Internal cooling channels with differing aspect ratios are typically found in gas turbine blades due to the varying thickness of the blade from the leading to trailing edge. These serpentine passages often contain several 180° bends, which are sharp edged in the region of the blade tip. The 180° bend has a pronounced effect on the heat transfer characteristics in the outlet channel and tip wall, where a strong influence is seen due to the divider wall-to-tip wall distance in the bend. The present study investigates the effect of the divider wall-to-tip wall distance for a ribbed two-pass cooling channel with a 2:1 inlet and 1:1 outlet channel. Spatially resolved heat transfer measurements were made using the transient thermochromic liquid crystal technique for a smooth and a ribbed configuration using parallel 45° ribs. Effects of the 180° bend on heat transfer and rib-induced enhancements were identified separately and bend effects were found to dominate the heat transfer increase in the outlet channel near the bend. Pressure losses due to the bend and ribs were also independently evaluated for a range of tip wall distances. Results show that the smaller tip wall distances increase heat transfer on the tip wall and outlet channel, but at the cost of an increased pressure loss. An optimum tip wall position is suggested, forming a compromise between heat transfer improvement and increased pressure losses.

Gas turbine blades are usually cooled by using ribbed serpentine internal cooling passages, which are fed by extracted compressor air. The individual straight ducts are connected by sharp 180 deg bends. The integration of turning vanes in the bend region lets one expect a significant reduction in pressure loss while keeping the heat transfer levels high. Therefore, the objective of the present study was to investigate the influence of different turning vane configurations on pressure loss and local heat transfer distribution. The investigations were conducted in a rectangular two-pass channel connected by a 180 deg sharp turn with a channel height-to-width ratio of H/W=2. The channel was equipped with 45 deg skewed ribs in a parallel arrangement with e/dh=0.1 and P/e=10. The tip-to-web distance was kept constant at Wel/W=1. Spatially resolved heat transfer distributions were obtained using the transient thermochromic liquid crystal technique. Furthermore static pressure measurements were conducted in order to determine the influence of turning vane configurations on pressure loss. Additionally, the configurations were investigated numerically by solving the Reynolds-averaged Navier–Stokes equations using the finite-volume solver FLUENT. The numerical grids were generated by 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 turbulence model. The results showed a significant influence of the turning vane configuration on pressure loss and heat transfer in the bend region and the outlet pass. While using an appropriate turning vane configuration, pressure loss was reduced by about 25%, keeping the heat transfer at nearly the same level in the bend region. An inappropriate configuration led to an increase in pressure loss while the heat transfer was reduced in the bend region and outlet pass.

Internal cooling channels with differing aspect ratios are typically found in gas turbine blades due to the varying thickness of the blade from the leading to trailing edge. These serpentine passages often contain several 180 deg bends, which are sharp edged in the region of the blade tip. The 180 deg bend has a pronounced effect on the heat transfer characteristics in the outlet channel and tip wall, where a strong influence is seen due to the divider wall-to-tip wall distance in the bend. The present study investigates the effect of the divider wall-to-tip wall distance for a ribbed two-pass cooling channel with a 2:1 inlet and 1:1 outlet channel. Spatially resolved heat transfer measurements were made using the transient thermochromic liquid crystal technique for a smooth and a ribbed configuration using parallel 45 deg ribs. Effects of the 180 deg bend on heat transfer and rib-induced enhancements were identified separately and bend effects were found to dominate the heat transfer increase in the outlet channel near the bend. Pressure losses due to the bend and ribs were also independently evaluated for a range of tip wall distances. Results show that the smaller tip wall distances increase heat transfer on the tip wall and outlet channel but at the cost of an increased pressure loss. An optimum tip wall position is suggested, forming a compromise between heat transfer improvement and increased pressure losses.

Gas turbine blades are usually cooled by using ribbed serpentine internal cooling passages which are fed by extracted compressor air. The individual straight ducts are connected by sharp 180° bends. The integration of turning vanes in the bend region lets one expect a significant reduction in pressure loss while keeping heat transfer levels high. Therefore, the objective of the present study was to investigate the influence of different turning vane configurations on pressure loss and local heat transfer distribution. The investigations were conducted in a rectangular two-pass channel connected by a 180° sharp turn with a channel height-to-width ratio of H/W = 2. The channel was equipped with 45° skewed ribs in a parallel arrangement with e/dh = 0.1 and P/e = 10. The tip-to-web distance was kept constant at Wel/W = 1. Spatially resolved heat transfer distributions were obtained using the transient thermochromic liquid crystal technique. Furthermore static pressure measurements were conducted in order to determine the influence of turning vane configurations on pressure loss. Additionally, the configurations were investigated numerically by solving the Reynolds-Averaged Navier-Stokes equations (RANS method) using the Finite-Volume solver FLUENT. The numerical grids were generated by 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 turbulence model. The results showed a significant influence of the turning vane configuration on pressure loss and heat transfer in the bend region and the outlet pass. While using an appropriate turning vane configuration pressure loss was reduced by about 25% keeping the heat transfer at nearly the same level in the bend region. An inappropriate configuration led to an increase in pressure loss while heat transfer was reduced in the bend region and outlet pass.

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 Wel/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.