Lift-off Height Research Articles

Flame lift-off height, velocity flow and mixing of hythane in oxy-combustion in a burner with two separated jets

The development of oxy-fuel burner with highly separated jets of fuel and oxidizer presents attractive perspectives because reactives separation generates a better thermal efficiency and a reduction of pollutant emissions by dilution effect. The aim of this work is to study the effects of the addition of hydrogen and the decrease of the equivalence ratio (Ф < 1) on the flame lift-off height and the mixing between the jets of hythane and oxygen. The oxy-fuel burner is made up of two nozzles, the first is hythane flow (mixing between natural gas and hydrogen) and the second is pure oxygen. Owing to the high diffusivity and reactivity of hydrogen in combustion, the use of hythane solves instability problems of flame linked to lean combustion. The OH radical emission results show that addition of hydrogen and reduction of equivalence ratio permit to decrease the flame lift-off height and to reduce its fluctuations. Thanks to the Particle Image Velocimetry of non-reacting and reacting flows, the comparison of 2-D mean velocity fields shows that the various parameters favour jets combination, a better mixing quality and a decrease of the size of the recirculation zone. In its kind of oxy-fuel burner, hydrogen and a lean equivalence ratio allow therefore an early combustion, a better flame stability and a decrease of amount of fuel used.

Transient response of the Delft jet-in-hot coflow flames

The ignition of impulsively started jets of natural gas in a hot coflow has been studied, with the aim of gaining insight into the processes that occur in a furnace equipped with regenerative burners. The development of the velocity field was measured in a time resolved manner with Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) at 10 kHz. Furthermore, simultaneous PIV and Planar Laser Induced Fluorescence of the OH-radical (OH-PLIF) were employed in combination with high-speed recordings of flame luminescence to map the ignition sequence. A characteristic feature of the impulsively started jets was that the turbulent phase was preceded by a laminar phase, during which no significant amount of combustion took place. After the sudden transition towards a turbulent state, significant ignition delay times were observed. As demonstrated in earlier work Oldenhof et al. (2011) [1], the radial temperature gradient of the coflow makes the entrainment of coflow fluid into the jet region very important to the jet stabilisation, influencing strongly the lift-off height. Using the time-resolved PIV data, the transport of fluid parcels in the coflow region was monitored during the transition towards the turbulent state. This transport is governed by the low velocities of the coflow and as a result, ignition delay times and the time until a steady state of combustion is attained are not solely related to jet time scales. It is thus concluded that in the presence of large-scale inhomogeneities, ignition delay times are several times greater than the time needed for the velocity field of the jet to reach its steady turbulent state.

Liftoff height behavior in a non-premixed turbulent hydrogen jet with coaxial air

To understand hydrogen lifted flames, the experimental approximation of liftoff height in non-premixed turbulent conditions was studied. The objectives were to analyze liftoff height behavior and to derive the normalized expression for lifted jet with the effective diameter (dF,eff). Hydrogen flow velocity varied from 100m/s to 300m/s. Coaxial air velocity was regulated from 12m/s to 20m/s. For the simultaneous measurement of velocity field and reaction zone, particle image velocimetry using hydroxyl radicals (PIV/OH) planar laser-induced fluorescence (PLIF) techniques with neodymium-droped yttrium–aluminum-gamet (Nd:YAG) lasers and charge-coupled device/intensified charge-coupled device (CCD/ICCD) cameras were used. Liftoff height decreased with increased fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The stabilization point was defined as the point where local flow velocity is balanced with turbulent flame propagation velocity. On the basis of the far field concept, we could derive the experimental approximation of the liftoff height divided by the effective diameter.

