Study of double-wall effusion cooling scheme for gas turbine blade applications
Keywords: Turbomachinery
aerodynamics
Turbine cooling
Simplified numerical analysis model
Gas turbine
Heat transfer
double-wall cooling
Film cooling effectiveness

Porous multi-wall cooling schemes such as double-wall cooling combined with effusion cooling offer a practical approximation to transpiration cooling which in turn present a potential for high cooling effectiveness. Most of the existing double-wall effusion cooling studies have been on flat plate geometries. There are varying external static pressure and secondary flow features in flow over an aerofoil which have an influence on overall cooling performance. Thus, there is need to study double-wall effusion cooling applied to an aerofoil. The main aim of this research was to extend the double-wall effusion-cooling technology research, that has long been undertaken on flat plates, onto a gas turbine blade.
In the present study, both numerical simulations and experiments were undertaken to study double-wall effusion-cooled (DWEC) turbine-representative aerofoils. The aerofoils were built from double-wall block elements that have been validated by another author. Both low porosity and high porosity circular and diamond pedestal designs were considered. A novel decoupled numerical analysis tool for preliminary cooling performance analysis of DWEC aerofoils was first developed. In this analysis method, a modified flat plate correlation from the literature was used to represent the two-dimensional distribution of film cooling effectiveness. The internal heat transfer coefficient was calculated from a validated conjugate analysis of a wall element representing an element of the aerofoil wall and the conduction through the blade solved using a finite element code in commercial CFD solver. The developed decoupled numerical analysis method was validated using results from fully coupled conjugate heat transfer (CHT) simulations. In addition, high-speed experimental tests at engine representative Mach and Reynold numbers flow conditions were carried out to study film cooling effectiveness over the full surface of three circular and six diamond pedestal DWEC blade designs using pressure sensitive paint. All the blades were tested within a range of representative modern engine coolant mass flow rate to mainstream mass flow rate ratios; 0.5% to 5.5%.
The novel simplified numerical analysis method offered good performance approximation particularly on the suction surface of the aerofoil. In addition, compared to CHT, the novel simplified numerical analysis method reduced computational time by approximately 50 times and therefore, computationally efficient for use during preliminary design and optimization stages. High effective porosity designs exhibited better film cooling effectiveness, than the low effective porosity counterparts, but this came at an expense of internal cooling efficiency. CFD results compared well with the experiments and were able to capture similar film effectiveness trends on both the pressure surface and the suction surface, however, inability of Reynolds-Averaged Navier–Stokes equations (RANS) models to correctly predict diffusion resulted in an overprediction of film cooling effectiveness around the vicinity of film cooling holes and an overprediction of film superposition on the suction surface.
This work has contributed knowledge of the DWEC aerofoils performance including overall cooling effectiveness predictions, internal cooling effectiveness predictions and film cooling effectiveness performance measurements. There is still much work to be done (including investigation into aerodynamic losses, weight and stresses associated with this cooling technology) to realise a practical double-wall effusion-cooled blade. The present author has included recommendations for future work.