Lift-off Height Research Articles

Lift-off of jet diffusion flame in sub-atmospheric pressures: An experimental investigation and interpretation based on laminar flame speed

This paper reveals lift-off behavior of jet diffusion flames in sub-atmospheric pressures less than 100kPa, in view of that the current knowledge on this topic is limited for normal pressure conditions. Physically, the variation of ambient pressure may have significant influence on the lift-off behavior of jet diffusion flames due to the change of some critical parameters such as laminar flame speed. In this work, experiments are conducted in a large pressure-controllable chamber of 3m (width)×2m (length)×2m (height) at different sub-atmospheric pressures of 60kPa, 70kPa, 80kPa, 90kPa as well as at normal pressure of 100kPa. Axisymmetric turbulent jet diffusion flames are produced by nozzles with diameters of 4mm, 5mm and 6mm using propane as fuel. It is revealed that the lift-off height increases as the pressure decreases and being much higher than that in normal pressure condition. The laminar flame speed with its dependency on pressure is introduced to interpret such behavior based on classic Kalghatgi model. It is found theoretically that the lift-off height has a power law dependency on pressure by P1−n, where n is overall reaction order of the fuel which is usually larger than 1 indicating a negative power law function with pressure (for example p−0.75 for propane as n=1.75) as well verified by the experimental correlation. Finally, a global model is proposed by including such pressure dependency function into the Kalghatgi model, which is shown to well collapse the experimental results of lift-off heights of different sub-atmospheric pressures.

Liftoff of a Co-Flowing Non-Premixed Turbulent Methane Flame: Effect of the Geometrical Parameters of a Circular Fuel Nozzle

This article reports an experimental study on the effect of the internal geometry of a circular nozzle on the stability of a turbulent methane diffusion flame with and without co-flow. The nozzle exit orifice is circular; however, the orifice diameter, length-to-diameter ratio, and contraction angle were varied. These geometrical parameters were aimed to create a wide range of conditions for the ensuing jet flow. The strength of the co-airflow was also varied to assess its impact on the jet flame stability parameters. The experimental results showed that the level of turbulence in the jet near field has a definite impact on the liftoff (i.e., local quenching) of an attached jet methane flame. This is obtained by systematically varying the nozzle geometry, where these variations led to the same conclusion that higher levels of jet near-field turbulence results in a lower flame liftoff velocity regardless of the nozzle geometry. The present results revealed also that there is a clear interplay between the flame liftoff height and the jet flow characteristics. That is, a jet flow that spreads faster and generates greater near-field turbulence would result in a flame base sitting closer to the nozzle exit. Finally, the presence of a moderate co-airflow results in the appearance of a hysteresis phenomenon at low jet velocity range but, in general, did not alter the relationship between the flame stability parameters and jet's characteristics.

Lagrangian Intermittent Modelling of a Turbulent Lifted Methane-Air Jet Flame Stabilized in a Vitiated Air Coflow

The present manuscript reports results of numerical simulations of turbulent lifted jet flames of methane with special emphasis placed on the autoignition effects. The impact of dilution with burned gases on the flame stabilization is analyzed under the conditions of a laboratory jet flame surrounded by a vitiated air coflow. In this geometry, mass flow rates, temperature levels and exact chemical composition of hot products mixed with air are fully determined. The effects of both finite rate chemistry and partially premixed combustion are taken into account within a Lagrangian intermittent framework. Detailed chemistry effects are incorporated through the use of chemical time scales, which are tabulated as functions of the mixture fraction. The concept of residence time (or particle age) and the transport equation for the mean scalar dissipation rate of the mixture fraction fluctuations, i.e., \(\widetilde{\varepsilon}_Z=\overline{\rho \mathcal{D} (\partial Z'' / \partial x_i) (\partial Z'' / \partial x_i) }/\overline{\rho}\), are also considered. This allows to improve the description of turbulent mixing including both the large scales engulfment processes through the consideration of the residence time scale, as well as the small scales molecular mixing processes the intensity of which is set by the integral scalar (mixing) time scale \(\tau_Z=\widetilde{Z''^2}/\widetilde{\varepsilon}_Z\). Numerical simulations of the turbulent diluted jet flame of methane studied by Cabra and his co-workers are performed. The flame liftoff height is reasonably well-captured and the computational results display a fairly satisfactory level of agreement with experimental data. They also confirm that the consideration of an additional transport equation for the mean scalar dissipation rate may significantly improve the computational results. The investigation is supplemented by a sensitivity analysis of the scalar to turbulence time scale ratio Cmix = τZ/τt often referred to as the mixing constant. Finally, the manuscript ends with the application of the proposed closure to experimental conditions which are characterized by variations of (i) the jet exit velocity, (ii) coflow velocity and (ii) coflow temperature.

