To investigate the influence of swirling air flow field on the spray characteristics and the droplet behaviors of the swirl cup airblast fuel injector, a single swirler cup and a double swirlers cup airblast injector were operated in a wide range of fuel mass flow rates (mf) and relative air pressure drops (ΔPa). Various laser based diagnostics, including planar Mie scattering, phase-Doppler particle analyzer, and high speed particle imaging velocimetry, were employed to provide the information of spray angle, droplet Sauter mean diameter (SMD), droplet velocity, and air flow velocity. The results show that the air flow field was characterized by central toroidal recirculation zone (CTRZ), the double swirlers cup airblast injector generated stronger CTRZ than single swirlers cup airblast injector. The spray characteristics and the droplet behaviors with different size classes were significantly affected by the air flow field. The larger droplets (25–100 μm) with higher initial momentum were likely to keep its velocity when passing through CTRZ, when the mf increased, more large fuel droplets could pass through the CTRZ and expanded to both sides, resulting an increase in a spray angle. On the other hand, smaller droplets (<25 μm) with lower initial momentum were trend to be enrolled by the CTRZ; therefore, the spray droplets at lower mf were confined in the CTRZ. These droplets trend to coalesce with each other resulting in an increase of SMD in this region.

The swirl cup airblast fuel injector is widely used in modern low-emission gas turbine combustors. It is important to understand the underlying physics in the interaction between fuel spray and complex swirling airflow. The present study investigates a swirl cup airblast fuel injector's swirling flow and spray atomization characteristics. Various laser-based diagnostics, including planar Mie scattering, phase-Doppler particle analyzer, and high-speed particle imaging velocimetry, were employed for this purpose. The swirl cup airblast fuel injectors, featuring different outer swirler swirl numbers (0.8<SN<1.8) and inner–outer relative swirling directions, were investigated in an air Reynolds number ranging from 0.77 × 106 to 1.16 × 106 and a 2 kg/h constant kerosene mass flow rate. The results showed that, since the swirler relative direction showed a significant influence on the inner swirler induced central toroidal recirculation zone and outer swirler induced swirling jet zone, the interaction between spray and swirling airflow was changed. Therefore, spray angle, Sauter mean diameter, and droplet velocity were changed by the swirler relative direction. Moreover, the effect of swirl number on the spray characteristics was found to be different for co- and counter-direction swirler configurations.

High-altitude relight is a critical challenge for aero-engines, directly impacting the safety and emergency response capabilities of aircraft. This paper systematically reviews the physical mechanisms, key factors, and relevant prediction models of high-altitude relight, highlighting the detrimental effects of extreme conditions such as low pressure and temperature on fuel evaporation rates, flame propagation speeds, and turbulent combustion processes. A comprehensive overview of the current state of high-altitude relight research is presented, alongside recommendations for enhancing the ignition performance of aero-engines under extreme conditions. This paper focuses on the development of ignition prediction models, including early empirical and semi-empirical models, as well as physics-based models for turbulent flame propagation and flame kernel tracking, assessing their applicability in high-altitude relight scenarios. Although flame kernel tracking has shown satisfactory performance in predicting ignition probability, it still overly relies on manually set parameters and lacks precise descriptions of the physical processes of flame kernel generation. Future studies on some topics, including refining flame kernel modeling, strengthening the integration of experimental data and numerical simulations, and exploring the incorporation of new ignition technologies, are needed, to further improve model reliability and predictive capability.

The dilution zone in modern aero-engine combustors is characterized by a strong swirling mainstream with weak transverse jets. This characteristic brings new challenges in homogenizing the temperature distribution at the combustor exit. Therefore, it is imperative to understand the temperature penetration and mixing process of the jet in swirling crossflow (JISCF). This investigation provides new insight in the temperature mixing process for a JISCF in nozzle exit diameter (D) at 7.4, 10.7, and 14 mm and jet to mainstream velocity ratio (VR) from 2.0 to 6.6. The temperature mixing process was measured in a specially designed optical assessable three-dome model gas turbine combustor by planar 1-methylnaphthalene (1-MN) tracer laser-induced fluorescence thermometry. A detailed quantitative measurement of temperature distribution is achieved through the spectral red shift in the fluorescence of 1-MN as the temperature increase. This diagnostic was employed to provide the first two-dimensional temperature distribution for the JISCF. The results showed that the swirling crossflows induce strong spanwise thermal advection, forming secondary low-temperature regions downstream. Generally, the flow structure and mixing process are governed by the interaction of jet and swirling flow. The jet flow parameters, including velocity ratio and diameter, changed the flow structures by changing the interaction between jet and swirling flow. Statistical results and proper orthogonal decomposition (POD) analyses showed a strong anisotropic mixing process in the downstream of the jet.

To investigate the physical phenomena interactions between airstream and liquid injection or droplets within a complex multi-stage swirl flow field, this study investigated the flow field and spray characteristics in a central stage direct injection combustor with a variety of optical diagnostic techniques, including using time-resolved particle image velocimetry (PIV) to measure the swirl flow field, using time-resolved planar Mie scattering (PMie) to measure the spray pattern, and using a laser particle size analyzer (LPSA) to measure the spray droplet size and its distribution. The results indicate that the lip recirculation zone (LRZ) and the swirl jet zone (SJZ) significantly influence droplet spatial and size distribution characteristics, such as spray penetration, cone angle, and droplet size. Due to the unique characteristics of the dual-stage swirl atomizer, the spray cone angle and penetration do not increase monotonically with the gas Weber number (${We}_g$). For the pilot stage, at a constant ${We}_g$, both the spray cone angle and penetration increase with higher fuel injection velocity. At different fuel injection velocities, the spray penetration increases with rising ${We}_g$. When the fuel injection velocity is low, the cone angle initially increases and then decreases as ${We}_g$ grows. The results about the effect of ${We}_g$ on droplet size distribution further support this conclusion. The Sauter mean diameter (SMD) of the main and pilot stage decreases with increasing relative pressure drop of air until reaching a stable state. The aerodynamic shear of the swirling airstream is sufficient to promote thorough fuel atomization, ensuring that the SMD remains low at the whole operating condition. Therefore, for the dual-stage swirl atomizer investigated in this study, good atomization can be achieved under low operating conditions, which provides a theoretical foundation and data support for the improvement and design of a low-emission, high-performance atomizer.