Analysis of sub-atmospheric pressure effects on flow characteristics of a twin-stage swirler combustor

Understanding of flow characteristics of combustor under sub-atmospheric conditions is critical for plateau ignition and high-altitude relight of aero-engine. In this study, the impacts of sub-atmospheric operating pressure pop on flow characteristics of a twin-stage swirler combustor have been experimentally and numerically investigated. Specifically, two reference velocities Vref, i.e., 3.0 and 5.5 m/s, are considered. The sub-atmospheric pop ranges from 0.08 to 0.02 MPa, while the pop of 0.11 MPa represents an atmospheric condition. Results show that as the pop decreases from the atmospheric pressure to sub-atmospheric ones, there exist two flow patterns, i.e., Y- and V-type. The critical pressure of their transition is associated with the Vref. A higher Vref has a lower critical pressure, indicating its higher capacity to withstand the sub-atmospheric pressure effect. A non-linear map between the radial pressure difference across the second stage swirler airflow and the pop is observed, except in the region near the swirler exit. The flow pattern transforming from Y-type to V-type leads to an expansion of the central toroidal recirculation zone and an increase in the reflux ratio. Further, the simplified force balance analysis by reductio for swirl flow shows that both the centrifugal and pressure differential forces should reduce linearly as decreasing the pop, but the pressure differential force actually decreases non-linearly and more rapidly than the centrifugal force when the pop approaches some certain value. Consequently, the swirl flow expands radially outward for force balances, accounting for the transition of the flow pattern from Y-type to V-type.