Past Research at the Gas Turbine Laboratory
TURBOMACHINERY
Inlet-Engine Integration for High Performance Aircraft
Current practice in aircraft inlet computations does not appear to account for, in a rigorous manner, the impact of flow redistribution due to the presence of engine compressors. This can be of import in the determination of inlet-engine dynamic behavior, e.g., ASTOVL vehicles at takeoff and landing. The research activity represents an effort to develop a consistent computational methodology for inlet-engine integration aerodynamics on high performance aircraft.
Performance and Flow Phenomena in Vaned Diffusers
Computational studies have been initiated to examine the impact of unsteady impeller-diffuser interactions on performance, specifically: (i) time-averaged effects of unsteady impeller-diffuser interactions and (ii) initiation and development of aerodynamic instabilities in a centrifugal compressor.
Analysis of Aerodynamically Mistuned Bladed Disks
In typical analyses of bladed disks, the problem is assumed to be tuned, that is all blades are assumed to have identical geometries, mass and stiffness characteristics. In reality, both the manufacturing process and engine wear create a situation where the blades differ slightly from one another. These blade-to-blade variations are known as mistuning and can significantly impact the operation of bladed disks. In particular, mistuning causes the forced response amplitudes of individual blades to be much larger than that predicted by a tuned analysis, which has serious implications for high cycle fatigue. The purpose of this research is to develop high-fidelity, low-order aerodynamic models suitable for use in a mistuning context. While significant progress has been made is the area of structural mistuning, the effects of blade geometric variations is not well understood. This research uses systematic model order reduction techniques (such as the proper orthogonal decomposition) to develop models suitable for the aerodynamic mistuning problem.
Modeling of Turbomachinery Aerodynamic Instabilities
The central focus of this research is the initiation and development of compressor instabilities in situations where the three-dimensionally of the flow is significant. Models have been developed to assess effects of: radial blade loading distribution on stability, inlet distortion on stall inception in three dimensional flows, and three-dimensional phenomena due to mismatching in multistage compressors. Procedures are also being developed to define the nonlinear evolution of compressor flow disturbances in situations in which compressibility has a strong role. Work has also been initiated to examine the effects of rotor stator interaction on blade row stalling behavior. The goal is to establish, in a rigorous manner, causal links between design characteristics and the stability of compressor flows .
Impeller-Diffuser Interaction On Aerodynamic Performance of Centrifugal Compressors
The work is aimed at a key problem on the aerodynamics of modern centrifugal compressor stages, the impact of unsteady impeller-diffuser interactions on performance and design. Three important technical issues are of engineering interest: (1) time-averaged effects of unsteady impeller-diffuser interactions on the performance of a modern centrifugal compressor stage; (2) initiation and development of aerodynamic instabilities in a centrifugal compressor; and (3) use of (1) to suggest design guidelines and potential innovative design for achieving step changes in performance. The current focus will be on technical issue (1) and (3); however the results will be used to infer their potential impact on issue (2). In addition to addressing the technical issues, assessment and validation of analytical/computational tool/tools for use in centrifugal compressor stage design and analysis will be carried out during the course of the research program.
Characterization of Aeromechanics Response and Instability in
High Performance Centrifugal Compressor Stage /Rocket Centrifugal Pump
The work here is address the role of impeller diffuser interactions on the forced response of impeller blades and as such is somewhat complementary to and synergistic with the effort on impeller-diffuser interaction on aero performance of centrifugal compressors.
Impact of End-Wall Flows and Wakes in Multistage Axial Compressor Performance
The effort proposes to examine the unsteady response of endwall flow, specifically the rotor tip leakage flow, to the flow conditions associated with the upstream stator (wakes and flow non-uniformity as set by design of upstream stator) and downstream stator (mostly unsteadiness due to potential interaction), and the potential time-average impact on multistage compressor performance.
Hydrodynamics of Centrifugal Pumps for Space Propulsion
While much useful work has been done on turbopumps for earth-to-space rocket propulsion, it has been mostly of a developmental nature. To raise the level of understanding and the design capability for turbopumps to the desired level, we see a need to address several specific areas of centrifugal pump fluid dynamics. One of these is the effect of tip clearance flows on performance and stability; others are the unsteady effects associated with fluid-structure interaction and with impeller-diffuser interaction. It is the first of these that we target here.
