Recent/Current Research at the Gas Turbine Laboratory

A Unified Approach for Vaned Diffuser Design in Advanced Centrifugal Compressors

Ruhou Gao
Advisor: Prof. Spakovszky

Modern internal combustion engines require high pressureratio turbochargers and NOx control strategies to improve efficiency and reduce emissions. For such applications, the centrifugal compressor has to simultaneously achieve high efficiency, high pressure ratio and a broad operating range. To meet these requirements, the design trend has been towards highly loaded compressors with high speed impellers and backswept blades for extended operating range and vaned diffusers for enhanced pressure recovery and compact geometry.

 

 

 

The transonic, unsteady and highly non-uniform flow out of the impeller presents multiple challenges for the vaned diffuser. A previous study has identified links between impeller outflow and diffuser performance based on appropriately averaged flow properties. However, the connection between the diffuser design parameters and overall diffuser system performance (which includes the downstream volute) needs yet to be established.

 

The objectives of the current investigation is to rigorously identify the key geometric parameters that govern diffuser performance and to establish design guidelines for different families of impeller-diffuser combinations. The technical approach combines first principles based modeling with high-fidelity calculations and full scale experiments for performance validation. The goal is to develop a set of unified design guidelines suitable for integration in the preliminary design process.

 

 

 

 

 

 

Investigation of Real Gas Effects in Supercritical CO2 Compression Systems

Derek Paxson, and Dr. Claudio Lettieri
Advisor: Prof. Spakovszky

Reduction of harmful CO2 emissions from power plants is becoming a major concern in industry and a high priority for research, development, and deployment projects. Carbon capture and sequestration in underground wells requires the fluid to be compressed to high pressures (exceeding 100 bar) reaching supercritical conditions. At this state the fluid has density similar to a liquid, but at the same time expends to fill up a volume just like a gas does. Although the properties of these working fluids can be characterized, little is known about their behavior in turbomachinery.

Unlike ideal fluids, thermodynamic properties of real fluids can vary significantly, especially near the critical point, altering the gas dynamics.  The main area of interest in the compressor is the leading edge of the impeller.  Here the flow is accelerated and sees a rapid drop in pressure.  The rapid rate of the expansion leads to non-equilibrium condensation and the fluid entering a metastable state.  This local condensation can lead to a reduction in efficiency and instability in the compressor.  Little is known about the behavior of CO2 in the metastable region, and consequently experimental data is needed to characterize the behavior before it can be modelled in multiphase CFD. 


A laboratory scale high pressure blowdown rig was constructed to assess the behavior of the supercritical CO2. Due to the complexity and cost of constructing and instrumenting a rotating compressor stage, a static test case was needed.  A converging-diverging nozzle with characteristic through-flow times and length scales similar to those of the compressor was chosen as a test case. Nozzles can easily be constructed and instrumented and provide a rapid pressure drop similar to those seen in the compressor.  Modifications to allow optical access to the test section are currently underway.  Optical measurement techniques such as interferometry will allow for the measurement of the density or velocity of the fluid.  This information, along with high speed pressure measurements along the nozzle, will allow for complete characterization of the thermodynamic properties of the fluid in both equilibrium and metastable states.  Testing on the rig will be used to calibrate 2-phase models which will be used for CFD of the entire compressor stage geometry.

 

Investigation of Transcritical Phenomena for Fuel Injection and Mixing Applications

Georgi Subashki , and Dr. Claudio Lettieri
Advisor: Prof. Spakovszky

Inefficient atomization in modern combustors leads to large quantities of unburned fuel which greatly reduces the combustion efficiency. In addition, inhomogeneous concentration of vapor fuel creates regions of higher localized flame temperature, enhancing the formation of harmful pollutants. These challenges can be efficiently addressed by using fuels at supercritical conditions. In particular, recent studies suggest that 80% reduction in NOx emissions and 10% improvement in brake thermal efficiency (Anitescu et al, 2009) can be achieved for injection and fluid mixing at supercritical conditions. In addition, the enhanced mixing of the fuel at supercritical state might enable more compact and lightweight combustor designs.

