Premixed Propane-air Flames Research Articles

Numerical Study of High-g Combustion Characteristics in a Channel with Backward-Facing Steps

High gravity (high-g) combustion can significantly increase flame propagation speed, thereby potentially shortening the axial length of aero-engines and increasing their thrust-to-weight ratio. In this study, we utilized the large eddy simulation model to investigate the combustion characteristics and flame morphology evolution of premixed propane–air flames in a channel with a backward-facing step. The study reveals that both the increase in centrifugal force and flow velocity can enhance pressure fluctuations during combustion and increase the turbulence intensity. The presence of centrifugal force promotes the occurrence of Rayleigh–Taylor instability (RTI) between hot and cold fluids. The combined effects of RTI and Kelvin–Helmholtz instability (KHI) enhance the disturbance between hot and cold fluids, shorten the fuel combustion time, and intensify the dissipation of large-scale vortices. The increase in fluid flow velocity can raise the flame front’s hydrodynamic stretch rate, thereby enhancing the turbulence level during combustion to a certain extent and increasing the fuel consumption rate. When a strong centrifugal force is applied, the global flame propagation speed can be more than doubled. Within a certain range, the increase in high-g field strength can enhance the intensity of RTI and accelerate the transition of RTI to the nonlinear stage.

Premixed Propane–Air Flame Propagation in a Narrow Channel with Obstacles

Flame interaction with obstacles can affect significantly its behavior due to flame front wrinkling, changes in the flame front surface area, and momentum and heat losses. Experimental and theoretical studies in this area are primarily connected with flame acceleration and deflagration to detonation transition. This work is devoted to studying laminar flames propagating in narrow gaps between closely spaced parallel plates (Hele–Shaw cell) in the presence of internal obstacles separating the rectangular channel in two parts (closed and open to the atmosphere) connected by a small hole. The focus of the research is on the penetration of flames through the hole to the adjacent channel part. Experiments are performed for fuel-rich propane–air mixtures; combustion is initiated by spark ignition near the far end of the closed volume. Additionally, numerical simulations are carried out to demonstrate the details of flame behavior prior to and after penetration into the adjacent space. The results obtained may be applicable to various microcombustors; they are also relevant to fire and explosion safety where flame propagation through leakages may promote fast fire spread.

Exhaust CO emissions of a laminar premixed propane–air flame interacting with cold gas jets

This study investigates a laminar premixed flame interacting with cold gas jets, at different cooling jet mass flow fractions (m˙jet*) and diluent types, namely air and N2. A novel burner and wall configuration is used to experimentally induce flame-cooling-air interaction (FCAI). Flame chemiluminescence imaging, exhaust temperature (Texh) and exhaust CO emissions ([CO]exh) measurements are conducted to characterise the flame shape and [CO]exh response to the cooling jets. Flame imaging reveals that the cooling jets greatly affect the flame shape. Measurements of [CO]exh demonstrate a direct correlation with Texh, as decreasing Texh is observed to occur with decreasing [CO]exh. Additionally, the air diluent case shows consistently lower [CO]exh values, relative to the N2 diluent case.Using a novel modelling approach, the cooling jets are simulated using one-dimensional (1D) fully resolved simulations (FRS). The effect of jet dilution, jet cooling and exhaust gas cooling are independently and jointly investigated in these simulations. The FRS results support the experimentally observed behaviour, and show that exhaust gas cooling and exhaust gas oxygenation produce decreased CO concentrations. Using a chemical reactor network (CRN), the jet mixing process is modelled by a perfectly stirred reactor (PSR), while the exhaust gas cooling process is modelled by a plug flow reactor (PFR). The CRN modelling shows that the jet mass flow rates dictated by m˙jet*, the dilution time (tdil) assumed for cooling jet mixing, and the exhaust gas cooling residence time (tcool), play an important role in determining the [CO]exh. An equilibrium analysis illustrates that the relationship between [CO]exh, Texh and exhaust O2, is due to the thermodynamically favoured equilibrium states. Timescale analyses demonstrate that appropriate modelling of jet mixing, and accounting for the rate of exhaust gas cooling, are important for estimations of [CO]exh.

