Effects of unsteady wakes on heat transfer of blade tip and shroud

An experimental study has been conducted to investigate the heat-transfer characteristics of blade tips and shrouds with and without unsteady wakes. Depending on the presence of unsteady wakes, the local heat/mass-transfer coefficients of the tip and shroud were measured using the naphthalene sublimation method. Wakes from unsteady blades were modeled as wakes generated from moving cylindrical rod bundles. Test conditions were set to a Reynolds number of 100,000, based on an inlet velocity of 11.4 m/s and the axial chord length. The Strouhal number was varied from 0 to 0.22. For the case without unsteady wakes (St = 0), high heat/mass-transfer coefficients appeared in regions where various flow patterns, such as flow reattachment, swirling flow, and vortices, occurred. Unsteady wakes (St = 0.22) made high turbulence intensity of the tip leakage flow, and flow patterns ranging from flow reattachment to tip leakage vortex in the tip and shroud were changed and dispersed in the presence of unsteady wakes, thus changing the heat/mass-transfer distributions in these areas. Due to the high thermal load on the tip and shroud under unsteady wake conditions, more detailed cooling designs must be considered, ranging from film cooling holes to the installation of additional structures on the blade tip, especially in the leading edge region and mid-region I of the tip, and the mid-region I and trailing edge region of the shroud.