Experimental study on self-excitations in nitrogen-diluted laminar lifted butane flames

Laminar lifted butane flames diluted with nitrogen have been investigated experimentally to determine distinctive self-excitation regimes in the flame stability maps and also to elucidate the individual self-excitation characteristics. Self-excitations of lift-off height are classified into five regimes in laminar free-jet lift-off butane flames diluted with nitrogen: a stationary lifted regime (regime I), a heat-loss-induced self-excitation (regime II), a buoyancy-induced self-excitation due to flame flicker as well as a heat-loss-induced self-excitation (III), a combined form of an oscillation prior to blow-out and a heat-loss-induced oscillation (regime IV), and a combined form of an buoyancy-induced self-excitation and a heat-loss-induced oscillation as well as an additional buoyancy-driven self-excitation due to flame flicker (regime V). Extremely low-frequency (<0.1Hz) self-excitation is caused by conductive heat loss from the premixed wings to the trailing diffusion flame and can be explained by a proposed mechanism. It is also found that the flame oscillation prior to flame blow-out is also caused by buoyancy and also significantly affected by the conductive heat loss from the premixed wings to the trailing diffusion flame, thereby showing that the frequency with nozzle exit velocity increases in the triple-flame propagation mode and then decreases in the flame-front propagation mode. Characterization of the individual self-excitation mode is presented and also discussed with Strouhal numbers and its relevant parameters through the analysis of power spectrum for temporal variation of lift-off height.

Damköhler Number Similarity for Static Flame Stability in Gaseous-Fueled Augmentor Flows

Afterburners (or augmentors) are used to increase thrust in aircraft engines. Static flame stability, or the robustness to flame blowoff, is an important metric in the performance assessment of combustion in aircraft engine afterburners, where bluff-body-type flame holders are typically used to stabilize the flame. The design of such flame holders is complicated by the operating conditions, which involve flows at high speed, high temperature, and low pressure. In this paper, experimental and computational studies of Damköhler (Da) number similarity are presented with relevance to augmentor flame stability. The Da number describes the ratio of flow and chemical time scales. Hence, as long as a reference Da number is kept constant, similar characteristics of static stability should be expected of the bluff-body stabilized flame at low and high speeds. Flame stability in high-speed vitiated flows could then be studied at low speed if the chemical time scale is reduced, for instance by lowering the oxidizer flow temperature. However, each chemical reaction has its own Da number and not all can be kept constant at the same time. Since different stabilization mechanisms are governed by different chemical reactions, it is not necessarily clear what the relevant Da number is. Here, a consistent method for defining the Da number is provided based on the analysis of the modeled governing equations. Numerical simulations are performed for three different velocities with the inflow temperature adjusted to keep the Da number constant. Results are validated by comparison with experimental PIV data and the reported flame liftoff height. For the same characteristic Da number and constant momentum ratio between the fuel jet and the vitiated cross flow, the three flames show similar mean features for the recirculation zone and the flame shape. The flow field is found to exhibit von Kármán vortex shedding with the same Strouhal number for all cases. The average nondimensional flame liftoff height is also found to be the same for all cases. These results suggest a method to properly define the Da number to test augmentor stability features in low-speed test facilities under the conditions of similarity.

Investigation of lifted jet flames stabilization mechanism using RANS simulations

The mean structure, stability and the lift-off heights of lifted methane-air flames in a turbulent round jet are computed using the Reynolds-averaged Navier-Stokes (RANS) approach coupled with the Eddy-Dissipation Model (EDM) for combustion. The computations are based on a 5-step methane-air reduced mechanism. The simulation results show that the EDM model reproduces a mean partially-premixed flame structure at the leading edge of the lifted flame. The fine-tuning of the main EDM parameter for experimental lifted flames with co-flow also results in very good predictions of the lift-off height for lifted flames without co-flow. Implications of the use of the EDM concept for partially premixed flames are discussed.

