Non-premixed Jet Research Articles

Global reaction mechanisms for MILD oxy-combustion of methane

This paper optimizes global reaction mechanisms under the MILD (Moderate and Intensive Low-oxygen Dilution) oxy-combustion combustion. Seven global mechanisms are compared and validated with two detailed mechanisms under the oxy-fuel combustion, MILD air-combustion, and MILD oxy-combustion conditions. The ability of these global models to capture the combustion process under different conditions is compared first using computational fluid dynamics (CFD) simulations of both the MILD oxy-combustion in a laboratory-scale furnace and non-premixed turbulent jet open flames, and later using a plug flow reactor (PFR) approach. Experiments of the MILD oxy-combustion are also carried out for the mechanism validation. The detailed comparison shows that the present optimized global mechanism significantly improves the prediction of temperatures, equilibrium concentrations of major species, and the peak CO concentration, relative to other global mechanisms, for the MILD oxy-combustion. The present refined mechanism is an appropriate global reaction mechanism for methane MILD oxy-combustion, if the computational cost of detailed reaction mechanism is unaffordable.

Plasma-Assisted Stabilization of Lifted Non-premixed Jet Flames

The present study experimentally investigates the effects of plasma discharges on the stabilization of lifted non-premixed jet flames in a stream of co-flow air. The plasma discharge is produced on the sharp edge of the fuel nozzle exit, facilitating its impact on flame stabilization. It is observed that the application of plasma discharges has an impact on the enhancement of flame lift-off velocity, lift-off height, and hysteresis phenomenon and leads to plasma-attached flames, plasma-enhanced lifted flames, and plasma-ineffective lifted flames, depending on flame lift-off conditions. A maximum enhancement of approximately 84% is observed for the flame lift-off velocity when the co-flow velocity is sufficiently low. As the co-flow velocity or the jet Reynolds number is low, the flame is anchored at the nozzle by the discharge. As the co-flow velocity or the jet Reynolds number is increased, the flame detaches but with a decrease in lift-off height compared to the flame without the discharge. If the co-fl...

Fuel pyrolysis and its effects on soot formation in non-premixed laminar jet flames of methane, propane, and DME

High-temperature combustion techniques have recently attracted interest with regard to the improvement of the thermal efficiency of combustion systems. Fuel pyrolysis is an important factor, as it can affect such flame structures at high temperatures. In this study, the pyrolysis of methane, propane, and dimethyl ether (DME) was measured and the results were compared with theoretical predictions. Pyrolyzed fuels were quenched to room temperature before being introduced onto the burner. Thus, the pyrolysis effects on laminar non-premixed jet flames could be distinguished from many other complex thermal effects. It was found that the flame length was not notably extended in spite of the great increase in the volumetric flow rates resulting from the pyrolysis. In contrast, fuel pyrolysis could significantly affect the soot formation process,and the number of smoke points could be sharply reduced depending on the pyrolysis temperature. Distributions of the luminous intensity and scattering intensity levels in the soot region were discussed in terms of the soot temperatures obtained with a two-color method. Although the adiabatic flame temperatures of the pyrolyzed fuels were theoretically increased, the actual soot temperatures could be reduced when the soot particles were excessively formulated, as in the case with propane flames.

Tomographic imaging of OH laser-induced fluorescence in laminar and turbulent jet flames

In this paper a new approach for 3D flame structure diagnostics using tomographic laser-induced fluorescence (Tomo-LIF) of the OH radical was evaluated. The approach combined volumetric illumination with a multi-camera detection system of eight views. Single-shot measurements were performed in a methane/air premixed laminar flame and in a non-premixed turbulent methane jet flame. 3D OH fluorescence distributions in the flames were reconstructed using the simultaneous multiplicative algebraic reconstruction technique. The tomographic measurements were compared and validated against results of OH-PLIF in the laminar flame. The effects of the experimental setup of the detection system and the size of the volumetric illumination on the quality of the tomographic reconstructions were evaluated. Results revealed that the Tomo-LIF is suitable for volumetric reconstruction of flame structures with acceptable spatial resolution and uncertainty. It was found that the number of views and their angular orientation have a strong influence on the quality and accuracy of the tomographic reconstruction while the illumination volume thickness influences mainly the spatial resolution.

