High Bypass Ratio Research Articles

A Calculation Tool of a Turbine Cooling Air Schedule for General Gas Turbine Simulation Algorithms

A tool to evaluate the turbine cooling air schedule during the flight path is presented. The method is suitable for complex gas turbine engine models where accurate calculation of thermodynamic parameters of a cooling flow used for studies such as specific fuel consumption (SFC), lifing, and operating cost analysis. The method is also applicable for land-based gas turbine engines where the flight path is replaced by load pattern. The method can be employed even if only a little information about the engine is known. The calculation method has been investigated on a model of a high bypass ratio turbofan for long haul application. Using it with an aircraft model and after comparing to conventional cooling air simulations, the reduction of SFC during the flight path has been observed. A comparison has been performed on engine manufacturer’s public domain data where the method shows comparable value of total cooling air needed. The tool returns similar results but estimates a higher proportion of cooling air for the high pressure turbine nozzle guide vanes (NGV) compared with Young and Wilcock’s method.

Applying heat pipes to a novel concept aero engine: Part 2 – Design of a heat-pipe heat exchanger for an intercooled-recuperated aero engine

AbstractWith the ever-increasing pressure for cleaner and more fuel efficient aero engines, gas turbine manufacturers are faced with a big challenge which they are bound to accept and act upon. The path from current high bypass ratio (BPR) engines to ultra high BPR engines via geared turbo fans will enable a significant reduction in SFC and CO2 emissions. However, in order to reach the emission levels set by the advisory council for aeronautics research in Europe (ACARE), the introduction of more complex cycles that can operate at higher thermal efficiencies is required. Studies have shown that one possibility of achieving higher core efficiencies and hence lower SFC is through the use of an intercooled recuperated (ICR) core. The concept engine, expected to enter into service around 2020, will make use of a conventional fin plate heat exchangers (HEX) for the intercooler and a tube type HEX as the recuperator. Although the introduction of these two components promises a significant reduction in SFC levels, they will give also rise to higher engine complexity, pressure losses and additional weight. Thus, the performance of the engine relies not only on the behaviour of the usual gas turbine components, but will be heavily dependent on the two heat exchangers. This paper seeks to introduce a heat pipe heat exchanger (HPHEX) as alternative designs for the intercooler and the recuperator. The proposed HPHEX designs for application in an ICR aero engine take advantage of the convenience of the geometry of miniature heat pipes to provide a reduction in pressure losses and weight when compared to conventional HEX. The proposed HPHEX intercooler design eliminates any ducting to and from the intercooler, offering up to 32% reduction in hot pressure losses, 34% reduction in cold pressure losses and over 41% reduction in intercooler weight. On the other hand the proposed HPHEX recuperator design can offer 6% improvement in performance, while offering 36% reduction in cold pressure losses, up to 80% reduction in hot pressure losses and over 31% reduction in weight. An ICR using HPHEX for the intercooler and recueprator may offer up to 2·5% increase in net thrust, while still offering 3% reduction in SFC and up to 7·7% reduction in NOX severity parameter, when compared to the ICR using conventional HEX.

Development of a Method for Enhanced Fan Representation in Gas Turbine Modeling

A challenge in civil aviation future propulsion systems is expected to be the integration with the airframe, coming as a result of increasing bypass ratio or above wing installations for noise mitigation. The resulting highly distorted inlet flows to the engine make a clear demand for advanced gas turbine performance prediction models. Since the dawn of jet engine, several models have been proposed, and the present work comes to add a model that combines two well-established compressor performance methods in order to create a quasi-three-dimensional representation of the fan of a modern turbofan. A streamline curvature model is coupled to a parallel compressor method, covering radial and circumferential directions, respectively. Model testing has shown a close agreement to experimental data, making it a good candidate for assessing the loss of surge margin on a high bypass ratio turbofan, semiembedded on the upper surface of a broad wing airframe.

