Tulip Flame Research Articles

Detailed investigation of flame transmission from a vessel to a discharge duct

The present paper deals with a problem of explosion initiated in a vessel and vented through a duct. On the basis of numerical simulation (CFD) and visualization by means of high speed camera it completes and discusses the existing results and hypotheses (especially those presented in the work of Ponizy and Leyer [1]) concerning the process of enhancement of vessel pressure rise during such an explosion. In particular, numerical simulation indicates that a secondary explosion in the duct, known as a “burn-up”, which is responsible for higher reduced explosion pressures during ducted venting, has its source in a highly turbulent zone generated at the duct entrance by the flame itself independently of the shape of the vessel/duct passage. Camera images confirm the hypothesis that the flame penetrating into the duct is strongly torn in this turbulent zone and mixed with fresh gases. On the other hand, these images show that the reverse flow created by the burn-up returns back to the vessel some amount of rapidly burning gases, which contributes to the vessel pressure increase in the same degree that the blockage of the outflow from the vessel and the intensification of combustion of the unburned mixture previously left in the vessel.Experimental results evidence also a strong dependence of the flame behaviour in the vessel on the vessel length and the duct diameter. They demonstrate that the final effect of the burn-up action on maximum explosion pressures reached in the vessel depends on the whole process of flame propagation in the vessel, which in turn is closely related to the vessel and duct dimensions. In particular, in some configurations, an apparition of the “tulip flame” combined with inertia of the gas column in the duct may in a spectacular manner reduce the effect of the burn-up action.

Numerical Simulation of Premixed Propane/Air Flame Propagation Using a Dynamically Thickened Flame Approach

A numerical study of premixed propane/air flame propagation in a closed duct is presented. A dynamically thickened flame (TF) method is applied to model the premixed combustion. The reaction of propane in air is taken into account using a single-step global Arrhenius kinetics. It is shown that the premixed flame undergoes four stages of dynamics in the propagation. The formation of tulip flame phenomenon is observed. The pressure during the combustion process grows exponentially at the finger-shape flame stage and then slows down until the formation of tulip shape. After tulip formation the pressure increases quickly again with the increase of the flame surface area. The vortex motion behind the flame front advects the flame into tulip shape. The study indicates that the TF model is quite reliable for the investigation of premixed propane/air flame propagation.

Numerical Simulation of Premixed Propane/Air Flame Propagation Using a Dynamically Thickened Flame Approach

A numerical study of premixed propane/air flame propagation in a closed duct is presented. A dynamically thickened flame (TF) method is applied to model the premixed combustion. The reaction of propane in air is taken into account using a single-step global Arrhenius kinetics. It is shown that the premixed flame undergoes four stages of dynamics in the propagation. The formation of tulip flame phenomenon is observed. The pressure during the combustion process grows exponentially at the finger-shape flame stage and then slows down until the formation of tulip shape. After tulip formation the pressure increases quickly again with the increase of the flame surface area. The vortex motion behind the flame front advects the flame into tulip shape. The study indicates that the TF model is quite reliable for the investigation of premixed propane/air flame propagation.

Experimental and numerical study of premixed flame propagation in a closed duct with a 90° curved section

The premixed flame propagation in a closed duct with a 90° bend is studied using high-speed schlieren photography and numerical simulation. The investigation provides basic understanding of the influence of the bend on the premixed flame propagation. The flame undergoes four stages in the propagation. The outer flame skirt influences significantly the flame dynamics. The flame surface area reaches its minimum value after the full formation of tulip flame. The flame dynamics observed in the experiment is well reproduced by the numerical simulation. The flame behavior at the later stage is in close connection with the hydrodynamics of the combustion-generated flow. The numerical simulation indicates that a single vortex is generated near the flame tip in the burnt gas just before the formation of flame indentation. The vortex remains in the vicinity of the flame tip and changes the flow field around the flame front. As a consequence, the effects of the circulating flow create the conditions required for the formation of a tulip flame.

Dynamics of premixed hydrogen/air flame in a closed combustion vessel

The dynamics of a premixed hydrogen/air flame propagating in a closed vessel is investigated using high-speed schlieren cinematography, pressure measurement and numerical simulation. A dynamically thickened flame approach with a 19-step detailed chemistry is employed in the numerical simulation to model the premixed combustion. The schlieren photographs show that a remarkable distorted tulip flame is initiated after a classical tulip flame has been fully produced. A second distorted tulip flame is generated with a cascade of indentations created in succession before the vanishing of the first one. The flame dynamics observed in the experiments is well reproduced in the numerical simulation. The burnt region near the flame front is entirely dominated by a reverse flow during the formation of the distorted tulip flame. The distorted tulip flame can be formed in the absence of vortex motion. The pressure wave leads to periodic flame deceleration and plays an essential role in the distorted tulip formation. The numerical results corroborate the mechanism that the distorted tulip flame formation is a manifestation of Taylor instability.

