Flame Interaction Research Articles

Characterization of a 5-nozzle array using premix/micromix injection for hydrogen

Hydrogen is one of the most promising fuels to decarbonize energy systems since it has a high specific energy, a carbon-free combustion process, and may be produced sustainably through electrolysis. Direct fuel drop-in replacement cannot be done in traditional lean, premixed combustion burners because of the high reactivity of the hydrogen-air mixture and its inherently unstable nature that might lead to flashback and hardware failure. Consequently, new injection strategies and burner geometries are investigated to mitigate these risks. Here we present hydrogen capabilities of a premix/micromix injector that relies on a two-staged fuel injection strategy to offer wide fuel flexible capability (methane to hydrogen) within a single design. Five injectors are placed in a cross-shaped array to simulate a sector found in multi-element combustion systems. Stability and combustion dynamics maps are obtained for the array and OH planar laser-induced fluorescence (PLIF) provides additional insight into the combustion process of these novel burners when placed in an array. The use of micromixing is shown to drastically improve the acoustics of this geometry and expand the flashback limits compared to the fully premixed configuration. Significant variability in flashback limits are observed for different additively-manufactured injector configurations. Phase-averaged OH-PLIF measurements, obtained by registering the acquisition with the acoustics module, and the 3D reconstruction of a partially-premixed flame highlight the complexity of the stabilization mechanisms for these highly three-dimensional, non-axisymmetric flames that may be subjected to large thermoacoustics. This first investigation into premix/micromix clustered injectors demonstrates the importance of better understanding the impact of flame–flame interaction in multi-element combustion systems with micromixed, or partially-premixed, combustion.

Numerical study on the shock wave and vortex and their coupling effect on the behavior of the flame induced by the spontaneous ignition of high-pressure hydrogen release

High-pressure hydrogen storage is currently the mainstream way of hydrogen storage. However, the risk of spontaneous ignition and subsequent jet flame are serious obstacles to the wide application of hydrogen energy. To further reveal the interaction of shock waves, vortices and flame induced by the spontaneous ignition of high-pressure hydrogen release under the effect of release process and burst pressure, a numerical study is conducted by employing LES, RNG, EDC models and detailed hydrogen/air combustion mechanism. The qualitative and quantitative comparison of flow field and flame splitting process outside the tube between this numerical study and previous experimental results validate the numerical results. It is found that an initial vortex is formed at the edge of the jet. Then, several vortices are generated at the edge of the jet and ahead of the Mach disk. The number of vortices in the outer side of the barrel shock is larger when the opening time is shorter. After moving out of the tube, the flame front becomes hemisphere and the lateral edge has a hemispherical bulge which is induced by the initial vortex. The temperature is low and the hydrogen concentration is high in the high-speed region which is induced by the reflected shock wave, leading to the splitting of the flame into two parts: the first and the second half of the flame. However, the flame cannot be formed at the initial stage when the opening time increases to 150 μs and the flame area is larger than with smaller opening times. After splitting, under the affection of the vortices generated at the lateral side, the first half of the flame rotates backward and gradually merges with the second half of the flame, finally forming a complete hydrogen jet flame.

Impact of spray interaction on ammonia/diesel dual-fuel combustion and emission under engine relevant conditions

This study investigates the combustion characteristics of ammonia and diesel sprays in a constant-volume vessel under conditions typical of internal combustion engines, focusing on the interplay between evaporation dynamics and flame interactions within the framework of the Direct Dual Fuel Stratification (DDFS) concept. Under non-evaporation conditions, ammonia and diesel sprays exhibit comparable evaporation profiles, but under evaporation scenarios, ammonia’s higher evaporation rate results in faster mixing with ambient gas than diesel despite the similar liquid penetration lengths of these two fuels. By utilizing meticulously designed arrangements of diesel and ammonia spray injectors, two distinct interaction scenarios between diesel spray and ammonia spray, early and late interaction, are explored. In the early interaction scenario, fuel-rich ammonia-air mixtures ignite directly by the diesel flame, achieving self-sustained propagation and significant heat release, thereby maintaining a high-temperature region for continuous combustion. Conversely, in late interaction scenarios, rapid ammonia evaporation leads to a fuel-lean ammonia/air mixture that cannot be ignited by the diesel flame, eventually leading to ammonia flame extinction. The study reveals that NOx and N2O emissions are sensitive to the diesel/ammonia flame interaction. N2O emissions, formed predominantly at the forefront of the quenching ammonia flame, pose a significant challenge due to the fast evaporation rate and slow oxidation rate in fuel-lean mixtures. These findings provide insights into the physics of ammonia–diesel combustion, highlighting the challenges and potential strategies for efficient and clean combustion in ammonia-fueled DDFS engines.

