LARGE EDDY SIMULATION OF TURBULENT PREMIXED FLAMES PROPAGATION IN A SMALL SCALE VENTING CHAMBER : INFLUENCE OF CHEMISTRY AND TRANSPORT MODELLING

Flame propagation in a semi-confined chamber with obstacles con titutes a representative configuration of gas explosion phenomena occurring in building s like power plants or refineries. In order to understand them, the small-scale combustion chamb er built at the University of Sydney [1] is studied by Large Eddy Simulation (LES). Four config urations which differ by the number of turbulence generating grids along the flame path ar e considered. The aim of this study is twofold : the first objective is to asse ss the capability of the Computational Fluid Dynamics (CFD) code AVBP to predict the critica l p rameters related to building safety. The second one is to investigate the influence of the c hemistry modelling on the results, in the context of reduced schemes. Results show the ability of AVBP to reproduce accurately the ov r-pressure generated in the explosion chamber in terms of timing and magnitude for the fo ur configurations. The analysis reveals that a correct description of the initial laminar pr opagation phase, which occurs between the ignition of the mixture and the interaction with the obst acles, is essential in the prediction of the over-pressure: (1) the burnt gas temperature must be a ccurately predicted by the reduced chemical scheme to obtain the right expansion ratio, (2) the use of realistic species Lewis numbers is mandatory to capture the flame response to curvature. Only the combination of these two ingredients allows a good prediction of the laminar phase, w hich largely influences the whole combustion process and the resulting over-pressure histor y. Introduction Accidents due to gas explosions have generated a growing int erest in safety aspects of design and operation condition in industrial buildings. A major co n ern for oil and chemical industries is to be able to locate precisely the hazardous areas in build ings, typically offshore oil and gas producing platforms, in order to design them as safe as possi ble. Thanks to the growing computational power, Computational Fluid Dynamics (CFD) appe ars as an interesting alternative to experiments which are expensive, and may be also dangerou s. Nowadays codes based on a URANS (Unsteady Reynolds Averaged Navier Stokes) approach are classically used to simulate gas explosion configurations at industrial scales [2, 3 ]. The emergence of the Large Eddy Simulation (LES) approach opens a new perspective on this fie ld of application. This technique has already shown its ability to give more reliable predicti ons than URANS in unsteady complex configurations, especially in gas explosion related st u ies [4, 5], even if this statement is most of the time restricted in practice to laboratory-scale experiments. The experimental setup of Sydney [1] is a typical example of t hese small-scale experiments built to study the mechanisms of gas explosions in semi-confi ned environments. It consists in a vented explosion chamber where a premixed flame propagates fi rst as a laminar spherical front, gets wrinkled by obstacles and becomes a turbulent flame prop agating at very high speeds. The interaction of the flame with the obstacles is known to lar gely control the over-pressure in the chamber, which is certainly the critical parameter in industrial safety. This configuration is particularly attractive to study gas explosion phen omena by LES: the dimensions of the chamber are low (0.625 litre) and a lot of published experime ntal and numerical data are available [1, 5, 6]. The main objective of this work is to assess the capability of the LES solver AVBP to capture the specific phenomena controlling flame propagation in semi-confined environment and to correctly predict the parameters of critical relevance f or safety related study, in particular the over-pressure peak in the chamber. A second objective is to h ghlight the influence of chemistry modelling in this kind of applications. The flame propagatio n in a stagnant flow is an academic problem mixing laminar phases (controlled by chemistry and thermodiffusive effects), and turbulent phases (where the influence of the flow structures will dominate). Constructing an LES which can handle both phases is a new problem, which has been a lmost untouched up to now. The impact of the number of steps used to describe chemistry w hen reduced schemes are used is studied. The importance of the unity Lewis assumption, of ten used in the context of reduced schemes, is also investigated using existing theories and D irect Numerical Simulations (DNS) of academic configurations. The test case The configuration studied in this work was set up at the Univer s ty of Sydney [1]. It consists in a square cross section premixed combustion chamber (25cm x 5c x 5cm) with solid obstacles. The obstacles consist in three removable turbulence g en rating gris and one fixed central obstacle, as shown in Fig. 1 (left). The bottom end of the cham ber is closed and the top end is opened out to the atmosphere. The vessel is initially fille d with a stoichiometric mixture of air and LPG (88% propane by volume) at atmospheric pressure a nd temperature. The mixture is then ignited by laser and the flame propagates past the obst acle . Additional details may be found in Kentet al. [1]. Config 0 Config 1 Config 2 Config 3