Turbulent mixing of a passive scalar in confined multiple jet flows of a micro combustor

The turbulent mixing characteristics of multiple jet flows in a micro can type combustor are investigated by means of large eddy simulation (LES). The micro combustor can be used for a micro gas turbine which is hybridized with solid oxide fuel cell. Attention is paid for a micro combustor having a circular disk baffle plate with a fuel injection nozzle in the center and oxidant injection holes allocated annularly. Downstream the baffle plate, a complex flow is produced from the interaction of multiple jet flows and study is made for three different configurations of the baffle plates resulting in different mixing pattern. From the results, it is substantiated that the turbulent mixing is promoted by complex flow fields caused by the jet flows and large vortical flow regions in the micro combustor. This is effective to accelerate the slow mixing between fuel and oxidant suffering from low Reynolds number in such a small combustor. In particular, the vortical flow region formed downstream the fuel jet core region plays an important role for rapid mixing coupled with another flow recirculation region. Discussion is made for the instantaneous and time and space averaged flow and passive scalar quantities which show peculiar turbulent flow and mixing characteristics corresponding to the different flow structures for each baffle plate shapes, respectively.     

Flame stabilization in a lifted non-premixed turbulent hydrogen jet with coaxial air

To understand hydrogen jet liftoff height, the stabilization mechanism of turbulent lifted jet flames under non-premixed conditions was studied. The objectives were to determine flame stability mechanisms, to analyze flame structure, and to characterize the lifted jet at the flame stabilization point. Hydrogen flow velocity varied from 100 to 300 m/s. Coaxial air velocity was regulated from 12 to 20 m/s. Simultaneous velocity field and reaction zone measurements used, PIV/OH PLIF techniques with Nd:YAG lasers and CCD/ICCD cameras. 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 non-premixed turbulent lifted hydrogen jet flames had two types of flame structure for both thin and thick flame base. Lifted flame stabilization was related to local principal strain rate and turbulent intensity, assuming that combustion occurs where local flow velocity and turbulent flame propagation velocity are balanced.     

Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging

Low NOx burner and air staged combustion are widely applied to control NOx emission in coal-fired power plants. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single low NOx swirl burner in an existing coal-fired boiler was numerically simulated to analyze the mechanisms of flame stability and in-flame NOx reduction. And the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize NOx emissions for the low NOx burner with air staged combustion of different burner stoichiometric ratios. The results show that the specially-designed swirl burner structures including the pulverized coal concentrator, flame stabilizing ring and baffle plate create an ignition region of high gas temperature, proper oxygen concentration and high pulverized coal concentration near the annular recirculation zone at the burner outlet for flame stability. At the same time, the annular recirculation zone is generated between the primary and secondary air jets to promote the rapid ignition and combustion of pulverized coal particles to consume oxygen, and then a reducing region is formed as fuel-rich environment to contribute to in-flame NO_X reduction. Moreover, the NOx concentration at the outlet of the combustion chamber is greatly reduced when the deep air staged combustion with the burner stoichiometric ratio of 0.75 is adopted, and the CO concentration at the outlet of the combustion chamber can be maintained simultaneously at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for lower NOx emission in the existing coal-fired boilers.     

Dynamics of an edge flame in a mixing layer

The dynamics of an edge flame in a mixing layer is considered. The flame, which stands at a well-defined distance from the plate separating the fuel and oxidizer streams, is stabilized by heat conducted back to the relatively cold plate. It has a tribrachial structure that consists of a lean premixed segment leaning toward the oxidizer side, a rich premixed segment leaning toward the fuel side, and a diffusion flame trailing behind. Within the context of a diffusive-thermal model, we describe numerically the steady and unsteady behavior of the flame. The primary objective is to systematically identify the conditions for the onset of oscillations examining, in particular, the influence of differential diffusion, mixture strength, flow rate, and radiative heat losses.     

The jet mixing effect on reaction flow in a bluff-body burner

This study investigates the mixing effects of primary and secondary jets on flame stability as fuel/air is injected into a bluff-body burner. A two-dimensional spray combustion model, based upon a SIMPLER method, is used for numerical studies of combusting flow. The influence of the jet-to-air velocity ratio on the recirculation zone behind the bluff body, the center axial velocity and the temperature profiles is studied in detail. The results show that mixing between the two jets is controlled by two vortex eddies on the inside and outside of the bluff-body. With a proper bluff-body blockage ratio, cone angle and jet-to-air velocity ratio a more stable flame can be achieved.     

