A Multi-Scale Numerical Model for Investigation of Flame Dynamics in a Thermal Flow Reversal Reactor

In this paper, the flame dynamics in a thermal flow reversal reactor are studied using a multi-scale model. The challenges of the multi-scale models lie in the data exchanges between different scale models and the capture of the flame movement of the filtered combustion by the pore-scale model. Through the multi-scale method, the computational region of the porous media is divided into the inlet preheating zone, reaction zone, and outlet exhaust zone. The three models corresponding to the three zones are calculated by volume average method, pore-scale method, and volume average method respectively. Temperature distribution is used as data for real-time exchange. The results show that the multi-scale model can save computation time when compared with the pore-scale model. Compared with the volumetric average model, the multi-scale model can capture the flame front and predict the flame propagation more accurately. The flame propagation velocity increases and the flame thickness decreases with the increase of inlet flow rates and mixture concentration. In addition, the peak value of the initial temperature field and the width of the high-temperature zone also affect the flame propagation velocity and flame thickness.

Numerical Simulation of the Deflagration to Detonation Transition in a Tube with Repeated Obstacles: Experimental Scale Simulation Using the Artificial Thickened Flame Method

The Artificial Thickened Flame (ATF) method, which involves artificially increasing the flame thickness so as to simulate with a coarse grid resolution, is applied to reduce the computational cost of predicting the Deflagration to Detonation Transition (DDT) in a tube with repeated obstacles. While simulation results depended on the parameter N (the number of grid points in laminar flame thickness), it was found that N values of more than 10 may be excessive. The results show that the chosen simulation method predicts the flame speed as compared to a reference experiment and captures the detail of the strong ignitions near the corner between the obstacle and the sidewall. The present simulation also captures the wrinkle flame front structure during the acceleration of flame.

A Combustion Regime-Based Model for Large Eddy Simulation

The aim of this work is to propose a unified (generalized) closure of the chemical source term in the context of Large Eddy Simulation able to cover all the regimes of turbulent premixed combustion. Turbulence/combustion scale interaction is firstly analyzed: a new perspective to look at commonly accepted combustion diagrams is provided based on the evidence that actual turbulent flames can experience locally several combustion regimes although global non-dimensional numbers would locate such flames in a single specific operating point of the standard combustion diagram. The deliverable is a LES subgrid scale model for turbulent premixed flames named Localized Turbulent Scales Model (LTSM). This is founded on the estimation of the local reacting volume fraction of a computational cell that is related to the local turbulent and laminar flame speeds and to the local flame thickness.

Numerical and Experimental Investigations on Combustion Characteristics of Premixed Lean Methane–Air in a Staggered Arrangement Burner with Discrete Cylinders

Premixed combustion of lean methane–air in an artificial porous media burner with staggered alumina cylinders was experimentally and numerically performed. Numerical simulations were conducted at gas mixture velocities of 0.43–0.86 m/s and equivalence ratios of 0.162 and 0.243, respectively. Through comparison with experimental results, temperature distribution, peak temperature and flame propagation velocity are analyzed and discussed in detail. The numerical calculated temperature profile over the axis of the combustor coincided well with test data in the post-flame zone, however a certain deviation was found in the preheated zone. A two-dimensional flame shape was observed and the flame thickness was the size of cylinder diameter. The peak temperature increased with the gas mixture inlet velocity at the certain equivalence ratio, and its peak value was about 1.8–2.16 times higher than the adiabatic combustion temperature under the desired equivalence ratio, which indicates that super-adiabatic combustion was the case for all the numerical simulations. The flame propagating velocity had a positive correlation with the gas mixture inlet velocity.

Large Eddy Simulation of a Turbulent Spray Jet Flame Using Filtered Tabulated Chemistry

This work presents Large Eddy Simulations of the unconfined CORIA Rouen Spray Burner, fed with liquid n-heptane and air. Turbulent combustion modeling is based on the Filtered TAbulated Chemistry model for LES (F-TACLES) formalism, designed to capture the propagation speed of turbulent stratified flames. Initially dedicated to gaseous combustion, the filtered flamelet model is challenged for the first time in a turbulent spray flame configuration. Two meshes are employed. The finest grid, where both flame thickness and wrinkling are resolved, aims to challenge the chemistry tabulation procedure. At the opposite the coarse mesh does not allow full resolution of the flame thickness and exhibits significant unresolved contributions of subgrid scale flame wrinkling. Both LES solutions are extensively compared against experimental data. For both nonreacting and reacting conditions, the flow and spray aerodynamical properties are well captured by the two simulations. More interesting, the LES predicts accurately the flame lift-off height for both fine and coarse grid conditions. It confirms that the modeling methodology is able to capture the filtered turbulent flame propagation speed in a two-phase flow environment and within grid conditions representative of practical applications. Differences, observed for the droplet temperature, seem related to the evaporation model assumptions.

A General Study of Counterflow Diffusion Flames for Supercritical CO2 Combustion

A counterflow diffusion flame for supercritical CO2 combustion is investigated at various CO2 dilution levels and pressures by accounting for real gas effects into both thermal and transport properties. The UCF 1.1 24-species mechanism is used to account the chemistry. The nature of important nonpremixed combustion characteristics such as Prandtl number, thermal diffusivity, Lewis number, stoichiometric scalar dissipation rate, flame thickness, and Damköhler number are investigated with respect to CO2 dilution and pressure. The results show that the aforementioned parameters are influenced by both dilution and pressure; the dilution effect is more dominant. Further, the result shows that Prandtl number increases with CO2 dilution and at 90% CO2 dilution, the difference between the Prandtl number of the inlet jets and the flame is minimal. Also, the common assumption of unity Lewis number in the theory and modeling of nonpremixed combustion does not hold reasonable for sCO2 applications due to large difference of Lewis number across the flame and the Lewis number on the flame drop significantly with an increase in the CO2 dilution. An interesting relation between Lewis number and CO2 dilution is observed. The Lewis number of species drops by 15% when increasing the CO2 dilution by 30%. Increasing the CO2 dilution increases both the flow and chemical timescales; however, chemical timescale increases faster than the flow timescales. The magnitudes of the Damköhler number signify the need to consider finite rate chemistry for sCO2 applications. Further, the Damköhler numbers at 90% sCO2 dilution are very small; hence, laminar flamelet assumptions in turbulent combustion simulations are not physically correct for this application. Also, it is observed that the Damköhler number drops nonlinearly with increasing CO2 dilution in the oxidizer stream. This is a very important observation for the operation of sCO2 combustors. Further, the flame thickness is found to increase with CO2 dilution and reduce with pressure.