Tulip Shape Research Articles

An experimental study on the flame behaviors of H2/CO/Air mixtures in closed tube with varying number of obstacles

In order to investigate the flame behavior of syngas/air mixtures after leakage in industrial scenarios. A series of fence-type obstacles with a blockage ratio of 0.3 were installed in a closed tube to simulate common blockage scenarios. The stoichiometric mixtures of H2/CO/Air with hydrogen fractions ranging from 10 % to 50 % were used to simulate typical syngas compositions. Results indicate that the increase in hydrogen fractions and the number of obstacles can enhance flame acceleration and oscillation downstream of obstacles. The flame inverts into the tulip shape after it passes the obstacle area, and then the distorted “tulip” flame appears. The obstacles number has no influence on the instant of the planar flame formation, because the flow competition initiates when the flame passes the first obstacle channel. However, the initial instant of the first “tulip” distortion decrease with the increasing obstacle number. The “tulip” flame is a hydrodynamic phenomenon, while the distortion of the “tulip” flame is related to pressure waves. At higher hydrogen fractions, the obstacle number increases the maximum overpressure more significantly. In practical scenarios, it is advisable to limit the number of obstacles as much as possible and prevent syngas leaks with higher hydrogen fractions.

Flame propagation dynamic characteristics and mechanisms of methane–syngas–air mixtures in a semi-enclosed duct

The composition of syngas is complex, often leading to the generation of methane, which impacts the explosive characteristics of syngas. The analysis focused on the kinetic characteristics and mechanisms of flame propagation in methane–syngas–air mixtures in a semi-enclosed duct under various equivalence ratios (φ = 0.8–1.4) and methane volume ratios (R = 0 %–70 %) under ambient temperature and pressure. The results indicated that the flame speed and pressure propagation of methane–syngas–air mixtures could be divided into three stages: a stable stage, an oscillatory increase stage, and an oscillatory decrease stage. In the stable stage, as the methane proportion in the premixed gas increased, the oscillations in the flame and pressure decreased. Turbulent flow of the mixed gas was observed due to restrictions and obstructions within the duct. This led to an increase in the peak pressure and flame speed of the mixed gas, thereby resulting in an oscillatory increase stage. Distorted tulip-shaped flames were also formed in the semi-enclosed duct. The flame of the pure syngas/air mixture exhibited a hemispherical shape and a finger-shaped and slope-shaped propagation structure. When 30 %, 50 %, or 70 % of the methane in the methane–syngas–air mixtures was added, the flame exhibited a propagation structure characterized by a hemispherical shape, finger shape, slope shape, flat shape, tulip shape, or distorted tulip shape. The flame front evolved with time from a linear growth stage to a nonlinear growth stage. The flame front speed of the methane–syngas–air mixtures was closely related to the pressure and flame structure. With the increase in the methane proportion in the methane–syngas–air mixtures, the concentration of key radicals and the rate of OH* generation and consumption gradually decreased. Sensitivity analysis revealed that with increasing methane proportion in the premixed gas, the most important reaction dominating OH* consumption changed from R15 (H + O2(+M) <=> HO2(+M)) to R138 (CH4 + OH <=> CH3 + H2O) and then to R112 (2CH3(+M) <=> C2H6(+M)); moreover, the most important reaction dominating OH* generation remained R1 (H + O2 <=> O + OH). The addition of methane interrupted the hydrogen–oxygen chain reaction of the methane–syngas–air explosion process.