Next Generation of Transport Engines

This article discusses the features of very high bypass ratio turbofans and open rotor engines. Each of these engine options has its pros and cons to consider. The very large bypass ratio turbofan engine maintains that the proven capability of containment of blade failures is inherently quieter due to ability to incorporate acoustic treatment in the fan duct and is not subject to high fan tip losses associated with direct exposure to higher cruise level flight speeds. The duct does not come for free, however, and installed weight becomes a primary concern as the increased bypass ratio drives up the engine diameter. Additionally, the fan is subject to higher local airfoil incidence when the fan nozzle un-chokes at low flight speed. The open rotor engine can achieve potentially greater improvements in propulsive efficiency than a turbofan but lacks the containment and noise reduction benefits of a duct. The rotor is also exposed to flight speed, driving up tip losses at today's accepted cruise flight speeds.

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.

Internal Losses and Flow Behavior of a Turbofan Stage at Windmill

The flow through a high-bypass ratio fan stage during engine-out conditions is investigated, with the objective of quantifying the internal losses when the rotor is at “windmill.” An analysis of altitude test data at various simulated flight Mach numbers shows that the fan rotational speed scales with the engine mass flow rate. Making use of the known values of the nozzle coefficients, we deduce the stagnation pressure loss of the fan stage, which rises significantly as the mass flow rate increases. In order to better understand this behavior, numerical simulations of the fan stage were carried out. The calculated losses agree well with the test data, and it is found that the bulk of the stagnation pressure loss occurs in the stator. A detailed examination of the flow field reveals that the relative flow leaves the rotor at very nearly the metal angle. Moreover, the rotational speed of the fan is such that the inboard sections of the fan blade add work to the flow, while the outboard sections extract work from it. The overall work is essentially zero so that the absolute swirl angle at the rotor exit is small, causing the stator to operate at a severely negative incidence. A gross separation ensues, and the resulting blockage of the stator passage accelerates the flow to high Mach numbers. The highly separated flow in the vane, together with the mixing of the large wakes behind it are responsible for the high losses in the vane. Based on the simulation results for the flow behavior, a simple physical model to estimate the windmill speed of the rotor is developed and is found to be in good agreement with the test data. The utility of this model is that it enables the development of a procedure to predict the internal drag at engine-out conditions, which is discussed.

Almost 40 Years of Airframe Noise Research: What Did We Achieve?

With the advent of low noise high bypass ratio turbofan engines airframe noise gained significant importance with respect to the overall aircraft noise impact around airports. Already around 1970 airframe noise, originating from flow around the landing gears and high-lift devices, was recognized as a potential “lower aircraft noise barrier” at approach and landing. Since then, the outcome of extensive acoustic flight tests and aeroacoustic wind tunnel experiments enabled a detailed description and ranking of the major airframe noise sources and the development of noise reduction means. In the last decade advances in numerical and experimental tools led to a better understanding of complex noise source mechanisms. Efficient noise reduction technologies were developed for landing gears while the benefits of high-lift noise reduction means were often compensated by a simultaneous degradation in aerodynamic performance. The focus of this paper is not on the historical sequence of airframe noise research but rather aims to provide a concise survey of the achievements in airframe noise source description and reduction over the last 40 years worldwide. Due to the vast amount of work focused on a variety of airframe noise problems, this review can only provide examples but does not claim to be complete.

Conceptual Study of Future Aero-Engine Concepts

The soaring fuel price and the burgeoning environmental concerns have compelled global research towards cleaner engines, aimed at substantial reduction in emission, noise and fuel consumption. In this context, the present research investigates the feasibility of some novel engine concepts, namely Geared Turbofan and Intercooled Recuperated Turbofan concepts, by hypothetically applying them into an existing state-of-the-art high bypass ratio engine. This paper assesses the effects on the baseline engine performances due to the introduction of these two concepts in it. By performing steady state simulations, it was found that incorporation of the Geared Turbofan concept into the baseline Turbofan engine reduced thrust specific fuel consumption, engine weight, and fan blade tip speed. However, when simulations were carried out by incorporating the Intercooler and Recuperator concept in the baseline turbofan engine, it did not demonstrate any substantial improvement in fuel consumption. It was observed that the engine performance was influenced to a large extent by the heat exchanger’s effectiveness and the associated pressure drop. In addition, the overall engine weight increased due to the inclusion of massive heat exchangers necessary for intercooling and recuperating.

