Vishal Sethi

On Intercooled Turbofan Engines

Public awareness and political concern over the environmental impact of the growth in civil aviation over the past 30 years have intensified industry efforts to address CO2 emissions [5]. CO2 emissions are directly proportional to aircraft fuel burn and one way to minimise the latter is by having engines with reduced Specific Fuel Consumption (SFC) and installations that minimise nacelle drag and weight. Significant factors affecting SFC are propulsive efficiency and thermal efficiency. Propulsive efficiency has been improved by designing turbofan engines with bigger fans to give lower specific thrust (net thrust divided by fan inlet mass flow) until increased engine weight and nacelle drag have started to outweigh the benefits. Thermal efficiency has been improved mainly by increasing the Overall Pressure Ratio (OPR) and Turbine Entry Temperature (TET) to the extent possible with new materials and design technologies.

Performance characteristics and optimisation of a geared intercooled reversed flow core engine

Intercooled turbofan cycles allow higher overall pressure ratios to be reached, which gives rise to improved thermal efficiency. In addition, intercooling allows for the size, weight and exhaust jet velocity of the core to be reduced. For an optimum jet velocity ratio and fixed thrust, the fan pressure ratio and specific thrust are also reduced, which benefits propulsive efficiency. A new intercooled core concept is proposed in this paper, which promises to alleviate limitations identified in previous intercooled turbofan designs. This concept facilitates the installation of the intercooler and reduces core losses at high overall pressure ratios. This engine concept takes advantage of intercooling and the arrangement of the high pressure spool to reach and exceed overall pressure ratios of 80. In addition, given the reduction in core size, bypass ratios beyond 14 have been considered. In order to identify efficiency gains and performance characteristics which are due to the novel arrangement alone, the geared intercooled reversed flow core engine has been compared with a geared intercooled engine with a more conventional core. Finally an optimisation exercise has been carried out to identify the best configuration for both the geared intercooled reversed flow core concept and the conventional core concept. In this paper, it is demonstrated that the geared intercooled reversed flow core concept allows for a 2.3% reduction in block fuel burn. The reductions are due to the improved core efficiency, higher overall pressure ratio as well as efficiency gains from the use of a mixed exhaust. The sensitivity analysis shows that the improvements are highly dependent on pressure losses in the core and bypass stream and that careful design of the mixer chutes and intercooler headers to achieve low losses is essential if the concept gains are to be realised.

On the trade-off between minimum fuel burn and maximum time between overhaul for an intercooled aeroengine

A large variety of promising power and propulsion system concepts are being proposed to reduce carbon dioxide and other emissions. However, the best candidate to pursue is difficult to select and it is imperative that major investments are correctly targeted to deliver environmentally friendly, economical and reliable solutions. To conceive and assess gas turbine engines with minimum environmental impact and lowest cost of ownership in a variety of emission legislation scenarios and emissions taxation policies, a tool based on a techno-economic and environmental risk assessment methodology is required. A tool based on this approach has been developed by the authors. The core of the tool is a detailed and rigorous thermodynamic representation of power plants, around which other modules can be coupled (that model different disciplines such as aircraft performance, economics, emissions, noise, weight and cost) resulting in a multidisciplinary framework. This approach can be used for efficient and cost-effective design space exploration in the civil aviation, power generation, marine, and oil and gas fields. In the present work, a conceptual intercooled core aeroengine design was assessed with component technologies consistent with 2020 entry into service via a multidisciplinary optimisation approach. Such an approach is necessary to assess the trade-off between asset life, operating costs and technical specification. This paper examines the influence of fuel consumption, engine weight and direct operating costs with respect to extending the engine life. The principal modes of failure such as creep, fatigue and oxidation, are considered in the engine life estimation. Multidisciplinary optimisation, comprising the main engine design parameters, was carried out with maximum time between overhaul as the objective function. The trade-off between minimum block fuel burn and maximum engine life was examined; the results were compared against the initial engine design and an assessment was made to identify the design changes required for obtaining an improved engine design in terms of direct operating costs. The results obtained from the study demonstrate that an engine optimised for maximum time between overhaul requires a lower overall pressure ratio and specific thrust but this comes at the cost of lower thermal efficiency and higher engine production costs.

