Flame structure and global flame response to the equivalence ratios of interacting partially premixed methane and hydrogen flames

Abstract

The interacting partially premixed methane and hydrogen flames established in a one-dimensional counterflow field were investigated numerically with the OPPDIF code and GRI-v3.0 was used to consider both fuels. The flame structure and response of the maximum flame temperature, heat-release rate, and flame speed to the equivalence ratios (Φ) and global strain rate (a_g) were investigated. The maximum temperature decreased with increasing a_g. The maximum temperature for cases with a stoichiometric hydrogen-side flame was higher than for other cases with the same a_g.The hydrogen-side flame played a key role in determining the maximum temperature. The maximum heat-release rates (MHRRs) for all cases show different trends. The MHRR of the methane-side flame was affected considerably by the interacting flame structure and hydrogen-side flame condition. However, the MHRRs of the hydrogen were independent of methane-side flame condition. For the cases where Φ of the methane-side flame was varied while the hydrogen-side flame was kept stoichiometric (Var-S), the MHRR and flame speed of the hydrogen-side flame were independent of the methane-side flame conditions. However, the methane-side flames had a negative flame speed except near-stoichiometric conditions. On the other hand, in the cases where Φ of the hydrogen-side flame was varied while the methane-side flame was kept stoichiometric (S-Var), the hydrogen-side flames had the MHRR and flame speed similar to those of an unstretched partially premixed hydrogen flame.

Introduction

For the last few decades, hydrogen has received increasing attention as a developing alternative and non-polluting fuel that can solve increasing environmental pollution problems and the limited availability of fossil fuels. Research on the safety problems that can occur during the production, storage, and utilization of hydrogen is in progress, increasing the expectation of hydrogen utilization as an alternative fuel. Hydrogen leakage and dispersion during hydrogen utilization can lead to fires or explosions by unidentified ignition sources, and accidents related to hydrogen safety have been recently reported [1]. In general, the combustibles in a combustion compartment consist of hydrocarbon fuel components. If a hydrogen fire occurs in the compartment, the fire propagates toward the ambient hydrocarbon fuel combustibles, leading to interacting hydrogen–hydrocarbon flames. This phenomenon could be often identified in real hydrogen accidents [1].

Fundamental investigation of hydrogen fire itself has not been performed thoroughly, even though many studies regarding hydrogen fires have been recently conducted. Hydrogen flame is near-invisible and does not emit soot, CO, or CO₂ because no carbon component is involved. The flame temperature of hydrogen fuel is much higher than that of conventional hydrocarbon flames. However, the radiation intensity from a hydrogen flame is less than that from a hydrocarbon flame because soot, CO, and CO₂ species, which play an important role in flame radiation, are generated only by hydrocarbon flames. Thus, interacting hydrogen and hydrocarbon flames can have peculiar flame structures and radiation behaviors. Studies of interacting hydrogen and hydrocarbon flames in a compartment are required to understand the fundamental characteristics of the interacting flames and improve the safety of the burning process.

Many studies on hydrogen utilization have focused on the combustion of mixed hydrogen and hydrocarbon fuels [2], [3], [4], [5]. The main objectives of these studies differed from those of other studies that have focused on safety-related problems of hydrogen. Schefer et al. [6] investigated the radiation characteristics of hydrogen jet flames to examine the fundamentals of hydrogen fires. In their study, a non-dimensional analysis was performed for the radiant fraction for a given fuel flow rate and heating value. The hydrogen jet flame length was found to be proportional to the flow rate and diameter of the jet nozzle. Mogi et al. [7] and Xu et al. [8] investigated the characteristics of auto-ignition and radiation heat transfer of hydrogen leaked from a highly pressurized vessel using two-dimensional numerical simulations.