Modeling and prediction of fan noise

Fan noise is a significant contributor to the total noise signature of a modern high bypass ratio aircraft engine and with the advent of ultra high bypass ratio engines like the geared turbofan, it is likely to remain so in the future. As such, accurate modeling and prediction of the basic characteristics of fan noise are necessary ingredients in designing quieter aircraft engines in order to ensure compliance with ever more stringent aviation noise regulations. In this paper, results from a comprehensive study aimed at establishing the utility of current tools for modeling and predicting fan noise will be summarized. It should be emphasized that these tools exemplify present state of the practice and embody what is currently used at NASA and Industry for predicting fan noise. The ability of these tools to model and predict fan noise is assessed against a set of benchmark fan noise databases obtained for a range of representative fan cycles and operating conditions. Detailed comparisons between the predicted and measured narrowband spectral and directivity characteristics of fan nose will be presented in the full paper. General conclusions regarding the utility of current tools and recommendations for future improvements will also be given.

Buzz‐saw noise : propagation of shock waves in aero‐engine inlet ducts

When high bypass ratio aircraft engines run at takeoff operating conditions, blade relative flow velocities can exceed sonic speed, thus generating forward propagating shock waves that spiral inside the intake before being radiated. “Buzz‐saw” or “multiple pure tone” noise then occurs, and measured acoustic spectra close to the fan display tones at the blade passing frequency and its harmonics, along with those of the engine shaft rotation frequency. This work first attempts to reformulate McAlpine and Fisher's frequency domain model for the propagation of a sawtooth waveform spiralling inside a hard‐walled and lined cylindrical duct with uniform flow. The nondissipative Burgers equation is solved, and modal attenuation and dispersion are added using a split‐step computational method. In practice, shocks do not only occur at blade tips, but on a significant portion of the blade span. The plane wave hypothesis being no longer valid, a new three‐dimensional model is presented for a no‐flow case. This model ...

Overview of coupling noise prediction for turbofans with engine and aircraft performance

In this work, a comparison of the noise levels of different aero-engines is presented. The coupling is based on integrating a gas turbine design and performance model with an aircraft model for long and short hauls and a noise model made of established empirical methods available in the open literature for turbofan noise prediction at design and off-design points. After joining together the performance models with the noise methods, the authors have studied three ultra high bypass ratio engine configurations vis-à-vis their baseline counterpart of equivalent thrust rating. Also, the effect of application of passive noise attenuators (liners) to reduce the noise is explored. The results obtained showed that though there are derivable benefits from the ultra high bypass ratio turbofans, such as reduction in fuel consumption, in jet noise and reduction of broadband noise for the fan, the new engines generate higher noise at lower frequencies in the fan.

Investigating the emissions and performance of high bypass ratio aero-engines

In the last decades the development of new technologies and methods of analysis has brought a continuous improvement of the performance of aeronautical jet engines among which are reductions of polluting substances and noise. In this vein, and in accordance with the general attention given environmental issues by the scientific community and governments, a first analysis has been conducted to assess the potential of new types of engines to meet the green environmental expectations. The objects of the study are two powerplants, one with bypass ratio (BPR) 5, used as baseline, and the other with BPR 12. A parametric analysis of the performance and the emissions of these engines was performed using software which directly coupled models of engine performance, emissions, and aircraft operations. Design-point (take-off) and off-design (whole of flight-mission) cases have been considered in order to show the different scenarios and information they could present. Lefebvre equations were used and calibrated to match reasonably the measured NO x and CO emissions of current combustors. Calibration method and errors are presented to clarify the approach. Trends of NO x and CO2 emissions relative to different operating parameters are presented for both cases. The principal differences in emissions for engines of high BPR are shown to be higher values of NO x associated with the off-design case.