Frequency jump of a flickering buoyant jet diffusion flame influenced by ambient coflow

Flickering of laminar diffusion flames is known to be caused by the buoyancy-induced toroidal vortices attached to the outer flame surface, making a classical model for understanding flame-vortex interactions. This work experimentally investigates the impacts of ambient coflow on the flickering behaviors of a buoyant jet diffusion flame. Utilizing a simultaneous high-speed flame imaging/OH⁎ chemiluminescence imaging/particle image velocimetry measurement system, the evolutions and interactions of coherent flame and flow structures are resolved. The results demonstrate that the periodic deformation of the flame surface (i.e., flame flickering) arises from the periodic formation, growth, and shedding of the buoyancy-induced outer vortex rings (OVRs), manifesting as a hydrodynamic instability developing along the outer shear layer (OSL). Upon applying a weak-to-moderate coflow, the flame bulge's size gradually reduces, and the flickering frequency slightly increases with increasing coflow velocity. The notable frequency jump phenomenon is confirmed as the coflow velocity rises to a threshold value such that the flickering frequency undergoes a sudden increase of 3–4 Hz; this is also accompanied by a significant reduction in the flame bulge's size and a delay in flame pinch-off. Moreover, a second frequency jump of 2–3 Hz occurs at a higher coflow velocity. Based on quantitative analysis of the OVR core's evolution, we find that the frequency jumps result from a sudden downstream shift of the initial OVR core, essentially indicating the jump of the instability onset point of the OSL. From a hydrodynamic instability perspective, this phenomenon can be explained as a stabilization effect of the coflow on the OSL, resulting in the re-establishment of the instability condition in a downstream location.

Non-periodic shear layer instabilities driving local extinction in premixed ramjet cavity flames

The local extinction characteristics of turbulent premixed ethylene-air flames are experimentally investigated in a high-speed cavity combustor to provide a full characterization of the temporally evolving non-periodic hydrodynamic instabilities that result in local extinction. The local flame extinction phenomenon is captured temporally using high-speed particle image velocimetry (PIV) and broadband chemiluminescence. The evolution of the flame topology, turbulent structures, and flame strain rate are analyzed through the extinction process to distinguish the turbulent shear layer mechanisms contributing to flame instabilities and extinction. A frequency analysis of the extinction event was performed and found the flame extinction to occur at non-periodic temporal intervals. A further wavelet decomposition revealed the presence of a non-periodic hydrodynamic instability in the shear layer causing extinction. The instability mechanism is related to the stability of vortex pairing in the shear layer, which were found to correspond to large-amplitude wave generation on the flame surface. In instances of flame extinction, the vortex pairs exhibit a core deformation from an elliptic instability mechanism, leading eventually to the shredding of the vortical cores, which is the primary mechanism for excessive turbulent flame strain and resulting extinction. Although the local flame extinction mechanism is driven from flame and shear layer interactions, the condition for local flame extinction to occur requires the flame to interact with an elliptical instability in the shear layer. The results can guide strategies to enhance combustion stability, performance, and efficiency.

