Laminar Jet Flames Research Articles

Effect of electric fields on reattachment and propagation speed of tribrachial flames in laminar coflow jets

The effects of electric fields on the reattachment of lifted flames have been investigated experimentally in laminar coflow jets with propane fuel by applying high voltages to the fuel nozzle. In case of AC, the frequency has also been varied. Results showed that reattachment occurred at higher jet velocity when applying the AC voltages, thus the stabilization limit of attached flames was extended by the AC electric field. Higher voltage and lower frequency of the AC were found to be more effective. On the contrary, the effect of DC was found to be minimal. To understand the early onset of the reattachment with the AC, occurring at higher jet velocity, the influence of AC electric fields on the propagation speed of tribrachial flame edge was investigated during the transient reattachment processes. The propagation speed increased reasonably linearly with the applied AC voltage and decreased inversely to the distance between the flame edge and the nozzle electrode. Consequently, the enhancement in the propagation speed of tribrachial flame edge was correlated well with the electric field intensity, defined as the applied AC voltage divided by the distance.

Effect of velocity gradient on propagation speed of tribrachial flames in laminar coflow jets

The effect of velocity gradient on the propagation speed of tribrachial flame edge has been investigated experimentally in laminar coflow jets for propane fuel. It was observed that the propagation speed of tribrachial flame showed appreciable deviations at various jet velocities in high mixture fraction gradient regime. From the similarity solutions, it was demonstrated that the velocity gradient varied significantly during the flame propagation. To examine the effect of velocity gradient, detail structures of tribrachial flames were investigated from OH LIF images and Abel transformed images of flame luminosity. It was revealed that the tribrachial point was located on the slanted surface of the premixed wing, and this slanted angle was correlated with the velocity gradient along the stoichiometric contour. The temperature field was visualized qualitatively by the Rayleigh scattering image. The propagation speed of tribrachial flame was corrected by considering the direction of flame propagation with the slanted angle and effective heat conduction to upstream. The corrected propagation speed of tribrachial flame was correlated well. Thus, the mixture fraction gradient together with the velocity gradient affected the propagation speed.

05/02675 Influence of oblique angle and heating height on flame structure, temperature field and efficiency of an impinging laminar jet flame

05/02675 Influence of oblique angle and heating height on flame structure, temperature field and efficiency of an impinging laminar jet flame

LIFTED LAMINAR JET DIFFUSION FLAMES

ABSTRACT This paper addresses the numerical description of lifted flames in axisymmetric laminar coflow jets. The analysis considers moderately large values of the Reynolds number, when the boundary-layer approximation can be used to describe the slender mixing region that extends between the jet exit and the flame, providing the profiles of velocity and mixture fraction that exist immediately upstream from the flame front region. The description of the nonslender flame front region, which provides the front propagation velocity as an eigenvalue, requires integration of the Navier-Stokes Equations with account taken of the reaction and thermal expansion effects. The limiting formulations corresponding to cases of practical interest, including large values of the air-to-fuel stoichiometric ratio, are briefly discussed. Illustrative numerical results are given for flames lifted or propagating at distances small compared with the jet development length, where the mixing layer is nearly planar.

04/02962 Effects of heating height on flame appearance,temperature field and efficiency of an impinging laminar jet flame used in domestic gas stoves: Hou, S.-S. and Ko, Y.-C. Energy Conversion and Management, 2004, 45, (9–10), 1583–1595

04/02962 Effects of heating height on flame appearance,temperature field and efficiency of an impinging laminar jet flame used in domestic gas stoves: Hou, S.-S. and Ko, Y.-C. Energy Conversion and Management, 2004, 45, (9–10), 1583–1595

EFFECTS OF DILUTION ON BLOWOUT LIMITS OF TURBULENT JET FLAMES

An extended database on blowout velocities of inert-diluted methane, propane, and hydrogen jet flames in the turbulent regime was experimentally established and used to examine and verify existing theories of blowout velocity estimation. Helium, argon, nitrogen, and carbon dioxide were used as the inert diluents to generate different initial properties at the jet exit. The theories of blowout velocity estimation by Kalghatgi (Combust. Sci. Technol., vol. 26, pp. 233–239, 1981) and Broadwell et al. (Proc. Combust. Instit., vol. 20, pp. 303–310, 1984) in the highly diluted regime were carefully examined using jet flames of different fuels diluted with inerts of different gas properties. The results showed that among the theories the blowout velocity estimation of Kalghatgi is more reliable in the extended region. On the other hand, the blowout velocity estimation of Broadwell et al. can do as well after proper modification by including the Reynolds number effect Re f. Based on the experimental results, modifications to the theories are proposed to accommodate for the deviation when they are used in the diluted regime. In addition, different from laminar jet flames, diffusive properties in terms of mass and thermal diffusivities are not the dominant parameters of blowout velocity in turbulent jet flames.

