The effect of DC electric field on the characteristics of flame spread over polyethylene-insulated twin electrical wires was studied by varying wire gap (S) and voltage (VDC). Under an applied electric field, the flame spread rate (FSR), flame width, leaning direction of the interacting twin flames varied substantially with varying the voltage and wire gap. The flame spread rate was initially larger for the wire with negative voltage (spreading flame with negative charge; SF−) than the wire with positive voltage (SF+), but the two eventually became the same in the developed region when a quasi-steady state was reached. The FSR behavior could be classified into two regimes; twin flame spread (regime I) and single flame spread (regime II) after the extinction of SF+. Under regime I, three sub-regimes were identified depending on the wire gap and voltage. For the twin flame spread, the flame spread rate initially decreased with increasing voltage as the flame leaned toward the burnt wire. As the two flames interacted, the flame spread rate increased because of the ionic wind effect, and eventually decreased because of the loss of molten PE mass and the electrospray phenomenon. In regime II after the extinction of SF+, the single flame spread showed a transient behavior since the influences of electric field from burnt and unburned wire sections of SF+ wire varied with flame spread. When the voltage was increased even further, SF– was extinguished by streamer generation and, at excessive voltages, an electrical short occurred. The flame spread rates for twin flame spread were best correlated with the electric field intensity in the form of |VDC|0.91/S0.72.

The blowout behavior of non-premixed turbulent coflow jet flames under microgravity environment was studied experimentally by utilizing a 3.6 s drop tower. Variations of flames leading to liftoff as well as blowout were examined by varying the coflow velocity and compared with those obtained under the normal gravity condition. A modeling work was conducted to incorporate the effects of the gravity (buoyancy) and coflow velocity on blowout behavior. Major findings include: (1) the flame length in microgravity was longer than that in normal gravity and decreased with increasing coflow velocity. The flame in microgravity showed more intense yellow luminosity with larger sooting zone; (2) the flame liftoff height increased with increasing coflow velocity in both gravity levels. The flame base was closer to the burner in microgravity as compared with that in normal gravity; (3) the blowout velocity in microgravity was appreciably larger than that obtained in normal gravity; and (4) a physical model based on Damköhler number was developed by using similarity solutions to characterize the differences in the blowout limits considering both the coflow and gravity (buoyancy) effects. The proposed model can successfully predict the experimental data. This work provided new data and basic scaling analysis for blowout limit of non-premixed turbulent jet flames considering both the coflow and gravity (buoyancy) effects.

The effect of AC electric field on flame spread over polyethylene (PE)-insulated electrical wire was experimentally investigated by varying the applied AC voltage (VAC) and frequency (fAC) with special attention to the effect of the thickness of the PE insulation material (Tins). The diameter of the Ni–Cr wire was 0.5 mm and Tins was 0.15, 0.3, and 0.5 mm, yielding outer diameters (Dout) of 0.8, 1.1, and 1.5 mm, respectively. For the baseline cases with no electric field, the flame spread rate (FSR) (flame size) decreased (increased) with insulation thickness. Both FSR and flame size were appreciably influenced by applied AC electric fields. The FSR behaviors under applied electric fields could be classified into three sub-regimes as AC frequency increased: regime I exhibited a decreasing FSR as AC frequency increased, regime II exhibited an increasing FSR, and regime III again exhibited a decreasing FSR. Molten PE dripped from the wire (resulting in mass loss); exhibited di-electrophoresis (some molten PE moved from the main molten zone toward the burnt wire, forming globules in the process); and developed electrosprays (ejection of small droplets from the molten PE surface). For Dout = 0.8 mm, the FSR behavior was similar to that of the flame width, such that the behavior could be explained by the thermal balance mechanism. When a low voltage and high frequency were applied to wires with Dout = 1.1 and 1.5 mm, molten PE droplets detached and moved to the burnt wires continuously (although sometimes intermittently) from the main body of molten PE; the FSR behavior thus deviated from that of regime I. Droplet detachment was attributable to a di-electrophoresis. The distance moved correlated well with the difference in electric field intensities of burnt and unburned wires. Appreciable dripping of molten PE occurred at high voltages and moderate frequencies in regime II. When the frequency was excessive, flame extinction occurred via two routes: appreciable reduction of flame size when Dout = 0.8 mm in regime I and appreciable fuel mass loss via dripping of molten PE during flame spread when Dout = 0.8 and 1.1 mm in regime III. These extinction frequencies correlated well with VAC/Dout. When high voltage and frequency were applied in the Dout = 1.5 mm case, droplets detached and moved to the burnt wire via di-electrophoresis; subsequently a series of fine droplets ejected from the surface via electrospraying, while the molten PE region grew and subsequently dripped. In such cases, flame extinction did not occur because di-electrophoresis increased the flame width and thereby the FSR over the experimental ranges of VAC and fAC.

