Variable Geometry Compressor Research Articles

On the Effect of Side Clearance in the Vaned Diffuser of a Centrifugal Compressor

AbstractEnvironmental and regulatory pressures have prompted original equipment manufacturers (OEMs) to explore highly boosted internal combustion (IC) engines as a means to minimize toxic air pollutants and meet strict legislative targets. The exhaust gas turbocharger is a key technology used to achieve the boost pressures necessary for advanced engines of the future. Centrifugal compressors with vaneless diffusers have been widely implemented in turbochargers for IC engines, particularly passenger vehicles, primarily due to their compact size and ability to deliver high-pressure ratios over a wide range of flowrates. Methods to improve compressor efficiency near surge without reducing the surge margin have received considerable interest in the past. In many situations, changes to the impeller blade design and/or diffuser are limited by the need to meet the strict engine-rated power target. Such competing demands have encouraged designers to develop variable geometry compressors with the aim of improving near surge performance without sacrificing rated power performance. The on/off-type vaned diffuser is a variable geometry compressor capable of delivering a pressure ratio benefit near surge by utilizing a vaned diffuser rather than a vaneless diffuser. The vanes are retracted prior to the diffuser choking to form a vaneless diffuser in order to reach the engine-rated power condition. Diffuser vane-shaped clearances in the hub or the shroud end wall, termed side clearances, enable axial sliding variability of the vanes into and out of the diffuser passage. Unfortunately, the clearances manifest themselves as a discontinuity in the flow field which gives rise to an efficiency penalty. It is not yet clear how the side clearance geometry and position affect the compressor performance i.e., efficiency and stability. Hence, a numerical and experimental investigation was carried out to determine the impact of side clearance geometry and end wall position on compressor performance. It was found that a side clearance positioned at the shroud end wall reduced the surge margin of the investigated compressor for both types of clearance geometry considered. In contrast to this, a clearance positioned at the hub end wall improved the compressor surge margin but had the lowest efficiency benefit near surge. An experimentally validated Computational Fluid Dynamics (CFD) model was used to inspect the flow field and to elucidate the reasons behind the changes in compressor performance.

An improved performance diagnostic method for industrial gas turbines with consideration of intake and exhaust system

Industrial gas turbines, as core equipment for efficient and clean utilization of energy, have been widespreadly applied in gas-steam combined cycle power plants. To ensure safe, stable, green and efficient operation, it is essential to establish a reliable industrial gas turbine fault diagnosis system. Industrial gas turbines need more flexible operation, due to requirements for power plant frequent frequency regulation and peak regulation in grid-supported mode. It is difficult to filter out quasi-steady-state operation data segment from collected operating data of industrial gas turbine, which makes the applicable boundary of traditional diagnostic methods narrower. For the actual problems to be solved urgently during diagnosis of industrial gas turbines, this paper proposes an improved performance diagnostic method for industrial gas turbines. First, a high-precision thermodynamic modeling method for industrial gas turbines was proposed with consideration of the effect of variable geometry compressor, and the established gas turbine thermodynamic model is validated by actual industrial gas turbine operation data. Next, an improved performance diagnostic method is established, and the health parameters for intake system and exhaust system is proposed in fault diagnosis for the first time and compressor variable geometry is also considered in the diagnostic process. Finally, through actual industrial gas turbine operation test, the effectiveness and reliability of the proposed diagnostic method to diagnose component performance degradation and component abrupt performance damage are verified. The proposed diagnostic method realizes real-time online quantitative diagnosis of all gas-path components under full operating conditions from the minimum IGV opening load to the IGV fully open base load.