Conically Stabilized Turbulent Premixed Lean-Flames Sustainability

Lean combustion is a very well-known technology for low carbon and NOx emissions and yet relatively high combustion efficiency. On the other hand, lean combustion flames may blow off in absence of well-designed flame stabilizers. This article presents an experimentally validated modelling approach to study lean premixed flames anchored on a conical bluff body. The structure of an unconfined premixed propane-air flame, stabilized on a conical bluff body, has been computationally examined for conditions of 0.8 to near blow off equivalence ratio (Ø). Reynolds number of about 30,600 is considered based on the conveying pipe’s diameter (10.4cm) and upstream velocity (4.5m/s). the effect of radiation heat losses on flame stability is examined as well as the flow and temperature profiles. Experiments were performed to measure the flame’s spatial temperature profile in order to validate the developed computational models. Our model’s result portrayed a good agreement with experimental measurements. As expected, calculated temperature profiles show that reduction in Ø reduces flame temperature. Results also show that the open-base cone generate a larger recirculation zone – which favours flame stability – than the closed-base cone. Overall, this article highlights the potential of computational models in providing greater insights in the study of flames sustainability nearby the lean combustion limit.

Large-eddy simulation of a bluff-body stabilised turbulent premixed flame using the transported flame surface density approach

A premixed propane–air flame stabilised on a triangular bluff body in a model jet-engine afterburner configuration is investigated using large-eddy simulation (LES). The reaction rate source term for turbulent premixed combustion is closed using the transported flame surface density (TFSD) model. In this approach, there is no need to assume local equilibrium between the generation and destruction of subgrid FSD, as commonly done in simple algebraic closure models. Instead, the key processes that create and destroy FSD are accounted for explicitly. This allows the model to capture large-scale unsteady flame propagation in the presence of combustion instabilities, or in situations where the flame encounters progressive wrinkling with time. In this study, comprehensive validation of the numerical method is carried out. For the non-reacting flow, good agreement for both the time-averaged and root-mean-square velocity fields are obtained, and the Karman type vortex shedding behaviour seen in the experiment is well represented. For the reacting flow, two mesh configurations are used to investigate the sensitivity of the LES results to the numerical resolution. Profiles for the velocity and temperature fields exhibit good agreement with the experimental data for both the coarse and dense mesh. This demonstrates the capability of LES coupled with the TFSD approach in representing the highly unsteady premixed combustion observed in this configuration. The instantaneous flow pattern and turbulent flame behaviour are discussed, and the differences between the non-reacting and reacting flow are described through visualisation of vortical structures and their interaction with the flame. Lastly, the generation and destruction of FSD are evaluated by examining the individual terms in the FSD transport equation. Localised regions where straining, curvature and propagation are each dominant are observed, highlighting the importance of non-equilibrium effects of FSD generation and destruction in the model afterburner.

Large Eddy Simulation of a premixed bluff body stabilized flame using global and skeletal reaction mechanisms

The increasing computational capacity in recent years has spurred the growing use of combustion Large Eddy Simulation (LES) for engineering applications. The modeling of the subgrid stress and flux terms is well-established in LES, whereas the modeling of the filtered reaction rate terms is under intense development. The significance of the reaction mechanism is well documented, but only a few computational studies have so far been conducted with the aim of studying the influence of the reaction mechanism on the predicted flow and flame. Such an investigation requires the availability of well documented, thoroughly tested, and accurate reaction mechanisms suitable for use in practical engineering simulations. Global and detailed reaction mechanisms are available for many fuel mixtures, whereas skeletal reaction mechanisms suitable for LES are in rather short supply. This research attempts to close this gap by using combustion LES to examine a well-known bluff-body stabilized premixed propane–air flame using two well-known global reaction mechanisms and a novel skeletal reaction mechanism, developed as part of this study. These reaction mechanisms are studied for laminar flames, and comparison with experimental data and detailed reaction mechanisms demonstrates that the skeletal mechanism shows improved agreement with respect to all parameters studied, in particular the laminar flame speed and the extinction strain rate. The LES results reveal that the choice of the reaction mechanism does not significantly influence the instantaneous or time-averaged velocity, whereas the instantaneous and time-averaged species and temperature are influenced. The agreement with the experimental data increases with increased fidelity of the reaction mechanism, and the skeletal reaction mechanism provides a more realistic basis for e.g. emission predictions.

