Modelling of behaviour of metals at high strain rates

The aim of the work presented in this thesis was to produce the improvement of the existing simulation tools used for the analysis of materials and structures, which are dynamically loaded and subjected to the different levels of temperatures and strain rates. The main objective of this work was development of tools for modelling of strain rate and temperature dependant behaviour of aluminium alloys, typical for aerospace structures with pronounced orthotropic properties, and their implementation in computer codes. Explicit finite element code DYNA3D has been chosen as numerical test-bed for implementation of new material models. Constitutive model with an orthotropic yield criterion, damage growth and failure mechanism has been developed and implemented into DYNA3D. Second important aspect of this work was development of relatively simple experimental methods for characterization of engineering materials, and extensive experimental work has been undertaken. Tensile test has been used for the characterisation of two aluminium alloys, at different levels of the strain rates and temperatures, and for three different orientations of materials. The results from these tests allowed derivation of material constants for constitutive models and lead to a better understanding of aluminium alloy behaviour. Procedures for derivation of parameters for temperature and strain rate dependant strength models were developed and parameters for constitutive relations were derived on the basis of uniaxial tensile tests. Taylor cylinder impact test was used as a validation experiment. This test was used to validate the implementation, and accuracy of material model in computer code. At the end of each incremental development, validation of the constitutive material model has been performed through numerical simulations of Taylor cylinder impact test, where simulation results have been compared with the experimental post-test geometries in terms of major and minor side profiles and impact-interface footprints. Plate impact test has been used to determine the material properties at high strain rate, and to investigate damage evolution in impact-loaded material. Initially the material model has been designed as a temperature and strain rate dependant strength model in a simple isotopic form, which then has been tested and verified against the experimental results. Coupling of the Hill’s orthotropic yield criterion with isotropic, temperature and strain rate dependant, hardening material model, has been chosen to suit the orthotropic behaviour. Method for calibration of orthotropic yield criterion has been developed and parameters have been identified for the orthotropic model under the associated flow rule assumption and in case of plane stress on the basis of tensile and cylinder impact tests. The complexity of the model has been further increased through coupling of hardening model with orthotropic yield criterion including damage evolution and failure criteria. The constitutive model was developed within the general framework of continuum thermodynamics for irreversible processes, and plate impact test and tensile tests have been used for determination of parameters for damage part of the new material model

GAS TURBINE PERFORMANCE DIGITAL TWIN FOR REAL-TIME EMBEDDED SYSTEMS

This contribution reports on the development of Performance Digital Twin for industrial Small Gas Turbines. The objective of this study was the development of automation systems with control and monitoring functionalities, capable of addressing the requirements of future gas turbine plants for increased availability and reliability by use of Digital Twin technology. The project explored development of Performance Digital Twin based on Real-Time Embedded computing, which can be leveraged with Internet-of-Things (IOT) Cloud Platforms. The proposed solution was provided in a form of modular software for a range of hardware platforms, with corresponding functionalities to support advanced control, monitoring, tracking and diagnostics strategies. The developed Digital Twin was designed to be used in off-line mode to assist the software commissioning process and in on-line mode to enable early detection of degradation and fault modes typical for gas path components. The Performance Digital Twin is based on a dynamic gas turbine model which was augmented with a Kalman tuner to enable performance tracking of physical assets. To support heterogeneity of gas turbine Distributed Control Systems (DCS), this project explored deployment of Digital Twin on multiple platforms. In the paper, we discuss model-based design techniques and tools specific for continuous, discrete and hybrid systems. The hybrid solution was deployed on PC-based platform and integrated with engine Distributed Control System in the field. Monitoring of gas turbine Performance Digital Twin functionalities has been established via Remote Monitoring System (STA-RMS). Assessment of deployed solution has been carried out and we present results from the field trial in this paper. The discrete solution was deployed on a range of Programable Logical Controller (PLC) platforms and has been tested by integrating Digital Twin in virtual engine Distributed Control System network. The Performance Digital Twin was embedded in Single Master PLC and Master-Slave PLC configurations, and we present results from the system testing using virtual gas turbine assets. The IoT Platform MindSphere was integrated within virtual engine network, and in this contribution, we explore expansion of the developed system with Cloud based applications and services

Distributed Network System for Real-Time Model Based Control of Industrial Gas Turbines

