Christopher A. Perullo

Effects of Advanced Engine Technology on Open Rotor Cycle Selection and Performance

Recent increases in fuel prices and increased focus on aviations environmental impacts have reignited focus on the open rotor engine concept. This type of architecture was extensively investigated in previous decades but was not pursued through to commercialization due to relatively high noise levels and a sudden, sharp decrease in fuel prices. More recent increases in fuel prices and increased government pressure from taxing carbon-dioxide production mean the open rotor is once again being investigated as a viable concept. Advances in aero-acoustic design tools have allowed industry and academia to re-investigate the open rotor with an increased emphasis on noise reduction while retaining the fuel burn benefits due to the increased propulsive efficiency. Recent research with conceptual level multidisciplinary considerations of the open rotor has been performed (Bellocq et al., 2010, “Advanced Open Rotor Performance Modeling For Multidisciplinary Optimization Assessments,” Paper No. GT2010-2963), but there remains a need for a holistic approach that includes the coupled effects of the engine and airframe on fuel burn, emissions, and noise. Years of research at Georgia Institute of Technology have led to the development of the Environmental Design Space (EDS) (Kirby and Mavris, 2008, “The Environmental Design Space,” Proceedings of the 26th International Congress of the Aeronautical Sciences). EDS serves to capture interdependencies at the conceptual design level of fuel burn, emissions, and noise for conventional and advanced engine and airframe architectures. Recently, leveraging NASA environmentally responsible aviation (ERA) modeling efforts, EDS has been updated to include an open rotor model to capture, in an integrated fashion, the effects of an open rotor on conventional airframe designs. Due to the object oriented nature of EDS, the focus has been on designing modular elements that can be updated as research progresses. A power management scheme has also been developed with the future capability to trade between fuel efficiency and noise using the variable pitch propeller system. Since the original GE open rotor test was performed using a military core, there is interest in seeing the effect of modern core-engine technology on the integrated open rotor performance. This research applies the modular EDS open rotor model in an engine cycle study to investigate the sensitivity of thermal efficiency improvements on open rotor performance, including the effects on weight and vehicle performance. The results are that advances in the core cycle are necessary to enable future bypass ratio growth and the trades between core operating temperatures and size become more significant as bypass ratio continues to increase. A general benefit of a 30% reduction in block fuel is seen on a 737-800 sized aircraft.

Development of a Suite of Hybrid Electric Propulsion Modeling Elements Using NPSS

NASA is actively funding research into advanced, unconventional aircraft and engine architectures to achieve drastic reductions in vehicle fuel burn, noise, and emissions. One such concept is being explored by Boeing, General Electric, Virginia Tech, and Georgia Tech under the Subsonic Ultra Green Aircraft Research (SUGAR) project [1]. A major cornerstone of this research is evaluating the potential performance benefits that can be attributed to using hybrid electric propulsion. Hybrid electric propulsion in this context involves a non-Brayton power generation or storage source, such as a battery or a fuel cell, which can be used to provide additional propulsive energy to a conventional Brayton cycle powered turbofan engine. Employing additional power sources for thrust production increases the number of degrees of freedom both from a design and configuration standpoint and from an operational one. In order to assess and understand the myriad number of potential new configurations a modeling and simulation tool is needed; however, current state of the art propulsion modeling tools such as the Numerical Propulsion System Simulation (NPSS) are not natively capable of assessing novel hybrid electric configurations.This research addresses the gap between hybrid electric propulsion and conventional cycle analysis tools by developing a suite of native NPSS elements suitable for hybrid electric engine cycle design and analysis. Elements have been developed for a fuel cell, battery, motor, generator, and electrical distribution system. Both room temperature and cryogenically cooled superconducting variants are developed. The elements are designed such that they can be seamlessly integrated into existing NPSS cycle models to assess any system configuration or architecture the designer can envision.© 2014 ASME

A review of hybrid-electric energy management and its inclusion in vehicle sizing

