Numerical and experimental investigation of flame dynamics in opposed-flow solid fuel burner

Abstract

Understanding the complex coupled physics of phase change and turbulent gas-phase combustion in solid fuel combustors is critical to the design of advanced propulsion systems. This study considers the combustion of hydroxyl-terminated polybutadiene (HTPB) in an opposed-flow burner (OFB) configuration. High-speed shadowgraph imaging on the OFB is conducted to capture the unsteady flame dynamics surrounding the burner. Unsteady two-dimensional axisymmetric simulations, performed using a high-fidelity 20-species, 109-reaction pressure comprehensive skeletal kinetics mechanism, show improved agreement in the prediction of flame thickness in comparison to previous quasi-one-dimensional modeling, attributed to the significant deviation from self-similarity in the OFB configuration. Experimental and numerical data are compared showing strong trend-wise agreement and providing novel insight into the hitherto unexplored complex dynamics of the OFB configuration. Power spectral density (PSD) profiles of the flame oscillations demonstrate strong agreement in the broad peak frequency between simulations and experiments. PSD of flame thickness, regression rate, and azimuthal vorticity from the computed flame dynamics show strong coupling between the instantaneous regression rate and flame thickness, which is largely driven by the low-frequency vortex shedding from the OFB nozzle lip. Snapshots of the flowfield show a secondary diffusion flame with the majority of $\mathrm{CO}$ to CO₂ oxidation downstream of the fuel grain edge. Variations in the species composition along the fuel surface highlight the complex balance between convective and diffusive forces arising from the proximity of the stagnation plane adjacent to the fuel surface.

Novelty and Significance Statement

This work utilizes large-eddy simulation (LES) to improve the prediction of flame thickness by 1000+% in an opposed flow solid fuel burner (OFB). Demonstration of significant deviation from self-similarity profiles highlight the limitations of current state-of-the art quasi one-dimensional modeling techniques and provides a viable strategy for predictive modeling of solid fuel combustion systems. Accurate prediction of heterogeneous combustion is a critical challenge limiting propellant and fuel discovery. Alongside validating experimental data, high-fidelity numerical simulations of heterogeneous combustion systems are of topical importance to advancing community understanding. High-fidelity modeling and experimental imaging gives unprecedented insight into the coupling between solid fuel combustion, unsteady flame dynamics, and unsteady fluid dynamics.