In the case of the increasing HL (including the increasing regime in U-shaped behavior), the HL behavior was correlated in terms of ScF, the density difference between fuel and air, ScF, cr1 and U0. To further elaborate the characteristics of the oscillating raised flames, experiments and numerical simulations were performed by measuring both the fuel density (by varying propane and n-butane mixtures) and the coflow density (by diluting air with N2/He- mixtures) to vary. I ∼ II in (a) and I ∼ V in (b) represent the time series of the OTLF and OMLF in Fig.
Jet similarity based on the cold jet theory
Unfortunately, however, this causes the reduction of thermal efficiency because the exhaust gas recirculation (EGR) requires an additional system from outside to inside. A laminar raised jet flame usually exhibits a tribrachial (or triple) flame structure consisting of lean and rich premixed flame wings and a trailing diffusion flame in between, all of which originate from the tribrachial edge as shown in Fig. As the tribrachial flame edge propagates toward unburned mixture along the stoichiometric mixture isoline, the lifted jet flame stabilizes where its edge propagation speed, Se, is balanced with local flow velocity, UL[7, 8]. Seis is generally affected by mixture concentration gradient, mixture strength, flame curvature, and Lewis number, while ULby jet momentum, flow redirection, and buoyancy.
If at least one of such flame and flow properties changes, the raised flame moves to a different stabilization point or may begin to oscillate. The takeoff height HL=x∗ and the jet velocity limit can be derived from Eq. Assuming constant Se and YF,st the take-off height ratio becomes:. From the studies [1, 6], it was theoretically and experimentally clarified that the rising flames of methane and ethane are immediately blown away when the high jet velocity lifts them from the fuel jet nozzle.
Also, contrary to the results of jet similarity, the hydrogen jet flame with ScF <0.5 remains nozzle-attached even when U0 becomes comparable to the speed of sound [6]. This is primarily attributed to the strong mass diffusion of hydrogen inducing a negative quenching distance, Hq, from the nozzle tip [6].
Stabilization mechanism
18] recently reported the strange behavior of a stationary lifted flame of CH4/N2 jets that HL decreases and then increases with increasing jet momentum, although it is supposed to increase with increasing jet momentum, which is called a U-shaped behavior of the lifted. flame. It was hypothesized to occur because the effect of buoyancy over convection becomes strong at relatively small jet speeds.
Oscillating mechanism: Competition between positive and negative buoyancies . 15
Experimental setups
To further investigate the effect of ScFon liftoff height, methane and ethylene blend fuels are tested. As shown in Table 2, the fuel in the swing flame burner is propane or propane/n-butane mixture diluted with N2, for which the fuel mole fraction, XF, varies from 0.12 to 0.17. The common stream is air diluted with N2/He mixture so that its density, ρco, can be controlled by varying the mole fractions of.
Binary fuel mixture of propane/n-butane was tested to further investigate the effect of fuel density on oscillatory flame behavior. We used the Chemkin Pro package [24] with GRI 3.0 Mech [25] to maintain the Tad with a similar range to that for the methane (propane) flames in the burner for stable lifted (oscillating) flame through the fuel mole fraction To adapt. The transport properties such as kinetic viscosity,ν, and fuel mass diffusivity in air stream, DF, are calculated from an open source program [26] to determine ScF= ν/DF.
The effective diffusivity of the methane and ethylene fuel mixture was determined from DF= XCH4DCH4+XC2H4DC2H4 [27].
Numerical methods
The fuel inlet velocity is specified as that of a fully developed pipeline flow for which the mean velocity, U0, is 6 cm/s and the cross-flow velocity, Vco, is fixed to be 8 cm/s in accordance with the present experiment. . No slip and impermeable boundary conditions are used for all walls and isothermal boundary condition is used at the fuel nozzle wall.
Stable lifted flame
This highlights the important roles of the Schmidt number and the Richardson number in the lift height behavior. It results from Ar as a diluent, which is heavier than the flowing air, implying that the raised flame can be affected by a negative motion exerted on the fuel in the downward direction [19]. Recognizing the importance of fuel density, we further examine the general lift flame behavior for methane diluted with N2, He, and Ar in terms of fuel Schmidt number and fuel density, and this will be discussed in Chapter IV .
Oscillating flame
Recognizing the importance of fuel density, we further investigate the overall behavior of the lifted flame for methane diluted with N2, He and Ar with respect to fuel Schmidt number and fuel density, and this will be discussed in Chapter IV. 7b-IV), and propagates downstream (7b-V), completing an oscillation cycle. First, for allXF, the OMLF develops at relatively low ρco and U0 such that its regime lies in the lower left corner of the OTLF regime. This implies that OMLF only occurs when the effect of the fuel's negative buoyancy on the flow field becomes significant (smallρco) at low U0.
Since there is only a ρco difference between the two oscillating flames, we can confirm that such flow structures are induced by a relatively strong negative buoyancy (ρF−ρco) of the OMLF, causing OMLF to move more radially outward than OTLF. 9b-II there is a small amount of the heat release rate of the rear diffusion flame. Since we focus not on a stationary but on an oscillating flame, the diffusion branch of the OMLF in the simulation cannot completely disappear, even for the premixed flame.