Propagation and topology in turbulent premixed flames

The mechanism of propagation close to flame–flame interaction events is analysed using direct numerical simulation of a turbulent premixed methane–air flame. Four canonical local topologies arising from flame–flame interaction are identified in the vicinity of critical points. These correspond to reactant pocket, tunnel closure, tunnel formation and product pocket. The two spherical topologies (reactant and product pockets) are found to propagate consistently with no change in direction. Reactant pockets tend to propagate in the direction normal to the flame while product pockets tend to diffuse in the counter–normal direction. In contrast, both cylindrical topologies (tunnel closure and formation) may propagate either normally or counter–normally. It is shown that the direction of propagation for these topologies is strongly linked to principal curvatures of the flame surface. In such cases, the direction of propagation may reverse as the topology evolves and the principal curvatures change over time. Thus the conditioning on topology allows for more accurate estimation of displacement speed which is central to modelling turbulent flame speed.

Experimental investigations on acoustic-velocity-flame dynamic characteristics of a stratified burner

To reveal the influence of interaction between pilot and main flames on flow and thermo-acoustic instability characteristics, high-frequency measurement devices such as High-Frequency Particle Image Velocimetry (HFPIV), high-speed camera, pressure sensors, and photomultiplier tube were applied to study the acoustic-velocity-flame dynamic characteristics of the stratified burner under ambient temperature and pressure. Results show that the pilot swirling flow significantly influences the acoustic-velocity-flame dynamic characteristics of the stratified burner. When the pilot and main stage are operated with a swirling air jet and swirling flame, the main recirculation zone disappears, the thermo-acoustic instability is strengthened and the frequency of thermo-acoustic instability is locked with the large-scale vortex shedding frequency. When the pilot stage is converted into a swirling flame, the main recirculation zone re-appears down stream of the nozzle outlet, and the high-temperature burned gas is rolled back to improve the combustion stability of the swirling flame. However, in this case, the interaction between the pilot and main flames makes the thermo-acoustic instability frequency not well consistent with large-scale vortex shedding frequency. The interaction between the main stage and pilot stage flames leads to the increase of the flame angle of the main stage. The heat release fluctuation at the flame interaction region is the most intense, and the large-scale vortex in the outer shear layer of the main stage also causes the flame to produce severe oscillation.

Interactions of turbulence and flame during turbulent boundary layer premixed flame flashback under isothermal and adiabatic wall conditions using direct numerical simulation

In this work, lean hydrogen/air premixed flame flashback in a turbulent boundary layer along an isothermal wall and an adiabatic wall was simulated using direct numerical simulation. The general characteristics of flame flashback were analyzed. It was found that the propagating speed along the adiabatic wall is higher than that along the isothermal wall. The displacement speed near the isothermal wall is notably lower than that near the adiabatic wall, primarily due to the smaller value of the reaction component of the displacement speed near the isothermal wall. Furthermore, the properties of the boundary layer turbulence during flame flashback were examined in terms of the flow topology and the anisotropy of Reynolds stress. It was observed that the focal topologies tend to prevail near the adiabatic wall, and the volume fraction of stable topologies increases near the isothermal wall due to the increased compressed region caused by the wall heat loss. Finally, the influence of turbulence on the flame structure was highlighted. It was suggested that the unstable topologies dominate the heat release rate away from the wall. The proportion of heat release rate in focal topologies increases near the adiabatic wall, and the proportion in stable topologies increases near the isothermal wall. The impact of the low temperature wall on the flame structure is more pronounced in topologies with compression compared to those with expansion.

Optimizing air distribution in floor and wall burners of an industrial steam cracking firebox: a CFD study

A 3D computational fluid dynamics model for an industrial scale steam cracking firebox is combined with a 1D reactor model for the cracking process within the reactor coil. This framework is validated using industrial tunable diode laser absorption spectroscopy (TDLAS) measurements in a naphtha steam cracking furnace. Industrial O2, H2O, CO and temperature data show good agreement with simulation results at different locations within the firebox. The influence of varying the primary-to-secondary air flow distribution to the floor and wall burners separately is investigated using the validated framework. Altering the primary-to-secondary air flow distribution of the floor burners from 75/25 to 25/75 results in a 66% increase in furnace run length and a 35% decrease in NOx formation. This shift in air ratio ensures a lower rollover height of the flame and a large recirculation zone of the flue gas, spreading the radiative heat flux on the reactor coil more evenly. Altering the primary-to-secondary air flow distribution of the sidewall burners from 50/50 to 100/0 results in a 13% increase in run length and a 30% decrease in NOx formation. More primary air towards the sidewall burners is found to lead to more evenly spread out flames along the wall, resulting in flame interactions between the different burners and a combined flow towards the center of the firebox. These interactions result in a more uniform tube metal temperature profile along the different passes of the coil.

