Reconcile Discrepancies of Current Syngas Kinetics Models by Considering Turbulence Effects on Ignition Delay at Gas-Turbine Relevant Operating Conditions

Abstract

With the increasing interest in utilizing syngas in gas turbine applications, the characterization of H2/CO 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 paper complements previous studies, and provides insight on the role of turbulence fluctuations and small-scale mixture inhomogeneities on the syngas combustion process. To this end, an analytic model has been developed that describes the turbulence amplification and combustion in the so-called adiabatic core region of a rapid compression machine (RCM). In this formulation, enhancement of turbulence and small-scale mixture fluctuations during the RCM compression phase are modeled using rapid distortion theory. The subsequent ignition and combustion processes are described using a Lagrangian Fokker-Planck model which considers turbulence/chemistry interaction and detailed reaction chemistry. The model was applied to different syngas mixtures and operating conditions, including pressures up to 20 atm and temperatures between 600 and 1300 K. Parametric studies showed that the model captures experimentally observed trends of reduced ignition delay and prolonged reaction progress during the ignition phase. A Damköhler criterion was introduced in order to characterize the sensitivity of the induction chemistry to turbulence fluctuations. Results suggest that syngas mixtures with Damhöhler numbers below 100 exhibit increasing sensitivity to turbulence and mixture fluctuations. Initial turbulence levels of less than 0.01 percent of the mean flow are sufficient in reducing the ignition delay times by several orders of magnitude. This study shows that the turbulence/chemistry interaction plays 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 combustion at low-temperature operating conditions.