Influence of oblique angle and heating height on flame structure, temperature field and efficiency of an impinging laminar jet flame

An experimental study is performed to determine the combined effects of oblique angle (θ) and heating height (H) on the flame structure, temperature field and thermal efficiency of a laminar premixed methane–air flame impinging on a plane surface. A double-flame burning of a Bunsen flame under a fuel-rich condition is used. It is found that the flame structure, temperature distribution and thermal efficiency depend strongly on the oblique angle and heating height. The influence of the oblique angle on flame structure is stronger for lower heating heights but is relatively weak for higher heating heights. For normal impingement, axisymmetric temperature distributions are found, which is attributed to the axisymmetric flame structure. However, a jet flame impinging at an angle onto the horizontal plate creates an asymmetric flame structure and, thus, results in asymmetric temperature distributions. The results show that as the oblique angle is reduced from 90° to 60°, the high-temperature zone shifts toward the major flow region. An optimum operating condition of the burner, identified by the widest high-temperature zone and the highest thermal efficiency, is achieved under the condition of θ = 60° and H = 12 mm. In general, a decrease of the oblique angle leads to a decrease in thermal efficiency. However, for the cases of lower heating heights (i.e. H ⩽ 12 mm), the trend may be reversed. Furthermore, for a fixed oblique angle, as the heating height is increased, the thermal efficiency first increases to a maximum value and then decreases.

음향 가진된 프로판 비예혼합 제트 화염의 부상 거동에 대한 실험적 연구

The lift-off characteristics of lifted laminar propane jet flames highly diluted with nitrogen are investigated introducing acoustic forcing with a fuel tube resonance frequency. A flame stability curve is obtained according to forcing strength and the nozzle exit velocity for N2 diluted flames. Flame lift-off behavior is globally classified into three regimes; 1) a weakly varying partially premixed behavior caused by a collapsible mixing for large forcing strength, 2) a coexistent behavior of the edge flame and a weakly varying partially premixed behavior for moderate forcing strength, and 3) edge flame or triple flame behavior for small forcing. It is shown that the laminar lifted flame with<br/> forcing affects flame lift-off behavior considerably, and is also clarified that the flame characteristic of flame base is well described with the penetration depth of the degree of mixing, r_d It is also confirmed that the weakly varying partially premixed flame caused by a collapsible mixing for large forcing strength behaves as that just near flame blow-out in turbulent lift-off flame.<br/>

Lifted flames in laminar jets of propane in coflow air

Characteristics of lifted flames in laminar axisymmetric jets of propane with coflowing air have been investigated experimentally. Approximate solutions for velocity and concentration accounting virtual origins have been proposed for coflow jets to analyze the behavior of liftoff height. From the measurement of Rayleigh scattering to probe the concentration field of propane, the validity of the approximate solutions was substantiated. From the images of OH PLIF and CH chemiluminescence and the Rayleigh concentration measurement, it has been shown that the positions of maximum luminosity in direct photographs of lifted tribrachial flames can reasonably be located along the stoichiometric contour. The liftoff height in coflow jets was found to increase highly nonlinearly with jet velocity and was sensitive to coflow velocity. The blowout and reattachment velocities decreased linearly with the increase in coflow velocity. These behaviors of liftoff height and the conditions at reattachment and blowout can be successfully predicted from the approximate solutions.

