Flow Field Structure Research Articles

The thrust balance model during the dragonfly hovering flight

In recent years, the micro air vehicle (MAV) oscillations caused by thrust imbalances have received more attention. This paper proposes a dual-wing thrust balance model (DTBM) that can solve the above problem by iterating the modified rotation angle formula. The core control parameter of the DTBM model is the au angle, which refers to the angle between the wing surface and the stroke plane at the mid-stroke position during the upstroke. For each degree change in the au angle, the range of variation in the dimensionless average thrust coefficient is between 0.0225-0.0268. A thrust coefficient of 0.0225 causes the dragonfly to move forward by 9.037 cm in one second, which is equivalent to 1.29 times its body length. By using DTBM, the average thrust coefficient can be reduced to below 0.001 in just a few iterations. No matter how complex the motion pattern is, the DTBM can achieve thrust balance within 0.278 s. Through our research, when selecting the deviation angle motion of real dragonflies, the dual-wing au angles exhibit a highly linear correlation with wing spacing, called linear motion. In contrast, the nonlinear variation of the au angle appears in the hindwing of the no-deviation motion and the forewing of the elliptical deviation motion. All of the nonlinear changes are referred to as nonlinear motion. Nonlinear variation of the au angle arises from larger disturbances of the lateral force during the upstroke. The stronger lateral force is closely related to the flapping trajectory. When the flapping trajectory causes the dual-wing to closely approach each other in the mid-stroke, a continuous positive pressure zone forms between the dual-wing. The collision of the leading-edge vortex and the shedding of the trailing-edge vortex is the special flow field structure in the nonlinear motion. Guided by the DTBM, future designs of MAVs will be able to better achieve thrust balance during hovering flight, requiring only the embedding of the iteration algorithm and prediction function of the DTBM in the internal chip.

Experimental study of performance enhancement in PEMEC with titanium mesh flow structure via in-situ temperature measurement

A proton exchange membrane electrolysis cell (PEMEC) is a promising method for hydrogen production. However, the flow-field structure of PEMEC presently has the disadvantages of poor fluidity and uneven heat distribution. A titanium mesh flow field was, therefore, proposed to replace the traditional parallel channel structure. A single-cell test platform was set up in this study, and a new in situ temperature measurement method was proposed. The temperature variation and electrolysis performance of titanium mesh structures were studied experimentally. The results demonstrated that the polarization performance of the titanium mesh structure was enhanced compared to that of the parallel structure. In situ measurements showed that the temperature increased and the voltage decreased with an increase in the inlet temperature and flow rate, with the rate of change gradually decreasing. The temperature distribution in the titanium mesh structure was more uniform than that in the parallel structure. When the current density and inlet flow rate step were changed, the voltage stabilized within 12 s. After 100 h of operation, the voltage of the parallel structure and titanium mesh structure increased by 0.12 V and 0.07 V, respectively, indicating good dynamic and duration performance.

Numerical research on flow field structure and droplets distribution of kerosene-fueled rotating detonation ramjet engine

In order to reveal the multiphase flow field structure and fuel droplets distribution under rotating detonation ramjet engine fueled by liquid kerosene, non-premixed simulations coupled with an Euler-Lagrangian approach is adopted. Supersonic air is used as oxidizer and the total pressure and total temperature at the entrance of isolation are set as 1.2 MPa and 1100 K, respectively, with a Mach number of 1.9. It is shown that a single-wave is formed and typical rotating detonation wave structures are established under two different orifice spacing conditions, namely 2 mm and 6mm. A "rich oil and poor oxygen band" is formed and attributed to the inconsistent supply of fuel and air after the passage of the detonation wave. When the orifices spacing is increased from 2 mm to 6 mm, both obvious strips after the detonation wave and “n-type” deflagration structures near the contact surface are observed. Besides, the detonation wave front becomes discontinuous, as well as from the deflagration heat release distribution. Despite of the effect of the circumferential propagation of detonation wave, kerosene droplets still propagate mainly along the downstream direction. However, Kerosene droplets distribution shows obvious difference along the detonation wave propagation direction.