Flame size measurements of premixed propane-air mixtures ignited by microwave-enhanced plasma

The effects of microwave-enhanced plasma ignition on unsteady, expanding, premixed propane-air flames were studied using Schlieren technique in a constant volume chamber at various equivalence ratios and different microwave exposure durations. The plasma was generated by a spark discharge in engine-like conditions. Under these conditions, the dominant factors affecting the flame were stretching, heat transfer, and wall confinement effects. The spark was enhanced using microwaves, which accelerated the electrons in the spark, generating a non-thermal plasma. The flame size, flame speeds were computed from the visualized flame fronts. The experimental results indicated that an unsteady ignition kernel precedes the flame propagation and that there are multiple regimes prior to the final steady regime, in which the flame propagation is sustained. There is a minimum flame size which spark kernel must achieve, below which the flame could not sustain for successful combustion. The measured minimum flame size is a function of mixture equivalence ratio, and it decreases with increasing equivalence ratio. Increasing the microwave energy resulted in higher initial flame velocity and a reduction in the time taken to reach the critical flame size. Measurement of microwave-enhanced plasma size at non-reactive conditions indicated that plasma size is larger than spark-only case. At lean mixture conditions, enhancing the spark by microwave addition proven to enlarge the size of the ignition kernel, and achieve the required minimum flame size, that successfully helps in transitioning the initial spark kernel to a propagating flame, whereas spark-only operation failed to sustain the flame.

Modelling flame turbulence interaction in RANS simulation of premixed turbulent combustion

Flame turbulence interaction is an important term for modelling scalar dissipation in premixed turbulent combustion. In order to obtain an accurate representation of the flame turbulence interaction phenomenon, an evolution equation for has recently been proposed. This equation gives a detailed insight into the flame turbulence interaction phenomenon and provides an alternative approach to modelling the important physics represented by . In this paper the evolution equation is used to model a premixed propane–air flame stabilised in a turbulent mixing layer. The simulations are carried out in the context of a Reynolds Averaged Navier–Stokes (RANS) framework and the results are compared with the experiments and also with Large Eddy Simulation (LES). It is found that the modelling strategy involving the evolution equation gives good approximations for the mean velocities and flame locations in the mixing layer stabilised flame when compared with other modelling strategies.

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.

Three-Dimensional Simulations of Steady Perforated-Plate Stabilized Propane–Air Premixed Flames

A numerical investigation of steady laminar premixed propane–air flames is presented. A three-dimensional simulation has been performed to examine the impact of operating conditions on steady-state characteristics of a perforated burner flame. A numerical simulation has been carried out using a reduced propane–air reaction mechanism having 30 species and 192 reactions. The results are validated against the one-dimensional flat-flame result obtained using PREMIX. Effects of the equivalence ratio, inlet velocity, hole–hole distance, and plate thermal conductivity on flame stability are examined. The flame stand-off distance increases with the increase in the inlet velocity. As the equivalence ratio increases, the heat flux to the plate increases as the flame moves closer to the plate. When the plate is adiabatic, the conical flame rests on the plate. The flame stand-off distance increases as the plate thermal conductivity is increased. The flame moves downstream of the plate as the distance between the adja...

