Vassilios Pachidis

A Fully Integrated Approach to Component Zooming Using Computational Fluid Dynamics

Background . This study focuses on a simulation strategy that will allow the performance characteristics of an isolated gas turbine engine component, resolved from a detailed, high-fidelity analysis, to be transferred to an engine system analysis carried out at a lower level of resolution. This work will enable component-level, complex physical processes to be captured and analyzed in the context of the whole engine performance, at an affordable computing resource and time. Approach . The technique described in this paper utilizes an object-oriented, zero-dimensional (0D) gas turbine modeling and performance simulation system and a high-fidelity, three-dimensional (3D) computational fluid dynamics (CFD) component model. The work investigates relative changes in the simulated engine performance after coupling the 3D CFD component to the 0D engine analysis system. For the purposes of this preliminary investigation, the high-fidelity component communicates with the lower fidelity cycle via an iterative, semi-manual process for the determination of the correct operating point. This technique has the potential to become fully automated, can be applied to all engine components, and does not involve the generation of a component characteristic map. Results . This paper demonstrates the potentials of the “fully integrated” approach to component zooming by using a 3D CFD intake model of a high bypass ratio turbofan as a case study. The CFD model is based on the geometry of the intake of the CFM56-5B2 engine. The high-fidelity model can fully define the characteristic of the intake at several operating condition and is subsequently used in the 0D cycle analysis to provide a more accurate, physics-based estimate of intake performance (i.e., pressure recovery) and hence, engine performance, replacing the default, empirical values. A detailed comparison between the baseline engine performance (empirical pressure recovery) and the engine performance obtained after using the coupled, high-fidelity component is presented in this paper. The analysis carried out by this study demonstrates relative changes in the simulated engine performance larger than 1%. Conclusions . This investigation proves the value of the simulation strategy followed in this paper and completely justifies (i) the extra computational effort required for a more automatic link between the high-fidelity component and the 0D cycle, and (ii) the extra time and effort that is usually required to create and run a 3D CFD engine component, especially in those cases where more accurate, high-fidelity engine performance simulation is required.

Towards a full two dimensional gas turbine performance simulator

In commercially available gas turbine performance simulation tools, individual engine components are typically represented with non-dimensional maps of experimental or default data. In those cases where actual component characteristics are not available and default characteristics are used instead, conventional tools can deviate substantially at off-design and transient conditions. Similarly, when real component characteristics are available, conventional engine cycle simulation tools can not predict the performance of the engine at other than nominal conditions satisfactorily, or account for the impact of changes in component geometry. This study looked into the full integration of two-dimensional streamline curvature component models with a low fidelity cycle program. Firstly, the obtained engine performance was compared against the one calculated based on default component characteristics. As a second case study, a range of flight Mach numbers and angles of attack were examined together with the effect of three different intake lip geometries on the performance of a notional, two-spool, low-bypass ratio, military engine. Two-dimensional models were used in the engine cycle analysis to provide a more accurate, physics- and geometry-based estimate of intake and fan performances. The analysis carried out by this study demonstrated relative changes in the predicted engine performance larger than 1%. For briefness, representative results are presented and discussed in this paper for one flight Mach number and angle of attack setting. More importantly, this research effort established the necessary methodology and technology required towards a full, two-dimensional engine cycle analysis at an affordable computational resource in the very short term.

Experimental and Numerical Investigation of a Compressor Cascade at Highly Negative Incidence

Abstract: The performance prediction of axial flow compressors and turbines still relies on the stationary testing of blade cascades. Most of the blade testing studies are done for operating conditions close to the design point or in off-design areas not too far from it. However, blade-and consequently engine-performance remain unexplored at relatively far off-design conditions, such as windmilling or sub-idle. Such regimes are dominated by blade operation under extremely low mass flows and rotational speeds that imply highly negative values of incidence angle, thus totally separated flows on the pressure side of the blades. Those flow patterns are difficult to be measured and even more difficult to be numerically predicted as the current modelling capability of separated internal flows is of limited reliability. In this paper, the performance of a 3-dimensional linear compressor cascade at highly negative incidence angle is initially experimentally investigated. The main objective of the study is to derive the total pressure loss and outlet flow angle through the blades and use the data for the validation-calibration of a numerical solver enhancing its capability to predict highly separated flows. The development of the CFD model and the simulation strategy followedare also presented.The numerical results are compared against the derived test data demonstrating a good agreement. In addition, most trends of the properties of interest have been captured sufficiently, therefore the physical phenomena are considered to be well captured, allowing the numerical tool to be used for further studies on similar test cases.