Color contours and arrow lines represent heat release rate, q; and streamlined, respectively; solid and dashed lines represent the isolines of the stoichiometric mixture fraction,ξst, and CO2 mass fraction,YCO2, of 0.10. Like the previous work [19], we can experimentally and numerically confirm the effect of the two driving forces on oscillating flame.
The critical Sc F on stable lifted flame
The lift height behavior in regime I and increasing range in the U-shaped behavior is characterized based on the jet velocity U0 scaled by stoichiometric laminar burning velocity S0L according to the stabilization mechanism [1, 2, 6] and the density difference (ρco - ρF) induces positive buoyancy (ρco is coflow -air density), thereby increasing the local flow rate and lift. The exponent (ScF−0.5) was chosen because for the fuel with ScF<1 the liftoff height behavior between the fuels having ScF<0.5 and ScF>0.5 is quite different based on the free jet theory [1, 2, 6 ]. An extraordinary case is for ΩHe=1.0, where the lift decreased with the increase in the above correlation, as shown in fig.
For the decreasing lift height with jet velocity in the U-shaped behavior, the relative role of buoyancy over convection in flame stabilization was emphasized for small jet velocities [18]. Note that in the U-shaped regime the difference (ρF−ρco) can be positive or negative, as shown in figure. Considering the importance of (ρF−ρco) and its sign, the launch altitude is correlated with a new Richardson number,Ri≡ |ρF−ρco|dg/ρFU02.
Current data are presented as three shaded regions defined as monotonically increasing take-off height behavior (mode I), U-shaped behavior (II) and flame oscillation (III). Note that (ρF−ρco)>0 results in negative buoyancy in the cold confluent air, implying a downward buoyancy direction.
Flame propagation speed on oscillating flame
S0L is measured by the stoichiometric mixture fraction, ξst, estimated from the mixture compositions of the fuel and oxidizer streams [35]. Note that V+/U0 represents the square root of the relative strength of the positive buoyancy to the fuel jet momentum, both of which push the flame downstream, while V−/SL0 indicates the negative buoyancy to flame spread or flame strength, both of which direct the flame upstream. It can be easily observed that the OMLF data are aligned in a narrow region around the marked straight line, showing that the OMLF regime adheres to the OTLF regime, and separates the OTLFs from the flame extinction regime.
An OMLF can also be extinguished when the negative buoyancy becomes relatively strong compared to S0L, further destabilizing it. Note that OMLFs of the propane/n-butane jet are relatively large V−/S0L and relatively small V+/U0 because the ρFi is much larger than ρco. For both OTLF and OMLF, Seis is measured at the flame base, which is defined as the intersection of the isolines of YCO2 = 0.10 and ξst.
In the figure, I∼II for the OTLF and I∼V for the OMLF correspond to the sequences of Figs. Although SeandUN generally behaves similar to that of the OTLF, they exhibit larger amplitudes and mean values than those of the OTLF.
Fuel concentration gradient on oscillating flame
As the OTLF propagates upstream toward the nozzle, the flow divergence becomes large due to the negative buoyancy, which subsequently increases the radial size and decreases the dYF/dy, leading to an increase in Se. As the OTLF approaches the nozzle, both flame area and reaction intensity increase, entraining more coflow to the flame due to positive buoyancy. As the OMLF first propagates upstream, Se increases due to the decrease in dYF/dy (Regime I; the tribrachial flame regime (B) in Figure 17), similar to the OTLF behavior.
Meanwhile, a strong toroidal vortex is formed by relatively large negative buoyancy, which subsequently induces a countercurrent-like structure upstream of the flame (see Fig. 9b-II and III). As the flame propagates further downstream, Se first increases substantially by an increase in the fuel concentration gradient normally on the flame edge propagation, dYF/dn, rather than dYF/dy due to positive buoyancy enhancement by breaking the counterflow-like structure and finally completes its cycle by return to its original starting point (Regime V). In different ranges of the fuel Schmidt number, nozzle attached, stationary lifted and oscillating lifted flames were observed.
The intersection of the two critical Schmidt numbers represented ScF= 1.05, which means that the experimental results agree with the jet similarity solution because it is close to unity [1, 2, 6]. From the oscillating tribrachial lifted flame (OTLF) and oscillating lifted flame with mode change (OMLF) regimes, it is found that OMLF occurs only when the effect of negative buoyancy on the flow field becomes significant at low U0, so that the OMLF regime lies in the lower left corner of the OTLF regime. From transient numerical simulations of both the OTLF and the OMLF, it was found that the OMLF occurs when a strong toroidal vortex and subsequent counterflow structure develops upstream of the flame due to relatively strong negative buoyancy.
Yoo, “Numerical study of pyrolysis effect on self-igniting laminar raised flames of dimethyl ether jet in heated coflow air,” Combust.
![Figure 1: Direct image (left) [1] and schematic (right) of a tribrachial flame.](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10497386.0/13.892.317.587.721.896/figure-direct-image-left-schematic-right-tribrachial-flame.webp)
![Figure 2 shows the schematic of the stoichiometric isolines and the iso-velocity lines of S e , where the stabilization points are located at closed circles [2]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10497386.0/15.892.277.613.669.961/figure-schematic-stoichiometric-isolines-velocity-stabilization-located-circles.webp)
