"水轮机蜗壳" can be translated into English as "spiral case of hydraulic turbine" or simply "turbine spiral case".
"Spiral case" accurately describes the spiral-shaped casing structure of the water turbine component.
"Hydraulic turbine" specifies the type of turbine (water-driven), though "turbine" alone is also commonly understood in context.
Both versions are acceptable, with the first being more precise.