Rotor-Dynamics of Different Shaft Configurations for a 6 kW Micro Gas Turbine for Concentrated Solar Power

Rotor-dynamics of Micro Gas Turbines (MGTs) under 30 kW have been a critical issue for the successful development of reliable engines during the last decades. Especially, no consensus has been reached on a reliable MGT arrangement under 10 kW with rotational speeds above 100,000 rpm, making the understanding of the rotor-dynamics of these high speed systems an important research area. This paper presents a linear rotor-dynamic analysis and comparison of three mechanical arrangements of a 6 kW MGT intended for utilising Concentrated Solar Power (CSP) using a parabolic dish concentrator. This application differs from the usual fuel burning MGT in that it is required to operate at a wider operating speed range. The objective is to find an arrangement that allows reliable mechanical operation through better understanding of the rotor dynamics for a number of alternative shaft-bearings arrangements. Finite Element Analysis (FEA) was used to produce Campbell diagrams and to determine the critical speeds and mode shapes. Experimental hammer tests using a new approach based on optical sensing technology were used to validate the rotor-dynamic models. The FEA simulation results for the natural frequencies of a shaft arrangement were within 5% of the measurements, while the deviation for the shaft-bearings arrangement increased up to 16%.

Altitude Effects on the Performance of Small Radial Turbomachinery: Part I — Compressor Impellers

Small unmanned aircraft are currently limited to flight ceilings below 20,000 ft due to the lack of an appropriate propulsion system. One of the most critical technological hurdles for an increased flight ceiling of small platforms is the impact of reduced Reynolds number conditions at altitude on the performance of small radial turbomachinery. The current article investigates the influence of Reynolds number on the efficiency and pressure ratio of two small centrifugal compressor impellers using a one-dimensional meanline performance analysis code. The results show that the efficiency and pressure ratio of the 60 mm baseline compressor at the design rotational speed drops with 6–9% from sea-level to 70,000 ft. The impact on the smaller 20 mm compressor is slightly more pronounced and amounts to 6–10%. Off-design changes at low rotational speeds are significantly higher and can amount to up to 15%. Whereas existing correlations show a good match for the efficiency drop at the design rotational speed, they fail to predict efficiency changes with rotational speed. A modified version is therefore proposed.

An Improved Test Facility for Studying Deposit Fouling on Steam Turbine Blades

A laboratory scale test facility has been developed to investigate deposition in steam turbines under conditions that are representative of those in steam power generation cycles. The facility is an advanced two-reactor vessel test arrangement, which is a more flexible and more accurately controllable refinement to the single reactor vessel test arrangement described previously in ASME Paper No. GT2014-25517 [1]. The commissioning of the new test facility is described in this paper, together with the results from a series of tests over a range of steam conditions, which show the effect of steam conditions (particularly steam pressure) on the amount and type of deposits obtained. Comparisons are made between the test results and feedback/experience of copper fouling in real machines.

Parametric Experimental and Numerical Study of LP Diffuser and Exhaust Hoods

The design of an effective diffuser for a given last stage blade of an LP turbine is known to be highly dependent on the size and shape of the exhaust hood in which it is located. For retrofit steam turbines in particular, where a new last stage blade and diffuser are fitted into an existing exhaust hood, the shapes and sizes of the exhaust box have been seen to vary significantly from one contract to the next. An experimental parametric study of diffuser lips and exhaust hood configurations has been run on a model test turbine rig at GE Power to investigate the impact of various geometric parameters on the performance of the diffusers. Improved testing and post-processing methodologies means the diffuser performance has been obtained for a greater number of geometric configurations than was previously typically possible. The results of these experiments are compared with numerical calculations and confirm the accuracy of the standard in-house diffuser design tools. Key geometric parameters are identified from the test data and used to generate improved diffuser design guidelines.

Low Pressure Steam Turbine Exhaust Flow: Part 1 — CFD Coupling of LP Turbine and Condenser Neck

