Parametric and statistical investigation of the behavior of a lifted flame over a turbulent free-jet structure

doi:10.1016/j.combustflame.2004.03.005

Combustion and Flame

Volume 137, Issue 4, June 2004, Pages 458-477

https://doi.org/10.1016/j.combustflame.2004.03.005 Get rights and content

Abstract

Partially premixed combustion is involved in many practical applications, due to partial premixing of combustible and oxidant gases before ignition, or due to local extinctions, which lead to mixing of reactants and burned gases. To investigate some features of flames in stratified flows, the stabilization processes of lifted turbulent jet flames are studied. This work offers a large database of liftoff locations of flames stabilized on turbulence-free jets for different fuels and nozzle diameters studied over their flame stability domains. Methane, propane, and ethylene flames are investigated for nozzle diameters of 2, 3, 4, and 5 mm. Blowout velocities are measured and compared with an approach based on large-scale structures of the jet. The axial and radial locations of the flame base are measured by planar laser-induced fluorescence (PLIF) of the OH radical through high sampling (at least 5000 points). From this large database the average locations of the flame base are analyzed for the fuels investigated. The pdfs exhibit an evolution of their shapes according to the region of the turbulent jet where the flame stabilizes (potential core, transition to turbulence, or fully developed turbulence regions). This dependence is probably due to the interaction of the flame with the jet structures. This is confirmed by the comparison between the amplitude of the height fluctuations and the local size of the large-scale structures deduced from particle image velocimetry measurements and self-similarity laws for velocity. The results show the flame can be carried over a distance equal to the local diameter of the jet within the region of fully developed turbulence for propane and ethylene, and over a slightly larger distance for methane.

Introduction

Flame stabilization is a phenomenon to be controlled in numerous industrial applications (furnace, boiler, rocket engine, turbine reactor, etc.). It requires the suitable mixing of fuel, oxidizer, and released heat. In industrial applications the flame is usually anchored at the nozzle to avoid flame instability (leading to pressure variation), extinction (leading to unburned hydrocarbon emission, etc.) and blowout for safety reasons. Partially premixed combustion is involved in more and more industrial applications to reduce NO_x emission and fuel consumption. This mode of combustion exhibits particular properties and needs to be better understood. From a fundamental point of view, the turbulent lifted flame is an interesting configuration with which to investigate the properties of partially premixed combustion, as this phenomenon results from a balance of the effects of combustion and turbulence involving particular interactions between aerodynamics and mass and heat transfers. Upstream of the flame base the jet fluid mixes with the ambient fluid, leading to partial premixing within the shear layer where the flame stabilizes. In this region the flame edge is submitted to high shear and mixture fraction gradients, leading to a complex phenomenon.

In this context, numerous theories have been proposed in the past, as reviewed by Pitts [1]; they fall into two categories. One approach is based on turbulent flame propagation [2], [3], [4], [5] and the other on extinction, considering either non-premixed flamelets [6], [7] or the role played by large-scale structures [8], [9]. More recently partially premixed propagation has been introduced in the description of turbulent lifted flame discussed in Peters' review [10].

The stabilization of turbulent flames on jets is controlled by specific combustion properties at the flame base induced by the local mixture fraction gradients and their superimposition on local turbulence upstream of the reaction zone. In laminar flows, the flame base develops into a triple flame structure (fuel-lean and fuel-rich premixed branches and a trailing diffusion flame) across a mixture fraction gradient, and flame stabilization is controlled by the combined effects of mixture fraction gradient and aerodynamical flow properties, which influence the heat release and mass fluxes at the flame base [11], [12]. In turbulent flows, these interactions and their consequences on flame behavior become very complex, and the triple flame becomes distorted, with branches overlapping into the trailing diffusion flame [13]. Numerous studies are still required to better understand the interaction between the flame edge and the turbulent flow, and to provide validation tools for numerical modeling. Recent experimental studies show that in turbulent flows the mixture at the flame base is within the flammable limits [14], [15], [16] and the velocity field in the stabilization region is slow enough to allow propagation of a leading-edge flame [17], [18]. This was confirmed by the experimental observation of a leading-edge flame in turbulent jet flames [19]. Nevertheless the properties of the flame edge in turbulent fields are not well characterized and further investigations are required.

The aim of the present study is to provide a large database of flame base locations (liftoff height and radius) for turbulence-free jets over their stability domain for different fuels (methane, propane, ethylene, and air-diluted ethylene) and different nozzle diameters (from 2 to 5 mm i.d.). The axial and radial locations of the flame base are measured by OH planar laser-induced fluorescence (PLIF). Blowout velocities are measured and their values are analyzed to determine whether the process can be described by a simplified model based on the large-scale structures of the jet. The locations of the flame base near the blowout condition are compared with those derived from the usual assumptions involved in the model proposed to describe the blowout condition [4], [8]. Axial and radial locations of the flame base are measured and statistically analyzed over the stability diagrams of the flames investigated. The large database of flame base locations (probability density function (pdf), average, standard mean deviation) is examined and exhibits a correlation between flame base behavior and the region of the jet where flame stabilizes. These results show the role played by the large-scale structures of the jet in the fluctuations of the flame base. This role is confirmed by comparing the amplitude of the flame base fluctuations with the local size of the large scales deduced from PIV measurements.

