Experimental and Computational Methods for the Evaluation of Double-Wall, Effusion Cooling Systems

Further improvements in gas turbine efficiency can be sought through more advanced cooling systems — such as the double-wall, effusion system — which provide high cooling effectiveness with low coolant utilisation. The double-wall system, as described here, comprises two walls, one with a regular array of impingement holes, the other with a closely-packed, regular array of film holes (characteristic of effusion systems). These walls are mechanically and thermally connected to one another via a bank of pedestals which increase coolant wetted area and turbulent flow features. However, a lack of data exists in the open literature on these systems. This study presents a novel experimental heat transfer facility designed with the intent of investigating flat plate versions of such double-wall geometries. Key features of the facility are presented including the use of recirculation to increase mainstream-to-coolant temperature ratio and the use of infrared thermography to obtain thermal measurements. Some rig commissioning characteristics are also provided which demonstrate well-conditioned, uniform flow. Both coolant and mainstream Reynolds numbers are matched to engine conditions, with Biot number within around 15% of engine conditions. The facility is used to assess the cooling performance of four double-wall effusion geometries which incorporate various geometrical features. Both overall effectiveness and film effectiveness measurements are presented at a range of coolant mass flows with conclusions drawn as to preferable features from a cooling perspective. The results from a fully conjugate CFD model of the facility are presented which utilised boundary conditions obtained during experimental runs. Additionally, a computationally efficient decoupled conjugate method developed previously by the authors was adapted to assess the experimental geometries with the results comparing favourably.