Lift-off Height Research Articles

Effects of Wall Heat Loss on Swirl-Stabilized Nonpremixed Flames With Localized Extinction

Large eddy simulation (LES) with three-dimensional conditional moment closure (CMC) subgrid model for combustion is applied to simulate a swirl-stabilized nonpremixed methane flame with localized extinction, with special focus on the effects of heat loss to the burner surface. The convective wall heat loss is modeled through introducing a source term in the conditionally filtered total enthalpy equation for the CMC cells adjacent to the wall. The mean heat flux is high on the middle surface of the bluff body, but relatively low near its edges. The turbulent heat flux based on the gradient of the resolved temperature is relatively low compared to the laminar counterpart, but increases with the turbulent intensity. The heat loss facilitates the occurrences of extinction and re-ignition for the CMC cells immediately adjacent to the wall, evidenced by comparing flame structures in the near-wall CMC cells. This can be directly linked to the increase of the mean conditional scalar dissipation near the wall in the heat loss case. Furthermore, the degree of local extinction near the bluff body measured by conditional reactedness at stoichiometry is intensified due to the wall heat loss. However, the results also show that there is negligible influence of wall heat loss on the probability density function (PDF) of the lift-off height, demonstrating the dominance of aerodynamic effects on flame stabilization. The results are in reasonable agreement with experimental measurements.

Flame lift-off height control by a combined vane-plasma swirler

In this study, a swirler combining the vane swirler and the plasma swirler is designed to control the flame lift-off height. The plasma swirler is located near the rim of the injector and the vane swirler is placed upstream of the plasma swirler. The vane swirler is employed to form a divergent flow to sustain the detached flame and the plasma swirler is adopted to control the flame lift-off height. The ionic wind clinging to the inner wall of the injector tube does not penetrate into the center, leaving the main stream flow in the central region largely undisturbed. Characteristics of the flow field are analyzed from the results of laser Doppler anemometry (LDA) measurement and mechanism of the flame lift-off control by the combined vane-plasma swirler is revealed. The flame lift-off locations calculated from the LDA measurement are consistent with those from direct observation. The dielectric barrier discharge (DBD) voltage influences the height of the flame lift-off with a linear relationship observed, which means the flame lift-off height can be controlled precisely by the DBD voltage without any mechanical movement or changing the mass flow rate. The combined vane-plasma swirler has the potential to improve the fuel flexibility, increase flame stability and attenuate the injector overheating.Highlights1. A combined vane-plasma swirler is designed to control the flame lift-off height.2. Flame lift-off height can be adjusted by changing the voltage of the dielectric barrier discharge.3. A linear relationship between flame lift-off height and the dielectric barrier discharge voltage is observed.4. Flame lift-off control by the combined vane-plasma swirler is mainly associated with the aerodynamic effect.5. The swirler can improve fuel flexibility, increase flame stability and attenuate injector overheating.

Characterization of ozone-enhanced propane cool flames at sub-atmospheric pressures

A recently developed experimental setup for the study of cool flames was employed to investigate low temperature combustion of propane. Cool flames were stabilized though ozone activation using a laminar flat flame Hencken burner at sub-atmospheric pressures. Propane cool flame stability was observed to be highly sensitive to pressure, and a pressure stability map is presented for a range of equivalence ratios (ϕ) from 0.17 to 1.0 in 0.17 increments and a range of reactant flow rates. Based on the stability map, a propane cool flame of ϕ = 0.17 at 17.3 kPa was chosen for further study. Flame lift-off height above the burner surface was measured and two operating regimes of burner-stabilized and freely propagating flames were observed. The intersection of these two regimes was used to estimate the cool flame propagation speed. Temperature measurements of the cool flame were taken and used in fixed-temperature numerical flame simulations. Additionally, flame temperatures and propagation speeds are presented for all equivalence ratios and pressures where stable, freely propagating flames were observed in the stability map. In order to enable non-fixed temperature, freely propagating cool flame simulations to be performed, reduction of a chemical kinetics model using the Directed Relation Graph method (DRG) in conjunction with reaction rate sensitivity analysis was performed. Experimentally determined propagation speed, temperature and species concentrations at partial equilibrium were compared to numerical simulations with reasonable agreement and results are discussed.

