A Novel Technique for the Internal Blade Cooling

Abstract

Development of an adequate air cooling system for the thermally highly loaded leading edge and tip of the blade, that is cost effective and also relatively insensitive to manufacturing tolerances and operating environment continues to be one of the major challenges in advanced gas turbine design. Extensive studies on the convective (including impingement) and film cooling techniques produced remarkable progress in achieving a high cooling effectiveness level for turbine airfoils. However, in the case of turbine blades, application of these techniques has severe limitations. Highly effective impingement cooling needs to be combined with film discharge of the spent air to avoid a negative impact of crossflow on internal heat transfer and also provide additional thermal protection of the surface downstream of the discharge holes. Noticeable aerodynamic penalties, stress concentration and significant increase in manufacturing cost limit application of blade film cooling, particularly for moderately high operating temperatures. Search for a highly effective robust design of internal airfoil cooling which can delay the use of film cooling resulted in the creation of a new technique which is described in this paper. This technique is based on generation of a swirling flow structure in the blade internal leading edge passage. Significant heat transfer augmentation can be achieved when the cooling air is delivered into the leading edge plenum tangentially to the inner concave surface. The best results can be obtained when the swirling flow is allowed to move radially, creating a three-dimensional screw-shaped flow in the plenum. The presented results of the flow and heat transfer studies performed for the practical range of Reynolds numbers for the internal flow show that the leading edge screw-shaped cooling technique provides internal heat transfer rate comparable with impingement coupled with film discharge of the spent air, is more effective than impingement with cross flow and is almost five times higher than heat transfer in the smooth channel.