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Bladeless or Tesla turbines consist of several flat parallel disks mounted on a shaft with a narrow gap between them. The analytical solution of Navier-Stokes equations for the flow between disks has been extensively studied in the past. However, there is a significant impact of the stator exit-flow conditions on the performance and flow behavior inside the rotor of the Tesla turbine. There has been limited research on flow characterization and performance evaluation of stator-rotor interaction of the Tesla expander using 3D numerical simulation. The challenge arises due to a very high aspect ratio of the Tesla rotor (diameter to gap ratio > 1000). In order to accurately evaluate the torque on the disks due to shear forces, a very fine resolution mesh is necessary. 3D numerical modeling with a hexahedral mesh of the nozzle/stator with the commercial software is presented. A steady-state solution is obtained using a density-based solver for solution stability. The simulation is performed for a wide range of inlet pressures and rotational speeds. The numerical solution does not consider the effects of ventilation losses, end-disk leakages, exit kinetic energy and bearing losses. This work focuses on the stator performance, the stator-rotor interaction, the rotor entry losses due to disk tip, the rotor tip velocity ratio and the degree of reaction on the performance of the Tesla expander. Challenges in modelling and key fluid dynamic features are extensively discussed. The peak efficiency of 58% is predicted for 3 bar inlet pressure and a rotational speed of 30000 rpm for a 3 kW machine with air as a working fluid. The 3D numerical analysis provides insights on flow characterization, mainly stator-rotor interaction and flow between disks at different mass flows, with the aim to contribute to the fundamental knowledge towards further improvement of the Tesla turbine performance. Numerical results are also compared with experimented test results performed on a 100-W and a 3-kW air expander.

Supercritical carbon dioxide plants are attracting strong interest, particularly for distributed power generation, thanks to the high-power density, allowing high compactness and efficiencies due to the particular features of the fluid conditions near the critical point. In the present work, the feasibility of innovative turboexpanders is evaluated for the first European demonstrator of MW size, coupling small volumetric flows with technological simplicity, typical of these types of plants. In particular, the possibility of replacing conventional turbines with bladeless expanders is studied, proposing a design in line with those achievable by small radial and axial turbomachines. The bladeless expanders consist of flat parallel disks mounted on a shaft, separated by spacers to maintain small gaps between them. The laminar flow inside the rotor makes it highly efficient: however, rotor-stator interaction losses reduce the overall performance. Such bladeless expanders maintain high interest for their capability to tackle low volumetric flows, their relatively simple design and ease of manufacturability. The design case presented in this paper is the feasibility study of a single modular bladeless expander using the existing conventional design (axial and radial stages) as the reference design. 3D numerical analysis is carried out using commercial computational fluid dynamic software. The results show ∼55% total static efficiency of the bladeless expander at 37000 rpm for approximately 1.25MW output power. The impact on performance at different nozzle throat cross-sections and rotor disks diameter has been discussed. The overall performance of the expander is presented by evaluating the losses and improvement strategies that are discussed.

Multidisk bladeless turbines, also known as Tesla turbines, are promising in the field of small-scale power generation and energy harvesting due to their low sensitivity to down-scaling effects, retaining high rotor efficiency. However, low (less than 40%) overall isentropic efficiency has been recorded in the experimental literature. This article aims for the first time to a systematic experimental characterization of loss mechanisms in a 3 kW Tesla expander using compressed air as working fluid and producing electrical power through a high-speed generator (40 krpm). The sources of losses discussed are stator losses, stator–rotor peripheral viscous losses, end-wall ventilation losses, and leakage losses. After description of experimental prototype, methodology, and assessment of measurement accuracy, the article discusses such losses aiming at separating the effects that each loss has on the overall performance. Once effects are separated, their individual impact on the overall efficiency curves is presented. This experimental investigation, for the first time, gives the insight into the actual reasons of 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.

Recent developments in Pressure Gain Combustion (PGC) technology have demonstrated its ability to achieve higher thermal efficiencies and lower carbon emissions as compared to conventional gas turbine counterpart working in Brayton cycle. Ongoing studies suggest the possibility of implementing PGC in aircraft engines by replacing the high-pressure section (HP compressor, combustion chamber and HP turbine) with a PGC system. This, coupled with research on advanced materials and cooling solutions, offers the potential for higher overall gas turbine efficiency and fuel economy, contributing towards emission reduction of the aviation sector. This paper aims at a comprehensive review of PGC technology solutions applied in the area of aero propulsion. Reported background covers the historic as well as ongoing research activities at the component level, the cycle level, and the propulsion application of Pulse Detonation Engine (PDE), Rotating Detonation Engine (RDE), Oblique Detonation Wave Engine (ODWE), Free Piston Composite Cycle Engine (FP-CCE), and wave rotor engines. The analytical, numerical, and experimental research work is reviewed, providing also a comparison of PGC engine conceptual designs with existing gas turbine engines used in aerospace propulsion.

