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Inflow-rotor interaction in Tesla disc turbines: Effects of discrete inflows, finite disc thickness, and radial clearance on the fluid dynamics and performance of the turbine

The article establishes the physics of the complex interaction of discrete multiple inflows with the stationary shroud and the rotating channel of a Tesla disc turbine. Using a large number (150) of separate, fully three-dimensional computational fluid dynamic simulations, we demonstrate the (sometimes dramatic) role of four important input parameters, namely the number of nozzles ( N nozzle ), rotational speed of the discs (Ω), radial clearance between the rotor and the shroud ( Δ rc ), and disc thickness (dt), in the fluid dynamics and performance of a Tesla turbine. An increase in N nozzle or Δ rc assists in the attainment of axisymmetric condition at rotor inlet. Ω influences significantly the distribution of radial velocity including the fundamental shape of its z-profile (parabolic, flat or W-shaped). The paper demonstrates the existence of an optimum Δ rc for which the efficiency of the rotor (η) is maximized. Present computational fluid dynamics simulations for many combinations of N nozzle and Ω establish that the η versus Ω curves, for each fixed value of N nozzle , are of the shape of an inverted bucket. With increasing N nozzle , the operable range of Ω decreases, the buckets become more peaky and the maximum possible η increases substantially (by a factor of 2 in the example calculation shown). The present systematic work thus demonstrates quantitatively, for the first time, that an axisymmetric rotor inflow condition represents the best possible design for the rotor. It is further shown that, as the disc thickness is increased, the efficiency may decrease substantially (even dramatically) and its maxima occur at lower rotational speeds. Chamfering of the disc edge or partial admission decreases the turbine efficiency. Thus, small disc thickness, flat disc edge, full nozzle opening, optimum radial clearance, and inlet condition as close to axisymmetry as is possible are recommended for the design of an efficient Tesla disc turbine.