Investigation of micro gas turbine system configurations for compact lightweight applications based on reversible bladeless Tesla machinery

Globally, the demand for low-emission, cost-effective, low-noise, lightweight and compact devices is rapidly increasing. In order to meet such market demands on a small scale, microgas turbines (MGTs) could play a crucial role. However, there are many challenges with MGT compatibility with internal combustion engines (e.g., low efficiency, fuel flexibility, light weight-compactness). As part of this study, the focus is on the component basis, where the study examines the Tesla or bladeless turbomachinery in both a compressor and expander configuration. Since little data is available on Tesla compressors/pumps, this study focuses primarily on the bladeless compressor, which is also analyzed as an expander due to its reversibility. The activity started from a 3-kW air Tesla expander prototype available at the University of Genoa. It is examined in compressor mode using a 3D CFD approach and its results are compared to experimental results. The CFD and experiments show good agreement for the pressure, with an error of less than 3% at zero flow condition. Despite the CFD analysis predicting a static efficiency of around 42% (without losses), the experiment did not meet that prediction due to significant leakage flows and other losses. Moreover, using LMS Test Lab software, the acoustic behavior of the Tesla compressor has been analyzed at different speeds and distances, and several aspects are compared with conventional or bladed technologies (same tip speed, same mass flow rate, and same pressure). It has been demonstrated that Tesla technologies are substantially quieter than bladed technologies. To improve Tesla compressor performance, a 3D numerical analysis has been carried out for the rotor only and coupled rotor-stator and volute configurations. The disk gap is optimized by relying on Ekman and Reynolds numbers. Based on the numerical analysis, the disk gap should be 3 times the thickness of the boundary layer with the best Reynolds number 9-11 and Ekman number 1.5-1.65. Moreover, numerical analysis has been performed for 2, 2.5, 3 and 4 diameter ratios in order to optimize the rotor diameter ratio. Higher diameter ratios indicate better performance than lower diameter ratios. Based on a CFD analysis, it has been predicted that at low mass flow rates, greater than 95 % efficiency can be achieved with the optimal disk gap and diameter ratio. In this case, the outlet flow angle would be around 89.9 degree, however, in practice, maintaining almost a tangential flow angle is difficult. In order to improve the performance of Tesla compressors, several stators have been studied. The stator outlet and inlet ratio between 2 and 4 is optimal for stator/diffuser performance. With an optimal rotor and eight stators, CFD analysis predicts a total stator efficiency of >53%; however, with a low number of stators, this efficiency will be somewhat improved. As part of an effort to enhance the performance of the Tesla compressor, a (stator-less) volute design has also been numerically analyzed, which shows better performance in terms of pressure ratio and efficiency than the stator configuration. Compared to a stator configuration, the total to static efficiency is estimated to increase by 3 to 5%. A new reversible Tesla prototype model has been developed using an optimized rotor (optimal disk gap and diameter ratio) and volute configuration for 22 krpm. CFD predicted total to static efficiency of 58% in compressor mode and 66% in expander mode without consideration of system losses. For this new reversible machine, leakage and end wall losses are also analyzed under a variety of conditions, including different end wall gaps and different exit radial clearances (with and without sealing systems). The implementation of the sealing system has resulted in a reduction in leakage, but the amount varies in accordance with the clearance of the radial exit. The end wall power loss varies with end wall gap, but usually ranges between 50 W-60 W for 22 krpm design speed, while power is around 600 W. During this dissertation writing, experimental work on a reversible bladeless machine is in progress. Preliminary results show the pressure ratio is 1.24 and the isentropic efficiency is 31%. Comparing these results with CFD simulations at closed valve conditions shows good agreement in terms of pressure ratio with < 3% error. It is observed that leakage occurs during the first test which passes through the narrow radial clearance of 0.3 mm. As a result of this preliminary investigation, some modifications have been made to the test-rig in order to ensure that good results are achieved with the least amount of leakage. An in-depth study of the reversible Tesla machine (compressor and expander) has shown that such technologies possess several attractive features (reversibility, low noise level, cost effective and operate with any kind of fluid), but are difficult to implement as Tesla envisioned (higher efficiency). Under ideal design conditions, however, such technologies could potentially achieve > 50% efficiency. The use of such technologies can be beneficial in areas where conventional technologies are not practical or less efficient.