The channels formed between adjacent blades in a turbine/compressor are nothing more than a variable section duct. Hence, the first step of turbomachinery design is to understand the physical processes experienced by a certain fluid when flowing through these nozzles and diffusers. In the main, nozzles are easier to understand since the fluid flows impelled by a favourable pressure gradient whereas for diffusers the flow has to face an adverse pressure gradient. This latter situation brings about the occurrence of stall (boundary layer detachment from the wall) which makes it more complicated to design and operate the component (both the individual stages and the entire compressor). It is thus essential to characterise the performance of diffusers of any type, which is influenced by several parameters such as geometry, Mach and Reynolds number, inlet total pressure and aspect of the boundary layer at the inlet section. Dolan and Runstadler generated very valuable information in 1973 (Pressure recovery performance of conical diffusers at high subsonic Mach numbers, NASA CR-2299) by providing performance maps for the flow of air in diffusers. This work is aimed at complementing the previous one by giving maps that apply to the flow of supercritical carbon dioxide in similar devices. By doing so, an important step towards the design of thermal turbomachinery specific of this singular fluid is taken.

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