interval of the even-symmetric 2f/1f harmonics signal with a self-convolution algorithm. The even-symmetric 2f/1f harmonics signal is obtained from the up-and-down laser optical frequency scanning WMS with lock-in amplification technology. A portable measurement system has been developed to achieve simultaneous velocity and temperature measurement of the gas in turbojet engine exhaust with different operation states. The maximum standard deviation of the velocity derived by the proposed method is 15.6 m/s, while that by traditional DAS and WMS are 69.2 m/s and 26.4 m/s, respectively. Experiment results have verified the improvement of velocity precision by the proposed method. It can provide reliable experimental data for improving the design of combustors and optimizing numerical combustion simulations.

A planar laser induced fluorescence (PLIF) using the third harmonic of the Nd-YAG laser at 355 nm is employed to visualize temperature and H ₂ O/CH ₂ O concentration distributions inside flames. Laser Absorption Spectroscopy (LAS) is employed for initial images of temperature and H ₂ O concentration, and the measured absorption offers constraints to update temperature and H ₂ O concentration images. A chemical reaction kinetics model based on ethanol combustion mechanisms is established, offering the production rate of H ₂ O and CH ₂ O, thereby bridging PLIF and LAS. The images of temperature and CH ₂ O/H ₂ O concentration is produced iteratively for quantitative imaging of the flame cross-section. A flame fueled with ethanol vapor was used and images of temperature, H ₂ O concentration and CH ₂ O concentration were given. Comparisons with thermocouples, the temperature difference was found to be no more than 40 K. The images of H ₂ O and CH ₂ O concentrations detailed the area of incomplete combustion within the flame and its temporal variations. Relative deviations from experiment results to the ideal values are respectively 3.05 % and 5.34 % for H ₂ O and CH ₂ O.

Tunable diode laser absorption spectroscopy (TDLAS) tomography is widely used for gas temperature and concentration imaging. Laser beam optimization is an efficient way to improve imaging quality with limited available projections for tomography. A beam optimization method independent of any specific distribution was proposed. Asymptotic point spread functions (PSFs) were generated with the Dirac function approximated by Gaussian functions, and the shape proximity between PSF and the Dirac function was estimated. Simultaneous algebraic reconstruction technique (SART) was applied for fan-beam layout optimization. Typical beam optimization methods were compared for numerical verifications. Reconstruction errors of the optimal beam layouts obtained from the proposed method were generally smaller than or equivalent to those from the literature. A TDLAS tomographic sensor with the optimal layout was realized, with only five pairs of distributed feedback (DFB) lasers used for nine projection angles. The flame of a Bunsen burner with a flat nozzle was detected, and beam layouts from the real sensor with projections missing were evaluated for experimental verifications. Averaged deteriorations of image quality indicated by the structural similarity index (SSIM) of temperature and H2O mole fraction were only 0.7% and 1.5%, implying the immunity of missing projections by the optimal beam layout. The SSIM variation with the number of missing projections was also well predicted by the proposed method, which verified the correlation between shape proximity and image quality.

Tunable diode laser absorption spectroscopy (TDLAS) is a popular technique for non-contact temperature measurement but is highly susceptible to beam-steering distortion due to the time-varying along-path inhomogeneity of refractive index in dynamic process monitoring. An adaptive beam-steering-distortion compensation method is proposed to eliminate the distortion in TDLAS measurement. A high-frequency modulated laser at a non-absorbing wavelength is coupled with a conventional low-frequency scanning laser at an absorbing wavelength and passes through the target field along an identical path. A synchronous demodulation method is proposed to demodulate the transmitted hybrid laser signal into the distorted absorption and distortion signals. The distortion signal is utilized to adaptively compensate the distortion parasitic on the distorted absorption signal. Experimental results obtained on a vibrated water thermostat show that the maximum deviation of temperature derived by the proposed method is 4.97 K, while that by the traditional DAS method is 34.64 K. Experimental results on a turbulent flame show that the relative standard deviation of temperature obtained by the proposed method has decreased from 36.7 % (DAS) to 4.20 %. Also, the temperature fitting success rate has increased from 88.0 % to 100 % in the turbulent flame case. The results show that the proposed method is more precise and robust than the traditional method. The proposed method will potentially play an important role in laser-based dynamic combustion monitoring and diagnosis.