Michele Gaffuri
With the future decarbonization of the electricity production, gas turbines will be an important tool for the stabilization of the electricity grid by meeting the peak of energy demand using zero-carbon fuels like synthetic gas and hydrogen.The combustion temperature in gas turbines has increased steadily over the years in a quest to increase the thermal efficiency of the machine, so that in modern gas turbines active cooling of blades and vanes is required to ensure safe operation.The need for cooling is exacerbated by the prospect of using hydrogen as fuel, since it has higher stoichiometric combustion temperature, and the combustion products include water vapor, which increases the load on parts exposed to the hot flow.For future turbines, therefore, the thermal management is of the utmost importance; this includes both the cooling system itself - internal cooling and film cooling - as well as protective coatings on the exposed surface of the parts.Additive manufacturing of metallic parts is already a reality for production of static turbine parts, and in the future rotating blades could also be produced by this method.This paradigm shift opens the design space of turbine cooling systems by reducing the manufacturing constraints of the parts, so that radically new cooling concepts can be implemented.For internal cooling, jet impingement achieves the highest local heat transfer and is therefore used in the most highly stressed regions of the turbine parts.The crossflow caused by the spent air of the jets, however, limits the number of jets that can be used in a channel while maintaining high cooling performances, especially in narrow channels with maximum crossflow configuration.The main objective of this work is the experimental investigation of a new impingement channel design that reduces the crossflow by stacking two consecutive shorter channels connected via a plenum.In total, 34 large scale models have been investigated at engine-relevant Reynolds numbers using a new transient liquid crystal technique. Experimental data include high resolution heat transfer coefficients on the target plates as well as pressure data at several locations, which allows to determine the pressure drop of each part of the channel.In a first, exploratory phase, the effect of the variation of the number of jets and of the length of the transition zone has been studied.Moreover, bypass holes that connect directly the two channels and allow for an initial crossflow in the second channel have been investigated.A low heat transfer region has been identified in the transition zone; therefore, several heat transfer enhancement devices have been installed in this zone. Ribs, pins, and cross-section reductions allow to increase the heat transfer coefficient levels.Combinations of these devices provide higher heat transfer enhancement with a marginal increase of the pressure drop.An analytical approach has been used to estimate the metal temperatures of a part cooled by the sequential channels in comparison to a conventional narrow impingement channel at the same massflow, with the new concept achieving lower temperature when using high performance heat transfer enhancement designs. With the coolant mass flow reduction allowed by the new design, an improvement of the thermal efficiency of 0.7% has been estimated when considering the thermodynamic cycle of a modern gas turbine.
EPFL2021
