The Physics of H-Darrieus Turbines Self-Starting Capability: Flapping-Wing Perspective

It has been widely reported that Darrieus turbines cannot self-start and that they require external assistance to accelerate to their operating tip speed ratios. However, recent experiments have shown conclusively that H-Darrieus rotors with fixed-pitch blades that employ a symmetrical aerofoil can reliably self-start in steady controlled environments. Previous attempts have also been made to model the starting characteristics but there still exists a significant discrepancy between the experimental data and model prediction, suggesting that our understanding of this starting characteristic remains weak. The investigation and explanation of the starting characteristics is the focus of this paper. The investigation was made through a careful analysis of aerofoils that undergo Darrieus motion, giving some insights on how the blade experiences different flow conditions and how driving force is developed over the flight path. The analysis reveals that the aerofoil in Darrieus motion is analogous to flapping wing mechanism; the mechanism that fish and birds employ to generate propulsion. The explanation of flow physics and torque development can then be made through a simple pitch-heave concept. The investigation using this concept together with observations of flapping creatures suggests that the key feature that promotes driving torque generation and the ability to self-start is the unsteadiness associated with the rotor. This unsteadiness is related to chord-to-diameter ratio. This, together with blade aspect ratio, and number of blades, is the reason why H-Darrieus turbines that employ a symmetrical aerofoil can self-start.

Component Testing and Prototype Commissioning of MAN’s New Gas Turbine in the 6 MW-Class

MAN Diesel & Turbo has developed a new gas turbine in the 6 MW-class for both mechanical drive and power generation applications. The lay-out of the Gas Turbine has been driven by opportunities in current and future markets and the positioning of the competition, and this has determined the characteristics and technical parameters which have been optimized in the 6 MW design. The design makes use of extremely high precision engineering so that the assembly of sub components to modules is a smooth flowing process and can guarantee both the high standards in quality and performance which MAN Diesel & Turbo is aiming for. Individual components have been tested and thoroughly validated. These tests include in particular the compressor of the gas turbine and the combustion chamber. The commissioning of the gas turbine prototype engine had been prepared with a numerous number of measuring probes and carried out at the Oberhausen plant gas turbine test field. Results of component and the gas turbine prototype tests will be presented and discussed.

Parameterization of High Solar Share Gas Turbine Systems

Existing solar thermal power plants are based on steam turbine cycles. While their process temperature is limited, solar gas turbine (GT) systems provide the opportunity to utilize solar heat at a much higher temperature. Therefore there is potential to improve the efficiency of future solar thermal power plants. Solar based heat input to substitute fuel requires specific GT features. Currently the portfolio of available GTs with these features is restricted. Only small capacity research plants are in service or in planning. Process layout and technology studies for high solar share GT systems have been carried out and have already been reported by the authors. While these investigations are based on a commercial 10MW class GT, this paper addresses the parameterization of high solar share GT systems and is not restricted to any type of commercial GT. Three configurations of solar hybrid GT cycles are analyzed. Besides recuperated and simple GT with bottoming Organic Rankine Cycle (ORC), a conventional combined cycle is considered. The study addresses the GT parameterization. Therefore parametric process models are used for simulation. Maximum electrical efficiency and associated optimum compressor pressure ratio πC are derived at design conditions. The pressure losses of the additional solar components of solar hybrid GTs have a different adversely effect on the investigated systems. Further aspects like high ambient temperature, availability of water and influence of compressor pressure level on component design are discussed as well. The present study is part of the R&D project Hybrid High Solar Share Gas Turbine Systems (HYGATE) which is funded by the German Ministry for the Environment, Nature and Nuclear Safety and the Ministry of Economics and Technology.

Analysis of the Operating Mode Influence Onto Energy Consumption of a Natural Gas Storage Plant

Natural gas carrying from production sites to users’ facilities is made by marine shipping in liquid phase or by terrestrial pumping in gaseous phase through long pipelines. In the latter case several storage stations are distributed along the pipeline nets to move the natural gas from its deposits to users’ terminals. Storage stations are set up to compensate seasonal fluctuations of the consumer demand versus methane supply, storing the gas in various kinds of reservoirs. In most of such plants centrifugal compressors are used, where the energy and the time that a complete charge takes are affected by the operation scheduling of the compressor from the minimum to the maximum storage levels. While the pressure in the reservoir enforces the instant operation pressure, the flow rate is limited within a quite wide range. Here an in-house code, based on the lumped parameter approach and a quasi-steady dynamics, is applied to a complete charge. The natural gas behavior is modeled by the pseudo-ideal gas in order to get a fair accuracy keeping the usual gas dynamics equations. The compression path has been parameterized and a multi objective optimization, embedding the simulation code, has been implemented to find the most suitable management of the compression station for the minimization of time and energy. The most significant paths are analyzed to pick out the effects of the compression strategy.

