On the role of turbulence and compositional fluctuations in rapid compression machines: Autoignition of syngas mixtures

摘要

With the increasing interest in utilizing syngas in gas turbine applications, the characterization of H_2/CO combustion and reaction chemistry under high-pressure and moderate-temperature operating conditions has been the focus of recent investigations. Different chemical-kinetics and hydrodynamic processes have been identified as being responsible for the discrepancies between experimental measurements and kinetic predictions of syngas ignition delay times. This contribution complements previous studies, and provides improved understanding about the role of turbulence and fluctuations in temperature and mixture composition on the syngas combustion process in rapid compression machines (RCMs). To this end, a self-contained model is developed that describes the ignition and combustion process by considering the interaction between turbulence, detailed reaction chemistry, and wall heat loss effects. Different mechanisms can be identified as being responsible for the generation of inho-mogeneities in flow-field, temperature, and mixture composition, including turbulence-generation during the filling process, corner vortices, wall-generated turbulence, and mixing between fluid in the core region and the boundary layer. In the present model, these contributions are parametrically represented in terms of initial turbulence levels, and the mean-strain amplification of these perturbations during the compression phase is described using rapid distortion theory. The model is applied to different syngas mixtures and operating conditions, including pressures up to 20 atm and temperatures between 600 and 1300 K. Parametric studies show that the model captures experimentally observed trends of reduced ignition delay and prolonged reaction progress during the ignition phase. A Damkohler criterion is proposed in order to characterize the sensitivity of the induction chemistry to turbulence fluctuations. Results suggest that syngas mixtures with Damkohler numbers below 50 exhibit increasing sensitivity to turbulence and mixture fluctuations. This parametric study indicates that the turbulence/chemistry interaction can play an equally important role in affecting the syngas ignition chemistry, and requires consideration in addition to chemical-kinetics and hydrodynamic processes previously identified as leading mechanisms for the observed discrepancy in syngas ignition.