Graduate Thesis Or Dissertation
 

The Global Consumption Speeds of Premixed Large-Hydrocarbon Fuel/Air Turbulent Bunsen Flames

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  • Large-hydrocarbon fuels are used for ground and air transportation because of their high energy-density and will be for the foreseeable future. However, combustion of large-hydrocarbon fuels in a turbulent environment is poorly understood and difficult to predict. The turbulent flame speed, which is the velocity at which a flame front propagates through a turbulent fuel and air mixture, is a key property in turbulent combustion. The turbulent flame speed can be used as a model input parameter for turbulent combustion simulations. However, turbulent flame speeds for large-hydrocarbon fuels are largely unknown. These values are needed to improve combustion models and enhance understanding of the physics and chemistry that control turbulent combustion of large-hydrocarbon fuels. The objective of this study is to measure the turbulent flame speed of large-hydrocarbon fuels and to identify key physics in the turbulent combustion of these fuels. This is motivated by the use of the turbulent flame speeds in modeling combustion in practical devices and the significant use of large-hydrocarbons in these devices. This research has broad implications for society and industry; both the Federal Aviation Administration and gas turbine engine companies have called for research on the turbulent flame speeds of large-hydrocarbon fuels. The turbulent flame speed in this work is defined as the global consumption speed, and is measured for three fuels on a turbulent Bunsen burner. The Reynolds number, turbulence intensity, preheat temperature, and equivalent ratio can be independently controlled using the burner. A conventional Jet-A fuel, known as A2, is used as a reference because of its common use in commercial and military aviation. A2 is compared to bi-modal and quadra-modal blends referred to as C1 and C5, respectively. These fuels are selected as they have similar heat releases and laminar flame speeds. Time-averaged line of site images of OH*, CH*, and CO₂* chemiluminescence are used to determine an the average flame front area. This flame area is used to determine the global consumption speed. The global consumption speed is measured for Reynolds number and equivalence ratio ranging between 5.000-10.000 and 0.7-1, respectively. Turbulence intensities are varied between 10% and 20% of the bulk flow velocity. The global consumption speed increases with turbulence intensity and Reynolds number for all fuels. Global consumption speeds for A2 and C5 match within 5% at all conditions. Conversely, the global consumption speed of C1 is up to 22% lower than A2 or C5. These results indicate the global consumption speed is sensitive to turbulent velocity fluctuations, bulk flow velocity, and fuel chemistry. These results together suggest the global consumption speed is additionally sensitive to flame stretch. Dimensional analysis is used to isolate and identify sensitivities of the global consumption speed to turbulent velocity fluctuations, bulk flow velocity, global stretch rate, and fuel chemistry. A clear sensitivity to fuel chemistry is observed and is affected by aromatic and alkane content. A2 and C5 have higher global consumptions speeds and increased stability; these fuels have shorter average hydrocarbon chain lengths and higher aromatic content than C1. In addition, the global consumption speed is highly sensitive to turbulence intensity of the flow; the turbulent flame speed increases an average of 30% for all fuels between the minimum and maximum turbulence intensity cases. Results are attributed to a strong sensitivity of the global consumption speed to flame stretch and a strong coupling of turbulence and fuel chemistry effects. These conclusions agree with the available literature and provide a foundational understanding of the sensitivities of the global consumption speed for large-hydrocarbon fuels.
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