Effects of vent layout and vent area on dynamic characteristics of premixed methane-air explosion in a tube with two side-vented ducts

To further elucidate the influence mechanism of side vents on the dynamic characteristics of gas explosions in tubes is helpful to design more reasonable vent layouts. In this paper, 9.5% methane-air explosion experiments were conducted in a tube with two side-vented ducts, and the effects of vent layouts and vent areas on the dynamic characteristics of explosion overpressure and flame propagation speed were investigated. The results demonstrate that under the same condition with a single vent area of 100 mm × 100 mm, when only the end vent is open, the maximum explosion overpressure and the maximum flame propagation speed are the highest among the five vent layouts. When the side vents 1 and 2 and the end vent are open, the maximum explosion overpressure is the lowest, and an unusual discovery is that the flame front changes into a hemispherical shape, finger shape, quasi-plane shape, tulip shape and wrinkled structure. When only side vent 1 is open, a unique Helmholtz oscillation occurs, and a new discovery is that there is a consistent oscillation relationship among the overpressure, flame propagation speed and flame structure. Helmholtz oscillation occurs only when a single vent area is 100 mm × 100 mm–60 mm × 60 mm, and the oscillation degree decreases with decreasing vent area. During the vent failure stage, the maximum explosion overpressure is generated, the flame front begins to appear irregular shape, and the flame propagation speed shows a prominent characteristic peak. After the vent failure stage, the driving effect of the end vent on the flame is higher than that of the side vent on the flame. Furthermore, the correlation equations of the mathematical relationships among the maximum explosion overpressure Pred, the static activation pressure Pstat and the vent coefficient Kv under four vent layouts are established, respectively.

Analysis of liquid surface deformation and breakups using three-dimensional high-speed data recorded with a single camera

Analyzing the deformation of liquid surfaces to better understand, for example, wave generation in oceanology or the formation of industrial spray systems, requires a series of three-dimensional snapshots that temporally resolve such events. This requirement is challenging, especially when applied to transient liquid surfaces that deform rapidly. A technique called Fringe Projection-Laser Induced Fluorescence (FP-LIF), developed by the authors, generates three-dimensional surface reconstructions of irregular liquid structures using snapshots recorded via a single camera only. In this article, FP-LIF is associated, for the first time, with a high-speed detection system, allowing the three-dimensional visualization of liquid surface deformation and breakups at a kHz frame rate. The technique is applied here at 20 kHz for imaging the complete development of a wide hollow-cone water spray and analyzing, in detail, the transition from early injection to stabilization. The three-dimensional image series covered a total time window of 300 ms (6000 frames). It is observed during the first 100 ms that the initial liquid jet deforms into a stable tulip shaped sheet. Then, between 100 and 180 ms, the tulip shape gradually grows until its stabilization corresponds to the final conical shaped sheet. Once the stabilization is reached, the fluctuation of the final spray-angle—ranging from 40° to 50°—is extracted by post-processing 1000 consecutive three-dimensional images, providing a detailed analysis of the radial symmetry of the spray over time and three-dimensional space. The results provided in this article are relevant for the validation of Computational Fluid Dynamics spray models.

Characterization of symmetric to non-symmetric flamefront transition in slender microchannels

The purpose of the present work is to analyze propagating two-dimensional flames confined in slender semi open channels, where the combustion process takes place towards the closed end. The study focuses on the calculation of the growth rate of the transition from symmetric to non-symmetric flames propagation identified by Jiménez et al. [1].The combustion cell is initially filled with a stoichiometric mixture of fuel and air at standard conditions. Ignition is induced close to the open end of the channel under planar and gaussian profiles in temperature and species mass fractions which activate a sustained combustion process. The gases inside the chamber, initially stagnant, are accelerated due to the heat generated in the chemical reactions, leading to the development of lateral boundary layers, so that the hot gases exit the channel following a Poiseuille velocity profile. This transverse flow velocity differences are accommodated by means of a symmetric tulip shape formed after a short initial transient.Acoustic waves generated in the ignition process, keep travelling along the channel, bouncing at the walls and interacting with the flame during all the combustion process. Additionally, the flame structure, curved by Darrieus-Landau instability, interacts with the pressure waves triggering small amplitude oscillations (primary oscillation mode), which under certain conditions can transition to higher amplitude oscillations (secondary mode).This transition is observed to be highly dependent not only on the cell geometry, but also in the initial conditions generated by the ignition procedure.The aim of this work is to improve the understanding of this process, complementing the work of Jiménez et al. [1], and to characterize the effect of the channel width in this transition.

