Impact of Cooling with Thermal Barrier Coatings on Flow Passage in a Gas Turbine

Inlet temperature is vital to the thermal efficiency of gas turbines, which is becoming increasingly important in the context of structural changes in power supplies with more intermittent renewable power sources. Blade cooling is a key method for gas turbines to maintain high inlet temperatures whilst also meeting material temperature limits. However, the implementation of blade cooling within a gas turbine—for instance, thermal barrier coatings (TBCs)—might also change its heat transfer characteristics and lead to challenges in calculating its internal temperature and thermal efficiency. Existing studies have mainly focused on the materials and mechanisms of TBCs and the impact of TBCs on turbine blades. However, these analyses are insufficient for measuring the overall impact of TBCs on turbines. In this study, the impact of TBC thickness on the performance of gas turbines is analyzed. An improved mathematical model for turbine flow passage is proposed, considering the impact of cooling with TBCs. This model has the function of analyzing the impact of TBCs on turbine geometry. By changing the TBCs’ thickness from 0.0005 m to 0.0013 m, its effects on turbine flow passage are quantitatively analyzed using the proposed model. The variation rules of the cooling air ratio, turbine inlet mass flow rate, and turbine flow passage structure within the range of 0.0005 m to 0.0013 m of TBC thicknesses are given.

Impact of Swirling Entropy Waves on a High Pressure Turbine

The harsh environment exiting modern gas turbine combustion chamber is characterized by vorticity and temperature perturbations, the latter commonly referred as entropy waves. The interaction of these unsteadiness with the first turbine stage causes non-negligible effects on the aerodynamic performance, blade cooling and noise production. The first of these drawbacks is addressed in this paper by means of an experimental campaign: entropy waves and swirl profile are injected upstream of an axial turbine stage through a novel combustor simulator. Two injection positions and different inlet conditions are considered. Steady and unsteady experimental measurements are carried out through the stage to address the combustor-turbine interaction characterizing the injected disturbance, the nozzle and rotor outlet aerothermal field. The experimental outcomes show a severe reduction of the temperature perturbation already at stator outlet. The generated swirl profile influences significantly the aerodynamic, as it interacts with the stator and rotor secondary flows and wakes. Furthermore, the clocking position changes the region most affected by the disturbance, showing a potential modifying the injection position to minimize the entropy wave and swirl profile impact on the stage. Finally, this work shows that in order to proficiently study entropy waves, the unsteady aerodynamic flow field stator downstream has to be addressed.

Exhaust Gas Turbine

As discussed in Chapter 2, the supercharger (basically, an air compressor) can also be driven by an exhaust gas turbine. In this case, the overall system is referred to as a turbocharger or turbosupercharger (Abgasturbolader in German). The focus in Kollmann’s manuscript is exclusively on radial compressors used as superchargers driven by a gear drive connected to the main engine shaft. This is not so surprising considering that, although significant R&D effort was spent on the turbine design (especially, turbine blade cooling), turbocharged German aircraft engines did not enter service until the end of the war. Even then, the service experience was limited to Junkers Ju 388 (mostly for high altitude reconnaissance) powered by two 1,500-HP BMW 801 J turbocharged engines. Many other designs (e.g., the DB 623) were eventually abandoned. The dilemma facing the German engineers at the time (1940s) was this: whether to develop an aircraft engine from the get-go with a turbocharger or to develop a turbocharger to be fitted into an existing engine (e.g., the DB 603). Since the need for the turbochargers arose during the war by the need for higher flight altitudes (10 to 14 km), e.g., to attack the Allied bomber formations and their fighter escort, the urgency of the situation made the choice for them1. Not surprisingly, they went with the latter option.

Low-Pressure Turbine Cooling Systems

Modern low-pressure turbine engines are equipped with casings impingement cooling systems. Those systems (called Active Clearance Control) are composed of an array of air nozzles, which are directed to strike turbine casing to absorb generated heat. As a result, the casing starts to shrink, reducing the radial gap between the sealing and rotating tip of the blade. Cooling air is delivered to the nozzles through distribution channels and collector boxes, which are connected to the main air supply duct. The application of low-pressure turbine cooling systems increases its efficiency and reduces engine fuel consumption.

