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Research in flammability limits and deflagration to detonation transition of ethanol

Bekius, H.M. (2013) Research in flammability limits and deflagration to detonation transition of ethanol.

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Abstract:The flammability limits can be divided in an upper and lower flammability limit. The lower flammability limit is defined as the minimum concentration of fuel for which flame propagation is possible. For the upper flammability limit this is the maximum concentration for which flame propagation is possible. The knowledge of the flammability range of a certain fuel is useful, because for safety reasons it is important to know whether a fuel/air mixture can explode or not under different circumstances like change in temperature. This report has the emphasis on ethanol as a fuel. The flammability limits for ethanol in air are 3.3%vol (lower) and 19.0%vol (upper) at atmospheric pressure and a temperature of 298 K, according to experimental data. There are different methods to determine the flammability limits for various temperatures. In the experiments done at INPE, a 20 L vessel was used with ignition energy of 90 J, the spark gap was 6.4 mm. The temperature in the vessel could vary between 20 ⁰C and 200 ⁰C. The lower flammability limits found in literature match with the ones determined by the experiments. The upper flammability limits determined in the experiments differ from the ones found in literature, the experimental values are 4.5% lower than the literature values. The phenomenon of deflagration to detonation transition (DDT) in a tube is explained in short. First a fuel/air mixture is ignited at one end of a tube. The flame front is propagating and pushes the flow in the fuel mixture with a weak compression wave. The compression wave gets stronger due flame acceleration, until it develops into a shock of considerable amplitude. This shock preheats the fuel/air mixture and the temperature behind the shock increases. The reaction time decreases extremely in the compressed local exothermic centers, called hot spots. This decrease in reaction may result in DDT ahead of the flame front, unless the compressed gas is burnt by the flame before active explosion is initiated. At the moment the basics of DDT are understood, previous studies in DDT treated the detonation cell size and the DDT run-up distance. For understanding the details of this phenomenon still a lot of research needs to be done. This is done with help of numerical models and experiments. A numerical model that uses not too complicated physical submodels to describe the fluid dynamics and combustion processes has to be used to get the right balance between the speed and accuracy in the DDT simulations. Two general types of experimental configurations to study DDT can be defined; one that allows the turbulence to evolve itself, and another with an initial shock to immediately create the turbulence. In the last type the flow will develop more quickly, because the turbulent flow is created immediately and the simulation will converge faster. Different schemes can be used for the simulation, of course an appropriate scheme that can capture shocks and large gradients in density and pressure is needed. Resolution tests should be carried out to check the convergence.
Item Type:Internship Report (Master)
Universidade Estadual Paulista, Brazil
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
Keywords:Flammability limits, ethanol, deflagration to detonation transition
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