Effects of heating height on flame appearance, temperature field and efficiency of an impinging laminar jet flame used in domestic gas stoves

Laminar jet flames are used in most domestic gas burners. However, the literature on combustion characteristics and thermal efficiencies of a single laminar impinging jet flame is very limited. Heating height is a significant operating parameter of a domestic gas stove, but it has received little attention in the literature. In this study, our aim is to simulate and examine the effect of heating height on the flame characteristics of a domestic gas stove. We emphasize the importance of heating height on flame structure, temperature distribution and thermal efficiency for low-Reynolds-number fuel-rich methane–air flames impinging normal to a plane surface, which has not been documented yet. Results show that flame structure, temperature distribution and thermal efficiency are greatly influenced by the heating height. With increasing heating height, the thermal efficiency first increases to a maximum value and then decreases. An optimum heating height, identified by the widest high temperature zone and the highest thermal efficiency, is achieved under the condition of Type-C flame burning, in which both the inner premixed flame and outer diffusion flame are open and diverge. Furthermore, we find that the optimum heating height increases with increasing methane concentration or injection velocity. Note that the maximum thermal efficiency occurs when the heating height is slightly lower than the tip of the inner rich premixed flame. This important characteristic can be applied to the design of domestic gas stoves, and it was not found in the available published work.

Visualization of lifted laminar jet flame by rayleigh scattering, OH PLIF, and CH chemiluminescence

A lifted flame in a laminar coflow jet has been vusualized, where the jet and coflow velocities were 9.0 and 4.0 cm/s, respectively, for the jet nozzle diameter of 0.25mm. To visualize the fuel concentration field for both cold (a) and reacting (b) flows, the Rayleigh scattering technique for propane fuel was adopted using an Nd:YAG laser sheet beam of 450 mJ at 532 nm. Two images of Abel transformed chemiluminescence of CH radical through a narrow band pass filter (480 nm, 10 nm FWHM) and OH PLIF image from Q1(6) line excitation are superposed on the Rayleigh image (b). The structure of the lifted flame behaving a tribrachial structure is demonstrated.

Propagation speed of tribrachial (triple) flame of propane in laminar jets under normal and micro gravity conditions

The propagation speed of tribrachial (triple) flames in laminar propane jets has been investigated experimentally under normal and micro gravity conditions. We found in the present experiment that the displacement speed varied nonlinearly with axial distance because the flow velocity along the stoichiometric contour was comparable to the propagation speed of tribrachial flame. Approximate solutions for the velocity and concentration accounting density difference and virtual origins have been used in determining the propagation speed of tribrachial flame and the concentration field was validated from the measurement of Raman scattering. Under the microgravity condition, the results showed that the propagation speed of tribrachial flame decreased with the mixture fraction gradient, in agreement with previous studies. The limiting maximum propagation speed under the microgravity condition is in good agreement with the theoretical prediction, ie, the ratio of maximum propagation speed to the stoichiometric laminar burning velocity is proportional to the square root of the density ratio of unburned to burnt mixture.

Measurements of scalar dissipation in a turbulent piloted methane/air jet flame

This paper describes the first results from a new experimental facility that combines line-imaging measurementsof Raman scattering, Rayleight scattering, and laser-induced CO fluorescence. Simultaneous single-shot line measurements of major species, temperature, mixture fraction, and the radial component of scalar dissipation are obtained in partially premixed methane/air jet flames (25% CH4, 75% air, by volume). The experimental setup and methods of data analysis are described. Results from a laminar jet flame and a piloted turbulent flame (Sandia flame D) are presented. The laminar and turbulent flames both show a local minimum in the average scalar dissipation near the stoichiometric mixture fraction. In the turbulent flame, the radial component of the instantaneous scalar dissipation near the stoichiometric condition displays a log-normal distribution at high values and an exponential distribution at low values. This is attributed to variation in the orientation of the three-dimensional mixture fraction gradient relative to the one-dimensional measurement. The effect of scalar dissipation on flame structure is examined, based on average species mass fractions doubly conditioned upon mixture fraction and scalar dissipation. The length scale of fluctuations in mixture fraction in the turbulent reaction zone is derived from the measured single-shot radial profiles at each streamwise location. This macro length scale is well resolved by the present methods, shows correlation over radial distances of a few millimeters, and grows with streamwise distance as the jet flame spreads. The length scale for fluctuations in scalar dissipation is less than 1 mm at each of the measured streamwise locations in the turbulent flame.