An experimental study of premixed hydrogen/air flame propagation in a partially open duct

High-speed schlieren cinematography and pressure records are used to investigate the dynamics of premixed hydrogen/air flame propagation and pressure build up in a partially open duct with an opening located in the upper wall near the right end of the duct. This work provides basic understanding of flame behaviors and the effects of opening ratio on the combustion dynamics. The flame behaves differently under different opening conditions. The opening ratio has an important influence on the flame propagation and pressure dynamics. When the opening ratio α ≤ 0.075 a significant distorted tulip flame can be formed after the full formation of a classical tulip flame. The propagation speed of flame leading tip increases with the opening ratio. The coupling of flame front with the pressure wave is strong at low opening ratio. Both the pressure growth rate and oscillation amplitude inside the duct increases as the opening ratio decreases. The formation times of tulip and distorted tulip flames and the corresponding distances of flame front increase with the increase of the opening ratio.

An experimental study of distorted tulip flame formation in a closed duct

The dynamics of a distorted tulip flame in a closed duct has been experimentally and numerically investigated [Xiao et al., Combust. Flame 159 (2012) 1523–1538]. In the present work, the distorted tulip flame formation and the flame evolution after its complete formation is studied to provide further insight into the formation process of the tulip flame distortions. The schlieren images show that a second distorted tulip flame forms with a cascade of secondary cusps on the primary tulip lips just before the collapse of the first one. The pressure wave triggered by the first contact of the flame with the sidewalls is responsible for the periodic flame deceleration and plays an important role in the formation of tulip flame distortions. The close analogy of the distorted tulip phenomenon to the flame inversion which results from the interaction of a convex flame with a shock wave suggests that the sudden flame deceleration acts in a similar way via the Taylor instability in the formation of the distorted tulip flames.

Numerical simulation of premixed methane–air deflagration in large L/D closed pipes

A three-dimensional numerical model in which the heat loss through pipe walls was included was developed to simulate gaseous deflagration in pipes of large length to diameter ratio ( L / D ). The simulated pressure time histories are in good agreement with published data, and the error between experimental and computed maximum pressure is less than 15.3%. The attention is focused on the flame propagation and flow field during deflagrations, as well as the effects of ignition point (at the center or at one end of the pipe) and L / D ( L / D = 6 − 10.35) of the pipe on them. The numerical results show that a tulip flame is formed during flame propagation, which is related to the reverse flow and vortices motion. The maximum fame speed is 30% higher with the ignition point at the end than that at the center, and it increases linearly with increasing L / D . The deflagration pressure decreases when the distance that flame travels increases due to the effect of the heat loss through pipe walls. ► Methane–air deflagrations in closed pipes of large L / D were simulated. ► The heat loss through pipe walls was included in the model. ► The results reveal flame propagation in long closed pipes. ► The maximum flame speed increases linearly with increasing L / D . ► The maximum flame speed is 30% higher for end ignition than for center ignition.

Experimental and numerical investigation of premixed flame propagation with distorted tulip shape in a closed duct

High-speed schlieren photography, pressure records and large eddy simulation (LES) model are used to study the shape changes, dynamics of premixed flame propagation and pressure build up in a closed duct. The study provides further understanding of the interaction between flame front, pressure wave and combustion-generated flow, especially when the flame acquires a “distorted tulip” shape. The Ulster multi-phenomena LES premixed combustion model is applied to gain an insight into the phenomenon of “distorted tulip” flame and explain the experimental observations. The model accounts for the effects of flow turbulence, turbulence generated by flame front itself, selective diffusion, and transient pressure and temperature on the turbulent burning velocity. The schlieren images show that the flame exhibits a salient “distorted tulip” shape with two secondary cusps superimposed onto the two original tulip lips. This curious flame shape appears after a well-pronounced classical tulip flame is formed. The dynamics of “distorted tulip” flame observed in the experiment is well reproduced by LES. The numerical simulations show that large-scale vortices are generated in the burnt gas after the formation of a classical tulip flame. The vortices remain in the proximity of the flame front and modify the flow field around the flame front. As a result, the flame front in the original cusp and near the sidewalls propagates faster than that close to the centre of the original tulip lips. The discrepancy in the flame propagation rate finally leads to the formation of the “distorted tulip” flame. The LES model validated previously against large-scale hydrogen/air deflagrations is successfully applied in this study to reproduce the dynamics of flame propagation and pressure build up in the small-scale duct. It is confirmed that grid resolution has an influence to a certain extent on the simulated combustion dynamics after the flame inversion.

