Gas Turbine Engine Durability Impacts of High Fuel-Air Ratio Combustors—Part I: Potential for Secondary Combustion of Partially Reacted Fuel

Demand for greater engine efficiency and thrust-to-weight ratio has driven the production of aircraft engines with higher core temperatures and pressures. Such engines operate at higher fuel-air ratios, resulting in the potential for significant heat release through the turbine if species, such as CO and HC, are emitted from the combustor in large quantities. This paper outlines the magnitude and potential for turbine heat release in current and future engines. The analysis suggests that high fuel-air ratio designs may have to consider changes to cooling strategies to accommodate secondary combustion. A characteristic time methodology is developed to evaluate the chemical and fluid mechanical conditions that lead to combustion within the turbine. Local species concentrations partly determine the potential for energy release. An energy release parameter, here defined as a maximum increase in total temperature $ΔTt,$ is used to specify an upper limit on the magnitude of impact. The likelihood of such impacts is set by the convective, mixing, and chemical processes that determine the fate and transport of species through the turbine. Appropriately defined Damko¨hler numbers (Da)—the comparative ratio of a characteristic flow time $τflow$ to a characteristic chemical time $τchem$—are employed to capture the macroscopic physical features controlling the flow-chemistry interactions that lead to heat release in the turbine.