Yamada et al. [9] and Briones and Aggarwal [10] investigated the combustion characteristics of nonpremixed and partially premixed hydrogen flames using flame conditions that were similar to those of hydrogen fires. These studies provide the fundamentals of the local structure and behavior of hydrogen flames, even though they did not directly consider hydrogen fires. Briones and Aggarwal [10] also numerically investigated the interaction between the premixed and nonpremixed reaction zones of partially premixed hydrogen–air flames formed in a counterflow field. In their study, the effects of the Lewis number on the structure of partially premixed hydrogen flames and the contribution of each reaction to the heat released were examined quantitatively through chemical kinetic analysis. Azzoni et al. [11] numerically and experimentally observed the interaction between two different rich and lean methane premixed flames established in a slot burner. The two-dimensional structure of partially premixed methane flames and the change in chemical kinetics during the flame interaction were discussed. Additionally, Lockett et al. [12] experimentally and numerically investigated the interaction of two rich and lean premixed methane flames in a one-dimensional counterflow field. In their study, the regions and conditions where a triple flame is stabilized were identified by determining the variation in the strain rate and equivalence ratio of each flame.

Cheng et al. [13], [14] used a one-dimensional counterflow configuration to simulate the combustion of a mixed hydrogen/hydrocarbon fuel. The structure of the interacting lean hydrocarbon (methane or propane) and counterflowing premixed hydrogen flames was examined experimentally and numerically. They provided the optimal chemical reaction mechanism from numerical simulations of the interacting premixed hydrocarbon and hydrogen flames. The methane flame had a negative flame speed when the lean premixed methane flame was affected by the high-temperature combustion products of the hydrogen flame. Katta et al. [15] numerically and experimentally studied on the Double-state behavior of the interacting lean methane premixed flame and lean hydrogen premixed flame for a counterflow configuration. Sohrab et al. [16] experimentally studied the interactions between two premixed hydrocarbon (methane or butane) flames with different equivalence ratios for a counterflow configuration. Various combustion modes existed when downstream interactions between the two counterflowing premixed flames occurred as the equivalence ratio of each premixed flame was varied. They revealed that the extent of the interaction between two premixed flames in a counterflow field was dependent on the separation distance between the two flames, which was related to the equivalence ratios of each flame, as well as the strain rate and effective Lewis number. Chung et al. [17] experimentally and theoretically studied the extinction conditions of interacting premixed hydrocarbon flames in a counterflow field. In their study, the effective Lewis number effects on the flame extinction and conditions during strong and weak interactions of two premixed flames were investigated. Additionally, Lee and Chung [18] numerically studied the structure and extinction conditions of interacting counterflowing premixed methane flames and discussed the role of chemical reactions in the amount of heat released.

Recently, our research group has studied the flame structure of interacting stoichiometric methane and hydrogen flames from the viewpoint of hydrogen fire safety [19]. For simplicity, a one-dimensional counterflow configuration was used for the interaction of the two flames. Studies of interactions between premixed methane and nitrogen-diluted hydrogen flames and syngas and methane flames have also been conducted for a counterflow geometry [20], [21], [22]. However, these studies focused mainly on the utilization of hydrocarbon fuels mixed with a syngas, including hydrogen. Thus, the amount of hydrogen supplied to the combustion zone was small because the premixed hydrogen flame was highly diluted with nitrogen. These studies examined the triple flame structure, with the fuel composition of each flame showing a negative flame speed, extinction limit, and Lewis number effects on the flame structure. To the authors’ knowledge, however, no study of the interaction between counterflowing hydrocarbon and hydrogen flames under partially premixed conditions, similar to the interaction of hydrogen and hydrocarbon fires, has been performed to date.

The main objective of this study was to examine the interaction between partially premixed methane and hydrogen flames. Numerical simulations were used because it is difficult to realize an experiment for the interaction of partially premixed methane and pure hydrogen flames. A one-dimensional counterflow configuration was considered, and the simulations included the detailed chemistry of the flame interaction. Parametric studies were conducted to examine the flame structure and chemical reaction behavior of the interacting partially premixed methane and hydrogen flames as the strain rate and equivalence ratios of each flame were varied.