Scarfing and Scalloping Effects on Lobed Forced Mixer at Low-Speed Conditions

This paper presents the findings on two aspects of geometric modification to a lobed forced mixer, namely scarfing and scalloping. Scarfing the lobes was achieved by alternately extending and cutting back the lobes at the trailing edge, and scalloping the lobes was achieved by removing a certain percentage of the straight sidewall area in the penetration region. Both modifications were intended to enhance the strength of streamwise circulation without causing any undesirable consequences, such as flow separation at the lobe troughs. Tests were conducted over a range of low-speed subsonic flow conditions (maximum Mach number of ?0.07) by using a three-hole pressure probe and a laser Doppler anemometer. The results showed that scarfing the lobes would be more beneficial than scalloping the lobes if the same level of enhancement for the streamwise circulation were to be achieved. Furthermore, for lobes at the same penetration angle and at the same percentage of surface area change, streamwise circulation generated at the trailing edge was actually higher (?15%) for the scarfed lobes than for the scalloped lobes. However, the associated mixing and boundary-layer losses incurred by the scarfed lobes would be higher than those of the scalloped lobes. Finally, the combined effects of the two modifications on a lobed mixer, that is, a scarfed and scalloped mixer, however, did not promote any significant streamwise-circulation enhancement.