An experimental and numerical analysis of the effects of methanol and ethanol addition on polycyclic aromatic hydrocarbon (PAH) and soot formation in non-premixed ethylene flames is reported here. Laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques were used to measure soot volume fractions and relative PAH concentrations in counterflow diffusion flames, respectively. A comprehensive chemical kinetic analysis was performed by modeling soot with detailed gas-phase chemistry and a sectional method. The results showed that although both methanol and ethanol are typically regarded as clean fuels, their presence in ethylene diffusion flames had the opposite effects on PAH and soot formation. The LIF and LII signals decreased significantly as methanol fraction increased, suggesting a soot-inhibiting role for methanol. Apart from the fact that methanol addition reduced the carbon supply for soot thus having a fuel-dilution effect (methanol converted primarily to CO), the increased H2 concentration from methanol decomposition was seen to chemically suppress incipient benzene ring formation and subsequent PAH and soot growth processes. In contrast, a small amount of ethanol addition enhanced soot formation, which was well captured by the numerical model. Reaction pathway analysis showed that ethanol decomposition produced a relatively large amount of methyl radicals, enhancing the chemical interaction between CH3 and C2 species and, thereby promoting the formation of propargyl and C4 species. As a result, benzene formation was promoted through reactions between C2H2 and C4 species and via C3H3 recombination reaction, leading sequentially to the enhancement of PAH growth and soot formation.

Stabilization of laminar lifted coflow jet flames of nitrogen-diluted methane was investigated experimentally and numerically. As the fuel jet velocity was increased, two distinct behaviors in liftoff height were observed depending on the initial fuel mole fraction; a monotonically increasing trend and a decreasing and then increasing trend (U-shaped behavior). The former was observed in the jet-developing region and the latter in the jet-developed region. Because the decreasing behavior of liftoff height with jet velocity has not been observed at ambient temperature, the present study focuses on decreasing liftoff height behavior. To elucidate the physical mechanism underlying the U-shaped behavior, numerical simulations of reacting jets were conducted by adopting a skeletal mechanism. The U-shaped behavior was related to the buoyancy. At small jet velocities, the relative importance of the buoyancy over convection was strong and the flow field was accelerated in the downstream region to stabilize the lifted flame. As the jet velocity increased, the relative importance of buoyancy decreased and the liftoff height decreased. As the jet velocity further increased, the flame stabilization was controlled by jet momentum and the liftoff height increased.

The effects of dimethyl ether (DME) addition to ethylene fuel on sooting tendencies with varying pressure were investigated in counterflow diffusion flames by using a laser scattering technique. Sooting limit maps were determined in the fuel (XF) and oxygen (XO) mole fraction plane, separating sooting and non-sooting regions. The results showed that when DME is mixed to ethylene, the sooting region was appreciably shrank, especially in the cases of soot formation/oxidation (SFO) flames as compared with the cases of soot formation (SF) flames. This indicated an inhibiting role of DME on sooting. An interesting observation was that the critical XO required for sooting initially decreased and then increased with the DME mixing ratio to ethylene β for the cases of SF flames, exhibiting a non-monotonic behavior. This implied a promoting role of DME on sooting when small amount of DME is mixed to ethylene. As the pressure increased, the sooting region generally expanded. Specifically, the range of β in promoting soot formation extended with pressure. This implies that a strategy in reducing soot by adding DME to ethylene at high pressures required a large amount of DME addition. To interpret the observed phenomena, kinetic simulations including reaction pathway and sensitivity analyses were conducted with the opposed-flow flames model using the KAUST-Aramco PAH Mech. The results showed that the thermal effect of DME addition on sooting tendency monotonically decreases with β. The chemical effect was found to be the main contributor to the DME addition effect on sooting tendency, resulting in the non-monotonic sooting limt behavior. The pathway analysis showed the role of methyl radicals generated from DME promoted incipient benzene ring formtion when small amount of DME was added, which can be attributed to the soot promoting role of DME addition for small β.

