Large Vortical Structures Research Articles

On Using the Distributor as a Multi Degree-of-Freedom System to Mitigate the Pressure Pulsation in an Axial Turbine at Speed-No-Load

Abstract Hydraulic axial turbines are more frequently utilized for grid regulation purposes. Sometimes, they must be operated at speed-no-load (SNL) conditions, which is characterized for some machines by a varying number of large vortical flow structures extending from the vaneless space to the draft tube, introducing detrimental pressure pulsations throughout the turbine. A recent study shows that the vortices can be mitigated by individually controlling the guide vanes. Since optimization of the distributor layout is linked with a large degree-of-freedom, machine learning is deployed to assist in finding an optimal setup cost-effectively. A reduced numerical computational-fluid-dynamics (CFD) model is built and used to generate input for Gaussian process regression surrogate models by performing 2000 steady-state simulations with varying distributor layouts. The surrogate models suggest that the optimal layout is to open seven out of 20 guide vanes in succession while keeping the remaining ones closed. However, this configuration induces large radial forces on the runner, and after implementing some modifications by trial and error, detailed time-dependent CFD simulations show that placing 4 + 3 opened guide vanes on opposite sides of the runner axis is better; it reduces the pressure peaks corresponding to a two- and three-vortex configuration, and the maximal pressure pulsations by as much as 88% in the vaneless space compared to regular SNL operation. Meanwhile, the radial force on the runner is reduced by more than 83%, and pressure pulsations on the runner blades by more than 55%, compared to the surrogate models' optimal layout prediction.

Effects of axisymmetric confinement on vortical structures emanating from round turbulent jets

Confined jets occur in many engineering applications including combustion chambers, jet pumps, and chemical reactors. The effects of axisymmetric confinement on the vortical structures identified in a turbulent jet are investigated using large eddy simulation at a Reynolds number of 30 000 (based on nozzle exit conditions) and expansion ratio (chamber-to-nozzle diameter ratio) of five. The results obtained from the confined jet are compared with those of a free jet under the same nozzle exit flow conditions. A prominent recirculation zone forms between the expanding jet and the confining wall, resulting in early shear layer distortion and a shorter interaction length in the confined jet (0.85 jet diameters) compared to the free jet (1.15 jet diameters). Using the λ2 criterion for vortex identification, two dominant structural modes are identified in the near-exit region of the free jet: ring and helical modes. However, in the confined jet, the helical mode is absent, and the turbulent confined fluid accelerates the breakup of the ring vortices. The interaction of the secondary line vortices with the breaking structures leads to the formation of new hairpin-like vortices, which also contribute to further vortex breakup. These results explain the enhanced mixing performance of confined jets as the mixing is directly tied to the breakup of large vortical structures. Proper orthogonal decomposition modes are also presented to identify the structures/events with the highest contribution to the total turbulent kinetic energy in both flow fields.

Experimental Validation of the Modified Holmén Vortex Identification Method

The iterative vortex identification method originally proposed by Holmén was modified to improve its performance and to add the ability to distinguish between counter-rotating vortices. Then, the resulting modified Holmén method (MHM) was evaluated using experimental data from five different experiments, including planar and three-dimensional flowfield data. The Q, λ2, and Γ2 criteria were used for comparison. No currently available vortex identification method is perfect (otherwise, proposing a new scheme would be a pointless exercise), and the MHM is no exception; however, the MHM was found to accurately identify vortical structures when the number of passes is chosen correctly while minimizing the experimental noise. In particular, it was found to be useful in capturing large vortical structures occurring in a noisy or high-shear region, cases where the Q and Γ2 criteria sometimes fail. On the other hand, the method is limited by the spatial resolution of the experimental data with respect to the structure’s size and its inability to resolve rotating structures near the edges of the measurement domain.

Numerical prediction of mean flow and acoustic field of a supersonic impinging jet

