Flame holder

In any gas burner some mechanism or device (flame holder or pilot) must be provided to stabilize the flame against the flow of the unbumed mixture. This device should fix the position of the flame at the burner port. Although gas burners vary greatly in form and complexity, the distribution mechanisms in most cases are fundamentally the same. By keeping the linear velocity of a small fraction of the mixture flow equal to or less than the burning velocity, a steady flame is formed. From this pilot flame, the main flame spreads to consume the main gas flow at a much higher velocity. The area of the steady flame is related to the volumetric flow rate of the mixture by equation 18 (81,82)... 

Partially Premixed Burners These burners have a premixing section in which a mixture that is flammable but overall fuel-rich is generated. Secondary combustion air is then supplied around the flame holder. The fuel gas may be used to aspirate the combustion air or vice versa, the former being the commoner. Examples of both are provided in Figs. 27-33 and 27-34. 

F.H. Wright and E.E. Zukowsky 1962, Flame spreading from bluff-body flame holders, Proc. Combust. Inst. 8 933-943. 

Considering the wake of a flame holder as a stirred reactor may be inconsistent with experimental data. It has been shown [66] that as blowoff is approached, the temperature of the recirculating gases remains essentially constant furthermore, their composition is practically all products. Both of these observations are contrary to what is expected from stirred reactor theory. Conceivably, the primary zone of a gas turbine combustor might approach a state that could be considered completely stirred. Nevertheless, as will be shown, all three theories give essentially the same correlation. 

Zukoski and Marble [70, 71] held that the wake of a flame holder establishes a critical ignition time. Their experiments, as indicated earlier, established that the length of the recirculating zone was determined by the characteristic dimension of the stabilizer. At the blowoff condition, they assumed that the free-stream combustible mixture flowing past the stabilizer had a contact time equal to the ignition time associated with the mixture that is, rw = ri( where rw is the flow contact time with the wake and r, is the ignition time. Since the flow contact time is given by... 

Stirred reactor theory was initially applied to stabilization in gas turbine combustor cans in which the primary zone was treated as a completely stirred region. This theory has sometimes been extended to bluff-body stabilization, even though aspects of the theory appear inconsistent with experimental measurements made in the wake of a flame holder. Nevertheless, it would appear that stirred reactor theory gives the same functional dependence as the other correlations developed. In the previous section, it was found from stirred reactor considerations that... 

From these correlations it would be natural to expect that the maximum blowoff velocity as a function of air-fuel ratio would occur at the stoichiometric mixture ratio. For premixed gaseous fuel-air systems, the maxima do occur at this mixture ratio, as shown in Fig. 4.56. However, in real systems liquid fuels are injected upstream of the bluff-body flame holder in order to allow for mixing. Results [60] for such liquid injection systems show that the maximum... 

Figure 12.3 shows some computational examples of nonreactive and reactive turbulent flows in a combustor with the bluff-body flame holder. The size of the combustor in Fig. 12.3 is 35 x 8 cm. The characteristic height and length of the bluff body is H = 2 cm. The left boundary is set as inlet, right boundary as outlet, and the upper and lower boundaries as rigid walls. 

Figure 12.5 Calculated mean temperature fields in combustors with a set of similar open-edge V-gutter flame holders of height H = 3 cm and apex angle of 60°. The isoterms divide the entire temperature interval from the initial temperature To to combustion temperature Tc into 10 uniform parts and correspond to t = 27.5 ms. The combustor is 1 m long and the distance between the planes of flame holders is 0.05 m. Flame holders are shifted in longitudinal direction by OH (no shift) (a), IH (6), 2H (c), 3H (d), and 5H (e). Combustion of stoichiometric methane-air mixture at the mean inlet velocity Ui = 20 m/s, po = 0.1 MPa, To = 293 K, ko = 0.24 J/kg, /o = 4 mm. The lower and upper boundaries of the computational domain are the symmetry planes...

For attaining higher combustion efficiencies with shorter tailpipes, a provision should be made for several flame holders in the combustion chamber. The optimum arrangement of the flame holders in the combustor in terms of the combustion efficiency, flame stability, and pressure loss should be found. The methodology suggested in sections 12.2 and 12.3 helps to solve this problem. 

As mentioned in section 12.1, Dunskii [12] was the first who put forward the phenomenological theory of flame stabilization. The theory is based on the characteristic residence time, L, and combustion time, tc, in adjoining elementary volumes of fresh mixture and combustion products in the recirculation zone behind the bluff body. Dunskii s condition for flame blow-off is U/tc = Mi, where Mi is the Mikhelson number close to unity (for example, for cone flame holder the measurements give Mi = 0.45 [36]). Residence time L is taken proportional to the flame holder size, H, and inversely proportional to the approach flow velocity, U, i.e., L = H/U. Combustion time is estimated as tc = at/Si, where... 