Effect of solid loading and particle size on bubble behavior and flow field structure in slurry bubble column

The complexity of fluid dynamics in a slurry bubble column reactor introduces significant uncertainty in reactor design and scale-up. This paper investigates the hydrodynamic performance of the gas–liquid–solid system within the reactor by employing computational fluid dynamics-population balance modeling numerical simulations alongside particle image velocimetry (PIV) experiments. The effect of superficial gas velocity and particle conditions on the overall gas holdup were analyzed, focusing on the effects of particle size and solid concentration on bubble size, bubble behavior, flow field structure, and local gas holdup distribution at high superficial gas velocities. Bubble size was evaluated using calibrated image measurements, and the impact of varying solid conditions was thoroughly explored. The results revealed that an increase in solid size correlated with higher gas holdup and smaller bubble sizes, whereas a greater solid concentration resulted in decreased gas holdup and larger bubble sizes. PIV experiments indicated that bubbles exhibited a tendency to migrate toward the central region of the reactor, leading to the formation of larger bubbles that accelerated the rise of surrounding bubbles, while smaller bubbles near the wall moved downward. As the slurry bed height increased, the range of local gas holdup distribution expanded, resulting in a symmetrical distribution of radial local gas holdup in the fully developed stage at a height of 0.16 m.

Laser ablation ignition modes in a cavity-based supersonic combustor

A numerical and experimental study was conducted to investigate the Laser Ablation (LA) ignition mode in an ethylene-fueled supersonic combustor with a cavity flameholder. The experiments were operated under a Mach number 2.92 supersonic inflow, with stagnation pressure of 2.4 MPa and stagnation temperature of 1600 K. Reynolds-averaged Navier-Stokes simulations were conducted to characterize the mixing process and flow field structure. This study identified four distinct LA ignition modes. Under the specified condition, laser ablation in zero and negative defocusing states manifested two distinct ignition modes termed Laser Ablation Direct Ignition (LADI) mode and Laser Ablation Re-Ignition (LARI) mode, correspondingly. LA ignition in a local small cavity, created by depressing the flow field regulator, could facilitate the ignition mode transforming from LARI mode to Laser Ablation Transition Ignition (LATI) mode. On the other hand, the elevation of the flow field regulator effectively inhibited the forward propagation of the initial flame kernel and reduced the dissipation of LA plasma, further enhancing the LADI mode. Based on these characteristics, the LADI mode was subdivided into strong (LADI-S) and weak (LADI-W) modes. Facilitating the transition of ignition modes through alterations in the local flow field could contribute to attaining a more effective and stable LA ignition.

Aerodynamic investigation of an S-shaped intake employing sweeping jet actuators based on proper orthogonal decomposition

The central curvature change of the S-shaped intake exacerbates the three-dimensional flow separation, resulting in total pressure and swirl distortions at the outlet. In this paper, an efficient active flow control technique, the sweeping jet actuator, was applied to control flow separation. The instantaneous characteristics of the flow field inside and outside the actuator were revealed by using validated numerical simulation methods. In addition, the effect of the sweeping jet on the aerodynamic performance of the S-shaped intake was investigated. Proper orthogonal decomposition (POD), an analysis method used to extract the main feature components of data, was applied to decompose the flow field structure at the S-shaped intake outlet in this study. The results highlight the external flow field characteristics of the actuator, with a jet sweeping range of up to ±40° and a sweeping frequency of 1 500 Hz, corresponding to a Strouhal number of 0.018. When the jet momentum coefficient is only 0.116%, the total pressure loss coefficient and pressure distortion index are reduced by 5.6% and 24.76%, respectively. This study demonstrates the critical role of momentum injection and discretization in inhibiting flow separation in the channel. The POD-based analysis indicates that a sweeping jet with high momentum can significantly enhance the energy in the flow passage and reduce the vorticity intensity of the first three modes. The stable mode structures exist near the outlet, exhibiting excitation frequency and frequency-doubling properties. The findings of this research provide essential input conditions for subsequent distortion studies.

Dynamic characterization of the flow field of air classifier disturbed structures and application to efficient separation of fly ash

Classification is crucial for the resource utilization of fly ash, impacting both utilization rates and product quality. This study introduces an enhanced flow field structure in a perturbed rotating centrifugal air classifier by optimizing toothed and tail blade designs for efficient fly ash classification. Using numerical simulations and experiments, we examine the flow field performance and separation efficacy at various speeds, and employ Dynamic Mode Decomposition (DMD) for modal analysis. Results show the toothed blade, angled at 35°, produces small-scale turbulence with high pressure and velocity, while the tail blade’s rightward bend induces large-scale vortices, aiding flow control. Modal analysis highlights that both Drs-types 1 and 2 predominantly exhibit weakly attenuated modes. Optimal classification occurs at Rs = 950rpm and fv = 0.3kg/s, with both Drs-types surpassing the original structure in effectiveness and pressure. These findings provide new directions for industrial applications and the design of cyclone air classifiers.

