Flow Effects on the Bend Region Heat Transfer Distribution of 2-Pass Internal Coolant Passages of Gas Turbine Airfoils: Influence of Film Cooling Extraction

A study of the flow and heat transfer in stationary models of a two-pass internal coolant passage is presented, which focuses on the flow characteristic effects on the wall heat transfer distribution. Results are given in the sharp 180deg bend region of the channel. The transient thermochromic liquid crystal technique was used. The technique allows full surface heat transfer coefficient measurements on all the walls, in the region of interest. Flow measurements were also conducted with a stereoscopic digital PIV system. The system measures all three-velocity components simultaneously. 3D-streamlines are extracted from the full data set. The coolant passage model consists of two square ducts, each having a 20 hydraulic diameter length. The ducts are connected by a sharp 180deg bend with a rectangular outer wall. 45deg ribs are mounted in a staggered arrangement on the bottom and top walls of both legs. The height of the ribs is equal to 0.1 hydraulic diameters. They are spaced 10 rib heights apart. One geometry is also equipped with extraction holes to simulate holes for film cooling. Two series of holes are placed solely in the bottom wall; four holes are located in the bend, and twelve in the downstream leg. The global extraction through the holes was set to 50% of the inlet massflow. The flow measurements were obtained at one flow condition with an inlet flow Reynolds number, based on the hydraulic diameter, of 50,000. The heat transfer measurements were obtained at the same flow conditions. The paper presents detailed measurement results of the heat transfer distribution on all the channel outer walls in the bend region. It describes how the main and secondary flows govern the heat transfer coefficient in the bend region. It relates the flow recirculating cell structures and the heat transfer distribution on the bend walls. It also shows how the film cooling extraction influences both the recirculating cells and the heat transfer distribution.