This paper aims at proposing a feasible method to determine an appropriate disc spacing distance in the design of Tesla turbines. Therefore, a typical Tesla turbine with seven different disc spacing distances was calculated numerically at different rotational speeds to investigate the influence of disc spacing distance on the aerodynamic performance and flow field of Tesla turbines and further to put forward the method. The results show that the isentropic efficiency of Tesla turbines peaks when the disc spacing distance gets its optimal value, and it decreases quickly as the disc spacing distance decreases from its optimal value. What’s more, the dimensionless parameter Ekman number is applied to determine an appropriate disc spacing distance in the design of Tesla turbines. There’s an optimal value of the Ekman number that Tesla turbines obtain its best performance, and it is influenced by the rotational speed. Meanwhile, the optimal value of the dimensionless rotor inlet tangential velocity difference which decides the rotational speed is also affected by the disc spacing distance. Thus, the determination of the optimal values of the dimensionless rotor inlet tangential velocity difference and the Ekman number is a cyclic iterative process to make them at their optimal values or in their optimal ranges respectively.

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