Free radical imaging techniques applied to hydrocarbon flames diagnosis

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Structure of a reacting hydrocarbon-air planar mixing layer

The structure of a reacting hydrocarbon-air two-stream planar mixing layer was investigated experimentally with non-premixed reactants under pressurized conditions. Propane and dimethyl ether (DME) diluted with argon or nitrogen was used as the fuel stream while heated air was used as the oxidizer. Experiments were performed at a range of Reynolds numbers in both the pre- and post-mixing transition portions of the mixing layer under conditions where the lean reactant (air) was placed in either the high-speed (AHS) or low-speed stream (FHS). The reacting mixing layer was visualized using a combined OH LIF/soot LII technique, wherein the reaction zone and the region of parent fuel entrainment and decomposition were simultaneously imaged. In both AHS and FHS cases at all Reynolds numbers examined, the mixing layer consisted of two regions: a high temperature reaction zone with a laminar appearance found on the oxidizer side of the mixing layer and an ‘internal’ mixing layer in which products mixed with pyrolized fuel in a manner reminiscent of a two-stream non-reacting mixing layer. The location and dynamics of the soot formed within the mixing layer were closely related to the mixing behavior of the large-scale structures. The regions of highest soot volume fraction were found in low temperature regions near the location of raw fuel entrainment. There was no significant broadening of the high-temperature reaction zone or increase in flame area under turbulent conditions due to the lack of dilution of the freestream conditions, unlike previous observations in jet flames. Changes in the inlet streams which affected chemistry did not appear to cause significant changes in the overall mixing layer structure shown in the OH/LII images. However, finite-chemistry effects were discernable with temperature measurements and indicated that reduced product formation was observed with reductions in a characteristic Damkohler number, Da. The point of flame lift-off was shown to occur at Da < 1 over a wide range of operating conditions that caused changes in mixing or chemistry.     

Numerical study for enhancement of laminar flow mixing using multiple confined jets in a micro-can combustor

To meet demands arising as a result of present trends towards miniaturization, an innovative design for promoting mixing enhancement in a miniature can combustor is investigated using an unstructured finite-volume technique. A multi-holed baffle plate is employed to create a ring of oxidizer jets surrounding a single fuel jet in parallel with the axis of the cylindrical chamber. The baffle plate is found to produce a dramatic improvement to the mixing performance when compared with simpler co-axial jet cases. Relatively small changes in geometry are found to have a major influence on mixing for laminar isothermal flow.     

Measurements of fuel mixture fraction oscillations of a turbulent jet non-premixed flame

This work describes new type of combustion instability for which the 3-way coupling between mixing, flame heat release, and acoustics is modified by local buoyancy effects. Measurements of fuel mixture fraction are made for a non-premixed jet flame in a combustion chamber to assess the dynamics of mixing under imposed acoustic oscillations (22-55 Hz). Infrared laser absorption and phase resolved acetone-planar laser induced fluorescence are used to measure the fuel mixture fraction and then the degree of fuel/air mixing is calculated by determining the unmixedness. Results show acoustic excitation causes oscillations in the degree of fuel/air mixing at the driving frequency, which results in oscillatory flame behavior. This oscillatory flame behavior couples to the buoyancy and this in turn affects the mixing. Results also show that the mixing becomes less effective when the excitation frequency is increased or when the flame is present, compared to the non-reacting case. This work describes a key coupling mechanism that occurs when buoyancy is a significant factor in the flow field.     

Experimental investigation of the effects of heat release on mixing processes and flow structure in a high-speed subsonic turbulent H2 jet

In this paper, we explore the effects of heat release on mixing and flow structure in a high-speed subsonic turbulent H₂ jet in an air coflow. Heat release effects are determined from the comparison of nonreacting and reacting jet behavior, boundary conditions being identical in both cases. Experiments are performed in a wind tunnel specifically designed for this purpose. Planar laser induced fluorescence on OH radicals and on acetone (seeded in the hydrogen jet) are used to characterize the cartography of scalars, and laser Doppler velocimetry is used to characterize velocity profiles in the far field of the H₂ jet. Results show significant effects of heat release on mixing and flow structure, indicating an overall reduction of mixing and entrainment in the reacting jet compared to the nonreacting jet. First, a change is observed in the orientation of coherent structures originating from Kelvin-Helmholtz type instabilities, and responsible for air entrainment within the jet, which appear “flatter” in the jet flame. Then, the flame length is increased over what would be predicted from the intersection of the mean stoichiometric contour with the centerline of the nonreacting jet. And finally, the longitudinal average velocity decrease along the jet axis is quicker in the nonreacting jet, and nondimensional transverse velocity fluctuations are about half as high in the reacting jet as in the nonreacting jet, indicating a reduction of the turbulence intensity of the flow in this direction in the jet flame.     

