Investigation of Innovative Means to Enhance Mist Cooling in Gas Turbines

One of the most common techniques to increase the thermal efficiency and output power of gas turbines is to increase the turbine inlet temperature with increased pressure ratio. The current turbine cooling schemes have almost reached a plateau with most of cooling advancements being incremental. Since the main challenge is to cool the turbine airfoils without using more cooling air, application of mist cooling is a very promising scheme that can provide significant cooling enhancement. In this study, mist (tiny water droplets 10-20 µm) is added to the conventional cooling air technique to improve the cooling performance. Computational studies have been performed to investigate employment of mist cooling a) in a complete, current gas turbine airfoils design under real gas turbine operating conditions including conjugate gas-solid heat transfer with internal passage cooling, impinging jet cooling, trailing edge cooling, and external film cooling and tip cooling in a rotating blade. b) in the first stator-rotor stage under complicated interactions of passing wakes and shock waves in a transonic gas turbine. c) through a row of novel sweeping jet design via fluidic oscillators without moving parts in the applications of impinging jet cooling and film cooling. Furthermore, an experimental study has been performed to investigate mist film cooling through the sweeping jet film cooling design in a wind tunnel. A Phase Doppler Particle Analyzer (PDPA) system was used to investigate the water droplet behavior with such a sweeping jet in conjunction with the temperature field using thermocouples and Infrared Thermography. The results show that using mist can achieve an average cooling enhancement of 20% to 80% with the local maximum enhancement up to 350%. For sweeping flow studies, due to the sweeping and feed-back flow behavior inside the fluidic oscillator, tiny droplets coalesce into bigger droplets, resulting in a phase lag between the air and mist flow, which makes cooling more uniform. The non-steady flow simulation has identified the fundamental vortex dynamics that explains the reason why the sweeping jet film cooling flow produces a time-averaged inward counter-rotating vortex pair (CRVP) against the outward CRVP presents in traditional steady jet film cooling flow