Purpose – The purpose of this study is to examine state-of-the-art in hybrid-electric propulsion system modeling and suggest new methodologies for sizing such advanced concepts. Many entities are involved in the modelling and design of hybrid electric aircraft; however, the highly multidisciplinary nature of the problem means that most tools focus heavily on one discipline and over simplify others to keep the analysis reasonable in scope. Correctly sizing a hybrid-electric system requires knowledge of aircraft and engine performance along with a working knowledge of electrical and energy storage systems. The difficulty is compounded by the multi-timescale dynamic nature of the problem. Furthermore, the choice of energy management in a hybrid electric system presents multiple degrees of freedom, which means the aircraft sizing problem now becomes not just a root-finding exercise, but also a constrained optimization problem. Design/methodology/approach – The hybrid electric vehicle sizing problem can be sub...

Assessment of Vehicle Performance Using Integrated NPSS Hybrid Electric Propulsion Models

NASA is actively funding research into advanced, unconventional aircraft and engine architectures to achieve drastic reductions in vehicle fuel burn, noise, and emissions. One such concept is being explored by Boeing, General Electric, Virginia Tech, and Georgia Tech under the Subsonic Ultra Green Aircraft Research (SUGAR) project. A major cornerstone of this research is evaluating the potential performance benefits that can be attributed to using hybrid electric propulsion. Hybrid electric propulsion in this context involves a non-Brayton power generation or storage source, such as a battery or a fuel cell, that can be used to provide additional propulsive energy to a conventional Brayton cycle powered turbofan engine. This research constructs an integrated NPSS hybrid electric propulsion model capable of predicting hybrid electric engine performance throughout the operational envelope. The system consists of a battery powered motor partially drving the low pressure shaft of a conventional turbofan engine. The applied motor power adds an additional degree of freedom, along with power setting, to the aircraft designer during mission analysis. Modeling features and issues unique to hybrid electric propulsion systems are described and a vehicle trade study is carried out to determine the optimum engine cycle for both a cryogenic and conventionally driven motor system.

Surrogate Modeling for Simultaneous Engine Cycle and Technology Optimization for Next Generation Subsonic Aircraft

This paper presents an engine sizing and cycle selection study of ultra high bypass ratio engines applied to a subsonic commercial aircraft in the N+2 (2025) timeframe. NASA has created the Environmentally Responsible Aviation (ERA) project to serve as a technology transition bridge between fundamental research (TRL 1–4) and potential commercial application (TRL 7). Specifically, ERA is focused on subsonic transport technologies that could reach TRL 6 by 2020 and can be integrated into an advanced vehicle concept to simultaneously meet the ERA project metrics for noise, emissions, and fuel burn. An important variable in exploring the technology trade space is the selection of the optimal engine cycle for use on the advanced aircraft. Previous literature demonstrated the cycle optimization using a design of experiments (DOE) to explore the engine cycle design space for a pre-defined technology package. However, since the optimal engine cycle is dependent upon the specific technology package, this process would have to be repeated to ensure optimal performance for each technology package. With more than 80 technologies to be analyzed, the combinatorial space of technology packages is enormous. As a result, executing a DOE to find the optimum engine cycle for each technology package is infeasible. To address this issue, it is proposed to use surrogate models that encompass the engine cycle and technology design space to enable fast and accurate optimization of the engine cycle for any given technology package.This paper describes the generation and analysis of surrogate models used for technology assessment and cycle optimization of an ultra high bypass geared turbofan engine architecture. The first study in the paper shows that a single surrogate model can be used to accurately simulate both a technology and cycle design space. To demonstrate the proposed surrogate modeling approach, the cycle design space for three different technology packages was analyzed. This study demonstrated that when an optimal cycle is found within the constrained interior of a design space, the surrogate modeling approach is quite accurate. The study also established that the surrogate models can also be used to assess potential cycles at the boundaries or even outside of the region for which they were trained.© 2012 ASME

Assessment of Engine and Vehicle Performance Using Integrated Hybrid-Electric Propulsion Models