Development of a Two-Dimensional Streamline Curvature Code

Two-dimensional (2D) compressor flow simulation software has always been a very valuable tool in compressor preliminary design studies, as well as in compressor performance assessment operating under uniform and non-uniform inlet conditions. This type of software can also be used as a supplementary teaching tool. In this context, a new streamline curvature (SLC) software has been developed capable of analyzing the flow inside a compressor in two dimensions. The software was developed to provide great flexibility in the sense that it can be used as: (a) a performance prediction tool for compressors of a known design, (b) a development tool to assess the changes in performance of a known compressor after implementing small geometrical changes, (c) a design tool to verify and refine the outcome of a preliminary compressor design analysis, and (d) a teaching tool to provide the student with an insight of the 2D flow field inside a compressor and how this could be effectively predicted using the SLC method combined with various algorithms and cascade models. Apart from describing in detail the design, structure, and execution of the SLC software, this paper also stresses the importance of developing robust, well thought-out software and highlights the main areas a potential programmer should focus on in order to achieve this. This text also highlights the programming features incorporated into the development of the software in order to make it amenable for teaching purposes. The paper reviews in detail the set of cascade models incorporated for subsonic and supersonic flow, for design and off-design operating conditions. More-over, the methods used for the prediction of surge and choke are discussed in detail. The code has been validated against experimental results, which are presented in this paper together with the strong and weak points of this first version of the software and the potential for future development. Finally, an indicative case study is presented in which the shift of streamlines and radial velocity profiles is demonstrated under the influence of two sets of compressor inlet boundary conditions.

Prediction of Engine Performance Under Compressor Inlet Flow Distortion Using Streamline Curvature

Traditionally, engine performance has been simulated based on nondimensional maps for compressors and turbines. Component characteristic maps assume by default a given state of inlet conditions that cannot be easily altered in order to simulate two- or three-dimensional flow phenomena. Inlet flow distortion, for example, is usually simulated by applying empirical correction factors and modifiers to default component characteristics. Alternatively, the parallel compressor theory may be applied. The accuracy of the above methods has been rather questionable over the years since they are unable to capture in sufficient fidelity component-level, complex physical processes and analyze them in the context of the whole engine performance. The technique described in this paper integrates a zero-dimensional (nondimensional) gas turbine modelling and performance simulation system and a two-dimensional, streamline curvature compressor software. The two-dimensional compressor software can fully define the characteristics of any compressor at several operating conditions and is subsequently used in the zero-dimensional cycle analysis to provide a more accurate, physics-based estimate of compressor performance under clean and distorted inlet conditions, replacing the default compressor maps. The high-fidelity, two-dimensional compressor component communicates with the lower fidelity cycle via a fully automatic and iterative process for the determination of the correct operating point. This manuscript firstly gives a brief overview of the development, validation, and integration of the two-dimensional, streamline curvature compressor software with the low-fidelity cycle code. It also discusses the relative changes in the performance of a two-stage, experimental compressor with different types of radial pressure distortion obtained by running the two-dimensional streamline curvature compressor software independently. Moreover, the performance of a notional engine model, utilizing the coupled, two-dimensional compressor, under distorted conditions is discussed in detail and compared against the engine performance under clean conditions. In the cases examined, the analysis carried out by this study demonstrated relative changes in the simulated engine performance larger than 1%. This analysis proves the potential of the simulation strategy presented in this paper to investigate relevant physical processes occurring in an engine component in more detail, and to assess the effects of various isolated flow phenomena on overall engine performance in a timely and affordable manner. Moreover, in contrast to commercial computational fluid dynamics tools, this simulation strategy allows in-house empiricism and expertise to be incorporated in the flow-field calculations in the form of deviation and loss models.