Flashback-induced flame shape transition in a two-stage LPP aeronautical combustor

Large-Eddy Simulations were performed to study the flashback-induced flame shape transition of a lean premixed M flame in a staged liquid-fuelled aeronautical lean-burner, as observed experimentally. The BIMER combustor is a Lean Premixed Prevapourised (LPP) burner composed of two stages, each with its own injector and swirler: the main outer stage, called multipoint, uses jet-in-crossflow injection to achieve the LPP regime, while the central stage, called pilot, uses a pressure swirl injector to create a hollow cone spray to stabilise the flame. During LPP operation, this M flame presents a strong acoustic activity, promoting a periodic flashback of its leading edge. When, aiming to stabilise the flame, the pilot injection is increased and the multipoint injection decreased, the oscillating leading edge (due to the longitudinal acoustic perturbations) attaches to the pilot spray, changing the flame into a Tulip shape. Two phenomena were identified as being the most relevant causes of this flame shape transition. First, the leading edge position and the thermoacoustic instability amplitude are directly linked to the combustion chamber final temperature. The higher the temperature in the chamber, the more upstream the leading edge stabilises, and the higher the acoustic oscillation amplitude, both increasing the risk of a successful flashback. Second, the injection regime with high pilot injection allows the leading edge to attach to the pilot spray, as the flame only transitions when the pilot spray is sufficiently high. The higher the pilot fuel flow, the higher the amount of fuel sprayed in the critical region where the flame might attach for a transition to the Tulip shape. Therefore, as the change in injection regime is the main mechanism lean staged burners use to reduce emissions while increasing operability, this works shows that an M flame is unsuitable to such burners with similar aerodynamic topology and properties.

Effect of obstacle location on explosion dynamics of premixed H2/CO/air mixtures in a closed duct

An experimental study was presented to investigate the effect of obstacle location (OL) on explosion dynamics of premixed H2/CO/air mixtures. A fence-type obstacle providing the blockage ratio of 0.5 was mounted in the closed duct, and the distance from the ignition end increased from 100 mm to 600 mm. Therefore, the effect of the obstacle on flame exponential acceleration stage, flame deceleration stage, and tulip flame stage had been comprehensively discussed. When OL ≤ 400 mm, the turbulent finger-shaped flame front will invert into the tulip shape after the flame propagates through the obstacle channel. The competition between the flamelets’ backward movements induced by the vortex motion and forward movement induced by the squish flow causes the turbulent finger-shaped flame front’s inversion. The pressure dynamics are discussed in conjunction with flame tip dynamics. The prominent flame deformation and oscillation after the tulip flame is caused by the interaction of flame and pressure waves. However, the pressure waves make no difference on the finger-shaped flame front inversion. This indicates that the flame front inversion after the obstacle is a pure hydrodynamic phenomenon. What’s more, for a given hydrogen volume fraction, the flame duration time first decreases and then decreases with obstacle location, the maximum flame tip speed and the maximum overpressure first increases and then decreases with obstacle location, and the maximum explosion severity occurs at OL = 400 mm.

Experimental study on the propagation characteristics of hydrogen/methane/air premixed flames in a narrow channel

