Effects of Fuel Density, Velocity5 and Composition on Flame-Acoustic Interaction

摘要

Experimental investigation was performed to determine the sensitivity of shear-coaxial injector flames to acoustic forcing from a transverse direction. A strong flame-acoustic interaction implies that the injector configuration and associated flow conditions are susceptible to combustion instability through favorable coupling between heat release and pressure oscillations. Flow parameters such as fuel-oxidizer density ratio, velocity ratio, and fuel composition were systematically varied and the resulting flame system was perturbed by fixed-amplitude acoustic forcing. The extent of flame perturbation was quantified by measuring the flame brush thickness through OH* chemiluminescence and CH* chemiluminescence imaging. By adding inert gases, helium and argon, to the hydrogen and oxygen lines, the density ratio between oxidizer and fuel was varied between 16 and 5, while holding the velocity ratio and the fuel composition constant. Similarly, the velocity ratio was varied between 3.0 and 5.3, while holding the density ratio and the fuel composition constant. Lastly, while holding both the density ratio and the velocity ratio constant, hydrogen-methane mixture fuel was tested by varying the mixture fraction. Throughout these tests, the temperature and the total pressure were held unchanged. In the ranges tested, flame-acoustic interaction was most sensitively affected by density ratio changes, and remained relatively unaffected by velocity ratio changes and the fuel mixture fraction changes. It is shown that strong flame-acoustic interactions are likely caused by intermittent baroclinic torques, which lowers the threshold of acoustic disturbance amplitude that develops into instability. The results support the notion that periodic baroclini(|vorticity strengthened by large density gradient in shear-coaxial injector element could be a problem, making the affected configuration more susceptible to combustion instability.