Combustion instability mitigation by magnetic fields

The present interdisciplinary study combines electromagnetics and combustion to unveil an original and basic experiment displaying a spontaneous flame instability that is mitigated as the non-premixed sooting flame experiences a magnetic perturbation. This magnetic instability mitigation is reproduced by direct numerical simulations to be further elucidated by a flow stability analysis. A key role in the stabilization process is attributed to the momentum and thermochemistry coupling that the magnetic force, acting mainly on paramagnetic oxygen, contributes to sustain. The spatial local stability analysis based on the numerical simulations shows that the magnetic field tends to reduce the growth rates of small flame perturbations.

Figure 1

(a) Schematic of the arrangement allowing the experiment: (1) coflow burner, (2) coils of the electromagnet, (3) continuous wave laser, (4) shutter, (5) neutral density filter, (6) set of beam expanding optics, (7) rotating diffusive disk, (8) set of collection optics, and (9) camera. (b) Close-up view of the laser beam crossing the flame in between both coils. (c) Typical frame captured by the camera as the shutter is open. The burner tip can be seen at the bottom and the soot layer produced in the flame right above.

Figure 2

Normalized soot volume fraction fields $f_{\text{v}}$/$f_{\text{v,max}}$. Experiments (a) and simulations (b). The leftmost four images show oscillating fields without magnetic field, the isolated image on the right shows the steady flame stabilized by a magnetic field. The phase angle is computed as $\varphi ={360}^{\circ }f\mathrm{\Delta }t$, where $f$ is the respective frequency (experiment: 12.6 Hz; simulation: 15.6 Hz) and $\mathrm{\Delta }t$ the physical time after the occurrence of the maximum flame height.

Figure 3

Normalized computed radial mean flow profiles. (a) Axial velocity $U_m$, (b) temperature $T_m$, and (c) ethylene $Y_{{\text{C}}_{\text{2}}{\text{H}}_{\text{4}},m}$ and (d) oxygen $Y_{{\text{O}}_{\text{2}},m}$ mass fractions at $z/D_f=0.4$ (circle), 0.6 (cross), and 1.0 (triangle) without (solid line) and with (dashed line) exposure to the steady magnetic-field gradient.

Figure 4

Perturbation growth rate $-{\alpha }_i$ as a function of the perturbation frequency $\beta$. Without (solid line) and with magnetic field influence (dashed line) for $z/D_f=0.4$ (circle), 0.6 (cross), and 1.0 (triangle).