Despite the large database of flame base locations, physical phenomena governing the stabilization of turbulent lifted flames are not yet fully understood and there are still several directions for future investigation.

Section snippets

Apparatus

The burner consists of a stainless-steel tube with an inside diameter, D, of 2, 3, 4, or 5 mm. It is now well established that the basic jet velocity profile is a determining parameter of the instability that controls the development of the turbulence within the jet (Fig. 1). The basic velocity profile depends on the experimental conditions (nozzle contraction, Reynolds number, roughness, jet exit configuration). To obtain a flat velocity profile and a thin basic mixing layer from the

“Blowout condition”

For all fuels and nozzle diameters, when possible, lifted flames are investigated over a wide range of initial velocities, U₀, from velocities below the “liftoff velocity” (flame in the hysteresis region) to velocities leading to flame blowout. Some experimental limitations have been encountered for the highest velocities when the flame dimensions become too large for the dimensions of the experiment. The experimental determination of “blowout velocity” is delicate, because at this stage the

Conclusions and summary

A parametric investigation of turbulent lifted flames has been carried out for three fuels and various nozzle diameters. Blowout velocities are determined and experimental data are compared with an approach proposed by Broadwell et al. [8] based on the role of inviscid large-scale structures of turbulent jets. From this approach it is possible to propose an expression of blowout velocity without a detailed description of the blowout phenomenon. Measurements of flame base location near blowout

