A study was made of the flow in radial nozzle cascades using an air test rig and a water test rig. In the air test rig, three cobra probes were used in circumferential and spanwise traverses to determine the total pressure variations in the flow field at three radii downstream of the nozzles at which static pressure was also measured. The tests were made on two sets of nozzle blades having heights of 0.148 in. (0.376 cm) and 0.200 in. (0.508 cm), at trailing edge angles (measured from circumferential direction) of 15, 20, and 25 deg, and at two flow Mach numbers of approximately 0.2 and 0.35. The test results presented in this paper, in the form of loss coefficients and flow angles, were flow-weighted and averaged. Flow visualization in the air test rig was made on the walls bounding the nozzle blades using the graphite power-oil mixture technique. Additional tests were made on the water test rig using dye injection technique. Photographs were obtained showing clearly formation of secondary flow around each nozzle blade in the form of the leading edge vortex. The test results confirm the existence of the leading edge vortices reported peviously, and extend their study to the radial nozzle cascades.
DISCUSSIONM. Kurosaka 1 . The authors' encounter with the "vortex whistle" appears to remind us, once again, that, whenever swirling in-flow is involved, one has to be always on the lookout for the possible contamination of flow field induced by these vigorous unsteady dynamic disturbances.As reported in the authors' references [8,9], in at least three instances of the radial flow test rigs known to the discusser, a loud screech sound, or the "vortex whistle," emerged when the swirl became sufficiently large; the frequency spectrum revealed the presence of spiky peaks of a pure tone and its harmonics, the frequency of which increased proportionately to swirl. As soon as the "vortex whistle" appeared, the timeaveraged, steady flow became distorted: the swirl angle distribution became drastically different from the one before the emergence of the whistle. At the same time, the total pressure near the outer casing exceeded its value upstream of the vanes(!), in exactly the same manner as reported by the authors, and this was compensated by the decrease of total pressure near the inner casing. Furthermore, the total temperature became separated in the radial direction, with hotter air found near the outer casing and colder air found near the inner wall, a phenomenon evocative of the Ranque-Hilsch effect. The degree of temperature separation was such that, in one instance, the outer casing became noticeably warm to the touch, while icing was observed on the inner casing.What is intriguing is the fact that once the vortex whistle is eliminated by the installation of acoustic suppressors, this distortion in the flow field disappears at once. This implies that the distortion in the steady flow is caused by none other than unsteady disturbances, through the mechanism of acoustic streaming. In fact, these observations led us to demonstrate ([17, 18] that the dominant cause of the Ranque-Hilsch effect, so far a little-understood phenomenon, is precisely this dimly foreseen mechanism of acoustic streaming.To all of this may be added a caveat: some of the "steady" flow data taken in the presence of intense disturbances in swirling flows of gas turbines may be suspect -they may be highly contaminated by the unsteady disturbances, as found by the authors.Parenthetically, the data showing the excess total pressure over its incoming value appear to have been measured also in [19]; although the author did not state any connection with the whistling sound and the subsequent discussions are not clear, the measurements taken downstream of vanes (Figure 26) unmistakably show a distribution similar to the one obtained in the presence of the vortex whistle.
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