Figure 12.8 Calculated trajectories of fluid particles in the combustor with flame holder (solid lines) and the curves of constant dimensionless residence time t/tr (dashed curves). The residence time tr is defined as the time taken for the fluid particle to reach the turning point at the limiting trajectory (marked by the arrow). Conditions are similar to Fig. 12.66... 

Table 12.1 Calculated characteristic reaction time tc, residence time tr, and the Mikhelson number Mi for the combustion of the stoichiometric methane-air mixture in a combustor with the open-edge V-gutter flame holder of height H and apex angle 60° at the mean inlet velocity Uin. Also presented is the maximum approach-stream velocity Um- Signs and correspond to stabilized flame and unstable flame, respectively... 

Figure 12.9 To the determination of the limiting inlet velocity for the stabilized combustion of the stoichiometric methane-air mixture in the combustor with open-edge V-gutter flame holders. Solid curves correspond to the calculated residence time b- Dashed curves correspond to the calculated reaction time tc. Flame holders with H = 10 cm (f), 5 cm (2), and 2 cm (3). Conditions are similar to those in Figs. 12.6 and 12.7...

The Presumed Probability Density Function method is developed and implemented to study turbulent flame stabilization and combustion control in subsonic combustors with flame holders. The method considers turbulence-chemistry interaction, multiple thermo-chemical variables, variable pressure, near-wall effects, and provides the efficient research tool for studying flame stabilization and blow-off in practical ramjet burners. Nonreflecting multidimensional boundary conditions at open boundaries are derived, and implemented into the current research. The boundary conditions provide transparency to acoustic waves generated in bluff-body stabilized combustion zones, thus avoiding numerically induced oscillations and instabilities. It is shown that predicted flow patterns in a combustor are essentially affected by the boundary conditions. The derived nonreflecting boundary conditions provide the solutions corresponding to experimental findings. 

Numerical studies of combustion control in simple combustors with flame holders have been made. The criterion of flame stabilization, based on the unambiguously defined characteristic residence and reaction times, is suggested and validated against numerous computational examples. The results of calculations were compared with available experimental findings. A good qualitative and reasonable quantitative agreement between the predictions and observations were attained. Futher studies are planned to include mixing between fuel jets with oxidizer and to extend the analysis to transonic and supersonic flow conditions. 

Winterfeld, G. 1965. On process of turbulent exchange behind flame holder. 10th Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 1265-75. 

The second device comprised a set of three circumferentially located pintle-type injectors Keihin, 10450-PG7-0031) to inject fuel radially into the main duct of the first flow arrangement as near-rectangular pulses. The frequency and duration of fuel injection were software controlled, and the fuel flow from each injector was delivered close to the outer edge of the annular ring flame holder by a cross-jet of air (1.2 x 5 mm), directed along the duct axis with exit velocity up to 100 m/s. The amplitude of the oscillated input was limited by the volume injection rate of the injectors. Propane, rather than methane, provided up to 3.5 kW of the total heat release of around 100 kW. With fluid dynamic damping, the RMS of the oscillated fuel flow corresponded to a heat release of around 1.8 kW. 

The tendency of premixed flames to detach from the flame holder to stabilize further downstream has also been reported close to the flammability limit in a two-dimensional sudden expansion flow [27]. The change in flame position in the present annular flow arrangement was a consequence of flow oscillations associated with rough combustion, and the flame can be particularly susceptible to detachment and possible extinction, especially at values of equivalence ratio close to the lean flammability limit. Measurements of extinction in opposed jet flames subject to pressure oscillations [28] show that a number of cycles of local flame extinction and relight were required before the flame finally blew off. The number of cycles over which the extinction process occurred depended on the frequency and amplitude of the oscillated input and the equivalence ratios in the opposed jets. Thus the onset of large amplitudes of oscillations in the lean combustor is not likely to lead to instantaneous blow-off, and the availability of a control mechanism to respond to the naturally occurring oscillations at their onset can slow down the progress towards total extinction and restore a stable flame. 

Although the oscillation of fuel in the annulus was limited by the actuator to 4% of the total fuel flow, the addition of the oscillated fuel close to the outer edge of the annular ring flame holder resulted in attenuations around 6 dB for the range of equivalence ratios considered. 

Air forcing had a significant effect on the flame structure. At the baseline conditions, the flame was lifted and sooty. With air forcing, the flame became fully reattached to the flame holder. Swirl had little effect on the structure of a highly forced flame. 

Metal particles incorporated into a gas-generating pyrolant act as flame holders to keep the flame in the ramburner. Fach metal particle flows with the combustible gas and becomes a hot metal or metal oxide particle. Since the flow velocity of such a hot particle is lower than that of the combustible gas, the flow velocity of the combustible gas just downstream of the hot particle is decreased due to the aerody-... 

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