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Film cooling is a cooling technique widely used in high-performance gas turbines to protect the turbine airfoils from being damaged by hot flue gases. Motivated by the need to further improve the turbine hot section cooling performance, a mist/air film cooling scheme is investigated. A small amount of tiny water droplets with an average diameter of about 7 µm (mist) is injected into the cooling air to enhance the cooling performance. One key feature in understanding mist cooling is the ability to capture droplet information. In order to thoroughly investigate a two-phase flow, experimental and computational studies are needed. Designing an experimental setup for a two-phase flow experiment is challenging, therefore, the authors will dedicate this paper as Part I to discuss all the challenges faced in order to create a successful experimental facility for air/mist film cooling while Part II will discuss how to conduct a high-fidelity computational model for air/mist film cooling accompanied with a validation study by including detailed experimental conditions. This paper presents the experimental facility and instrumentation of an air/mist film cooling study with both heat transfer and droplet measurements. A wind tunnel system and test facilities are built. A phase doppler particle analyzer (PDPA) system is employed to measure the two-phase flow characteristics, including droplet size, droplet dynamics, velocity, and turbulence. An infrared camera and thermocouples are both used for temperature measurements. An extensive uncertainty analysis is performed to assist in identifying large uncertainty sources and planning for an experimental procedure. It was found during the experiment design process that resolving the mist agglomeration problem is the key in successfully generating a well-controlled mist/air mixture and reducing experimental uncertainties. The test apparatus has proven to serve the purpose well to investigate mist/air film cooling with both heat transfer and droplet measurements. Selected experimental data are presented.

In this Part II of the paper, a numerical study has been performed to validate the multiphase computational fluid dynamics (CFD) model by comparing its results to the experimental data of an air/mist film cooling study presented in Part I. The complete experimental test section was simulated including all the details such as five holes, side walls effect, partially opened top wall, the conjugate bottom wall, the initial droplet distributions from the actual data, and the long boundary layer developing region upstream of the cooling holes. The effects of different particle distributions, the breakup and coalescence models, and the conjugate wall on the adiabatic film cooling effectiveness were investigated. The multiphase CFD model employs an Eulerian–Lagrangian approach. The Eulerian method is used for the continuous phase including air and water vapor, and the Lagrangian method in terms of the discrete phase model (DPM) is used to simulate the dispersed phase of liquid droplets in a continuous phase of air–water vapor mixture. The overall consistency between the computational prediction and experimental data is within 0.2% for the air-only case and 7% for the mist case. The results showed that considering the wall using the conjugate heat transfer technique significantly provided better agreement with the experimental data than without including the wall. The non-uniform particle distribution provided better agreement with the experimental results near the film cooling hole exit.

This paper describes a numerical investigation to study the effect of injecting mist (tiny water droplets) into the cooling air used to cool down rotating gas turbine blades. In this study, the conjugate heat transfer method is used which consists of the simulation of the air/mist fluid flow inside and outside the blades as well as the heat conduction through the blade body. The complete 3D blade with internal cooling passages and external film cooling holes on the surface and blade tip is simulated in a rotating, periodic sector of the blade. The discrete phase model (DPM) is used to simulate and track the evaporation and movement of the tiny water droplets. The rotation effect of the turbine blade is included in the computational fluid dynamics (CFD) simulation by using the moving reference frame method. The effects of different parameters such as the mist/air ratio (10–20%) and the mist droplets size (20–40 µm) on mist cooling enhancement are investigated. The results show that the mist cooling enhancements are about 10–25% on the outer surface of the blade and reach 50% in some locations inside the blade on the internal cooling passages walls. Most of the liquid droplets completely evaporate inside the internal cooling passages; only a limited amount of mist is able to escape from the film cooling holes to enhance the blade outer surface and blade tip cooling. The effect of 10% mist on enhanced cooling can be converted to an equivalent of a 30% reduction in cooling air flow.