Recirculating Flow and Turbulence in a Low Aspect Ratio Dump Combustor

Re-circulating flows are established in dump combustors at the dump plane due to the sudden expansion. However, given enough length, the separated flow at the dump plane attaches itself inside the combustor and a fully developed, non-circulating, attached flow field is established. But, if the length of the combustor is less than the free-stream reattachment length, then the flow does not re-attach inside the combustor. Instead, a portion of the flow is reflected from the exit section, causing stronger re-circulation that modifies the flow structure inside the combustor. This paper describes an experimental study of turbulent flow field inside a dump combustor for a range of flow Reynolds numbers. The focus of this effort is to study the interaction between the flow re-circulation and the large-scale turbulence. Detailed measurements of the wall pressure transients were taken using strain-gage pressure sensors. The fluctuating component of the pressure was isolated and analyzed. The signals were analyzed using FFT, Auto-Correlation and Cross-correlation to distinguish the re-circulating flow and the large-scale turbulence. The re-circulating flow, identified by low frequency fluctuations in pressure (∼ 0.5 Hz), was seen to be strongest inside the combustor almost half way through the combustor length. At the same time, the large-scale turbulence intensity (identified by high frequency fluctuations in the range of 460 Hz) level is seen to be lower inside the combustor than in the incoming pipe. This can be attributed to the turbulence cascading due to the re-circulating flow, which increases the small-scale energy and reduces the large-scale energy. These results show turbulence modulation due to re-circulating flow and can have far reaching applications in swirling turbulent flows.