ENGINEERING AND CFD

Introduction

Combustion in most engineering applications, such as spark ignition engines and gas turbines, often involves elevated pressure conditions and non-unity Lewis number Le fuel-blends. However, under these extreme conditions, the flame morphology becomes increasingly complex and turbulent, persistent with convoluted structures arising due to the presence and interactions of inherent flame instabilities [1]. In this project, direct numerical simulation (DNS) analysis is performed to evaluate and complement existing modelling approaches to account for realistic operating conditions for combustion applications. In this regard, a wide parameter database of Bunsen flames is generated using the higher-order compressible DNS code SENGA [1-5]. The database is representative of wrinkled/corrugated flamelets and thin reaction zones regimes, comprising variations of operating pressure, turbulence intensity and Lewis number conditions.

Results and Methods

DNS, where the flame front is adequately resolved, are used to generate a comprehensive numerical database to analyze and quantify the flame structure and physics under varying operating conditions. Compressible Navier-Stokes equations coupled with a scalar transport equation for the reaction progress variable are numerically solved using the finite difference-based DNS code SENGA, which implements 10^th-order finite differences for space and a third-order Runge-Kutta method for time discretization. The code is fully parallelized using domain decomposition. A generic single-step Arrhenius-type irreversible chemistry was shown to be accurate enough for the analysis in this project. The numerical configuration comprises a burner of diameter n_d in a cubic domain of sides 2d_n and characteristic mesh sizes of about 750³ cells, with all boundaries modelled as partially non-reflecting outlets (except the burner inlet). At the inlet, a hyperbolic-tangent like mean velocity distribution with superimposed pseudo turbulence is imposed.

The database has been established in [1] and analyzed [2] and extended later on with focus on modelling turbulent premixed combustion [3-5]. The present report illustrates a more recent comparison between DNS of a lab scale burner (see Fig. 1) with experiments conducted at RWTH Aachen for three hydrogen diluted atmospheric methane flames. The instantaneous snapshots in Fig. 2 show an excellent qualitative agreement between experiment and DNS confirming the validity of the chosen approach. The development of flame instabilities with increasing hydrogen content which results in an increased turbulent flame speed and a shortening of the flame brush, can be clearly seen. A more quantitative analysis in terms of turbulent flame wrinkling, turbulent flame speed and validity Damköhler’s hypothesis [2] is shown in Fig. 3. The effects of hydrogen dilution can be well captured with the present methodology and illustrate a very good agreement between experiment and DNS at laboratory scale.

References and Links

M.Klein, H.Nachtigal, M.Hansinger, M.Pfitzner, N. Chakraborty. FLOW TURBUL COMBUST, 101(4): 1173-1187, Dec 2018.

[2] N. Chakraborty, D. Alwazzan, M. Klein, R.S. Cant. Proc. Combust. Inst., 37(2): 2231-2239, 2019.

[3] R. Rasool, N. Chakraborty, M. Klein. Combust. Flame, 231:111500, 2021.

[4] R. Rasool, M. Klein, N. Chakraborty. Combust. Flame, 239, 111766, 2022.

[5] V. Mohan, M. Herbert, M. Klein, N. Chakraborty. Energies, 16:2590, 2023.