Numerical simulations on non-reactive multiphase flow during the transition from vent activation to venting with vent cap fragments in 18650 lithium-ion batteries

Mandatory safety devices, prevalent in lithium-ion batteries (LIBs), enable current interrupt device (CID)-and vent-activated power-off protection by sensing the electrochemical gas production boost triggered by thermal runaway (TR). For an in-depth understanding of the early warning of safety devices, the thermodynamic and multiphase flow behaviour of gases-vent particles interaction under the non-combustion venting phase is numerically investigated. From the gas phase simulation results, the flow field structure of the counter-rotating vortex pairs (CVPs) bounded by vent cap fragments enhances the heat transfer between ambient air and the venting gas, and the local venting gas temperature can reach a drop of >100 K. The transient three-dimensional spatial evolution characteristics of vent particles interacting with vent cap fragments at the early stage of the venting process after 18650 thermal runaway-induced vent-activation is originally demonstrated, including gas temperature and velocity as well as particle size and temperature. The spatial distributions of polydisperse particle size and temperature under the process of non-combustion venting are used to characterize the ejected particles' behaviour, which reveals the particle trajectories in particle-laden streams and their heat-transfer characteristics. The results show that ejected particles with relatively high temperatures and large sizes are mainly clustered within the cell diameters, and possible thermal runaway propagation inside the module can be triggered by the smaller-size particles of higher temperature sprayed in the periphery when the blocking effect of the vent cap fragment is weakened.

On the large-scale vortices in an L/D = 2.30 serpentine diffuser with internal bump

In this paper, the generation and spatial evolution of large-scale vortices in an ultra-compact serpentine diffuser (L/D = 2.30) featuring an integrated internal bump are experimentally and numerically investigated. Due to reduction of the local flow area caused by the internal bump, a favorable streamwise pressure gradient is induced first and then followed by an adverse one. Concurrently, a high-pressure region exists on the side of the diffuser as it contracts, yielding a sideward circumferential pressure gradient and then an inward one. Such circumferential pressure gradients divert the boundary layer on the windward side of the internal bump to the side and induce a spanwise U-shaped vortex on the leeward side, which constrains the development of flow separation in this region. The circumferential pressure gradients further induce two pairs of streamwise vortices located at the top and bottom of the diffuser, respectively. These vortex pairs entrain the low-energy fluids and transport them to the aerodynamic interface plane (AIP), resulting in two pairs of low-total-pressure regions and swirling flows. Moreover, in the experimentally studied range of the exit Mach number MAIP from 0.293 to 0.546, the distributions of low-total-pressure regions and swirl angles at the AIP exhibit a small variation, indicating that the internal flow field structures are insensitive to MAIP.

Aerodynamic study of a bifurcated turboprop engine inlet with a propeller for flow at ground suction conditions

An advanced turboprop inlet with a bypass duct plays a crucial role in preventing foreign object damage and ensuring a high-quality airflow to the engine. However, it also introduces an increased flow complexity and design challenge due to the interaction between the propeller and the bifurcated ducts. To address these issues, a combined experimental and computational fluid dynamics (CFD) study was conducted on the aerodynamic performance and flowfield characteristics of a turboprop inlet equipped with a bypass duct considering the propeller interference. A ground suction test bench was utilized for generating the working conditions and the performance was measured by using total pressure rakes and pressure scanners. It is found that the rotational propeller on the one hand does work on and thus increases the total pressure recovery of the inlet, however, on the other hand causes a turning effect on the inlet flowfield structure along the direction of rotation and increases total pressure and swirling flow distortions in the engine duct. Besides, the engine duct and the bypass duct interact with each other. The combined influence of suction effect and the profile induction of the inlet leads to the majority of the shed vortices being drawn into the engine duct. Lastly, the presence of deflectors installed in the engine duct is found to effectively mitigate the secondary flow, thereby reducing the swirl distortion within the engine duct. This study may provide a significant reference to the design and optimization of advanced turboprop inlets with bypass ducts.