DES analysis on effects of variable geometry inlet orifice on a centrifugal compressor

A variable geometry inlet configuration can improve the off-design aerodynamic performance of a turbocharger compressor by actively modulating the internal flow field. As the core part of the configuration, the variable geometry orifice is arranged in front of the compressor inducer. Its novelty is to actively regulate the compressor inlet flow area at an off-design point without causing significant impact at the design point. In this paper, the effects of the orifice on compressor performance and internal flow field are investigated using experiments and numerical simulations. The experimental and the numerical results confirm the variable inlet configuration to be a promising method for extending the compressor operation range. The DES calculations reveal incidence angle reduction at inducer inlet, shock wave migration to a lower radius from blade tip, formation of the aft-loaded impeller, reduced difference between jet and wake flows at impeller exit, and turbulent flow enhancement in the diffuser. Based on understanding the orifice’s effects on the centrifugal compressor, a design suggestion is proposed for the variable geometry compressor of this sort.

Gas turbine gas‐path fault diagnosis in power plant under transient operating condition with variable geometry compressor

AbstractTo extend useful life, improve availability, and reduce gas turbine maintenance costs, scholars have proposed many gas‐path fault diagnostic approaches on the basis of steady‐state operating conditions. However, gas turbines increasingly have to operate more flexibly in grid‐supported modes. In transient operating mode, the service life of the gas turbine will be consumed faster compared with that in base‐load operating mode. In addition, the use of a variable geometry compressor has introduced new uncertainty in the actual diagnostic process, making the applicable boundaries of the original gas‐path diagnosis methods narrower and narrower. Regarding the issue above, a novel approach is proposed for fault diagnosis of gas turbine gas‐path components in power plants under a transient operating condition with the variable geometry compressor for the first time. First, the mathematical relationship of the influence of the compressor inlet guide vane position on the compressor flow and efficiency characteristics is deduced. And a high‐precision thermodynamic model for the purpose of performance analysis and gas‐path fault diagnosis is established, which comprehensively considers the above interference factors and includes health parameters of the main gas‐path components. Second, the thermodynamic model‐based diagnosis strategy under transient operating conditions with the variable geometry compressor is proposed and component health parameters can be quantitatively obtained. At last, the effectiveness and reliability of the method proposed are validated through both simulation test and actual operation test with good accuracy and real‐time performance to diagnose single and multiple component performance degradations, and component abrupt fault.

Influence of Variable Geometry Compressor on Transient Performance of Counter-Rotating Open Rotor Engines

This work describes a methodology used for counter-rotating (CR) propellers performance estimation. The method is implemented in an in-house program for gas turbine performance prediction, making possible the simulation of the counter-rotating open rotor (CROR) architecture. The methodology is used together with a variable geometry compressor control strategy to avoid surge conditions. Two cases are simulated under transient operation for both fixed and variable geometry compressor. The influence of the variable geometry control on the transient performance of CROR engines is evaluated and a comprehensive understanding on the transient behavior of this type of engine could be obtained. It is shown that the use of the variable geometry compressor control does not significantly affect the overall engine performance, while avoiding the surge conditions, thus ensuring the engine operation safety.

A prediction model of compressor with variable-geometry diffuser based on elliptic equation and partial least squares.

To achieve a much more extensive intake air flow range of the diesel engine, a variable-geometry compressor (VGC) is introduced into a turbocharged diesel engine. However, due to the variable diffuser vane angle (DVA), the prediction for the performance of the VGC becomes more difficult than for a normal compressor. In the present study, a prediction model comprising an elliptical equation and a PLS (partial least-squares) model was proposed to predict the performance of the VGC. The speed lines of the pressure ratio map and the efficiency map were fitted with the elliptical equation, and the coefficients of the elliptical equation were introduced into the PLS model to build the polynomial relationship between the coefficients and the relative speed, the DVA. Further, the maximal order of the polynomial was investigated in detail to reduce the number of sub-coefficients and achieve acceptable fit accuracy simultaneously. The prediction model was validated with sample data and in order to present the superiority of compressor performance prediction, the prediction results of this model were compared with those of the look-up table and back-propagation neural networks (BPNNs). The validation and comparison results show that the prediction accuracy of the new developed model is acceptable, and this model is much more suitable than the look-up table and the BPNN methods under the same condition in VGC performance prediction. Moreover, the new developed prediction model provides a novel and effective prediction solution for the VGC and can be used to improve the accuracy of the thermodynamic model for turbocharged diesel engines in the future.