NASA is actively funding research into advanced, unconventional aircraft and engine architectures to achieve drastic reductions in vehicle fuel burn, noise, and emissions. One such concept is being explored by The Boeing Company, the General Electric Company, Virginia Polytechnic Institute and State University, and the Georgia Institute of Technology under the Subsonic Ultra Green Aircraft Research Project. A major cornerstone of this research is evaluating the potential performance benefits that can be attributed to using hybrid-electric propulsion. Hybrid-electric propulsion in this context involves a non-Brayton power generation or storage source, such as a battery or a fuel cell that can be used to provide additional propulsive energy to a conventional Brayton-cycle-powered turbofan engine. This research constructs an integrated Numerical Propulsion System Simulation hybrid-electric propulsion model capable of predicting hybrid-electric engine performance throughout the operational envelope. The syste...

Analysis of the Effect of Cruise Speed on Fuel Efficiency and Cost for a Truss-Braced Wing Concept

Modern day commercial aviation has a strong incentive to pursue the design of advanced aircraft concepts, motivated both by growing environmental concerns and more challenging airline economics. In this regard, the Truss-Braced Wing concept has significant potential to achieve appreciable improvements with regard to fuel efficiency, emissions, noise, and operating cost, but without entailing as much technological risk and uncertainty as more exotic designs. The performance advantage of a Truss-Braced Wing design over a conventional cantilever wing design stems from the fact that the truss allows a wing of much higher aspect ratio, and thus higher aerodynamic efficiency, to be achieved without a correspondingly high weight penalty. In a previous investigation by the authors, a Multidisciplinary Design Optimization was performed to identify the most promising TrussBraced Wing architecture from among candidates that included a Strut-Braced Wing design in addition to one-jury and two-jury Truss-Braced Wing designs. The insights gained from that investigation are built on in this paper, which analyzes the mission-level impact of the aircraft’s cruise speed. A lower cruise speed may allow a reduction in mission fuel consumption, but this is not the only concern as airline operators must also consider the aircraft utilization. In this work therefore, consideration is given to both mission block fuel and the operating cost, and both fuel-optimal and cost-optimal designs are arrived at for a range of potential cruise speeds. The fuel and cost-optimal designs for a single aisle N+3 Truss-Braced Wing concept at the same Mach numbers were contrasted, and impact of cruise speed on operating costs was quantified.

An Integrated Assessment of an Organic Rankine Cycle Concept for Use in Onboard Aircraft Power Generation

It is well established that there are advantages in moving towards non-pneumatic engine secondary systems. Such systems are used primarily to provide pressurization, cabin climate control, and de-icing; however, as bypass ratios continue to grow and engine cores become more efficient, the engine fan diameter is increased and core size is diminished. As a consequence, pneumatic off-takes require a larger percentage of the core flow leading to larger performance penalties. One current solution is to drive the aircraft environmental control systems (ECS) with large engine driven electric compressors rather than to use high pressure air from the core. Since cores are generally less sensitive to electrical power off-takes than to pneumatic off-takes this results in a smaller performance penalty. [F1] Using electrical air compressors also ensures fresh, clean air is delivered to the ECS thereby eliminating the risk of engine bleed contaminated cabin air.This research uses the Environmental Design Space (EDS) to examine the feasibility of recovering engine core exhaust heat to perform useful work within the aircraft. EDS serves to capture interdependencies at the conceptual design level of fuel burn, emissions, and noise for conventional and advanced engine and airframe architectures [F2]. Recovering exhaust heat is accomplished through a novel concept that makes use of an organic Rankine cycle (ORC).The concept is similar in principle to heat recovery steam generators used in power plant applications to improve combined cycle efficiency [3]. The main difference is the ORC system is relatively lightweight and appropriate for use onboard an aircraft. The waste heat in this application is used to generate electricity to drive external air compressors to supply flow to the ECS. As a result pneumatic bleeds within the engine can be eliminated, thereby eliminating growing performance penalties associated with shrinking core size and increased fan diameters. An ORC is considered because ORC cycles are ideal for extracting low grade heat. As an additional benefit the ORC vapor cycle can use the fan inlet and wing leading edge anti-ice devices as a condensation heat transfer mechanism that could also allow the system to provide anti-icing capabilities, further reducing engine pneumatic off-takes. The current research focuses on the system as applied the ORC concept to a CFM56 sized engine and has analytically demonstrated from a 0.9% to a 2.5% benefit in vehicle fuel burn relative to a conventional, pneumatically driven ECS. Actual fuel burn savings are dependent on the net installation weight of the ORC cycle.Copyright

