Influence of Porosity on Double-Walled Effusion-Cooled Systems for Gas Turbine Blades

Double wall effusion cooling (DWEC) systems for gas turbine blades utilise two skins connected by pedestals and take advantage of cooling benefits provided by impingement jets and film holes. The latter exhausts coolant externally onto the blade surface forming a protective layer against the high external heat loads, which can be enhanced via the beneficial influence of adjacent films. Consequently, increasingly porous outerskins are being considered in order to provide greater thermal protection and/or reduce the required coolant mass consumption. To realise such systems, further research must understand how the internal aerothermal field is affected by high porosity. A semi-decoupled unit-cell computational fluid dynamics (CFD) method is applied to a range of DWEC systems to understand overall cooling effectiveness as well as internal characteristics. A comparison of internal convection highlights a shift in the breakdown of cooling performance, due to the large changes in wetted surface area of the outerskin. For low porosity, most of the internal cooling occurs through the jet impingement on the internal outerskin wall, while the addition of more film holes provides an increasingly greater proportion of convective heat transfer. On the external surface, porosity increased film effectiveness due to film superposition, provided a more uniform film coverage, and reduced the likelihood of jet-lift-off. Coupling the benefits of internal cooling and film effectiveness resulted in a reduction of mean metal temperature, peak temperature and temperature gradient between the outer and inner walls. Criteria reflecting the main drivers for thermal fatigue. Despite these benefits, for the most porous DWEC configuration a variation in mass flow between film holes was observed, and in some cases the risk of hot gas ingestion was evident. DWEC components would benefit from further understanding of the susceptibility to blockage, the pressure margin limits and the extent of flow migration.