Gas turbine basically consists of compressor, combustor and turbine. The gas turbine draws atmospheric
air to the compressor and the compressed air is premixed with fuel to be combusted in the combustion chamber.
The combusted gas with high pressure and high temperature passes through the turbine to rotate the turbine blades by expanding itself. Gas turbine engines are being applied to various industries include aircrafts, power stations and ships.
For the improved gas turbine performance, the Turbine Inlet Temperature (TIT) is steadily increased for decades and it causes high thermal load on turbine parts. To prevent turbine part from exceeding the allowable temperature of turbine material, various cooling technics are developed such as internal cooling, film cooling, impinging jet cooling, etc.
The gas turbine is a widely used power generating system in these days due to its wide capacity, from kilo- to mega- watts, and reliable performance. Like other thermal power systems, its efficiency is directly proportional to operating temperature which is specifically called the turbine inlet temperature.
Therefore a lot of efforts were made to increase the turbine inlet temperature and major components of gas turbine, such as combustor and turbine blades, are exposed to extremely hot combustion gas. But there is a limitation in raising the turbine inlet temperature because of the maximum allowable temperature of material. And continuous thermal stress is the most frequent cause in failure of gas turbine engine and also it effects significantly in their lifespan. To increase the efficiency and lifespan of gas turbine engine, designing an effective cooling system has become a prerequisite process in development of high performance gas turbine engines. The incipient idea of cooling technique is to make an internal passage inside the components and let cooling fluid flow. This is very common technic in these days and various advanced internal cooling passages are implemented to improve gas turbine performance.
Hot components in a recent gas turbine engine have been operated over conditions of maximum material temperature for enhancing energy efficiency.
Because of these environments, various cooling schemes have been used and investigated. Moreover, accurate prediction of heat transfer coefficients on external and internal blade surface is important in gas turbine cooling design and optimization.
Furthermore, the high temperature and the temperature gradient in each hot component generate thermal damages by high thermal stresses. In other words, as an operation condition is unsuitable, local thermal cracks and structural failure may occur due to the high thermal stress. Therefore, it is necessary to estimate the stress distributions and the lifetime prediction in the cooling system due to the safety of hot components such as combustors, vanes, and blades. Thermal analysis as well as a failure analysis is required definitely for the effective thermal design.
In modern gas turbine engines for aircraft propulsion and land-based power generation or industrial application, turbine inlet temperature has been increased steadily because of improvement of gas turbine performance.
Accordingly, new material, thermal barrier coating (TBC) and advanced combined cooling methods have been developed for a few decades to improve reliability and durability of the hot component. In combustion liner cooling, various cooling methods like impingement cooling, film cooling and internal passage cooling are applied. Recently, NOx is environmental problem and it has been attract attention of the world. High temperatures (above 1900K) of traditional gas turbine combustors result in relatively large NOx emissions.
One approach to reducing NOx emissions has been to premix the maximum possible amount of compressor air with fuel.
The result in lean premixed combustion produces cooler flame temperatures and lower NOx emissions.
But, because the advanced combustors premix the maximum possible amount of air with the fuel for NOx reduction, little or no cooling air is available making film cooling of the combustion liner impossible. Therefore, impingement jet cooling and internal passage cooling tend to be adopted in combustion liner cooling instead of film cooling.