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Heat Transfer from Multiple Row Arrays of Low Aspect Ratio Pin Fins

A major consideration in the operation of a gas turbine engine is the temperature at which the gas exiting the combustor enters the first stage of the turbine. To maximize the performance of an engine, the turbine inlet temperature should be as high as possible. Today’s turbine inlet temperatures are higher than the melting point of the material used to construct the turbine blades and vanes, so improved methods, such as the insertion of pin fin arrays to internal cooling channels, are necessary. In most turbine designs, coolant air is extracted from the compressor and routed through internal cooling channels in each turbine blade.

Figure 1. Schematic of the overall test facility for pin fin testing

Cooling the trailing edge of a turbine blade is a particularly difficult engineering challenge because of its thin cross-section. It is a challenge because the external heat transfer coefficients associated with the mainstream core flow through the turbine are very high and the thin trailing edge section does not allow cooling technologies, such as film-cooling, because of the needed structural integrity. The cooling technology most commonly used for the trailing edge section of the blade are cylindrical pin fins with relatively low aspect ratios (pin height-to-diameter). These pin fins, which can be used in staggered or in-line configurations, are placed in the internal cooling channels of the airfoils to increase the wetted heat transfer area and to increase the turbulence levels of the channel flow thereby increasing the convection. The most common cooling designs for the trailing edge employ pin fins with aspect ratios between 0.5 and 4.0. Because of the large range of pin aspect ratios that lead to differing flow effects, such as the pin fin wake interaction with the endwalls, interpolation between aspect ratios do not provide accurate results. As the aspect ratio is decreased, the pins no longer provide the advantage of increasing the wetted heat transfer area; therefore, it is of interest to find the most effective aspect ratio to satisfy both heat transfer and structural requirements. The heat transfer characteristics of low aspect ratio pin fin arrays has not been as widely studied as the heat transfer in high aspect ratio pin fin arrays commonly used in heat exchanger applications.

Studies were performed to develop new and improved correlations to predict Nusselt number for a duct flow having a low aspect ratio with an array of pin fins. The duct and pin fin geometries are applicable to the cooling channel technology within gas turbine engines. In these studies, the heat transfer from the surface of the pins as well as the heat transfer from the duct walls, also known as endwalls, is measured. Heat transfer coefficients are measured for multiple row arrays of pin fins to determine the effects of changing the aspect ratio as well as the spanwise and streamwise spacings of the pin fins.

Figure 2. Test section for pin fin array.