This paper describes the development of a distributed network system for real-time model based control of industrial gas turbine engines. Distributed control systems contribute toward improvements in performance, testability, control system maintainability and overall life-cycle cost. The goal of this programme was to offer a modular platform for improved model based control system. Hence, another important aspect of this programme was real-time implementation of non-linear aero-thermal gas turbine models on a dedicated hardware platform. Two typical applications of real-time engine models, namely hardware-in-the-loop simulations and on-line co-simulations, have been considered in this programme. Hardware-in-the-loop platform has been proposed as a transitional architecture, which should lead towards a fully distributed on-line model based control system. Distributed control system architecture offers the possibility of integrating a real-time on-line engine model embedded within a dedicated hardware platform. Real-time executing models use engine operating conditions to generate expected values for measured and non-measured engine parameters. These virtual measurements can be used for the development of model based control methods, which can contribute towards improvements in engine stability, performance and life management. As an illustration of model based control concept, the example of gas turbine transient over-temperature protection is presented in this study

Quick Start of an Industrial Gas Turbine Engine through the Development of “Silent Start” VGV Schedule

In this paper, research that was carried out to optimise an initial variable guide vane schedule of a high-pressure ratio, multistage axial compressor is reported. The research was carried out on an extensively instrumented scaled compressor rig. The compressor rig tests carried out employing the initial schedule identified regions in the low speed area of the compressor map that developed rotating stall. Rotating stall regions that caused undesirable non-synchronous vibration of rotor blades were identified. The variable guide vane schedule optimisation carried out balancing the aerodynamic, aeromechanical and blade dynamic characteristics gave the ‘Silent Start’ variable guide vane schedule, that prevented the development of rotating stall in the start regime and removed the non-synchronous vibration. Aerodynamic performance and aero-mechanical characteristics of the compressor when operated with the initial schedule and the optimised ‘Silent Start’ schedule are compared. The compressor with the ‘Silent Start’ variable guide vane schedule when used on a twin shaft engine reduced the start time to minimum load by a factor of four and significantly improved the operability of the engine compared to when the initial schedule was used

Integrated model-based control and health management for industrial gas turbines

This paper describes technology programme that considered development of closely integrated gas turbine control and health monitoring system. This programme addresses the current requirements of gas turbine engines for increased availability, reliability and reduced life-cycle cost. Unified control and health management framework was formulated considering objective such as: increased flexibility and optimized asset management of industrial gas turbines

Modelling and simulation of transient thermal loading in a gas turbine

The nozzle guide vane of the gas turbine is considered to be one of the most critical engine components from a thermal stress point of view. Hot gases from the combustor exhaust heat the outer surfaces of the vane, and because the turbine vane is cooled internally by air diverted from the compressor, a temperature gradient results between the inner and outer surfaces of the vane. These high gradients often make the nozzle guide vane the most likely engine component to fail due to low cycle fatigue. During engine transients each engine acceleration and deceleration induces a cycle of thermal stress, which can eventually cause component failure. Therefore it is very important to predict the thermal loading of turbine components and assess how that can affect component life. For that purpose a heat transfer model for a cooled turbine component has been developed taking advantage of simplifications in heat flow analysis. Numerical simulations have been carried out using the generic gas turbine simulation tool GasTurboLib. This Simulink library enables 0-D modelling, which is the simplest level of modelling, but most widely used in industry. The developed model of a cooled turbine has been implemented into turbine block of GasTurboLib library, enabling simulation and analysis of heat transfer effects within turbine components in more details. The transient behaviour of a gas turbine during full load acceptance has been simulated to validate the proposed model against engine test data, and good agreement with test results has been observed. Analysis of thermal loading for a turbine component during engine rapid transients has been carried out and numerical results are presented in this paper

MODEL-BASED CONTROL AND DIAGNOSTIC TECHNIQUES FOR OPERATIONAL IMPROVEMENTS OF GAS TURBINE ENGINES

Gas turbines operational requirements continue to become more demanding in response to the need for extended component life, increased reliability and improved overall efficiency. To support these requirements, new model-based gas turbine control and diagnostics concepts have been introduced. Traditionally gas turbine control system transforms real engine limits, into limits which are based on measured engine variables. As a result of that, engines operate with increased safety margins and thus with non-optimal performance. To overcome this problem model based control concepts have been proposed. Model based control approach exploits real-time on-line engine models to estimate control feedback signals, enabling the implementation of novel control methods. Model-based diagnostics employs engine models tuned to match the observed engine state in the same manner as model-based control. The residual deviations between predicted and sensed parameters are modelled, again usually as variations in component losses and flow capacity, and the best match is used to identify likely component degradation modes and faults. The use of model based techniques to diagnose and adaptively manage degradation of engine component characteristics is crucial for operational effectiveness of gas turbines. This paper gives overview of current and evolving model-based techniques and discusses benefits of these concepts in operational management of the gas turbines