For small-medium heat pumps and small-scale energy storage (e.g., hydro) and other small-scale applications (up to 10kWe), an economical, reliable, durable, robust, and acceptable performing bladeless expander is an attractive technology. In this article, experiments are performed on a bladeless expander of 1kW design power with water as a working fluid. The complete expansion is in the subcooled liquid phase with an overall pressure drop across the expander in the 2-14 bar range. The water expander is designed as a similitude case study for a butane heat pump, where such a bladeless expander could replace the expansion valve recovering untapped energy from isenthalpic to isentropic expansion. Unlike conventional bladed expanders, the present bladeless expander consists of several co-rotating compact disks, closely spaced and parallelly mounted on the shaft which transmits torque using wall shear forces. The present expander design is an improved version resulting from the detailed loss characterisation done on earlier air expanders. The article begins with the definition of design conditions for water expander starting from the expected butane expansion in the heat pump, fully inside the liquid region. The rotor design of a bladeless expander is outlined using dimensionless parameters that dictate the performance features. The turbine is designed for 1kW of power output and 2kg/s mass flow with an overall pressure drop of 14 bar with a rotational speed of 8000 rpm. The resulting turbine rotor consists of an 80mm disk outer diameter, 120 disks and a 0.1mm gap between them. An experimental test rig employing water as a working fluid is described. Experiments are conducted for overall pressure drop ranging in the 2-14 bar interval, with a maximum rotational speed of 6500 rpm. The performance is recorded with two different stator configurations, having two different throat dimensions for varying mass flow at maximum inlet pressure. Peak total to static efficiency of 30% is obtained with a net power of 670 W at ∼3000 rpm. An experimental ventilation loss (end wall viscous disk friction) is performed with both water and air as working fluids to estimate the power loss. It is found that ventilation loss is the major source of loss in the present turbine prototype with a power loss of 250W@3000 and 1100W@8000 rpm, varying quadratically with rotational speed. It is finally concluded that the expander performance is promising because ventilation losses can be potentially reduced with established strategies used in conventional expanders.

In this paper an industrially established 1D model for two-phase nozzles design and analysis (Elliott, 1968) has been extended and validated with a wider range of experimental data, focusing on single component two-phase fluid expansion from initial quality in the 0%-25% range. The Authors focused on the correlations of the gas-liquid slip velocity and wall friction for two-flow regimes. The upgraded model has been tested on a converging nozzle showing accurate results under subcritical conditions (Ma 1, demonstrating that the new model obtained a significant reduction in error compared to the original Elliott model and to the well-known isentropic homogeneous approach (IHE). The extended model was also tested on a convergent-divergent nozzle produced by Carrier Corporation for the 19-XRT chiller, obtaining a satisfactory performance prediction. The validation process allowed to assess the limits of validity of the new model, which can be effectively used as design tool for subsonic or supersonic two-phase nozzles. In particular, the model capability to identify critical mass flow and critical expansion ratio has been investigated, showing good match for the critical expansion ratio, while margins of improvement remain for the critical mass flow prediction.

In Turbo machines, there are many components which are provided with interference to transmit torque and to maintain contact between them. There are transient scenarios where impeller and shaft lose interference and limits the speed of the impeller. In this paper, a case study on centrifugal compressor has been presented to see the effect of taper pairing of shaft with impeller. When the centrifugal compressor starts then a transient condition is dominant wherein the impeller of the centrifugal compressor gains higher temperature quickly compared to shaft. Since the thermal expansion of the impeller will be much more than the shaft, interference between impeller and shaft is likely to reduce. With reduced interference, the impeller should retain minimum required contact pressure with the shaft and the torque transmitting capability. For conventional fitted impeller, stress built up is a major concern especially at higher speeds. This paper gives a proposal about the importance of tapered interference fit method compared to cylindrical fitted impeller assembly. An effective analysis between the conventional straight fitted impeller and impeller having tapered bore is presented for the same torque transfer capability. The analysis reveals that for the impeller with tapered bore, the stress for impeller reduces and a healthy contact pressure exists.