Coordinated Performance Optimization of a Variable Geometry Compressor With Model Predictive Control for a Turbocharged Diesel Engine

The concept of variable geometry compressor (VGC) has been recently proposed as a future improvement for automotive turbochargers to further increase the efficiency and boosting at low engine speed, and to provide a more direct control over the stability margin at medium–high engine load. This paper presents a control design for the air path system of an automotive diesel engine equipped with exhaust gas recirculation (EGR), variable geometry turbine (VGT), and VGC systems. Starting from a validated model, a study of the steady-state and dynamic responses is conducted to understand the influence of the VGC combined with VGT–EGR actuators on the performance of the system. Then, a feedback control strategy that coordinates the VGC actuator with the existing VGT–EGR controller based on model predictive control is proposed to optimize the compressor performance. The new methodology shows how the VGC can be used to increase the stability range of the compressor while providing the opportunity to improve its efficiency, by means of shifting the surge limit and reshaping the peak efficiency region of the machine. Simulation results on a vehicle acceleration profile with gear shift transients are provided to verify the effectiveness of the proposed methodology.

Model-based optimisation and predictive control of a turbocharged diesel engine with variable geometry compressor

This paper presents a model-based optimisation and preliminary control design for a diesel engine equipped with variable geometry turbocharger (VGT), exhaust gas recirculation (EGR), and variable geometry compressor (VGC) systems. Starting from a validated model of the engine air path dynamics, a model-based optimisation is conducted at steady state conditions. The optimisation results define a simple feed-forward control strategy, which shows how the VGC can be used to increase the stability range of the compressor by means of shifting the surge limit of the machine while providing opportunity to improve fuel efficiency and turbo-lag. A design of feedback control is then investigated by coordinating the available actuators to track the desired performance variables derived from the steady state optimisation, and ensure the stability of the compressor during transient. The regulation is formulated as a receding horizon optimisation problem, which is cast as a constrained quadratic programming to reduce the computational load. The proposed controller is implemented into the nonlinear engine model, followed by an analysis of the simulation results.

Performance Evaluation of a Variable Geometry Gas Turbine in a CHP Plant

This study is to develop mathematical models and evaluate the performance of a gas turbine with variable geometry compressor working in a CHP plant. A single shaft gas turbine plant can maintain the exhaust gas temperature if the load is not below 50 % of the full load by simultaneously regulating the compressor variable vanes position and fuel flow. For load less than 50% the engine is running to meet the power demand. This is achieved by controlling the fuel flow and air bleed at the downstream of the compressor to avoid surge formation while variable vanes are opened fully. To accommodate change of compressor parameters during variable vanes re-stagger correction coefficients are introduced. A behavior of a 4.2 MW gas turbine performance was evaluated. The effect of variation of load and ambient temperature on the gas turbine specific fuel consumption, temperature, pressure ratio, variable vanes opening and efficiency were examined. Comparison between the field data and simulation results demonstrate good agreement. The off-design calculation was done by in-house developed program in MATLAB environment.

Development of a turbocharger compressor with variable geometry for heavy-duty engines

This article describes the first development phase of a centrifugal compressor with variable geometry which is designed to match the needs of future heavy-duty engines. Requirements of truck engines are analyzed, and their impact on the properties of the compressor map is evaluated in order to identify the most suitable kind of variable geometry. Our approach utilizes the transformation of engine data into pressure ratio and mass flow coordinates that can be displayed and interpreted using compressor maps. One-dimensional and three-dimensional computational fluid dynamics fluid flow calculations are used to identify loss mechanisms and constraints of fixed geometry compressors. Linking engine goals and aerodynamic objectives yields specific recommendations on the implementation of the variable geometry compressor.

