In this paper the efficiency of an axial flow contrarotating turbine is expressed in terms of two dimensionless parameters. From this expression the efficiency of a single runner tubular turbine is obtained by considering it as a special case of the first arrangement, i.e., having one runner stationary. The effects and the interactions of various parameters on the optimum efficiency for both cases are analyzed and equations for the maximum efficiencies for a given flow coefficient are established. The maximum efficiency under a fixed specific speed is also investigated. The possibility of extension to the three dimensional case is indicated. The comparison of both types of turbines with and without limitations shows that on the basis of optimum efficiency contrarotating turbines are more favorable than single runner turbines for low head installations.

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