Flame Shape Changes Research Articles

Experimental study on flame propagation over horizontal electrical wires under varying pressure

Flame propagation behavior over the polymer insulation of electrical wires can ignite adjacent combustibles and increase the scale of the fire. In this study, the flame propagation behavior over polyethylene (PE)-insulated wires was investigated theoretically and experimentally under varying pressures. The experiments were conducted in three cities with different altitudes (64, 76, and 100 kPa) and in a newly designed hypobaric chamber (7.5–101.3 kPa) for the first time. The flame morphology, structure, and spread rate were examined and discussed. The results show that pressure significantly affects flame propagation behavior. With increasing pressure, the flame illumination increases, and the flame shape changes, while the average flame height increases monotonously with decreasing average flame width. The weaker reaction rate and reduced heat transfer are the essential factors that result in a decreasing flame spread rate at lower pressures. In addition, a simplified correlation between the flame spread rate, pressure, and wire configuration in the form of Vf∝(P2L3)0.65 is proposed to predict flame propagation along an electrical wire. Moreover, the influence of the metal core on the flame behavior was observed based on the different heat transfer mechanisms in the flame front zone. All results contribute to the safe operation of electrical wires.

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.

Numerical investigations on behaviour bifurcation of premixed H2-air flames in mesoscale tubes

Unsteady numerical simulations with detailed kinetic chemistry have been reported for premixed H2-air flames in straight mesoscale tubes of various diameters to understand the effect of wall heat transfer conditions on flame transition and behaviour bifurcation leading to a change in flame shape and flame propagation velocity. A parabolic velocity profile is applied with no-slip boundary conditions at the walls. For certain wall heat transfer conditions, it was observed that the flame transforms from a concave- to convex-shaped flame during propagation through the channel, accompanied by a change in flame propagation speed, heat of reaction and heat release rates. The transition in flame shape is attributed to thermal wall coupling effect through wall heat transfer to surroundings, which affects the flow redirection in the vicinity of the flame due to heat-loss, resulting in flame behaviour transition. Due to these flame behaviour transitions, the heat release rates and flame propagation velocities change significantly as discussed in this paper.

Experimental study of premixed syngas/air flame propagation in a half-open duct

The flame propagation characteristics of premixed syngas/air mixtures are investigated in a half-open duct over a wide range of hydrogen fractions at various equivalence ratios. A high-speed camera and a pressure transducer are used to acquire the flame images and overpressure dynamics. The results indicate that the flame shape changes as the hydrogen fraction and equivalence ratio vary. There are two different styles of tulip flame inversion. When the equivalence ratios, Ф, are 2.5 and 3.0 and the hydrogen fraction, φ, is higher than 0.3, the flame inversion is more complicated than the classical tulip flame inversion. Moreover, the distorted tulip flame, which is always obtained in closed duct, is observed in a half-open duct in three cases, i.e., Ф = 1.0 and φ = 0.1, Ф = 1.0 and φ = 0.3, and Ф = 3.0 and φ = 0.1. Both the equivalence ratio and the hydrogen fraction can affect the flame propagation velocity and the pressure dynamics. Dramatic flame reacceleration and pressure oscillation occur after the flame inversion. The hydrogen fraction has a significant influence on the characteristics of premixed syngas/air flame. The influence is significant when hydrogen fraction was small (φ < 0.5), and the influence is moderate when hydrogen fraction was large (φ ≥ 0.5).

Experimental Study of Transient Mechanisms of Bistable Flame Shape Transitions in a Swirl Combustor

Sudden changes of flame shape are an undesired, yet poorly understood feature of swirl combustors used in gas turbines. The present work studies flame shape transition mechanisms of a bistable turbulent swirl flame in a gas turbine model combustor, which alternates intermittently between an attached V-form and a lifted M-form. Time-resolved velocity fields and two-dimensional flame structures were measured simultaneously using high-speed stereo-particle image velocimetry (PIV) and planar laser-induced fluorescence of OH (OH-PLIF) at 10 kHz. The data analysis is performed using two novel methods that are well adapted to the study of transient flame shape transitions: First, the linear stability analysis (LSA) of a time-varying mean flow and second, the recently proposed spectral proper orthogonal decomposition (SPOD). The results show that the transitions are governed by two types of instability, namely a hydrodynamic instability in the form of a precessing vortex core (PVC) and a thermoacoustic (TA) instability. The LSA shows that the V-M transition implies the transient formation of a PVC as the result of a self-amplification process. The V-M transition, on the other hand, is induced by the appearance of a TA instability that suppresses the PVC and thereby modifies the flowfield such that the flame re-attaches at the nozzle. In summary, these results provide novel insights into the complex interactions of TA and hydrodynamic instabilities that govern the shape of turbulent swirl-stabilized flames.