Numerical studies of CO formation during biogas combustion

This work deals with the LES (Large Eddy Simulation) of methane (CH4) and biogas non-premixed jet flames with the goal of examining the impact of CO2 addition on the formation of carbon monoxide. To model the chemistry, Equilibrium Chemistry (EC) as the cheapest approach was considered first, followed by a global kinetic model and a reduced reaction mechanism describing the combustion of syngas (CO + H2) combined with the reaction CH4 + 0.5 O2 → CO+ 2 H2. Highly diluted laminar jet flames were studies first to decouple the chemistry from the turbulence. All three models predict a considerable increase in the mass fractions of CO in the presence of CO2 in the fuel. Turbulent jet flames were subsequently considered which could not be simulated using equilibrium chemistry owing to numerical problems. The two chemical kinetic models predict a strong increase in CO.

A conservative, thermodynamically consistent numerical approach for low Mach number combustion. Part I: Single-level integration

We present a numerical approach for low Mach number combustion that conserves both mass and energy while remaining on the equation of state to a desired tolerance. We present both unconfined and confined cases, where in the latter the ambient pressure changes over time. Our overall scheme is a projection method for the velocity coupled to a multi-implicit spectral deferred corrections (SDC) approach to integrate the mass and energy equations. The iterative nature of SDC methods allows us to incorporate a series of pressure discrepancy corrections naturally that lead to additional mass and energy influx/outflux in each finite volume cell in order to satisfy the equation of state. The method is second order, and satisfies the equation of state to a desired tolerance with increasing iterations. Motivated by experimental results, we test our algorithm on hydrogen flames with detailed kinetics. We examine the morphology of thermodiffusively unstable cylindrical premixed flames in high-pressure environments for confined and unconfined cases. We also demonstrate that our algorithm maintains the equation of state for premixed methane flames and non-premixed dimethyl ether jet flames.

The effect of exit Reynolds number on soot volume fraction in turbulent non-premixed jet flames

Soot volume fraction (SVF) was measured in five attached turbulent non-premixed jet flames of a C2H4H2N2 fuel mixture using the Laser-Induced Incandescence (LII) technique. The five flames comprise two sets with exit strain rates of 4100 and 7500 s−1, respectively. Within each set, the exit Reynolds number was changed both by varying the jet diameter and the fuel exit velocity of the flames. Measurements of the mean, instantaneous and integrated SVF reveal a weak inverse dependence on the exit Reynolds number. A minor dependence of the axial and radial profiles of soot intermittency on the exit Reynolds number is also observed. The total soot yield is found to scale linearly with both the jet exit diameter and the fuel flow rate for the two flame sets of different exit strain. The total soot yield is also found to be a strong function of both the exit strain and the flame volume, but to be almost independent of the exit Reynolds number. A non-negligible effect of buoyancy on SVF is also deduced from the global correlations.

The influence of pilot injection on high-temperature ignition processes and early flame structure in a high-speed direct injection diesel engine

Simultaneous high-speed natural luminosity and OH* chemiluminescence imaging is used to characterize high-temperature ignition processes in conventional diesel combustion with a pilot-main injection strategy in a single-cylinder, light-duty optical diesel engine. High-speed imaging provides temporally and spatially resolved information in terms of high-temperature ignition processes and flame structure during the combustion. Using these imaging measurements, the high-temperature inflammation and the diffusion flame development processes are analyzed. The chemiluminescence signal shows a hot, reactive mixture, which gradually decreases after the peak release of the pilot combustion and lasts long after the apparent heat release has ended. Therefore, when the reactive pilot mixture exists near the main injection jets, the high-temperature ignition of the main injection is apparently initiated through interactions with the reactive pilot mixture. High-temperature autoignition, another process by which ignition of the main injection occurs, is observed in main injection plumes where the chemiluminescence signal of the reactive pilot mixture becomes very weak or is absent at the start of main injection. As the reaction of the main injection continues, the non-premixed main injection jet structure is developed and the high-temperature reacting region expands throughout the jet.

