There are three main parts of a gas turbine, namely: the compressor, the combustor and the turbine, as represented in Figure 1.1. The function of the compressor is to pressurize the air before the air goes into the combustion chamber, mixed with the fuel and ignited. This fuel-air mixture burns at high temperatures and expands. Thereafter the hot gas enters the turbine and strikes the vane, which directs the incoming gas to the blade. The blade deflected by the oncoming gas stream and thus a torque produced on the shaft causing it to rotate which is then converted into useful work.
One use of this rotational movement is to produce electricity by rotating a generator and then stepping up by using a transformer. Fig 1.1 Gas Turbine 1.2 Turbine Inlet Temperature (TIT) Due to the nature of its working, the power generated by a turbine increases with increasing the temperature at which the gas enters, called the turbine inlet temperature. An increased power output results in a higher efficiency.
However, the turbine inlet temperature cannot increase arbitrarily because of the limits imposed due to the temperature at which the blade material melts. Although advances have been made in material science to make new alloys having high melting points that can withstand operation at such high temperatures without failing, these materials are expensive and are difficult to machine. Gas turbines have a wide application area from commercial and military aircraft engines to naval propulsion and power generation. These entire aerials, marine or land based application areas are growing day by day with greater need of power output. Increasing power output demands made thermal efficiency one of the most important issues about the gas turbine engines due to environmental and economic concerns. Gas turbine engine applications, aircraft engines to industrial utilizations, can be idealised as a Brayton cycle. In an ideal Brayton cycle, under the assumptions of isentropic compression and expansion with no friction losses and constant Cp, main controlling factor of thermal efficiency is turbine inlet temperature. Through the decades, engineers increased the turbine inlet temperatures of gas turbines to receive greater thermal efficiency as well as greater power output and interest in elevating the temperatures to higher values remains. The turbine component of the gas turbine engines mainly deals with the highest temperature values of which are well beyond the allowable material limit. Although, more durable alloys have been developed and coating technologies have made important progress, material endurance and operational live under intense temperatures are still the limiting factor to increase turbine inlet temperatures. A plot of approximate turbine inlet temperatures of large aircraft engines throughout last half century shown at Figure 1.2. Figure 1.2 Improvements on turbine inlet temperature through time As can be seen from Figure 1.1 the allowable material temperature line has no interest with the current needs of gas turbine engines. Figure 1.2 also reveals that turbine inlet temperatures of 2000 K are typical for current gas turbines. It is apparent that to cope with the thermal efficiency and power output requirements, using turbine-cooling techniques is inevitable. 1.3 Turbine Cooling Strategies 1.3.1 Gas turbine blade A turbine blade is the individual component, which makes up the turbine section of a gas turbine or steam turbine. The blades are responsible for extracting energy from the high temperature, high pressure gas produced by the combustor. The turbine blades are often the limiting component of gas turbines. To survive in this difficult environment, turbine blades often use exotic materials like super alloys and many different methods of cooling, such as internal air channels, boundary layer cooling, and thermal barrier coatings. Blade fatigue is a major source of failure in steam turbines and gas turbines. Fatigue is caused by the stress induced by vibration and resonance within the operating range of machinery. To protect blades from these high dynamic stresses, friction dampers are used. Fig 1.3 Gas turbine blade The air extracted from the compressor of the engine cools the turbine blades. The task is keeping maximum metal temperature below the values specified by material capabilities and besides avoid from high temperature variations within the turbine blade to maintain acceptable life and operational requirements with extracting the minimum cooling air possible. To achieve this task, special techniques implemented to optimize turbine cooling. The cooling strategies mainly divided into two groups as internal cooling and external cooling. 1.3.2 Internal Cooling Various cooling arrangements employed to enhance heat removal from the blade. These arrangements chosen according to which zone of the turbine blade cooled. Figure 1.4 describes the most effective application of internal cooling in turbine blades used in modern turbine blades. There are three major internal cooling zones in a turbine blade. Regions near leading edge cooled using impingement cooling, at central regions air ducted through serpentine passages, which are often ribbed, and trailing edge region is equipped with an array of pin-fins. Figure 1.4 Cooling concepts of a modern gas turbine engine Among all internal turbine-cooling techniques, jet impingement is the most effective at increasing heat transfer coefficient. In jet impingement method, air jets created by forcing air through perforated plates, impinge on hot interior blade regions. Modern cooling configurations generally use jet impingement method at leading edge due to its structural constraints. Serpentine passages with ribs turbulators on the inner walls are used near the middle portion of the turbine blades. Repeated rib turbulators are cast into the serpentine passage to increase heat transfer coefficient. Close to trailing edge, turbine blade becomes thinner and pin-fins are preferred because of its structural advantages. They generally have staggered array and extend interior suction to pressure surfaces. Both rib turbulators and pin-fins have vast variation of shape, height, width, placement in the internal cooling channel and angle with the air flow. Each configuration has its own advantages and disadvantages in terms of heat transfer coefficient enhancement and created loses. 