This work is focused on the explosion characteristics of premixed gas containing different volume fractions of hydrogen in a narrow channel (1000 mm × 50 mm × 10 mm) under the circumstance of stoichiometric ratio. The ignition positions were set in the closed end and the middle of the pipeline respectively. The results showed that when the gas was ignited at the pipeline closed end, the propagating flame was tulip structure for different premixed gas. When the hydrogen volume fraction was less than 40%, the flame propagation speed increased significantly with the rise of hydrogen volume fraction, and the overpressure peak also appeared obviously in advance. However, when the volume fraction of hydrogen was more than 40%, the increase of flame propagation speed and the overpressure peak occurrence time varied slightly. Furthermore, when the ignition position was placed in the middle of the pipeline, the flame propagation speed propagating to the opening end was much faster than that propagating to the closing end, and there was no tulip shape when the flame propagates to the opening end. The flame propagating to the closed end appeared tulip shape under the influence of airflow, and high-frequency flame oscillation occurred during the propagation. This work shows that the hydrogen volume fraction and ignition position significantly affected the flame structure, flame front speed, and explosion overpressure.

Numerical study of premixed flame dynamics in a closed tube: Effect of wall boundary condition

Numerical simulations were conducted to study the dynamics of premixed flames propagating in a closed tube by solving the fully compressible reactive Navier–Stokes equations using a high-order numerical method on a dynamically adapting grid. A simplified chemical-diffusive model was used to describe the reactions and energy release in a stoichiometric hydrogen-air mixture. The influence of wall boundary condition on the flame dynamics was explored by considering three different types of condition on the walls: adiabatic no-slip, adiabatic free-slip, and isothermal. The calculations show that the wall boundary condition has a significant effect on the generation and amplification of pressure waves and consequently on the flame dynamics. In the early stages of flame propagation, the flame behaves in a similar manner for different boundary conditions, that is, the flame develops a tulip shape that further evolves into a distorted tulip flame (DTF) through Rayleigh-Taylor instability arising from acoustic-flame interaction. Significant differences, however, arise after DTF formation in the late stages, especially when the primary acoustic wave is amplified to form a shock wave in the adiabatic free-slip and isothermal cases. The shock-flame interactions facilitate the formation of a series of increasingly corrugated flames by triggering the Richtmyer–Meshkov instabilities. The way how the lateral flame fronts touch the tube sidewalls to generate the primary acoustics and the heat conduction through the tube sidewalls play an important role in the generation and amplification of the pressure waves.

An experimental study on premixed syngas/air flame propagating across an obstacle in closed duct

This paper studies the effect of obstacle on the propagation of the premixed flame. Experiments are conducted in a closed rectangular duct that house a pair of fence-type obstacles with various blockage ratios (BR = 0, 0.3, and 0.5) mounted both on top and bottom walls of the duct employing the stoichiometric syngas/air mixtures with various hydrogen volume fractions (0% ≤ x(H2) ≤ 50%). High-speed camera and piezoelectric pressure gauges are respectively employed to track the propagation of the premixed flame complex and overpressure dynamics. The presence of obstacles changes the flame front shape and breaks the flame surface. The flame will still invert and the tulip flame forms after propagation through the obstacle opening. There is a new pattern of tulip flame development because of the vortex motion caused by the obstacles. After inversion, obstacles and hydrogen volume fractions are coupled to affect the propagation dynamics. The flame maintains in tulip shape as x(H2) = 0%, but the flame oscillates and undergoes a more drastic shape change as x(H2) ≥ 10%. The characteristic time decreases, but the maximum flame tip speed increases, as hydrogen volume fraction increases significantly as 0% ≤ x(H2) < 10% and gradually as x(H2) ≥ 10%. The presence of the obstacles has no effect on the flame touching the duct walls, but accelerates the flame inversion and propagation to the discharge vent. The maximum flame tip speed increases with blockage ratios. Finally, the corresponding of flame tip speed and overpressure profile is specially observed and discussed.