Since many years the diffuser and exhaust of low pressure (LP) turbines have been in the focus of turbine development and accordingly broadly discussed within the scientific community. The pressure recovery gained within the diffuser significantly contributes to the turbine performance and therefore plenty of care is taken in investigations of the flow as well as optimization within this part of the turbine. However on a plant level the component following the LP turbine is the condenser, which is connected by the condenser neck. Typically the condenser neck is not fully designed to provide additional enthalpy recovery. Due to plant arrangement reasons, often it is full of built-ins like stiffening struts, feed-water heaters, extraction pipes, steam dump devices and others. It is vital to minimize the pressure losses across the condenser neck, in order to keep performance benefit, previously gained within the diffuser. As a general rule, each mbar of total pressure loss in a condenser neck may reduce the gross power output up to 0.1%. While turbines usually follow a modular approach, the condenser is typically designed plant specific. Therefore, on a plant level it is crucial to identify and evaluate the loss contributors and develop processes and tools which allow an accurate and efficient design process for an optimized condenser neck design. This needs to be performed as a coupled modelling approach, as both, turbine and condenser flow interact with each other. 3-D CFD tools enable a deep insight into the flow field and help to locally optimize the design, as they help to identify local losses and this even for small geometrical design changes. Unfortunately these tools are costly with respect to computational time and resources, if they are used to analyze a full condenser neck with all built-ins. Here 1-D modelling approaches can help to close the gap, as they can provide fast feedback, e.g. in a project tender phase, or can allow to quickly analyze design changes. For this they need a proper calibration and validation. This publication discusses the CFD modelling of a LP steam turbine coupled to a condenser neck and the validity of such calculations against measurement data. In the second publication (Part 2) a simplification of the gained information to a 1-D modelling approach will be discussed.

A Simple Recuperated Ceramic Microturbine: Design Concept, Cycle Analysis, and Recuperator Component Prototype Tests

Ceramic recuperators could enable microturbines to achieve higher fuel efficiency and specific power. Challenges include finding a suitable ceramic fabrication process, minimizing stray heat transfer and gas leakage, mitigating thermal stress, and joining the ceramic parts to neighboring metal components. This paper describes engine and recuperator design concepts intended to address these obstacles. The engine is sized to produce twelve kilowatts of shaft power, and it has a reverse-flow compressor and turbine. Motivations for this layout are to balance axial thrust forces on the rotor assembly; to minimize gas leakage along the rotating shaft; to reduce heat transfer to the compressor diffuser; to enable the use of a simple, single-can combustor; and to provide room for lightweight ceramic insulation surrounding all hot section components. The recuperator is an annular, radial counterflow heat exchanger with the can combustor at the center. It is assembled from segmented wafers made by ceramic injection molding (CIM). These are housed in a pressure vessel to load the walls mainly in compression, and are joined together by flexible adhesives in the cool areas to accommodate thermal expansion. A representative wafer stack was built by laser-cutting, laminating, and sintering tapecast ceramic material. The prototype was tested at temperatures up to 675°C, and the results were used to validate analytical and computational fluid dynamics (CFD) models, which were then used to estimate the effectiveness of the actual design. Turbomachinery efficiencies were also calculated using CFD, and allowances were made for additional losses like bearing friction and gas leakage. Based on these component performance estimates, a cycle model indicates the engine could achieve a net fuel-to-electrical efficiency of 21%, at a core weight including the recuperator of 11 kg, or about 1 kg/kW electric output.

Investigation of Cross Flow in Double Entry Turbocharger Turbines

The downsizing of combustion engines has become the major strategy within the automotive industry to meet the increasing demands in terms of fuel economy and harmful emissions. Furthermore, it is important to fulfil the customers expectations in terms of drivability by increasing the power density and transient performance of the engines. The key technology to reach these ambitious targets is the enhanced utilization of exhaust pulses on turbocharged engines. In four cylinder gasoline engine applications this is mainly realized by the use of double entry turbines or variabilities in the exhaust valve train. During the designing and matching process of double entry turbines it is still a major challenge to predict the turbine power output and accurately model its interaction with the engine. In the past few years, several authors have published measurement and simulation technologies aimed at enhanced modelling quality. Most of these approaches are based on the introduction of different flow conditions which help to describe the thermodynamic performance under various pulsating boundary conditions. Within an average engine cycle, the turbine operates under equal, single and unequal admissions. Furthermore, the evaluation of a turbine interacting with a four cylinder gasoline engine shows that cross flow between both turbine scrolls can occur during the blow-down phase of the cylinders. In this phase, the temperature and pressure upstream of the turbine reach their peak values within the complete engine cycle. Therefore, this phase is most crucial for the conversion of the exhaust energy into mechanical energy, which drives the compressor impeller of the turbocharger. This work focuses on the results of stationary hot gas measurements and 3D CFD simulations of the cross flow phenomena to gain a deeper understanding of the scroll interaction in double entry turbines and its impact on engine performance. The findings were used to improve the modeling quality of double entry turbines in 1D engine process simulations, especially during the exhaust blow down where cross flow between the dividing wall and the turbine wheel occurs. The new methodology to quantify the amount of cross flow with a hot gas test has shown that the cross flow rate of a twin scroll turbine can reach values as high as 35% of the overall flow rate entering the turbine housing, whereas this value can be significantly reduced by using a segment turbine housing. The new map based turbine model, which enables predictive simulations, covers all engine relevant flow conditions of the turbine including cross flow. This is important because the cross flow has a large impact on the exhaust pulse separation and thus on the residual gas fraction of the cylinders after the gas exchange.