The role of precursors on the stabilisation of jet flames issuing into a hot environment

This paper seeks to address unusual flame stabilisation behaviour observed in experimental jet flames which issue into a hot coflow. It has been observed that increasing the temperature and/or oxygen concentration in the coflow can lead to an increase in flame liftoff height. The paper isolates the role of chemistry, and in particular flame intermediates, on the observed phenomenon with a view to better understand how the behaviour changes over a range of conditions. A descriptive theory for this behaviour is proposed, which is based on the well-established theory that a build-up of radicals and intermediate species is responsible for autoignition of these flames. This paper systematically examines the role of these precursors with a view to better understanding of the chemical kinetics and to assess if the observed behaviour is chemistry-dominated. To this end, laminar flame calculations and ignition delay curves are presented, and the findings are validated with experiments. The results indicate that chemical effects alone are insufficient to fully explain the observations, but the calculations support the general trends noted in the experiments and highlight the importance and relative effects of some key precursors. In particular, the production and consumption of formaldehyde in a low oxygen environment supports the unusual flame behaviour observed experimentally.

Experimental Study of Oxygen Enrichment Effects on Turbulent Non-premixed Swirling Flames

The current paper describes the effects of oxygen enrichment on flame stability and pollutant emissions for turbulent non-premixed swirling flames. The study is motivated by CO2 capture applications further to the increase of the CO2 concentration by the O2 addition. The burner configuration consists of two concentric tubes with a swirl placed in the annular part for air or oxygen–air. The exhaust gas compositions are measured using gas analyzers. OH chemiluminescence experiments are conducted to investigate the stability of flames. The measurements were performed for oxygen concentrations ranging from 21 to 30% by volume, with swirl numbers of 0.8 and 1.4 and global equivalence ratios of 0.8, 0.9 and 1. Results show that oxygen enrichment enhances combustion efficiency and flame stability. The increase of the oxygen concentration in air leads to a decrease of lift-off heights and fluctuations of the flame base. Increasing the swirl number significantly improves the flame stability. Experiments demonstrat...

A new approach to model turbulent lifted CH4/air flame issuing in a vitiated coflow using conditional moment closure coupled with an extinction model

In this article, conditional moment closure model (CMC) with detailed chemistry is used to model lifted turbulent methane flame in a high temperature and vitiated coflow and to predict flame lift-off height. The flow and mixing field are predicted by a 2D in-house code employing a k–ε turbulence model (RANS) with modified constant Cε2. The first-order CMC model on its own could not capture the behavior of the lifted flame. Large eddy simulations (LES) coupled with second-order CMC model would be a promising alternative but the objective here was to improve low-cost simulations based on RANS and first-order CMC to address realistic problems. Hence, an extinction model has been incorporated in the first-order CMC to improve its predictions and is referred in this paper as CMCE. In the CMCE model, flame is assumed to be extinguished when the ratio of flow time scale to the chemical time scale falls below a critical value. Predicted lift-off height by the CMCE model agrees very well with the experimental results. There is a significant improvement in temperature and species distributions in both axial and radial directions with the implementation of the CMCE model. Further, the model is extended to predict the flame lift-off height for various coflow temperatures and jet velocities by using scaling ratios. With these modifications, the lift-off heights predicted by the CMCE model match well with the experimental results for a wide range of jet velocities and coflow temperatures. Results from both CMC and CMCE models are compared against the experimental data to show the importance of the extinction model. Flame stabilization process indicates that flame stabilizes on the contour of mean stoichiometric mixture fraction where axial mean velocity equals the turbulent burning velocity.