At supercritical state, fluids exhibit peculiar characteristics: vanishing surface tension, liquid-like density, and gas-like diffusivity. Supercritical fluids can provide potential benefits in a variety of engineering applications but there is little information available in the literature about the transition from subcritical to supercritical state and the related underlying mechanisms. A key challenge is that vanishing surface tension makes standard two-phase modeling inapplicable due to the increased difficulty in defining a phase front. Also, the large variations in fluid properties near the critical point can result in non-ideal, non-equilibrium gas behavior which require accurate and computationally efficient equation of state models.


This project aims to characterize the fluid behavior in transcritical fuel injection and mixing. For the purpose, subcritical and supercritical single-droplet evaporation is studied first. Appropriate non-dimensional groups are determined from a scaling analysis and parametric numerical multi-phase flow calculations are carried out to establish a general fluid-independent model for transcritical phase change. Ultimately, it is desired to integrate such models in the industrial design process for more efficient combustors.

 

A New Modeling Approach for Rotating Cavitation Instabilities in Rocket Engine Turbopumps

Vincent Wang , and Dr. Claudio Lettieri
Advisor: Prof. Spakovszky

Axial inducers are often used in high performance turbopumps for rocket engines in order to reduce system weight and cost. The inducer allows operation at high rotational speeds and low inlet pressures, which in return can lead to cavitation within the inducer. The dynamic cavitation behavior is often unsteady and periodic, leading to several distinct cavitation instabilities.


Of particular concern among the various instabilities is rotating cavitation, in which cavities propagate circumferentially at frequencies between 1 and 2 times the rotor frequency. In some cases, the propagation frequency can approach the rotor shaft natural frequency, which can cause severe vibrations that present a significant risk to the turbopump. Unfortunately, the physical mechanisms of rotating cavitation are not yet well understood, and as such no general design guidelines exist to mitigate its onset. Existing low-fidelity modeling efforts, which are largely two dimensional, have no predictive capability and fail to capture the three-dimensional nature of the phenomenon.


The goal of this project is to establish a new capability to guide the design of the inducer and related casing treatment to suppress rotating cavitation instabilities. The approach exploits a recently developed body-force based modeling approach that has been used successfully in aerodynamic instability modeling of jet engine axial and centrifugal compressors as well as in acoustic descriptions of non-uniform flow in transonic fans. The feasibility of employing this method to determine the frequencies and mode shapes of cavitation instabilities will be examined. The technical objectives are to (1) define a representative inducer geometry of modern design exhibiting the typically observed dynamic behavior including rotating cavitation, (2) identify the required inducer blade loading distributions and endwall flow forcing requirements to mitigate rotating cavitation, (3) determine the casing treatment type and geometry that can render the required forcing, (4) assess the effect of inlet distortion on cavitation dynamics, and (5) investigate the linkages between rotordynamics and cavitation.

 

 

An inducer was designed, manufactured, and experimentally tested. The MIT inducer meets the predicted performance requirements and captures the hypothesized dynamic behavior. At low flow coefficients, cavitation surge is observed while at higher flow coefficients alternate blade cavitation and rotating cavitation are identified.

 

Modeling Instabilities in High-Pressure Pumping Systems

Dr. Claudio Lettieri
Advisor: Prof. Spakovszky

Pumps are used in the process industry and in power plant applications. They must be able to operate over a wide range of flow rates resulting from variations in load. The particular pumping system under consideration is unstable near its best efficiency point. The challenges are to determine the physical mechanism leading to this system-wide instability and how to redesign the system to ensure stable operation at all relevant flow rates. The system is modeled using a previously-established dynamic modeling framework. This is combined with appropriate boundary conditions to obtain eigenvalues, which are natural frequencies of the system, and their associated growth rates. This determines the stability characteristics of the system. The pumping system model is schematically shown below and includes a plenum containing a gas spring, various area changes, and piping both upstream and downstream of the pump.

 

Some system components dissipate energy, while others act as energy sources. The pump is an active element, which can dissipate or add energy to the system. The working hypothesis is that the pump is the cause of the observed instabilities. The pump is comprised of an impeller (rotating part) and a volute (stationary part). The impeller dynamics are known. The volute dynamics, however, are not known. It is therefore necessary to obtain a dynamic model of the volute to complete the system stability model. To obtain such a model and to identify the source of the unsteadiness, unsteady RANS CFD calculations of the flow in the volute are conducted. These revealed that the key mechanism governing the self-excited unsteady flow in the volute is bluff body flow separation in the return channel. At the best efficiency operating point, the dynamic behavior in the volute due to this bluff body separation is such that energy is fed into the system. Overall there is insufficient damping, leading to dynamic instability. The figure below illustrates the mechanism which leads to the unsteady behavior in a simplified planar diffuser geometry and relates it to the flow in the volute.