Analysis and Comparison of the Performance of an Inverted Brayton Cycle and Turbo-Compounding With Decoupled Turbine and CVT Driven Compressor for Small Automotive Engines

For an internal combustion engine, a large quantity of fuel energy (accounting for approximately 30% of the total combustion energy) is expelled through the exhaust without being converted into useful work. Various technologies including turbo-compounding and the pressurized Brayton bottoming cycle have been developed to recover the exhaust heat and thus reduce the fuel consumption and CO2 emission. However, the application of these approaches in small automotive power plants has been relatively less explored because of the inherent difficulties, such as the detrimental backpressure and higher complexity imposed by the additional devices. Therefore, research has been conducted, in which modifications were made to the traditional arrangement aiming to minimize the weaknesses. The turbocharger of the baseline series turbo-compounding was eliminated from the system so that the power turbine became the only heat recovery device on the exhaust side of the engine, and operated at a higher expansion ratio. The compressor was separated from the turbine shaft and mechanically connected to the engine via CVT. According to the results, the backpressure of the novel system is significantly reduced comparing with the series turbo-compounding model. The power output at lower engine speed was also promoted. For the pressurized Brayton bottoming cycle, rather than transferring the thermal energy from the exhaust to the working fluid, the exhaust gas was directly utilized as the working medium and was simply cooled by ambient coolant before the compressor. This arrangement, which is known as the inverted Brayton cycle was simpler to implement. Besides, it allowed the exhaust gasses to be expanded below the ambient pressure. Thereby, the primary cycle was less compromised by the bottoming cycle. The potential of recovering energy from the exhaust was increased as well. This paper analysed and optimized the parameters (including CVT ratio, turbine and compressor speed and the inlet pressure to the bottoming cycle) that are sensitive to the performance of the small vehicle engine equipped with inverted Brayton cycle and novel turbo-compounding system respectively. The performance evaluation was given in terms of brake power output and specific fuel consumption. Two working conditions, full and partial load (10 and 2 bar BMEP) were investigated. Evaluation of the transient performance was also carried out. Simulated results of these two designs were compared with each other as well as the performance from the corresponding baseline models. The system models in this paper were built in GT-Power which is a one dimension (1-D) engine simulation code. All the waste heat recovery systems were combined with a 2.0 litre gasoline engine.

Hermetic 40-kW-Class Steam Turbine System for the Bottoming Cycle of Internal Combustion Engines

An entirely new 40-kW-class steam turbine prototype has been constructed and successfully tested for more than 6,500 hours. The two-stage Curtis wheel with a nominal pressure ratio of about 130 has been designed for applications in bottoming water/steam cycles. The axial turbine runs at a rated rotational speed of 36,000 rpm, utilizing magnetic bearings and a permanent magnet synchronous generator which is coupled to the grid by frequency inverters. The integral turbine-generator set has been designed as a hermetically sealed assembly group. The turbine design allows both the turbine’s oil supply system and the bottoming cycle’s feed-water treatment system to be eliminated. The turbine has been designed to allow unmanned operation of the entire cycle with minimum maintenance requirements and reduced costs. Extensive turbine testing, including rated power, overload and load rejection tests was carried out to verify functionality. Long-term operational capability was also demonstrated, giving particular attention to generator performance. Exemplarily a bottoming cycle, utilizing the turbine, matching the exhaust conditions of internal combustion engines was designed, increasing electrical efficiency from 40.4% to 43.4%, which represents a relative net improvement of 7.5%. In this application a turbine isentropic efficiency of more than 55% is expected, based on the congruence of measurement and calculation.

Influence of Turbulence Modelling to Condensing Steam Flow in the 3D Low-Pressure Steam Turbine Stage

With the tremendous role played by steam turbines in power generation cycle, it is essential to understand the flow field of condensing steam flow in a steam turbine to design an energy efficient turbine because condensation at low pressure (LP) turbine introduces extra losses, and erosion in turbine blades. The turbulence has a leading role in condensing phenomena which involve a rapid change of mass, momentum and heat transfer. The paper presents the influence of turbulence modelling on non-equilibrium condensing steam flows in a LP steam turbine stage adopting CFD code. The simulations were conducted using the Eulerian-Eulerian approach, based on Reynolds-averaged Navier-Stokes equations coupled with a two equation turbulence model, which is included with nucleation and droplet growth model for the liquid phase. The SST k-ω model was modified, and the modifications were implemented in the CFD code. First, the performance of the modified model is validated with nozzles and turbine cascade cases. The effect of turbulence modelling on the wet-steam properties and the loss mechanism for the 3D stator-rotor stage is discussed. The presented results show that an accurate computational prediction of condensing steam flow requires the turbulence to be modelled accurately.