Aeromechanic Response of High Speed Compressor Stage to Inlet Distortion
The focus is on using data (both experimental from CRF at AFRL and computations) to delineate the response of high speed compressor stage to inlet distortion in terms of the flow processes/drivers responsible for the observed unsteady blade loads and response. Also effort will be undertaken to assess the observations/understanding from CRF test rigs together with the accompanying numerical simulations against those from engine in flight test situations; this is needed to answer the question of what constitute an adequate ground/model rig test for representing the forced response on engine compressor encountering inlet distorted flows in flight. The intellectual challenge here is how to organize the results so that physical insight can be extracted to aid future design of fan/compressors with fewer aeromechanics difficulties, including HCF (High Cycle Fatigue) failure.
Characterization of Wakes/Tip Vortex/Secondary Vortex Induced Blade Excitations
in Multi-Stage Environment
The overall goal here is to understand and predict two- and three-dimensional sources of unsteady loads on fans, compressors and turbine blades under representative operating environments and to show their impact on forced vibration and component flutter. The key objectives are to: (1) define key links between unsteady blade load and its sources which include rotor tip vortices, discrete secondary flow vortices, wakes, blade surface unsteady boundary layer separation, and blade motion over the operating range of interest; (2) quantify the effects of adjacent blade rows, or adjacent stages (i.e. multi-stage influence); and (3) develop a predictive physical model that includes aero-structural coupling effects and hence can provide data to predict dynamic stress and strain. In the larger picture, it is important to quantify parametric trends which fix aeromechanics response and instability (flutter) onset of turbomachinery blades, and to assess the feasibility of tailoring the aerodynamic and structural design of blades
Computational Model for Multistage Compressor Aerodynamics:
Performance Map Generation and Stability Characterization
A framework for an effective computational methodology was developed for multi-stage compressor map generation. The methodology consists of using a few isolated-blade row Navier-Stokes solutions for each blade row to construct a body force database. The purpose of the body force database is to replace each blade row in a multi-stage compressor by a body force distribution to produce same pressure rise and flow turning. To do this, each body force database is generated in such a way that it can respond to the changes in local flow conditions. Once the database is generated, no further Navier-Stokes computations are necessary. The process is repeated for every blade row in the multi-stage compressor. The body forces are then embedded as source terms in an Euler solver. The method is developed to have the capability to compute the performance in a flow that has radial as well as circumferential non-uniformity with a length scale larger than a blade pitch; thus it can potentially be used to characterize the stability of a compressor under design. It is these two latter features as well as the procedure to obtain the body force representation that distinguish the present methodology from the streamline curvature method.
Spectral Computations of Three-Dimensional Flow for Axial Turbomachinery
A three-dimensional spectral code has been developed for computation of 3-D flow through an axial turbomachinery blade row. It is currently being used to examine the response of the blade passage flow field to the incoming moving wakes and streamwise vortices.
Computational Studies of Flutter and Forced Response in Compressors/Fans
Research is underway to explore the capability and the role of computational fluid dynamic tools for: (i) addressing blade flutter phenomena and associated controlling mechanisms as well as the parametric trends; and (ii) assessing the effect of density nonuniformities on the unsteady response of turbomachinery blading.
Experimental Investigations of Axial Turbine Fluid Physics
Transient (blowdown) testing of advanced performance turbines is being conducted at realistic Mach and Reynolds numbers, ratios of turbine inlet temperature to metal and coolant temperature, and inlet temperature profiles. A current topic under investigation is the effect film cooling on aerodynamic performance and heat transfer.
Aspirated Compressors
Aspirated compressors employ suction on the blading and endwalls to realize greatly increased pressure rise and efficiency from axial turbomachinery. Single stage compressors scheduled for test include a 700 fps design with a pressure rise of 1.5:1 and a 1500 fps design with a pressure rise of 3.5:1. This work is in partnership with NASA GRC, PW, and Honeywell.
Evaporative Cooling for Gas Turbines
A new approach to the problem of using two phase flow to internally cool gas turbine hot parts is under investigation. Experiments on a rotating turbine blade simulator have demonstrated the cooling rates needed for the engine environment. Work is proceeding toward demonstrating this concept in the engine environment.
Modeling and Integration of Active Internal Flow Control in Aero-Engine Compressors
This project constitutes a research program on the key technical issues relating to the modeling and integration of flow control devices in aero-engine compressors. The project focuses on the physical understanding of the governing flow phenomena, their implementation and experimental application with the goal to further enhance the performance and operability of axial-flow compressor stages beyond the current loading and stability limits. The theme of the technical approach is a unique analytical, reduced-order dynamic system modeling methodology, which incorporates a system rather than a component view of this multi-disciplinary problem. This work is in collaboration with the NASA Glenn Research Center where a proof-of-concept experiment with synthetic jets will be conducted in the Low Speed Axial Compressor test facility.