1.3.3 Types of Internal Cooling There are various types of internal cooling which have developed over the years. No particular type of cooling is suitable for all blades for all applications. Thus, the cooling scheme must be selected according to operating conditions and requirements of the application at hand. 126.96.36.199 Impingement Cooling It is generally used near the leading edge of the aerofoil where the jet of cooling air strikes the inside of the blade surface and hence the name impingement cooling. These techniques can also be used in the middle part of the blade. The heat transfer characteristics of this kind of cooling depends on the size and distribution of jet holes, cross-section of the cooling channel and the surface area of the target face. 188.8.131.52 Pin Fin Cooling Since the trailing edge of the blade is very narrow, it is difficult to manufacture holes and passages in this portion, thus pin fin cooling is applicable in this region. The flow around the pins is similar to flow around a cylinder. The airflow separate and the wakes shed downstream. Moreover, a horseshoe vortex also forms wrapping around the fins and creating additional mixing and thus enhancing heat transfer. The heat transfer characteristics largely depend on the type of fin array and the spacing of the pins in the array, the pin shape and size. 184.108.40.206 Dimple Cooling This type of cooling occurs due to the presence of concave depressions or indentations on the surfaces of the blade passage. They induce flow separation and reattachment and thus enhance heat transfer. They are a very desirable cooling technique as they have low-pressure losses. 220.127.116.11 External (Film) Cooling In cooling arrangements, which solely use internal cooling applications, circulated air in the internal cooling passages ejected from trailing edge ejection slots. However, in external cooling, the idea is about injecting coolant air from inside the blade to the hot gas path by discrete holes other than trailing edge ejection slots and protecting blade from hot gasses by forming a film layer on the blade. Injected fluid introduces a secondary flow and protects both immediate region of injection and downstream of it so compared to internal cooling techniques, which remove heat from inside surface, external cooling applications directly, protects the blade surface. External cooling configuration success depends on many parameters. The geometrical aspects of the hole like shape, length-to-diameter ratio and injected flow angle relative to the main flow are very important. Besides, location and distribution of the holes on the surface have also primary importance for blade protection from hot gases. Figure 1.5. External (film) cooling holes locations The holes responsible from leading edge and tip cap cooling are the most critical ones. The reason is that, first deals with the highest temperatures due to the stagnation of hot gases at the leading edge and second has a location where lacks durability and difficult to cool because of the tip leakage. Other common film cooling holes locations are blade platform cooling holes and gill holes (depicted at Figure 1.5). Although, there are common locations for turbine cooling holes as presented at Figure 1.5, some quantities which are evaluated to decide their exact locations. Most useful measures of quantifying effectiveness of external cooling configuration are coolant to mainstream temperature ratio (Tc/Tg) and pressure ratio (Pc/Pg). In general lower temperature ratio and higher-pressure ratio are favourable while the other parameter kept constant. However, a too high coolant to mainstream pressure ratio may cause jet penetration to mainstream and reduce the external cooling effectiveness. Therefore, amount of coolant used for internal and external cooling needs to optimise under engine operating Conditions. 18.104.22.168 Rib Turbulated Cooling Ribs are protrusions which are placed in a controlled way along specific target walls of the internal duct. Repeated rib turbulence promoters are cast on the two opposite walls (pressure and suction sides) of the internal duct to enhance the heat transfer. Having ribs on the opposite walls, maximizes the interaction with the coolant and wall and helps cool the turbine blade effectively. Various cases see ribs only on one side of the duct because they need to match the external thermal loads which are different for the pressure and suction sides. While it causes friction for the flow in the channel which is correlated 7 to increased heat transfer, its drawback is pressure drop, which will be higher than a regular smooth channel. Hence having the right rib-height, rib pitch and the angle at which the ribs hit the flow is very important to minimize pressure loss and maximize benefits. Ribs have conventionally been arranged orthogonally to the flow, i.e., the extension of the rib is situated 90o to the streamwise flow direction. Different configurations for the rib placement like the angle of attacks have different characteristics and hence different heat transfer rates. As per previous studies, the rib pitch-to-height ratio (P/e), the rib height-to-hydraulic diameter (e/D) and rib angle of attack are the main factors effecting the heat transfer coefficients and friction factors. The function of a rib is to detach a layer of the oncoming flow as the rib is placed in such a way that it obstructs a part of the flow. Due to the presence of the rib, the flow trips and separates at the top of the rib and reattaches to the wall between the ribs. The boundary layer is disturbed increasing the turbulence of the flow due to the separation and reattachment. Due to this phenomenon, the flow mixes the fluid elements near the wall with the cooler air in the middle of the flow, enhancing the overall heat transfer. The fact that it only disturbs the near ” wall flow, the overall heat transfer of the flow is enhanced and the pressure drops are relatively low. This can be clearly visualized in Fig. 1.6 which shows how the flow is separated and reattached between two ribs. Figure 1.6 Mechanism of Rib Turbulated Cooling The flow around a rib is characterized by several re-circulating zones which involves shear, mixing, and impinging flow which increases the turbulence level and hence the heat transfer to 2-4 times that experienced in a smooth channel. The main effect is the large re-circulating zone downstream of the rib and the reattachment length. The physics within this region is similar to that behind a backward-facing-step. The ribs are represented as short, rectangular or square channels with different aspect ratios (AR). In case of a rectangular channel, the enhancement of heat transfer is dependent on the rib turbulators’ geometry, such as rib size, shape, distribution, flow angle of attack, and the flow Reynolds number. To obtain higher Reynolds number (Re) flows, the rib height should be constrained in order to receive higher efficiencies with a hit on the heat transfer coefficient and increased pressure drop. Rib spacing is widely effective on the performance of the channel as closely spacing the ribs would enhance the heat transfer but spacing them wider would increase the overall area for convection. Hence optimization of the rib is a key aspect in order to obtain optimal results for heat transfer. 9 In order to assess the performance of the rib in a smooth channel, different comparison factors are studied to evaluate the best results. In most cases Nusselt number (Nu) and friction factor (f) are analysed because small changes in the rib geometry can be easily tracked. In case of the rib, thermal energy is transferred from the external pressure and suction surfaces of the turbine blades to the inner zones through conduction which is then removed by internal cooling.