Dissipation elements method provides analysis to understand turbulent non-premixed jet flames

Through direct numerical simulation, supercomputer aids dissipation elements analysis to bridge the gap between chemistry and turbulence.

Dissipation element analysis of a turbulent non-premixed jet flame

The objective of the present work is to examine the interaction between turbulent mixing and chemistry by employing the method of dissipation elements in a non-premixed turbulent jet flame. The method of dissipation elements [L. Wang and N. Peters, J. Fluid Mech. 554, 457–475 (2006)] is used to perform a space-filling decomposition of the turbulent jet flow into different regimes conditioned on their location with respect to the reaction zone. Based on the non-local structure of dissipation elements, this decomposition allows us to discern whether points away from stoichiometry are connected through a diffusive layer with the reaction zone. In a next step, a regime based statistical analysis of dissipation elements is carried out by means of data obtained from a direct numerical simulation. Turbulent mixing and chemical reactions depend strongly on the mixture fraction gradient. From a budget between strain and dissipation, the mechanism for the formation and destruction of mean gradients along dissipation elements is inspected. This budget reveals that large gradients in the mixture fraction field occur at a small but finite length scale. Finally, the inner structure of dissipation elements is examined by computing statistics along gradient trajectories of the mixture fraction field. Thereby, the method of dissipation elements provides a statistical characterization of flamelets and novel insight into the interaction between chemistry and turbulence.

Numerical modeling for structure and NOx formation characteristics of oxygen-enriched syngas turbulent non-premixed jet flames

The present study numerically investigated the effect of oxygen enrichment on the precise structure and NOx formation characteristics of turbulent syngas non-premixed flames. The turbulence-chemistry interactions were represented by a Lagrangian flamelet model. In context with the Lagrangian flamelet model, the NO concentration was obtained directly from the flamelet calculation based on full NOx chemistry, with radiative heat loss being accounted for through the flamelet energy equation. Computations were performed for three different syngas compositions with a designated nitrogen dilution level. Numerical results indicated that, for the CO-rich composition with the lowest LHV yielding the highest scalar dissipation rate and shortest flight time, the flame structure was dominantly influenced by turbulence-chemistry interactions. On the other hand, with regard to the H2-rich composition with the highest LHV yielding the lowest injection velocity and longest flight time, the flame structure was strongly influenced by radiative cooling. The peak NO level was remarkably elevated by increased oxygen level due to the elevated temperature of the oxygen-enriched flame. In the enhanced oxygen level (30%), the H2-rich case produced the highest NO level due to a higher temperature and longer residence time within the hot flame zone, while the CO-rich case yielded the lowest NO level due to a lower temperature and shorter residence time. It was also found that, by enhancing the oxygen level, contributions of NNH and N2O to total NO emission rapidly decreased while the contributions of the thermal NO path were progressively dominant for all cases.

Flame downwash length evolution of non-premixed gaseous fuel jets in cross-flow: Experiments and a new correlation

