Preliminary Design and Performance of Counter Rotating Turbines for Open Rotors: Part II — 0-D Methodology and Case Study for a 160 PAX Aircraft

Direct Drive Open Rotors (DDORs) have the potential to significantly reduce fuel consumption and emissions relative to conventional turbofans. However, this engine architecture presents many design and operational challenges both at engine and aircraft level. At preliminary design stages, a broad design space exploration is required to identify potential optimum design regions and to understand the main trade offs of this novel engine architecture. These assessments may also aid the development process when compromises need to be performed as a consequence of design, operational or regulatory constraints. Design space exploration assessments are done with 0-D or 1-D models for computational purposes. These simplified 0-D and 1-D models have to capture the impact of the independent variation of the main design and control variables of the engine. Historically, it appears that for preliminary design studies of DDORs, Counter Rotating Turbines (CRTs) have been modelled as conventional turbines and therefore it was not possible to assess the impact of the variation of the number of stages (Nb) of the CRT and rotational speed of the propellers. Additionally, no preliminary design methodology for CRTs was found in the public domain. Part I of this two-part publication proposes a 1-D preliminary design methodology for DDOR CRTs which allows an independent definition of both parts of the CRT. A method for calculating the off-design performance of a known CRT design is also described. In Part II, a 0-D design point efficiency calculation for CRTs is proposed and verified with the 1-D methods. The 1-D and 0-D CRT models were used in an engine control and design space exploration case study of a DDOR with a 4.26m diameter an 10% clipped propeller for a 160 PAX aircraft. For this application: • the design and performance of a 20 stage CRT rotating at 860 rpm (both drums) obtained with the 1-D methods is presented. • differently from geared open rotors, negligible cruise fuel savings can be achieved by an advanced propeller control. • for rotational speeds between 750 and 880 rpm (relatively low speeds for reduced noise), 22 and 20 stages CRTs are required. • engine weight can be kept constant for different design rotational speeds by using the minimum required Nb. • for any target engine weight, TOC and cruise SFC are reduced by reducing the rotational speeds and increasing Nb (also favourable for reducing CRP noise). However additional CRT stages increase engine drag, mechanical complexity and cost.

Sensitivity Analysis and Experimental Validation of Transient Performance Predictions for a Short-Range Turbofan

Transient effects are important features of engine performance calculations. The aim of this paper is to analyze a new, fully transient model implemented using the PRopulsion Object Oriented Simulation Software (PROOSIS) for a civil, short range turbofan engine. A transient turbofan model, including the mechanical inertia effect has been developed in PROOSIS. Specific physical effects such as heat soakage, mass storage, blade tip clearance and combustion delay have been implemented in the relevant components of PROOSIS to obtain a fully transient model. Since a large number of components are concerned by all the transient effects, an influence study is presented to determine which are the most critical effects, and in which components. Inertia represents the relevant phenomenon, followed by thermal effects, combustion delay and finally mass storage. The comparison with experimental data will provide a first validation of the model. Finally a sensitivity study is reported to assess the impact of uncertain knowledge of key input parameters in the response time prediction accuracy.

Analysis of Droplet Impingement Characteristics of Aero-Engine Nose Cone With Hot Air Film

A new nose cone ice protection configuration with hot air film slot was investigated, where the surface need to be protected could be heated with interior impingement heat transfer and exterior hot air film. Numerical simulation methods using computational fluid dynamics code were developed and validated to find the effects of the jetted air film on the droplet impingement characteristics. Combination of two-dimensional axisymmetric algorithm and Lagrangian method were adopted to solve the air flow field and the droplet trajectories. The simulation methods were validated with the results from the experimental data. The droplet impingement characteristics on two structures were investigated respectively, the intact cone without film slot and the slotted cone. Results show that the surface local collection coefficient changes significantly behind the film slot because of the blowing effect of the air film on the incoming droplets, and the variation of the local collection coefficient is very dependent on the droplet diameter for a given blowing ratio, or the blowing ratio for a given droplet size. Some different effects, such as “fully blowing-off”, “blowing behind” and “limited blowing-off”, may happen for different combination of droplets size and blowing ratio. Compared with the cone without film slot, the blowing effect is more significant for smaller droplets or higher blowing ratio. Besides that, the total collection coefficient maybe only half of those without film for some conditions.