Design Optimization of a Low Pressure Steam Turbine Radial Diffuser Using an Evolutionary Algorithm and 3D CFD

The design of the radial exhaust hood of a low pressure (LP) steam turbine has a strong impact on the overall performance of the LP turbine. A higher pressure recovery of the diffuser will lead to a substantial higher power output of the turbine. One of the most critical aspects in the diffuser design is the steam guide, which guides the flow near the shroud from axial to radial direction and has a high impact on the pressure recovery. This paper presents a method for the design optimization of the steam guide of a steam turbine for industrial power generation and mechanical drive of centrifugal compressors. This development is in the frame of a continuous effort in GE Oil and Gas to develop more efficient steam turbines. An existing baseline exhaust and steam guide design is first analyzed together with the last LP turbine stage with a frozen rotor full 3D Computational Fluid Dynamics (CFD) calculation. The numerical prediction is compared to available steam test turbine data. The new exhaust box and a first attempt new steam guide design are then first analyzed by a CFD computation. The diffuser inlet boundary conditions are extracted from this simulation and used for improving the design of the steam guide. The maximization of the pressure recovery is achieved by means of a numerical optimization method that uses a metamodel assisted differential evolution algorithm in combination with a 3D CFD solver. The profile of the steam guide is parameterized by a Bezier curve. This allows for a wide variety of shapes, respecting the manufacturability constraints of the design. In the design phase it is mandatory to achieve accurate results in terms of performance differences in a reasonable time. The pressure recovery coefficient is therefore computed through the 3D CFD solver excluding the last stage, to reduce the computational burden. Steam tables are used for the accurate prediction of the steam properties. Finally, the optimized design is analyzed by a frozen rotor computation to validate the approach. Also off-design characteristics of the optimized diffuser are shown.

Dynamic Stress Prediction in Centrifugal Compressor Blades Using Fluid Structure Interaction

A computational model is developed that predicts stresses in the blades of a centrifugal compressor. The blade vibrations are caused by the wakes coming off stationary inlet guide vanes upstream of the impeller, which create a periodic excitation on the impeller blades. When this excitation frequency matches the resonant frequency of the impeller blades, resonant vibration is experienced. This vibration leads to high cycle fatigue, which is a leading cause of blade failure in turbomachinery. Although much research has been performed on axial flow turbomachinery, little has been published for radial machines such as centrifugal compressors and radial inflow turbines. A time domain coupled fluid-structure computational model is developed. The model couples the codes unidirectionally, where pressures are transferred to the structural code during the transient solution, and the fluid mesh remains unaffected by the structural displacements. A Fourier analysis is performed of the resulting strains to predict both amplitude and frequency content. This modeling method was first applied to a compressor in a single stage centrifugal compressor test rig. The analysis results were then validated by experimental blade strain measurements from a rotating test. The model correlated very well with the experimental results. In this work, a model is developed for a liquefied natural gas (LNG) centrifugal compressor that experienced repeated blade failures. The model determined stress levels in the blades, which helped to predict the likely cause of failure. The method was also used to investigate design changes to improve the robustness of the impeller design.