Large-Eddy Simulation of the lean-premixed PRECCINSTA burner with wall heat loss

Swirl burners are widely used in aeronautical engines and industrial gas turbines. These burners enhance the flame stabilization by bringing back hot products to the reactive zone and their topology ensures a good compactness. However, the outer recirculation zones created in these burners induce wall heat loss that affects the flame structure. This paper proposes a numerical strategy based on Large-Eddy Simulation (LES) and non-adiabatic boundary conditions with a skeletal chemistry approach coupled to the Dynamic Thickened Flame model (TFLES). Simulations were performed on the PRECCINSTA burner with meshes up to 877 millions elements. Results demonstrated the impact of the addition of wall heat loss on the lift-off of the external flame front, leading to a flame topology change from a M-shape to a V-shape. This lift-off height increases with the grid resolution showing the strong influence of the mesh resolution on the flame topology. Comparisons of the non-adiabatic LES results on the finest mesh with the experimental data show an unprecedented agreement. The flame quenching process is analyzed and exhibits the role of strain rate which controls the level of penetration of cooled products within the inner reaction zone.

Three-dimensional Linear Eddy Modeling of a Turbulent Lifted Hydrogen Jet Flame in a Vitiated Co-flow

A new methodology for modeling and simulation of reactive flows is reported in which a 3D formulation of the Linear Eddy Model (LEM3D) is used as a post-processing tool for an initial RANS simulation. In this hybrid approach, LEM3D complements RANS with unsteadiness and small-scale resolution in a computationally efficient manner. To demonstrate the RANS-LEM3D model, the hybrid model is applied to a lifted turbulent N2-diluted hydrogen jet flame in a vitiated co-flow of hot products from lean H2/air combustion. In the present modeling approach, mean-flow information from RANS provides model input to LEM3D, which returns the scalar statistics needed for more accurate mixing and reaction calculations. Flame lift-off heights and flame structure are investigated in detail, along with other characteristics not available from RANS alone, such as the instantaneous and detailed species profiles and small-scale mixing.

Theoretical studies on fuel dispersion and fireball formation associated with aircraft crash

ABSTRACTThe accidental or intentional crash of an aircraft causes the fuel dispersion followed by a fireball formation that may affect the integrity of structures by thermal hazards. The fireball arising from the aircraft crash has been studied and analyzed using Eddy Dissipation Concept (EDC) combustion model in the computational fluid dynamic (CFD) platform openFOAM. The simulations have been performed with two methods. In first method, the fuel is injected in complete vapor form and in the second method, fuel is injected as dynamic fuel spray. The input to analyze fireball by dynamic spray model has been obtained from droplet spray model reported earlier (Shelke et al., 2018). The diameter and lift-off height obtained in CFD analyses are compared with a video footage reported in literature and it is found to be in good agreement. To illustrate the transient behavior of fireball, evolution of mass fractions of fuel and combustion products and developed pressure are studied. It has been found that both methods give similar predictions for fireball diameter and lifting height.

Quantifying kinetic uncertainty in turbulent combustion simulations using active subspaces

Uncertainty quantification in expensive turbulent combustion simulations usually adopts response surface techniques to accelerate Monte Carlo sampling. However, it is computationally intractable to build response surfaces for high-dimensional kinetic parameters. We employ the active subspaces approach to reduce the dimension of the parameter space, such that building a response surface on the resulting low-dimensional subspace requires many fewer runs of the expensive simulation, rendering the approach suitable for various turbulent combustion models. We demonstrate this approach in simulations of the Cabra H2/N2 jet flame, propagating the uncertainties of 21 kinetic parameters to the liftoff height. We identify a one-dimensional active subspace for the liftoff height using 84 runs of the simulations, from which a response surface with a one-dimensional input is built; the probability distribution of the liftoff height is then characterized by evaluating a large number of samples using the inexpensive response surface. In addition, the active subspace provides a global sensitivity metric for determining the most influential reactions. Comparison with autoignition tests reveals that the sensitivities to the HO2-related reactions in the Cabra flame are promoted by the diffusion processes. The present work demonstrates the capability of active subspaces in quantifying uncertainty in turbulent combustion simulations and provides physical insights into the flame via the active subspace-based sensitivity metric.