A Noise Assessment Framework for Subsonic Aircraft and Engines

This paper proposes a preliminary subsonic aircraft and engine noise assessment framework, capable of computing the aircraft total noise level at all three certification points (i.e. Approach, Lateral, and Flyover) defined by the International Civil Aviation Organisation. The proposed framework is numerically integrated to account for the complete aircraft noise sources (i.e. the fuselage, wings, landing gear, as well as noise sources resulting from the engine component level, (i.e. fan, compressor, combustor, turbine, and jet). The developed framework is based on a wide-range of empirical and semi-empirical correlations collected from the public domain literature. The fidelity of the framework also caters for flight effects such as atmospheric attenuation, spherical spreading, Doppler shift, lateral attenuation, retarded time and ground reflection. A conversion between the sound pressure level SPL [SPLdB] to effective perceived noise level EPNL [EPNdB] is also included to allow for a consistent comparison with the certification procedure. Through the successful deployment of the proposed framework a generic aircraft model, representative of a modern commercial carrier aircraft has been investigated, operating under representative operational conditions. The sound pressure level corresponding to various aircraft and engine component have been thoroughly investigated and verified with trends acquired based on the theory. Furthermore, the predictions made by the framework corresponding to the aforementioned three certification points have also been verified against the noise level measurements provided by the International Civil Aviation Organization. The results acquired exhibit good correlation against the verification data for total noise levels at the microphones. Furthermore, a component level comparison is also presented which exhibit good agreement with verification data. The deployed methodology can essentially be regarded as an enabling technology to support the effective and efficient implementation of framework(s) (i.e. Technoeconomic, Environmental and Risk Assessment) targeted to evaluate the existing and advanced aircraft and engine architectures in terms of operational performance and environmental impact.

Potential of Micro Turbine Based Propulsion Systems for Civil UAVs: A Case Study

There is a high potential for civil applications of Unmanned Aerial Vehicles (UAV) in areas such as goods transport, telecommunication, remote monitoring and sensing, surveillance, search and rescue, and disaster management. Developments in areas such as telecommunication, control and information technology offer opportunities for long range remotely or automatically piloted missions. This requires efficient and light-weight small propulsion systems. The potential of turboprop propulsion for civil UAVs using micro turbine technology has been explored and compared with existing concepts, such as piston engine driven propellers. Different propulsion concepts have been analyzed and the application areas where advanced turboprops would be superior to other systems such as reciprocating engines and electric motors, identified. However, turboprop engines of the small power capacity required for the aircraft concepts and missions considered are not currently available with competitive performance. A conceptual design study of a micro turboprop engine has been performed by downscaling an existing reference engine. Scale effects on efficiency have been taken into account, as well as effects of technological progress. Engine cycle optimization has been carried out and the effects of turbine inlet temperature, compressor pressure ratio, engine size, and component efficiency have been investigated. An aerodynamic and flight performance model of a baseline UAV has been developed in order to predict mission performance. This model has been coupled to a turboprop model to evaluate system performance with different engine configurations for the selected mission. The outcome of the study provides information about the technological improvements in terms of cycle efficiency required to make the micro-turboprop a competitive solution. The Propulsion and Power group of Delft University of Technology will pursue these R&D goals in an attempt to contribute to the development of civil UAV technology.

Wake and Loss Analysis for a Double Bladed Swept Propeller

Inspired by Prandtl’s theory on aircraft wings with minimum induced drag, the authors introduced a double-bladed propeller, the Boxprop, intended for high-speed flight. The basic idea is to join the propeller blades pair-wise at the tip to improve aerodynamics and mechanical properties compared to the conventional propeller. The rather complex geometry of the double blades gives rise to new questions, particularly regarding the aerodynamics. This paper presents a propeller wake energy analysis method which gives a better understanding of the potential performance benefits of the Boxprop and a means to improve its design. CFD analysis of a five bladed Boxprop demonstrated its ability to generate typical levels of cruise thrust at a flight speed of Mach 0.75. The present work shows that the near tip velocity variations in the wake are weaker for this propeller than a conventional one, which is an indication that a counter rotating propeller designed with a Boxprop employed at the front may exhibit lower interaction noise.

Full-Scale Turbofan Demonstration of a Deployable Engine Air-Brake for Drag Management Applications

This paper presents the design and full-scale ground-test demonstration of an engine air-brake (EAB) nozzle that uses a deployable swirl vane mechanism to switch the operation of a turbofan’s exhaust stream from thrust generation to drag generation during the approach and/or descent phase of flight. The EAB generates a swirling outflow from the turbofan exhaust nozzle, allowing an aircraft to generate equivalent drag in the form of thrust reduction at a fixed fan rotor speed. The drag generated by the swirling exhaust flow is sustained by the strong radial pressure gradient created by the EAB swirl vanes. Such drag-on-demand is an enabler to operational benefits such as slower, steeper, and/or aeroacoustically cleaner flight on approach, addressing the aviation community’s need for active and passive control of aeroacoustic noise sources and access to confined airports. Using NASA’s Technology Readiness Level (TRL) definitions, the EAB technology has been matured to a level of 6, i.e., a fully functional prototype. The TRL-maturation effort involved design, fabrication, assembly, and ground-testing of the EAB’s deployable mechanism on a full-scale, mixed-exhaust, medium-bypass-ratio business jet engine (Williams International FJ44-4A) operating at the upper end of typical approach throttle settings. The final prototype design satisfied a set of critical technology demonstration requirements that included (1) aerodynamic equivalent drag production equal to 15% of nominal gross thrust in a high-powered approach throttle setting (called dirty approach), (2) excess nozzle flow capacity and fuel burn reduction in the fully deployed configuration, (3) acceptable engine operability during dynamic deployment and stowing, (4) deployment time of 3–5 seconds, (5) stowing time under 0.5 second, and (6) packaging of the mechanism within a notional engine cowl. For a typical twin-jet aircraft application, a constant-speed, steep approach analysis suggests that the EAB drag could be used without additional external airframe drag to increase the conventional glideslope from 3 to 4.3 degrees, with about 3 dB noise reduction at a fixed observer location.