A Spatially Progressive Neural Network for Locally/Globally Prioritized TDLAS Tomography

Tunable diode laser absorption spectroscopy tomography (TDLAST) has been widely applied for imaging two-dimensional distributions of industrial flow-field parameters, e.g., temperature and species concentration. Two main interested imaging objectives in TDLAST are the local combustion and its radiation in the entire sensing region. State-of-the-art algorithms were developed to retrieve either of the two objectives. In this paper, we address the both by developing a novel multi-output imaging neural network, named as Spatially Progressive Neural Network (SpaProNet). This network consists of locally and globally prioritized reconstruction stages. The former enables hierarchical imaging of the finely resolved and highly accurate local combustion, but coarsely resolved background. The later retrieves a fine-resolved image for the entire sensing region, at the cost of slightly trading off the reconstruction accuracy in the combustion zone. Furthermore, the proposed network is driven by the hydrodynamics of the real reactive flows, in which the training dataset is obtained from large eddy simulation. The proposed SpaProNet is validated by both simulation and lab-scale experiment. In all test cases, the visual and quantitative metric comparisons show that the proposed SpaProNet outperforms the existing methods from the following two perspectives: a) the locally prioritized stage provides ever-better accuracy in the combustion zone; b) the globally prioritized stage shows turbulence-indicative accuracy in the entire sensing region for diagnosis of heat radiation from the flame and flame-air interactions.

Flame behavior and temperature distribution of coupled fire induced by the interaction of hydrogen jet flame and pool flame

Once fires occur unexpectedly at the oil-hydrogen integrated refueling stations, there is a high likelihood that they will escalate into a coupled fire, resulting in unpredictable casualties and property losses. In this work, experiments on coupled fire induced by hydrogen jet flame and a heptane pool flame were conducted with different nozzle diameters and release pressures. It has been found that there is a critical diameter above which the hydrogen jet can be ignited instantly. This critical diameter is determined by the release pressure and the location of the pool flame. The heat release rate is greater than that of the single pool flame, and it reaches its highest point at 0.2 MPa. Compared to a single fire, the height of the flame decreases while the length of the flame increases, a prediction model is proposed for the dimensionless flame length and width, taking into account the ratio of the heat release rate of the hydrogen jet flame to the heat release rate of the pool flame. The temperature of the coupled fire reaches its highest point at 0.2 MPa and increases as the nozzle diameter. And it decreases with the increase in the distance when Y= 0 m. However, it increases with the increase of the distance when Y= 0.05 m due to the inclination of the flame.

Experimental and numerical investigation of transverse combustion instability in a rectangle multi-injector rocket combustor

Rocket engines are prone to suffer from transverse combustion instabilities, manifesting as periodic oscillations in pressure and heat release rate. Self-excited transverse instabilities in a rectangle multi-injector rocket combustor powered by n-C12H26 and O2, are investigated experimentally and numerically in this paper. Transverse combustion instabilities are captured during repeatable experiments, while dynamic characteristics in the combustion chamber and driving mechanisms of instabilities are analyzed via numerical simulation based on stress-blended eddy simulation. Dynamic mode decomposition analysis reveals coupling behaviors between the transverse mode in the combustion chamber and longitudinal mode in the oxidizer post, resulting in propellant mass flow rate oscillations. Substantial evidence demonstrates that the intensities of heat release rate near the end walls are higher than that detected around the chamber center due to its intense coupling to the local first transverse (1W) acoustic mode. Flow field animations indicate strong deflection of oxygen posts and liquid particles by a transverse moving gas mixture leading to flame interactions and collision between adjacent share injectors. The propellant injected into the chamber does not burn immediately and is transported to the combustor end wall. Propellant mixing performance is significantly enhanced during interactions between acoustic pressure and end wall, causing a sudden heat release near 1W pressure anti-node, thereby strengthening pressure oscillations. Finally, a combustion instability driving mechanism associated with propellant mass flow rate oscillations, and interactions between unsteady acoustic pressure and heat release rate is proposed. The insights obtained from this work can aid comprehension of transverse combustion instabilities encountered by rocket engines.

