The present paper investigates the performance of a Tesla disk turbine used as a turboexpander for small waste heat recovery applications. The geometry of the stator is slightly involuted and the admission of the fluid takes place through a two-convergent-nozzle configuration with supersonic flow conditions close to the nozzle outlet. Three cases with varying disk tip clearance are simulated for the entire operating range. The preliminary results suggest that a small decrease in the tip clearance can lead to a considerable increase in the performance of the turbine up to 57%. Specific adverse flow characteristics are analyzed: i) The circumferential extend of the supersonic region inside the stator, ii) flow asymmetry in the axis perpendicular to the flow and iii) flow reversal inside the rotor due to the local over-expansion close to the nozzles. The current simplified investigation is directed towards the improvement of Tesla turbines performance operating in the two-phase regime and under local supersonic conditions.