Numerical Simulation of Delft-Jet-in-Hot-Coflow (DJHC) Flames Using the Eddy Dissipation Concept Model for Turbulence–Chemistry Interaction

In this paper we report results of a numerical investigation of turbulent natural gas combustion for jet in a coflow of lean combustion products in the Delft-Jet-in-Hot-Coflow (DJHC) burner which emulates MILD (Moderate and Intense Low Oxygen Dilution) combustion behavior. The focus is on assessing the performance of the Eddy Dissipation Concept (EDC) model in combination with two-equation turbulence models and chemical kinetic schemes for about 20 species (Correa mechanism and DRM19 mechanism) by comparing predictions with experimental measurements. We study two different flame conditions corresponding to two different oxygen levels (7.6% and 10.9% by mass) in the hot coflow, and for two jet Reynolds number (Re=4100 and Re=8800). The mean velocity and turbulent kinetic energy predicted by different turbulence models are in good agreement with data without exhibiting large differences among the model predictions. The realizable k-e model exhibits better performance in the prediction of entrainment. The EDC combustion model predicts too early ignition leading to a peak in the radial mean temperature profile at too low axial distance. However the model correctly predicts the experimentally observed decreasing trend of lift-off height with jet Reynolds number. A detailed analysis of the mean reaction rate of the EDC model is made and as possible cause for the deviations between model predictions and experiments a low turbulent Reynolds number effect is identified. Using modified EDC model constants prediction of too early ignition can be avoided. The results are weakly sensitive to the sub-model for laminar viscosity and laminar diffusion fluxes.

층류제트 부상화염에서 열손실에 의한 자기진동에 관한 연구

질소 희석된 프로판 부상화염에서 열손실에 의한 자기진동을 기초로 화염안정화선도를 도출하기 위하여 노즐직경 0.3 ㎜, 1.0 ㎜에서 실험적 연구를 수행하였다. 예혼합화염에서 확산화염으로의 전도 열손실에 의한 자기진동 및 매연 복사에 의한 자기진동을 관찰하였다. 0.1 ㎐보다 낮은 주파수 성향을 띄는 열손실에 의한 자기진동은 제안된 메커니즘에 의해 잘 묘사되었고 반면 매연복사에 의한 자기진동은 O(0.1 ㎐)의 주파수 범위를 나타내었으며 제안된 메커니즘은 항온항습실 실험을 통해 입증하였다. 질소 희석된 프로판 부상화염에서 관찰된 열손실에 의한 자기진동의 특성화는 관련된 변수 및 스트라훌 수에 의해 잘 묘사되었다.

Sooting turbulent jet flame: characterization and quantitative soot measurements

Computational fluid dynamics (CFD) modelers require high-quality experimental data sets for validation of their numerical tools. Preferred features for numerical simulations of a sooting, turbulent test case flame are simplicity (no pilot flame), well-defined boundary conditions, and sufficient soot production. This paper proposes a non-premixed C2H4/air turbulent jet flame to fill this role and presents an extensive database for soot model validation. The sooting turbulent jet flame has a total visible flame length of approximately 400 mm and a fuel-jet Reynolds number of 10,000. The flame has a measured lift-off height of 26 mm which acts as a sensitive marker for CFD model validation, while this novel compiled experimental database of soot properties, temperature and velocity maps are useful for the validation of kinetic soot models and numerical flame simulations. Due to the relatively simple burner design which produces a flame with sufficient soot concentration while meeting modelers’ needs with respect to boundary conditions and flame specifications as well as the present lack of a sooting “standard flame”, this flame is suggested as a new reference turbulent sooting flame. The flame characterization presented here involved a variety of optical diagnostics including quantitative 2D laser-induced incandescence (2D-LII), shifted-vibrational coherent anti-Stokes Raman spectroscopy (SV-CARS), and particle image velocimetry (PIV). Producing an accurate and comprehensive characterization of a transient sooting flame was challenging and required optimization of these diagnostics. In this respect, we present the first simultaneous, instantaneous PIV, and LII measurements in a heavily sooting flame environment. Simultaneous soot and flow field measurements can provide new insights into the interaction between a turbulent vortex and flame chemistry, especially since soot structures in turbulent flames are known to be small and often treated in a statistical manner.

