1 DRAFT Proceedings of ASME Turbo Expo 2014: Turbine Technical Conference and Exposition GT2014 June 16-20, 2014, Düsseldorf, Germany GT2014-27103 HEAT TRANSFER TO AN ACTIVELY COOLED SHROUD WITH BLADE ROTATION Onieluan Tamunobere, Christopher Drewes, Sumanta Acharya Turbine Innovation and Energy Research (TIER) Center Louisiana State University Baton Rouge, Louisiana 70803 ABSTRACT In this paper, an experimental study of the shroud heat transfer behavior and the effectiveness of shroud cooling under low-speed rotational conditions is undertaken in a single stage turbine. The shroud consists of 1696 cooling holes that are 1 mm in diameter. The holes are angled at 45 degrees in a repeating pattern consisting of 5 unique hole pitches around the shroud circumference. Measurements of the normalized Nusselt number and film cooling effectiveness are done using liquid crystal thermography. These measurements are reported for the no coolant case, blowing ratios of 1.0, 1.5, 2.0, 2.5 and 3.0, and rotation speeds of 300, 400, 500, 600 and 700 RPM. The results with no coolant injection show that the high Nu/Nu0 region migrates upstream toward the shroud leading edge with increasing rotation. The cooling results show that increasing the blowing ratio increases the area-averaged film cooling effectiveness in the shroud hole region for all rotation speeds studied. The cooling effect further downstream of the injection hole locations is marginal indicating rapid mix-out of the coolant jets. Furthermore, increasing the blade rotation speed increases the area-averaged Nusselt numbers and decreases the area-averaged film cooling effectiveness in the shroud hole region for all blowing ratios studied. As in the no-coolant case, with increasing rotation speeds, the high Nu/Nu0 region migrates upstream toward the shroud leading edge and disrupts the cooling effectiveness in this region. Finally, the results show that decreasing the shroud coolant hole spacing changes the lateral heat transfer profile from a periodic sinusoidal distribution for a shroud hole spacing of P/D = 10 to a more even distribution for a shroud hole spacing of P/D = 5. INTRODUCTION In order to achieve higher thermal efficiencies, gas turbine inlet temperatures have increased significantly in recent years to values that exceed the material limits of the blade and shroud materials. Thus, the high pressure turbine components have to be cooled to prevent material failure and to increase reliability. Of particular interest is the heat transfer and cooling in the tip and shroud region that experience high thermal loading. Multiple cooling strategies have been developed to sustain these high temperatures and for the past few decades, film cooling has been extensively studied to develop more efficient ways of cooling turbine components including the tip and shroud [1]. For the shroud, various methods of cooling have been studied. Kanjirakkad et al. [2] investigated two passive film cooling concepts known as platform cooling which involves direct coolant injection and rail cooling which involves coolant injection using compound holes to cool a rotor shroud. Their results show that platform cooling was effective in cooling the turbine blade shroud while the rail cooling concept was not as effective. Michel et al [3] studied the effect of full coverage film cooling using compound angle holes in a multi-perforated ring. Results were compared between axially oriented holes and swirl inducing holes. They found that in both cases, the film remained attached to the wall with higher effectiveness for the swirling cases. However, the effects of rotation on the heat transfer behavior and cooling effectiveness have not been studied in the majority of the literature. The effects of rotation are important toward understanding the heat transfer on the shroud. Centrifugal forces act on the flow as it enters the rotor domain, and result in the increase of the radial component of the flow acceleration as the rotation speed increases. Changing the blade rotation speeds result in different flow incidence angles. The incidence angle affects the flow profile into the rotor and leads to changes in the acceleration and impingement of the flow at changing locations on the shroud surface [4]. Further, the high pressure gradients near the tip create a high temperature, high speed leakage flow in the tip-gap. Thus, both the tip and the shroud are subject to high thermal loadings and material failure. As the leakage flow passes through the tip gap over to the suction side of the blade, the tip leakage vortex is created. This tip leakage vortex interacts with secondary vortices created by the relative motion of the shroud causing additional pressure losses and potentially high heat transfer coefficients on the suction surface near the tip