Interest in small scale turbines is growing mainly for small scale power generation and energy harvesting. Conventional bladed turbines impose manufacturing limitations and higher cost, which hinder their implementation at small scale. This paper focuses on experimental and numerical performance investigation of Tesla type turbines for micro power generation. A flexible test rig for Tesla turbine fed with air as working fluid has been developed, of about 100W net mechanical power, with modular design of two convergent-divergent nozzles to get subsonic as well as supersonic flow at the exit. Seals are incorporated at the end disks to minimize leakage flow. Extensive experiments are done by varying design parameters such as disk thickness, gap between disks, radius ratio and outlet area of exhaust with speeds ranging from 10000 rpm to 40000 rpm. A quasi-1D model of the whole setup is created and tuned with experimental data to capture the overall performance. Major losses, ventilation losses at end disks, nozzle and exhaust losses are evaluated experimentally and numerically. Effect of design parameters on the performance of Tesla turbines is discussed.

The reduction of CO2 and, more generally, GHG (Green House Gases) emissions imposed by the European Commission (EC) and the Environmental Protection Agency (EPA) for passenger cars has driven the automotive industry to develop technological solutions to limit exhaust emissions and fuel consumption, without compromising vehicle performance and drivability. In a mid-term scenario, hybrid powertrain and Internal Combustion Engine (ICE) downsizing represent the present trend in vehicle technology to reduce fuel consumption and CO2 emissions. Concerning downsizing concept, to maintain a reasonable power level in small engines, the application of turbocharging is mandatory for both Spark Ignition (SI) and Diesel engines. Following this aspect, the possibility to recover the residual energy of the exhaust gases is becoming more and more attractive, as demonstrated by several studies around the world. One method to recover exhaust gas energy from ICEs is the adoption of turbo-compounding technology to recover sensible energy left in the exhaust gas by-passed through the waste-gate valve. In the paper, an innovative option of advanced boosting system is investigated through a bladeless micro expander, promising attractive cost-competitiveness. The numerical activity was developed on the basis of experimental data measured on a waste-gated turbocharger for downsized SI automotive engines. To this aim, mass flow rate through the by-pass valve and the turbine impeller was measured for different waste-gate settings in steady-state conditions at the turbocharger test bench of the University of Genoa. The paper shows that significant electrical power can be harvested from the waste-gate gases, up to 94 % of compressor power, contributing to fuel consumption reduction.

This paper presents the experimental campaign on Tesla turbo expanders carried out at Thermo-chemical Power group (TPG) of University of Genoa, Italy. An experiment system is established using compressed air as a working fluid. A 200 W turbine is tested with rotational speed up to 40000 rpm. 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. An improved version of 3 kW air Tesla turboexpander is built. Preliminary experimental results are discussed along with the effect of number of nozzles on the performance of the turbine.

This paper summarizes the development of fully 3D Computational Fluid Dynamics (CFD) analysis for bladeless air micro expander for 200 W and 3 kW rated power. Modelling of nozzle along with rotor is done using structured mesh. This analysis, for the first time, demonstrates the interaction between nozzle and rotor using compressible flow density-based solver. The Shear Stress Transport (SST) turbulence model is employed to resolve wall effects on the rotor and to determine the shear stress accurately. The results illustrate the flow field inside stator and rotor along with complicated mixing zone between stator and rotor. The comparison of rotor-stator CFD simulation results is done with experiments to preliminary validate the model. The losses in the turbine are discussed with the help of experimental and numerical data.

This study aims to investigate the reversible operation of a bladeless air expander prototype operated reversibly in compressor mode to understand the performance by numerical method and compare its results experimentally. A bladeless machine can reverse its operation by simply inverting the rotational speed. However, expander and compressor performance may differ significantly since losses are exacerbated in the compressor mode. The prototype was previously tested as an expander (experimental highest isentropic efficiency of 36.5%). In this work, the reverse mode is discussed, when the prototype is actuated as a compressor, with and without diffuser at variable rotational speeds. In compressor mode, the fluid enters through the center axially, passes radially outwards through disk gaps, and exits throughout the diffuser. The momentum transfer and pressure gain are carried out by the shear force produced on the surface of the rotating disk. An experimental/theoretical analysis focused on the pressure ratio, mass flow, and efficiency of bladeless compressor is conducted. High losses (main leakage across the rotor) were noticed during the experiments, affecting the overall Tesla compressor performance. Numerical calculations are carried out to estimate leakage losses by comparison with experimental results. It is shown that the original expander design would require specific modifications to reduce end disk leakages, which are higher in compressor mode than in expansion mode, significantly affecting the elaborated net mass flow. Improved diffuser, scroll, disk end gaps, and sealing mechanisms are discussed in order to augment overall performance of the bladeless prototype in compressor mode.