Flow Survey of a Forward Curved Blades Centrifugal Fan for HVAC Applications

The advancements in fan technology are nowadays animated by two major drivers: the legal requirements that impose minimum fan efficiency grades for fans sold within European Union (and soon US and Asia), and the market request for better air performance and lower sound emissions. Within HVAC (Heating, Ventilating and Air Conditioning) applications, centrifugal fans with forward curved blades are widely used due to the higher total pressure rise capability and lower acoustic emissions with respect to more efficient backward curved blades. However the continuous rise of minimum fan efficiency grades pushes the manufacturers to develop a new generation of forward curved centrifugal fans, improving previous design. Here the challenge is not only on aerodynamics, but in the overall production process, as squirrel cage fans are characterised by a cost-effective consolidated technology, based on simple blade geometries and easy series manufacturing. For example, the blades usually have circular camber lines, as results of cut cylinders. Thus, once the number of blades and the angle at the leading edge are selected, the chord and the deflection capability are constrained as well. These concurring aspects led industry to include in the design process new tools, in particular CFD, to analyse the flow features of the current generation of fans in order to understand which phenomena are to be either controlled or exploited to increase efficiency and total pressure rise. Here we present a numerical investigation on a forward curved blade centrifugal fan for HVAC applications, to highlight the flow features inside the impeller and in the critical region of coupling with the volute. The analysis was carried out with OpenFOAM, an open-source library for CFD. Computations were performed with the frozen rotor approach and validated against available experimental data.

A Turbofan-Engine Nacelle Shape Design and Optimization Method for Natural Laminar Flow Control

A correctly profiled engine nacelle can delay the transition in the boundary layer and allow laminar flow to extend back, resulting in a substantial drag reduction. Therefore, the laminar flow nacelle has lower fuel consumption than current turbulent designs. In this paper, aerodynamic shape optimization of natural laminar flow nacelle has been studied by using a novel nacelle shape design method and transition prediction with CFD. First, the 2D longitudinal profile-line of nacelle is optimized, in order to extend its laminar region and achieve minimum drag coefficient within the design space. Second, the optimized longitudinal profile-line is then circumferentially stacked to construct the 3D nacelle aerodynamic shape. At last, the aerodynamic improvement of the new shape is evaluated by 3D CFD simulation. A nacelle geometry generator has been developed where the deflection angle (related to the curvature) along the cord is controlled by using Non-Uniform Rational B-Splines. It is then analytically integrated to obtain the longitudinal profile-line. And also a leading edge matching function is involved in the generator. This technique improves the smoothness of nacelle profile-line, which ensures the curvature and slope of curvature to be continuous all over the nacelle surface. The pressure distribution over the nacelle surface has been improved with no spikes in Mach number. A transition model coupling with shear stress transport turbulent model is used in solving Navier-Stokes equations for transition prediction. An optimization system has been established in combination with the geometry generator, the transition prediction model with CFD, a Kriging surrogate model and a Multi-Island Genetic Algorithm. As a result, the aerodynamic improvement, with one profile-line optimized, is obvious against the original nacelle shape by CFD validation in 3D simulation. The optimized nacelle can achieve a laminar flow up to 23% and its drag coefficient has reduced by 6.5%. It is indicated that the optimization system is applicable in nacelle aerodynamic shape design.

Study on the Effect of Inlet Geometry on the Noise of an Axial Fan, With Involvement of the Phased Array Microphone Technique

The paper presents a comparative case study in which a free-inlet, free-outlet industrial ventilating fan has been equipped with various inlet geometries. The original short-tapered entry has been replaced by a standardized bellmouth entry, resulting in remarkable noise reduction. The experimentation presented herein is adaptable to industrial onsite diagnostics. The upstream-radiated broadband noise associated with rotating sources has been mapped in a spatially resolved manner, by means of a Phased Array Microphone system and a Rotating Source Identifier beamforming algorithm. The acoustic measurements have been supplemented with aerodynamic measurements on the inlet velocity profile, and with Computational Fluid Dynamics studies. The acoustic data have been processed for enabling their evaluation in association with the aerodynamic operation of the elemental rotor cascades in a two-dimensional approach, and also for their interpretation in relationship to three-dimensional flow phenomena such as tip leakage flow. For this purpose, the acoustic data have been presented in the form of circumferentially-averaged noise profiles along the blade span, as well as noise source imaging maps. The studies reveal the following acoustic benefits of reconfiguring the original short-tapered entry to the bellmouth entry. A peripheral separation zone is characteristic for the short-tapered entry, provoking double-leakage tip clearance flow, being the dominant source of noise at higher frequencies. Such a peripheral separation zone is suppressed by the bellmouth inlet, and thus, the double-leakage flow and the related noise is eliminated. Farther away from the tip, along the dominant portion of the span, the moderation of endwall blockage due to suppressing the peripheral separation zone has led to the reduction of the rotor inlet velocity, thus moderating the noise associated with the suction side boundary layer developing on the blades.