Laminar Non-premixed Jet Flames Research Articles

Vortex formation mechanism within fuel streams in laminar nonpremixed jet flames

A vortical structure occurring at the fuel stream in laminar nonpremixed jet flames was recently found and shown to have both a fluid-dynamic impact on the flow field and a possible influence on the flame stability and soot formation. We designed a systematic experiment and numerical simulation to investigate the physical mechanisms of this recirculation phenomenon in a coflow system. We hypothesized that a negative buoyancy, caused by the fuel jet being heavier than the ambient air, may play a significant role in the recirculation. Therefore, we experimentally varied the density of the fuel jet using a binary mixture of methane and n-butane, and tested the density of the coflow oxidizer by replacing nitrogen with carbon dioxide. Several fuel jet velocities, flame temperatures, and nozzle diameters were also studied to thoroughly investigate all parameters that might possibly affect the recirculation. As a result, we found that our modified Richardson number, which is based on the cold density difference between the fuel and the coflow, the flame length, and the jet momentum flux, explained the physical mechanism of the recirculation well, with Ri ∼60 being the critical value for formation of the recirculation. The negative buoyancy was the primary driving force behind the recirculation, while the jet momentum mitigated its formation.

Soot evolution and flame response to acoustic forcing of laminar non-premixed jet flames at varying amplitudes

New details regarding the soot evolution and its controlling parameters in steady and forced flames have been studied using high spatial resolution laser diagnostic techniques. Steady laminar non-premixed ethylene/nitrogen flames with three different diameters burners were acoustically forced using a loudspeaker. 10-Hz-sinusoidal signals of different amplitudes were transmitted to the loudspeaker to drive the flames. The results reveal that the spatial correlation between the soot field and the temperature profile is influenced by the burner diameter and forcing conditions. The soot field in steady laminar flames is confined to a relatively narrow temperature range, 1500–2000 K. In contradiction, the soot field in forced flames spread across a wider range of temperature, 1400–2100 K. Furthermore, the spatial correlation between the normalised soot concentration and primary particle size can be described with an exponential function. While it is observed that the exponential coefficients vary with burner diameter and forcing conditions, further study is necessary for a better understanding. In general, laminar flames forced at a lower amplitude (α=25%) tend to produce less soot than moderately forced (α=50%) flames. Further increasing the forcing amplitude to α=75% does not increase the soot production in laminar flames; conversely, lower peak and volume-integrated soot volume fraction are observed in the strongly forced flame (α=75%) as relative to the moderately forced counterpart. These findings shed new light on the seemingly contradictory results published in the literature regarding the effect of the forcing intensity on the soot production.

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.

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.

Experimental and numerical studies on NOx emission characteristics in laminar non-premixed jet flames of ammonia-containing methane fuel with oxygen/nitrogen oxidizer

The study investigated the formation characteristics of nitrogen oxides (NOx) in non-premixed coflow methane jet flames experimentally and numerically. Ammonia was added to the fuel stream, while varying the oxygen ratio in the oxygen/nitrogen oxidizer stream. In the coflow jet flame experiment using pure oxygen oxidizer, NOx emissions increased monotonically with respect to the mixing ratio of ammonia. When using oxygen/nitrogen oxidizers, NOx emissions reached a maximum at an oxygen ratio of 0.7, with non-monotonic changes occurring in response to variations in the oxygen ratio. Depending on the ammonia addition, increasing NOx emissions appeared in the two ranges of either relatively low or high ratio oxygen in the oxidizers. However, a reversal phenomenon of decreasing NOx emissions was noted within the section between the two ranges of the oxygen ratio. To elucidate the characteristics of NOx formation under various conditions of fuel and oxidizer compositions, 1-D and 2-D numerical simulations were conducted using a detailed chemical kinetics (GRI-Mech 3.0). The 2-D simulation results for pure oxygen and oxygen/nitrogen oxidizers provided reasonable predictions of trends in experimentally measured NOx emissions with the ammonia addition. Moreover, the cause of the reversal phenomenon was successfully explained through a comparative analysis of reactions leading to production and destruction of nitrogen monoxide (NO), based on the results of the 1-D and 2-D simulations. The results provide fundamental information to help in the design of oxygen-enriched combustor.