Numerical analysis of effect of ignition methods on flame behavior during passing through a sudden contraction near the quenching conditions

The present study numerically investigates the propagation of a laminar premixed propane–air flame in various tubes with a sudden contraction and isothermal cold walls in the stoichiometric mixture. Two different ignition methods are examined: plane and round shaped ignition. These investigations show that for relatively small tubes, the method of ignition does not influence the final flame shape in the tube – it is mushroom-shaped. For wider tubes and the plane ignition method, the flame transforms to a tulip-shaped one. The numerical results show that a flame can enter a smaller diameter tube from a wider one and propagate in a stable manner in all configurations of diameters of both tubes as long as the diameter of the narrow tube is far from the quenching diameter. Decreasing the diameter of the narrow tube, we approach the conditions where the dimension of the wide tube determines if the flame will propagate or quench. This is associated with a temporarily increased ratio of heat losses to thermal energy generation caused by a greater heat loss area. It is also shown that a tulip-shaped flame occurring at large diameters of the wide tube is particularly exposed to quenching during propagation through the sudden contraction.

Flame dynamics of a meso-scale heat recirculating combustor

The dynamics of premixed propane–air flame in a meso-scale ceramic combustor has been examined here. The flame characteristics in the combustor were examined by measuring the acoustic emissions and preheat temperatures together with high-speed cinematography. For the small-scale combustor, the volume to surface area ratio is small and hence the walls have significant effect on the global flame structure, flame location and flame dynamics. In addition to the flame–wall thermal coupling there is a coupling between flame and acoustics in the case of confined flames. Flame–wall thermal interactions lead to low frequency flame fluctuations (∼100 Hz) depending upon the thermal response of the wall. However, the flame–acoustic interactions can result in a wide range of flame fluctuations ranging from few hundred Hz to few kHz. Wall temperature distribution is one of the factors that control the amount of reactant preheating which in turn effects the location of flame stabilization. Acoustic emission signals and high-speed flame imaging confirmed that for the present case flame–acoustic interactions have more significant effect on flame dynamics. Based on the acoustic emissions, five different flame regimes have been identified; whistling/harmonic mode, rich instability mode, lean instability mode, silent mode and pulsating flame mode.

High-Speed Visualization of Flame Reaction Zone Using a Positive Point-to-Plane Corona Discharge

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We have used a pulsed corona discharge to obtain time-resolved spectrally filtered 337.1-nm <formula formulatype="inline"><tex>$N_{2}(C \rightarrow B)$</tex></formula> emission images for the visualization of the reaction zone (flame front) of a premixed propane–air flame. The low-density region at the flame front allows one to guide the streamer along the reaction zone during the initial stages of the pulsed discharge, while the temperature-dependent electron-detachment/attachment rate allows for the increased conductivity of the plasma after the discharge has been fully established. Images with 500-shot accumulations are presented to clearly illustrate the concept, while the ability for greater time resolution is discussed as an attractive means for imaging small-scale turbulent instabilities in a flame reaction zone. </para>

Electrical control of the thermodiffusive instability in premixed propane–air flames

This work focuses upon the effects of DC electric fields on the stability of downward propagating atmospheric pressure premixed propane–air flames under experimental conditions that provide close coupling of the electric field to the flame. With the appropriate electrode geometry, modest applied voltages are shown to drive a stable conical flame first into a wrinkled-laminar flamelet geometry, and then further toward either a highly unstable distributed flamelet regime or a collective oscillation of the flame front. Applied potentials up through + 5 kV over a 40-mm gap encompassing the flame front have been used to force the above transition sequence in flames with equivalence ratios between 0.8 and 1.3 and flow velocities up to 1.7 m/s. Experiments are reported that characterize the field-induced changes in the geometry of the reaction zone and the structure of the resulting unstable flame. The former is quantified by combustion intensity enhancement estimates derived from high-speed two-dimensional direct and spectroscopic imaging of chemiluminescence signals. The flame fluid mechanical response to the applied field, brought about by forcing positive flame ions counter to the flow, drives the effective flame Lewis number to values suitable for the onset of the thermodiffusive instability, even near stoichiometric conditions. Possible field-driven flame ion recombination chemistry that would produce light reactants near the burner head and precipitate the onset of the thermodiffusive instability is proposed. Electrical measurements are also reported that establish that minimal electrical power input is required to produce the observed flame instabilities. Current continuity-based calculations allow estimates of the level of deficient light reactant necessary to cause the flame to become unstable. This applied-electric-field-induced modification of the thermodiffusive effect could serve as a potentially attractive means of controlling flame fluid-mechanical characteristics and validating combustion instability models over a wide range of equivalence ratios.