Manifold geometry optimization and flow distribution analysis in commercial-scale proton exchange membrane fuel cell stacks

The manifold structure in high-power Proton Exchange Membrane Fuel Cell (PEMFC) stacks critically influences airflow distribution, thereby affecting stack performance, temperature uniformity, and longevity. Therefore, this paper explores how manifold geometry affects fluid distribution within the manifold of a fuel cell stack. In addition, a new step-shaped manifold structure is established to improve the manifold flow field structure and enhance the uniformity of gas flow distribution in each single cell. In our study, each cell within the stack is modeled as a porous medium. We examine how varying the height at different positions within the manifold impacts the uniformity of gas flow distribution, focusing on different length ratios and heights. Our results demonstrate that adjusting the manifold shape improves airflow uniformity, with length ratios having a more pronounced impact than height variations. Specifically, the mass flow coefficient of variation decreased by 22.62 % and 23.07 % for cases 9 and 12, respectively. Similarly, the velocity inhomogeneity coefficient in the inlet manifold decreased by 21.22 % and 21.34 % for cases 9 and 12, respectively. Additionally, pressure distribution in case 9 showed greater uniformity. Our findings indicate that a manifold shape with a length ratio of 2/3 and a height of 2.5 mm delivers optimal performance.

Numerical Study of the Combustion Process in the Vertical Heating Flue of Air Staging Coke Oven

To investigate the combustion process and reduce Nitric Oxide (NO) emissions in the vertical heating flue of air-staged coke ovens, a three-dimensional computational fluid dynamics method was applied to simulate the combustion process. The model integrates the k-ε turbulence model with a multi-component transport combustion model. The impact of air staging on the flow field and NO emissions in the vertical fire chamber was assessed through comparative validation with experimental data. The impact of air staging on the flow field and NO emissions in the vertical fire chamber was assessed through comparative validation with experimental data. Based on this research, the effects of the excess air coefficient and air inlet distribution ratio on NO emission levels at the flue gas outlet were further investigated. Analysis of the flow field structure, temperature at the center cross-section, component concentration, and NO emission levels indicates that as the excess air coefficient increases, the NO emission levels at the flue gas outlet initially decrease and then increase, accompanied by corresponding changes in outlet temperature. At an air excess factor of 1.3 and an air inlet distribution ratio of 7:3, NO emission levels are at their lowest—53% lower than those in a conventional coke oven—and the temperature distribution in the riser channel is more uniform. These results provide a theoretical foundation for designing the air-staged coke oven standing fire channel structure.

Development Status of Flow Field Design for Bipolar Plates in Hydrogen Fuel Cells

This article provides a detailed introduction to the current development status of flow field design for hydrogen fuel cell bipolar plates, analyzes the impact of traditional and new flow field designs on fuel cell performance, and explores innovative designs such as biomimetic flow fields and three-dimensional flow fields through numerical simulation and experimental verification by domestic and foreign researchers. Some scholars have also studied the effect of adding protrusions, sub channels, or groove structures in the flow channel on improving the performance of fuel cells.This article provides the latest progress and recent research results in the flow field design of hydrogen fuel cell bipolar plates for future researchers. These contents have important reference value and guiding significance for researchers to further explore new flow field designs and improve flow field structures.

Study on Vibration and Failure of Transverse Flow Turbine Based on Fluid–Structure Interaction

During the operation of a turbine, the water level drop can cause vibration and damage to the flow components, severely threatening its stability and reliability. Investigating the impact of operational parameters on the internal flow field and flow structures of tidal turbines under low flow conditions is crucial for peak shaving and deployment of tidal power stations. This paper takes a 24[Formula: see text]MW turbine as the research object, using numerical calculations to analyze the effects of tailwater height on the flow characteristics and structural properties of the unit under low flow conditions. It studies the hydraulic impact on the flow component walls under different operating parameters and verifies the reliability of numerical calculations through vibration and stress tests. The results show that the increase in tailwater level affects the turbine’s flow characteristics, forming large-scale, high-intensity vortices in the internal flow field and causing pressure pulsations near the wall surface. Modal analysis reveals that under different tailwater heights, the maximum modal effective mass exists along the axis of the impeller, with modal frequencies higher than the main frequencies of pressure pulsations. The impeller region corresponds to the turbine chamber wall bearing significant stress, which induces strain. The magnitude of stress and the degree of strain are positively correlated with the tailwater height. The research findings provide guidance for tailwater regulation and stable operation of tidal power stations.