A Multidisciplinary Approach for the Comprehensive Assessment of Integrated Rotorcraft–Powerplant Systems at Mission Level

This paper presents an integrated methodology for the comprehensive assessment of combined rotorcraft–powerplant systems at mission level. Analytical evaluation of existing and conceptual designs is carried out in terms of operational performance and environmental impact. The proposed approach comprises a wide-range of individual modeling theories applicable to rotorcraft flight dynamics and gas turbine engine performance. A novel, physics-based, stirred reactor model is employed for the rapid estimation of nitrogen oxides (NOx) emissions. The individual mathematical models are implemented within an elaborate numerical procedure, solving for total mission fuel consumption and associated pollutant emissions. The combined approach is applied to the comprehensive analysis of a reference twin-engine light (TEL) aircraft modeled after the Eurocopter Bo 105 helicopter, operating on representative mission scenarios. Extensive comparisons with flight test data are carried out and presented in terms of main rotor trim control angles and power requirements, along with general flight performance charts including payload-range diagrams. Predictions of total mission fuel consumption and NOx emissions are compared with estimated values provided by the Swiss Federal Office of Civil Aviation (FOCA). Good agreement is exhibited between predictions made with the physics-based stirred reactor model and experimentally measured values of NOx emission indices. The obtained results suggest that the production rates of NOx pollutant emissions are predominantly influenced by the behavior of total air inlet pressure upstream of the combustion chamber, which is affected by the employed operational procedures and the time-dependent all-up mass (AUM) of the aircraft. It is demonstrated that accurate estimation of on-board fuel supplies ahead of flight is key to improving fuel economy as well as reducing environmental impact. The proposed methodology essentially constitutes an enabling technology for the comprehensive assessment of existing and conceptual rotorcraft–powerplant systems, in terms of operational performance and environmental impact. [DOI: 10.1115/1.4028181]

Simulation Framework Development for Helicopter Mission Analysis

Helicopter mission performance analysis has always been an important topic for the helicopter industry. This topic is now raising even more interest as aspects related to emissions and noise gain more importance for environmental and social impact assessments. The present work illustrates the initial steps of a methodology developed in order to acquire the optimal trajectory of any specified helicopter under specific operational or environmental constraints. For this purpose, it is essential to develop an integrated tool capable of determining the resources required (e.g. fuel burnt) for a given helicopter trajectory, as well as assessing its environmental impact. This simulation framework tool is the result of a collaborative effort between Cranfield University (UK), National Aerospace Laboratory NLR (NL) and LMS International (BE). In order to simulate the characteristics of a specific trajectory, as well as to evaluate the emissions that are produced during the helicopter’s operation within the trajectory, three computational models developed at Cranfield University have been integrated into the simulation tool. These models consist of a helicopter performance model, an engine performance model and an emission indices prediction model. The models have been arranged in order to communicate linearly with each other. The linking has been performed with the deployment of the OPTIMUS process and simulation integration framework developed by LMS International. The optimization processes carried out for the purpose of this work have been based on OPTIMUS’ built-in optimizing algorithms. A comparative evaluation between the optimized and an arbitrarily defined baseline trajectory’s results has been waged for the purpose of quantifying the operational profit (in terms of fuel required) gained by the helicopter’s operation within the path of an optimized trajectory for a given constraint. The application of the aforementioned methodology to a case study for the purpose of assessing the environmental impact of a helicopter mission, as well as the associated required operational resources is performed and presented.Copyright

