The effect of charge stratification on the combustion of lean methane-oxygen mixtures under constant volume. conditions

Abstract

A constant volume cylindrical combustion bomb of 100 mm diameter, and of variable length (from 250 mm to 1000 mm) has been built. The effect of charge stratification on the ignitability and subsequent flame propagation of lean (1 > 6) methane-oxygen mixtures has been investigated. The test mixtures were quiescent with initial conditions of 25°C and 1.5 bar absolute. The stratified charge was created by injecting small quantities of a relatively rich (λ = 4.57) premixed methane-oxygen mixture through a modified commercially available spark plug so that an easily ignitable mixture formed in the vicinity of the spark electrodes. The injector used was a commercially available Bosch gasoline injector, suitably modified for gas operation. The injection pressure was 5 bar gauge. The size of the injected puff could be altered by adjusting the duration (from 0-100 ms) for which the injector was opened, and the timing of the spark could be adjusted so that it occurred either before, after or at the end of injection. Results show that the injected premixed puff is an efficient high energy ignition source for very lean methane-oxygen mixtures. For the most reliable ignition performance a delay of 10 ms between the end of injection and the occurrence of the spark has been found to be desirable. This is attributed to the decay of the turbulence produced by the puff. Long injection durations (greater than 20 ms) also improved ignition reliability, due to the larger puff size. The use of charge stratification did not enable combustion to continue below the ideal flammability limit. It did extend the equipment lean limit of flammability from λ = 7.3 (spark alone) to λ = 8.35, and thus demonstrated that it could be useful as a limit extender in non-ideal combustion situations. Results from the longest bomb used (1000 mm) show that the flame dies out after successful ignition has been achieved, and that a distinct lean flammability limit does not exist. Experimental evidence suggests that the flame is generating turbulence as it propagates, and this turbulence causes the flame to become self accelerating. Further, it is thought that the flame generated turbulence is the primary cause of flame extinction (in the form of turbulence induced gas phase quenching) after successful ignition in the 1000 mm bomb.