Aero-Thermal Design and Numerical Investigation of an Additively Manufactured High-Pressure Turbine Vane

Abstract

The turbine inlet temperature of modern jet engines is constantly being increased in order to improve thermal efficiency. The hot gas temperature considerably exceeds the operating limits of the typically used single-crystal nickel-based alloys, requiring cooling of the thermally highly loaded components. This includes the first stage nozzle guide vane, which is internally cooled with compressor discharge air. For external cooling, the cooling air is ejected through film cooling holes. This results in irreversible pressure losses in the secondary flow path and mixing losses when the cooling air is injected into the main flow. The primary objective is to minimize these losses through the reduction of the required cooling air mass flow rate. This can be achieved through the utilization of innovative cooling systems or the use of advanced materials. As part of the DLR project 3DCeraTurb, both routes were considered by investigating various material and manufacturing concepts in terms of their applicability for vanes subject to high thermal loads. The stator of the first high-pressure turbine stage of the DLR's internal UHBR engine design serves as the object of investigation. Within the scope of this work, two high-pressure turbine vane cooling designs were developed. The first design is a conventional cooling concept that represents the state of the art known from the literature and serves as a basis for comparison. Impingement cooling is implemented in the form of a perforated sheet metal insert, while pin-fins are incorporated directly into the trailing edge. Film cooling is used to create a protective coolant film on the vane surface. The respective geometric details, such as diameter and shape, are designed based on constraints that apply to the corresponding manufacturing process. The second cooling design is optimized for additive manufacturing. Compared to the conventional vane, this enables an integral design of the otherwise multi-part assembly. In a double-walled cooling concept, impingement cooling can be integrated into an inner wall. The inner wall connects to the outer wall through turbulators, which transform the negative effect of cross-flow within the vane into a way to enhance the internal heat transfer. In addition, the internal surface area for heat transfer is increased. This allows the material temperature to be lowered with the same coolant mass flow rate, which enables more efficient use of the cooling air. Manufacturing the cooled high-pressure turbine vane using additive manufacturing presents certain challenges, particularly in creating thin internal cooling channels that are not aligned with the main build direction of the vane. To overcome this challenge, different variations of the overhang area geometry, orientation and diameter were evaluated by computed tomography of the resulting samples. Quantitative and qualitative comparative analysis methods were used to verify the CAD models. An optimization study was conducted to ensure that the desired channel configuration can be achieved with sufficient approximation using SLM. The designed vane concepts were evaluated using conjugate CFD-FEM simulations. These simulations were used to optimize the cooling air distribution while ensuring that the materials remained within their temperature limits. The results show improvements in cooling performance of the additive manufactured cooling design compared to the conventional counterpart.