Effect of Hydrogen Addition on the Flammability Limit of Stretched Methane/Air Premixed Flames

A computational study is performed to investigate the effects of hydrogen addition on the fundamental characteristics of stretched methane/air premixed flame in an opposed flow configuration. The problem is of interest as a potential application to gas turbines and spark-ignition engines, where it has been anticipated that addition of a small amount of hydrogen will extend the lean flammability limit, allowing combustion at leaner conditions to achieve lower NOx emission. The flame response is first studied under steady conditions with different levels of hydrogen addition. The results show that the extinction strain rate and the lean flammability limit are significantly extended due to the presence of hydrogen in the mixture. On the other hand, the consumption speed and time scale of the flame close to extinction were found to be insensitive to the extent of blending. Further simulations were performed in an unsteady opposed-flow configuration to study the effects of mixture stratification at various time scales. The dependence of the dynamic flammability limits on the mean composition was determined at various frequencies and compared with a pure methane-air flame. Introduction In many practical applications for power generation, such as gas turbines, there has been strong interest in achieving lean premixed combustion. The advantages of operating at very lean mixture conditions are high thermal efficiency and low emissions of NOx due to lower flame temperatures. However, operating very close to the lean flammability limit has the drawback of local extinction, emissions of unburnt hydrocarbons, and large-amplitude oscillations in pressure that can result in mechanical damage [1]. These considerations demand the mixture to be significantly richer than what would otherwise be desirable. It has been shown in earlier studies that blending of hydrogen with hydrocarbon fuels improves its lean flammability limit and flame propagation speed [2, 3, 4], thereby enabling stable combustion at lean mixture conditions. In the case of natural gas engines, enriching the fuel with hydrogen has the proven benefits of improving the combustion stability and reducing the emissions [5, 6, 7]. The available results indicate a definite advantage in blending hydrogen, provided an economical and efficient source of hydrogen can be established. In this paper, we study the effects of hydrogen addition on stretched premixed methane-air flames. It is known from earlier studies [2, 3, 4] that blending of hydrogen causes an increase in the laminar flame speed ( ) of a freely propagating flame due to an increase in the flame temperature (thermal effect) and an increased supply of active radicals (chemical effect). In a more recent work that considered the effect of strain rate [8], Ren et al. concluded that the increase in extinction strain rate due to hydrogen addition outweighs Corresponding author: [email protected] Proceedings of the Third Joint Meeting of the U.S. Sections of The Combustion Institute the effects on flame speed and flammability limit. The primary objective of this paper is to quantify the increased immunity of lean premixed flames to flow strain due to the blending of hydrogen. In particular, the mixing between diffusive-thermally neutral methane and highly diffusive hydrogen can introduce unique dynamic behavior of stretched premixed flames. Another focus of the study is the basic characteristics of premixed combustion in an inhomogenous mixture field. Due to the lack of understanding of the flame response under a spectrum of various mixture composition due to the initial stratification and subsequent turbulent mixing process, several earier simulations of the direct injection spark ignition (DISI) engine have relied on empirical assumptions for the combustion models [9, 10, 11]. In the last symposium [12], we demonstrated that a stretched flame with transient composition fluctuation is able to sustain combustion even if the equivalence ratio temporarily becomes lower than the steady flammability limit for a certain duration of time. This observation is further examined in the present study by exploring the effect of mixture stratification in a system with multi-component fuels and the dependence of the dynamic flammability limit on the characteristic time scale of the flame. Formulation and Numerical Method The computational configuration is a counterflow premixed flame between two opposing axisymmetric nozzles separated by a distance . The governing equations for this configuration and details of the numerical implementation can be found in Ref. [13]. The steady solutions are obtained using a modified version of OPPDIF [14] and the unsteady solutions using OPUS [15]. These codes are interfaced with Chemkin [16] and Transport [17]. The full methane-air kinetic mechanism (GRI 3.0) [18] including NOx chemistry has been used. The radiative heat transfer term is included by using the optically-thin gas approximation, where the Planck mean absorption constants are taken from Ju et al. [19]. To understand the strained premixed flame characteristics, a symmetric back-to-back premixed flame is established by supplying premixed fuel-air mixture at identical conditions from both nozzles. Only a half of the domain is actually solved by applying the symmetry boundary condition at the stagnation plane. A zonal grid refinement method is used to capture the flame moving in the computational domain [13]. The grid convergence is fully tested to ensure the numerical accuracy of the solutions. The reactant stream consists of a premixed -air mixture. The temperature at the inlet is set to 300K and the pressure is 1 atm for all the test cases. Due to the presence of multiple fuels in the mixture, it is necessary to suitably define the equivalence ratio such that it takes into account the overall stoichiometry of the mixture. Following Yu et al. [2], we define two parameters, and , characterizing the effective equivalence ratio and extent of hydrogen blending respectively, namely