Measurements and modeling of turbulent consumption speeds of syngas fuel blends
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Increasingly stringent emission requirements and dwindling petroleum reserves have generated interest in expanding the role of synthesis gas (syngas) fuels in power generation applications. Syngas fuels are the product of gasifying organic-based feedstock such as coal and biomass and are composed of mainly H₂ and CO. However, the use of syngas fuels in lean premixed gas turbine systems has been limited in part because the behavior of turbulent flames in these mixtures at practical gas turbine operating conditions are not well understood. This thesis presents an investigation of the influence of fuel composition and pressure on the turbulent consumption speed, ST,GC, and the turbulent flame brush thickness, FBT, for these mixtures. ST,GC and FBT are global parameters which represent the average rate of conversion of reactants to products and the average heat release distribution of the turbulent flame respectively. A comprehensive database of turbulent consumption speed measurements obtained at pressures up to 20 atm and H₂/CO ratios of 30/70 to 90/10 by volume is presented. There are two key findings from this database. First, mixtures of different H₂/CO ratios but with the same un-stretched laminar flame speeds, SL,0, exposed to the same turbulence intensities, u'rms , have different turbulent consumption speeds. Second, higher pressures augment the turbulent consumption speed when SL,0 is held constant across pressures and H₂/CO ratios. These observations are attributed to the mixture stretch sensitivities, which are incorporated into a physics-based model for the turbulent consumption speed using quasi-steady leading points concepts. The derived scaling law closely resembles Damkhler's classical turbulent flame speed scaling, except that the maximum stretched laminar flame speed, SL,max, arises as the normalizing parameter. Scaling the ST,GC data by SL,max shows good collapse of the data at fixed pressures, but systematic differences between data taken at different pressures are observed. These differences are attributed to non-quasi-steady chemistry effects, which are quantified with a Damkhler number defined as the ratio of the chemical time scale associated with SL,max and a fluid mechanic time scale. The observed scatter in the normalized turbulent consumption speed data correlates very well with this Damkhler number, suggesting that ST,GC can be parameterized by u'rms/SL,max and the leading point Damkhler number. Finally, a systematic investigation of the influence of pressure and fuel composition on the flame brush thickness is presented. The flame brush thickness is shown to be independent of the H₂/CO ratio if SL,0 is held constant across the mixtures. However, increasing the equivalence ratio for lean mixtures at a constant H₂/CO ratio, results in a thicker flame brush. Increasing the pressure is shown to augment the flame brush thickness, a result which has not been previously reported in the literature. Classical correlations based on turbulent diffusion concepts collapse the flame brush thickness data obtained at fixed u'rms/U₀ and pressure reasonably well, but systematic differences exist between the data at different u'rms/U₀ and pressures.