Session: 12-11 Code Development

Paper Number: 81923

81923 - Effects of Freestream Turbulence on Air-Mist Film Cooling: Two-Phase Flow Simulations 

Air-mist film cooling is a potential technique to protect the blade surface of a gas turbine operating at high temperatures for improved thermal efficiency and power output at the cost of aerodynamic losses. In a real gas turbine, high freestream turbulence (fst) is encountered, which may influence the performance of air-mist film coolant. A systematic study documenting the variation in the flow field and adiabatic air-mist film cooling effectiveness for a wide range of fst varying from 0.2 to 10% is conducted here to account for the real operating condition of the gas turbine. However, numerical simulation is restricted to a flat plate, where the flow physics of air-mist coolant is resolved for varying fst.

The investigated domain consists of a flat plate with a series of discrete holes of 35° streamwise orientation and connected to a common delivery plenum chamber via a pipe of diameter D=12.7mm. A two-phase mist containing finely dispersed water droplets of 10.0µm in an airstream at a mist concentration of 3.0% is introduced as a secondary flow. The blowing ratio and density ratio are 0.5 and 1.2, respectively, where the Reynolds number based on the diameter of the hole is 1.0×10⁴. The Reynolds Averaged Navier Stokes equation in the Eulerian-Lagrangian frame is used to simulate the two-phase flow by ANSYS Fluent 15.0 with the k-ɛ realizable model.

 The numerical simulation successfully resolves the mean thermal-flow field along with the droplet dynamics. The tiny droplets in the air-mist coolant behave like a small heat sink in the coolant layer as they evaporate while advecting downstream and provide improved protection relative to the conventional film cooling. High turbulence intensity significantly enhances the mixing of droplets with the crossflow, thereby improving the spanwise diffusion of droplets. This, in turn, broadens the coolant coverage in the spanwise direction with improved lateral uniformity of the coolant layer with high fst. Although film cooling effectiveness increases spanwise with an increase of fst from 0.2 to 10%, it remains almost invariant in the streamwise direction, sustaining its values of 81.78% to 64.94% along the hole centerline. Further, a reduction of the strength of the counter-rotating vortex pair is also evident. The area-averaged film cooling effectiveness is evaluated by integrating effectiveness over the area from the hole's exit to project a single value as an overall signature of the air-mist film cooling. It increases by 21.5% with a change of turbulence intensity 0.2 to 10%. However, aerodynamic losses also increase by almost 30% with the fst. The analysis is generic and could be extended to assess the air-mist film cooling over a turbine blade for varying freestream turbulence.

Presenting Author: Anjali Dwivedi Indian Institute of Technology Kanpur

Presenting Author Biography: PhD Scholar, IIT Kanpur, India

Authors:

Subrata Sarkar Indian Institute of Technology Kanpur
Anjali Dwivedi Indian Institute of Technology Kanpur