Turbulent flame development in a high-pressure combustion vessel

The objective of this work was to extend the range of turbulent burning velocities presented in the literature by performing new measurements at elevated pressures over a range of turbulent velocities, u' (r. m. s. deviation from the mean velocity). The influence of equivalence ratio was investigated for a fixed u'= 2 m/s at 0.5 MPa for methane, a 70 % methane/ 30 % hydrogen mixture, methanol, isooctane and a gasoline (Shell dutch-pura). The mixtures were varied from the lean ignition limit to rich limit up to a maximum equivalence ratio of 2. Further measurements were performed with iso-octane and the effect of r. m. s. turbulent velocity (u' = 0.5,1,2,4,6 m/s), pressure (P = 0.1,0.5,1 MPa) and equivalence ratio (0= 0.8,1.0,1.2 and 1.4) were investigated, to produce a database. Turbulent flames were centrally ignited in isotropic turbulence in the Leeds fan stirred bomb. Flame progress was monitored using high-speed schlieren photography and pressure measurements. The turbulent burning velocity based on the production of burned mass, ur was obtained from both techniques. The burning velocities obtained from schlieren imaging and pressure measurements were in good agreement. The turbulent flames continually accelerated throughout the time that they were monitored. This was the result of turbulent flame development, with the range of turbulent scales wrinkling the flame surface increasing as the flames grew. It was shown that flame development occurred primarily as a result of the kernel radius, rather than the time from ignition. For each turbulent condition, corresponding spherically expanding laminar flames were ignited and imaged with schlieren photography. The measurements were processed to give the stretch free laminar burning velocity and also the Markstein number, a measure of the influence of stretch on the burning velocity. The peak laminar burning velocity was found to be in the range 1>o>1.2 depending on the fuel. At 0.5 MPa a number of the flames were observed to become cellular, in some cases this occurred from ignition, this has been linked to negative Markstein numbers which were observed with lean methane and rich iso-octane - air mixtures. The peak in the turbulent burning velocity with equivalence ratio varied considerably with the different fuels, and did not correspond to that for the laminar burning velocity. In the case of methane and the 70 % methane/ 30 % hydrogen mixture the peak turbulent burning velocity was lean of stoichiometric. In contrast, for iso-octane and the gasoline, the peak in %, was beyond 0=1.3. It was concluded that the shift in the peak could be explained by comparison with the measured Markstein numbers. In the iso-octane database, pressure was not observed to have a significant influence on the turbulent burning velocity for some conditions, however, when fuel rich higher pressure gave an increase in utr. Turndown of utr, (the burning velocity does not increase as with the turbulent velocity) was observed at high turbulence velocities, although this depended on the equivalence ratio. The measurements were then compared with existing turbulent burning velocity expressions and correlations. In general these expressions were found not to predict the effect of equivalence ratio well.