A Numerical Study of Lean Propane-Air Flame Acceleration at the Early Stages of Burning in Cold and Hot Isothermal Walled Small-Size Tubes

In this study, the problem of lean propane-air premixed flame acceleration from closed to open end during the early stages of burning in small-size tubes with isothermal walls was considered. In particular, the effects of tube radius, slip/non-slip wall conditions, and the wall temperature on the flame propagation and shape were investigated numerically. Five stages of flame propagation are identified: 1) spherical expansion of the flame front; 2) finger shape expansion of the flame front before touching the wall; 3) flame propagation in the tube subjected to flame-wall interactions; 4) transformation of the flame shape into tulip form; 5) conversion of the tulip shape flame to finger. Our results show that the tube radius, wall condition and its temperature significantly affect flame propagation regimes even in the first instance of the flame propagation in the tubes. We find that increasing tube radius has, overall, the effect of increasing the flame propagation speed in isothermal tubes. Also, depending on tube radius and wall condition, the wall temperature can increase or decrease the flame propagation speed in the isothermal tubes. Furthermore, the results demonstrate that imposing either slip or non-slip condition on the tube’s walls impressively affects flame acceleration and its configuration in the early stages. We observe that, unlike flame propagation forms in the tubes with slip walls, the early exponential flame propagation phase in the tubes was generally followed by a linear flame propagation phase in the tubes with a non-slip wall condition. We obtain that flame propagation in tubes with slip wall conditions are more sensitive to variations in tube radius and wall temperature compared to non-slip conditions. We also see that, contrary to the exponential flame propagation phase, increasing the non-slip wall temperature reduces the flame propagation speed in the linear part of the flame propagation, while such an increase in temperature leads to oscillations in the flame propagation speed in the tubes with slip walls.

Experimental study of hydrogen/air premixed flame propagation in a closed channel with inhibitions for safety consideration

An experimental study of hydrogen/air premixed flame propagation in a closed rectangular channel with the inhibitions (N2 or CO2) was conducted to investigate the inhibiting effect of N2 and CO2 on the flame properties during its propagation. Both Schlieren system and the pressure sensor were used to capture the evolution of flame shape and pressure changes in the channel. It was found that both N2 and CO2 have considerable inhibiting effect on hydrogen/air premixed flames. Compared with N2, CO2 has more prominent inhibition, which has been interpreted from thermal and kinetic standpoints. In all the flames, the classic tulip shape was observed. With different inhibitor concentration, the flame demonstrated three types of deformation after the classic tulip inversion. A simple theoretical analysis has also been conducted to indicate that the pressure wave generated upon the first flame-wall contact can affect the flame deformation depending on its meeting moment with the flame front. Most importantly, the meeting moment is always behind the start of tulip inversion, which suggests the non-dominant role of pressure wave on this featured phenomenon.

A comparative investigation of premixed flame propagation behavior of syngas-air mixtures in closed and half-open ducts

A comparative investigation has been developed in conjunction with high-speed cameras and piezoelectric gauge to gain deep insight into the propagation behavior of premixed syngas-air flames. Experiments were carried out in two constant rectangular ducts, one closed duct (C-D) and one half-open duct (HO-D). One important finding is that the flame behavior in the C-D is different than that in the HO-D. The repeatable distorted tulip flame forms in the C-D while an unrepeatable distorted tulip shape forms in the HO-D. The repeated pressure-flame interactions result in a repetitive process of the tulip distortion. The downstream opening end of the duct has a significant influence on the flame propagation characteristics after the flame surface reaches the lateral walls. The opening end results in a higher maximum flame tip velocity, a lower overpressure and a longer plane formation time in the HO-D. Meanwhile, the position of the plane flame formation decreases as hydrogen fraction increases in the C-D while the opposite is true in the HO-D. Furthermore, the pressure build-up in both ducts are examined, and the maximum flame tip velocity is of importance for distorted tulip flame formation.