Optimization Study on Nozzle Selection Based on the Influence of Nozzle Parameters on Jet Flow Field Structure

Currently, the primary method for controlling red tides in the ocean involves spraying water solutions with special chemicals as solutes. High-pressure spraying results in the formation of typical jet structures. In this study, numerical simulation methods are employed to investigate the velocity variations, turbulent characteristics, and gas content distribution of jet flow fields under different initial jet flow pressures, cone angles, and nozzle diameters. Based on practical application scenarios, cluster analysis is used to explore the similarities and differences in jet equivalent diameters under different parameter conditions. The research findings indicate the following. (1) The difference of jet velocity distribution at the far field exit will be enlarged with the increase in the nozzle cone angle. When the nozzle cone angle is 4 mm, the velocity uniformity at the outlet is the best. (2) The TKE of the flow field has no consistent change law along the central axis. At the jet exit, the TKE shows an obvious multi-peak structure. (3) The gas content demonstrates a typical “double-valley” feature at the jet outlet cross-section. Increasing the initial pressure leads to a decrease in the gas content within the jet due to reduced entrainment, while the entrainment range remains largely constant. (4) Cluster analysis reveals that the similarity of jet flow width when it reaches the water surface is minimal compared to other operating conditions when the initial pressure is 0.36 MPa, the cone angle is 115°, and the nozzle diameter is 2 mm. All conditions can be categorized into two or three groups to ensure jet effectiveness. The study results provide scientific guidance for selecting spray devices for controlling red tides in the ocean.

Development and Experimental Study of Supercritical Flow Payload for Extravehicular Mounting on TZ-6.

This paper provides a detailed description of the development and experimental results of the supercritical flow experiment payload carried on the TZ-6 cargo spacecraft, as well as a systematic verification of the out-of-cabin deployment experiment. The technical and engineering indicators of the payload deployment experiment are analyzed, and the functional modules of the payload are shown. The paper provides a detailed description of the design, installation location, size, weight, temperature, illumination, pressure, radiation, control, command reception, telemetry data, downlink data, and experimental procedures for the out-of-cabin payload in the extreme conditions of space. The paper presents the annular liquid surface state and temperature oscillation signals obtained from the space experiment and conducts ground matching experiments to verify the results, providing scientific references for the design and condition setting of space experiments and comparisons for the experimental results to obtain the flow field structure under supercritical conditions. The paper provides a specific summary and discussion of the space fluid science experiment project, providing useful references for future long-term in-orbit scientific research using cargo spacecraft.

Performance of the multi-U-style structure based flow field for polymer electrolyte membrane fuel cell

The design of the reactant gas flow field structure in bipolar plates significantly influences the performance of proton exchange membrane fuel cells (PEMFCs). In this study, we introduced four innovative U-shaped flow field designs, namely: In-Out Multi-U, Out-In Multi-U, Distro In-Out Multi-U, and Distro Out-In Multi-U. To investigate the impact of these various flow fields on PEMFC performance, we conducted computational fluid dynamics (CFD) numerical simulations, validated through model experiments. Our results indicate that the Distro Out-In Multi-U flow field offers notable advantages compared to the conventional parallel flow field (CPFF) and conventional serpentine flow field (CSFF). These benefits include reduced inlet and outlet pressures, lower liquid water content, more uniform liquid water distribution, and a more even current density distribution. Furthermore, the Distro Out-In Multi-U design demonstrates improved efficiency, consuming less H2 (91.9%) than the CSFF while achieving a higher net power density output (10.1%). As a result, for the same power output, the Distro Out-In Multi-U utilizes only 83.5% of the H2 consumed by the CSFF. In summary, the U-shaped structured flow field exhibits superior output performance, enhanced energy efficiency, and improved resistance to flooding. These findings suggest that the U-shaped flow field design holds significant potential as a reactive flow field for PEMFCs.