Flame downwash behavior (flame pulled by the wake flow and attached to the leeward side of the nozzle) of gaseous fuel jets in cross-flow is of much practical importance in the design of burners as well as industrial flare, and thus important for energy conversion and conservation; however, the studies are still very limited. The critical condition (i.e., critical cross-flow air speed) for flame downwash occurrence as well as the evolution of flame downwash length for various fuel jet exit velocities has not been quantified yet. In this work, the flame downwash length evolution of non-premixed jets for different pipe nozzle diameters at various fuel discharge velocities and cross-flow air speeds as well as the critical condition under which the flame downwash occurs have been quantified comprehensively. Pipe nozzles with inner diameters of 8, 10, 13 and 15mm were employed in the experiments, using propane as the fuel and with fuel jet velocities ranging between 0.38 and 2.42m/s. The experimental results showed that the flame downwash length increased with increasing cross-flow air speed for a given fuel jet exit velocity. It was also found that, with increasing cross-flow air speed, the downwash length increased more rapidly for the higher fuel jet exit velocity than that for the lower fuel jet velocity. The critical cross-flow air speed, when flame downwash occurs, was found to be little dependent on the nozzle diameter but increase with the fuel jet exit velocity following a linear relationship. A new correlation for flame downwash length was proposed based on physically the coupling effects due to the competition of the fuel jet momentum to the cross-flow momentum and the total fuel mass supply. The proposed correlation was shown to well characterize the flame downwash length in terms of nozzle inner diameters, dimensionless fuel mass flow rates and the jet momentum ratio. The findings of this work provide basic knowledge and have potential practical applications for burner design and flare implementation, which allow predictions to be made regarding the possible threat and the establishment of the necessary safety length for the stack to prevent the damage for reducing the potential risk.

Flame heights of line-source buoyant turbulent non-premixed jets with air entrainment constraint by two parallel side walls

This paper investigates the flame height of line-source buoyant turbulent non-premixed jets with air entrainment constraint by two parallel side walls at various separation distances. Laboratory-scale experiments are carried out by employing two line-source nozzles (2mm×142.5mm and 2mm×217mm) using propane as fuel. The nozzles are arranged with two kinds of orientations relative to the side walls, i.e. with its longer or shorter side perpendicular to the side wall. The flame heights are measured for various fuel supply exit velocities and side walls separation distances. Results show that the flame height changes little with side walls separation distance when the longer side of the nozzle is perpendicular to the side walls; meanwhile decreases with increase in side walls separation distance and finally approaches the value of a free jet after the separation distance beyond a critical value when the shorter side of the nozzle is perpendicular to the side walls. This confirms the physics that the air entrainment into the buoyant turbulent non-premixed jets for such line-source nozzle is mostly from the longer side; meanwhile rarely from the shorter side. The air entrainment per height is reduced hence the flame height increases with decrease in side walls separation distance when the shorter side of the nozzle is perpendicular to the side walls. The critical side walls separation distance normalized by the nozzle width is found to be linear function of the non-dimensional fuel exit velocity normalized by the buoyancy induced air entrainment velocity using the flame height of the free jet as characteristic length. Finally, a non-dimensional correlation is proposed to characterize the flame heights for various side walls separation distances when the shorter side of the line-source nozzle is perpendicular to the side walls.

Non-Premixed Burner-Attached Jet Flames in Crossflow Pulsed at Resonance Frequency

The flame behavior, flow patterns, and thermochemical structures of a combusting propane gas jet in crossflow excited acoustically at resonance frequency were studied in a wind tunnel. The jet and crossflow Reynolds numbers were 1530 and 1399, respectively. The jet-to-crossflow momentum flux ratio was 0.192. The flame behavior was found to be closely related to the shear-layer vortical flow structures in the near-tube region. Six characteristic flame modes were observed in the domain of jet pulsation intensity . In flame modes 1 and 2 (), the shear-layer vortices were dominant. In flame modes 3–6 (), the flames were dominated by a puffing bent jet. Vortical structures increased in size with increasing . Temperature and combustion product measurements showed significant improvements in mixing and combustion, as well as a reduction of combustion-induced pollutants at the near-tube region of the flame. However, pulsing the jet flame in crossflow at pulsation intensities greater than 0.9 led to a significant increase in the maximum values of carbon monoxide and nitric oxide concentrations at the middle- to far-tube flame regions. Therefore, strongly pulsing a jet flame in crossflow may not necessarily lead to an improved combustion.