Dynamics of and noise radiated by a perturbed impinging premixed jet flame

This paper presents an experimental study of an acoustically excited, premixed, laminar-jet flame impinging on a water-cooled plate. Without excitation, different types of flame shapes are identified. With excitation, a strong noise is produced. The mechanism of flame-induced noise generation is investigated in detail. The results show that the modulation of the flow upstream of the flame produces strong variations in the flame’s area and also intense noise emission. The amplitude of the sound exceeds by one or two orders of magnitude that associated with a flame under the same flow modulation, but without the plate. The source of sound is identified as the cyclic extinction of flame area as the jet interacts with the cool plate. The link between the far-field pressure, the chemiluminescence from the flame and the flame surface is successfully compared with classical results of combustion-noise theory.

Feasibility of Controlling NOx Emissions from a Jet Flame by Acoustic Excitation

This paper investigates the feasibility of using acoustic excitation and coaxial air to control NOx production from a jet flame and tries to scale the NOx emission index of the jet hydrogen flame in the laminar regime. Applying both coaxial air and excitations through coaxial nozzle at low frequencies are found to be effective in reducing the flame length and the NOx emissions. A simple scaling law for NOx emission index from hydrogen jet flames with no coaxial air in the laminar regime is developed. The scaling, based on the flame length, nozzle diameter and exit velocity, for the residence time in the laminar jet flames is derived. The normalized NOx emission index, EINOX normalized by the residence time, can be scaled as the 1/2 power of the global strain rate in the laminar regime that is identical with Chen and Driscoll's (1990) scaling in the turbulent regime. This scaling result implies that effects of the buoyancy vortices, which prevail in the laminar regime, on the NOX production in a laminar hydrogen jet flame take place only through the residence time. With low frequency excitation, close to the flame flickering frequency, the normalized EINOX data have a similar power dependence, about 0.6, on the global strain rate, (UF/dF). The effect of the coaxial air on EINOX comes through reduction of the flame length. The derived simple scaling is not suitable for the coaxial-air cases. Reduction of both EINOX and EICO using acoustic excitaion with a suitable combination of frequency and amplitude is possible for propane jet flame; however, no simple scaling law is yet obtained

Reaction zone structure in turbulent nonpremixed jet flames—from CH-OH PLIF images

It is shown that simultaneous images of the CH and OH concentration fields can be obtained throughout a high-Reynolds-number (18,600) turbulent nonpremixed, nonsooting jet flame, and that the CH-OH boundary is a useful marker of the instantaneous stoichiometric contour. Previous CH-OH imaging was confined to the flame base. The structure of the fuel-decomposition zone—identified by the CH images—includes the following regions: those with high-curvature cusps; those with low CH concentration; and those where the flame “pinches” due to oxidizer being entrained to the centerline. It is found that the reaction zone that is associated with fuel decomposition (i.e., the CH layer) remains thin and rarely exceeds 1 mm, even near the tip of the high-Reynolds-number flame. CH layers in the turbulent flame are not thicker than the CH layers in the laminar jet flame at the same x/ d location. In fact, CH layer thickness is relatively insensitive to Reynolds number and the level of turbulence. This implies that turbulence does not broaden the CH reaction zone, and that flamelet concepts are justified in modeling the reaction zone associated with fuel decomposition. The CH layers become thicker in the streamwise ( x) direction, which is expected because scalar gradients and the dissipation rate are expected to decrease in the streamwise direction. Imaging the CH layer makes it possible to measure the flame surface density (Σ), which has a typical value of 0.2 mm −1. Surface density is shown to be related to the turbulent brush thickness and the degree of wrinkling.

Reaction zone structure of non-premixed turbulent flames in the “intensely wrinkled” regime

Simultaneous images of the CH and OH reaction zones are reported for “Intensely Wrinkled” nonpremixed flames, to determine whether reaction zones retain their thin “laminar flamelet” structure or become “distributed reaction zones”. Intensely Wrinkled Flames (IWFs) were achieved by using a special burner with large coflow air velocities to obtain a normalized turbulence intensity ( u′/Ū ) of 3.6, which is 10 times greater than the turbulence intensity within jet flames. The images were used to measure profiles of the flame surface density (Σ) and the average CH layer thickness ( δ¯ - CH ) it is argued that these parameters are the ones that should be used to assess new large eddy simulations (LESs), rather than insensitive parameters such as mean concentrations. In the regime of IWFs, the CH reaction zones remained as thin as those measured in laminar jet flames (i.e., less than 1 mm thick) and had the appearance of flamelets. These thin reaction zones were extinguished before they became thickned by intense turbulence, which provides experimental evidence to support laminar flamelet modeling concepts. “Shredded flames” occurred, within which the reaction zones were short, discontinuous segments, and the degree of flame wrinkling was significantly larger than in jet flames. Shredded flames have not been observed previously. There is no evidence of small-scale wrinkling of the reaction zones at scales less than half the integral scale. The images showed where the instantaneous stoichiometric contour is located, since it exists at the boundary between the CH and OH layers. Flame surface densities were typically 0.3 mm −1 .