Effect of pressure on the characteristics of lifted flames

The effects of pressure on the characteristics of lifted flames in a coflow with propane fuel were investigated experimentally in a pressure chamber. Changing the pressure influenced the density, reaction kinetics, and flame propagation speed. The pressure range tested was P = 0.5–5.5 atm. As the fuel jet velocity increased, a nozzle-attached flame transitioned to a lifted flame before blowout occurred. Depending on pressure, the onset conditions of liftoff and blowout occurred in the laminar, transition, or turbulent regimes. When P < 1.6 atm, the flame was lifted and had a tribrachial edge structure in the laminar regime, and the liftoff height (HL) increased with increasing pressure. Both the liftoff and blowout velocities decreased with decreasing pressure, and they merged at 0.5 atm. A correlation was derived in terms of the Schmidt number (Sc) and the Reynolds number (Re): Pn/2(HLPn/2)(Sc−1)/(2Sc−1)∝Re. The reattachment velocity in the laminar and transition regimes linearly decreased with pressure. The liftoff height had two local minimum points at given Reynolds number in the transition regime, and then increased linearly in the turbulent regime.

Nondestructive Measurement of Al Alloy Surface Strengthening Layer Depth Using Swept Frequency Eddy Current Testing

An innovative nondestructive measurement method of Al alloy surface strengthening layer depth using frequency sweep eddy current testing (SFECT) technology is proposed. Based on the analysis of the forming mechanism of Al alloy surface strengthening layer and the comparison of several possible detection methods, the applicability of SFECT method is discussed theoretically. By means of electromagnetic simulation software Ansoft Maxwell and orthogonal experimental design method, the SFECT coil model is built. The optimal design and simulation of the parameters of the SFECT coil are carried out, and optimal coil structure parameters and lift-off height are obtained. Simulation of depth detection of Al alloy surface strengthened layer using SFECT coil is implemented, and then preliminary experiment of Al alloy surface strengthening layer depth measurement with the self-fabricated SFECT coil is completed. The simulation and experiment results show that SFECT method can be applicable for nondestructive testing of Al alloy surface strengthening layer depth.

Numerical simulations of turbulent lifted jet diffusion flames in a vitiated coflow using the stochastic multiple mapping conditioning approach

Lifted turbulent jet diffusion flames of H2/N2 issued into a hot coflowing stream of combustion products from a lean premixed H2/air mixture are simulated using the stochastic multiple mapping conditioning (MMC) approach within the Reynolds-averaged Navier–Stokes (RANS) framework. The underlying turbulent flow is modeled using the two-equation k–ɛ model with modified constants. In the MMC approach, large-scale turbulent fluctuations are emulated by introducing suitable reference variables. A single reference variable is used here to describe the evolution of mixture fraction via a mapping function. The modified Curl's model has been adapted to model the micro-mixing term. A detailed chemical kinetic mechanism involving 9 species and 21 reversible reactions is used for H2/O2 combustion. The computed results from the present simulations are found to be in excellent agreement with the available experimental data. Numerical results are found to be sensitive to change in the minor dissipation timescale, which certainly controls the conditional fluctuations around the conditional mean. Also, the predicted flame lift-off heights obtained using the minor mixing time constant Cmin = 0.25 are found to be in close proximity with the experimentally observed values for the entire range of coflow temperatures.

Decreasing liftoff height behavior in diluted laminar lifted methane jet flames

Abstract Stabilization of laminar lifted coflow jet flames of nitrogen-diluted methane was investigated experimentally and numerically. As the fuel jet velocity was increased, two distinct behaviors in liftoff height were observed depending on the initial fuel mole fraction; a monotonically increasing trend and a decreasing and then increasing trend (U-shaped behavior). The former was observed in the jet-developing region and the latter in the jet-developed region. Because the decreasing behavior of liftoff height with jet velocity has not been observed at ambient temperature, the present study focuses on decreasing liftoff height behavior. To elucidate the physical mechanism underlying the U-shaped behavior, numerical simulations of reacting jets were conducted by adopting a skeletal mechanism. The U-shaped behavior was related to the buoyancy. At small jet velocities, the relative importance of the buoyancy over convection was strong and the flow field was accelerated in the downstream region to stabilize the lifted flame. As the jet velocity increased, the relative importance of buoyancy decreased and the liftoff height decreased. As the jet velocity further increased, the flame stabilization was controlled by jet momentum and the liftoff height increased.

