The first bladeless turbine, also known as a friction turbine, was designed and manufactured by a Serbian engineer and inventor Nicola Tesla in 1913. This unusual device makes use of viscous effects which occur in the boundary layer flow. Opposite to classical bladed turbines where viscous effects in flow are undesirable as a source of efficiency loss, these effects enable the rotational move of the rotor. The rotor consists of up to a few dozens of thin disks locked on a shaft perpendicular to its axis of revolution. The supply of a Tesla turbine is accomplished by one or several nozzles discretely located along the circumference. The nozzles are tilted under a certain angle to the disk tangent. Working fluid flows between the disks spirally from the outer to inner radius and transfers energy to the rotating disks. The medium flows out in the axial direction through a number of holes in the disks situated near the turbine shaft. The efficiency of the Tesla turbine depends on many parameters, namely on: pressure, temperature and velocity conditions between the disks, number, diameter, thickness and distance between the disks as well as on the state of the disk surface, rotational speed of the rotor, number and arrangement of the supply nozzles, etc.

Rotor of a multidisc Tesla bladeless turbine
[patent documentation – Hicks Kenneth USA].

The flow efficiency of Tesla turbine models operating in an Organic Rankine Cycle for a small biomass cogeneration plant of heat capacity 20kW was investigated with the help of the code Fluent. Solkatherm®SES36S was assumed to be a working medium. The nominal operating conditions are for the mass flow rate of 0.13 kg/s and pressure drop from 14.8 bar to 1.9 bar).

The calculations exhibit interesting features of transonic flow in the high-load Tesla turbine. Fluid elements pass through shock wave zones several times along their pathlines within the interdisk space. It was found that  the Tesla turbine operates as a reaction turbine with a large pressure drop within the interdisk space. For Tesla models with a disk diameter 100 mm the highest value of isentropic efficiency was achieved in a model supplied from four nozzles for the rotational velocity of 18 000 rpm. It reached 30% for the nominal load, giving the power output of 1200 W.

Streamlines in flow through the four-nozzle and two-nozzle models of the Tesla turbine;
n=18 000 rpm, pin= 7,8 bar, Tin=400 K.

Contours of static pressure and velocity in flow through the four-nozzle model of the Tesla turbine;
δ=0.25 mm, n=18 000 rpm, pin= 14,8 bar, Tin=410 K, G = 0,132 kg/s, P = 1177 W.

The power output, inlet pressure, flow efficiency and reaction of an 11-disk turbine as a function of flow rate;
the system supplied from four nozzles - δ=0.25 mm.