Impact of Swirling Entropy Waves on a High Pressure Turbine

The harsh environment exiting modern gas turbine combustion chamber is characterized by vorticity and temperature perturbations, the latter commonly referred as entropy waves. The interaction of these unsteadiness with the first turbine stage causes non-negligible effects on the aerodynamic performance, blade cooling and noise production. The first of these drawbacks is addressed in this paper by means of an experimental campaign: entropy waves and swirl profile are injected upstream of an axial turbine stage through a novel combustor simulator. Two injection positions and different inlet conditions are considered. Steady and unsteady experimental measurements are carried out through the stage to address the combustor-turbine interaction characterizing the injected disturbance, the nozzle and rotor outlet aerothermal field. The experimental outcomes show a severe reduction of the temperature perturbation already at stator outlet. The generated swirl profile influences significantly the aerodynamic, as it interacts with the stator and rotor secondary flows and wakes. Furthermore, the clocking position changes the region most affected by the disturbance, showing a potential modifying the injection position to minimize the entropy wave and swirl profile impact on the stage. Finally, this work shows that in order to proficiently study entropy waves, the unsteady aerodynamic flow field stator downstream has to be addressed.

Multi-Objective Optimization on the Fluid Flow and Heat Transfer of Semi-Attached Rib-Channels

Rib-turbulators have been widely used in turbine blade cooling designs to enhance internal heat transfer. However, application of the ribs in blade cooling channels inevitably causes large pressure losses and lower heat transfer areas behind the ribs and at the joint parts of the ribbed wall with sidewalls in comparison to smooth channels. Semi-attached rib having two rectangular holes at the bottom of the ribs at the joint regions of the ribbed wall and two sidewalls is able to reduce friction losses and alleviates lower heat transfer in the channels. This paper presented a method of combining computational fluid dynamic simulation with an optimization method to optimize the performance of a channel with semi-attached ribs and to discuss the effects of hole geometric variables on both pressure losses and heat transfer enhancement. A fully automatic multi-objective genetic algorithm based on the Non-dominated Sorted Genetic Algorithm-II is applied for a one-pass squared channel with 60° inclined semi-attached ribs. Four geometrical variables are widths and heights of the two rectangular holes, and two objective functions are the maximum of area-averaged Nusselt number on the rib roughend wall and the minimum of friction factor. The Kriging model is constructed by using the Latin hypercube sampling method to choose 15 experimental points in the design space. The results showed that the optimization method helps to increase the channel performance and to eliminate nearly all low heat transfer areas by appropriately adjusting the heights and widths of the holes. Compared to solid ribs, the optimal semi-attached rib increases the area-averaged Nu number on the rib-roughened walls by 9.45%, whereas the friction factor in the channel is decreased by 6.95%, which corresponds to an increase of 12.13% in thermal performance. In the optimal model, the existence of large holes on the right side of the optimal semi-attached ribs moves the vortex core of the secondary flow downstream of the ribs from the right side of the solid-ribbed channel to the middle, which causes a large increase in Nu number on the middle and left side of the rib roughened walls and only a moderate reduction in Nu number on the right part of the walls. In the whole range discussed, heat transfer coefficient on the rib-roughened walls increased with the height of the right holes, but it increases with the width of the hole only in the range of 8–10mm. For the holes on the left, heat transfer coefficient deteriorates as the height and width increase. Increasing width and height of the holes on both sides of the semi-attached ribs reduces the channel friction losses.

Impact on Cycle Efficiency of Small Combined Heat and Power Plants From Increasing Firing Temperature Enabled by Additive Manufacturing of Turbine Blades and Vanes

Using an analytical cooled gas turbine model and a steam cycle model, this study estimates the impact on combined heat and power (CHP) cycle performance from increasing the turbine firing temperature by 180°F (100°C) and improving the turbine blade cooling for a 6-MW scale gas turbine. A sensitivity analysis was performed to understand the impact of increasing the internal cooling effectiveness, thermal barrier coating performance, and blade material upgrades on gas turbine and CHP cycle efficiency. The impacts of turbine blade cooling improvements were studied for three common CHP cycle configurations identified from the literature. Various definitions for CHP cycle efficiency from the literature are used in the comparisons. The results show that a 180°F (100°C) increase in firing temperature can increase the gas turbine efficiency by 1 percentage point without improving cooling effectiveness and add 2 additional percentage points in efficiency by using advanced turbine blades with higher internal cooling efficiency. The engine upgrades evaluated in this study show potential for increasing the CHP cycle efficiency by 3 percentage points while increasing the steam generation rate by 8%.

Comprehensive Review on Leading Edge Turbine Blade Cooling Technologies

Developments in the gas turbine technology have caused widespread usage of the Turbomachines for power generation. With increase in the power demand and a drop in the availability of fuel, usage of turbines with higher efficiencies has become imperative. This is only possible with an increase in the turbine inlet temperature (TIT) of the gas. However, the higher limit of TIT is governed by the metallurgical boundary conditions set by the material used to manufacture the turbine blades. Hence, turbine blade cooling helps in drastically controlling the blade temperature of the turbine and allows a higher turbine inlet temperature. The blade could be cooled from the leading edge, from the entire surface of the blade or from the trailing edge. The various methods of blade cooling from leading edge and its comparative study were reviewed and summarized along with their advantages and disadvantages.