Liftoff behavior and combustion characteristic of jet flame in a coflow burner

ABSTRACTStabilization and autoignition mechanisms of lifted flames have been widely investigated to improve combustion efficiency and safety of combustion equipment. This paper focuses on liftoff behavior and combustion characteristic of methane and propane flames under various coflow conditions in a coflow burner. Unlike the case of free jet flame in ambient air, the different tendencies of liftoff height changes with jet velocity for both methane and propane flames in vitiated coflow illustrate a transition from conventional combustion to Moderate & Intense Low Oxygen Dilution (MILD) combustion. Flame temperature difference with radial position measured by primary spectrum pyrometry proves the transition regime.

A Pressurized Vitiated Co-Flow Burner and Its Preliminary Application for a Methane Lifted Flame

A new pressurized vitiated co-flow burner (PVCB) was designed and built for the investigation of lifted flame properties and was supported by a vitiated co-flow of hot combustion products from a lean H2/air flame at a controllable pressure; its preliminary application for a methane lifted flame was tested. The distribution of the co-flow temperature, oxygen mole fraction, flow rate, and pressure of the PVCB was measured and calculated. The research results show that the co-flow temperature range is from 300 to 1300 K, the background pressure range is from 1 to 1.5 bar, the stable temperature field of the PVCB is wider, and the background pressure of the PVCB can be controlled. The simulation results show that the PVCB provides a controllable, pressurized co-flow of hot and vitiated gases, which makes it possible to investigate flame stabilization mechanisms. The PVCB has the advantages of controllable background pressure and a stable temperature field. The well-defined uniform boundary conditions and simplified flow of the PVCB simplify the establishment of a numerical model and decouple the turbulent chemical kinetics from the complex recirculating flow. It can be widely used in the research on lifted flames. A lifted flame of methane was recorded under conditions of a co-flow temperature of 1133 K and pressure from 1 to 1.043 bar. The lift-off height decreased and stabilized with the increase in the background pressure. The laminar flame speed and the autoignition delay time were tested and simulated at the same time by Chemkin; the influence of background pressure on the lift-off height, laminar flame speed, and autoignition delay were analyzed. The results show that the autoignition, as well as the flame propagation, dominated the stabilization mechanism of the lifted flame in the PVCB.

Evaluation of Ramp-Type Micro Vortex Generators Using Swirl Center Tracking

The evolution of the primary vortex pair downstream of micro vortex generators placed in a supersonic stream is compared using a swirl center tracking technique, for standard, slotted, and split-ramp-type vortex generators. Evolution of vortex strength, determined using circulation, and distribution of vortices, determined using contours of helicity, are also compared. Results show that secondary vortices emerge stronger, and liftoff heights drop, with the introduction of slot (slotted micro vortex generator) or gaps (split-ramp micro vortex generator). Near-surface contours of axial velocity and integral properties of the boundary layer are then compared to evaluate the effects of the evolution of the vortical structures on the flowfield. Results shows that the split-ramp device results in the most energetic near-surface flow but the worst outgoing boundary-layer integral properties, whereas the slotted-ramp devices with taper fare well on both counts. The computations performed in this work involve supersonic flow past single micro vortex generators at Mach 2.5 and use an immersed-boundary method to render the control devices. The flow solver is suitable for high-speed turbulent flows and has been extensively validated in earlier works. The turbulence model used is Menter’s shear-stress transport version.

Effect of Toluene Addition on the PAH Formation in Laminar Coflow Diffusion Flames of n-Heptane and Isooctane