Improvement of diesel combustion with suppression of mutual fuel spray flame interactions with staggered nozzle hole arrangement and a spatially divided combustion chamber

A combination of a fuel injector with staggered nozzle hole arrangement with a combustion chamber divided into upper and lower layers by placing a lip at the middle of the side wall is proposed to improve diesel combustion with suppression of interactions among fuel spray flames. The fuel injector has twelve alternately arranged spray holes ( ϕ0.09 mm), staggered at two injection angles, 79° and 55° from the central axis of the injector. Two ordinary injectors with eight ( ϕ0.113 mm) and twelve ( ϕ0.092 mm) holes with an injection angle of 78° from the central axis with a conventional re-entrant combustion chamber were also examined as references. The experimental results showed that the improvements in thermal efficiencies and the reduction in smoke emissions can be established over a wide IMEP range with the proposed combination of the staggered nozzle hole injector and the divided combustion chamber. The exhaust loss is reduced with the combination of the injector and the combustion chamber while the cooling loss increases slightly. The rate of heat release from the combination of the injector and the combustion chamber has a more active main combustion and smaller afterburning than conventional systems. The three-dimensional CFD simulations showed that the combination of the injector and the combustion chamber can efficiently suppress the interactions among fuel spray flames at the combustion chamber wall after the impingement of fuel spray flames and also suitably distribute the fuel mixture, resulting in reductions in the afterburning and the smoke emissions. The slight increase in cooling loss with the proposed combination may be due to the increase in contact area of hot burned gas on the cylinder head; further improvements in thermal efficiency can be expected with optimization of the design factors related to the fuel injection and the combustion chamber configuration.

Exploring the effects of turbulent field on propagation behaviors in confined hydrogen-air explosion using OpenFOAM

Explosion always happens in turbulent field, which is hard to measure in reality. This paper explores the influence of turbulent kinetic energy on the propagation behaviors of flame and pressure of stoichiometric hydrogen-air explosion in confined vessel, using large eddy simulation. The momentum equation and progressive variable equation are adapted to consider interaction of turbulence and flame. The results reveal that with initial turbulent kinetic energy increasing, its promoting effects on flame and pressure propagation weaken. For flame at low initial turbulent kinetic energy and low initial pressure, it undergoes three propagating stages: acceleration, steady propagation and deceleration. In addition, flame deceleration is caused by rapid decrease of flow velocity which falls faster with initial turbulent kinetic energy increasing. Furthermore, explosion pressure propagates faster with initial turbulent kinetic energy increasing. Sharp fluctuation of flow velocity appears in the flame zone where big pressure gradient exists.

Flame Interaction Characteristics of Two Adjacent Liquid Pool Fires Under the Influence of Cross Wind

Flame Interaction Characteristics of Two Adjacent Liquid Pool Fires Under the Influence of Cross Wind

Influence of Flow Configuration and Thermal Wall Boundary Conditions on Turbulence During Premixed Flame-Wall Interaction within Low Reynolds Number Boundary Layers