Ultra Low Emission Technology Innovations for Mid-century Aircraft Turbine Engines

Commercial transport fuel efficiency has improved dramatically since the early 1950s. In the coming decades the ubiquitous turbofan powered tube and wing aircraft configuration will be challenged by diminishing returns on investment with regards to fuel efficiency. From the engine perspective two routes to radically improved fuel efficiency are being explored; ultra-efficient low pressure systems and ultra-efficient core concepts. The first route is characterized by the development of geared and open rotor engine architectures but also configurations where potential synergies between engine and aircraft installations are exploited. For the second route, disruptive technologies such as intercooling, intercooling and recuperation, constant volume combustion as well as novel high temperature materials for ultra-high pressure ratio engines are being considered. This paper describes a recently launched European research effort to explore and develop synergistic combinations of radical technologies to TRL 2. The combinations are integrated into optimized engine concepts promising to deliver ultra-low emission engines. The paper discusses a structured technique to combine disruptive technologies and proposes a simple means to quantitatively screen engine concepts at an early stage of analysis. An evaluation platform for multidisciplinary optimization and scenario evaluation of radical engine concepts is outlined.

Generic Framework for Multi-Disciplinary Trajectory Optimization of Aircraft and Power Plant Integrated Systems

Engineering improvements, technology enhancements and advanced operations have an important role to play in reducing aviation fuel consumption and environmental emissions. Currently several organizations worldwide are focusing their efforts towards large collaborative projects whose main objective is to identify the best technologies or routes to reduce the environmental impact and fuel efficiency of aircraft operations. The paper describes the capability of a multi-disciplinary optimization framework named GATAC (Green Aircraft Trajectories under ATM Constrains) developed as part of the Clean Sky project to identify the potential cleaner and quieter aircraft trajectories. The main objective of the framework is to integrate a set of specific models and perform multi-objective optimization of flight trajectories according to predetermined operational and environmental constraints. The models considered for this study include the Aircraft Performance Model, Engine Performance Simulation Model and the Gaseous Emissions Model. The paper, further discusses the results of a test case to demonstrate trade-offs between fuel consumption, flight time and NOx emissions that the trajectory optimization activity achieves at a primary level. It thereby forms the basis of a complete reference base-line trajectory which will be used to determine more accurate environmental gains that can be expected through optimization with the integration of more models within the framework in the future.

Concept description and assessment of the main features of a geared intercooled reversed flow core engine

Intercooled turbofan cycles allow higher overall pressure ratios to be reached which gives rise to improved thermal efficiency. Intercooling also allows core mass flow rate to be reduced which facilitates higher bypass ratios. A new intercooled core concept is proposed in this paper which promises to alleviate limitations identified with previous intercooled turbofan designs. Specifically, these limitations are related to core losses at high overall pressure ratios as well as difficulties with the installation of the intercooler. The main features of the geared intercooled reversed flow core engine are described. These include an intercooled core, a rear-mounted high-pressure spool fitted rearwards of the low-pressure spool as opposed to concentrically as well as a mixed exhaust. In these studies, the geared intercooled reversed flow core engine has been compared with a geared intercooled straight flow core engine with a more conventional core layout. This paper compares the mechanical design of the high-pressure spools and shows how different high-pressure compressor and high-pressure turbine blade heights can affect over-tip leakage losses. In the reversed configuration, the reduction in high-pressure spool mean diameter allows for taller high-pressure compressor and turbine blades to be adopted which reduces over-tip leakage losses. The implication of intercooler sizing and configuration, including the impact of different matrix dimensions, is assessed for the reversed configuration. It was found that a 1-pass intercooler would be more compact although a 2-pass would be less challenging to manufacture. The mixer performance of the reversed configuration was evaluated at different levels of mixing effectiveness. This paper shows that the optimum ratio of total pressure in the mixing plane for the reversed flow core configuration is about 1.02 for a mixing effectiveness of 80%. Lower mixing effectiveness would result in a higher optimum ratio of total pressure in the mixing plane and fan pressure ratio.