A shutter-based line-imaging system for single-shot raman scattering measurements of gradients in mixture fraction

A line-imaging system is described that uses a high-speed, rotating shutter, to reject background luminosity and allow one-dimensional Raman scattering measurements to be obtained without the limitations in dynamic range and signal-to-noise ratio that are associated with image intensifiers. The rotating shutter replaces the entrance slit of an imaging spectrograph, which is combined with a cryogenically cooled, backilluminated CCD array. This system was developed for the purpose of obtaining simultaneous single-shot measurements of the major species concentrations, the mixture fraction, and the gradients of these scalars along a line in turbulent non-premixed flames, including those with simple hydrocarbon fuels. The performance of the shutter-based system was demonstrated through experiments in a laminar jet flame having the same fuel composition (25% CH4 and 75% air) used in previous experiments on piloted jet flames. Time-averaged, high-resolution spectra are presented to document the nature of hydrocarbon fluorescence interference in such flames. Single-shot measurements are obtained using on-chip binning to form superpixels, a process that minimizes instrument noise. The demonstration experiment made use of f/3.45 collection optics and 450 mJ/pulse of laser energy at 532 nm. The precision of the measurement of species concentrations, the mixture fraction, and the radial gradient of mixture fraction in the laminar flame is analyzed in terms of signal-to-noise ratios and standard deviations. This analysis indicates that planned experiments using f/2 optics and approximately 2.6 J/pulse should yield relative standard deviations of the conditional scalar dissipation of roughly 13% at xst=10s−1 and 6% at Xst=160 s−1. This level of precision will be useful in validating models for conditional scalar dissipation in turbulent nonpremixed flames.

Triple flame structure and dynamics at the stabilization point of an unsteady lifted jet diffusion flame

We present results of a numerical study of a forced lifted laminar two-dimensional jet flame using a single-step irreversible global mechanism with particular emphasis on the structure and dynamics of the flame base. A coupled Lagrangian-Eulerian low Mach number numerical scheme was developed to solve the governing equations. Finite difference discretization was used with adaptive mesh refinement for the scalar conservation equations, while the vortex method was adopted for the momentum equation. The flame base stabilized in a region where the flow velocity was sufficiently small, and there was adequate premixing of the fuel and oxidizer streams. A triple flame was observed at the flame base and was studied with respect to its global structure, dynamics, and modulation by an unsteady vortex-generated strain field. We studied the unsteady flow field and heat release rate of the flame base as it was entrained, stretched, and contorted by the passing vortex before returning to the original configuration at the termination of the interaction. We observed stretching of the rich triple flame branch associated with the entrainment and isolation of a pocket of coflow air in the jet. We correlated velocity and strain-rate fluctuations at the flame base with changes in peak heat release rate. Given the size of the triple flame, neither the dilatational nor the temperature field were found appropriate for experimental measurement of the triple flame.

Propagation of unsteady tribrachial flames in laminar non-premixed jets

The characteristics of propagating tribrachial flames in non-premixed laminar jets have been investigated experimentally. Free jets of methane are ignited downstream with a Nd:YAG laser to eliminate possible flow disturbances during ignition. The flame propagation is visualized and its speed measured by a high-speed shadowgraphy and schlieren technique. Results show that the edge of the propagating flame in a laminar jet regime has a tribrachial flame structure: a lean premixed flame, a rich premixed flame, and a diffusion flame, all extending from a single location. The flame displacement speed maintains a constant value for a specified flow rate and decreases with jet velocity. Flame propagation speeds of tribrachial flames have been determined from the velocity fields measured by laser Doppler velocimetry and from jet velocity modeling. This flame speed correlates well with flame curvature and mixture fraction gradient. It is found that the unsteady curvature term dominates over the steady curvature term in flame stretch and that the flame propagation speed correlates well and decreases with flame stretch. © 1998 by The Combustion Institute