Stability and Liftoff of Non-premixed Large Hydrocarbon Combustion in MILD Conditions

摘要

New combustion approaches and technologies are desired to reduce pollutant emissions. High thermal efficiencies and low emissions have been achieved in combustors using a distributed reaction zone, which occurs when reactants diluted with combustion products are burned above their autoignition temperature. These conditions can be referred to as moderate or intense low-oxygen dilution (MILD) combustion. Extensive work has been performed to characterize the MILD combustion of gaseous fuels, while comparatively few research efforts have considered how practical large hydrocarbon fuels behave under MILD conditions. Moreover, such studies have typically focused on the development of MILD combustors. This work investigates how the different fuel chemistry of large hydrocarbon fuels affects MILD combustion. A jet in hot co-flow (JHC) burner is used to emulate MILD conditions in a nonpremixed flame by issuing vaporized fuel into a hot and highly diluted annular co-flow. By altering the balance of nitrogen and air in the secondary burner, the co-flow volumetric oxygen concentration can be varied between 3 and 9 percent while maintaining a constant exit temperature between 1200K to 1500K. The MILD reaction zone extends 100 mm downstream of the central tube. OH~* is used to probe the MILD reaction zone. Chemiluminescence is used to measure flame liftoff height and evaluate the flame structure overall. OH~* measurements show a sensitivity of flame liftoff height to the co-flow oxygen concentration and fuel jet Reynolds number. The liftoff height increases as oxygen concentration is decreased and decreases as the jet Reynolds number increases. Both trends are counter to trends for conventional nonpremixed jet flames, but agree with existing MILD literature for small hydrocarbon fuels.