Measuring laminar burning velocities

Thesis

The laminar burning velocity of a fuel is the rate of normal propagation of a 1D flame front relative to the movement of the unburned gas. This is a fundamental property of a fuel that affects many aspects of its combustion behaviour. Experimental values are required to validate kinetic simulations, and also to provide input for models of flashback, minimum ignition energy and turbulent combustion. Burning velocity affects burn duration and consequently power output in spark ignition engines. Burning velocities are affected by pressure, temperature, equivalence ratio, residuals, additives, and stretch rates. The constant volume vessel has been used as it is considered both the most versatile and accurate method of measuring laminar burning velocities. An existing combustion vessel and oven were refurbished and new systems built for fuel injection, ignition, experiment control, data acquisition and high speed schlieren photography. An existing multi-zone model was used to allow calculation of burning velocity from pressure and schlieren data, allowing the user to select data uncorrupted by heat transfer or cellularity. A twelve term correlation for burning velocity was validated using methane modelling data. The chosen data from all the experiments was then fitted to the correlation. Methane, n-butane, n-heptane, iso-octane, toluene, ethylbenzene and ethanol were tested over a wide range of initial pressures (0.5, 1, 2 and 4 barA), temperatures (289-450 K) and equivalence ratios (0.7-1.4). For liquid fuels, tests with real residuals at mole fractions of up to 0.3 were also conducted. Stoichiometric mixture tests were performed at two initial temperatures (380 and 450 K) and the same four initial pressures. For mixtures of iso-octane and ethylbenzene, percentage volumes of 12.5, 25, 50 and 75% iso-octane were tested. It was found that the the percentage of iso-octane affected burning velocity non-linearly. For iso-octane/ethanol, a single 50:50% mixture was tested.

Authors: Marshall, S; Stephen Marshall

• Publication date: 2010
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: residuals; combustion; flame speeds; laminar burning velocities
• Subjects: Combustion; Mechanical engineering