Modeling and Analysis of a Turbocharged Diesel Engine with Variable Geometry Compressor System

Modeling and Analysis of a Turbocharged Diesel Engine with Variable Geometry Compressor System

Stochastic axial compressor variable geometry schedule optimisation

The design of axial compressors is dictated by the maximisation of flow efficiency at on design conditions whereas at part speed the requirement for operation stability prevails. Among other stability aids, compressor variable geometry is employed to rise the surge line for the provision of an adequate surge margin. The schedule of the variable vanes is in turn typically obtained from expensive and time consuming rig tests that go through a vast combination of possible settings. The present paper explores the suitability of stochastic approaches to derive the most flow efficient schedule of an axial compressor for a minimum variable user defined value of the surge margin. A genetic algorithm has been purposely developed and its satisfactory performance validated against four representative benchmark functions. The work carries on with the necessary thorough investigation of the impact of the different genetic operators employed on the ability of the algorithm to find the global extremities in an effective and efficient manner. This deems fundamental to guarantee that the algorithm is not trapped in local extremities. The algorithm is then coupled with a compressor performance prediction tool that evaluates each individualʼs performance through a user defined fitness function. The most flow efficient schedule that conforms to a prescribed surge margin can be obtained thereby fast and inexpensively. Results are produced for a modern eight stage high bypass ratio compressor and compared with experimental data available to the research. The study concludes with the analysis of the existent relationship between surge margin and flow efficiency for the particular compressor under scrutiny. The study concludes with the analysis of the existent relationship between surge margin and flow efficiency for the particular compressor under scrutiny.

Real time simulation of medium size gas turbines

A tool for gas turbine dynamic simulation is presented in this work. First, a thermodynamic model of the engine is presented, capable of predicting engine performance at full and part load conditions. This model considers modern features as variable geometry compressor and air cooled turbine expansion. Then, two different control techniques are assessed in order to control the engine during transient operation. After analyzing the stability and robustness of both controllers, one of them is selected due to its faster response and better stability. The complete model is validated and then applied to two different transient conditions. First, some perturbations are applied in order to see how the control system is able to compensate these abnormal situations in order to maintain the frequency of the shaft. Then, a ramp up from idle to peak load is analyzed. This last study is aimed at assessing the stability of the model for the widest possible load range. According to the results shown, the stability and robustness of the model is satisfactory.

Mathematical modeling and simulation of a cogeneration plant

This work aims to develop mathematical models to simulate a single shaft gas turbine based cogeneration plant with variable geometry compressor. At off-design the variable vanes are re-staggered to improve the cogeneration performance. Two modes of operation are identified with the first mode being for part load of less than 50% running to meet the part load demand. This is achieved by controlling the fuel flow and air bleeding at the downstream of the compressor to avoid surge formation. The second mode of operation is for part load greater than 50%. It is running to meet both the part load demand and the exhaust gas temperature set value by simultaneously regulating the fuel flow and the variable vanes opening. To accommodate change of compressor parameters during variable vanes re-stagger correction coefficients are introduced. A behavior of a 4.2 MW power generating cogeneration plant is simulated. The effect of variation of power and ambient temperature on cogeneration parameters like fuel consumption, temperature, pressure, variable vanes opening, efficiency and steam generated is studied. Comparison between the field data and the simulation results is in good agreement. To support the calculations required for off-design analysis, a computer program is developed in MatLAB environment.

