Subsonic and supersonic diffusers have the following geometry:

A Hydro turbine is required to give $$25$$ $$MW$$ at $$50$$ $$m$$ head and $$90$$ $$rpm$$ runner speed. The laboratory facilities available, permit testing of $$20$$ $$kW$$ models at $$5$$ $$m$$ head. What should be the model runner speed and model to prototype scale ratio?