Analysis of Flame Characteristics in a Laboratory-Scale Turbulent Lifted Jet Flame via DNS

A fully compressible 3D solver for reacting flows has been developed and applied to investigate a turbulent lifted jet flame in a vitiated coflow by means of direct numerical simulation (DNS) to validate the solver and analyze the flame characteristics. An eighth-order central differencing scheme is used for spatial discretization and a fourth-order Runge-Kutta method is employed for time integration. The DNS results agree well with the experimental measurements for the conditional means of reactive scalars. However, the lift-off height is under predicted. The mean axial velocity develops into a self-similar profile after x/D = 6. The normalized flame index is employed to characterize the combustion regime. It is found that at the flame base the gradients of the reactants are opposed and diffusion combustion is dominant. Further downstream, the contribution of premixed combustion increases and peaks at x/D = 8. Finally, the stabilization process is examined. The turbulent lifted flame is proved to stabilize in the lean mixtures and low scalar dissipation rate regions.

Nitrogen-Diluted Methane Flames in the Near-and Far-Field

With the increased utilization of multicomponent fuels, such as natural gas and biogas, in industrial applications, there is a need to be able to effectively model and predict the properties of jet flames for mixed fuels. In addition, the interaction of these diluted fuels with outside influences (such as differing levels of coflow air) is a primary consideration. Experiments were performed on methane jet flames under the influence of varying levels of nitrogen dilution, from low Reynolds number lifted regimes to blowout, observing the influence of the nitrogen on lifted flame height and flame chemiluminesence images. These findings were analyzed and compared with existing lifted jet flame relations, such as the flammable region approximation proposed by Tieszen et al., as well as to undiluted flames. The influence of nitrogen dilution was seen to have an effect on the liftoff height of the flame, as well as the blowout velocity of the flame, but was seen to have a less pronounced effect compared with flames with coflowing air.

A Comparison of the Blow-Off Behaviour of Swirl-Stabilized Premixed, Non-Premixed and Spray Flames

Confined short turbulent swirling premixed and non-premixed methane and heptane spray flames stabilized on an axisymmetric bluff body in a square enclosure have been examined close to the blow-off limit and during the extinction transient with OH* chemiluminescence and OH-PLIF operated at 5 kHz. The comparison of flames of different canonical types in the same basic aerodynamic field allows insights on the relative blow-off behaviour. The flame structure has been examined for conditions increasingly closer to blow-off. The premixed flame was seen to change from a cylindrical shape at stable burning condtions, with the flame brush closing across the flow at conditions close to blow-off. The PLIF images show that for the gaseous non-premixed flame, holes appear along the flame sheet with increasing frequency as the blow-off condition is approached, while the trend is less obvious for the spray flame. Non-premixed and spray flames showed randomly-occurring lift-off, which is further evidence of localised extinction. The mean lift-off height increased with increasing fuel jet velocity and decreased with increasing air velocity and approaches zero (i.e. the flame is virtually attached) just before the blow-off condition, despite the fact that more holes were evident in the flame sheet as extinction was approached. It was found that the average duration of the blow-off event, when normalised with the characteristic flow time d/U b (d being the bluff-body diameter and U b the bulk velocity) was in the range 9–38 with the spray flame extinction lasting a shorter time than the gaseous flames. Finally, it was found that correlations based on a Damkohler number collapse the blow-off velocity data for all flames with reasonable accuracy. The results can help the development of advanced turbulent combustion models.

Experimental study of industrial gas turbine flames including quantification of pressure influence on flow field, fuel/air premixing and flame shape

A commercial swirl burner for industrial gas turbine combustors was equipped with an optically accessible combustion chamber and installed in a high-pressure test-rig. Several premixed natural gas/air flames at pressures between 3 and 6bar and thermal powers of up to 1MW were studied by using a variety of measurement techniques. These include particle image velocimetry (PIV) for the investigation of the flow field, one-dimensional laser Raman scattering for the determination of the joint probability density functions of major species concentrations, mixture fraction and temperature, planar laser induced fluorescence (PLIF) of OH for the visualization of the flame front, chemiluminescence measurements of OH* for determining the lift-off height and size of the flame and acoustic recordings. The results give insights into important flame properties like the flow field structure, the premixing quality and the turbulence–flame interaction as well as their dependency on operating parameters like pressure, inflow velocity and equivalence ratio. The 1D Raman measurements yielded information about the gradients and variation of the mixture fraction and the quality of the fuel/air mixing, as well as the reaction progress. The OH PLIF images showed that the flame was located between the inflow of fresh gas and the recirculated combustion products. The flame front structures varied significantly with Reynolds number from wrinkled flame fronts to fragmented and strongly corrugated flame fronts. All results are combined in one database that can be used for the validation of numerical simulations.