Hydrogen combustion under conditions of a high-temperature supersonic flow

Some results of studying the combustion of hydrogen in a high-velocity air flow at stagnation temperatures below 3000 K are given. Flame shape and combustion zone length change at temperatures above 2000–2200 K due to deterioration in the fuel–oxidizer mixing process. It has been shown that dissociation processes have an effect on the process of hydrogen combustion at a high temperature. In experiments, they can be detected by a change in the intensity of radiation from the intermediate ОН* radical.

Experimental study on the influence of multi-layer wire mesh on dynamics of premixed hydrogen-air flame propagation in a closed duct

Hydrogen, which is considered to be a promising clean energy source, has been studied and applied extensively in industries. In order to improve the safety of hydrogen energy application, an experimental study on the influence of multi-layer wire mesh on dynamics of premixed hydrogen-air flame propagation in a closed duct is conducted. Four different kinds of wire mesh with 40, 45, and 50 layers are chosen in the experiments. High speed schlieren photography is applied to capture the flame shape changes and determine the flame tip speed. Pressure transducer is used to measure the pressure transient. It is found that flame quenches in the cases of adding wire mesh of 60, 80, and 100 mesh with 45 and 50 layers, while for the wire mesh of 40 mesh, 50 layers cannot even quench the flame. Moreover, the multi-layer wire mesh can effectively suppress the flame tip speed, maximum pressure, and sound waves during premixed hydrogen-air flame propagation in the duct. The attenuated maximum pressure reaches approximately 78.6% in the case of adding wire mesh of 100 mesh-50 layers.

Premixed methane/air flame propagating in an obstructed chamber with different BRs and spatial configurations

The effects of obstacle blockage ratio (BR) and spatial configurations on premixed methane/air flame propagation are experimentally investigated by the high-speed video photography and pressure transducer. The results demonstrate that the flame shape changes, flame tip speed and deflagration overpressure in the initial stage are not affected by the obstacles with different BRs and spatial configurations. After the moving flame feels the obstacle, flame tip speed and overpressure increase significantly with increasing obstacle BR for center-installed obstacle. The first peak value of flame tip speed with bottom-installed obstacle is slightly larger than that with center-installed obstacle. Two-peak structure is observed in the curves of flame tip speed, and overpressure variation depends strongly on the second flame acceleration, which also explains the fact that deflagration overpressure with bottom-installed obstacle is smaller than that with center-installed obstacle with time ranging from t = 36 m to t = 90 m.

Experimental and LES investigation of premixed methane/air flame propagating in an obstructed chamber with two slits

The paper aims at revealing the interaction of twin premixed methane/air jet flames in a closed duct, in which a thin obstacle with two slits is mounted to generate two free jet flames. In the experiment, a high-speed video camera and pressure transducers are used to study the flame shape changes and pressure dynamics. In the numerical simulations, large eddy simulation (LES) with a Power-Law model is applied to investigate the interaction between the moving flame and vortices induced by the obstacle. The results indicate that the flame propagation for all obstacle configurations in a closed duct can be divided into four typical stages, i.e. hemispherical flame, finger-shaped flame, jet flame and bidirectional propagation flame. For three obstacle configurations, the merged jet flames, paralleling jet flames, and separated jet flames are observed after twin jet flames are formed downstream of the two slits. The Power-Law model can well reproduce the flame shape changes, the dynamics of flame front, and the pressure growth rates. By analyzing the predicted flow structure and vorticity magnitude, the premixed methane/air flame propagation in an obstructed tube can be explained in the view of pure hydrodynamics. In addition, the transition from a “corrugated flamelets” to a “thin reaction zones” is observed in the simulation.

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.

Bistable behaviour and thermo-acoustic instability triggering in a gas turbine model combustor

In a liquid-fueled two-stage gas turbine model combustor, a bistable point where two different flame states can exist is studied in non-reacting and reacting conditions with the help of fast spray-PIV and chemiluminescence imaging. The first flame, named sV, is stabilized thanks to an internal reaction zone that modifies the air flow compared to non-reacting conditions. The second state, associated with a lifted flame, exhibits a strong thermo-acoustic instability linked to the quarter-wave mode of the chamber and piloted by a varying liquid-fuel supply to the flame. Using well-controlled air flow rate modulations, the switch from the quiet sV state to the lifted one is studied in a time-resolved fashion. The analysis shows that the sensitivity of the internal reaction zone of the sV state to perturbations is responsible for the change in flame shape which then triggers the onset of the thermo-acoustic instability.

