An Experimental and Modelling Study of the Oxidation of n-Propylbenzene Over a Wide Range of Temperatures and Pressures and its Comparison with n-Butylbenzene
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This study presents an experimental and modeling investigation of the ignition of n-propylbenzene and a mixture of n-propylbenzene and n-heptane. This mixture was chosen to represent a surrogate for n-decylbenzene a common constituent of transport fuels which is extremely difficult to study in the vapour phase due to its low vapour pressure. n-Propylbenzene was chosen to study as it has a sufficiently high vapour pressure to study alkyl aromatic compounds commonly found in transport fuels. This study was carried out over a wide variety of temperatures, pressures and equivalence ratios in a shock tube and rapid compression machine. Ignition delay times were measured in the shock tube over the temperature range of approximately 1000-1600 K at pressures of 1, 10, 30 and 50 atm and for equivalence ratios of 0.29, 0.48, 0.96 and 1.92 for pure n-propylbenzene while the equivalence ratios of 0.29, 0.49, 0.98 and 1.95 were used for the n-propylbenzene and n-heptane mixtures. This study is the first of its kind as it describes the ignition characteristics of n-propylbenzene in both the high and low temperatures while also describing the effects of varying pressures from very low pressures (1 atm) to high engine like pressures (50 bar). This wide range of conditions studied help to validate the chemical kinetic mechanism produced in this study. The high pressure shock tube is a 8.7 m long stainless steel tube divided into a driver and driven section by a double diaphragm section. Upon rupture of the diaphragm section a shock wave rushes from the high pressure driver section into the relatively low pressure driven section which contains the test gas. This test gas is pressurized and heated by the shock wave which results in chemical reactions which ultimately result in the ignition of the fuel mixture. The time from compression of the fuel to the desired pressure to ignition is defined as the ignition delay time which is measured in each experiment. The time scale of ignition occurring in the shock is between 50 microseconds and 3 milliseconds. Ignition delay times were also measured in the unique twin-opposed piston rapid compression machine at the same equivalence ratios as described for the shock tube. The pressures studied in the rapid compression machine were 10, 30 and 50 atm and the temperature range was approximately 600-1000 K. For pure n-propylbenzene ignition only occurred after 750 K thus limiting the range of experiments available. The time scale of these experiments is from 3 to approximately 500 milliseconds but there is a greater discrepancy in the longer ignition delay times due to greater heat loss. Experiments were carried out on both pure n-propylbenzene and with a 57 % / 43 % mixture of n-propylbenzene and n-heptane. n-Propylbenzene was chosen as it is expected to exhibit sufficiently similar chemistry to longer n-alkylbenzenes of which there is not a great deal of experimental data. The mixture conditions were chosen with similar molecular weight and carbon / hydrogen ratio to a larger n-alkylbenzene, namely n-decylbenzene, a possible surrogate for diesel fuel. Additionally ignition delay times for n-propylbenzene and n-butylbenzene were compared. This comparison was made with data obtained at similar conditions in a supplemental study. It was found that at lower temperatures the reactivity of n-butylbenzene was greater than that of n-propylbenzene while at higher temperatures they are quite similar. Finally a chemical kinetic model was derived for both fuels using a previously published mechanism and updating this with the most up to date C0-C4 chemistry, including pressure dependent rate constant expressions for the unimolecular decomposition of n-propylbenzene and n-butylbenzene, including a previously published n-heptane mechanism and a full low temperature reaction scheme for n-propylbenzene and n-butylbenzene which were previously not included. Overall very good agreement was observed between shock tube and rapid compression machine experiments while the key trends in relation to effects of pressure and equivalence ratio were described. Simulations performed using the chemical kinetic mechanism showed good agreement with the experimental data.