Large-Eddy Simulation of Supercritical Combustion: Model Validation Against Gaseous Injector

Supercritical combustion has attracted significant interest due to its applications in high-pressure combustion devices. Validation of supercritical combustion modeling, however, has not been well reported because of the lack of experimental data with sufficient spatial and temporal resolution as well as the complexity associated with modeling and simulations. The present work begins to bridge this gap with a systematic examination of gaseous combustion of a shear coaxial injector at supercritical conditions, using both large-eddy simulation and detached-eddy simulation approaches. The formulation accommodates the full conservation laws and real-fluid thermodynamics and transport theories. Turbulence/chemistry interactions are treated by means of the flamelet and flamelet/progress-variable approaches. A nonpremixed jet flame (Sandia flame D) was first considered for code validation at ideal-gas conditions. The gaseous combustion at supercritical conditions was then studied systematically using different combinations of turbulence closure and combustion models. Special attention was given to comparison with measured wall heat flux. The large-eddy simulation/flamelet, detached-eddy simulation/flamelet, and large-eddy simulation/flamelet/progress-variable approaches produced qualitatively similar results in terms of flow and flame structures as well as wall heat flux. The present work was also compared with studies conducted by other research groups.

Blow-out of nonpremixed turbulent jet flames at sub-atmospheric pressures

Blow-out limits of nonpremixed turbulent jet flames in quiescent air at sub-atmospheric pressures (50–100kPa) were studied experimentally using propane fuel with nozzle diameters ranging 0.8–4mm. Results showed that the fuel jet velocity at blow-out limit increased with increasing ambient pressure and nozzle diameter. A Damköhler (Da) number based model was adopted, defined as the ratio of characteristic mixing time and characteristic reaction time, to include the effect of pressure considering the variations in laminar burning velocity and thermal diffusivity with pressure. The critical lift-off height at blow-out, representing a characteristic length scale for mixing, had a linear relationship with the theoretically predicted stoichiometric location along the jet axis, which had a weak dependence on ambient pressure. The characteristic mixing time (critical lift-off height divided by jet velocity) adjusted to the characteristic reaction time such that the critical Damköhler at blow-out conditions maintained a constant value when varying the ambient pressure.

The effect of exit strain rate on soot volume fraction in turbulent non-premixed jet flames

Laser-Induced Incandescence (LII) measurements from soot were used to determine the soot volume fraction (SVF) in three attached turbulent non-premixed jet flames, of a similar C2H4H2N2 fuel mixture, with different exit strain rate. For all three cases, the jet exit Reynolds number was maintained at 15,000 while varying both the jet diameter and the fuel exit velocity. Measurements are reported of the mean, instantaneous, and radially integrated SVF, together with soot intermittency, for all three flames. They reveal an inverse relationship between the exit strain rate and all of the aforementioned quantities. This observation is consistent with previous trends reported both in laminar flames and turbulent flames, although this is the first systematic assessment for flames that are also well-suited to model validation. The probability density functions of maximum instantaneous SVF exhibit similar trends with regards to exit strain rate, although the most probable SVF increases with decreasing exit strain. The volume integrated SVF reveals an inverse relationship with the exit strain rate, which can be characterized by an exponential fit. A linear correlation was also found between the exit strain rate and the maximum instantaneous SVF, the maximum mean SVF, and the total soot volume per flow rate of fuel.

Flow instability in laminar jet flames driven by alternating current electric fields