Auto-ignition and stabilization mechanism of diluted H2 jet flame

The controllable active thermo-atmosphere combustor (CATAC) has become a utilizable and effective facility because it benefits the optical diagnostics and modeling. This paper presents the modeling research of the auto-ignition and flames of the H2/N2 (H2/CH4/N2, or H2/H2O2/N2) mixture on a CATAC, and shows curves varying with temperatures of auto-ignition delay, the height of the site of auto-ignition of lifted flames, and flame lift-off height. The results of auto-ignition delay and the lift-off height are compared the experimental results to validate the model. A turning point can be seen on each curve, identified with criterion temperature. It can be concluded that when the co-flow temperature is higher than the criterion temperature, the auto-ignition and lifted flame of the mixture are not stable. Conversely, below the criterion temperature, the mixture will auto-ignite in a stable fashion. Stabilization mechanisms of auto-ignition and lifted flames are analyzed in terms of the criterion temperature.

Large Eddy Simulation of flame stabilisation dynamics and vortex control in a lifted H2/N2 jet flame

Flame stabilisation in (highly) preheated mixture is common in several industrial applications. When the reactants are injected separately in the device (usually at high-speed), the flame is lifted so that the fuel and oxidant first mix to give an ignitable mixture. If the temperature of the mixture is adequate, it auto-ignites stabilizing the flame. Here we focus on an academic lifted jet flame and Large Eddy Simulation (LES) is used to capture the flame and auto-ignition dynamics. Comparisons with experimental data show that LES simulates accurately high OH fluctuation levels at the stabilisation location. The vortex dynamics linked to these fluctuations is analyzed and it is found that small scale coherent structures play a vital role in the auto-ignition process. These structures are axial vorticity tubes (braids) and are located relatively far (in the radial direction) from the shear-layer. As a consequence, the lift-off height varies dramatically in time leading to OH fluctuations of the order of the mean OH concentration. This scenario is monitored in the compositional space highlighting the simultaneous evolution of OH, HO 2 and temperature. Further, different strategies for open-loop control of the flame lift-off height are tested. In order to anchor the flame at different positions downstream of the nozzle, the vortex dynamics in the shear-layer was modified. Promoting successively vortex ring and braids, the auto-ignition region was moved significantly. In particular, modified nozzle geometries impacted the formation of braids and ensured a good premixing very close to the nozzle. As a consequence, it was possible to reduce significantly the lift-off height and stabilise the flame few diameters downstream of the nozzle.

Computer simulation of liquid jet fire with various injection conditions

Jet fire is one of the most frequent fire accidents occurring due to leakage in storage and transportation process of combustible gas and liquids. For fire safety consideration, computer simulation was performed on liquid jet fire with various injection conditions, based on LES and mixture-fraction combustion model. In liquid jet fire simulation, it is suggested that D*/δx reach 18 or more. With increasing, the fire length and lift-off height decreases as injection angle increases. Maximum centerline temperature is not dependent on injection angle. Large injection pressure causes the fire length and lift-off height to rise, and also leads maximum centerline temperature to drop slightly. Various nozzle heights above the ground have little influence on jet fire.

117 交番磁場を利用した非破壊検査における新検出法の提案(セッション5 構造物診断技術)

This paper describes a new eddy-current inspection method that enables noncontact detection of the corrosion wastage of a pipe and the flaw/defect on a back of thick-walled magnetic and non-magnetic metal specimens. This paper proposes a method for detecting the faint signal component from the eddy current induced on the back surface of the specimen, by offsetting the strong signal from the surface eddy current with the external sinusoidal signal which amplitude and phase are equal to the detection signal of the sensor. Using this method, it is possible to detect the flaw/defect of the inner surface of a thick-walled pipe from the outer surface and to detect the corrosion wastage from the outer surface of a pipe with lift-off height of tens mm, and to detect the internal crack on the weld zone of a seam weld pipe from the outer surface of the pipe.