Autoignited and non-autoignited lifted flames of pre-vaporized n-heptane in coflow jets at elevated temperatures

The characteristics of laminar lifted flames of pre-vaporized n-heptane in coflow jets were investigated under both non-autoignited and autoignited conditions by varying the initial temperature. The fuel tested was n-heptane considering the importance as a primary reference fuel for gasoline and its low temperature ignition behavior at relatively low pressure. The results showed that the lifted flame edge in the non-autoignited regime had a tribrachial structure with lean and rich premixed flame wings together with a trailing diffusion flame. The liftoff heights correlated reasonably well with the fuel jet velocity scaled by the stoichiometric laminar burning velocity regardless of the initial temperature and the nitrogen dilution. The liftoff velocity multiplied by the buoyancy-induced velocity and the blowout velocity scaled by the mole fraction of the fuel correlated well with the stoichiometric laminar burning velocity. When the initial temperature was above 900K, flames were autoignited without any external ignition source. Autoignited lifted flames with both tribrachial edges and mild combustion characteristics were observed. The correlation of the liftoff height with the calculated adiabatic ignition delay time was weak, unlike in cases with gaseous fuels of C1–C4 hydrocarbons in which the liftoff height of the autoignited flames correlated well with the square of the adiabatic ignition delay time. When the mole fraction of the fuel was small, mild combustion behaviors were exhibited with edge flames without distinct tribrachial structures. The liftoff height was correlated with the fuel jet velocity scaled by the initial fuel mass fraction, while the dependence on the ignition delay time was weak when compared with the gaseous fuels.

DNS analysis of a three-dimensional supersonic turbulent lifted jet flame

A fully compressible solver for direct numerical simulation of supersonic combustion has been developed and applied to investigate a three-dimensional spatially-developing supersonic turbulent jet flame. High-resolution bandwidth-optimized weighted essentially non-oscillatory scheme of spatial discretization and total variation diminishing temporal integration are used to capture the intricate phenomena with an accurate characteristic system and an updated detailed H2/air reaction mechanism. The numerical algorithms are first validated by simulating some canonical problems, and then used to analyze the global features, compressibility effects, and the stabilization mechanism of the supersonic lifted jet flame. It is found that the lifted flame consists of a stable laminar flame base with auto-ignition as the stabilization mechanism, a violent mixing region in which vigorous turbulent combustion occurs with both premixed and diffusion flames, and a far-field outer diffusion flame as well as an inner weaker premixed flame. Although diffusion combustion dominates the flame in general, premixed combustion is significant within the violent mixing region. The heat release rate in the high-speed region can be significantly influenced by the compressibility. In the region of fuel compressing, the heat release rate increases as the compressive power increases. While in the region of fuel expanding, the heat release rate also increases as the expansive power increases albeit with a smaller magnitude. The auto-ignition in a fuel-lean mixture near the most reactive condition is the main stabilization mechanism, which is determined from the statistical analysis of the heat release rate and the Damköhler number as well as the deduction of the lift-off height of the flame.

Evaluation of an Unsteady Flamelet Progress Variable Model for Autoignition and Flame Lift-Off in Diesel Jets

An unsteady flamelet progress variable (UFPV) model is evaluated for modeling autoignition and flame lift-off in diesel jets. Changes in injection pressure, orifice diameter, ambient temperature, density, and O2 concentration are considered. In implementing the model in a Reynolds-averaged Navier–Stokes (RANS) code, a look-up table of reaction source terms is generated as a function of mixture fraction Z, stoichiometric scalar dissipation rate χst and progress variable Cst by solving the unsteady flamelet equations. It is assumed that the probability density functions (pdfs) of Z, χst, and Cst are statistically independent, and presumed functions are employed for the pdfs. Comparisons with experimental results show that the model is able to predict ignition delay and flame lift-off with reasonable accuracy in the RANS simulations. The quantitative agreement between computed and measured results depends on the definitions employed to quantify autoignition time and lift-off height, but, in general, the agreement is within 25%.

