Experimental and Numerical Investigations of Heat Transfer related to Gas Turbine Applications

Sammanfattning: Gas turbines are broadly used in aerospace and marine propulsion and power production etc.Investigation of heat transfer in gas turbines is a long-lasting and still an active subject ofresearch due to its tremendous importance. Thermal efficiency, durability and fatigue lifetimeof a gas turbine greatly rely on the heat transfer characteristics of gas turbine components.According to the first law of thermodynamics, efficiency of a gas turbine can be increased byincreasing the gas turbine inlet temperature (TIT) and minimizing the losses occurring in thegas turbine. To permit high TIT in gas turbines, a sophisticated gas turbine system with highcooling ability and minimum losses is required.With the objective of high thermal efficiency, initially as a fundamental study, a pair of vortexgenerator (VG) was imbedded in a boundary layer upstream of a cylindrical obstacle. Thereare well established vortical structures associated with an obstacle positioned in the flow andsimilarly with a vortex generator (VG). This has been designed with an idea how the vorticesaccompanied by the obstacle and VG interact with each other and consequently how the heattransfer characteristics on the endwall is affected. The orientation, position and yaw angle ofattack were studied experimentally using the steady state liquid thermography method. Thestudy of orientation of VG revealed that counter flow inward (CFI) orientation is useful forendwall heat transfer augmentation whereas, counter flow outward (CFO) orientation isappropriate for reduction of heat transfer on the endwall. Moreover, the position of the VGrelative to the obstacle also influenced the endwall heat transfer and more importantly, a 45oangle of attack provided larger augmentation of heat transfer than a lower angle of attack.Employing the results of this study, a VG pair installed in CFO orientation upstream of anairfoil was investigated with the objective to suppress the heat transfer and horseshoe vortex(HV) in the junction region of a nozzle guide vane. The high heat transfer in the junctionregion results a hot spot and moreover, the HV increases the secondary losses in the gasturbine. The VG pair was separated apart at different gaps in the spanwise direction makingdifferent cases. The experimental results showed reduction of endwall heat transfer in thejunction region with the VG pair. Moreover, numerical simulations using ANSYS Fluentpredicted similar behavior of heat transfer and the flow features showed suppression of thesecondary flow particularly the HV with the presence of the VG compared to without VGcase.Similarly, utilizing the knowledge gained in the fundamental study, VGs were used forthermal performance enhancement of pin fins which are usually employed in the heat transferaugmentation for internal cooling of the trailing part of a gas turbine blade/vane. Vortexgenerators were mounted in pairs upstream of the first row of in-line pin fins arrays. Thermalperformance obtained numerically was compared without VGs and with VGs positioned atvarious locations with reference to the pin fins and yaw angles for various Reynolds numbers.The results illustrated that thermal performance was enhanced with VGs irrespective of the VG angle and position compared to the case without VGs. However, the level of enhancementis dependent on the VGs angle and position. The VG placed at 45o with a moderate gapbetween VGs and pin fins showed greater thermal performance.In advanced gas turbines, the assembly of different components of the gas turbine may leadto some modifications in the flow paths. In an aero engine, when a low pressure turbine isconnected to the nozzle section which has outlet guide vanes, a cavity is generated as a resultof the assembly process. This cavity is formed upstream of an outlet guide vane. Differentstreamwise distances between the cavity and outlet guide vane (in this study an airfoil isconsidered) were investigated experimentally as well as numerically. Results demonstratedthat the cavity located at a small distance is favorable as it considerably reduced the endwallheat transfer in the junction region of the airfoil. Similarly, the meeting parts of combustorand turbine are assembled via an axisymmetric contouring to match the diameters of bothparts. So the effect of an axisymmetric endwall contouring on film cooling/heat transfer andsecondary losses was numerically investigated for a nozzle guide vane (NGV) in a singlepassage region. It was revealed that an axisymmetric contouring enhanced the film coolingeffectiveness. The heat transfer increased with the addition of film cooling which is obviousbut the level of enhancement degraded with endwall contouring. Flow structures highlightedthe suppression of the secondary flow.