Evolution of premixed stoichiometric hydrogen/air flame in a closed duct

Experiments were performed in a narrow duct to scrutinize the evolution of premixed stoichiometric hydrogen/air flame propagation at varied initial pressures. Besides the formation of the classic tulip flame, new stages of flame deformation have been observed using high speed Schlieren photography, e.g. elongated tulip flame and T-shape flame. With decreasing initial pressure, the flame propagation tends to be milder, but the classic tulip shape is of robust appearance at all conditions. The measured characteristic times of flame deformation were compared with the predictions of Bychkov's model. The pressure wave generated upon the flame-wall contact at very early stage does not trigger the classic tulip inversion, but will collide with the flame front modifying the tulip structure after reflection on the end wall. Thereafter, three different types of pressure dynamics are initiated depending on the initial pressure, P0≥0.7atm,0.5≤P0<0.7atm, or P0<0.5atm. The Rayleigh–Taylor instability primarily accounts for the flame deformation, whose amplitude growth rate, ω is periodic and determined mainly by the flame acceleration, g according to the linear theory. The measured pressure dynamics by sensor is a combined manifestation originating from the flame dynamics, flame-induced flow and pressure waves excited during combustion.

A study on the dynamic behavior of premixed propane-air flames propagating in a curved combustion chamber

Experiments and numerical simulations were performed to study the flame dynamics and pressure build-up in a curved combustion chamber filled with propane-air mixture. The configuration of the system allows a direct comparison of flame propagation in a bend to that in a straight duct. The comparison helps reveal the role of the bend in the flame dynamics. In the experiments, a high-speed schlieren system and a pressure transducer were used to record the flame evolution and pressure dynamics, respectively. In the simulations, a dynamically thickened flame (TF) model with a single-step chemistry was adopted to represent the premixed combustion. One important finding is that the bend of the chamber has a significant influence on the flame dynamics by promoting the growth of instabilities. The flame behavior in the bend is different than that in the straight section of the chamber. A salient tulip flame is formed in the bend while a much less pronounced tulip shape in the horizontal straight section. The lower part of flame dominates the burning process in the bend while the opposite is true in the horizontal straight section. Furthermore, the influence of heat transfer and gravity were examined using numerical simulations. It was found that heat transfer has an important effect on the combustion dynamics when the flame interacts with the sidewalls of the chamber. The flame propagation speed and pressure rise are overestimated in the absence of heat losses at the chamber walls. Although the gravity plays a role in the flame evolution in the straight configuration, its effect on flame dynamics is damped by the bend. The gravity effect on pressure build-up in the entire process is minor. In addition, the good agreement between the numerical simulations and the experiments supports the validity and reliability of the TF model with the one-step chemical kinetics for calculating propane-air flame propagation.

Experimental study on premixed flame propagation of hydrogen/methane/air deflagration in closed ducts

Experimental investigations of premixed hydrogen/methane/air flame propagation in closed ducts were conducted in six ducts for stoichiometric mixtures with five hydrogen fractions (i.e., 0%, 25%, 50%, 75%, and 100%). In the experiments, the flame propagation behavior associated with the pressure dynamic was studied, and the results show that the flame shape changes as the aspect ratio and hydrogen fraction vary. In the duct with the smallest aspect ratio, only the first four stages of the “tulip” flame were observed. As the aspect ratio increased, the flame structure evolved differently. In cases with lower hydrogen fractions, the flame front became asymmetric, and the lower lateral lip propagated slower than the upper one in a classic “tulip” flame inversion. As the hydrogen fraction increased but before reaching pure hydrogen, the flame propagated while maintaining its tulip shape, accompanied by small oscillations; furthermore, a cellular flame appeared in the duct with the largest aspect ratio before the end of propagation. In those ducts, a distorted “tulip” flame with indented tulip lips was observed for pure hydrogen. For all configurations, the flame speed dynamic is closely related to the pressure, and they both significantly increase with increasing hydrogen.