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This paper presents an experimental study on horizontally jet flames with rather high velocities (58 m/s-167 m/s, Reynolds number up to 3.0 × 10⁵), which mainly fall in the momentum-controlled region. Both high-speed schlieren and color imaging techniques have been adopted to obtain the time-resolved images, based on which the flame and flow dynamics has been discussed comprehensively. Under the test conditions, all the flames have shown lift-off characteristics. The dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD) analysis has been performed to reveal the flame and flow fluctuating frequency and the corresponding spatial distribution. The results have shown two high frequency modes at 269 Hz and 537 Hz in the lifted-off jet region, which may come from the test bench vibration. The statistics of flame length, height and lift-off distance have been presented, which have been compared with previously reported data at lower velocities. The comparison indicates that the flame behaviors have shown quite dramatic differences in the buoyancy and momentum-controlled regions. The present experiments have extended the research of horizontally placed jet flames to a wider velocity range, which has greatly broadened our knowledge of the lift-off jet flame behaviors under high velocity conditions.
Assessment of the stabilization mechanisms of turbulent lifted jet flames at elevated pressure using combined 2-D diagnostics
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Combined 2-D laser diagnostics have been used extensively in the past to examine lifted flames. In [11,12,14,18], planar imaging velocimetry (PIV) was combined with planar laser induced fluorescence (PLIF) of one or more flame markers (OH, CH, and CH2O). In [12,16,25,27], the mixture fraction was measured using Rayleigh scattering in combination with OH-PLIF, CH-PLIF, or CH2O-PLIF.
The stabilization mechanisms of turbulent lifted jet flames in a co-flow have been investigated at a pressure of 7 bar. The structure of the flame base was measured with combined OH and CH₂O planar laser induced fluorescence (PLIF) and the spatial distribution of equivalence ratio was imaged, simultaneously, with CH₄ Raman scattering. The velocity field was also measured with particle imaging velocimetry (PIV). Different bulk jet velocities U_j and co-flow velocities U_c were examined. Data show that flames with U_c = 0.6 m/s stabilize much further away from the nozzle than those with U_c = 0.3 m/s and that their structure does not resemble that of the edge-flames found closer to the nozzle. In addition, for U_c = 0.6 m/s, the measured lift-off height decreases with increasing bulk jet velocity, which is opposite to what is typically observed for lifted flames. Statistical examination of CH₄ Raman images shows that the flames with U_c = 0.6 m/s propagate through regions of the flow where the equivalence ratio is not always stoichiometric but, instead, spans the whole flammability range. This is not consistent with edge-flames and is, instead, indicative of premixed burning. This is corroborated by PIV results which show that the flame base velocity exceeds that typically reported for edge-flames. Measurements of relevant flow properties were also conducted in non-reacting jets to predict the turbulent burning velocity of these lifted flames burning in a premixed mode. For U_c = 0.6 m/s and relatively large bulk jet velocities (U_j = 10 and 15 m/s), the predicted turbulent burning velocities are sufficiently high to counter the incoming flow of reactants and, in turn, allow flame stabilization. However, for a lower bulk jet velocity of U_j = 5 m/s, the predicted turbulent burning velocity is much less, leading to blow-out. This explains why the lift-off height decreases with increasing jet velocity for methane at 7 bar and U_c = 0.6 m/s. Data also shows that increasing pressure promotes transition from edge-flames to premixed flames due to reduced laminar burning velocity and enhanced mixing.
An experimental study of turbulent lifted flames at elevated pressures
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Studying the lift-off behavior of non-premixed jet flames is important to understanding the flame stabilization mechanisms in practical systems, including gas flares and combustors, and to improving the safety of pressurized fuel tanks in case of fuel leaks. This is typically done with the canonical configuration of an axisymmetric fuel jet issuing into a quiescent or co-flowing oxidizer and abundant data are available in the literature. However, most of these data were collected at normal or sub-atmospheric pressure and little data are available at elevated pressure and high Reynolds numbers, conditions relevant to practical configurations. The present study fills this gap by reporting lift-off height measurements of methane and ethane non-premixed jet flames for pressures up to 7 bar and Re = 57,500 in the presence of an air co-flow. Data are interpreted using Kalghatgi's model for the dimensionless lift-off height, which was previously proven successful for sub-atmospheric to normal pressures, as well as elevated pressure but only for low turbulent Reynolds numbers and propane. In this contribution, Kalghatgi's model has been shown to accurately predict the slope of the lift-off height vs. jet velocity curves at elevated pressure and large Reynolds number for methane and ethane. A new term, a function of the stoichiometric mixture fraction, the laminar burning velocity, and a turbulent Schmidt number, is also introduced to extend the predictive capabilities of Kalghatgi's model to configurations featuring a co-flow.
Pressure effects and transition in the stabilization mechanism of turbulent lifted flames
2019, Proceedings of the Combustion Institute
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Figure 5 shows the measured probability distributions of the axial (left column) and radial (middle column) positions of the flame edge as well as the edge flame speed (right column) for the 2 bar (top row) and 6 bar (bottom row) conditions. All the distributions are unimodal and the distributions of flame-edge position are consistent with those measured by others in the near-field at atmospheric pressure [17,18]. Regardless of pressure, the edge-flame speed is bounded by 0 and 3 times the laminar flame speed SL, whose values are computed here with Chemkin-Pro [19] and the USC-II chemistry mechanism [20].
This study reports novel measurements on the effects of pressure on the lift-off behavior and the stabilization mechanism of turbulent non-premixed methane jet flames. A high-pressure combustion duct (HPCD) was operated within the range of pressure P = 1–7 bar using jet velocities of 4 m.s⁻¹ ≤U_j≤30 m.s⁻¹ and co-flow velocities of 0.23 m.s⁻¹ ≤U_c≤0.60 m.s⁻¹. Lift-off heights were measured from chemiluminescence pictures while joint images of hydroxyl and velocity, performed using joint PLIF-OH/PIV, were used to extract information about the stabilization mechanism. It is shown that while the lift-off height generally increases with pressure, the impact of pressure depends on the magnitude of the co-flow velocity. For U_c = 0.30 m.s⁻¹, the flame's base remains near the nozzle over the entire pressure range and the measured flame speeds indicate that edge-flame stabilization is dominant. The slope of the lift-off height vs. jet velocity curves is positive. For U_c = 0.60 m.s⁻¹ and P > 2 bar, the flame stabilizes further downstream and a transition to turbulent premixed flame propagation appears to have occurred. At these conditions, the slope of the lift-off height vs. jet velocity curves becomes negative. This reversal at high pressure is a new result for methane. More importantly, the transition in the stabilization mechanism with increasing U_c is consistent with results reported earlier for ethylene and appears to be independent of the fuel.
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Blow-out limits of nonpremixed turbulent jet flames in quiescent air at sub-atmospheric pressures (50–100 kPa) were studied experimentally using propane fuel with nozzle diameters ranging 0.8–4 mm. Results showed that the fuel jet velocity at blow-out limit increased with increasing ambient pressure and nozzle diameter. A Damköhler (Da) number based model was adopted, defined as the ratio of characteristic mixing time and characteristic reaction time, to include the effect of pressure considering the variations in laminar burning velocity and thermal diffusivity with pressure. The critical lift-off height at blow-out, representing a characteristic length scale for mixing, had a linear relationship with the theoretically predicted stoichiometric location along the jet axis, which had a weak dependence on ambient pressure. The characteristic mixing time (critical lift-off height divided by jet velocity) adjusted to the characteristic reaction time such that the critical Damköhler at blow-out conditions maintained a constant value when varying the ambient pressure.

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Parametric and statistical investigation of the behavior of a lifted flame over a turbulent free-jet structure

Abstract

Introduction

Section snippets

Apparatus

“Blowout condition”

Conclusions and summary

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