Influence of Preferential Diffusion in Turbulent Lean Premixed Hydrogen-Rich Syngas Spherical Flames at Elevated Pressure

The objective of this work was to investigate the influence of preferential diffusion on flame structure and propagation of lean-premixed hydrogen-carbon monoxide syngas-air flame at elevated pressure using direct numerical simulation (DNS) and detailed chemistry. The physical problem investigated is lean-premixed H2/CO outwardly propagating turbulent spherical flame at constant pressure of 4bar and at constant turbulent Reynolds number of 150. It is observed that the local flame structure, heat release rate tangential train rate are strongly altered by preferential molecular diffusion at elevated pressure.  Corresponding author: [email protected] Proceedings of the European Combustion Meeting 2015 Introduction Combustion technology is the most important energy conversion method which produces over 80% of the world energy by burning fossil fuels, such as petroleum, coal, and natural gas [1]. However, attention has increasingly turned towards emission control technologies of combustion for the reduction of greenhouse gases (GHGs). In an effort to reduce the GHGs of combustion processes while maintaining high efficiency power generation, development of combustion technology using more environmentally friendly fuels such as high hydrogen content (HHC) syngas becomes important [2]. However, the possibility of burning HHC syngas in modern lean premixed combustion engines can impose challenging constraints which need detailed investigations. Preferential diffusion [3] is one such important physical phenomenon for HHC lean premixed combustion. Preferential diffusion affects chemical reaction and heat transfer that can play a significant role in hydrogen or hydrogen-rich combustion , and it is often described by the Lewis number, Le , defined as the ratio of thermal to fuel mass diffusivity. In the reacting flow field, non-unity Lewis numbers correspond to the potential presence of preferential diffusion effects, while different values of the species Lewis numbers correspond to differential diffusion effects. Direct numerical simulation (DNS) in which the complete spectrum of scales is resolved, produces realistic realisation of turbulent combusting flames which can potentially help to identify preferential diffusion effects in turbulent lean premixed high hydrogen content combustion as well as turbulence/chemistry interactions. The integration of DNS and theoretical formulations enables us to analyse the mechanisms in detail to reveal the underlying physics and therefore systematically link the information obtained from numerical simulations to characterise preferential diffusion effects. The effect of preferential diffusion on hydrogen-air premixed combustion has been investigated in a number of DNS studies. For example, Im and Chen [4] investigated preferenatial diffusion effects on the burning rate of turbulent premixed hydrogen-air flames. Bell et. al. [5] discussed the effect of Lewis number on flame morphology and local flame propagation speed on flame curvature in lean premixed hydrogen turbulent flame. Bisetti et. al. [6] examined the effect of temperature stratitification on the occurance of preferential diffusion during the auto-ignition of lean premixed hydrogen-air mixture. Aspden et. al. [7, 8] investigated characterisation of low Lewis number and the role of Lewis number on flames in the distributed burning regime in turbulent lean premixed hydrogen-air flames. Although much DNS investigations have been carried out for preferential diffusion effects on structure and propagation of hydrogen-air premixed flames at atmospheric pressure, the influence of preferential diffusion on structure and propagation of hydrogen or hydrogen-rich syngas premixed flames at elevated pressure has not been fully investigated. This paper describes DNS investigation of effects of preferential diffusion on flame structure and propagation of lean premixed HHC syngas flame at elevated pressure. Numerical Details In test cases considered here, two-dimensional DNS were performed for HHC lean premixed H2/CO syngas fuel mixture with 70% of H2 and 30% of CO by volume with an equivalence ratio of 0.7. DNS computations involving complete reaction schemes and multicomponent diffusion models remain extremely demanding in terms of computing time and memory.