On the role of heat and mass transport during the mutual annihilation of two premixed propane–air flames

The unsteady process of mutual annihilation of two stoichiometric propane–air flames in one dimension is investigated numerically in the presence of preferential (the diffusion of heat relative to mass diffusion of species) and differential diffusion (the relative mass diffusions of species) effects. These effects are found during the early stages of mutual annihilation, corresponding to preheat layers’ interactions, as well as during the merger of the reaction layers. The diffusive mobility of heat relative to the reactants results in the preheating of the reactants and associated increases in the rates of reactants’ consumption. These rates are sustained during the merger of the reaction layers due to the relative mobility of the secondary fuels, especially H 2, which results in the build-up of radicals in the reaction zone prior to the completion of the mutual annihilation process. Preferential and differential diffusion effects also result in the formation of products of incomplete combustion at the end of this process.

Electric-field-induced flame speed modification

The effects of pulsed and continuous DC electric fields on the reaction zones of premixed propane–air flames have been investigated using several types of experimental measurements. All observed effects on the flame are dependent on the applied voltage polarity, indicating that negatively charged flame species do not play a role in the perturbation of the reaction zone. Experiments designed to characterize the electric-field-induced modifications of the shape and size of the inner cone, and the concomitant changes in the temperature profiles of flames with equivalence ratios between 0.8 and 1.7, are also reported. High-speed two-dimensional imaging of the flame response to a pulsed DC voltage shows that the unperturbed conical flame front (laminar flow) is driven into a wrinkled laminar flamelet (cellular) geometry on a time scale of the order of 5 ms. Temperature distributions derived from thin filament pyrometry (TFP) measurements in flames perturbed by continuous DC fields show similar large changes in the reaction zone geometry, with no change in maximum flame temperature. All measurements are consistent with the observed flame perturbations being a fluid mechanical response to the applied field brought about by forcing positive flame ions counter to the flow. The resulting electric pressure decreases Lewis numbers of the ionic species and drives the effective flame Lewis number below unity. The observed increases in flame speed and the flame fronts trend toward turbulence can be described in terms of the flame front wrinkling and concomitant increase in reaction sheet area. This effect is a potentially attractive means of controlling flame fluid mechanical characteristics. The observed effects require minimal input electrical power (<1 W for a 1 kW burner) due to the much better electric field coupling achieved in the present experiments compared to the previous studies.

Plasma supported combustion

Oxidation of molecular hydrogen and different hydrocarbons in stoichiometric mixtures with air and oxygen in the pulsed nanosecond discharges was studied at room temperature, and the detailed kinetics of the process has been numerically investigated. In the discharge afterglow, the reactions including electron-excited particles play a dominant role for the time up to 100 ns, ion–molecular reactions—for the time of microsecond range, and reactions including radicals mostly contribute for the time interval of several milliseconds. The principal role of processes with formation of excited components that support the development of the chain mechanism of oxidation has been shown. The spatial uniformity of the gas-mixture combustion initiated by a high-voltage nanosecond volume discharge is investigated at gas pressures of 0.3–2.4 atm and temperatures of 1000–2250 K. The self-ignition time and the time of discharge-induced ignition are determined. It is found that the discharge significantly (by 600 K) decreases the ignition temperature with very low energy in the discharge (∼10 −2 J/cm 3). The influence of gas excitation by a pulsed nanosecond discharge with a high-voltage pulse amplitude up to 25 kV on the properties of a premixed propane–air flame has been investigated over a wide range of the equivalence ratios (0.4–5). It was experimentally found that the flame’s blow-off velocity increased more than twice at a discharge energy input less than 1% of the burner power. Efficient production of active radicals under the action of a barrier discharge has been observed. The increase in the flame’s propagation velocity is explained by the production of atomic oxygen in a discharge by the quenching of electronically excited molecular nitrogen N 2 and the dissociation of molecular oxygen on electron-impact. A numerical model has been developed, which describes the influence of pulsed electric discharges on the ignition, combustion, and flame propagation.