Experimental and LES investigation of premixed methane/air flame propagating in a tube with a thin obstacle

In this paper, an experimental and numerical investigation of premixed methane/air flame dynamics in a closed combustion vessel with a thin obstacle is described. In the experiment, high-speed video photography and a pressure transducer are used to study the flame shape changes and pressure dynamics. In the numerical simulation, four sub-grid scale viscosity models and three sub-grid scale combustion models are evaluated for their individual prediction compared with the experimental data. High-speed photographs show that the flame propagation process can be divided into five stages: spherical flame, finger-shaped flame, jet flame, mushroom-shaped flame and bidirectional propagation flame. Compared with the other sub-grid scale viscosity models and sub-grid scale combustion models, the dynamic Smagorinsky–Lilly model and the power-law flame wrinkling model are better able to predict the flame behaviour, respectively. Thus, coupling the dynamic Smagorinsky–Lilly model and the power-law flame wrinkling model, the numerical results demonstrate that flame shape change is a purely hydrodynamic phenomenon, and the mushroom-shaped flame and bidirectional propagation flame are the result of flame–vortex interaction. In addition, the transition from “corrugated flamelets” to “thin reaction zones” is observed in the simulation.

Effect of external heat loss on the propagation and quenching of flames in small heat-recirculating tubes

This study investigates the effect of external heat loss on the slow propagation of strongly-burning flames inside narrow heat-recirculating tubes. The system is studied using a two-dimensional numerical model for reactive flow, including conjugate heat transfer, over a range of tube diameters spanning the micro- and the meso-scale. Increasing external heat loss decreases the propagation speed of the slowly-propagating flames, leading to a transition from upstream to downstream propagation, until a heat loss limit is reached. Flames in micro-scale tubes are subjected to an extinction limit, while flames in larger diameter tubes, in the meso-scale range, are subjected to a blowout limit. While the absolute value of the external convective heat loss coefficient at the extinction/blowout limit increases with diameter, the dimensionless volumetric heat loss coefficient decreases with an increase in diameter until an approximately constant value, for the studied conditions, is reached at the blowout limit in the mesoscale range. Discrepancies in the predicted trends of 1-D and 2-D models indicate that 2-D effects play a significant role at larger diameters, not only at the meso-scale, but also in the upper-range of the micro-scale. These 2-D effects, associated with changes in flame shape that allow an increase in burning surface area, are seen to promote stability of the system. Results have implications on the choice of tube diameter to be used in the design of a stable burner optimized for heat transfer to an external heat load.

Study on the combustion characteristics of non-premixed hydrogen micro-jet flame and the thermal interaction with solid micro tube

The combustion characteristics of non-premixed hydrogen micro-jet flames in co-flow air were investigated in a wide fuel flow velocity range experimentally and numerically to extend the knowledge on non-premixed micro flame. The direct flame images were captured by a digital reflex camera with long exposure time, and the OH radical distributions in the flames were measured by the laser induced fluorescence technique. The experimental observation shows that the flame height and shape change with the deceasing fuel flow velocity. A detailed chemical reaction mechanism was employed in the numerical computations with considering the heat transfer between gas and solid micro-tube. The computed distributions of OH concentrations agree qualitatively with those obtained by the experiments. The flame structure and thermal interaction between the flame and solid tube were inspected in detail. It was found that the thermal interactions at high, intermediate and low fuel flow velocities are different. The details of the thermal interactions between flames and solid tube are depended on the shape of the micro flame, which essentially affects the temperature distribution.

