Investigations On Conjugate Heat Transfer Performance Of Gas Turbine Internal Channel With Ribs And Film Holes

In order to acquire the higher turbine performance and thermal efficiency, increasing the inlet temperature of modern aviation gas turbine engine is an important approach. However, the inlet temperature of advanced gas turbine has far exceeded the capacity of metal materials. Therefore the complex cooling technologies have to be developed and adopted to protect the thermal components of the advanced gas turbine from the high temperature. To improve the thermal efficiency and extend the operating life of gas turbine, it is essential to predict accurately the temperature distribution at the surface to be cooled. In this regard, numerous researchers have done a lot of research, and the corresponding cooling techniques have been developed through several generations, from simple cooling via smooth internal channel, enhanced internal cooling by coolant impingement and ribbed channel, external film cooling, to nowadays widely used combined cooling of internal ribbed channel with external film cooling, even laminated plate cooling and transpiration cooling. It is clear that the single cooling method has been not meet the requirements of the increasing inlet temperature, the combined cooling technology has been the hot topic in the field of cooling technology in the modern gas turbine designs. Due to the limits of experimental conditions and measuring technology, one cannot get a complete information only through experiment, in this case, numerical simulation can make up for the lack of experimental work.In the previous researches, most researchers investigated the heat transfer characteristics of internal ribs and external film cooling separately. But in the real turbine blade, the combined cooling of internal ribbed channel with external film cooling has been widely used. The previous research approaches cannot give the actual cooling results. In this work, the heat transfer characteristics of four combined cooling structures of internal ribbed channel with external film cooling have been investigated through experimental and numerical approaches. The real serpentine channel of turbine blade was simplified into a rectangular straight channel with different ribs and film holes. This work focused on the cooling characteristics of the combined cooling structures with oblique ribs and compound angle film holes.In the previous investigations on heat transfer characteristics of internal ribs and external film cooling, most experiments were carried out at the adiabatic wall conditions and low experimental temperature. However, this experiment was carried out in the wind tunnel with high temperature, and four different combined cooling structures were made of real turbine blade material, i.e. super-alloy. The first structure has one row of film holes located at the rear region of rib, the second structure has one row of film holes located at the middle of rib, the third structure has one row of film holes located at the front of rib, the last structure has two rows of film holes located at the front and the rear of rib separately. Furthermore, the angle of rib is60°, both of the compound angles of film hole are20°, and the inlet of film holes are arranged on the central axis of the internal cooling channel.In the experiment, the combined cooling structures were surrounded by the mainstream with high temperature, the cooling air passed through the internal ribbed cooling channel to generate an enhanced heat convection effect, and the cooling air was ejected from the discrete film holes, and fully mixed with the hot mainstream. In the experiment, the surface temperature was measured by an infrared thermal imaging system, and the cooling air temperature was measured by thermocouples. The thermal efficiency was evaluated by the modified cooling effectiveness formula, which is defined by the temperature difference of the initial and cooled surface temperatures. With this experimental method, the actual working environment of gas turbine blade can be fully simulated. Through the measurements and analysis of the experimental data, the cooling performances of four different structures were compared.In the simulation work, the heat transfer characteristics of the first structure were investigated with the commercial software Ansys CFX. In the simulation, the SST κ-ω turbulence model was adopted, and the simulation results were verified with the experimental data. The velocity fields of the mainstream and cooling air, Nusselt number distribution and cooling effectiveness profile at the surface to be cooled were fully investigated. In order to emphasize the advantages of the combined structure with ribs and film holes, as a reference, the heat transfer characteristics of purely ribbed internal channel without film holes were also investigated numerically, and the numerical results of the two structures were compared.