Dynamic Simulation of Active Compressor Stability Control for a Gas Turbine Engine

The type of fuel, ambient conditions and gas turbine engine variations are among many variables, which determine the amount of metered fuel required for efficient and reliable gas turbine start-up with minimized thermal stresses and without compressor instabilities. Hence, scheduling of fuel for gas turbine engine start-up without dynamic adjustments for unpredictable influencing variables cannot assure reliable start-up. To address this variability, implementation of the active compressor stability control for the start-up scheduling was considered in this study. Generic active control method has been proposed, and this method can be applied on both open and closed start-up scheduling loops. Active control philosophy is based on the control of stability margin using corrective action to avoid or recover engine from compressor instabilities. Active control method is designed to sense incipient stall using stall detection method and subsequently to initiate engine control system corrective action to avoid compressor instabilities by adjusting the fuel flow schedule. Centre casing dynamic pressure signal is used for synthesis of fast and reliable measure of compressor destabilization. When compressor instability is detected, engine control system initiates adjustment of fuel flow schedule defined by corrective function, which is based on the synthesized measure of compressor destabilization. To assess applicability of proposed control method, dynamic simulations of engine start-up have been carried out using generic gas turbine simulation tool GasTurboLib. Nonlinear mathematical model of transient compressor dynamics has been developed to describe instability behaviour of axial compressor. During simulation of start-up sequence, compressor instabilities have been induced to study respond of proposed active control method. Description of implemented compressor model and numerical simulation results are given in this paper

GASTURBOLIB – SIMULINK LIBRARY FOR GAS TURBINE ENGINE MODELLING

A new Simulink library, called GasTurboLib, containing blocks specialized for gas turbine modelling has been developed. Different engine configurations can be generated using GasTurboLib components and these models can be used for steady state and transient performance analysis. This paper describes the newly developed generic gas turbine simulation tool and presents experiences with modelling and simulation of single and twin shaft gas turbine engines. This library enables 0-D modelling, which is the simplest level of modelling but the most widely used in industry. This component based modelling environment can be used to simulate start-up sequence, load change, control system design, power-system stability studies and real-time modelling. Traditionally, control method improvements are developed and validated through engine testing. The goal was to develop a functional engine model, which can be started, operated and shut down by a governor model, for the purposes of development of control methods and protection algorithms, thus providing considerable cost savings, as well as enabling better project progress through independence from the availability of test beds. It has been demonstrated that rapid model generation and reusability of components along with user-friendly graphical user interface make this simulation environment a valuable tool for gas turbine system performance analysis

NUMERICAL SIMULATION OF PARTIAL FLAME FAILURE IN GAS TURBINE ENGINE

A mathematical model for the simulation of engine start-up thermodynamics has been developed and validated against engine test data. This numerical model has been validated using engine test results for both single and multiple combustor flameouts, and reasonable agreement between test and simulation data has been observed. Numerical simulations have then been generated for flameout cases that have not been available from engine tests, such as flame failure in different combinations of combustors, and at different engine operating conditions. The mathematical model features object modeling of engine components with three gas compositions, being air, fuel, and combustion products. The combustion system has been represented by six combustors, and the gas stream from each combustor has been divided according to the number of the gas path thermocouples downstream from the combustion system. The effects of heat transfer within the combustors and turbine have been modeled. Two sets of thermocouples have been considered, the first being thermocouples installed in multiple combustor burners, and the second being an array of thermocouple probes which are circumferentially positioned in the engine hot gas path. All thermocouples have been modeled as first order dynamic systems. The numerical simulations have been successfully used to support development of a new partial flame failure detection method, which is based on the combined measurements from both sets of thermocouples. A range of numerical simulations have been conducted in order to assess the ability of this new detection algorithm to detect different partial flame failure scenarios, and to examine the sensitivity of the detection algorithm with respect to thermocouples faults