Pressure And Equivalence Ratios Essay

3.2 Effect of Pressure and Equivalence Ratio

Fig. 3 (1) - (3) give the effects of pressure and equivalence ratio on ignition delay times of DME/air, n-butane/air and 50%DME50%n-butane/air binary fuel. Note that for all mixtures, ignition delay times decreased with the increase of pressure, meaning that the increase of pressure can promote fuel ignition in current conditions. This is mostly due to the increased fuel concentration and enhanced molecule collision probability at elevated pressures.

The influences of equivalence ratio on the ignition delays of DME/air and n-butane/air mixtures were investigated at pressures of 2 and 10 atm. For the DME/air mixture, as shown in Fig. 3 (1), both experiment and simulation exhibit shorter ignition delay times under the fuel stoichiometric (= 1.0) condition than the fuel lean (= 0.5) condition from low to high temperatures, meaning that the ignition delay

However, the almost linear promoting effects of DME addition were found in propane and n-butane ignition delays [23, 25]. This is mainly due to that the reactivity of methane is much lower relative to that of DME. Therefore, even with a small amount of DME addition, the ignition was strongly promoted by the decomposition of DME accompanied by the rapid build-up of free radicals, thus lead to the nonlinear promoting effect on methane ignition [21, 22]. However, for the higher order alkanes such as propane and butane, the reactivity of which are higher and the ignition delay times are much shorter relative to methane. Moreover, in the high temperature oxidation of methane, the rate of the governing reaction CH4 + O2 CH3 + HO2 is much slower than of the similar reactions of the higher order alkanes [22]. Therefore, the nonlinear promoting of DME addition on ignition is not observed for higher order alkanes at high