GPPS-TC-2019-089 CFD ANALYSIS OF A TESLA TURBOEXPANDER USING SINGLE PHASE STEAM

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. INTRODUCTION The Tesla turbine owes its name to its creator Nikola Tesla who first patented the turbine in 1913 (Tesla, 1913). The turbine rotor consists of multiple flat concentric disks that are fixed on a shaft and rotate together. The flow admission takes place through one or more nozzles and fluid enters the rotor almost tangentially. The fluid moves spirally through the tight gap between the disks transferring energy and forcing the disks to rotate before exiting through the outlet located at the center of the rotor. Other names for the Tesla turbine is "viscous turbine" or "boundary layer turbine", as the sole mechanism of energy transfer between the fluid and the rotor disks is the viscous force. The main reason that the Tesla turbines are not commercially available for industrial applications is their low efficiency. Most of the efficiencies experimentally derived are in the region of 25% (Lemma, et al., 2008), (Rice, 1965). Despite, its low efficiency the Tesla turbine offers many advantages such as the simple design, the low cost of production and maintenance as well as the ability to operate with various working fluids or even two-phase flows. (Guha & Smiley, 2010). In addition, such turbines tend to become competitive compared to the conventional bladed turbines as the size decreases (Epstein, 2004). Considering the above, a suitable application of the Tesla turbine is the waste heat recovery as often done using Organic Rankine Cycles. (Manfrida, et al., 2017) proposed an improved modular design for the power range of 500W to 5kW and developed an analytical model for two working fluids. (Song, et al., 2017) assessed the performance of a Tesla turbine in the kW range for various organic fluids, by using a 0D thermodynamic model. Small power generation applications may require the Tesla turbine to work with a two phase fluid or a particle laden flow (Neckel & Godinho, 2015). Such turbines are supposed to have the characteristic of "selfcleaning" by forcing heavier droplets or particles to their periphery (Sengupta & Guha, 2012). A parameter that often influences shrouded turbomachinery performance is rotor tip clearance, which is the size of the gap between the rotor and the shroud or casing.