International Journal of Research in Engineering and Science (IJRES) ISSN (Online): 2320-9364, ISSN (Print): 2320-9356 www.ijres.org Volume 2 Issue 2 ǁ Feb. 2014ǁ PP.48-55 www.ijres.org 48 | Page Numerical Experiments of Hydrogen-Air Premixed Flames M.A. Abdel-Raheem 1 , S.S. Ibrahim 2 , W. Malalasekera 1 1 Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, UK 2 Department ofAeronautical and Automotive Engineering, Loughborough University, UK ABSTRACT : Numerical experiments have been carried out to study turbulent premixed flames of hydrogen- air mixtures in a small scale combustion chamber. Flow is calculated using the Large Eddy Simulation (LES) Technique for turbulent flow. The chemical reaction is modeled using a dynamic procedure for the calculation of the flame/flow interactions. Sensitivity of the results obtained to the computational grid, ignition source and different flow configurations have been carried out. Numerical results are validated against published experimental data. It was found that the grid resolution has very small effect on the results after a certain grid. Also, the ignition source has influenced only the time where the peak overpressure appears. Finally, the different configurations are reported to affect both the peak overpressure and flame position. Keywords–Dynamic Flame Surface Density, Hydrogen, LES, Explosion, Reaction Rate I. INTRODUCTION The deployment of hydrogen as a clean fuel and energy carrier brings into consideration the safety problems related to its use. The current interest in hydrogen is due its availability from many resources and the fact that has no carbon emissions. However, to enable its widespread usage in practical applications, tough challenges must be overcome regarding hydrogen and further studies are needed to develop an improved understanding of the issues affecting the generation, storage, distribution as well as combustion of hydrogen. The objective of the present work is to contribute to hydrogen safety by developing numerical capabilities to compute the overpressures resulting from itsexplosion. The study uses the large eddy simulation (LES) technique to calculate the structure of lean hydrogen flames propagating inside a vented combustion chamber while interacting with solid obstructions. The results are validated against experimental measurements reported by Masri et al.[1] and Al-Harbi et al. [2]. The LEStechnique is now accepted as a reliable computational tool to study turbulent flames both premixed and non-premixed [3-8] despite its added computational cost. A key advantage of LES lies in its ability to compute the complex dynamics of turbulent flows and resolve transient processes such as flame propagation, instability, extinction, as well as ignition. The cost and accuracy of LES solutions lie between direct numerical simulation (DNS) and Reynolds Averaged Navier-Stokes (RANS) techniques. A crucial challenge to the advancement of LES lies in the development of adequate sub-grid-scale (SGS) models, which are capable of representing combustion over a wide range of flow and combustion conditions. This paper makes a contribution towards this objective. As the reaction zone thickness of the premixed flame to be resolved is thin, with a characteristic length scale much smaller than a typical LES filter width, an appropriate SGS model is needed to account for chemical reaction. Earlier studies [9, 10] using the dynamic flame surface density (DFSD) model based on laminar flamelets were promising in predicting key characteristics of propagating turbulent premixed flames with built- in solid obstructions. The work presented in this paper is a continuation of previous research [9, 10] where progress has been made in the development of the DFSD model to account for the SGS chemical reaction rate. Here, the same strategy is applied where the DFSD model is used to simulate transient turbulent premixed flames of hydrogen-air mixtures with equivalence ratio of 0.7, propagating in a small vented chamber having baffles and a square solid obstacle. The experimental test facility [1] offered the capability to configure various flow configurations with a range of turbulent flow conditions. The simulations are carried out using different grid resolution and the results are examined for a number of ignition source strength as well as different flow conditions. The numerical results are then validated against experimental data reported in[1, 2]. II. REACTION RATE CLOSURE The experimental chamber used in this study was developed by the University of Sydney, Australia [1, 2]. The combustion chamber has dimensions of 50 x 50 x 250 mm and consists of 3 baffle plates located at equidistance and a solid square obstacle of size 12 x 12 mm at about 96 mm from the ignition end (Fig. 1a). In the experiments the hydrogen-air mixture enters the atmospheric pressure chamber, where the mixture is