Effect of stretch on the burning velocity of laminar and turbulent premixed flames

Steady and unsteady laminar premixed methane/air and hydrogen/air plane-jet flames with different equivalence ratios ranging from fuel-lean to fuel-rich are investigated under atmospheric conditions using DNS with detailed molecular transport and chemistry. The objective is to gain a deeper understanding of the influence of unsteady and nonuniform stretching on flame propagation. A nonuniform velocity profile used at the inlet leads to a stretched flame. For steady-state flames, consumption speeds, flame stretch, curvature, strain and Markstein numbers are evaluated. By increasing the mass flow rate at the inlet, the flames become longer and different Markstein numbers are obtained. The inflow is then harmonically excited with different frequencies and the flames oscillate in the unsteady flow. For these unsteady flames, flame relaxation times are evaluated from the phase shift between the movement of the flame and the fluid flow velocity at the flame surface. The amplitude of the flame front movement is attenuated with increasing frequency and chemical time scale. Also, the phase shift between the movement of the flame and the local flow field becomes larger with increasing frequency or chemical time scale. Due to the flame relaxation time, different Markstein numbers are obtained from different phase angles within one oscillation period. Time averaged and frequency dependent Markstein numbers are computed which become smaller with decreasing frequency. This behavior can be reproduced by a power function in dependence on the Damköhler number

Hybrid eulerian-lagrangian approach for dense spray simulations

In this work, a hybrid Euler-Lagrangian solver for dense spray systems is developed specifically for cases where film creation by accumulation of liquid droplets at the walls plays a crucial role. EulerLagrangian solvers are commonly used to describe the spray with predefined spray characteristics. The Lagrangian particles represent liquid drops moving along the continuous gaseous phase. This approach assumes a small particle size compared to the cell size and it is unable to capture the breakup behavior of liquid jets in the presence of instabilities. VOF methods, on the other hand, are not a computationally feasible option when it comes to small droplet sizes as a result of liquid atomization because they have to be fully resolved by the computational mesh. Hence, multiscale simulations are required to bridge the gap between the two methods combining subgrid droplets in Lagrangian approaches and large liquid structures in VOF methods. In the present work, a multiscale approach is developed where Lagrangian particles representing small droplets are tracked through the continuous phase until they hit a wall or a liquid-gas interface represented by a continuous VOF field. At the time of impact, the Lagrangian particles are removed and the mass and momentum of these particles are transferred to the VOF field. This allows having a spray consisting of subgrid droplets computed with a Lagrangian particle tracking (LPT) approach and liquid films at the walls with VOF method. The method represents a one-way coupling, converting Lagrangian particles to Eulerian liquid phase (VOF) and has been implemented into the open-source CFD code OpenFOAM. Subsequently, the solver has been tested in different scenarios to ensure mass and momentum conservation. Positive test results encouraged its use to gain insight on the fluid flow in a real cylindrical compression-dissolution unit where the liquid is sprayed from the top while simultaneously the gas is compressed from the bottom. Dynamic mesh technique is used to account for the compression with a moving piston

Development of an Openfoam Solver for Numerical Simulation of Carbonization of Biomasses in Rotary Kilns

Carbonization is a key process to increase the energy density of high moisture-containing biomasses and biogenic wastes and to provide multipurpose raw chemicals for further applications. Steam-assisted carbonization is a kind of slow pyrolysis, in which wet biomass is treated continuously in superheated steam at elevated temperature and atmospheric pressure. Rotary kiln reactors due to their flexibility and easy control of operating conditions are well suited for this process. In this work, a numerical simulation tool based on an Eulerian-Langrangian approach has been developed to simulate the carbonization of biomasses in rotary kiln reactors resolved in time and space by combining existing OpenFOAM features and developing new physical models. This study demonstrates the features of this extended and validated Eulerian-Lagrangian approach for simulating dense particulate multiphase flows in large-scale rotary kiln reactors. The focus is to use the new tool to aid the design of large-scale rotary kiln reactors by performing parameter studies. The simulations of this kind of large-scale reactors require large computational resources on supercomputers. Therefore, a further focus lies in different approaches to reduce the computational effort while keeping the accuracy at an acceptable level. By using the MP-PIC model, computing time increases linearly with the number of biomass particles instead of exponentially with the DPM model. The optimal cell size has been found to be about twice the largest particle diameter. By choosing the optimal domain decomposition method, simulation time can be reduced by a factor of 1/10. Introducing a solver frequency parameter to the DOM radiation model can help to reduce simulation times further by a factor of 1/8 while decreasing the accuracy by only 2%. Parallel scaling tests show good performance with over 1000 CPU cores. These results show that simulations with a total of 40,000 CPU-hours per studied case become feasible proving the developed solver to be an efficient tool for the design of rotary kiln reactors

