Auto-Ignition and Numerical Analysis on High-Pressure Combustion of Premixed Methane-Air mixtures in Highly Preheated and Diluted Environment

Abstract

The present work investigates both the autoignition and the combustion characteristics on highly preheated and diluted combustion of laminar premixed stoichiometric CH4/O2/N2 mixture in a cylindrical combustor operating at elevated pressure. The analysis has been carried out for a range of operating parameters such as reactant preheat temperature of 1100-1500 K, combustor pressure of 1-10 atm, and highly diluted mixture, which is achieved by decreasing the oxygen content in the oxidizer from 21 to 3% in volume basis. The simulations have been carried out using the laminar premixed adiabatic PFR (plug flow reactor) model of Ansys Chemkin Pro. Twodimensional pictorial representation is carried out using finite volume-based CFD code Ansys Fluent 19.2. Finite-rate chemistry with detailed chemical mechanism GRI Mech 3.0 is used for combustion analysis. The results show that OH and HCO mole fractions are decreased with increased combustor pressure and N2 dilution (or decreased O2 content) while it increases with the reactant's temperature. Also, it has been found that by reducing the oxygen content in the mixture, the flame gets stabilized far away from the combustor inlet. In contrast, an increase in combustor pressure and reactant temperature stabilize the flame towards the combustor inlet. The flame stabilization characteristics at different locations of the combustor are explained in terms of ignition delay time, which is calculated using the closed homogenous reactor (CHR) model available in the Ansys Chemkin Pro package. The flame peak temperature decreases with an increase in N2 dilution and increases with increasing the reactant temperature. Moreover, the peak temperature varies marginally with increasing the combustor pressure. Finally, a regime diagram is prepared to show the various combustion mode such as HiTAC, MILD combustion, and No ignition region as a function of O2 content and reactant temperature for different operating pressure. The CO and NO emission are reduced with an increase in pressure in the MILD combustion region.

Modified Reacting Solver: A Simplified Approach for Capturing the Molecular and Flow Diffusivities for the Nonpremixed MILD Flames

“…Since there is no built-in solver in OpenFOAM to feed the individual Lewis number, the required new variables are created in the library and the above equations are modified in reactive solver in the OpenFOAM 8. In addition to the above governing equations, the direct integration approach is used to find the chemical source term in the reactive flow . The discrete ordinate (DO) radiation model is used in the present study with a weighted sum of the gray gas model (WSGGM) to compute the radiative heat transfer through appropriate prediction of the emission and absorption coefficients.…”

“…In addition to the above governing equations, the direct integration approach is used to find the chemical source term in the reactive flow. 53 The discrete ordinate (DO) radiation model 54 is used in the present study with a weighted sum of the gray gas model (WSGGM) to compute the radiative heat transfer through appropriate prediction of the emission and absorption coefficients.…”

Numerical and Chemical Kinetic Analyses on the Formation of CO and CO2 for C1–C4 Hydrocarbon Alkanes in a Hot Co-Flow under MILD Combustion

“…Flow and velocity fields are modeled using the standard κ–ε turbulence model. The value of C ε1 in this model is changed from its default value of 1.44 to 1.6 following the previous investigations. ,,− The optimized eddy dissipation concept (EDC) model and a reduced chemical mechanism recently developed by Sharma et al are considered for turbulence and chemistry interaction. The success of the EDC model in capturing MILD combustion characteristics has been verified in the previous research works. ,,, This model assumes that the chemical reaction takes place in the fine-scale structures, which are in the order of the Kolmogorov length scale.…”

Numerical study of inlet air conditions on methane flameless combustion characteristics

“…Flameless combustion has a lot of different names and expressions in reviewed articles. Some of researches called the flameless phenomenon as HiTAC [2], CDC [3,4], FLOX [1,5] and MILD combustion [6,7]. In flameless mode, the mixture shall be diluted by internal recirculated combustion products.…”