Quiet Supersonic Platform (QSP) Propulsion
We are examining the propulsion requirements for very quiet long range, MACH 2+ cruise vehicles. As part of this effort we are designing a 2 stage aspirated compressor with a pressure ratio of 10:1.
MICRO ENGINES
Microengine Materials, Structures and Packaging
Includes tasks of materials characterization and constitutive modeling and the overall thermal and structural design of the microengine and associated devices. Packaging includes the design and fabrication of the interfaces of these micro-devices with the macro-world, including fuel supplies, air intakes and electrical contacts.
Micro Bearing Rig Rotordynamics
A fundamental enabling technology needed for all the high-power-density micro devices is the ability to spin silicon rotating elements, supporting the turbomachinery and electrical components, at peripheral speeds of several hundred meters per second, over two orders of magnitude faster than silicon rotors have previously achieved. This research focuses on experimental studies on the rotordynamic and hydrostatic journal gas bearings and thrust bearings of the micro devices.
MicroEngines (MEMS Gas Turbines, Generators, & Rocket Engines)
A multidisciplinary effort, in cooperation with the MIT Micro Technology Laboratory, is underway to develop gas turbine and rocket engines a few millimeters in diameter spinning at 2-5 x 106 rpm. The devices would be capable of producing 10-50 watts of power or 10-30 grams of thrust. Applications include battery replacement and micro-airplane propulsion. A subset of this effort is a program to build micro electric motor driven compressors of similar size. Also, a bipropellant, turbopump equipped, centimeter sized micro rocket engine, producing 3-5 lb. of thrust, is under development. There efforts encompasses all aspects of gas turbine and rocket propulsion engineering, including the fluid mechanics of turbomachinery, mechanical design, structures and materials, combustion, bearings, electric generators, materials, and microfabrication.
MEMS in Turbomachinery
An analytical and experimental program which is evaluating the use of large arrays of high frequency response micro-fabricated flow valves arranged on the tip casing of a compressor to alter the tip flowfield in an advantageous manner.
ENVIRONMENTAL IMPACT
A Functionally Silent Aircraft to Transform Commercial Air Transportation
Aircraft noise is a major inhibitor of the growth of air transport. This project focuses on revolutionary enabling technologies for a functionally silent aircraft. Silent in this context means sufficiently quiet that the aircraft noise is less than that of the background noise in a typical well-populated environment. In order to make aircraft operations quiet enough such that the noise is not perceived as annoying by the community, airframe and propulsion system noise have to be reduced dramatically and beyond the current noise reduction goals. The proposed work introduces revolutionary concepts for a functionally silent aircraft and focuses on feasibility and quantitative assessment of the system integration of these enabling technologies.
Post-Combustion Flow Field Effects on Engine Exhaust Composition
Aircraft emissions are implicated in a diverse range of local and global atmospheric effects. This ongoing effort endeavors to make significant contributions towards understanding the role of engine hot section aerodynamics and kinetics in determining the composition of engine exhaust, specifically as these processes influence trace species constituents. A combined chemistry/flow model has been developed in collaboration with Aerodyne Research, Inc., that for the first time has allowed detailed 1-D, 2-D, and 3-D simulations of intra-engine trace species evolution. This program is currently pursuing detailed investigations of sulfur emissions, model development, and direct validation using planned experiments.
Gas Turbine Combustor Research
Some of the most complex flows within a gas turbine engine can be found in the combustor. As a result, design methods have historically been based largely on empiricism. We are working in collaboration with Pratt and Whitney and United Technologies Research Center to develop reduced-order models of various phenomena in gas turbine combustors. These models are being developed to provide insight into flow physics and chemistry, and to provide a basis for design tools that would serve as complements to empirical data and 3-D CFD.
High Fuel-Air Ratio Combustor and Turbine Research
As temperatures and overall fuel-to-air ratios increase in commercial and military aircraft engines, the potential for significant heat release due to post-combustor oxidation of partially reacted fuel is increased. Film-cooling flows can be the site of significant heat release, potentially altering surface heat transfer characteristics and damaging sections of the internal gas path. A shock tunnel test facility, numerical simulations and analytical modeling are being exercised to understand and provide design options to alleviate adverse effects that high fuel-air ratio combustion may have on hot-section durability, turbine performance, and pollutant emissions. The work is funded by Pratt and Whitney.
Low Greenhouse Gas Emissions Aircraft
We are performing systematic assessments of cost and emissions impacts of future aircraft technologies designed to reduce greenhouse gas emissions. Tools are being developed for use in a global, multiple transport mode context to conduct inter-modal comparisons of relative cost per unit emissions reduction potential for various technologies under different emissions regulation and demand scenarios. This work is jointly funded by MIT's Center for Environmental Initiatives and the MIT Cooperative Mobility Program.
