Improved Performance of Lean-Burn Combustion

摘要

The focus of the research described here is on establishing and controlling the causes of combustion instabilities in fuel lean combustion systems. The frequencies of the dominant instability modes are typically < 500 Hz and a comprehensive closure at second moment level, which accounts for the effects of persistent non-gradient transport in a consistent manner, has been applied to compute the combusting flow characteristics in the plane duct configuration of De Zilwa et al (2001). Results indicate that aerodynamic strain and the treatment of pressure related terms have a major impact under stable and oscillating combustion conditions and a consistent first order redistribution model has been derived and applied. The calculations indicate that the treatment of the duct exit boundary condition is critical in the context of the distribution of energy between different acoustic modes. An investigation into alternative boundary treatments for time-dependent flows suggests that the most appropriate technique for reproducing experimental impedances is to extend the computational domain to include the induced flow outside the duct. The calculation method is shown to represent combustion of methane and air at equivalence ratios that give rise to stable combustion and the low-amplitude oscillations that exit at low Reynolds numbers and equivalence ratios between lean extinction and the lean stability limit. Experiments have been performed with methane, propane and ethylene fuels, premixed with air and include the determination of flammability and stability limits with hot and cold ducts, the nature of combustion instabilities and some aspects of control. They fulfill the obligations of the contract and provide information that has and will continue to assist the development of the above computer method.