Simulation Framework Development for Aircraft Mission Analysis

Since the very beginning of first commercial flight operations, aircraft mission analysis has played a major role in minimizing costs, increasing performances and satisfying regulations. The operational trajectory of any aircraft must comply with several constraints that need to be satisfied during its operation. The nature of these constraints can vary from Air Traffic Control (ATC) regulations, to emissions regulations and any combination between these two. The development of an integrated tool capable of determining the resources required (fuel and operational time) for a given aircraft trajectory, as well as assessing its environmental impact, is therefore essential. The present work illustrates the initial steps of a methodology developed in order to acquire the optimal trajectory of any specified aircraft under specific operational or environmental constraints. The simulation framework tool is the result of a collaborative effort between Cranfield University (UK), National Aerospace Laboratory NLR (NL) and LMS International (BE). With this tool, the optimal trajectory for a given aircraft can be computed and its environmental impact assessed. In order to simulate the characteristics of a specific trajectory, as well as to evaluate the emissions that are produced during the aircraft operation within it, three computational models developed at Cranfield University have been integrated into the simulation tool. These models consist of an aircraft performance model, an engine performance model and an emission indices model. The linking has been performed with the deployment of the OPTIMUS process and simulation integration framework developed by LMS International. The optimization processes carried out were based on OPTIMUS’ built-in optimizing algorithms. A comparative evaluation between an arbitrarily defined baseline trajectory and optimized ones has been waged for the purpose of quantifying the operational profit (in terms of fuel required or operational time) gained by the aircraft operation within the path of an optimized trajectory. Trade-off studies between trajectories optimized for different operational and environmental constraints have been performed. The results of the optimizations revealed a substantial margin available for reduction in fuel consumption as well as required operational time compared to a notional baseline. The optimal trajectories for minimized environmental impact in terms of produced emissions have been acquired and their respective required resources (fuel required and operational time) have been evaluated.© 2010 ASME

Advanced Performance Simulation of a Turbofan Engine Intake

Introduction T HE majority of today’s engine simulation software is of a low fidelity (nondimensional). Such tools can offer a good prediction of the performance of a whole engine but are incapable of analyzing the performance of individual engine components in detail, or capturing extreme/complex physical phenomena, that is, inlet flow distortion. On the other hand, computational fluid dynamics (CFD) tools can predict the performance of individual engine components satisfactorily, especially close to design point (DP) operating conditions, but do not offer whole engine performance prediction. Two issues prevent modeling the entire geometry of a propulsion system at the highest level of resolution (three dimensional) from being a practical solution. First, for a complete three-dimensional system simulation, the amount and level of detailed information needed as boundary and initial conditions would be extremely difficult to obtain. Second, the computational time and cost will be extremely high for effective and practical use.1,2

Effect of Combustor Geometry on Performance of Airblast Atomizer Under Sub-Atmospheric Conditions

Abstract One of the certification requirements that a jet engine has to fulfil is its altitude relight capability. Relighting an aero gas turbine engine at high altitudes is more challenging than at sea level conditions. The pressure, air velocity, and temperature within the combustor at such conditions are very low, hindering the fuel atomization and evaporation process. After ignition, combustion efficiency can be relatively low due to the poor fuel atomization quality, leading to slow shaft acceleration rates. Further studies in this field can help determine and predict the fuel spray characteristics, which limit the relight and pull-away capability of the engine at these sub-idle conditions. Reported in this paper is the CFD analysis of a typical airblast atomizer, simulated at different sub-idle operating conditions. Two sets of models were used; one with a simple liner-only combustor with co-flow air, and the other with a more detailed geometry, including wall cooling slots, primary and secondary dilution holes, and co-flow air. For the simpler model, three different inner and outer liner wall spacing were modelled to examine the effect of the chamber volume on the fuel spray behaviour. The CFD models were then run at a chamber pressure of 101, 41 and 3 1 kPa, typical of sub-idle conditions. The effect of such conditions on the atomization quality of the fuel spray was analysed. The study carried out indicates how the chamber pressure, chamber volume and AFR (through amount of co-flow air introduced) significantly affect the resulting spray characteristics. A parametric analysis was performed to extract a correlation between the spray SMD (Sauter Mean Diameter) and fuel flow rate.