Characterization of soot formation is becoming more important for gasoline surrogate fuels, requiring a deeper understanding of the effect of toluene ratio on PAHs formation. In this work, the different size PAHs distribution in laminar diffusion flames was measured by PLIF technique, and the chemiluminescences of OH and CH radicals were recorded by ICCD coupled with band-pass filters. The effect of toluene addition to n-heptane and isooctane on the flame and PAHs formation was comparatively studied. Then, the chemical kinetic of PAHs was analyzed using the CHEMKIN Diffusion Flame model. The experimental results showed that the cool flame area ranks as n-heptane > isooctane > toluene may due to the NTC action. The OH intensity and flame lift-off height show a monotonous increasing trend with the toluene ratio. At the same toluene ratio, the peak OH intensity and flame lift-off height of n-heptane/toluene is lower than that of isooctane/toluene. The CH intensity in n-heptane, isooctane, and toluene flames shows completely different distribution characteristics. As the toluene ratio increases, the 320 nm-LIF in n-heptane and isooctane flames shows a weakening monotonous increasing trend with different inflection points. The 360/400/450 nm-LIF in n-heptane and isooctane flames show a nonmonotonic trend with different peak points. The peak point is 50% toluene ratio in n-heptane/toluene flames and 40% in isooctane/toluene flames, and it corresponds exactly to the smoke point of the flame. The kinetic analysis showed that the benzyl has a significant effect on the formation of A2–A4 through the generation of C10H9, C9H7, and C14H12. A1 trend is dominated by C6H5CH3 + H = A1 + CH3 and A1– + CH4 → A1 + CH3. The trends of A2, A3 and A4 are mainly dominated by the reactions between their own dehydrogenation groups with H2.

Comparison of droplet evaporation models for a turbulent, non-swirling jet flame with a polydisperse droplet distribution

The current investigation aims to present the numerical results of a turbulent spray jet flame in the framework of Large Eddy Simulation. The injection of n-heptane liquid fuel is achieved through the use of a pressure-swirl atomiser generating a lifted spray flame. The evolution of the sub-grid probability density function of relevant scalars is accounted for by the Eulerian stochastic field method. A non-reactive spray computation is conducted in a preliminary stage. The simulation allows for a stochastic breakup formulation in combination with a stochastic dispersion model to confirm their predictive capabilities. This is achieved as the liquid properties such as droplet diameter and velocity profiles are found to be in excellent agreement between the simulated results and measurements. The main target of the work is to assess the performance of the most widely accepted evaporation models in a turbulent droplet-laden flame. In this paper, a detailed comparison of the evaporation models is conducted thanks to a recent development in measurement techniques which allow to permit the spray temperature profiles. The time-averaged droplet temperature across the spray flame is therefore investigated in order to validate several droplet vaporisation models. All the models under consideration are found to capture the formation of a double reaction zone flame and the measured lift-off height is reproduced within a satisfactory level of accuracy. This good reproduction of the flame morphology additionally confirms the performance of the pdf approach as a closure for the unknown turbulence–chemistry interaction in this type of spray flames. However, the simulated wet-bulb temperature in the hot burnt gas between the two reaction zones in addition to the profile along the spray centreline show a large discrepancy when compared to measurements. This large difference thus requires further investigation both in the modelling of evaporation and in the accuracy of measurement techniques.

Autoignited lifted flames of dimethyl ether in heated coflow air

Autoignited lifted flames of dimethyl ether (DME) in laminar nonpremixed jets with high-temperature coflow air have been studied experimentally. When the initial temperature was elevated to over 860 K, an autoignition occurred without requiring an external ignition source. A planar laser-induced fluorescence (PLIF) technique for formaldehyde (CH2O) visualized qualitatively the zone of low temperature kinetics in a premixed flame. Two flame configurations were investigated; (1) autoignited lifted flames with tribrachial edge having three distinct branches of a lean and a rich premixed flame wings with a trailing diffusion flame and (2) autoignited lifted flames with mild combustion when the fuel was highly diluted. For the autoignited tribrachial edge flames at critical autoignition conditions, exhibiting repetitive extinction and re-ignition phenomena near a blowout condition, the characteristic flow time (liftoff height scaled with jet velocity) was correlated with the square of the ignition delay time of the stoichiometric mixture. The liftoff heights were also correlated as a function of jet velocity times the square of ignition delay time. Formaldehydes were observed between the fuel nozzle and the lifted flame edge, emphasizing a low-temperature kinetics for autoignited lifted flames, while for a non-autoignited lifted flame, formaldehydes were observed near a thin luminous flame zone.For the autoignited lifted flames with mild combustion, especially at a high temperature, a unique non-monotonic liftoff height behavior was observed; decreasing and then increasing liftoff height with jet velocity. This behavior was similar to the binary mixture fuels of CH4/H2 and CO/H2 observed previously. A transient homogeneous autoignition analysis suggested that such decreasing behavior with jet velocity can be attributed to partial oxidation characteristics of DME in producing appreciable amounts of CH4/CO/H2 ahead of the edge flame region.