Experimental Study on Lifted Flames Operated with Liquid Kerosene at Elevated Pressure and Stabilized by Outer Recirculation

This study deals with the impact of the operating conditions, e.g. pressure, preheating temperature, pressure drop across the nozzle, nozzle size and stoichiometry, on the reaction zone location and spray evaporation progress in case of a lifted flame. Lifted flames are highly valued for their NOx reduction potential and for their low susceptibility to flash-back and thermo-acoustic instabilities. These advantageous features arise from the improved homogeneity of the fuel-air mixture provided to the reaction zone. One distinctive feature of the lifted flames is the presence of the so called lift-off zone located between nozzle outlet and main reaction zone. Within the lift-off zone fuel and oxidizer remain a certain time in contact and mix together prior to the onset of the combustion reaction. This leads to a more uniform heat release distribution and lowers the nitrogen oxides emissions at lean conditions by reducing the temperature spikes. In contrast to many other studies the subject of investigation was not a plain jet flame, but a modified version of the airblast nozzle, widely used in industrial applications. The nozzle was operated with liquid kerosene. As liquid fuels are easier to handle than gaseous or solid, it is expected that many efforts in the future will focus on the development of liquid fuels surrogates. Our previous investigations have shown, that the nozzle is well suited to be operated with gaseous fuels as well (Fokaides et al, J Eng Gas Turbine Power 130, 011508 2008). The position of the reaction zone was determined by means of chemiluminescence of the OH∗ radicals and from its location the lift-off height was derived. In addition the fuel evaporation progress was measured by means of light scattering, revealing that fuel droplets and main reaction zone are well separated. It was found that the operating conditions have a versatile impact on the length of the lift-off zone and spray cone and thus on the degree of pre-evaporation and premixing. Thus, it may be concluded, that through a proper choice of operating conditions and combustor size a desired lift-off height can be adjusted in accordance with criteria, like available space, required emission levels etc.

Flame height and lift-off of turbulent buoyant jet diffusion flames in a reduced pressure atmosphere

This paper reports new experimental findings at a reduced atmospheric pressure (at high altitude in Tibet) for turbulent buoyant jet diffusion flames and global correlations for both normal and this reduced atmospheric pressure. Comparative experiments are carried out in Hefei (50m, 100kPa) and Lhasa (3650m, 64kPa) in China to measure the mean flame height and lift-off behaviors. The turbulent jet diffusion flame is produced by nozzles with diameters of 4, 5, 6, 8 and 10mm using propane as fuel. A series of new findings are revealed and their interpretations are presented in this work. Results show that the normalized mean flame height is higher in the lower pressure atmosphere. A theory of diffusion flame height based on flame Froude number for the transition from buoyancy to momentum controlled turbulent jets can still successfully collapse the flame height data, although a 0.8 factor is needed globally to include effects of reduced entrainment and larger fluctuation in reduced pressure. The lift-off heights are revealed to be higher, while the lift-off velocities are smaller, in the reduced pressure atmosphere. The lift-off heights are correlated based on different theories. The present work provides new findings supplementary over previous classical knowledge on buoyant turbulent jet diffusion flame behaviors.

Kinetic and fluid dynamic modeling of ethylene jet flames in diluted and heated oxidant stream combustion conditions

This paper reports the results of a numerical investigation of several turbulent nonpremixed ethylene jet flames, either undiluted or diluted with hydrogen, air or nitrogen, which have been experimentally studied using a jet in hot coflow (JHC) burner. The fuel jet issues into a hot and highly diluted coflow at two O2 levels (3% or 9%) and a temperature of 1100 K. These conditions emulate those of moderate or intense low oxygen dilution (MILD) combustion. The attention is mainly focused on assessing the performance of different models for the turbulence–chemistry interaction. The model predictions are compared to experimental measurements. The Eddy dissipation concept (EDC), flamelet and PDF transport models were used in combination with a two-equation turbulence model (k–ε) and two different kinetic mechanisms (GRI-Mech 3.0 and POLIMI). The effect of the oxygen concentration in the coflow is well captured by all models. A modified version of the EDC model, recently proposed for the modeling of methane MILD combustion conditions, is also used. This model significantly improves the predictions of the EDC in the case of the ethylene flames studied in this paper. The agreement between measurements and predictions of the flamelet model is generally not as good as the PDF transport and modified EDC models, whose predictions are in good agreement with the measurements, and are improved especially for the apparent liftoff heights and the peak flame temperature. The effect of the fuel composition on the CH2O formation is also discussed, showing generally good agreement.