Heat transfer in a PEMFC flow channel

A numerical method was applied to the heat transfer performance in the flow channel for a proton exchange membrane fuel cell (PEMFC) using the finite element method (FEM). The heat transfer enhancement has been analyzed by transversely installing a baffle plate and a rectangular cylinder to manage flow pattern in the flow channel of the fuel cell. Case studies include baffle plates (gap ratios from 00.05 to 0.2) and the rectangular cylinder (width-to-height ratios from 0.66 to 1.66 with a constant gap ratio of 0.2; various gap ratios from 0.05 to 0.3 with a constant width-to-height ratio 1.0) at constant Reynolds number. The results show that the transverse installation of a baffle plate and a rectangular cylinder in the flow channel can effectively enhance the local heat transfer performance of a PEMFC. The installation of a rectangular cylinder has a better effective heat transfer performance than a baffle plate; the larger the width of the cylinder is the better effective heat transfer performance becomes.     

Effects of incident shock wave on mixing and flame holding of hydrogen in supersonic air flow

The effects of incident shock wave on mixing and flame holding of hydrogen in supersonic airflow have been studied numerically. The considered flow field was including of a sonic transverse hydrogen jet injected in a supersonic air stream. Under-expanded hydrogen jet was injected from a slot injector. Flow structure and fuel/air mixing mechanism were investigated numerically. Three-dimensional Navier–Stokes equations were solved along with SST k-ω turbulence model using OpenFOAM CFD toolbox. Impact of intersection point of incident shock and fuel jet on the flame stability was studied. According to the results, without oblique shock, mixing occurs at a low rate. When the intersection of incident shock and the lower surface is at upstream of the injection slot; no significant change occurs in the structure of the flow field at downstream. However when the intersection moves toward downstream of injection slot; dimensions of the recirculation zone and hydrogen-air mixing rate increase simultaneously. Consequently, an enhanced mixing zone occurs downstream of the injection slot which leads to flame-holding.     

Reexamination on methane/oxygen combustion in a rapidly mixed type tubular flame burner

To fundamentally elucidate the requirement for an inherently safe technique of rapidly mixed type tubular flame combustion, experiments have been made to investigate (1) the mixing process of fuel and oxidizer, and (2) the appearances of methane flames under various oxygen mole fractions. Three optically accessible quartz burners of different slit widths were made for measuring the mixing layer thickness with a PIV system. Under various rates of flow of the oxidizer to the fuel, a boundary layer type flow is recognized to dominate the mixing of fuel and oxidizer around the exit of the injection slit, namely the mixing layer thickness is inversely proportional to the square root of mean injection velocity. Using two stainless steel burners, combustion tests were conducted with the oxidizers of oxygen/air mixtures. To quantitatively investigate the requirement for tubular flame establishment, the Damköhler number, which is the ratio of characteristic mixing time to characteristic chemical reaction time, has been discussed in detail. The mixing time was calculated according to estimated mixing layer thickness, while the chemical reaction time was computed with the Chemkin code. The Damköhler number has proved to be a useful measure for success/failure of tubular flame combustion. When the Damköhler number is larger than unity, chemical reaction starts before complete fuel/air mixing and the tubular flame fails to be established; when the Damköhler number is much smaller than unity, the fuel and the oxidizer are completely mixed before the onset of reaction, resulting in successful tubular flame combustion. The results confirm our hypothesis in a previous study. Furthermore, based on the concept of Damköhler number, the minimum flow rate to achieve the tubular flame combustion could be estimated.     