Optimization of Aero Gas Turbine Maintenance Using Advanced Simulation and Diagnostic Methods

Engine maintenance costs are a major contributor to the direct operating costs of aircraft. Therefore, the minimization of engine maintenance costs per flight-hour is a key aspect for airlines to operate successfully under challenging market conditions. Minimization can be achieved by increasing the on-wing time or by reducing the shop-visit costs. Combining both provides optimum results and can only be achieved by thorough understanding of the engine. In the past, maintenance optimization was mainly an experience-based process. In this work, a novel analytical approach is presented to optimize the maintenance of commercial turbofan engines. A real engine fleet of more than 100 long-haul engines is used to demonstrate the application. The combination of advanced diagnostic and simulation methods with classical hardware-based failure analysis enables linking of overall engine performance with detailed hardware condition and, thus, an effective optimization of the overall maintenance process.

Advanced Open Rotor Performance Modelling for Multidisciplinary Optimization Assessments

As a consequence of increased stringent engine emission regulations, in a highly competitive market, it has become necessary to explore innovative, economic and environmentally friendly cycles to sustain competitive advantages. Among these innovative cycles, both the geared and the direct drive counter-rotating open rotors, due to their relatively higher propulsive efficiency, have the potential to significantly reduce fuel consumption and emissions relative to conventional high bypass ratio turbofans. A detailed TERA (Technoeconomic Environmental Risk Analysis), multidisciplinary optimisation framework, can be used to optimise both engines and thereby assess their potential as well as quantify their risks on a formal and consistent basis. This technique is based on detailed and rigorous engine performance, aircraft performance, engine geometry, engine weight, noise, gaseous emissions and environmental impact simulation models. No specific performance simulation methodology for counter rotating open rotors is available in the public domain. An innovative technique is introduced, comprising novel models of: • Counter-rotating propellers (including their interaction); • Counter-rotating turbines; • Planetary differential gearboxes. A thorough description of the modelling methodology (with a justification of the main assumptions) of each of these three components is presented and an indication of work in progress is provided. These components are then used to develop direct drive and geared open rotor performance models. The results of steady state design point and off design performance simulations of these two engine models are subsequently presented via two case studies. Some of the differences in the performance of the low pressure system of geared and direct drive open rotors are highlighted. It was observed that the impact of the key OR performance DP parameters is different for the two engines. Consequently the optimal design and control strategies of theses two configurations will differ. The flexibility of the new simulation technique makes it a suitable candidate to perform multi-disciplinary TERA design space exploration and optimisation studies assess and optimise open rotor designs and control strategies in a multidisciplinary framework.Copyright

Evaluation of aero gas turbine preliminary weight estimation methods

The estimation of gas turbine engine weight during the preliminary or conceptual design phase is a key part of a Techno-economic Environmental Risk Analysis (TERA). Several methods that are available in the public domain are analysed and compared, in order to establish the physics driving them and their suitability for the weight estimation of modern gas turbine engines. Among the tested methods, only WATE managed to achieve acceptable accuracy for engine optimisation studies. This work demonstrates that the age and restrictions of existing ‘whole engine based’ methods, along with their dependency on old engine databases make them unsuitable for future and novel aero engines. A hybrid weight modelling approach is proposed as a solution permitting the creation of simple ‘whole engine based’ methods that do not depend on the availability of existing engine data, which are also subject to uncertainties and incoherencies.

Towards the Development of a Multi-Disciplinary Flight Trajectory Optimization Tool: GATAC

Reducing the impact on the environment and the associated commercial implications are two major challenges that the global commercial aviation industry is addressing with significant commitment today. In this respect, Clean Sky, which is a 1.6 billion Joint Technology Initiative part funded by the European Commission is the largest ever programme addressing the greening of air transportation in response to the Advisory Council for Aeronautics Research in Europe (ACARE) goals of reducing CO2 and perceived noise emissions by 50% and NOx by 80% by 2020 compared to 2000 condition. This paper presents research work carried out within the Systems for Green Operations Integrated Technology Demonstrator (ITD) of Clean Sky that is associated with GATAC, a trajectory and route planning tool to enable the multi-objective optimization of flight trajectories and missions. The design and operational methodology of the tool, the optimization algorithms and models are discussed and the results of a preliminary application for a long-range commercial flight are presented.