Micro gas turbines (MGTs) have been pivotal in sectors such as electricity generation, military applications, and commercial uses over the past five decades. With increasing global concerns about climate change, there is a pressing need to reduce emissions, including CO2 and NOx, from power generation mechanisms like aircraft engines. In alignment with the ACARE (Advisory Council for Aviation Research and Innovation in Europe) goals, there is a mandate to reduce NOx emissions by 75% per passage kilometre for aircraft engines by 2050, using the year 2000 as the baseline. MGTs, representing scaled models of larger gas turbines, offer a cost-effective platform for aerodynamic exploration and technological advancements in gas turbines at large.
This study utilised both numerical and experimental methodologies to explore the aerodynamics within MGT blade passages, specifically using the Wren44 and Wren100 models supplied by Turbine Power Solutions Ltd. The research was aimed at developing and comparing different reverse-engineering (RE) strategies to enhance the aerodynamic efficiency of the main gas flow paths of the micro turbomachinery. Two unique RE strategies based on laser-scanning and iterative CFD techniques were evaluated. The discrete ("what the part really is") approach proved superior for detailed aerodynamics analysis, while the parametric ("what the part could be") was more suited for rapid design modifications, capturing overall blade performance with fewer parameters.
The entire MGT gas path was simulated using the renowned CFX software suite, employing both RANS and LES modelling techniques. The fidelity of these models was rigorously verified and validated against empirical data from jet engine tests and wind tunnel cascade experiments. The baseline performance of the MGT under various RPMs is documented with an error margin of ±4.45%, as derived from thrust sensor specifications. Subsequent design enhancements and parametric evaluations offered invaluable insights into the intricate physics governing flow dynamics within turbomachinery. For the Wren100 stator, modifications including reducing one vane, halving trailing edge thickness, and increasing the aspect ratio by 11% resulted in a thrust increase from 24.25N to 29.95N and an improvement in rotor isentropic efficiency from 80.1% to 81.1%. Additionally, halving the tip clearance of the rotor, coupled with the redesigned stator, potentially further enhanced isentropic efficiency to 83.4% while maintaining higher thrust levels.
The investigation into varying surface roughness levels through wind tunnel cascade experiments and LES WALE model simulations extended from normal (120,000RPM) to peak (160,000RPM) operational conditions for the Wren100 MGT. These studies culminated in actionable recommendations for optimal surface roughness maintenance. It was observed that surface roughness could eliminate laminar separation bubbles on the suction side near the leading edge of the Wren100 stator under different operational conditions, potentially delaying transition onset and enhancing turbulence intensity in the main flow, thereby reducing secondary losses. For the MGT rotor blade tip, increased surface roughness not only reduced the size of separation bubbles but also delayed their onset, potentially lower secondary effects like tip leakage flows and corner vortices.
This research stands as a pioneering analysis into the different blade characteristics and the effects of surface roughness on boundary layer evolution in MGTs under relatively low Reynolds number conditions, emphasising the importance of tailored roughness maintenance alongside other parameters to maximise thrust and aerodynamic efficiency.