DNS of Near Wall Dynamics of Premixed CH4_4/Air Flames

This work presents a numerical study on the effect of flame-wall interaction (FWI) from the viewpoint of flame dynamics. For that purpose, direct numerical simulations (DNS) employing detailed calculations of reaction rates and transport coefficients have been applied to a 2D premixed methane/air flame under atmospheric condition. Free flame (FF) and side-wall quenching (SWQ) configurations are realized by defining one lateral boundary as either a symmetry plane for the FF or a cold wall with fixed temperature at 20 oC for the SWQ case. Different components of flame stretch and Markstein number regarding tangential, normal (due to curvature) and total stretch, Ka$_s$ , Ka$_c$ and Ka$_tot$ = Ka$_s$ + Ka$_c$, as well as their correlations with respect to the local flame consumption speed SL have been evaluated. It has been shown that the FWI zone is dominated by negative flame stretch. In addition, S$_L$ decreases with decreasing normal stretch due to curvature Ka$_c$ while approaching the cold wall. However, S$_L$ increases with decreasing Ka$_c$ while approaching the symmetry boundary for the free flame case, leading to an inversion of the Markstein number Ma$_tot$ based on Ka$_tot$ from positive in the free flame case to negative in the SWQ case. The quenching distance evaluated based on wall-normal profiles of S$_L$ has been found to be approximately equal to the unstretched laminar flame thickness, which compares quantitatively well with measured data from literature. The flame speed has been confirmed to scale quasi-linearly with the stretch in the FWI zone. The results reveal a distinct correlation during transition between FWI and FF regarding flame dynamics, which brings a new perspective for modeling FWI phenomena by means of flame stretch and Markstein number. To do this, the quenching effect of the wall may be reproduced by a reversed sign of the Markstein number from positive to negative in the FWI zone and by applying the general linear Markstein correlation (S$_L$/S$_{L,0}$ = 1− Ma · Ka), leading to a decrease of the flame speed or the reaction rate in the near-wall region

The Eulerian Stochastic Fields Method Applied to Large Eddy Simulations of a Piloted Flame with Inhomogeneous Inlet

Large Eddy Simulations of the Sydney mixed-mode flame with inhomogeneous inlet (FJ200-5GP-Lr75-57) are performed using the Eulerian Stochastic Fields (ESF) transported probability functions method to account for the sub-grid scale turbulence–chemistry interaction, to demonstrate the suitability of the ESF method for mixed-mode combustion. An analytically reduced 19-species methane mechanism is used for the description of the chemical reactions. Prior to the reactive case, simulation results of the non-reactive setup with cold and hot pilot stream are presented, which show differences in the jet breakup and radial species mass fluxes. The reactive case simulations are compared to experimental data and a recently conducted model free quasi-DNS (qDNS), showing very good agreement with the qDNS in terms of scatter data and radial mean values of temperature and species distribution, as well as mixture fraction conditional statistics. Further analysis is dedicated to sub-grid scale statistics, showing that mixture fraction and reaction progress variable are strongly correlated in this flame. The impact of the number of stochastic fields on the filtered temperature and species distribution is investigated; it reveals that the ESF method in conjunction with finite-rate chemistry is very insensitive to the number of employed fields to obtain highly accurate simulation results