The characteristics of autoignited laminar lifted methane/hydrogen jet flames in heated coflow air are numerically investigated using laminarSMOKE code with a 57-species detailed methane/air chemical kinetic mechanism. Detailed numerical simulations are performed for various fuel jet velocities, U0, with different hydrogen ratio of the fuel jet, RH, and the inlet temperature, T0. Based on the flame characteristics, the autoignited laminar lifted jet flames can be categorized into three regimes of combustion mode: the tribrachial edge flame regime, the Moderate or Intense Low-oxygen Dilution (MILD) combustion regime, and the transition regime in between. Under relatively low temperature and high hydrogen ratio (LTHH) conditions, an unusual decreasing liftoff height, HL, behavior with increasing U0 is observed, qualitatively similar to those of previous experimental observations. From additional simulations with modified hydrogen mass diffusivity, it is substantiated that the unusual decreasing HL behavior is primarily attributed to the high diffusive nature of hydrogen molecules. The species transport budget, autoignition index, and displacement speed analyses verify that the autoignited lifted jet flames are stabilized by autoignition-assisted flame propagation or autoignition depending on the combustion regime. Chemical explosive mode analysis (CEMA) identifies important variables and reaction steps for the MILD combustion and tribrachial edge flame regimes.

Although ionic wind has been observed to play important roles in the effects of electric fields on flames, there is a lack of systematic quantification of ionic wind that allows interpretation of a flame's responses to electric fields. Here, we report on various responses of nonpremixed flames, such as the flame's dynamic responses and the generation of bidirectional ionic wind, in relation to the applied voltage and frequency of an alternating current (AC) in a counterflow burner. We find that although the Lorentz force acting on charged molecules initiates related effects, each effect is both complex and different. When the applied voltage is in the sub-saturated regime (small) as determined by the voltage-current behavior, flame movements and flow motion are minimally affected. However, when the applied voltage is in the saturated regime (large), flame oscillation occurs and a bidirectional ionic wind is generated that creates double-stagnation planes. The flame's oscillatory motion could be categorized in the transport-limited regime and in the oscillatory decaying regime, suggesting a strong dependence of the motion on the configuration of the burner. We also observed bidirectional ionic wind in visibly stable flames at higher AC frequencies. We present detailed explanations for flame behaviors, electric currents, and flow characteristics under various experimental conditions.

A light extinction technique is widely-adopted for quantitative measurement of soot volume fractions. The measurement accuracy is dependent on the optical properties of soot, which are expected to vary with the wavelength of incident light and physicochemical environments in which soot is formed. In the present study, a diode laser based light extinction setup, capable of providing light with variable wavelengths ranging from 405 to 1064 nm, was utilized to investigate the in-situ spectral dependence of light absorption for soot formed in counterflow diffusion flames. Soot volume fractions (FV) were inferred from the extinction level of these laser beams for a series of flames parameterized by oxygen/fuel mole fractions, nozzle exit velocities, and fuel types. Special attention was given to distinguish between the soot formation (SF) and soot formation/oxidation (SFO) flames, considering their notable differences in soot evolutions. It was found that the inferred FV as measured with visible light (405−670 nm) was always significantly higher than those measured with near-infrared light (> 780 nm). In addition, the quantitative decrease of FV with the increase in light wavelength (λ) was found to be different for soot particles formed at different flame locations and/or flame conditions, even in the spectral range above 780 nm for which polycyclic aromatic hydrocarbon (PAH) interferences are expected to be minimal. This confirms the wavelength dependence of the soot optical property E(m). In particular, the value of E(m) tends to decrease with increasing wavelength and the rate of decrease is lower for more mature soot particles. Furthermore, by fitting the extinction coefficient with wavelength in the near-infrared range, the quantitative relation of E(m) with λ was derived and compared among various flame conditions. The present study demonstrates that soot formed at different conditions have different optical properties. The results are also expected to provide essential information for uncertainty evaluation in literature FV data in counterflow diffusion flames as measured with light extinction, especially for those performed with visible light sources where PAH interference may not be negligible.

The oscillating lifted flame in a laminar nonpremixed nitrogen-diluted fuel jet is known to be a result of buoyancy, though the detailed physical mechanism of the initiation has not yet been properly addressed. We designed a systematic experiment to test the hypothesis that the oscillation is driven by competition between the positive buoyancy of flame and the negative buoyancy of a fuel stream heavier than the ambient air. The positive buoyancy was examined with various flame temperatures by changing fuel mole fraction, and the negative buoyancy was investigated with various fuel densities. The density of the coflow was also varied within a certain range by adding either helium or carbon dioxide to air, to study how it affected the positive and negative buoyancies at the same time. As a result, we found that the range of oscillation was well-correlated with the positive and the negative buoyancies; the former stabilized the oscillation while the latter triggered instability and became a source of the oscillation. Further measurements of the flow fields and OH radicals evidenced the important role of the negative buoyancy on the oscillation, detailing a periodic variation in the unburned flow velocity that affected the displacement of the flame.