The three-dimensional turbulent mean flow and acoustic field of a supersonic jet impinging on a solid plate is studied computationally using the general purpose CFD code Ansys Fluent. A pressure-based coupled solver formulation with the second order weighted central-upwind spatial discretization is applied to compute transient solutions. Cold and hot jet thermal conditions are considered. Mean flow characteristics are investigated by a steady-state modeling approach. Acoustic radiation of impingement tones is simulated using a transient time-domain formulation. The effects of turbulence in steady-state are modeled by the SST k-ω turbulence model. The Wall-Modeled Large-Eddy Simulation (WMLES) model is applied to compute transient solutions. The near-wall mesh on the impingement plate is fine enough to resolve the viscosity-affected near-wall region all the way to the laminar sublayer. Nozzle-to-plate distance is parameterized in the model for automatic re-generation of the mesh and results. Steady-state predictions of hover lift loss and mean jet velocity distributions are compared with experimental data, and favorable agreement is reported. The transient solution reproduces the mechanism of impingement tone generation by the interaction of large scale vortical structures with the impingement plate. The acoustic near-field is directly resolved by Computational Aeroacoustics (CAA) to accurately propagate impingement tone waves to near-field microphone locations. Calculated impingement tone frequencies and sound pressure levels agree with experimental values.

Numerical analysis of turbulent nitrogen-diluted hydrogen flames stabilised by star-shaped bluff bodies

In this paper, we investigate the flow and flame dynamics within a combustion chamber supplied with nitrogen-diluted hydrogen fuel and air as oxidiser. The chamber is equipped with a bluff body that serves as a flame stabiliser. We focus primarily on the influence of the bluff body shape on the formation of large and small-scale vortical structures generated at the bluff body edge. We analyse how the geometry of the bluff body alters these structures and examine the impact of resulting modifications on flame characteristics, including key features such as flame length and radial extent, temperature and species field, and completeness of the combustion process. We consider four bluff body geometries, namely a typical cylindrical shape, a star-shaped wall, and a small triangular sharp and smooth wavy structure. The research employs the Large Eddy Simulation (LES) method, providing deep insight into the complex physics of unsteady flow. In the cylindrical bluff body configuration, the key mechanism driving the fuel-oxidiser mixing process is the periodic occurrence of large toroidal vortices shed at an exit of the oxidiser duct and on the bluff body edge. They exert the flow towards and away from a recirculation zone formed above the bluff body. When its geometry is irregular, the appearance of the large vortices is suppressed, however, the wall irregularities induce small-scale flow phenomena that enhance mixing in the shear layer formed between the recirculation zone and the oxidiser stream. The mixing efficiency is largely dependent on the shape of the bluff body wall, and it turns out that seemingly small changes (sharp vs. smooth small triangular parts), which in non-reactive regimes have only a minor effect on the flow dynamics, significantly change the flame length and fuel consumption rate. Compared to the cylindrical bluff body configuration, the small triangular parts cause an axial shift of the maximum temperature downstream and slow down fuel consumption. Contrary, large triangular elements in the star-shape bluff body clearly influence the flow both in non-reactive and reactive regimes. They cause the formation of local oxidiser streams directed towards the fuel-rich mixture. Consequently, the injected fuel burns at a rate similar to that observed in the case of the cylindrical bluff body, but the combustion process occurs at an average temperature approximately 200 K lower and its fluctuations are reduced. These factors may influence the thermal reduction of NOx. On the other hand, a negative aspect is the highest level of hydrogen in the closest vicinity of the bluff body, which in practical applications may accelerate the embrittlement of its surface.

Direct numerical simulation of a pressurized thermal shock scenario at higher reynolds number

Direct Numerical Simulation (DNS) is performed for a pressurized thermal shock scenario in a reactor downcomer geometry. In the present work, a simplified representative geometry is adopted where cold fluid injected from a square duct is injected in a planar downcomer. A secondary hot inlet at the top of the downcomer is employed to enforce a downward flow of the injected emergency core cooling water, to mimic the effect of density driven flow. The Reynolds number of the impinging flow, based on bulk velocity and duct height, equals 12,500, while a unity Prandtl number fluid is used. The simulation is performed for two thermal boundary conditions, namely an isotemperature condition and an adiabatic condition, representing the two extreme scenarios of a conjugate heat transfer problem. The instantaneous and mean fields are first analysed in order to study the flow pattern and formation of vortical structures within the downcomer. The impinging flow is deflected radially outwards in the vicinity of the barrel wall, which then interacts with the surrounding hot fluid forming large vortical structures and bringing the cold fluid back to the vessel wall. The mixing of flow and penetration of colder temperatures in the downcomer is observed to be greatly influenced by interaction of these vortical structures. The descending cold plume in the downcomer is observed to form three branches separated by low-speed regions. The maximum values of turbulent kinetic energy are observed only in the vicinity of flow impingement, while high values of its production and dissipation are also observed within the shear layers of the descending plume. The maximum values of temperature fluctuations are found at the interface of the descending cold plume and surrounding hot fluid, while relatively high values are also seen at the locations of large vortical strucutres. Anisotropy in turbulence is also analysed using the componentality contour approach and a modified barycentric colour mapping scheme. Also reported herein is a comparison of mean and statistical profiles with those at lower Reynolds number reported in the literature. At higher Reynolds number, the cold impinging jet is deflected farther outwards in the downcomer. The locations of peak fluctuations in velocity and temperature are also farther away from the impingement. The magnitudes of fluctuating temperature and turbulent kinetic energy are observed to be higher for the present higher Reynolds number case. The peak Nusselt number at the barrel wall is observed to scale with a factor of Re1/2 in the present configuration.

