Unified scaling and shape laws for turbulent premixed methane and hydrogen jet flames

Abstract

The scaling of turbulent premixed flames is typically described by correlations derived for unity-Lewis-number fuels. However, their validity for hydrogen (H$_{2}$) remains uncertain due to the thermodiffusive effects associated with its low Lewis number. In this study, turbulent premixed H$_{2}$ and methane (CH$_{4}$) jet flames are systematically compared over a wide range of operating conditions. Experiments were conducted for Reynolds numbers between 5000 and 60000 and effective Karlovitz numbers spanning 3-368. Flame structure and global flame geometry were characterized using spatially resolved OH$^{*}$ chemiluminescence imaging, allowing consistent comparison between the two fuels across different turbulence intensities. The results are interpreted via a unified framework that incorporates two thermodynamic- and fuel-dependent parameters: a flame speed factor, $α$, representing the enhancement of local burning rates, and a shape factor, $γ$, describing the scaling of mean flame geometry. Despite significant fuel-specific thermodiffusive effects associated with preferential diffusion and intrinsic reactivity, which lead H$_{2}$ flames to exhibit enhanced sensitivity to turbulence and more compact flame configurations, both H$_{2}$ and CH$_{4}$ flames are found to exhibit robust and consistent turbulent scaling behavior when analysed within the proposed unified framework. The resulting correlations provide a generalised description of turbulent burning velocity and flame structure, demonstrating that key turbulence-chemistry interactions can be captured within a common model across fuels with widely different Lewis numbers. Overall, the dataset spans multiple turbulence regimes and flame geometries for both fuels, providing a valuable experimental benchmark for the validation of turbulent combustion models across different regimes.

Figures (7)

• Figure 1: Schematics of the round turbulent jet burner showing the central premixed jet surrounded by a pilot and coflow.
• Figure 2: Ensemble-averaged OH* chemiluminescence showing the effect of varying $Re_{j}$ number and injector diameter for H2 (left) and CH4 (right). Each fuel was tested at different $Re_{j}$ levels and three diameters ($d_{j}$ = 6, 9, 12).
• Figure 3: Normalized turbulent burning velocity as a function of $Re_{j}$ for H2 (top) and CH4 (bottom) flames.
• Figure 4: Scaling parameter $\alpha_i$ as a function of $Re_j$ for H2 (top) and CH4 (bottom).
• Figure 5: Comparison between measured $s_{T} / u_{j}$ and modeled $s_{T}^{m} / u_{j}$ for H2 (top) and CH4 (bottom) flames. The solid line indicates the one-to-one correlation ($y = x$). The zoomed in panel for the H2 cases illustrates the relatively large difference ($\sim 20\%$) between the model and measured $s_{T}$ in the high Karlovitz cases.