Performance Investigation of Bladeless Micro-expanders

Interest in small scale turbines is growing mainly for small scale power generation and energy harvesting applications. Conventional bladed turbines impose manufacturing limitations, lower performance and higher cost, which hinder their implementation at a small scale. Tesla bladeless turbomachines are recently being investigated due to many advantages such as their simple design and ease of manufacturing with acceptable performance. If an efficient design is achieved, this will be a promising machine in the area of small-scale power generation and energy harvesting. However, low (less than 40%) overall experimental isentropic efficiency has been recorded in the literature for Tesla turbines. In this study, firstly, a 0-D model is developed to design the expander rotor. A systematic algorithm is presented, and results of the model are compared with 2-D rotor model results. A 3D computational fluid dynamic (CFD) analysis of rotor and stator with real fluid gas is performed to characterize flow in the Tesla micro expander (all three prototypes studied). The experimental turbine efficiencies were found to be lower compared to the CFD results. The gap in the efficiency is discussed by analyzing CFD and experimental results. A systematic experimental, numerical performance and losses investigation of Tesla turbines for micro-power generation is carried out combining experimental and numerical approaches. In the first prototype, a flexible test rig for the Tesla turbine fed with air is developed of about 100W net mechanical power, with a modular design of two convergent-divergent nozzles to get subsonic as well as supersonic flow at the nozzle exit. Extensive experiments are done by varying design parameters such as disk thickness, the gap between disks, radius ratio and outlet area of exhausts with speeds ranging from 10000 rpm to 40000 rpm. Major losses such as stator and ventilation losses at end disks together with mechanical, leakage and exhaust losses are evaluated experimentally and numerically. The effect of design parameters on the performance of Tesla turbines is discussed. The experimental analysis focused mainly on the efficiency features of this expander, showing the impact on performance of different disk gaps, disk thickness, discharge holes, exhaust geometry, as a function of speed and mass flow. Maximum adiabatic efficiency of 18% has been measured, with many other points in the 10- 15% range. Results show that the three largest sources of losses are the nozzle losses (which include the nozzle and nozzle-disk tip interaction losses), the leakages losses (due to flow bypassing the rotor at the extremes gaps), and ventilation losses. The nozzle losses account, alone, for about 2/3 of overall losses. In the second prototype, the experimental performance investigation of a 3 kW (rated power) expander with a high-speed integrated generator is carried out. The Tesla expander and electric generator are housed in a single casing making it the first of its kind to be tested with such configuration. The expander is fed with air and operated at high rotational speeds up to 40000 rpm. The test is carried out with a different number of nozzles (1, 2, 4 and 8) to understand its effect on performance. Results show that the peak efficiency for two nozzles is better than one- nozzle and four-nozzles configurations for the same inlet pressure conditions. Experimental tests revealed that this turbine is the most efficient Tesla turbine till now with air as a working fluid. Furthermore, one of the most important losses in Tesla turbomachines, nozzle loss, is experimentally characterized. Maximum isentropic efficiency is obtained for two-nozzle case which is 36.5% at 10000 rpm. This is the highest Tesla turbine efficiency recorded till now for actual prototypes with air as a working fluid. Using such 3-kW Tesla expander air prototype, systematic experimental characterization of loss mechanisms is performed. The sources of losses discussed are stator losses, stator-rotor peripheral viscous losses, end wall ventilation losses and leakage losses. Once the effects of losses are separated, their impact on the overall efficiency curves is presented. This experimental investigation, for the first time, gives insight into the actual reasons for the low performance of Tesla turbines, highlighting critical areas of improvement, and paving the way to next-generation Tesla turbines, competitive with state-of-the-art bladed expanders. The third prototype is designed with an innovative concept “Ultra-high Tesla expander” which is developed based on findings of the first two prototypes aiming to minimize stator-rotor losses and then potentially approaching the rotor-only efficiency for the overall machine. The prototype is 1 kW (rated power) with water as working fluid. The 3D numerical results show very high total to static efficiency (70 -75%). The preliminary experimental tests results are discussed and ventilation loss, which is major source of loss in this expander, is characterized.