A de-coupled approach to component high-fidelity analysis using computational fluid dynamics

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. The technique described in this paper is called ‘de-coupled’ high-fidelity analysis and utilizes an object-oriented, zero-dimensional gas turbine modelling and performance simulation system and a three-dimensional computational fluid dynamics (CFD) component model. The technique involves the generation of a component characteristic map without the parallel or iterative execution of the non-dimensional cycle and the three-dimensional CFD model. Therefore, a faster high-fidelity engine performance simulation can be achieved at run time. This paper demonstrates the ‘de-coupled’ approach to component high-fidelity analysis by using a three-dimensional CFD intake model of a high by-pass ratio turbofan as a case study. The CFD model is based on the geometry of the intake of the CFM56-5B2 engine. The CFD-generated performance map can fully define the characteristics of the intake at several operating conditions and power settings, and is subsequently used to provide a more accurate, physics-based estimate of intake performance (i.e. pressure recovery) and hence, engine performance, replacing the default, empirical values within the non-dimensional cycle model. A detailed comparison between the baseline engine performance (empirical pressure recovery) and the engine performance obtained after using the CFD-generated map is presented in this paper. The analysis carried out by this study demonstrates relative changes in the simulated engine performance > 1 per cent.

Measurements of Intake Separation Hysteresis in a Model Fan and Nacelle Rig

A 1/20-scale, low speed model rig representing the fan and nacelle of a high bypass ratio jet engine has been tested under crosswind conditions. The flow conditions under which the intake flow separates and reattaches have been found to exhibit considerable hysteresis. This phenomenon has been examined by a careful test procedure in which the crosswind angle has been slowly increased and then decreased. Measurements of the hysteresis associated with separation and reattachment are presented for independent variations in stream-tube contraction ratio, ground clearance, fan operating point and Reynolds number. The results reveal that particular care must be taken to allow for any hysteresis when testing intakes under crosswind conditions. They also indicate that separation hysteresis is particularly sensitive to fan operating point and the position of the ground plane. These findings suggest that it is important for high Reynolds number intake tests and calculations to include a ground plane and a model of the downstream turbomachinery. © 2002 by the author(s).

Movement of Deposited Water on Turbomachinery Rotor Blade Surfaces

A theoretical approach for calculating the movement of liquid water following deposition onto a turbomachine rotor blade is described. Such a situation can occur during operation of an aero-engine in rain. The equation of motion of the deposited water is developed on an arbitrarily oriented plane triangular surface facet. By dividing the blade surface into a large number of facets and calculating the water trajectory over each one crossed in turn, the overall trajectory can be constructed. Apart from the centrifugal and Coriolis inertia effects, the forces acting on the water arise from the blade surface friction, and the aerodynamic shear and pressure gradient. Nondimensionalization of the equations of motion provides considerable insight and a detailed study of water flow on a flat rotating plate set at different stagger angles demonstrates the paramount importance of blade surface friction. The extreme cases of low and high blade friction are examined and it is concluded that the latter (which allows considerable mathematical generalization) is the most likely in practice. It is also shown that the aerodynamic shear force, but not the pressure force, may influence the water motion. Calculations of water movement on a low-speed compressor blade and the fan blade of a high bypass ratio aero-engine suggest that in low rotational speed situations most of the deposited water is centrifuged rapidly to the blade tip region.