Burned Gas Characteristics of Laminar Lifted Flame at the Beginning of Stream Heating

This research experimentally investigates the exhaust gas characteristics of a methane-air lifted-jet flame with elevated stream temperatures up to 450 K. Emission indices of NOx and CO were measured by direct gas sampling using a gas detector tube. The index of NOx decreased or did not change with increasing stream temperature up to 350 K, and then increased with further temperature increases. The index of CO changed in an opposite manner to that of NOx with increasing stream temperature. These tendencies demonstrated that the generation of emission gases did not monotonically increase with stream temperature. These non-monotonic properties of the emission indices were caused by particular variations in the lift-off height of flame and flame length. In order to examine these flame characteristics, we measured the flame curvature with OH-PLIF and related it to the lift-off height of triple flame. These flame properties could explain the unique emission characteristics of a lifted-jet flame with stream heating.

Stability of lifted flames in centerbody burner

The centerbody burner was designed with the objective of understanding the coupled processes of formation, growth, and burn-off of soot through decoupling them using recirculation zones (RZs). Experimentally it was found that the sooting characteristics of the centerbody burner could be altered dramatically via simple changes in the operating conditions. One of the interesting operating regimes in which a flame lifts off and forms a column of soot was identified when oxygen in the annulus air jet was reduced sufficiently. This paper describes the numerical studies performed to aid the understanding of lifted flames in the centerbody burner. A time-dependent, axisymmetric, detailed-chemistry CFD model is used. Combustion and PAH formation are modeled using the Wang–Frenklach (99 species and 1066 reactions) mechanism, and soot is simulated using a two-equation model of Lindstedt. Calculations have predicted the structure of the lifted flame very well. Two RZs [outer (ORZ) and inner (IRZ)] are formed between the fuel and air jets. A diffusion flame that is lifted-off the centerbody plate anchors steadily to the outer periphery of the ORZ. A near-perfect match between the computed and measured flame lift-off heights is achieved. RZs transport soot that is formed in the flame toward the face of the centerbody and create the soot column. Ethylene and its lighter fuel fragments that are formed in the RZs diffuse toward the annulus air jet and establish a mixing layer with the inwardly diffusing oxygen. Heat diffusing away from the RZs initiates autoignition reactions in the mixing layer. A flame with a triple-flame-base structure becomes established at a location where the ignition-delay time matches the residence time. Soot that is transported into the RZs is found to have a significant effect on the flame lift-off height. Numerical experiments are performed to aid the understanding of the relationship between soot and flame lift-off. Radiation from the soot decreases the temperature, slows the autoignition process, and increases the lift-off height. Soot oxidation consumes O and OH radicals, slows the autoignition reactions, and increases the lift-off height.

Characteristics of Oscillating Flames in a Coaxial Confined Jet

Flame characteristics when a non-premixed n-butane jet is ejected into a coaxial cylindrical tube are investigated experimentally. Flame stability depends mainly on the characteristics of flame propagation as well as air entrainment which depend on the jet momentum and on the distance between the nozzle exit and the base of a confined tube. As flow rate increases, the flame lifts off from a nozzle attached diffusion flame and a stationary lifted flame can be stabilized. The liftoff height increases nearly linearly with the average velocity at the nozzle exit. The lifted flame has a tribrachial flame structure, which consists of a rich premixed flame, a lean premixed flame, and a diffusion flame, all extending from a single location. As flow rate further increases, periodically oscillating flames are observed inside the confined tube. Once flame oscillation occurs, the flame undergoes relatively stable oscillation such that it has nearly constant oscillation amplitude and frequency. The criteria of flame oscillation are mapped as functions of nozzle diameter, the distance between nozzle and tube, and jet velocity. This type of flame oscillation can be characterized by Strouhal number in terms of flame oscillation amplitude, frequency, and jet velocity. Buoyancy driven flame oscillation which is one of the viable mechanism for flame oscillation is modeled and the results agrees qualitatively with experimental results, suggesting that the oscillation is due to periodic blowoff and flashback under the influence of buoyancy.

