Three-dimensional structure and burning speed of turbulent premixed H2–air and H2/CH4–air Bunsen flames using high-speed tomographic imaging

Abstract

The deployment of lean premixed hydrogen combustion for carbon-free power generation necessitates a better understanding of flame structure and burning speed, where volumetric information plays a crucial role. This work presents a novel tomographic imaging approach to reconstruct the volumetric Mie scattering signal distribution from seeded droplets in Bunsen flames, enabling detailed measurements of 3D flame surface topology, surface area, and turbulent flame speed. A series of lean turbulent premixed H₂-air and H₂/CH₄-air flames from the Cambridge piloted Bunsen burner were investigated, systematically varying the equivalence ratio, Lewis number, and Karlovitz number. A high-speed tomographic imaging system, consisting of eight simultaneous views was employed to capture volumetrically illuminated Mie scattering within an approximately 20(x)$\times$40(y)$\times$12(z) mm${}^3$ probe volume. The reconstructed 3D signals using the SMART algorithm enables reliable flame front detection and surface triangulation. Based on this reconstruction, flame structures were analyzed by computing mean and Gaussian curvatures, as well as principal curvatures. Results reveal that hydrodynamic instabilities (HDI) induce regular surface oscillations near the Burner exist, while thermodiffusive instabilities (TDI) enhance surface fluctuations near the flame tip. The HDI is found to be more prominent at low-$Ka$ and near-unity $Le$ conditions, whereas TDI dominates in moderate-$Ka$ and sub-unity $Le$ flames, leading to increased surface wrinkling. Additionally, both instantaneous flame surfaces and surfaces based on the mean progress variable were examined and used to derive global and local flame speeds. It was observed that the normalized turbulent flame speed ratio, $S_{\text{T}}/S_{\text{L}}$, can be effectively scaled with turbulence intensity and Lewis number. However, the accuracy of the surface area calculation significantly affects the precise determination of $S_{\text{T}}/S_{\text{L}}$. Overall, the tomographic laser diagnostic technique demonstrated in this study provides valuable insights into the flame structures and burning characteristics of lean turbulent premixed H₂-air and H₂/CH₄-air combustion.