Large Eddy Simulation of Combustion Instabilities in a Jet Engine Afterburner Model

Though turbulent reacting flows are often encountered in technical applications reliable prediction methods are lacking as a result of the complex nature of the physics. A promising approach to flows of practical interest is Large Eddy Simulation (LES). The philosophy behind LES is to explicitly simulate the large scales of the flow, directly affected by boundary conditions whilst modelling the smaller scales. The effects of the small unresolved scales appear as extra unknown terms in the LES equations that must be modelled. Subgrid models for reacting LES are more difficult to formulate than for non-reacting LES since also filtered reaction rates and other terms resulting from non-linear state equations need to be modelled. With the advent of transported PDF and linear-eddy models the situation has improved; due to the complexity and computational cost of similar models there is, however, still much to be gained from more conventional models. This study focuses on the development and application of a flame-wrinkling LES combustion model in which transport equations for a reaction coordinate, a modelled flame-wrinkling density and the laminar flame speed are derived, modelled and solved for. The unresolved transport terms in the momentum and energy equations are not particular to reacting flows and are modelled by a one-equation eddy-viscosity model. A centred second order accurate finite volume based scheme is used to solve the governing equations. The model is applied to a lean premixed propane air flame stabilised in the wake behind a triangular-shaped flameholder. Besides comparing with experimental data a discussion of different modes of combustion found to occur in this combustor is presented.

Burning Velocities of Flamelets in a Turbulent Premixed Flame

Burning velocities of flamelets in a vertically oriented, burner-stabilized turbulent premixed propane–air flame in the reaction-sheet regime are investigated by use of a microelectrostatic probe and a laser-doppler velocimeter (LDV) that give components in a vertical plane of the flame-front velocity and the gas velocity, respectively. Under the experimental conditions investigated, the average turbulent fluctuating velocity is of the same order of magnitude as the laminar burning velocity of the planar, unstrained, adiabatic laminar flame. The component of the flamelet motion with respect to the gas in the vertical plane is found from these measurements to be distributed over all directions and to range in magnitude from nearly zero to a few times the planar, unstrained, adiabatic laminar burning velocity. It may be concluded from these observations that the turbulence may exert substantial influences on the flamelets.

Application de la réduction systematique de schemas cinetiques au calcul de flammes landnaires etirees de premelanges airpropane

The study of strained laminar flames is of fundamental importance and it is also of great interest for turbulent combustion modeling. The calculations are usually performed either with extremely simple global kinetic schemes or with elementary and complex detailed chemistry. A new approach based on the systematic reduction of kinetic schemes has been recently proposed by Peters. Our goal is to evaluate this method in the context of an atmospheric premixed propane-air flame. An augmented reduced scheme involving 9 species and 6 reactions is proposed. All the computations are compared with those of a reference scheme due to Warnatz. The results obtained are generally satisfactory. It appears that the differences between the reduced and complex schemes concerning the flame position vary with the flame parameters such as the equivalence ratio, while flames structures are always well predicted. The extinction phenomenon is correctly reproduced by the reduced mechanism and he critical values of the strain rate and the chemical composition at extinction are determined with reasonable accuracy