Hysteresis and transition in swirling nonpremixed flames

Strongly swirling nonpremixed flames are known to exhibit a hysteresis when transiting from an attached long, sooty, yellow flame to a short lifted blue flame, and vice versa. The upward transition (by increasing the air and fuel flow rates) corresponds to a vortex breakdown, i.e. an abrupt change from an attached swirling flame (unidirectional or with a weak bluff-body recirculation), to a lifted flame with a strong toroidal vortex occupying the bulk of the flame. Despite dramatic differences in their structures, mixing intensities and combustion performance, both flame types can be realised at identical flow rates, equivalence ratio and swirl intensity. We report here on comprehensive investigations of the two flame regimes at the same conditions in a well-controlled experiment in which the swirl was generated by the rotating outer pipe of the annular burner air passage. Fluid velocity measured with PIV (particle image velocimetry), the qualitative detection of reaction zones from OH PLIF (planar laser-induced fluorescence) and the temperature measured by CARS (coherent anti-Stokes Raman spectroscopy) revealed major differences in vortical structures, turbulence, mixing and reaction intensities in the two flames. We discuss the transition mechanism and arguments for the improved mixing, compact size and a broader stability range of the blue flame in comparison to the long yellow flame.     

Stratified jet flames in a heated (1390 K) air cross-flow with autoignition

Measurements are reported of the heat release profiles, the flame lengths, flame structure and other properties of a reacting jet-in-cross-flow (JICF) for two fuels. The air was heated to a static temperature of 1390 K, which is above the autoignition temperature, and the air velocity was 468 m/s, which is much larger than values that were considered previously. Aerodynamic strain rates are so large that the flame was expected to fall into either the “distributed reaction”, “thickened flamelet”, or “shredded flamelet” regimes. Fluorescence images of CH, OH and formaldehyde identified the flame structure. The jet-in-cross-flow is a unit physics problem that occurs in turbojets and scramjets. While scaling relations are known for the non-reacting case, more information about the reacting case is needed, especially when autoignition and strain rates become important. Three regions were identified. In the liftoff region autoignition reactions occur which create a strong formaldehyde PLIF signal. However, flames and heat release do not occur in the liftoff region since CH and CH¹ signals were negligible. The second region is the lifted flame base, which has the character of a premixed flame, as evidenced by a very rapid rise in the heat release rate as indicated by the CH¹ and OH¹ signals. The third region contains a turbulent non-premixed flame and the CH images indicate the presence of thickened and shredded flamelets. The 2–3 mm thickness of each CH layer is more than 10 times the laminar flamelet thickness. In the third region the heat release rate decays slowly downstream, which is typical of a non-premixed flame. Because both upstream autoignition and downstream thickened flamelets were observed, we classify this combustion to be an “autoignition-assisted flame”. Flame lengths increase linearly with fuel mass flow rate, indicating that mixing is controlled by the air velocity rather than the fuel velocity.     

Experimental and numerical analysis of oxy‐fuel combustion in a porous plate reactor

The present work focuses on studying experimentally and numerically the oxy‐fuel combustion characteristics inside a porous plate reactor towards the application of oxy‐combustion carbon capture technology. Initially, non‐reactive flow experiments are performed to analyze the permeation rate of oxygen in order to obtain the desired stoichiometric ratios. A numerical model is developed for non‐reactive and reactive flow cases. The model is validated against the presently recorded experimental data for the non‐reacting flow cases, and it is validated against the available literature data for oxy‐fuel combustion for the reacting flow cases. A modified two‐step oxy‐combustion reaction kinetics model for methane is implemented in the present model. Simulations are performed over wide range of operating oxidizer ratios (O₂/CO₂ ratio), from OR = 0.2 to OR = 0.4, and over wide range of equivalence ratios, from φ = 0.7 to φ = 1.0. The flame length was decreased as a result of the increase of the oxidizer ratio. Effects of CO₂ recirculation amount on the oxy‐combustion flame stability are examined. A reduction in combustion temperature and increase in flame fluctuations are encountered while increasing CO₂ concentration inside the reactor. At high equivalence ratio, the combustion temperature and flame stability are improved. At low equivalence ratio, the flame length is increased, and the flame was moved towards the reactor center line. Copyright © 2015 John Wiley & Sons, Ltd.     

Investigations of swirl flames in a gas turbine model combustor: II. Turbulence–chemistry interactions

The thermochemical states of three swirling CH₄/air diffusion flames, stabilized in a gas turbine model combustor, were investigated using laser Raman scattering. The flames were operated at different thermal powers and air/fuel ratios and exhibited different flame behavior with respect to flame instabilities. They had previously been characterized with respect to their flame structures, velocity fields, and mean values of temperature, major species concentrations, and mixture fraction. The single-pulse multispecies measurements presented in this article revealed very rapid mixing of fuel and air, accompanied by strong effects of turbulence–chemistry interactions in the form of local flame extinction and ignition delay. Flame stabilization is accomplished mainly by hot and relatively fuel-rich combustion products, which are transported back to the flame root within an inner recirculation zone. The flames are not attached to the fuel nozzle, and are stabilized approximately 10 mm above the fuel nozzle, where fuel and air are partially premixed before ignition. The mixing and reaction progress in this area are discussed in detail. The flames are short (<50 mm), especially that exhibiting thermoacoustic oscillations, and reach a thermochemical state close to adiabatic equilibrium at the flame tip. The main goals of this article are to outline results that yield deeper insight into the combustion of gas turbine flames and to establish an experimental database for the validation of numerical models.     

