Investigating the effects of hydrogen enrichment in a partially premixed methane-air flame

Abstract

In the modern era, the rapidly growing demand for energy brings major challenges such as environmental pollution and the depletion of energy resources. This crucial situation requires a reconsideration of existing energy generation methods and a push toward discovering sustainable alternatives. Among the prominent alternatives, hydrogen has emerged as a key energy source and spearheads the current research initiatives. Due to the environmental drawbacks and finite nature of hydrocarbon-based fuels, there is a rising interest in implementing hydrogen in the aviation industry. The potential for hydrogen use in aircraft propulsion systems has become a significant topic of research, primarily due to its benefits in environmental sustainability and system efficiency. Hydrogen's wide flammability range, high energy density, and high laminar flame speed make it a highly attractive fuel for enhancing the efficiency and thrust performance of next-generation aircraft propulsion systems. Its ability to combust over a wide range of fuel–air mixtures allows for flexible engine design, while its fast flame speed contributes to stabilize the combustion even under lean-burn conditions, like ammonia combustion, thereby improving thermal efficiency and reducing emissions. Crucially, hydrogen produces only water vapor when combusted, making it a zero-carbon fuel which is an essential characteristic for meeting stringent global aviation emission targets. Given these advantages, experimental and numerical investigations are essential to optimize hydrogen's integration into hydrocarbon based engines. Research efforts must address challenges such as fuel storage, flame stability, and NOx formation to safely accommodate hydrogen's unique combustion properties. Gas turbines are a commonly used engine type in power plants for electricity generation and aviation for thrusting the aircraft which are typically powered by hydrocarbon fuels. Recent scientific studies have highlighted that partially premixed combustion methods can yield higher efficiency and reduced emissions in these types of engines. In this method, a limited quantity of air is premixed with fuel just before entering the combustion chamber, resulting better efficiency and emission control. The technique facilitates improved fuel use and helps lower pollutant levels. In propulsion system design, the significance of numerical simulations has grown alongside experimental studies, particularly due to the increasing availability of computational power. Today, computational fluid dynamics (CFD) methods serve as effective tools in analyzing aviation propulsion systems, as they mitigate the high costs and avoid the safety concerns associated with experimental testing. CFD approaches such as Reynolds-averaged Navier-Stokes (RANS), large eddy simulation (LES), and direct numerical simulation (DNS) are commonly employed. However, because DNS demands excessive computational resources and memory, it remains impractical for large combustion applications. Therefore, RANS and LES are widely adopted for their balance between accuracy and computational feasibility, with LES offering more detailed and large-scale insights. The enrichment of conventional hydrocarbon fuels with hydrogen has emerged as a prominent research area due to its potential to enhance combustion efficiency and reduce emissions. A survey of existing literature reveals that the majority of past investigations have concentrated on premixed and non-premixed combustion configurations, where the dynamics of hydrogen addition are relatively well-characterized. In contrast, partially premixed combustors remain a new area to explore, particularly with regard to hydrogen-enriched fuel blends. The combustion behavior in such systems is inherently complex, involving intricate interactions between turbulent mixing and chemical kinetics. Despite its practical relevance in aero-engines and industrial gas turbines, limited research has been dedicated to understanding how hydrogen supplementation alters the flame structure, ignition characteristics, heat release zones, and flow field patterns in partially premixed environments. This gap highlights the need for detailed experimental and computational studies to elucidate the influence of hydrogen on flame anchoring, pollutant formation, and overall combustor performance in these transitional combustion modes. This thesis aims to carry out numerical analyses of a partially premixed methane-air flame. The deepen our understanding of the flame dynamics using an experimental combustor geometry to validate our basis. Moreover, by enriching the methane fuel with hydrogen, this research aims to contribute to literature by comparing key parameters such as flame characteristics, flow dynamics, flame stability, and carbon emissions of partially premixed methane-air flame under hydrogen enrichment. In the first part of the thesis, a computational framework is established to simulate the methane-air flame in a swirl-stabilized partially premixed combustor. The open-source flow solver OpenFOAM is used. The large eddy simulations approach with Smagorinsky subgrid scale model is employed. The partially stirred reactor (PaSR) combustion model is utilized to observe the turbulence-chemistry interactions. For detailed chemistry, a reduced GRI-Mech 3.0 mechanism is utilized to get accurate species formation. P1 radiation model is added into the framework to observe the formation of species for emission characteristics away from the flame. The simulations yield detailed insights into the combustor's internal flow and the dynamics of the methane-air flame. The central recirculation zone and the shear layers are successfully captured. The comparison of mean velocity and temperature fields validated the results from the framework with the experimental study. The flame front is investigated using temperature fluctuations and heat release rates. The heat release rate, and temperature fluctuations are analyzed to determine the position of the reaction zone, and it is compared with the hydroxyl chemiluminescence recordings obtained from experimental observations. This comparison showed that the computational framework accurately simulated the reaction zone of the methane-air flame. Lastly, the formation of species such as carbon monoxide and carbon dioxide is investigated. In the final part of the thesis, the hydrogen enrichment to methane-air flame is investigated by changing the fuel composition from 100% methane to volumetrically 40% hydrogen and 60% methane. The hydrogen enrichment did not alter the flow field features such as position of the shear layers, and recirculation zone primarily. However, significant changes are observed in the flame dynamics specifically the temperature distribution, formation of the species, and the position of the flame. The shift of global equivalence ratio to leaner end altered the flame shape from an M-shaped flame to a V-shaped flame. The lift-off height of the flame is decreased and the length of the flame is increased showcasing the effect of the hydrogen enrichment on the flame speed and the anchoring position of the flame. Furthermore, the carbon-monoxide formation is reduced due to the shift of equivalence ratio and the increase in combustion efficiency, and the carbon-dioxide formation is reduced. The energy output of the hydrogen enriched combustor is decreased primarily due to the significant decrease in the fuel mass flow rate compared. The findings of this thesis confirm that hydrogen enrichment improves combustion efficiency and reduces emissions of carbon-based gases. For future work, further analysis of the impact of varying equivalence ratios for hydrogen enriched methane on both flow and flame structures is recommended. Additionally, a deeper exploration of NOx formation behavior would be beneficial. These efforts would advance understanding of hydrogen's potential in energy conversion and support the development of cleaner and more efficient combustion systems.