Methodology for gas turbine performance improvement using variable-geometry compressors and turbines

Poor part-load performance is a well-known undesirable characteristic of gas turbines. Running off-design, both compressor and turbine lose performance. Flow misalignment at the various rows causes losses to increase sharply, thereby decreasing net output faster than decreasing fuel consumption. To bring the flow to alignment with the blade passages, it is required to restagger the blades both at the compressor and at the turbine. To avoid mechanical complexities, it is generally accepted to restagger only the stators. This work deals with a numerical approach to the simulation of a gas turbine equipped with variable stators at the compressor and at the turbine, enabling the search for better-performance operation. A computer program has been developed to simulate virtually any gas turbine having variable stators at the compressor stages and turbine nozzle guide vanes. Variable-inlet guide vanes (VIGVs), variable-stator vanes (VSVs), variable-nozzle guide vanes (VNGVs), variable-geometry compressors (VGCs) and variable-geometry turbines (VGTs) are the focus in this work, which analyses a one-shaft free power turbine for power generation in the search for performance improvement at part load.

The Use of Neural Nets for Matching Fixed or Variable Geometry Compressors With Diesel Engines

A technique which uses trained neural nets to model the compressor in the context of a turbocharged diesel engine simulation is introduced. This technique replaces the usual interpolation of compressor maps with the evaluation of a smooth mathematical function. Following presentation of the methodology, the proposed neural net technique is validated against data from a truck type, 6-cylinder 14-liter diesel engine. Furthermore, with the introduction of an additional parameter, the proposed neural net can be trained to simulate an entire family of compressors. As a demonstration, a family of compressors of different sizes is represented with a single neural net model which is subsequently used for matching calculations with intercooled and nonintercooled engine configurations at different speeds. This novel approach readily allows for evaluation of various options within a wide range of possible compressor configurations prior to prototype production. It can also be used to represent the variable geometry machine regardless of the method used to vary compressor characteristics. Hence, it is a powerful design tool for selection of the best compressor for a given diesel engine system and for broader system optimization studies.

The Effect of Vaneless Diffuser Geometry on the Surge Margin of Turbocharger Compressors

A study into the effect of two methods of changing the geometry of a vaneless diffuser on the performance of the compressor of a road haulage diesel engine turbocharger is described. The development of compressor variable geometry will enable the full potential of variable geometry turbines to be realized. This will give a more flexible power unit which will provide, for example, better low-speed torque and hence a smaller gearbox, and shorter journey times or larger payloads than are currently the practice. The disadvantages are added complexity and cost in the relatively simple turbocharger, and the need for an engine management system. The latter is currently being implemented on many vehicles to meet tight emissions regulations in Europe and elsewhere, and is thus not a drawback limited to variable geometry turbocharging. A compressor test facility, including appropriate instrumentation and a computer-based data-acquisition system, was constructed with the specific aim of investigating the unstable flow regime prior to and including surge. Alternative fixed vaneless diffuser geometries were designed to simulate a variable geometry diffuser which could be achieved through a flexing diffuser wall and a sliding throttle ring. Both the converging wall and throttle ring arrangement moved the peak pressure ratio to lower flowrates, and at the near surge flowrates (where the device would be introduced, when operating in a variable geometry mode) improvements in both pressure ratio and efficiency are shown. While the converging wall concept exhibited slightly better aerodynamic performance than the throttle ring, it has implementation difficulties with respect to material integrity under continuous flexing when developed to a fully variable geometry device. The simplicity of the sliding throttle ring makes it a more viable proposition. Prototype variable geometry (VG) devices have been constructed with a view to further rig and engine testing.

Health Monitoring of Variable Geometry Gas Turbines for the Canadian Navy

The Canadian Department of National Defence has identified a need for improved Engine Health Monitoring procedures for the new Canadian Patrol Frigate (CPF). The CPF propulsion system includes two General Electric LM2500 gas turbines, a high-pressure-ratio engine with multiple stages of compressor variable geometry. A general method for predicting the thermodynamic performance of variable geometry axial compressors has been developed. The new modeling technique is based on a meanline stage-stacking analysis and relies only on the limited performance data typically made available by engine manufacturers. The method has been applied to the LM2500-30 marine gas turbine and the variations in engine performance that can result from a malfunction of the variable geometry system in service have been estimated.