Experimental Study of Reaction and Vortex Breakdown in a Swirl-Stabilized Combustor

Abstract

Non-reacting and lean reacting flow experiments are conducted in a swirl-stabilized combustor with several configurations of a Triple Annular Research Swirler (TARS) fuel injector. The test chamber is composed of the TARS swirler at the inlet of a straight cylindrical fuel pre-mixing section, followed by a sudden expansion and a finite-length concentric pipe. The combustor chamber exit is open to the atmosphere, hence pressure is ambient for all test cases. Non-reacting flow tests are conducted with air at 300K and 600K. The reacting flow tests use premixed air (preheated to 600K) and commercial grade gaseous propane fuel (injected at ambient temperature) at low equivalence ratios. Particle imaging velocimetry (PIV) is used to measure the distribution of axial, radial and circumferential velocity fields from which the swirl ratio at a cross section near the expansion plane and the position and size of the vortex breakdown zone are determined for a certain swirler configuration with non-reacting cold and pre-heated flows and with reacting flows at various equivalence ratios. Simultaneous OH chemiluminescence is taken and used to identify the location of the reaction zones and hence the flame anchoring point for each reacting-flow case. Results show the complex dynamical interaction between the flame and the breakdown zone, and the appearance of oscillations in the position of both. In the non-reacting cold flow case, the breakdown zone appears near the expansion plane. In the non-reacting pre-heated flow case on the other hand, it is pushed downstream of the dump plane. For reacting flows with low equivalence ratios (near the lean blow out point) the breakdown zone is anchored to the dump plane and expansion corners and is relatively stable, while the flame oscillates inside it. On the other hand, at higher equivalence ratios, the flame is anchored to the dump plane and expansion corners while the breakdown zone oscillates behind it. The swirl numbers measured near the expansion plane exhibits nice correlation with the position of the vortex breakdown and satisfies the necessary theoretical conditions for the first appearance of breakdown in a premixed reacting swirling flow.