The Effect of Stagger Variability in Gas Turbine Fan Assemblies

Fan blades of high bypass ratio gas turbine engines are subject to substantial aerodynamic and centrifugal loads, producing the well-known phenomenon of fan blade untwist. The accurate prediction of the running geometry, as opposed to the cold geometry at rest, is crucial in the assessment of aerodynamic performance, vibratory response, and noise production of the fan. The situation is further complicated by the fact that some geometric variation is inevitable even for the state-of-the-art manufacturing processes used. The aim of this paper is to investigate the effect of static stagger variability on the dynamic untwist behavior of fan assemblies. An aeroelastic model was used to show that under certain conditions the stagger pattern changes significantly, both in form and amplitude, relative to the static configuration. At other conditions, a strong correlation between the running and static patterns is demonstrated.

Status of duct liner technology for application to aircraft engine nacelles

The peculiar technical requirements that constrain the design of acoustic liners used to suppress turbo‐machinery noise emission from commercial aircraft engine nacelles continue to make the design of such liners a challenge. The need for increased liner efficiency and suppression bandwidth is particularly critical in the face of ever decreasing nacelle wall area available for liner treatments in modern high bypass ratio engines. This paper reviews some of the key advances in the enabling measurement technologies that have been critical to creating and validating impedance models for liner structures suitable for the harsh aeroacoustic environments encountered on the inside of nacelle inlets and exhaust ducts. Specifically, the paper describes recent advances and outstanding issues related to determining liner impedance beneath high speed grazing flows combined with high sound levels. Evolution of this technology has resulted from the need for more reliable/accurate impedance determinations, and from the impetus provided by modern data acquisition systems and computational methodologies. Outstanding issues include discrepancies between impedances as educed in different laboratories under similar aeroacoustic conditions. For single layer, perforate over honeycomb type structures, these discrepancies are mainly confined to the normalized resistance component and may be as large as 0.50.

05/00337 Gas-turbine diagnostics using artificial neuralnetworks for a high bypass ratio military turbofan engine: Joly, R. B. et al. Applied Energy, 2004, 78, (4), 397–418

05/00337 Gas-turbine diagnostics using artificial neuralnetworks for a high bypass ratio military turbofan engine: Joly, R. B. et al. Applied Energy, 2004, 78, (4), 397–418

Unsteady Aerodynamics and Aeroacoustics of a High-Bypass Ratio Fan Stage

A numerical investigation of the unsteady aerodynamics of a fan stage comprised of a transonic rotor, swept fan exit guide vane (FEGV), and low-pressure compressor inlet guide vane (IGV) is described, with emphasis on acoustics. It is shown that the effects of the two downstream stator rows on the time-mean blade flow field are negligible, permitting its investigation using isolated rotor calculations. Simulations of this type are carried out along the engine operating line to quantify the acoustic sources associated with the upstream shock field and wake turbulence-stator interaction. The shock noise achieves its maximum value near the flyover acoustic certification condition, while the wake turbulence is least at this condition owing to its proximity to the design point. The behavior of these noise sources is explained physically by carrying out a detailed examination of the rotor flow field. The unsteady interaction between the rotor and stator rows at a high-power setting is investigated next. It is shown that the time-mean IGV flow is significantly affected by this interaction. Moreover, the unsteady loading on the IGV is found to be large. The behavior of the upstream-propagating acoustic field generated by rotor-IGV interaction is examined. The interaction between the rotor and FEGV is found to be linear in nature. The FEGV surface unsteady pressure and far-field acoustic field behavior are investigated.

Numerical Simulation of Fore and Aft Sound Field of a Turbofan

A framework for predicting fan related noise from high bypass ratio engines is presented. The methodology accounts for both fore and aft radiation and includes the effects of liner treatment on the engine walls. Solutions are obtained through a combination of Euler and linearized Euler solvers, coupled with a Kirchhoff formulation for far-field noise prediction. The nacelle region is solved using the Euler solver, whereas the linearized Euler solver is used for the fan exhaust stage. Switching to the linearized Euler solver in the aft region has been necessary to overcome difficulties experienced due to the coupling of the acoustic modes with flow near the engine exhaust boundary and appearance of flow instabilities in the shear layer trailing the exhaust shroud. Results are presented for spinning modes from a turbofan engine with acoustic treatment.