High-fidelity numerical investigation of a real gas annular cascade with experimental validation

This study aims at investigating real gas flow in the complex geometry of the Cambridge University annular turbine cascade using numerical simulations. The objectives include validating the numerical approach and understanding the loss mechanisms in this configuration. The numerical results are compared to experimental measurements obtained at various locations in the domain. Two turbulence modeling techniques, large Eddy simulation (LES) and Reynolds-averaged Navier–Stokes (RANS), are employed to assess the influence of turbulence models and inlet turbulence levels. The results show good agreement between numerical simulations and experimental measurements in regions upstream of the trailing edge. However, discrepancies arise in the transition region of the suction side boundary layer, and RANS results are influenced by the choice of turbulence injection. In the wake of the blade, both RANS and LES accurately predict the stagnation pressure ratio, with some slight differences in shock positions and total pressure levels. The analysis reveals that large vortical structures at the hub contribute significantly to the overall losses in this annular configuration. The study quantifies losses due to boundary layers, the wake, and vortical structures using a loss coefficient, with RANS and LES producing slightly different results. These differences, while calling for further experimental measurements, also hint at the possible inaccuracy of the present turbulence models in the context of real gas flows for which a dedicated modeling effort is required.

Aerodynamics and Wake Flow Characteristics of a Four-Cylinder Cluster

The aerodynamic behaviour and wake flow of a cluster of two-dimensional sharp-edged bluff bodies exhibits extremely complex unsteady phenomena in both near and far fields. Due to the high cost of wind-tunnel experiments and numerical simulations, a complete understanding of wake flows and a description of their characteristics are lacking. This paper presents large-eddy simulations (LES) in different flow/wind directions for a cluster of 2 times 2 aligned square cylinders, at a separation distance in streamwise and cross-wind directions equal to cylinder side length, and at Reynolds number {Re}=22,000 based on the single cylinder side length D. The case at 0^circ incidence shows an evident channel-type flow in the along-wind street/gap, and at its exit an irregularly pulsing jet with an intense shedding of large vortices. The wavelet analyses of the side force/lift coefficient and instantaneous velocities in the wake show that the characteristic length and time scales of the large vortical structures in the far-field wake are close to the cluster size 2D; this is the so called ‘cluster effect’. The cluster effect increases monotonically as the flow incidence angle increases. At a large incidence angle in the near-field wake, the cylinder-scale flow structures are much weaker compared to the cluster-scale structures. At the incidence angle of 45^circ, the overall wake flow and the aerodynamic characteristics are well scaled by the scale approximately equal to 2D. Nevertheless, the interaction between cylinders significantly affects the aerodynamics performance of the individual cylinders. The drag and lift coefficients of the individual cylinders differ substantially from each other in the cluster, and are significantly different from observations on a single isolated cylinder too.

Influence of pilot H[formula omitted] injection on methane-air swirled flame stabilization and acoustic response

