Turbine Blade Tip Cooling With Blade Rotation-Part II: Shroud Coolant Injection

In this paper, blade tip cooling is investigated with coolant injection from the shroud alone and a combination of shroud coolant injection and tip cooling. With a nominal rotation speed of 1200 rpm, each blade consists of a cut back squealer tip with a tip clearance of 1.7% of the blade span. The blades also consist of tip holes and pressure side (PS) shaped holes, while the shroud has an array of angled holes and a circumferential slot upstream of the rotor section. Different combinations of the three cooling configurations (tip and PS holes, shroud angled holes, and shroud circumferential slot) are utilized to study the effectiveness of coolant injected from the shroud as a complementary method of cooling the blade tip. The measurements are done using liquid crystal thermography. Blowing ratios of 0.5, 1.0, 2.0, 3.0, and 4.0 are studied for shroud slot cooling, and blowing ratios of 1.0, 2.0, 3.0, 4.0, and 5.0 are studied for shroud hole cooling. For cases with coolant injection from the blade tip, the blowing ratios used are 1.0, 2.0, 3.0, and 4.0. The results show an increase in film cooling effectiveness with increasing blowing ratio for shroud hole coolant injection. The increased effectiveness from shroud hole coolant is concentrated mainly in the tip region below the shroud holes and toward the blade suction side and the suction side squealer rim. Slot coolant injection results in increased effectiveness on the blade tip near the blade leading edge up to a maximum blowing ratio, after which the cooling effectiveness decreases with increasing blowing ratio. The combination of the different cooling methods results in better overall cooling coverage of the blade tip with the shroud hole and blade tip coolant combination being the most effective. The level of coolant protection is strongly dependent on the blowing ratio and combination of blowing ratios.