Abstract

For unshrouded high pressure turbines, the design of rotor blade tips is a dynamic multi-purpose process. The model should have least aerodynamic and heat transfer losses caused by the flow through the tip gap and the blade. Recent studies show that by the modification of the blade tip, there is some improvement in efficiency and reduced tip gap flow. These include mainly parametric study of cavity tip and winglet tip modifications. Previous studies confirm the benefit of overhang in reducing tip leakage loss and heat transfer by changing the location of the tip leakage vortex away from the blade. But, to the best of authors’ knowledge none of the study reports the effect of casing relative motion on modified winglet and squealer tip. In the present study, novel modified tip blade geometry is introduced named as Top Squealer with Bottom Winglet (TSBW). Tip gap physics and loss generation has been investigated on three other different designs of squealer and winglet geometries and compared with the novel design. These designs are named as Flat Winglet, Cavity Squealer, Top Squealer with Bottom Winglet (new design) and Top Winglet with Bottom Squealer (TWBS). The flat tip rotor blade is considered as the base case for comparison. Three-dimensional computational study using ANSYS CFX 18.2 has been performed in order to examine the effect of casing relative motion on various designs of winglet and squealer tip. Structured mesh is created using ANSYS ICEM 18.2. At the downstream of trailing edge, distinct regions of momentum deficits named as the tip leakage vortex (LV), tip passage vortex (TPV), wakes and hub passage vortex (HPV) has been observed. Wakes formed due to the interaction of scraping vortex (SV), Tip passage vortex (TPV) and the leakage vortex. It has been found that Cavity Squealer and Top Squealer with Bottom Winglet (TSBW)gave the lowest total pressure loss coefficient and lowest tip leakage flow rate. But in casing relative motion case, cavity squealer with bottom wingletout runs the cavity squealer aerodynamic performance. This is due to the enlarged cavity at the tip. The incoming pass over flow got blocked because of enhanced interaction of LV and Scraping vortex (SV) in the tip cavity. As a result, tip leakage losses and tip leakage mass flow rate decreased.

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