High-fidelity resolution of the characteristic structures of a supersonic hydrogen jet flame with heated co-flow air

In the present paper, direct numerical simulation (DNS) is performed to analyze the characteristic structures of a supersonic jet lifted hydrogen-air flame with Reynolds number of 22, 000, and Mach number of 1.2. The fuel consisting of 85% H₂ and 15% N₂ by volume is injected into hot co-flow air from a round orifice. Overall 975 million grids are used to compute the complex multi-scales phenomena. A Damköhler number and a flame index are defined to analyze combustion modes and the mixedness of the flame. Complicated characteristic elements of the supersonic jet lifted flame are observed, i.e. a stable laminar flame base with auto-ignition as the stabilization mechanism, a violent mixing region in which vigorous turbulent combustion occurs with both fuel-lean and fuel-rich mixtures, and a flame region consisting of outer diffusion combustion and inner weaker premixed combustion in the far field. At the leading edge of the fame base, auto-ignition takes place primarily in the fuel-lean mixture where the mixedness mode is opposed. Downstream of the laminar flame base, the combustion becomes turbulent due to the intensified mixing of fuel and air, which results in the subequilibrium values of temperature and OH concentration. Detonation occurs near the sonic layer, and then sustains the combustion in higher dissipative mixture. The flame near the stochiometric condition keeps non-premixed, and the other non-premixed flame elements could be observed in the very fuel-rich region. Through the reacting field the premixed flame appears near the shear layer. The combustion intensity decreases in the far field where the inner non-premixed flame disappears gradually.     

Characteristics of flame stability and gaseous emission of biocrude-oil/ethanol blends in a pilot-scale spray burner

A burner system with capacity of 30,000 kcal/h was designed for the combustion of biocrude-oil and ethanol blends. An air atomizing spray nozzle with larger fuel orifice was adopted to prevent nozzle clogging, with swirl flow introduced to the combustion air for flame stabilization. Biocrude-oil was prepared from the fast pyrolysis of woody biomass and was blended with ethanol to improve flame stability and ignition characteristics. At various mixing ratios of biocrude-oil and ethanol, flame stability was determined, and gaseous emissions of CO and NO were measured. It was found that stable combustion could be achieved with up to 90 vol% of biocrude-oil. CO emissions of biocrude-oil/ethanol blends were smaller than those of pure ethanol, whereas CO concentration increased significantly in case of pure biocrude-oil due to incomplete combustion. Pollutant NO emission increased slightly with the biocrude-oil mixing ratio. The biocrude-oil burner in this study could provide a design database for industrial burner development.     

Experimental investigation of the morphology and stability of diffusion and edge flames in an opposed jet burner

The stability of methane/air and hydrogen/air flames in an axisymmetric counterflow burner was investigated experimentally for different burner geometries, degrees of fuel dilution, and combinations of flow velocities. Both planar diffusion flames and edge flames were observed, and the transitions between these flame types were studied. The experimental results confirmed previously published numerical predictions on diluted hydrogen/air flames: the existence of two distinct stable flame types; the possibility of switching between the two flame types by perturbing the flames, e.g., by suitably changing a flow velocity; and the strong hysteresis for the transition from one flame type to the other. Flame stability diagrams were compiled which delineate the range of fuel and air flow velocities for which the planar diffusion flame and the toroidal edge flame are stable. The lower boundary curve for the edge flame stability exhibits a characteristic minimum at a well-defined value of the fuel velocity. For fuel velocities lower than this value, the transition between the edge and the diffusion structure is reversible, and the flames exhibit bistable behavior. For higher fuel velocities, the decrease of air velocity leads to the extinction of the edge flame. An investigation of both the cold and the reactive flow field identified bistable behavior for the flow field as well. Except for very low flow rates, the stagnation plane stabilizes in two positions, close to either of the two nozzles. Detailed numerical simulations of hydrogen flames capture the essentials of this behavior. The observed flame extinction results from the interaction of the flame dynamics with the dynamics of the flow field.