Numerical Analysis of Flow and Combustion Characteristics in a Rotary Kiln with Varying dp/ds Ratios

Abstract

The combustion process inside rotary kilns plays a crucial role in determining flame stability, heat transfer performance, and overall thermal efficiency. This study investigates the effect of the dp/ds ratio, defined as the ratio between the diameters of the primary and secondary air nozzles, on jet flame flow characteristics and thermal performance in a rotary kiln using methane (CH₄) as fuel. Numerical simulations were conducted using Computational Fluid Dynamics (CFD) in ANSYS Fluent. The Realizable k–ε turbulence model, non-premixed combustion model with steady diffusion flamelet, and P-1 radiation model were employed to simulate turbulent combustion and radiative heat transfer. A two-dimensional axisymmetric domain was used with three dp/ds ratios of 0.044, 0.070, and 0.096. Model verification against reference data resulted in a Mean Absolute Percentage Error (MAPE) of 1.01%, indicating excellent agreement and reliability of the numerical approach. The results show that increasing the dp/ds ratio enhances fuel-air mixing, shifts the combustion zone closer to the burner inlet, and shortens the flame development region. The highest peak temperature of approximately 2600 K was obtained at dp/ds = 0.044, while increasing the dp/ds ratio to 0.096 reduced the peak temperature to approximately 2000 K due to enhanced air entrainment and lean combustion conditions. Furthermore, higher dp/ds ratios accelerated CH₄ consumption and promoted earlier CO₂ formation, indicating faster combustion completion. The flow analysis revealed stronger jet momentum and entrainment intensity at higher dp/ds ratios, resulting in a more compact flame structure and improved mixing characteristics. Overall, the dp/ds ratio significantly influences the flow field, flame behavior, and thermal performance within the rotary kiln, making it an important parameter for burner design and combustion optimization.