Methane/Air-Lifted Flames in Magnetic Gradients

The authors analyzed the behavior of a laminar CH4/air flame with a central methane jet and surrounding air jet in a large range of fuel and air flow rates. Different regimes of flame stability are observed from an anchored flame to a stable lifted flame, which is destabilized before extinction. It is shown that the flame instabilities are coupled to the production of vortices on the air jet/ambient air side and is depending on the injected air velocity. Influence of an upward increasing magnetic field is studied by setting the flame inside the bore of an electromagnet. Measurements at different values of methane and air flow rates show that the flame liftoff height is decreased by the magnetic gradient. It is attributed to the magnetic force, which develops on air via its action with paramagnetic oxygen. The magnetic force interacts with the air jet structures and influences the flame stability.

A Numerical Study of the Magnetic Influence on Coaxial Jets' Flow Upstream From Lifted Flames

The authors conducted a numerical investigation of the laminar flow issuing from a methane/air coaxial burner submitted to a magnetic field. They intend to understand the mechanisms involved in the magnetic stabilization of the lifted flames. The model is based on the coupled equations of conservation of mass, species, and momentum in which the magnetic force acting on paramagnetic oxygen is introduced in the source term. A CFD code is used to solve the steady-state equations in an axisymmetric domain. Air jet velocity is shown to decrease locally in front of the magnetic gradient. Calculation of the stoichiometric radius reveals a lateral displacement of the stoichiometric line and a decrease of the fuel mass fraction gradient when the magnetic force is applied. These results explain the observed reduction of the liftoff height of flames under magnetic gradient due to an increase of the flame propagation velocity.

Modeling of Lifted Methane Jet Flames in a Vitiated Coflow Using a New Flame Extinction Model

In this article, flame liftoff height predictions have been carried out for diluted methane jet flames with high temperature and vitiated coflow. Three-dimensional mass, momentum, energy, and species equations are solved with modified k–ϵ and modified k–ω turbulence models to predict the behavior of lifted jet flames. The effect of turbulence model parameters, fuel jet velocity and coflow velocity has been investigated for jet flames. The flame is assumed to be quenched when the fluid time scale is less than the chemical time scale (Da = τ f /τ ch < 1). The predicted results show that substantial premixing of fuel occurs with surrounding oxidizer in axial and radial directions. Predicted flame liftoff height agrees well with the experimental measurements at 100 m/s jet velocity and it is significantly higher than the predictions reported in earlier work. However, predicted variation of flame liftoff height with jet velocity agrees well with Kalghatgi's (1984) nondimensional parameters. The flame liftoff height, temperature distribution, species distribution, and velocity distributions in radial and axial directions are in good agreement with the experimental results.

The stabilization characteristics of turbulent lifted diffusion flames of CH4/CO blended fuels

The outstanding stabilization characteristics of lifted turbulent jet diffusion flames of CH4/CO blended fuels in still air were investigated experimentally by igniting the flame from downstream in the initial condition of a nonreacting fuel jet. In this study, we concentrated on the effects of the shift of outstanding reaction paths on the stabilization characteristics as the composition of the blended CH4/CO is varied. Systematic experiments were conducted using the nonintrusive diagnostics of shuttered laser particle image velocimetry (PIV), OH∗ chemiluminescence, and digital image processing to study the flame stabilization characteristics in terms of the key parameters such as the instantaneous flame-base location, lift-off height, and streamline velocity distribution in the vicinity of the edge flame leading point by varying the exit velocity and blended-fuel concentration. A conditional leading point from shutter PIV measurement is defined to calculate the instantaneous streamline velocity profile at the flame base. For the CO concentration in fuel blends lower than 80%, the stabilization is similar in characteristics to that proposed by the triple flame model. As the CO concentration is increased to 90%, the flame stabilizes on the fuel-rich side and the stabilization characteristics may be related to the maximum burning velocity at the rich equivalence ratio. This is confirmed by the location of the instantaneous flame stabilization point defined by the instantaneous leading point of the lifted flame with respect to the fuel concentration profiles.