Hydrogen-enriched nonpremixed jet flames: Effects of preferential diffusion

Abstract

Influence of preferential diffusion on the dynamics of hydrogen and syngas nonpremixed impinging jet flames was studied using direct numerical simulation and flamelet generated manifolds based on detailed chemical kinetics. The results presented in this study were obtained from a uniform Cartesian grid with 768 × 768 × 768 points. Reynolds number used was Re = 2000, based on the reference quantities. Results reported here indicate that the preferential diffusion significantly affects the structures and the maximum temperature of the hydrogen flame, which deviates significantly from the results obtained without considering the preferential diffusion. The preferential diffusion results in a shift in the equivalence ratio in the reaction zone to leaner conditions. Moreover, the numerical results suggest that the preferential diffusion influences the flame–wall interaction and thus wall heat transfer, which is critical for the design of combustion equipment for clean combustion applications with high hydrogen contents in the fuel.

Introduction

Global climate change is receiving increasing attention worldwide. International community and national governments are strengthening their efforts to tackle the global climate change across several sectors in the short, mid and long terms. In this context, the technology of clean energy conversion can play a significant role in reducing the greenhouse gas emissions into the atmosphere [1]. 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 [2]. 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 hydrogen and synthesis gas (or syngas) becomes important [3]. However, the possibility of burning hydrogen and syngas in modern combustion devices such as gas turbine combustors or automotive engines can impose challenging constraints which need detailed investigations. Preferential diffusion [4] is an important physical phenomenon for hydrogen-enriched fuels, while near-wall phenomenon [5] can be crucial for all practical combustion applications. Both phenomena can have strong effects on the characteristics of hydrogen-enriched combustion systems and need detailed investigations.

Preferential diffusion affects chemical reaction and heat transfer that can play a significant role in hydrogen combustion [6], and it is often described by the Lewis number, Le, defined as the ratio of thermal to fuel mass diffusivity. The high diffusivity and reactivity of hydrogen may lead to high flame temperatures in combustion [7]. Nonpremixed hydrogen or syngas jet flames are generally mixing controlled due to the fast chemistry of hydrogen combustion. The high diffusivity of light chemical species such as H and H₂ affects flame characteristics through preferential diffusion. 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. Many theoretical, experimental and computational efforts have been devoted to identify the roles of preferential and differential diffusions. Experimental investigations on differential molecular diffusion have been carried out using spontaneous Raman scattering by measuring major species mass fraction, combined with Rayleigh scattering to measure temperature. Bilger [4] reported early work on the influence of differential diffusion on turbulent nonpremixed flames and significant effects of differential diffusion on temperature and species concentrations in a H₂/Air at low Reynolds numbers were later identified [8]. The influence of differential diffusion effects close to the jet nozzle in turbulent H₂/N₂/Air flames was investigated [9] and similar findings relevant to H₂/CO₂/Air and H₂/Air were discussed [10], [11]. Barlow and Frank [12] and Dibble and Long [13] stated that modelling of turbulent nonpremixed flames including preferential diffusion effects represents a challenge, while modelling of differential molecular diffusion in round jet has been carried out, e.g. [14]. Computational modelling of species preferential diffusion and differential diffusion of heat and mass often requires the use of detailed chemistry in multi-dimensional simulations. As a promising tool to provide fine details of the reacting flow field, direct numerical simulation (DNS) of differentially diffusing reacting scalars in isotropic decaying turbulence including the quantification of non-unity Schmidt number effects was reported [15], [16]. Since the pioneering work of laminar flamelet approach by Peters [17], the flamelet formulations for nonpremixed combustion has been developed to account for non-unity Lewis number effects [18], [19]. Investigation on the influence of differential diffusion on the maximum flame temperature of H₂/Air flame was also carried out using two-dimensional (2D) DNS [20]. Nevertheless, there is still a lack of detailed three-dimensional (3D) DNS study on preferential and differential diffusion effects on nonpremixed jet flames.

Investigation on the effects of preferential diffusion on hydrogen-enriched combustion including near-wall flame characteristics is an important area of research particularly for the high hydrogen content fuels, which would improve the current understanding of near-wall flame characteristics of hydrogen-enriched combustion. Obtaining reliable and quantitatively accurate results for preferential diffusion and near-wall combustion are challenging, as a broad range of fluid flow and chemical scales are involved. The complexity of preferential diffusion and near-wall combustion can be better understood through high fidelity numerical simulations such as DNS. The objective of this paper was to investigate the effects of preferential diffusion (non-unity Lewis numbers) on hydrogen and then examine the flame characteristics of hydrogen-enriched syngas flames by accounting preferential diffusion using DNS and flamelet generated manifolds (FGM) chemistry [21]. This work is a continuation of our previous investigation in which the vortical structures due to instabilities in the flame [22] and wall heat fluxes of H₂ nonpremixed flame [23] and fuel variability effects on nonpremixed syngas impinging flames which involves H₂, CO, CO₂ and N₂ [24] were carried out using DNS. In this work, an extended model equation for reaction progress variable which accounts for preferential diffusion has been considered. Given the indications on flame dynamics from previous studies performed using a relatively small nozzle-to-plate distance of the impinging geometry [22], [23], [24], larger distance is adopted in this work since it is important to identify the preferential diffusion effects on the flame characteristics in a more developed primary jet region and secondary wall jet region for the impinging flame. The rest of the paper is organised as follows. Section 2 provides mathematical formulation, chemistry and numerical implementation. Section 3 then discusses the results with respect to H₂ and H₂/CO fuel mixtures. Finally, Section 4 summarises key findings and conclusions.