Large Eddy Simulation is used to investigate the effect of localized pilot H2 injection on the Flame Transfer Function (FTF) of a premixed CH4-air swirled flame. The response of a perfectly premixed methane-air flame is compared to the one with an additional pilot hydrogen injection that supplies 10% of the original power. Numerical simulations are validated against experiments in terms of global FTF values at selected forcing frequencies, acoustic pressure and velocity signals, CH* flame images and flame root position dynamics. The unforced cases are first considered showing that, when H2 is injected in the center, the flame becomes slightly lifted with a localized diffusion front above the pilot injection. As in the experiments, LES retrieves that hydrogen pilot injection leads to a global redistribution of the heat release rate towards the flame root due to higher burning rates which translates to an overall FTF gain reduction over the entire frequency range explored. Then, the flame acoustic response for the two injection strategies is scrutinized at two distinct forcing frequencies: 240 Hz where the FTF gain difference is maximum, and 590 Hz where the FTF phase shift is maximum. LES reveals that, despite H2 pilot injection does not modify the dynamics of the large vortical structures shed in the external shear layer of the swiling jet which are synchronized by the acoustic forcing, the redistribution of the heat release rate towards the flame base weakens their interaction with the flame tip, explaining partly the FTF gain reduction at the two selected frequencies. In addition to that, a marked axial movement of the internal recirculation zone is observed at 240 Hz during the forcing cycle. For the pilot injection, it leads to an oscillation of the lifted flame root while, for the CH4-air case, the flame anchoring point is not affected. This additional oscillation leads for the pilot case to heat release rate fluctuations acting in phase opposition with respect to those observed at the flame tip, generating a further drop of the FTF gain at this specific frequency. The increased burning rate at the flame root and the flame length reduction of the pilot hydrogen flame also affect the characteristic time lag of the flame response. For both frequencies f = 240 Hz and 590 Hz, the phase shift between the two injection strategies is proportional to the flame length reduction caused by hydrogen injection. These simulations confirm that pilot hydrogen injection is an efficient way to reduce the acoustic response of swirled flames over a large frequency bandwidth.

On Dynamics of Flow past a Stage Axial Air Turbine

The air flow inside a single-stage test turbine is studied experimentally using the Particle Image Velocimetry technique. The measurements were performed at the axial-tangential plane just behind the rotor at the middle blade radius. The large vortical structures are identified in the shear layers between the blade wakes and channel jet. The phase-locked structures topology of random dynamical structures in the flow behind the stage are to be analysed using the Proper Orthogonal Decomposition method.

Numerical analysis of influence of various bluff-body shapes on diffusion flame dynamics

In this paper, various bluff-body shapes (cylindrical, square, star) and two different surface topologies (smooth, wavy) are applied as passive tools for controlling a non-premixed hydrogen flame in a combustion chamber. We focus on the dynamics of the flame and its time-averaged characteristics in the close vicinity of an injection system within formed recirculation zones and also in a far-field. The research is performed with the help of large-eddy simulations (LES) method using the ANSYS Fluent software and a high-order academic code SAILOR. Flame behaviour is found to be strongly dependent on the geometry of the bluff-body whereas its wall topology affects the flame characteristics only slightly. In the cases with the square and star bluff-body, small vortical structures originating at the corners deform large vortical structures created by the Kelvin-Helmholtz instability mechanism. This intensifies the mixing and combustion process and, in the configuration with the square shape bluff-body, translates to the shortening of the recirculation zone by 15% of the equivalent bluff-body diameter and the flame, which in the axis develops closer to the bluff-body. The star shape leads to the most uniform flame at the radial border or the recirculation zone.

Control of the noise production in a supersonic jet impinging an inclined plate using grooved surface

This paper presents an experimental investigation on free jet and jets impinging on smooth and grooved surfaces. Acoustic studies are performed using free-field microphones, while two dimensional (2D)-particle imaging velocimetry (PIV) is utilized to investigate the velocity fields. The effects of nozzle-plate spacing distance, nozzle pressure ratio (NPR) and plate inclination are also investigated. The results demonstrate that a high-speed jet impinging on a grooved plate generates lesser acoustic radiation on an inclined plate surface when compared to a smooth plate with 2.16 < NPR < 3.1 in the range of L/d = 3 ∼ 5. Reduction of the amplitude of discrete tones is an important factor in its noise reduction. To present a detailed explanation of this phenomenon, this paper further coupled the acoustic discrete tones and mean fluctuating flow velocities. Results showed that screech tone is mainly caused by the interaction between the rear edge of the fourth shock cells and the large vortical structures in the mixing layer, and the shear layer of the wall-jet has a certain shielding effect on the near-wall acoustic power, and the acoustic power decreases at high-speed impinging jets.

Experimental and Numerical Study on Vortical Structures and Their Dynamics in a Pump Sump

Research on water flow in a pump inlet sump is presented. The main effort has been devoted to the study of the vortical structures’ appearance and their behavior. The study was conducted in a dedicated model of the pump sump consisting of a rectangular tank 1272 × 542 × 550 mm3 with a vertical bellmouth inlet 240 mm in diameter and a close-circuit water loop. Both Computational Fluid Dynamics (CFD) and experimental research methods have been applied. The advanced unsteady approach has been used for mathematical modeling to capture the flow-field dynamics. For experiments, the time-resolved Particle Image Velocimetry (PIV) method has been utilized. The mathematical modeling has been validated against the obtained experimental data; the main vortex core circulation is captured within 3%, while the overall flow topology is validated qualitatively. Three types of vortical structures have been detected: surface vortices, wall-attached vortices and bottom vortex. The most intense and stable is the bottom vortex; the surface and wall-attached vortices are found to be of random nature, both in their appearance and topology; they appear intermittently in time with various topologies. The dominant bottom vortex is relatively steady with weak, low-frequency dynamics; typical frequencies are up to 1 Hz. The origin of the vorticity of all large vortical structures is identified in the pump propeller rotation.