Flow-field analysis and performance assessment of rotating detonation engines under different number of discrete inlet nozzles

This study explores in depth rotating detonation engines (RDEs) fueled by premixed stoichiometric hydrogen/air mixtures through two-dimensional numerical simulations including a detailed chemical kinetic mechanism. To model the spatial reactant non-uniformities observed in practical RDE combustors, the referred simulations incorporate different numbers of discrete inlet nozzles. The primary focus here is to analyze the influence of reactant non-uniformities on detonation combustion dynamics in RDEs. By systematically varying the number of reactant injection nozzles (from 15 to 240), while maintaining a constant total injection area, the study delves into how this variation influences the behavior of rotating detonation waves (RDWs) and the associated overall flow field structure. The numerical results obtained here reveal significant effects of the number of inlets employed on both RDE stability (self-sustaining detonation wave) and performance. RDE configurations with a lower number of inlets exhibit a detonation front with chaotic behavior (pressure oscillations) due to an increased amount of unburned gas ahead of the detonation wave. This chaotic behavior can lead to the flame extinguishing or decreasing in intensity, ultimately diminishing the engine's overall performance. Conversely, RDE configurations with a higher number of inlets feature smoother detonation propagations without chaotic transients, leading to more stable and reliable performance metrics. This study uses high-fidelity numerical techniques such as adaptive mesh refinement (AMR) and the PeleC compressible reacting flow solver. This comprehensive approach enables a thorough evaluation of critical RDE characteristics including detonation velocity, fuel mass flow rate, impulse, thrust, and reverse pressure waves under varying reactant injection conditions. The insights derived from the numerical simulations carried out here enhance the understanding of the fundamental processes governing the performance of RDE concepts.

Study on the flow characteristics of double-cone in hypersonic flows

Reusable spacecraft is one of the hot topics in the field of aerospace, attracting wide attention from researchers. Due to the high-Mach-number flight of re-entry, the air near the wall exhibits significant thermochemical nonequilibrium effects. Besides, the existing shock wave/boundary layer interaction (SWBLI) leads to severe aerodynamic heating issues. This study utilizes the two-temperature model to conduct simulations of hypersonic laminar flows around a canonical 25°/55° double-cone with low and high enthalpy. By varying the wall temperature and the aft cone angle, the evolution mechanism of the flow under the high-enthalpy and hypersonic freestream is explored. Our findings illustrate that while the thermodynamic nonequilibrium model closely reflects the separation zone evidenced in the experiments, it habitually overpredicts the peak heat transfer, particularly under high-enthalpy conditions. For the thermochemical nonequilibrium model, the flow field structure appears more uniform, with a reduced standoff distance of the detached shock. An incremental rise in wall temperature correlates with a proportional augmentation of the separation bubble, though its impact on the overall flow field is negligible. Increasing the aft cone angle intensifies the shock/shock interaction (SSI), transitioning from a lower-intensity Type VI to a more intense Type IV shock interplay. The examination reveals that the increase in temperature and cone angle amplifies the interaction between the separation region and shock waves, drastically escalating the peak heat transfer and fostering a secondary peak. The hypersonic flow of the double-cone demonstrates a multifaceted interaction of phenomena, notably SWBLI and SSI, with our analyses providing pivotal insights for the aerodynamic and thermal protection design of the high-Mach-number spacecraft.

Investigation of combustion mode conversion driven by fuel flow variation in a cavity-based scramjet

This work aimed to investigate combustion mode conversions by rapid variation of the fuel flow rate in a cavity-based scramjet combustor. The experiments were carried out on a direct-connected facility with an inflow condition of Mach number 2.52, a total pressure of 1.35 MPa, and a total temperature of 1650 K. The fuel injector consisted of two injection ports: fuel was continuously injected from one port while the other controlled the fuel flow for mode conversions by switching it on or off. Simultaneous schlieren and CH* imaging techniques were used to characterize the dynamics of combustion mode conversions. It was recognized that the combustion modes characterized by different flow field structures and heat release distributions can be classified into three types: the shear-layer mode, the transition mode, and the jet-wake mode. During the combustion mode conversion, the mixing region of the transverse jet and air became thicker with the increase in fuel flow rate, and the gradient of the flow field density and the flame area increased, making the flame more likely to propagate upstream. The combustion suppression induced by rapid fuel addition was observed at low equivalence ratios. It was speculated that the weak heat supply was insufficient to provide adequate heat for the rapid ignition of the added fuel. Furthermore, it was found that the flame-flow matching process with frequent flame propagation upstream occurred during the combustion mode conversion. This process was attributed to the mismatch between the increasing heat release and the original flow field structure.