The effect of electric fields on the instability of laminar nonpremixed jet flames was investigated experimentally by applying the alternating current (AC) to a jet nozzle. We aimed to elucidate the origin of the occurrence of twin-lifted jet flames in laminar jet flow configurations, which occurred when AC electric fields were applied. The results indicated that a twin-lifted jet flame originated from cold jet instability, caused by interactions between negative ions in the jet flow via electron attachment as O2+e→O2– when AC electric fields were applied. This was confirmed by conducting systematic, parametric experiment, which included changing gaseous component in jets and applying different polarity of direct current (DC) to the nozzle. Using two deflection plates installed in parallel with the jet stream, we found that only negative DC on the nozzle could charge oxygen molecules negatively. Meanwhile, the cold jet instability occurred only for oxygen-containing jets. A shedding frequency of jet stream due to AC driven instability showed a good correlation with applied AC frequency exhibiting a frequency doubling. However, for the applied AC frequencies over 80Hz, the jet did not respond to the AC, indicating an existence of a minimum flow induction time in a dynamic response of negative ions to external AC fields. Detailed regime of the instability in terms of jet velocity, AC voltage and frequency was presented and discussed. Hypothesized mechanism to explain the instability was also proposed.

Mixture fraction, soot volume fraction, and velocity imaging in the soot-inception region of a turbulent non-premixed jet flame

An experimental study is performed to investigate the feasibility of conducting simultaneous mixture fraction, soot volume fraction and velocity imaging in sooting jet flames. The measurements are performed in the soot-inception region of ethylene jet flames, where the yellow luminous region first appears in the flame. Three-component velocity and soot volume fraction are measured by stereoscopic particle image velocimetry and laser-induced incandescence, respectively. The mixture fraction is inferred from laser-induced fluorescence of krypton gas seeded into the fuel stream. To obtain mixture fraction from the fluorescence signal, the signal must be corrected for density and fluorescence quenching effects. This correction is accomplished by invoking an assumed state relationship that is derived from a laminar strained-flame calculation. Once properly calibrated, the krypton planar laser-induced fluorescence data give the mixture fraction, temperature and major species near the regions of soot formation. The krypton is seeded into the fuel jet at a mole fraction of approximately 4%. The fluorescence of krypton is achieved by two-photon absorption at 214.7nm and the resulting fluorescence is collected at 760.2nm. The krypton fluorescence signal is rather weak, particularly near the reaction zones where density is lowest, and so adequate signal-to-noise ratios could be achieved in a region only about 1mm in height, which effectively limited this study to a line measurement of mixture fraction. The temperature field derived from the mixture fraction field was compared to temperatures obtained from thermocouple measurements. The mean radial temperature profiles using the different techniques show excellent agreement and this serves to validate the methodology used to map from fluorescence signal to mixture fraction and temperature. The resulting data are of high enough quality as to allow the investigation of the kinematics, thermo-chemical state and even the dissipation fields near regions of soot formation.

Semi-implicit iterative methods for low Mach number turbulent reacting flows: Operator splitting versus approximate factorization

Two formally second-order accurate, semi-implicit, iterative methods for the solution of scalar transport–reaction equations are developed for Direct Numerical Simulation (DNS) of low Mach number turbulent reacting flows. The first is a monolithic scheme based on a linearly implicit midpoint method utilizing an approximately factorized exact Jacobian of the transport and reaction operators. The second is an operator splitting scheme based on the Strang splitting approach. The accuracy properties of these schemes, as well as their stability, cost, and the effect of chemical mechanism size on relative performance, are assessed in two one-dimensional test configurations comprising an unsteady premixed flame and an unsteady nonpremixed ignition, which have substantially different Damköhler numbers and relative stiffness of transport to chemistry. All schemes demonstrate their formal order of accuracy in the fully-coupled convergence tests. Compared to a (non-)factorized scheme with a diagonal approximation to the chemical Jacobian, the monolithic, factorized scheme using the exact chemical Jacobian is shown to be both more stable and more economical. This is due to an improved convergence rate of the iterative procedure, and the difference between the two schemes in convergence rate grows as the time step increases. The stability properties of the Strang splitting scheme are demonstrated to outpace those of Lie splitting and monolithic schemes in simulations at high Damköhler number; however, in this regime, the monolithic scheme using the approximately factorized exact Jacobian is found to be the most economical at practical CFL numbers. The performance of the schemes is further evaluated in a simulation of a three-dimensional, spatially evolving, turbulent nonpremixed planar jet flame.