Gas Turbine Cooling: Moving From Macro to Micro Cooling

Up to the present time, virtually all production gas turbine engines utilize high-pressure turbine components that are cooled by various modes of bulk, or macro, internal flow passages and networks, many of which are then linked to external film cooling by holes through the walls. Typical examples of macro cooling include serpentine passages, swirl chambers, pin banks, and impingement jets. All cooled turbine airfoils in commercial operation today utilize cooling channels and film holes that are considered to be macroscopic in physical magnitude. The conventional delimiters between macro and micro cooling are (1) whether the feature (internal passages and holes) can be repeatedly manufactured via investment casting methods, and (2) whether the flow passage (film hole, impingement orifice, dust hole) may become plugged by particles in the cooling fluid. The acceptable sizes are relative to the turbine size and the operating environment. This study examines the limitations and weaknesses inherent in macro cooling and the reasoning that demands gas turbine cooling change to micro cooling methods. A brief history of the developments in micro cooling for turbine airfoils is presented, including the reasons why it has not yet become commercially viable. A simple cooling study is used to demonstrate the very significant performance gains that can be obtained with micro cooling, such as cooling flow reductions approaching 40%, and thermal stress (gradient) reductions of 50%. The key to realizing these gains in full lies in cost effective manufacturing and durability in operation.