Experimental investigations on the stabilization of lifted kerosene spray flames with coflow air

ABSTRACTIn the present work, detailed experimental investigations have been carried out to understand the effect of coflow air on the stabilization of lifted spray flames with kerosene fuel. Two full cone type pressure swirl nozzles N1 and N2 ( = 1.7 and 7.08 kg/h at ΔPinj = 9 bar) are considered for experiments with a moderate fuel injection pressure range (4–12 bar). The flame liftoff height is observed to increase with fuel injection pressure for lower coflow velocity range (≤ 0.2 m/s). At higher coflow velocities (0.2–0.4 m/s), the flame liftoff height decreases with an increase in fuel injection pressure. At lower injection pressures, the flame liftoff height has been observed to increase significantly for higher coflow velocities. Detailed droplet velocity measurements using PIV and droplet size measurement using the shadowgraphy technique in cold flow helped understand the stabilization of lifted spray flames in the domain. At higher coflow velocities and lower injection pressure, smaller droplet number density allows additional air entrainment and flame stabilizes at the downstream location, where SMD and droplet flow velocities are small. At higher injection pressure, high droplet number density reduces the additional air entrainment, which stabilizes lifted spray flame at a relatively downstream location. At lower coflow velocity and lower injection pressure, flame stabilizes upstream, where mixture burning velocity is nearly equal to flow velocity irrespective of higher SMD and droplet flow velocity. At higher injection pressure, flame stabilizes only after the end of high velocity zones, as observed from PIV measurements.

Analysis of the Eddy Dissipation Concept formulation for MILD combustion modelling

Performance of the Eddy Dissipation Concept (EDC) in the regime of Moderate and Intense Low-oxygen Dilution (MILD) combustion is investigated. The special MILD features, where chemical and turbulence time scales are comparable (Damköhler number close to unity), have led several researchers to suggest modifications of EDC, mainly by changing model constants. EDC with standard and modified constants are compared, and the importance of each effect is outlined. Different fine-structure reactor models and their inflow/initial conditions are discussed and found to play a significant role. The reacting fraction of fine structures, which in virtually all other numerical studies is set to unity, is also discussed and found to be important. We observe better agreement with experiment when the reacting fraction is reduced below unity, which is also described by the original EDC. The results obtained with the variable reacting fraction are found to improve both the temperature distributions and the lift-off height predictions. The calculations are carried out with the use of open source software OpenFOAM. The main test case was the Delft Jet-in-Hot-Coflow burner emulating MILD regime at three different flow conditions (jet Reynolds numbers of 2500, 4100 and 8800).

Measurements in swirling spray flames at blow-off

Various characteristics of swirling spray flames of ethanol, n-heptane, n-decane, and n-dodecane have been measured at conditions far from and close to blow-off using phase Doppler anemometry and OH* chemiluminescence, OH-planar laser-induced fluorescence, and Mie scattering at 5 kHz. The blow-off transient has also been examined. The OH* showed that the two main heat release regions lie around the spray jet at the inner recirculation zone and along the outer shear layer between the inner recirculation zone and the annular air jet. The heat release region is shortened and more attached as the flame approached blow-off. Mie images and phase Doppler anemometry data showed a wider dispersion of the ethanol spray compared to the other fuels. Similar spatial distributions of the Sauter mean diameter were observed for the four fuels for identical flow conditions, with the Sauter mean diameter value increasing with decreasing fuel volatility, but with the exception of significant presence of droplets in the nominally hollow cone for the ethanol spray. The OH-planar laser-induced fluorescence measurements showed an intermittent lift-off from the corner of the bluff body and the average lift-off height decreased with increasing air velocity, with less extinction along the inner flame branch especially for the heavier fuels. At the blow-off conditions, local extinctions appeared at both flame branches. The blow-off process followed a gradual reduction of the size of the flame, with the less volatile fuels showing a more severe flame area reduction compared to the condition far from blow-off. The average blow-off duration, [Formula: see text], calculated from the evolution of the area-integrated OH* signal, was a few tens of milliseconds and for all conditions investigated the ratio [Formula: see text] /( D/ UB) was around 11, but with large scatter. The measurements provide useful information for validation of combustion models focusing on local and global extinction.