The need of a continuous improvement in gas turbine efficiency for propulsion and power generation, as well as the more demanding operating conditions and power control required to these machines, still ask for great efforts in the design and analysis of the high pressure section of the turbo-expander. To get detailed insights and improve the comprehension of the flow physics, a wide experimental campaign has been performed in the last ten years at Politecnico di Milano on the unsteady aerodynamics of a high-pressure turbine stage considering several operating conditions. This paper presents and discusses the experimental results obtained for the stage operating with different expansion ratios and rotor loading. The turbine stage under study is representative of a modern high-pressure turbine and can be operated in both subsonic and transonic conditions. The experimental tools applied for the current research represents the state of the art when unsteady investigations are foreseen. The detailed flow field, the blade–rows interaction and the overall performance are described and discussed; efforts have been devoted to the discussion of the various contribution to the overall stage efficiency. The direct effects of the expansion ratio, affecting the Reynolds and the Mach numbers, have been highlighted and quantified; similarly, the indirect effects, accounting for a change in the rotor loading, have been commented and quantified as well, thanks to a dedicated set of experiments where different rotor loadings at the same expansion ratio have been prescribed.
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