The influence of flow configuration on flame-wall interaction (FWI) of premixed flames within turbulent boundary layers has been investigated. Direct numerical simulations (DNS) of two different flow configurations for flames interacting with chemically inert isothermal and adiabatic walls in fully developed turbulent boundary layers have been performed. The first configuration is an oblique wall interaction (OWI) of a V-flame in a turbulent channel flow and the second configuration is a head-on interaction (HOI) of a planar flame in a turbulent boundary layer. These simulations are representative of stoichiometric methane-air mixture under atmospheric conditions and the non-reacting turbulence for these simulations corresponds to the friction velocity based Reynolds number of Re_{tau }=110. It is found that the mean wall shear stress, mean wall friction velocity and the mean velocity statistics are affected during FWI and the behaviour for these quantities varies under the different flow configurations as well as for the different thermal wall boundary conditions. The behaviour of the quenching distance and mean wall heat flux under isothermal wall conditions is found to be significantly different between the two flow configurations. The variation of the non-dimensional temperature in wall units for cases with isothermal walls suggests that the temperature in the log-layer region is significantly altered by the evolving wall heat flux in both flow configurations. Statistics of the mean Reynolds stresses and turbulence dissipation rate show that the flame significantly alters the behaviour of turbulence due to thermal expansion effects and flow configuration plays an important role.

Experimental and numerical studies on the interaction between two identical slot jet H2/CO diffusion flames

Hydrogen offers a unique opportunity in achieving a CO2-neutral future, but great challenges exist for the successful transition from conventional fossil fuels to H2, due to the great component variation and safety issues of H2 energy. The multiple-injection combustor concept effectively addresses these problems, with the merits of flexible nozzle distance satisfying the varying H2 content in the fuel and low flash-back risk comparable to diffusion flames. In such combustors, flame interaction plays an essential role in the performance of multiple-injection combustors. This work focused on the flame interaction behavior of two identical slot jet H2/CO diffusion flames. OH* chemiluminescence was captured by an intensified CCD camera and the flame temperature was measured using a type-B thermocouple. The combustion process was modeled using a Fortran flame code by incorporating the detailed thermal and transport properties as well as the chemical kinetic mechanism. With these experimental and numerical efforts, the effects of burner separating distance, fuel composition, and fuel velocity were parametrically studied. Results showed that flame interaction enables a more uniform temperature distribution. Increasing H2 content in the fuel decreases the flame length for the merged flames, but this effect is less distinguished for the merging and separated flames. In addition, flames with a higher H2 content merge more easily because of the enlarged flame radius, featured with a linear relationship between the merging distance and H2 content. When increasing the fuel flow rate, the twin flames contact and merge at farther separating distances, and this influence of fuel flow rate on flame interaction was observed to be fuel-composition-dependent. Increasing the burner separating distance lowers the flame length, which is attributed to the enhancement of CO oxidization rather than the H2 reactions. Moreover, burner separating distance plays a critical role in NO emissions, with a non-monotonical impact due to the combined effect of flame temperature and O2 concentration.

Characteristics of flame acceleration and deflagration-to-detonation transition enhanced by SF6 jet-in-cross-flow/flame interaction

Jet-in-crossflow (JICF), as a turbulence generator, is proposed to promote the flame acceleration and deflagration-to-detonation transition (DDT) process. Previous work has suggested that JICF using CO2 or Ar was in favor of flame acceleration under optimal conditions. The competition mechanism between the positive combustion-enhancement effect caused by jet-induced turbulence and the negative combustion-inhibition caused by non-reactive gas has been demonstrated. It is worth noting that the difference in molar weight between the medium of JICF and the combustible mixture may significantly impact deflagration propagation prior to DDT. Thus, in this work, to investigate the flame propagation perturbed by the enhancement on the difference in molar weight between the JICF medium and combustible mixture, an experimental study of the flame propagation perturbed by SF6 JICF is carried out. By adjusting the injection pressure (Pj) of the SF6 JICF, the characteristics of flame propagation and DDT are analyzed. The results showed that the flame can accelerate to the onset of detonation within a short run-up distance in the cases with SF6 JICF. Schlieren images of flame evolution show that the flame disturbed by SF6 JICF rapidly transitions to a wrinkled flame. It is suggested that the SF6 JICF-enhanced turbulence promotes flame acceleration and DDT. The effect of SF6 JICF on the flame characteristics can be concluded in two aspects: one is that the inhomogeneously distributed vortices that are induced by JICF cause a nonuniform diffusion of SF6 in the tube, resulting in a nonuniform dilution of the combustible mixture near the jet; another one is that the vortices induced by JICF stretch the flame surface to increase the flame area. Increasing the injection pressure could promote the formation of complex wrinkles in the flame, however, it shows few further improvements in promoting DDT. Different jet mediums are also tested, and the results show that SF6 JICF can shorten the run-up distance significantly compared with helium and nitrogen JICF. The small and densely distributed wrinkles significantly increase the flame area to enhance heat release. The flame disturbed by SF6 JICF could accelerate to the local sound speed of the unburnt mixture, which results in the facilitation of the precursor shock wave formation. After the flame passes through the SF6 JICF, it remains a larger acceleration rate and then transitions to quasi-detonation rapidly. The conclusion of this work could help to qualitatively understand the positive effect of JICF-induced turbulence on flame acceleration and DDT. It also provides a relatively efficient and safe technology to promote detonation initiation in the design of detonation engines.