Autoignition characteristics of laminar lifted jet flames of pre-vaporized iso-octane in heated coflow air

The stabilization characteristics of laminar non-premixed jet flames of pre-vaporized iso-octane, one of the primary reference fuels for octane rating, have been studied experimentally in heated coflow air. Non-autoignited and autoignited lifted flames were analyzed. With the coflow air at relatively low initial temperatures below 940K, an external ignition source was required to stabilize the flame. These lifted flames had tribrachial edge structures and their liftoff heights correlated well with the jet velocity scaled by stoichiometric laminar burning velocity, indicating the importance of the edge propagation speed on flame stabilization. At high initial temperatures over 940K, the autoignited flames were stabilized without requiring an external ignition source. These autoignited lifted flames exhibited either tribrachial edge structures or mild combustion behaviors depending on the level of fuel dilution. Two distinct transition behaviors were observed in the autoignition regime from a nozzle-attached flame to a lifted tribrachial-edge flame and then to lifted mild combustion as the jet velocity increased at a certain fuel dilution level. The liftoff data of the autoignited flames with tribrachial edges were analyzed based on calculated ignition delay times. Analysis of the experimental data suggested that ignition delay time may be much less sensitive to initial temperature under atmospheric pressure conditions as compared with predictions.

Interactive transient flamelet modeling for soot formation and oxidation processes in laminar non-premixed jet flames

The Interactive Transient Flamelet (ITF) model has been devised to realistically simulate slow processes such as soot formation and radiation in a laminar non-premixed flame with a relatively long residence time. In context with the ITF model, soot formation in the laminar non-premixed jet flame is modeled by the two-equation soot model to simulate physical processes such as nucleation, surface growth, oxidation and agglomeration. In the present ITF approach, gaseous chemistry is effectively coupled with soot chemistry. The present ITF model accounts for radiative cooling induced by gaseous species and soot particles in the same mixture fraction space simultaneously while it treats the unphysical diffusion of soot to be practically zero. Moreover, the present ITF procedure can maintain consistency to extend the present transient flamelet model to the simulation of turbulent non-premixed sooting flames. To validate the present ITF approach together with the two-equation soot model, in terms of soot volume fraction, number density, temperature, OH and C2H2 mass fractions, as well as reaction rates for nucleation, surface growth, and oxidation, numerical results are compared with those obtained by the full transport equation approach. Furthermore, to assess the applicability of the present ITF approach and to evaluate the effects of gaseous differential diffusion on the soot formation processes, numerical results obtained by the present ITF model with equal and differential diffusion are also compared with experimental data in terms of temperature; CH4, OH, H2O, CO2 and C2H2 mole fractions; and soot volume fraction. Based on these numerical results, the detailed discussion has been made for the capability and limitations of the present ITF approach to predict the precise flame structure and soot formation characteristics in the laminar non-premixed methane/air jet flames.

Simulation of Non-Autoignited and Autoignited Laminar Non-Premixed Jet Flames of Syngas in Heated Coflow Air

The characteristics of non-autoignited and autoignited flames of syngas (CO/H2 mixture) fuels with heated coflow air have been investigated numerically. The simulations accounting for the differential diffusion have been performed by adopting several kinetic mechanisms to test the models’ ability in predicting the flame behaviors observed previously. The results agreed well with the observed nozzle-attached flame characteristics in case of non-autoignited flames. For autoignited lifted flames in high temperature regime, a unique autoignition behavior can be predicted having HO2 and H2O2 radicals in a broad region between the nozzle and stabilized lifted flame edge. Species’ profiles as a function of temperature showed a similarity with a homogeneous autoignition case. The autoignited liftoff height was correlated well in terms of the jet velocity multiplied by the square of ignition delay time, in agreement with previous experimental data. The simulation results tested with various kinetic mechanisms, however, could not predict the non-monotonic liftoff height behavior with jet velocity for autoignited flames observed experimentally at relatively low temperature. The initial temperature in the simulation should be higher at about 80 K as compared to experimental conditions to have comparable liftoff heights. The result suggested a necessity to improve kinetic mechanisms at atmospheric pressure conditions.

On the characteristics of laminar lifted flames in a nonpremixed jet

Characteristics of laminar lifted flames stabilized in a nonpremixed jet issuing from a small nozzle into ambient air have been studied experimentally. Results show that near the flame base the lifted flame has a tribrachial structure composed of a lean premixed flame, a rich premixed flame, and a diffusion flame extending downstream from the same point. A simple theoretical formula for the liftoff height of nonpremixed jet flames based on a boundary layer type of jet theory predicts that the liftoff height increases with increasing flow rate for Schmidt numbers in the range Sc > 1 or Sc < 0.5, while it decreases for 0.5 < Sc < 1, which is physically unrealistic. This is consistent with the experimental observation that propane and n-butane jets (Sc > 1) have lifted flames while methane and ethane (0.5 < Sc < 1) exhibit blowout directly from the attached flames. A correlation for the explicit dependence of the liftoff height on the flow rate and the nozzle diameter was derived from the experiments for propane and butane and shown to agree well with the theory. This finding emphasizes the importance of the tribrachial flame, approximated as a stoichiometric premixed flame, in determining the characteristics of lifted laminar nonpremixed jet flames.