Dynamic Analysis of Afterburner Transient Characteristics in Variable Cycle Engines: Insights into RVABI Performance and Combustion Stability

Among the advanced propulsion systems under development, the Variable Cycle Engine (VCE) stands out as a highly promising candidate. The VCE is unique because it combines the characteristics of both turbofan and turbojet engines, which allows it to dynamically adjust its operational mode to optimize performance across a wide range of flight conditions. This adaptability makes the VCE a versatile and efficient solution for modern aircraft, capable of achieving superior performance in diverse mission profiles. However, the inherent complexity of VCEs presents significant engineering challenges. The engine’s ability to switch between different operational modes requires  adjustments in Mode Selector Valve (MSV), Rear Variable Bypass Injector (RVABI), and Front Variable Bypass Injector (FVABI) which can lead to substantial changes in thermodynamic cycle parameters and result in unsteady characteristics and strong pneumatic coupling between components which complicates the design and optimization processes, specially to ensure efficient and stable combustion in the afterburner which is an important component located at the rear of the VCE. The afterburner in a VCE must cope with the variable aerodynamic conditions induced by the RVABI, which regulates the mixing of bypass and main air streams. This regulation leads to time-varying flow structures and combustion characteristics, posing challenges such as local flow separation, flame instability, and combustion inefficiency. Additionally, the coupling between unsteady heat release and acoustic waves can result in undesirable phenomena like thermoacoustic oscillations and structural vibrations, which negatively affect the reliability and lifespan of the engine. To this account, new study published in the Journal Applied Thermal Engineering and led by Professor Shilong Zhao and Professor Liang Xie from the Sun Yat-sen University and conducted by graduate students Hui Xiao, Yafan Li, and Haihong Chen, investigated the transient characteristics of the afterburner under varying inlet conditions   to understand the matching law of bypass ratio and guiding angle, flow structure, and combustion behavior during mode conversion in a VCE.

The research team analyzed the air-entrainment strength and efficiency with different guiding angles and bypass ratios and found that larger guiding angles and higher bypass ratios significantly enhanced air-entrainment strength. Specifically, a guiding angle of nearly 5° had the same effect on air-entrainment strength as a bypass ratio of 0.2. The air-entrainment efficiency, however, was more influenced by the guiding angle than the bypass ratio. The results indicated that increasing the guiding angle by 10° could raise the air-entrainment efficiency by 20% which is important in understanding how to optimize the configuration of the RVABI to achieve efficient air mixing and combustion. The authors also analyzed velocity distribution was analyzed at different guiding angles and bypass ratios to further investigate the impact of the RVABI on the flow structure  and observed complex velocity profiles, including an “inverted S” distribution at certain configurations. For example, at a guiding angle of 31.92° and a bypass ratio of 1.1, an “inverted S” velocity profile was observed which indicate an interplay between boundary layers and shear forces. This profile suggested that shear forces from velocity differences could enhance droplet breakup and atomization, improving combustion efficiency. Conversely, positive local velocity distributions with a “U” shape, generated by small guiding angles or large bypass ratios, provided larger turbulence kinetic energy, which increased swirl strength and mass and heat transfer capacity in the afterburner. Moreover, they measured the total pressure recovery coefficient along the axial direction to understand the pressure loss characteristics and found that pressure losses decreased with increasing bypass ratios at larger guiding angles. However, the pressure losses were highest at a guiding angle of 9.31° and a bypass ratio of 1.1. The pressure loss was kept below 5% in general which indicates efficient flow management. Turbulence intensity and dissipation rates were also examined, revealing that the peak of turbulence intensity typically appeared at the intersection of two streams. According to the authors, higher turbulence intensities were beneficial for mass and heat transfer because it enhanced combustion stability and these findings highlights the importance of managing turbulence characteristics to maintain efficient combustion.

The research team used Proper Orthogonal Decomposition (POD) to analyze the coherent structures in the transient flow field. The first two POD modes captured the dominant flow features, with energy fractions quickly converging to about 98% which indicates that a few leading modes contained most of the kinetic energy and suggested stable and dominant flow structures. The red-blue alternating vortex structures captured by the first two POD modes highlighted the complex interactions between the backpressure from the pilot and the blunt body effect of the strut. When the researchers increased the guiding angle, they observed enhanced vortex diffusion and breakup which facilitated better droplet fragmentation and mixing. These POD analysis studies provided important information into the aerodynamic performance and stability of the afterburner under different configurations.  Furthermore, the researchers looked at the temperature and species distribution at the outlet and demonstrated a close match between the numerical simulations and experimental data, with biases within 8%. Higher outlet temperatures were generally achieved with smaller guiding angles and bypass ratios. The presence of local low-temperature zones, due to flow separation or high velocity distortion, could disrupt flame propagation and stability. They analysed the formation of CO₂ and CO components and found that the proportion of CO₂ decreased with increasing radial altitude, while the CO proportion increased, particularly in regions with strong flow separation. Finally, the researchers examined the combustion efficiency and stability under various guiding angles and bypass ratios and reported that velocity distortions above 1.2 led to combustion vibrations, decreased efficiency, and unstable flames. Conversely, velocity distortions below 0.5 caused flow separation and incomplete radial flame transmission. Increasing air-entrainment intensity generated high turbulent energy, which could improve flow separation but also increased the instability of radial flame development.

In conclusion, the research work of Professor Shilong Zhao, Professor Liang Xie, Hui Xiao, Yafan Li, and Haihong Chen is an important advancement of VCE technology, which is critical for modern and future aerospace applications because it provided a deeper understanding of the transient aerodynamic and combustion phenomena in afterburners, specifically in relation to RVABI. The study findings can offer valuable guidelines to optimize the configuration of VCEs which can lead to improved air-entrainment strength and efficiency and directly enhance the engine’s overall performance and fuel efficiency.  Moreover, the authors’ findings on the effects of velocity distortions on combustion stability is essential in better design of afterburners that can maintain stable and efficient combustion under varying flight conditions which make more reliable and safer aircraft. Furthermore, the study’s investigation into the air-entrainment efficiency and the impact of different guiding angles and bypass ratios provides practical guidelines for developing effective fuel supply schemes and ensures optimal mixing of fuel and air which leads to more complete combustion and reduced emissions.