Geometrical features and global radiant heat of double turbulent jet flames

In energy industry, double jet fires are often reported to occur with a large release momentum at the leakage exit. The available literature considered the double buoyancy-controlled jet flames, and also neglected the restrictive effect of ground or tank surface. In this paper, an experimental setup is built to model the interaction of double turbulent jet fires with different spacings. Different nozzle diameters and exit velocities are chosen to achieve the transition from flame buoyancy to exit momentum for the dominance of turbulent jet flames. Comparative tests are also conducted between with and without a plate near the nozzle exit. It is found that the flame buoyancy, exit momentum and nozzle spacing significantly affect the flame merging behavior. A new dimensionless parameter coupling the Froude number (Fr) and the spacing is proposed to well fit the merging probability (Pm) of double jet flames dominated by the flame buoyancy (Fr < 105 for propane) and the exit momentum (Fr ≥ 105 for propane), respectively. The interaction of double jet flames leads to a less lift-off height than a single jet flame, and the lift-off height can also be reduced by a plate near the nozzle exit. A correlation coupling the spacing, the exit diameter and Froude number, is developed for the mean flame height of double jet fires. The distributions of radiant heat flux are intensively measured around the double jet flames, which can help to calculate the radiative fraction. The radiative fraction of double jet flames at the full merging state (Pm = 1) follows the law of a single jet flame, and it decreases at the intermittent (0 < Pm < 1) and no merging (Pm = 0) states, as compared to that of a single jet flame. In addition, the available line source radiation model of a single jet flame is revised to predict the thermal radiation of double jet flames in the three different merging states.

Interaction of a turbulent flame with the very-large-scale structures in a channel flow

Combustion processes taking place in large-scale burners may need to consider the impact of the recently identified very-large-scale motions (VLSM) appearing in wall-bounded turbulent flows. While VLSM in non-reacting, cold flows have been found to be important and noticeably impact turbulence statistics, the possible influence of VLSM on turbulent premixed flames has not been considered up to now to the best of our knowledge. The present study investigates the interaction of a turbulent flame with VLSM in a channel flow using direct numerical simulations (DNS). Detailed chemical kinetics have been used to compute a H2 flame. The turbulent channel flow corresponds to Reτ= 280, while the premixed flame is associated to a Damköhler number Da = 0.167. First, VLSM have been identified for the non-reacting flow by proper orthogonal decomposition (POD). Interestingly, the propagation of the turbulent flame is found to have a negligible effect on the VLSM, highlighting their resilience. At the same time, the VLSM do not modulate strongly the flame structure, while it is noticeably affected by small-scale vortices. This decoupling might be explained by the very different length scales of both processes, the VLSM covering the full channel length, while the turbulent flame is characterized by small-scale structures. In the same manner, the characteristic time scales also differ widely, the flame time scale being much faster than the characteristic time of the VLSM. The flame curvature is positively correlated with the fluctuating streamwise velocity when the flame is still far from the wall. When approaching the wall, flame quenching takes place in a complex process. Overall, it appears for the present conditions that VLSM do not impact directly the turbulent flame propagation, but there is still an indirect impact through the modifications of the turbulence statistics.