Flow characteristics and performance of the propulsion system with wavy duct

The present study applies the wavy shape on a wake equalizing duct before a propeller. The numerical simulations are performed to investigate the effect of wavy duct on a propulsion performance in the ranges of angles of attack (0°≤α≤15°) and the wavenumber of the duct ((4≤Wn≤32)). All ducts contribute to improving the propeller efficiency. The effect of the wavy duct on the force coefficients and their dependence on Wn become significant with increasing α. When α=10°, the wavy duct gives larger force coefficients and propeller efficiency with increasing Wn. The better performance of the propeller at the largest Wn is physically explained by the increase of the axial velocity in the slipstream region, more uniform distribution of the upstream velocity, and wider distributions of the low and high pressures in the suction and pressure side surfaces, respectively. The vortical structures of the wavy ducts and those size depend on the circumferential plan and Wn. The largest Wn breaks large vortical structures into smaller structures. The increase of the local momentum near the troughs contributes to the modification of vortical structures. Also, the appearance of additional axial vortices over the troughs is associated to modify the vortical structures with the largest Wn.

Droplet nuclei caustic formations in exhaled vortex rings

Vortex ring (VR) structures occur in light or hoarse cough configurations. These instances consist of short impulses of exhaled air resulting to a self-contained structure that can travel large distances. The present study is the first implementation of the second order Fully Lagrangian Approach (FLA) for three-dimensional realistic flow-fields obtained by means of Computational Fluid Dynamics (CFD) and provides a method to calculate the occurrence and the intensity of caustic formations. The carrier phase flow field is resolved by means of second order accurate Direct Numerical Simulation (DNS) based on a Finite Difference approach for the momentum equations, while a spectral approach is followed for the Poisson equation using Fast Fourier Transform (FFT). The effect of the undulations of the carrier phase velocity due to large scale vortical structures and turbulence is investigated. The evaluation of the higher order derivatives needed by the second order FLA is achieved by pre-fabricated least squares second order interpolations in three dimensions. This method allows for the simulation of the clustering of droplets and droplet nuclei exhaled in ambient air in conditions akin to light cough. Given the ambiguous conditions of vortex-ring formation during cough instances, three different exhale (injection) parameters n are assumed, i.e. under-developed (n=2), ideal (n=3.7) and over-developed (n=6) vortex rings. The formation of clusters results in the spatial variance of the airborne viral load. This un-mixing of exhumed aerosols is related to the formation of localised high viral load distributions that can be linked to super-spreading events.

The Impact of Steady Blowing from the Leading Edge of an Open Cavity Flow

Cavity flows occur in a wide range of low-speed applications (Mach number ≤0.3), such as aircraft wheel wells, ground transportation, and pipelines. In the current study, a steady jet is forced from a cavity leading edge at different momentum fluxes (0.11 kg/ms2, 0.44 kg/m·s2, and 0.96 kg/m·s2). The investigation was performed for an open cavity with length to depth ratio of 4 at the Reynolds number based on a cavity depth of approximately 50,000. Particle image velocimetry, surface oil flow visualisation, constant temperature anemometry, and pressure measurements were performed in this investigation. The aim of the jet blowing is to separate the cavity separated shear layer from the recirculation zone to reduce the cavity return flow, and hence stabilise the cavity separated shear layer. It was found that increasing the jet momentum flux causes an increase in the cavity return flow due to the increase in the thickness of the cavity separated shear layer. The study also found that the jet populates the separated shear layer with a large number of small-scale disturbances. These disturbances increase the broad band level of the pressure power spectra and Reynolds shear stress in the cavity separated shear layer. On the other hand, the jet disturbances make the shedding of the large vortical structures more intermittent.

