Hydraulic efficiency of Francis turbine whose vanes are radial at inlet is (α – guide blade angle)

• A. \(\frac{{2 + {{\tan }^2}\alpha }}{2}\)
• B. \(\frac{{{{\tan }^2}\alpha }}{2}\)
• C. \(\frac{2}{{2 + {{\tan }^2}\alpha }}\)
• D. \(\frac{2}{{ta{n^2}\alpha }}\)