An Integrated Assessment of an Advanced Open Rotor Configuration Using the Environmental Design Space

Government and industry have been renewing the investigation of an open rotor concept to reduce the future environmental impact of aviation. The open rotor was extensively developed in previous decades including more than one successful flight test demonstration. The concept never evolved into a mature commercial application due to a sudden drop in fuel prices in the late 1980s and because of the relatively high noise levels of the open rotor. More recent increases in fuel prices and increased government pressure in the form of carbon-dioxide taxes have made the open rotor a concept of interest once again. There has been a wealth of recent information and data generated on both the historical open rotor testing and updated design configurations. Recent advances in computational fluid dynamics and aeroacoustics have allowed designers to lower the noise levels of the open rotor without undue reduction in the large fuel burn benefit that results from the increased propulsive efficiency. The Environmental Design Space (EDS), developed at the Georgia Institute of Technology, is a conceptual level tool capable of capturing the interdependencies of fuel burn, noise, and emissions for both conventional and advanced engine and airframe architectures. Recently EDS has been updated to include an open rotor model capable of capturing the fuel burn, noise, and emissions impacts associated with an open rotor and the corresponding interdependencies between the metrics. This research capitalizes on these modeling efforts to investigate the operational space of an open rotor engine on a conventional tube and wing vehicle configuration. Trades between fuel burn and noise for a sized configuration are assessed. Benefits to noise and fuel burn are shown by changing the power management of the propeller during different phases of flight. Assessments in this research have been performed using a louder historical baseline blade set that is currently available in the public domain (F7/A7); however, the assessment will be repeated with a quieter historical baseline (F31/A31) in the future. Benefits of up to two percent fuel burn or five EPNdB cumulative noise are found through varying propfan rotational speed during different phases of operation.

Bayesian Belief Network for Robust Engine Design and Architecture Selection

Designing propulsion system architectures to meet next generation requirements requires many tradeoffs be made. These trades are often between performance, risk, and cost. For example, the core of an engine is the most expensive and highest risk area of a propulsion system design. However, a new core design provides the greatest flexibility in meeting future performance requirements. The decision to upgrade or redesign the core must be justified by comparison with other lower risk options. Furthermore, for turboshaft applications, the choice of compressor, whether axial or centrifugal, is a major decision and trade with the choice being heavily driven by both current and projected weight and performance requirements. This problem is confounded by uncertainty in potential benefits of technologies or future performance of components. To address these issues this research proposes the use of a Bayesian belief network (BBN) to extend the more traditional robust engine design process. This is done by leveraging forward and backward inference to identify engine upgrade paths that are robust to uncertainty in requirements performance. Prior beliefs on the different scenarios and technology uncertainty can be used to quantify risk. Forward inference can be used to compare different scenarios.The problem will be demonstrated using a two-spool turboshaft architecture modeled using the Numerical Propulsion System Simulation (NPSS) program. Upgrade options will include off the shelf, derivative engine (fixed core) with no technologies, derivative engine with new technologies, a new engine with no technologies, and a new engine with new technologies. The robust design process with a BBN will be used to identify which engine cycle and upgrade scenario is needed to meet performance requirements while minimizing cost and risk. To demonstrate how the choice of upgrade and cycle change with changes in requirements, studies are performed at different horsepower, ESFC, and power density requirements.Copyright