Proper orthogonal decomposition of turbulent swirling flow of a draft tube at part load

A large scale vortical structure, i.e., rotating vortex rope (RVR), generally forms at part load (PL) operation of a reaction turbine due to the combined effect of swirling flow at the runner outlet and the adverse pressure gradient in the draft tube. Even with the data analysis through advanced measurement techniques like Laser Doppler velocimetry (LDV) and Particle Image Velocimetry (PIV), the information available about the flow instabilities developed due to RVR is still incomplete. This paper presents an application of the proper orthogonal decomposition (POD) method to analyze the flow field inside the draft tube cone at PL operation of a high-head model Francis turbine. The POD analysis is performed on 250 PIV snapshots containing the axial and radial velocities. The results show that the first eight modes contain more than 95% of the total kinetic energy (KE) of the flow field and are associated with the organized motion of the flow. The first mode contains more than 50% of the total energy, and the axial velocity profile reconstructed with the first mode is identical to the mean axial velocity flow field. The maximum dissipation of the turbulent kinetic energy (TKE) occurs through unorganized motion.

Fluid Dynamics of a Bistable Diverter Under Ultrasonic Excitation—Part II: Flow Visualization and Fundamental Mechanisms

Abstract The switching mechanism and underlying flow physics of an actively controlled fluidic device are investigated using both large eddy simulation (LES) and particle imaging velocimetry (PIV). The fluidic device considered herein uses acoustic excitation of inherent flow instabilities to control the movement of the jet. Acoustic excitation at the preferred frequency is shown to yield high saturation amplitudes resulting in the formation of large vortical structures that do not undergo pairing. Basic flow features including the shear layer instabilities are further examined to explain why the excitation mode that triggers the switching process changes from a shear layer-based mode (Stθ=0.012) to a jet orifice mode (Sth=0.25) as the Reynolds number increases.

Numerical Flow Characterization around a Type 209 Submarine Using OpenFOAM

The safety of underwater operation depends on the accuracy of its speed logs which depends on the location of its probe and the calibration thoroughness. Thus, probes are placed in areas where the flow of water is smooth, continuous, without high velocity gradients, air bubbles, or vortical structures. In the present work, the flow around two different submarines is numerically described in deep-water and near-surface conditions to identify hull zones where probes could be installed. First, the numerical setup of a multiphase solver supplied with OpenFOAM v7 was verified and validated using the DARPA SUBOFF-5470 submarine at scaled model including the hull and sail configuration at H/D=5.4 and Fr=0.466. Later, the grid sensitivity of the resistance was assessed for the full-scale Type 209/1300 submarine at H/D=0.347 and Fr=0.194. Free-surface effect on resistance and flow characteristics was evaluated by comparing different operational conditions. Results shows that the bow and near free-surface regions should be avoided due to high flow velocity gradient, pressure fluctuations, and large turbulent vortical structures. Moreover, free-surface effect is stronger close to the bow nose. In conclusion, the probe could be installed in the acceleration region where the local flow velocity is 15% higher than the navigation speed at surface condition. A 4% correction factor should be applied to the probe readings to compensate free-surface effect.

A 2-D DNS study of the effects of nozzle geometry, ignition kernel placement and initial turbulence on prechamber ignition

A parametric direct numerical simulation study was conducted to investigate the effects of the initial flow field (quiescent or turbulent), nozzle inlet sharpness and width, main chamber composition (lean and stoichiometric), and ignition kernel placement in a two-dimensional prechamber (PC) ignition system. The strongly coupled operating and geometric parameters determine the time at which the flame exits the prechamber, the transient structure and penetration of the initially cold and subsequently hot reactive jet and their impingement on the lower main chamber (MC) wall, affecting the combustion mode and the fuel consumption rate. The temperature of the flame reaching and crossing the nozzle is affected by the flame exit time and is significantly lower than the adiabatic flame temperature of the planar flame, although no quenching is observed. Interaction with the flow field (strong small scale vortices for narrow and sharp entry nozzles, large vortices for wide nozzles) generated close to the exit increases the surface area of the flame and its interaction with the MC mixture. Jet penetration and impingement on the lower MC wall is determined by combustion in the PC and the flow field it generates in the main chamber. Impingement results in large scale vortical structures, which further contribute to the flame area increase and accelerate the consumption of the MC charge at later times. For the conditions studied, budget analysis shows that the main combustion mode is premixed deflagration with locally enhanced or reduced reactivity. Local flame–flame interactions which are more pronounced close to the nozzle exit and the lower MC wall can increase the propagation speed up to six times compared to the planar flame. The evolution of the probability density functions of different quantities is used to characterize the strongly transient process.