A comparative study on premixed hydrogen–air and propane–air flame propagations with tulip distortion in a closed duct

The premixed hydrogen–air and propane–air flames in a closed duct were experimentally studied for a deep insight into the tulip distortion phenomenon. High speed schlieren photography and pressure sensors recorded the flame images and pressure dynamics at different equivalence ratios for detailed analyses and comparisons. The distorted tulip shape which was first scrutinized and distinguished as an exclusive feature in premixed hydrogen–air flame, was also observed in stoichiometric premixed propane–air flame with a similar behavior on flame shape changes. Tulip distortions are always accompanied by remarkable flame tip velocity fluctuations with near-constant amplitudes. But the pressure dynamics is totally different. There are no observable oscillation and stepped rise in pressure trajectory of stoichiometric premixed propane–air flame. The pressure wave effect is definitely not the incitation factor, but an additional enhancement of flame acceleration, deceleration and deformation.

The Used of Infrared Sensor for 3D Image Reconstruction

In this paper, new development of a sensor rig device with infrared sensors is used to reconstruct the 3D image of an object’s surfaces. Five sensors have been used to acquire data that measure distance from the object and sensors. A stepper motor controls the object rotation with 2 degrees per turn. A few objects such as tulip, curvilinear, flower and star shape have been selected for data collection, and the results proved that this sensor rig device is capable of reconstruct a 3D image of an object’s surface. A prosthetic model with distinct size in diameter was tested to prove that this device is capable of measuring distance with different size. From the results obtained, this sensor rig device shows a capability to reconstruct 3D image of an object surfaces with simple post processing technique. A prosthetic object was tested and results show that the accuracy was around 75%.

An investigation of premixed flame propagation in a closed combustion duct with a 90° bend

In this work, the premixed flame propagation and pressure dynamics in a closed combustion tube with a 90° bend are investigated using experiment and numerical simulations to provide further knowledge of premixed combustion process in a curved chamber. In the experiment, high-speed schlieren photography is employed to record the changes in flame shape and position as a function of time. And a pressure transducer is used to measure the pressure rise in the chamber. In the numerical simulations, a dynamically thickened model is applied to allow a detailed insight into the flame dynamics under both isothermal and adiabatic wall conditions. It is revealed that the flame evolves into a notable tulip shape in the straight horizontal section which takes on a four-tongue appearance in the numerical calculations. The lower tongues dominate the flame propagation in the bend. The flame remains concaved after the tulip disappearance and rounds along the inner wall in the bend. It is found that the heat losses to the walls have a great impact on the combustion dynamics, including the flame front evolution and pressure build-up. The numerical combustion dynamics with isothermal walls agrees well with the experimental results. Furthermore, the analytical analysis demonstrates that the flame mechanism in the horizontal section is consistent with that in a straight duct in spite of the presence of the bend.

Experimental and numerical study of premixed hydrogen/air flame propagating in a combustion chamber

An experimental and numerical study of dynamics of premixed hydrogen/air flame in a closed explosion vessel is described. High-speed shlieren cinematography and pressure recording are used to elucidate the dynamics of the combustion process in the experiment. A dynamically thickened flame model associated with a detailed reaction mechanism is employed in the numerical simulation to examine the flame-flow interaction and effect of wall friction on the flame dynamics. The shlieren photographs show that the flame develops into a distorted tulip shape after a well-pronounced classical tulip front has been formed. The experimental results reveal that the distorted tulip flame disappears with the primary tulip cusp and the distortions merging into each other, and then a classical tulip is repeated. The combustion dynamics is reasonably reproduced in the numerical simulations, including the variations in flame shape and position, pressure build-up and periodically oscillating behavior. It is found that both the tulip and distorted tulip flames can be created in the simulation with free-slip boundary condition at the walls of the vessel and behave in a manner quite close to that in the experiments. This means that the wall friction could be unimportant for the tulip and distorted tulip formation although the boundary layer formed along the sidewalls has an influence to a certain extent on the flame behavior near the sidewalls. The distorted tulip flame is also observed to be produced in the absence of vortex flow in the numerical simulations. The TF model with a detailed chemical scheme is reliable for investigating the dynamics of distorted tulip flame propagation and its underlying mechanism.