Anthony Davis

Rotating Heat Transfer Measurements on a Multi-Pass Internal Cooling Channel: II — Experimental Tests

The present contribution is focused on heat transfer measurements on internal cooling channels of a high pressure gas turbine blade in static and rotating conditions. A novel rig designed for the specific purpose was used to assess the heat transfer coefficients on a full internal cooling scheme of an idealized blade. The channel has a multi-pass design. Coolant enters at the blade hub in the leading edge region and move radially outwards inside a two-sided ribbed channel. The second passage is again a two-sided ribbed channel with a trapezoidal cross section of high aspect ratio, while inside the third leg low aspect-ratio cylindrical pin fins are arranged in a staggered configuration to promote flow turbulence. Inside the third passage, the coolant is progressively discharged at the blade trailing edge and finally at the blade tip. The test model differs with respect to the real design only because there is no curvature due to the blade camber. Conversely, the correct stagger angle of the real blade with respect to the rotation axis is preserved. Experiments were performed for static and rotating conditions with engine similar conditions of Re=21000 and Ro=0.074, both defined at the channel inlet. Transient liquid crystal technique was used for the measurement of the heat transfer coefficient (HTC) on both pressure and suction sides internal surfaces of the channel. From the spatially resolved HTC maps available, it is possible to characterize the thermal performances of the whole passage and to highlight the effect of rotation. INTRODUCTION Many different cooling methods have been developed to ensure that the turbine blade metal temperatures are maintained at a level consistent with safe and economic airfoil service life. Adoption of internal cooling is the starting point to achieve this task. Coolant air is routed to serpentine passages within the airfoil and convectively removes heat from the blade. The spent coolant can be then exhausted in multiple ways: at the blade tip, through cooling holes or slots at the trailing edge or by film cooling holes in multiple locations on the airfoil surface. This necessary cooling flow has a strong impact on the turbine efficiency, therefore improving cooling technology is always a focus point of gas turbine industry research activity, as demonstrated by the large literature available. Inside multi-pass channels different types of turbulent promoters are used to enhance heat transfer. The choice of which kind of turbulator to install is constrained by the channel cross section dimensions and aspect ratio, which in turns depends upon the blade region where the passage is located. Inclined ribs are usually found in the first legs, which have the task to cool leading edge and main body regions. Conversely, inside the thin trailing edge of the airfoil, pin-fin channels are prevalently adopted with combined benefits of structural integrity and heat transfer enhancements. ROTATING HEAT TRANSFER, CURRENT STATE-OF-ART Another important aspect that has to be taken into account in the design process of a cooling passage is the combined effect of turbulators, rotation, and channel orientation. The experimental and numerical results obtained by Hart [1], Lezius and Johnston [2], Speziale [3], and Speziale and Thangam [4] can be considered as the fundamental contributions describing the Coriolis effects on the flow field inside basic channel geometries