| Thermal efficiency of a gas turbine is largely dependent on the turbine inlet temperature (TIT). Modern gas turbines work at operating temperatures as high as 2000K which is higher than the melting point of the material used. Thus, thermal protection of these components is the biggest challenge for engineers in this industry. Film cooling is the most common cooling technique used in this application. In film cooling, coolant is injected at discrete locations along the metal surface, which forms a layer of cool air immediately over the hot surface, thus, protecting it from direct contact with hot mainstream air. The cooling is strong along the centerline of the hole in the downstream region and rapidly decreases over the spanwise direction. This results in large thermal gradients inducing thermal stresses in the material. To overcome this effect, a new method of injection, called backward injection, is proposed in this work. Conventionally, both mainstream and coolant are injected in the same direction. In the new scheme, the direction of the coolant is opposite to that of the hot gas, which results in large interactions between them and spreads the coolant in the lateral direction. This scheme is explored in detail in the current work under major influencing parameters at operating conditions. Different hole shapes are investigated to obtain best possible results. Fluent, a commercial CFD software, is extensively used as tool in the current work for numerical simulations. Numerical results are validated with experimental results. Pitot tubes are used to measure velocity and thermocouples and infrared camera are used to capture thermal fields. Results show that backward injection can result in uniform cooling along spanwise direction. Under certain conditions, this scheme results in better cooling in the downstream regions when compared to forward injections. Aerodynamic loss induced in this scheme is a major drawback. However, it can be compensated with the reduced coolant consumption, and lesser thermal gradients. |
