A Shock Tube Study Of Ignition Delay Characteristics Of Ethylene And Comtamination Effect

Supersonic combustion ramjet engine （scramjet engine） has a widespreadapplication prospect in future space vehicle field. Under supersonic conditions the fueldwell time became short in combustion chamber, thus ignition delay characteristics offuels have been the focus of many investigations. Shock tube is the main experimentalfacility for measurements of ignition delay time for hydrocarbon fuels. A wide range oftemperature and pressure can be extended by adjusting speeds of incident shock wave.Based on the newly built shock tube, an experimental assembly was constructed toinvestigate fuel ignition characteristics. In this thesis，the ignition delay characteristicsof ethylene were studied at relatively high pressures and air-like oxygen concentration.Furthermore，the contamination effect of water and carbon dioxide on ethylene ignitioncharacteristics was investigated.A new experimental assembly consisted of shock tube, intake, gas preparation,vacuum, pressure measurement and spectroscopic measurement Thereafter, theoperation and shock tube were completed preparing for ignition delay measurement.The constant temperature system was also installed to maintain the shock tube at135℃constantly for reducing the influence of water adsorption on the shock tube wall in thecontamination experiment.The ignition delay characteristics of ethylene/O2/Ar mixtures were obtained bymonitoring the steepest rise of the characteristic emission of OH（306.5nm） orCH（431.5nm） radical. The oxygen content of the test mixtures was about20%（vol）. Theexperimental conditions cover pressures of2,7and12atm, stoichiometric ratios of0.5,1.0and2, and temperature range from800to1600K, respectively. The impact ofpressure and stoichiometric ratio on the ignition delay characteristics of ethylene wasinvestigated, and the dependence relation of ignition delay time with temperature,pressure, stoichiometric ratio, fuel concentration, oxidant concentration and otherparameters was obtained by least squares fitting as follows:where the ignition delay time is in microseconds, reactant concentrations in mol/cm³,pressure P in atm, Φ is the stoichiometric ratio, and the ignition activation energy is in J/mol. The transition to detonation in ethylene ignition was observed，and the transitionprobability with initial temperature, ignition temperature and stoichiometric ratio wasalso discussed.On the basis of ignition delay time data of ethylene at2atm, the impact of thecontaminants, water and carbon dioxide, on ethylene ignition delay was investigated.The variation of ignition delay of the test mixtures was measured, with addedcontaminant H₂O7.5%,15%and25%（vol）, CO₂10%（vol） and25%H₂O+10%CO₂（vol）, covering a temperature range from800to1400K, and atstoichiometric ratio of0.5and1.0. The experimental results showed that effect of wateron ethylene ignition characteristics was small. There were almost no effect at Φ=0.5andslight inhibitory effect with25%water at Φ=1（32%growth at800K）. The carbondioxide showed little effect on ethylene ignition characteristic at Φ=0.5and someinhibitory effect at Φ=1（53%growth at850K）.When adding25%H₂O+10%CO₂（vol）, an inhibitory effect on ethylene ignitionwas observated over a wide temperature range. The collaborative presence of H₂O andCO₂makes35－50％increase of ethylene ignition time at Φ=0.5over a temperaturerange of900－1400K and15－33％increase at Φ=1over a temperature range of800－1300K. Below the critical temperature1200K， the collaborative presence of H₂Oand CO₂showed an inhibitory effect on the ethylene ignition delay at Φ=1, and theethylene ignition times were increased by15－33％over the temperature range of800－1250K.The reason of pressure plateau drop behind reflected shock waves with addingwater was briefly discussed. The bifurcation effect was excluded by comparingcalculation and experiment. and two probable contributions were discussed.