Experimental and LES investigation of premixed methane/air flame propagating in a chamber for three obstacle BR configurations

The paper aims at revealing the effect of blockage ratio (BR) on the flame acceleration process and the flame-vortex mechanism in an obstructed chamber based essentially on the experimental and numerical methods. In the experiments, high-speed video photography and pressure transducer are used to study the flame shape changes and pressure dynamics. In the numerical simulations, large eddy simulation (LES) with the flame surface density (FSD) model is applied to investigate the interaction between the moving flame and vortices induced by obstacle. The results demonstrate that the flame propagation process can be divided into four stages, namely spherical flame, finger-shaped flame, jet flame and volute flame for three obstacle BR configurations, and a small recirculation zone is observed above the obstacle only for BR = 0.5. The peak of flame tip speed and pressure growth rate increases with the blockage ratio. The generation and evolution of the vortex behind the obstacle can be attributed to the initial flame acceleration, while the subsequent flame deceleration is due to the flame-vortex interaction. In addition, the transition from a “thin reaction zones” to a “broken reaction zones” is also observed in the simulation.

Experimental and numerical study of premixed methane/air flame propagating in various L/D closed ducts

The article aims at explaining the effects of L/D (the ratio of length to diameter) on premixed methane/air flame propagation in the closed duct, which is based essentially on the experimental and numerical methods. High-speed camera, pressure transducer, and large eddy simulation model are used to study the flame shape changes and pressure build-up in the closed ducts with various L/D. The results demonstrate that the premixed flame propagation undergoes four typical stages, namely spherical flame, finger-shaped flame, flat flame, and tulip flame. The pressure growth rate and the flame tip speed reach the maximum value simultaneously when the flame lateral sides touch the sidewalls in the closed duct. The dynamic synchronization of the flame tip speed and the pressure growth rate indicates the tulip flame is a purely hydrodynamic phenomenon resulting from the interaction of the flame front and the pressure wave. Particularly, the maximum flame tip speed increases linearly with increasing L/D. © 2015 American Institute of Chemical Engineers Process Saf Prog 35: 185–191, 2016

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.

Interaction of Two Micro-slot Flames: Heat Release Rate and Flame Shape

This paper studies the interaction between two identical micro-slot diffusion flames. Here, we define a micro-slot flame as a slot flame of which the slot width is less than about 1 mm. Because of its smallness, a micro-slot flame has a high heating density and can be used as a small heat source. However, the heat release rate of a single micro-slot flame is limited, and therefore, multiple micro-slot flames may be used to increase total heat release rate. As a first step, this paper considers a situation in which two micro-slot flames are used with certain burner spacing. When two diffusion flames are placed closely, flame shape changes from that of an isolated flame. Studying such flame shape change and resultant change in total heat release rate is the topic of this paper. Experiment is conducted and total heat release rate is measured by integrating CH* chemiluminescence recorded using a CCD camera and an optical filter of the wavelength of 430 nm. Two different burner materials, copper and glass, are tested to study the effect of heat loss to burners. An analytical model is applied to predict flame shape. In addition to the classical Burke-Schumann assumptions, two slot flames are modeled as line sources with zero width, enabling a simple analytical solution for the critical burner spacing at which two flames touch each other. The critical burner spacing is a key parameter that characterizes the interaction between two micro-slot flames. Computational fluid dynamics (CFD) simulations are then conducted to test the validity of the present theory. CFD results are favorably compared with the theoretical prediction.

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.

Phase Resolved Analysis of Flame Structure in Lean Premixed Swirl Flames of a Fuel Staged Gas Turbine Model Combustor

A scaled model of a gas turbine (GT) burner with coaxially mounted swirlers has been used to study the effects of fuel staging on the behavior of lean premixed methane air flames. Lean flames are known to be susceptible to instabilities that can lead to unsteady operation, flame extinction, and thermo-acoustic oscillations. High speed (10 kHz) laser and optical diagnostic techniques have been used to investigate the fuel staging effect on the mechanisms involved in such instabilities. Methane air flames at atmospheric pressure have been investigated at a constant thermal power of 58 kW. The global equivalence ratio was kept constant, while the fuel staging was varied. The bulk flow velocity at the exit plane was kept constant at 20 m/s. Simultaneous high speed OH PLIF, OH* CL, and acoustic measurements were performed at kHz repetition rate to characterize the flames and determine the operability limits of the combustor. The characterization measurements reveal significant changes in flame shape for various staging ratios as well as onset of self-excited thermo-acoustics in flames with more than 55% fuel injection in the outer swirler. The phase resolved analysis of the OH* CL revealed pulsation in the heat release due to acoustics in flames with higher percentage of fuel in the outer swirler. Comparison of the pressure oscillation in the combustion chamber with the heat release yielded a clear picture regarding the feedback mechanism that sustains the self-excited thermo-acoustic pulsations. The variation of local equivalence ratio of the mixture seems to be the driving force that initiates the onset of acoustics pulsations.