The Effect of Swirl Number On Lean Blow Out Limits of Lean Direct Injection Combustors

Abstract

This is the first study where a single variable sweep of swirl number (SN) is conducted to assess its impact on lean blowout limits (LBO) in a liquid fueled Lean Direct Injection (LDI) combustor. This study uses a scaled NASA SV-LDI (Swirl Venturi - Lean Direct Injection) hardware and is concerned with the impact of swirl number on the lean blow out limit of a single element LDI system at atmospheric pressure. The SN was varied from 0.31 to 0.66 using continuously variable active SN control system that was developed in-house. It is shown that the minimum operating equivalence ratio is a linearly increasing function of swirl number. While previous literature agrees with the positive slope for this correlation, past work has insisted that the LBO limit is proportional to the swirler vane angle of swirl cup flame holders which is shown to be untrue for LDI systems. By actively varying the swirl number, it is proven that LBO is proportional to SN, and it is well known that SN is not proportional to swirler vane angle. Increased SN reduces LBO margin because the better-mixed, high swirl cases dilute locally rich pockets of fuel air mixture. In addition to a baseline venturi, which was scaled from NASA's geometry, two other venturis were tested. A low pressure loss venturi with a large throat diameter showed poor blow out performance where as a parabolically profiled venturi improved LBO over the baseline for the same swirl number.