Design Parameters of Brush Seals and Their Impact on Seal Performance

Brush seals, which were originally designed for gas turbine applications, have been successfully applied to large-scale steam turbines within the past decade. From gas turbine applications, the fundamental behavior and designing levers are known. However, the application of brush seals to a steam turbine is still a challenge. This challenge is mainly due to the extreme load on the brush seal while operating under steam. Furthermore, it is difficult to test brush seals under realistic conditions, i.e. under live steam conditions with high pressure drops. Due to these insufficiencies, 2 test rigs were developed at the University of Technology Braunschweig, Germany. The first test rig is operated under pressurized air and allows testing specific brush seal characteristics concerning their general behavior. The knowledge gained from these tests can be validated in the second test rig, which is operated under steam at pressure drops of 45 bar and temperatures up to 450 °C. Using both the air test rig and the steam test rig helps keep the testing effort comparably small. Design variants can be pre-tested with air, and promising brush seal designs can consequently be tested in the steam seal test rig. The paper focuses on a clamped brush seal design which, amongst others, is used in steam turbine blade paths and shaft seals of current Siemens turbines. The consequences of the brush assembly on the brush appearance and brush performance are shown. The clamped brush seal design reveals several particularities compared to welded brushes. It could be shown that the clamped bristle pack tends to gape when clamping forces rise. Gapping results in an axially expanding bristle pack, where the bristle density per unit area and the leakage flow vary. Furthermore, the brush elements are usually assembled with an axial lay angle, i.e. the bristles are reclined against the backing plate. Hence, the axial lay angle is also part of the investigation.

Experimental and Numerical Validation of Conical Strainer Fluid/Structural Performance Model

Temporary conical strainers are widely employed in the Oil & Gas industry as filtering devices in the Centrifugal Compressors suction line. They protect compressor stages from the ingestion of foreign objects whether coming from dirty process gas or left in the pipeline after its construction. Very few literature and research papers are available on the fluid dynamic and structural performance of conical strainers. The purpose of this work is to plug this gap by the definition of a theoretical-experimental model for the characterization of the pressure drop and mechanical resistance of these devices. Starting from the definition of the main fluid dynamics and geometric variables which influence the performances, an experimental campaign has been performed in order to derive the relationship governing the pressure drop behavior. The model efficacy has been confirmed by a CFD analysis, which also allowed a qualitative insight review into the dynamics of velocity and turbulence intensity fields. Further tests have been performed in order to validate the model at off-design points. As far as the structural analysis is concerned, several FEM models and DOE techniques have been implemented in order to define relationships for bust pressure computation and feasible design improvements with respect to the current state of the art. Besides fluid dynamic and structural correlations, the notable achievements of this work are the definition of best pressure static probes positioning and the maximum clogging level that a strainer can withstand before collapse. Furthermore, some guidelines are given in order to prevent pipeline resonance and acoustic fatigue caused by the interaction between strainer turbulence and compressor inlet flow.

Impact of Secondary Flow on the Accuracy of Simplified Design Methods for Steam Turbine Stages

The subject of the presented paper is the validation of a design method for HP and IP steam turbine stages. Common design processes have been operating with simplified design methods in order to quickly obtain feasible stage designs. Therefore, inaccuracies due to assumptions in the underlying methods have to be accepted. The focus of this work is to quantify the inaccuracy of a simplified design method compared to 3D Computational Fluid Dynamics (CFD) simulations. Short computing time is very convenient in preliminary design; therefore, common design methods work with a large degree of simplification. The origin of the presented analysis is a mean line design process, dealing with repeating stage conditions. Two features of the preliminary design are the stage efficiency, based on loss correlations, and the mechanical strength, obtained by using the beam theory. Due to these simplifications, only a few input parameters are necessary to define the primal stage geometry and hence, the optimal design can easily be found. In addition, by using an implemented law to take the radial equilibrium into account, the appropriate twist of the blading can be defined. However, in comparison to the real radial distribution of flow angles, this method implies inaccuracies, especially in regions of secondary flow. In these regions, twisted blades, developed by using the simplified radial equilibrium, will be exposed to a three-dimensional flow, which is not considered in the design process. The analyzed design cases show that discrepancies at the hub and shroud section do exist, but have minor effects. Even the shroud section, with its thinner leading-edge, is not vulnerable to these unanticipated flow angles.

Optimal Gas-Turbine Design for Hybrid Solar Power Plant Operation

A dynamic simulation model of a hybrid solar gas-turbine power plant has been developed, allowing determination of its thermodynamic and economic performance. In order to examine optimum gas-turbine designs for hybrid solar power plants, multi-objective thermoeconomic analysis has been performed, with two conflicting objectives: minimum levelized electricity costs and minimum specific CO2 emissions. Optimum cycle conditions: pressure-ratio, receiver temperature, turbine inlet temperature and flow rate, have been identified for a 15 MWe gas-turbine under different degrees of solarization. At moderate solar shares, the hybrid solar gas-turbine concept was shown to provide significant water and CO2 savings with only a minor increase in the levelized electricity cost.