The capability to raise the turbine entry temperature (TET) is a major driver for increasing the efficiency of gas turbines. For this reason, active cooling is needed for turbine stator and rotor blades, as well as endwalls, to cope with temperatures higher than what the material can withstand.However, the amount of cooling air, generally extracted from the high pressure compressor, needs to be minimized, since the efficiency of the machine is reduced when increasing the cooling air.In this work, we present a sequential impingement cooling concept in which two consecutive narrow impingement channels are connected in a cascading fashion via a plenum. This arrangement allows for an increase of the effectiveness of the cooling flow and/or a reduction of the cooling mass flow. Additionally, this allows to limit the degradation of the performances due the crossflow from the upstream jets, which reduces the penetration of the following jets, resulting in a reduction of their cooling capacity.Experimental heat transfer tests have been carried out using the transient liquid crystal thermography (LCT) technique on perspex model channels having a rectangular cross-section with a jet-to-plate distance of 3D (D=Jet Diameter), and a channel width of 5D. The jets, arranged in a single array, are placed at a spacing of 5D. Reynolds numbers, based on the mean flow velocity, range from 10,000 to 40,000. The collected data include Nusselt number maps for the target plate of both channels and pressure drops along the channel.Particular attention has been placed on the so called transition zone, which connects the first impingement channel with the second plenum. Here the target plate is not cooled by impingement jets, resulting in lower heat transfer coefficients that can cause stresses due to high temperature gradients in the material. For this reason this region is analyzed in detail, with modifications to the baseline geometry ranging from cross section reductions to addition of heat transfer enhancing features like pin-fins or ribs.In addition to the experiments, steady-state RANS numerical simulations are carried out on selected geometries, which allows to gain an insight into the complex flow features present in the transition zone.Results show that the area averaged Nusselt number in the problematic transition zone can be increased by 50% (at Reynolds number 20,000) when using a combination of pins and ribs compared to a smooth channel, with a modest increase of 7.6% of the pressure drop.

Heat transfer enhancement in a double sequential impingement channel

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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