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Experimental investigation of stabilization mechanisms in turbulent, lifted jet diffusion flames
With Prof. M. Godfrey Mungal, Stanford University
(Figures: flow schematic sample images sample quantitative results stabilization model) (Publications)
Figure 1. The lifted, co-flowing jet diffusion flame. The high-temperature interface is marked by the large temperature gradients between the unburned gases and the hot combustion products.
The high-temperature interface separates the unburned gases and hot combustion products. We define the most upstream points on that interface to either side of the jet centerline as the leading points. It is observed in experiments that the position and shape of the high-T interface fluctuates strongly. This work is concerned with measuring the fuel-air mixing field and the velocity field upstream of these leading points via planar laser imaging. The measurements provide direct insight into the mechanism by which the lifted flame stabilizes against the incoming flow. In particular, we will be able to compare the merits of stabilization theories based on full fuel-air premixing, partial premixing, and combustion in diffusion flame structures.
Figure 2. (a) A sample fuel mole fraction field, determined from the PLIF measurements. (b) The corresponding particle Mie scattering image used for PIV, showing the absence of signal in high-temperature regions. The high-T interface from (b) is superimposed on the scalar field in (a). The jet Reynolds number is 6200.
Figure 2a shows a sample fuel mole fraction, X, field, and Fig. 2b shows the corresponding Mie scattering field used for PIV. The fog particles evaporate at elevated temperatures, so the high-temperature interface is evident from the Mie scattering images as the surface of large scattering intensity gradient. The imaging window and high-T interface from Fig. 2b are shown superimposed on the X field in Fig. 2a. These high-T interfaces then allow the determination of the leading point positions.
A significant part of this work concerns the compilation of mole fraction and velocity field data upstream of the flame location. Figure 3 shows averaged radial profiles of axial velocity, u, and fuel mole fraction, X, conditional on the instantaneous leading point positions found as shown in Fig. 2, compared with Gaussian fits to the unconditional profiles at the mean leading point position.
Figure 3. Averaged radial profiles of (a) axial flow velocity, u, and (b) methane mole fraction, X, for the four jet Reynolds numbers. Shown are conditional profiles (in blue) compiled immediately upstream of the instantaneous leading point, and Gaussian curves representing the unconditioned profiles (red) compiled at the mean leading point downstream position.
Sets I through IV represent increasing Reynolds numbers (4400, 6200, 7400 and 10700, respectively). It is significant that the conditional profiles (shown in blue) in Fig. 3 reflect the instantaneous leading point positions. Owing to the fluctuations in the flame location, statistics compiled at the mean leading point positions do not accurately reflect the true flame inflow conditions. For example, the unconditional u and X profiles upstream of the flame are consistent with the expected self-similar Gaussian form, but the conditional profiles are both slightly wider and shallower than the similarity form.
Other observations from Fig. 3 are that the mean axial velocity at the instantaneous leading points tends to be near the laminar flame speed, S_L, which argues for a significant role of fuel-air premixing; and that the methane mole fraction at the leading points tends to be at or below the lean flammability limit (X=0.044), which indicates that combustion takes place toward the inside of the high-T interfaces. Further analysis of the data(details are in the references) shows that the scalar dissipation rates upstream of the leading points are well below quenching values for diffusion flames, again supporting stabilization models that incorporate premixing.
Figure 4. Proposed model for the flame stabilization process, in terms of the large-scale organization of the scalar mixing field. The instantaneous stabilization point (in red) oscillates in order to maintain axial velocities near S_L, and fuel mole fractions within the flammability limits.
From the full analysis of the measurement data, we propose a model for flame stabilization that is rooted in the large-scale organization of the mixing field. Prior studies have established the presence of axisymmetric, helical, and mixed modes. Figure 4 depicts the stabilization model in terms of the axisymmetric mode. The `blobs' in the figure represent coherent mixing structures. The general outline of the model is that the stabilization point tends toward the outside of the mixing structures, where the fuel mole fraction is in the flammable range. When the stabilization point is relatively far from the centerline, the local axial velocity is low, the flame propagates upstream, and the stabilization point moves radially inward to maintain a flammable mixture; with the inward motion, the local axial flow velocity increases, causing the flame to recede downstream, and the stabilization point then moves outward to find a flammable mixture. This results in oscillatory motion that has a period equal to the passage time of the coherent mixing structures. Ongoing work is focused on confirming these observations, including through simulations of partially premixed flames, in which the simulation inflow conditions can be specified using the present experimental measurements.
Selected publications (inquiries: email Lester Su, lsu: jhu.edu)
All materials © 2005 Applied Fluid Imaging Laboratory. Last page update 3.17.05.
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