Experimental study on the behaviors and shape changes of premixed hydrogen–air flames propagating in horizontal duct

The behaviors and shape changes of premixed hydrogen–air flames at various equivalence ratios propagating in half-open and closed horizontal ducts are experimentally investigated using high-speed schlieren imaging and pressure sensors. The study shows that the premixed hydrogen–air flame undergoes more complex shape changes and exhibits more distinct characteristics than that of other gaseous fuels. One of the outstanding findings is that obvious distortion happens to tulip flame after its full formation when equivalence ratio ranges from 0.84 to 4.22 in the closed duct. The salient tulip flame distortions are specially scrutinized and distinguished from the classical tulip collapse and disappearance. The dynamics of distorting tulip flame is different from that of classical tulip flame. The normal tulip flame can be reproduced after the first distortion followed by another distortion. The initiation of flame shape changes coincides with the deceleration both of pressure rise and flame front speed for flames with tulip distortions. And the formation and dynamics of tulip/distorting tulip flames depend on the mixture composition.

Experimental and numerical study on premixed hydrogen/air flame propagation in a horizontal rectangular closed duct

Hydrogen is a promising energy in the future, and it is desirable to characterize the combustion behavior of its blends with air. The premixed hydrogen/air flame microstructure and propagation in a horizontal rectangular closed duct were recorded using high-speed video and Schlieren device. Numerical simulation was also performed on Fluent CFD code to compare with the experimental result. A tulip flame is formed during the flame propagating, and then the tulip flame formation mechanism was proposed based on the analysis. The induced reverse flow and vortex motion were observed both in experiment and simulation. The interactions among the flame, reverse flow and vortices in the burned gas change the flame shape and ultimately it develops into a tulip flame. During the formation of the tulip flame, the tulip cusp slows down and stops moving after its slightly forward moving, and then, it starts to move backward and keeps on a longer time, after that, it moves forward again. The structure of the tulip flame is becoming less stable with its length decreasing in flame propagation direction. The flame thickness increases gradually which is due to turbulence combustion.

Experimental study of flame microstructure and propagation behavior of mine-gas explosion

The high speed cameral and schlieren images methods were used to record the photograph of flame propagation process. Meanwhile, the ionization current probes were set up to detect the reaction intensity of the reaction zone. The characteristics of methane/air flame propagation and microstructure were analyzed in detail by the experimental results coupled with chemical reaction thermodynamics. The high speed schlieren image showed the transition from laminar flame to turbulence combustion. The ion current curves disclosed the reaction intensity and combustion characteristic of flame front. In the test, the particular tulip flame was formed clearly, which was induced to some extent by turbulent combustion. Based on the schlieren images and iron current result, it can be drawn that the small scale turbulence combustion also appears in laminar flame, which thickens the flame front, but makes little influence on the flame front shape. During the laminar-turbulent transition, the explosion pressure plays an important role on the flame structure change.

Flame acceleration in the early stages of burning in tubes

Acceleration of premixed laminar flames in the early stages of burning in long tubes is considered. The acceleration mechanism was suggested earlier by Clanet and Searby [Combust. Flame 105 (1996) 225]. Acceleration happens due to the initial ignition geometry at the tube axis when a flame develops to a finger-shaped front, with surface area growing exponentially in time. Flame surface area grows quite fast but only for a short time. The analytical theory of flame acceleration is developed, which determines the growth rate, the total acceleration time, and the maximal increase of the flame surface area. Direct numerical simulations of the process are performed for the complete set of combustion equations. The simulations results and the theory are in good agreement with the previous experiments. The numerical simulations also demonstrate flame deceleration, which follows acceleration, and the so-called “tulip flames.”