Momentum-dominated Methane Jet Flame at Sub-atmospheric Pressure

The influence of pressure and flow rate on momentum-dominated methane laminar flames’ flickering and radiation characteristics were experimental studied to provide a focused description of diffusion flame behavior at sub-atmospheric pressure of 50-100kPa. Measurement system consisted of CCD camera, high speed camera and schlieren system, radiation flux meter. Parameters including flame length, liftoff height, flickering frequency, and radiation intensity were compared and analyzed over the pressure range of 50-100kPa in a low pressure combustion cabin. The controlled fuel mass flow rates provided diffusion flames of 5.95-23.81mg/s stable at all ambient pressures considered. Experiment results indicated that for momentum-dominated jet flame, flame lengths were not dependent on pressure but linearly dependent on mass flow rate. As soot volume fraction was strongly influenced by pressure, when pressure descended, the radiation intensity declined resulted from reduction of soot formation. Lift off phenomenon of diffusion flames was not significant, however, liftoff heights also increased with increasing pressure and mass flow rate gradually. A semi-quantitative explanation on combined effect of pressure and velocity on flickering frequency was given as f = 0.227u0.18P∞1/3. All experiment data was in good agreement with theoretical analysis.

LES-CMS simulations of a turbulent lifted hydrogen flame in vitiated co-flow

Results from a study of turbulent lifted jet flame with Conditional Moment Closure (CMC) turbulent combustion model are reported. First, a qualitative description of the simulation results is given, by examining the instantaneous resolved species mass fraction fields. The structure of the lifted flame is also assessed by comparing the measured temperature and species mass fraction profiles and the simulation results. The model is able to capture the axial and radial profiles of mixture fraction, temperature and major species. The sensitivity of prediction to boundary conditions is also explored and the lift-off height is found to be very sensitive to the co-flow temperature. Finally, the stabilisation mechanisms, auto-ignition or premixed flame propagation, are addressed. No evidence of premixed flame propagation is found: the diffusion in physical space is negligible for all studied conditions.

Numerical study of turbulent normal diffusion flame CH4-air stabilized by coaxial burner

The practical combustion systems such as combustion furnaces, gas turbine, engines, etc. employ non-premixed combustion due to its better flame stability, safety, and wide operating range as compared to premixed combustion. The present numerical study characterizes the turbulent flame of methane-air in a coaxial burner in order to determine the effect of airflow on the distribution of temperature, on gas consumption and on the emission of NOx. The results in this study are obtained by simulation on FLUENT code. The results demonstrate the influence of different parameters on the flame structure, temperature distribution and gas emissions, such as turbulence, fuel jet velocity, air jet velocity, equivalence ratio and mixture fraction. The lift-off height for a fixed fuel jet velocity is observed to increase monotonically with air jet velocity. Temperature and NOx emission decrease of important values with the equivalence ratio, it is maximum about the unity.

Analysis of autoignition of a turbulent lifted H2/N2 jet flame issuing into a vitiated coflow

Due to energy crisis and concern regarding the environmental emission, hydrogen as an alternative clean fuel has received more attention. To develop new devices or upgrade the conventional combustion systems for hydrogen flames, fundamental concepts necessary for burner design need to be investigated. In the present work, characteristics of flame stabilization for a turbulent lifted H2/N2 jet flame issuing into a hot coflow of lean combustion are investigated using the Scalar probability density function (PDF) approach. Calculations are carried out for different coflow temperatures, concentrations of species and equivalence ratio. Reaction rate analyses are used to investigate the dominant chemistry at the flame base for a variety of conditions. The results show the occurrence of autoignition at the flame base that is responsible for the stabilization of the lifted turbulent flame. The coflow temperature plays an important role in the relative contribution of elementary reactions and the determination of the dominant chemistry at the flame base. This leads to a high sensitivity of lift-off height to the coflow temperature. Oxygen and water content in the hot coflow could affect the ignition process and lift-off height depending on the dominant chemistry at the flame base. Furthermore, the effect of oxygen content in hot coflow is found to be very important on the reactions controlling the high temperature combustion.