Abstract
The convective ignition of solid fuel (PMMA) in a sudden-expansion combustor is investigated from the perspective of flame-vortex interactions. Most of the studies of sudden-expansion combustors focus on the non-reacting flow and patterns of heat transfer. For the reacting flow, the premixed cases are more comprehensively discussed. Literatures dedicated to the phenomena of solid fuel ignition within the sudden-expansion combustors are rare. Adequate mixing between the fuel vapor and the oxidizing stream is critical to the ignition. It has been demonstrated from jet diffusion-flame that the mixing is approached via vortex roll-up. Moreover, the corresponding strain effects associated with vortices have also been investigated. Accordingly, the connections between vortices and the ignition/flame spread of the present study are expected to be significant. Nonetheless, for the transient flame spread over the surface of solid fuel in actual sudden-expansion combustors, the effects of shedding vortices and the turbulence behavior have rarely been addressed.A connected-pipe test facility for ramjet applications was established in this study. The heated oxidizing stream required for the experiment (Reh = 6200, U0 = 22 m/s, [O2] ~ 11.7 %, and T0 = 810oC) is generated by the combustion of air-LPG mixture within the vitiator. A PMMA (polymethylmethacrylate) slab (thickness = 6 mm) serves as the fuel grain, and the step height (h) is 35 mm. Also, a cold flow with wall mass injection was conducted to simulate the effects of fuel vapor on the flow behind a step. Transient flow visualization and measurements with particle image velocimetry (PIV) of both nonreacting and nonpremixed reacting flows were undertaken.While large-scale coherent vortical structures dominate mixing and thus the nonpremixed reaction, dilatation from the reaction affects back to these structures. The mixing-induced reaction results in a great difference of large vortical structures between the nonpremixed reacting and nonreacting flows. Small eddies form large-scale coherent vortical structures in the nonreacting flow, whereas no apparent small eddies were found in moderate-scale coherent vortical structures in the nonpremixed reacting flow. Vortical characteristics that differ among nonreacting, premixed and nonpremixed reacting flows are demonstrated and indicated.Three phases of the transient flame spread are identified via the diagnostics of flow visualization and particle image velocimetry (PIV). The dominance of small/large vortices is revealed respectively in the pre-/post-ignition regimes, which demonstrates the small-to-large vortex transformation due to heat release. Attributed to the decreased characteristic reaction time and enhanced mixing, the first ignition is observed at the downstream end of the fuel, after which a primitive flame is formed and initiates the opposed flame spread. During the spread, the rolling behavior of flame kernels are considered to be dominated by the small eddies. The combined effects of broken vortices and continuing pyrolysis introduce the periodical extinction-reignition around the reattachment region. At the final phase, the entrainment of flame kernels into the shear layer is facilitated by the large shedding vortices, and a sustained diffusion flame is established.The transient dynamic interactions before a new steady state between a wall mass injection and the sudden-expansion flow were revealed. The results can simulate the transient effects on the SFRJ flow due to the high-temperature unsteady vaporized fuel gas. The uniform injection into the recirculation is nonuniform due to the dynamic interactions between the corner eddy and the recirculation bubbles. The initial intruding of wall mass injection always stars at the near-wall region 0.5 ~ 1.5 h downstream the step. The mixing in the recirculation region is reduced with increasing wall mass injection rate.The interactions after a new steady state between a wall mass injection and the sudden-expansion flow were investigated. The results can simulate the effects on the SFRJ flow due to the high-temperature vaporized fuel gas before ignition and after ignition. Low mass injection rate (Qw = 50 L/min) reduces the fluctuations with inlet small/large Reynolds numbers. The region of 1 h downstream the step is the most affected due to the wall mass injection. As a perspective of wall cooling, a compromise condition is found to conserve the coolant and sustain the mixing characteristics of the designed SFRJ.The study not only provides novel insights into the convective ignition of solid fuel in the separation-reattachment flow, but also serves as a basis for the advancement of ignition control.