The Effect of Side Wall Mass Extraction on Pressure Loss and Heat Transfer of a Ribbed Rectangular Two-Pass Internal Cooling Channel

Gas turbine blades are often cooled by using combined internal and external cooling methods, where for internal cooling purposes usually serpentine passages are applied. In order to optimize the design of these serpentine passages it is inevitable to know the influence of mass extraction due to film cooling holes, dust holes or due to side walls for feeding successive cooling channels as for the trailing edge on the internal cooling performance. Therefore, the objective of the present study was to analyse the influence of side wall mass extraction on pressure loss and heat transfer distribution in a two-pass internal cooling channel representing a cooling scheme with flow towards the trailing edge. The investigated rectangular two-pass channel consisted of an inlet and outlet duct with a height-to-width ratio of H/W = 2 connected by a 180° sharp bend. The tip-to-web distance was kept constant at W_el/W = 1. The mass extraction was realized using several circular holes in the outlet pass side wall. Two geometric configurations were investigated: A configuration with mass extraction solely in the outlet pass, and a configuration with mass extraction in the bend region and outlet pass. The extracted mass flow rate was 0%, 10%, and 20% of the inlet channel mass flow. Spatially resolved heat transfer distributions were obtained using the transient thermochromic liquid crystal technique. Pressure losses were determined in separate experiments by local static pressure measurements. Furthermore, a computational study was performed solving the Reynolds-Averaged Navier-Stokes equations (RANS method) using the commercial Finite-Volume solver FLUENT. The numerical grids were generated using the hybrid grid generator CENTAUR. Three different turbulence models were considered: the realizable k-ε model with two-layer wall treatment, the k-ω-SST model, and the v²-f model. The experimental data of the investigation of side wall ejection showed that the heat transfer in the bend region slightly increased when the ejection was in operation, while the heat transfer in the section of the outlet channel with side wall ejection was nearly not affected. After this section a decrease in heat transfer was observed which can be attributed to the decreased mainstream mass flow rate.