Fuel and Combustion

Fuel and Combustion

Impact of Diluent Flow Rate on Flame Stability and Pollutant Emission in Coaxial Non-Premixed Burner: A Large Eddy Simulation Study

Document Type : Original Article

Authors
Hamed Helmi
Younes Bakhshan
Pouyan Adibi
Mechanical Engineering Department, University of Hormozgan, Bandar abbas, Iran
10.22034/jfnc.2026.593583.1467
Abstract
Flame stabilization in coaxial burners using steam dilution is an effective strategy for reducing pollutants and stabilizing combustion systems. This study numerically investigates the effect of the steam dilution mass flow rate (SFR) on dynamic stability and pollutant profiles in a coaxial burner configuration using large eddy simulation. The results indicate that increasing the dilution rate directly influences the shear layer structure and hydrodynamic instabilities including the Darrieus-Landau instability by modulating the local Lewis number and the thermal expansion ratio. Simulations confirm that within specific SFR ranges, increased dilution attenuates pressure oscillations and drives the system from a limit-cycle regime to a stable state. While this approach yields significant NOx reduction, analyses reveal that exceeding critical dilution thresholds leads to increased concentrations of unburned hydrocarbons and a risk of flame extinction due to local cooling and impacts on chemical kinetics. The findings of this research provide a numerical framework for determining the optimal burner operating range to achieve a balance between flame stability and environmental performance.

Highlights

This study quantitatively assesses the role of steam dilution in controlling flame stability, emission profiles, and propagation characteristics in non-premixed concentric flames using Large Eddy Simulation (LES). The results demonstrate that increasing the steam-to-fuel mass flow ratio ( ) is an efficient mechanism for suppressing combustion instabilities. Spectral analysis confirms that at low dilution, the flame exhibits strong limit-cycle oscillations at a characteristic frequency of 20 Hz; however, increasing enhances thermo-diffusive damping—driven by modifications in the local Lewis number ( ) and thermal expansion ratio ( )—thereby transitioning the system from a high-amplitude fluctuating regime to a stable, acoustically damped state.

Furthermore, the data indicate that steam injection strengthens the Central Recirculation Zone (CRZ) and modifies shear layer dynamics, which significantly reduces flame-front wrinkling. While this approach effectively regulates the peak flame temperature to suppress formation, the results highlight a critical operating window: dilution levels exceeding  lead to local thermal quenching, characterized by an increase in unburned hydrocarbons, a reduction in oxidation efficiency, and an increased risk of local extinction.

Based on these findings, the following directions are proposed for future research and development:

·         Optimal Burner Operation: Development of stability maps based on non-dimensional parameters (Reynolds, Lewis, and Damköhler numbers) to define optimal operating points for industrial applications.

·         Active Control Strategies: Investigation of active control for thermoacoustic oscillations, specifically through pulsed steam injection techniques.

·         Feedback Control Systems: Implementation and evaluation of real-time feedback control systems based on oscillation indices, such as the Prandtl number or density fluctuations, in both experimental and numerical studies.

·         Geometric Optimization: Improvement of burner and concentric nozzle geometries to reinforce the CRZ structure and extend the flame stability limits under high-dilution conditions.

Keywords
Burner
instability
combustion
Subjects
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