Abstract: An impeller having an axis about which the impeller is rotatable in a working direction of rotation A, with a plurality of aerofoil blades spaced from and arranged about the axis; the inwardly axis-facing surface of each blade defines a longer fluid flow path across the blade than the opposite outwardly-facing surface, and each blade has an angle of attack from 0° up to a positive angle of attack less than that at which the blade will induce turbulent fluid flow when the impeller is rotated in a fluid at a working speed in the working direction of rotation A so that during such rotation the impeller induces an inlet fluid flow generally axially toward the impeller and an outward fluid flow away from the impeller, where the outward flow is in directions generally inclined at an angle which lies between substantially radial and axial directions relative to the axis.