System for Assessing Global Emissions of Aviation
Working with a team of researchers from the MIT Flight Transportation Laboratory and the DOT Volpe National Transportation Systems Center, we are developing what will become FAA's primary tool for assessing various policy options for regulating pollutant emissions from aircraft.
The Economic Value of Silence
Rising congestion and delays throughout the air transport system are in part associated with noise-related operational restrictions and airport expansion delays. The full societal costs of the constraints that these restrictions impose on service to the flying public, the quality of life for local communities, regional economic growth, and expansion of aerospace-related industries have never been rigorously documented. As a result, policy decisions regarding both regulatory strategies and federal expenditures to address aviation noise have not been adequately informed. We are collaborating with researchers from Cambridge University to articulate and provide a survey of the explicit and implicit costs of aircraft noise. Through this account, the potential benefits that follow reduced external, direct, investment, and opportunity costs associated with aircraft noise can be assessed. The work is thus a step towards enabling a rational account of the comparative utility of operational or technologically-oriented regulatory strategies, federal expenditures on noise reduction research, and local noise abatement.
SMART ENGINES
Dynamic Control of Compressor and Compression System Aerodynamic Instability
A multi-disciplinary research area is "smart engines," in which the components (inlet, compressor, turbine, etc.) are under local feedback control. A three-stage axial compressor, two helicopter engines, a subsonic and a supersonic inlet diffuser are instrumented to support this research.
The Active Rotor
The active rotor is a composite transonic fan with embedded actuators capable of altering both the static shape and dynamic response of the fan blades. It is designed to be a research tool for flutter and forced response, aerodynamic performance, noise, and compressor stability. The program is currently focused on the very challenging rotor mechanical design and construction.
Active Control of Shocks in Supersonic Inlets
A major design driver for supersonic diffusers is the requirement to prevent unwanted shock formation and shock blow-out (supersonic unstart). As part of the Quiet Supersonic Platform (QSP) program to create a new efficient supersonic vehicle in the business jet size range, efficient supersonic inlets are being designed that rely on feedback control, rather than traditional design methods, to meet the unstart requirement. Small-scale experiments at MIT will use high-speed schlieren imaging to study the dynamics and control of shock formation and movement.
Diffuser Separation Control
Serpentine inlets common in tactical aircraft introduce significant levels of distortion to the flow into the compressor. Open loop and feedback methods to reduce this distortion, mitigate associated unsteadiness, and improve the pressure recovery of the diffuser are being investigated. A 1/6th scale Unmanned Combat Aerial Vehicle (UCAV) inlet is being tested at MIT, with plans for large scale testing at NASA Glenn.
Modeling and Integration of Active Internal Flow Control in Aero-Engine Compressors
Active flow control is one possible strategy to enhance both the performance and the stability of compressor and turbine flow systems. Synthetic jets are a means of injection actuation and consist of an orifice or neck that is driven by an ocillating wall in a cavity. This research program focuses on the key technical issues relating to the modeling and integration of flow control devices in aero-engine compressors with the goal to further enhance the performance and operability beyond the current loading and stability limits. The theme of the technical approach is a unique analytical, reduced-order dynamic system modeling methodology, which incorporates a system rather than a component view of this multi-disciplinary problem. Preliminary test will be conducted in MIT GTL's linear compressor cascade wind tunnel and final experiments are planned to be carried out in NASA GRC's Low Speed Axial Compressor (LSAC) test facility.
ROBUST AEROTHERMAL DESIGN
The Robust Jet Engine Project
Variability in gas turbine engine performance due to manufacture and in-service wear is a key factor in the competitiveness in the gas turbine industry. Aircraft engines are subject to a number of competing requirements and current designs often represent a compromise between efficiency and many of the other "ilities" (affordability, reliability, operability, etc). Further, while a given design may be highly efficient, if small perturbations from in geometry or operating state lead to large variations in engine performance, then the engine is not performing robustly. A continuing need exists for (1) engines that are robust to variability with greater reliability and efficiency yet at lower costs and (2) next generation design tools to develop these improved engines. The Robust Jet Engine project is a response to these needs with a particular focus on aerothermal robustness. The goals of this program are: Identification and quantification of key drivers for uncertainty and engine-to-engine variability in aerothermal quality including validation against data; Definition of criteria for the design of engines with a commercially-significant reduction in sensitivity to uncertainty and variability including analysis of cost trade-offs; Development of improved processes for monitoring and controlling the effects of variability on aerothermal quality; Implementation of one or more of the above elements in an industrial setting.