Microstructure of premixed propane/air flame in the transition from laminar to turbulent combustion

In order to explore the flame structure and propagation behavior of premixed propane/air in the transition from laminar to turbulent combustion, the high speed camera and Schlieren images methods were used to record the photograph of flame propagation process in a semi-vented pipe. Meanwhile, the super-thin thermocouple and ionization current probe methods were applied to detect the temperature distribution and reaction intensity of combustion reaction. The characteristics of propane/air flame propagation and microstructure were analyzed in detail by the experimental results coupled with chemical reaction thermodynamics. In the test, the particular tulip flame behavior and the formation process in the laminar-turbulent transition were disclosed clearly. From the Schlieren images and iron current results, one conclusion can be drawn that the small-scale turbulent combustion also appeared in laminar flame, which made little influence on the flame shape, but increased the flame thickness obviously.

Flame behavior and flame-induced flow in a closed rectangular duct with a 90° bend

A 3-D premixed laminar flame propagation, ignited at one end in a closed rectangular duct with a 90° bend, has been investigated by both experimental and numerical methods. A high-speed camera was used to capture the flame shapes and propagation speed in the experiments, and a computational fluid dynamics (CFD) code KIVA-3V was employed to simulate the 3-D transient laminar reacting flow in the numerical studies. The results showed that: (1) a flame tends to avoid the inner and outer surfaces when it propagates through the first 45° of the bend and this phenomenon was termed “flame shedding”; (2) the numerical and experimental results were in good agreement for the basic physical phenomena, such as the tulip flame and “flame shedding”; (3) a flame could behave as a thrust resource for the unburned mixture to generate or induce very complex flow fields which could facilitate an environment that subsequently affects flame behavior. This interaction between the flame and the flame-induced flows, once the flame front begins to flatten, results in a higher dynamic pressure acting forward on the corner portions of the flame front than the middle and therefore a tulip-shape flame could be formed; (4) “flame shedding” is due to bend effects: the presence of a streamwise-oriented vortex pair near the inner surface of the bend and the high-pressure region near the outer surface.

The transition from deflagration to detonation in thin channels

A numerical study of a two-dimensional model for premixed gas combustion in a thin, semi-infinite and thermally-insulated channel is performed. The work is motivated by recent theoretical advances revealing the important role of hydraulic resistance in deflagration-to-detonation transition, one of the central yet still poorly understood phenomena of gaseous combustion. The two-dimensional formulation reproduces the formation of the so-called tulip flame and its predetonational acceleration, well-known experimentally but unattainable within the quasi-one-dimensional approach employed previously. It is shown that the detonation first develops in the boundary layer where the effect of hydraulic resistance is stronger, and thereupon spreads over the channel's interior. However, the second stage of the transition does not proceed gradually but rather through a localized auto-ignition within the tulip.

Cellular and tulip flame configurations

Flame propagation in a plane channel with the formation of tulip and cellular configurations of the combustion front is simulated. The near-flame flow structure and the thermal flow structure are determined. An analogy is found between the tulip configuration and flame inflections at cell interfaces.

Premixed flames in closed cylindrical tubes

We consider the propagation of a premixed flame, as a two-dimensionalsheet separating unburned gas from burned products, in a closedcylindrical tube. A nonlinear evolutionequation, that describes the motion of theflame front as a function of its meanposition, is derived. The equation containsa destabilizing term that results from thegas motion induced by thermal expansionand has a memory term associated withvorticity generation. Numerical solutionsof this equation indicate that, whendiffusion is stabilizing, the flameevolves into a non-planar form whose shape, and its associated symmetry properties, aredetermined by the Markstein parameter, andby the initial data. In particular, weobserve the development of convexaxisymmetric or non-axisymmetric flames, tulip flames and cellular flames.

The Hydrodynamic Structure of a Methane-Air Tulip Flame

The flame propagation and structure of a stoichiometric methane-air flame in a closed vessel is investigated. For the parameter range examined a tulip flame is formed. A qualitative agreement with experiments is obtained for different length to width ratios of the vessel. The results indicate an enhanced tulip effect with increasing vessel length.

Tulip flames: changes in shape of premixed flames propagating in closed tubes

The experimental results that are the subject of this communication provide high-speed schlieren images of the closed-tube flame shape that has come to be known as the tulip flame. The schlieren images, along with in-chamber pressure records, help demonstrate the effects of chamber length, equivalence ratio, and igniter geometry on formation of the tulip flame. The pressure/time records show distinct features which correlate with flame shape changes during the transition to tulip. The measurements indicate that the basic tulip flame formation is a robust phenomenon that depends on little except the overall geometry of the combustion vessel.