The next step is to conduct a numerical parametric study of the effects of velocity and pressure perturbations on the flow in the volute. This system identification study will result in a dynamic model for use in the existing modeling framework. The framework can then be used to predict at which flow rates unstable operation will occur. Such a prediction tool, coupled with the insight gained into the unsteady flow in the volute, will be used to guide design changes to render the system stable.


Ported Shroud Operation in Turbochargers

George Christou
Advisor: Dr. Tan

In recent years, due to environmental regulations, automotive turbochargers have been increasingly implemented to accomplish high powering and downsizing of internal combustion engines. The operability of the compressor is bound at low mass flow rate by the surge line. Surge is characterized as a breakdown of the flow with large pressure fluctuations that can cause rapid deterioration and in some cases failure of the compressor and the bearing system. A technique used to control the development of surge is by implementing a ported shroud at the inlet of the compressor. The ported shroud configuration is used to improve both the choke and surge lines on the compressor performance map.

Garret by Honeywell, Turbo Tech 103 (Expert), 2006

The overall goal of this research project, in collaboration with Honeywell Turbo Technologies, is to improve the performance of ported shroud centrifugal turbochargers. Specific goals include: providing an explanation of changes in the flow processes with and without ported shroud relative to compressor operation; identifying and quantifying loss mechanisms present in ported shroud centrifugal compressors; increasing the effective operating range by increasing surge and choke margins; and increasing the efficiency at off-design operating points.

 

Aerodynamic Benefits of Boundary Layer Ingestion for the D8 Double-Bubble Aircraft

Cécile Casses
Advisors: Prof. Greitzer, , Dr. Uranga, Dr. Titchener

Boundary Layer Ingestion (BLI) -- passing a portion of an airframe's wake through the engine -- has been suggested as a means of reducing civil aircraft energy consumption. An aerodynamic performance benefit results from the reduction of viscous dissipation in the airframe wake and in the propulsor jet downstream of the aircraft. A number of challenges exist in the design and performance assessment of BLI aircraft configurations. One is that definition of the propulsion system requirements becomes more difficult because the concepts of thrust and drag, conventionally associated with the engines and airframe, respectively, become ambiguous with a tightly integrated propulsion system. Further, the engine performance itself may be adversely affected by the presence of inlet distortions arising from the ingested airframe boundary layer.


The goal of this research is to determine the aerodynamic BLI benefit of the D8, advanced civil aircraft design (see picture below). The metric used is the mechanical flow power, which is the volume flux of stagnation pressure for the flow regime of interest. Two independent experimental methods have been applied to wind tunnel investigations using a 1:11 scale powered model. The experiments were carried out during two entries in the NASA Langley Research Center 14'x22' Wind Tunnel. The first method is the direct integration of stagnation and static pressure measurements from a five-hole probes at the inlet and exit of the propulsors. This enables information on flow angles, velocity components, and pressure coefficients at the inlet of the propulsors, thereby informing about inlet distortion. The second method assesses the conversion of electrical power into mechanical flow power. To carry out the assessment, experiments to characterize the fans and the electric motors that drive them have been conducted in the MIT GTL 1'x1' Wind Tunnel. Computational assessment is also being carried out by NASA Ames Research Center.


The main results are that the aerodynamic BLI benefit is approximately 8-10%. This comes mainly from the increase in propulsive efficiency (85%) with a small contribution from the decrease in airframe dissipation.

 

Credit: NASA George Homich


Operability and Engineering Attributes of Artificial Lift System for Oil-Production

Sebastien Mannai
Advisor: Dr. Tan

Oil extraction on many reservoirs requires the use of artificial lift systems to pump the fluid to the surface. While conventional rod pump system work well for vertical well, there is an increasing push towards horizontal wells that is showing the limits of conventional low cost pumping techniques such as rod pump systems. Today there are few effective artificial lift solutions for sweeping fluids through lateral sections. The movements of the rod pump downhole are difficult to evaluate and pumping becomes inefficient and difficult to predict for low flow rate systems.


     

As such there is a need to assess and define the engineering attributes/requirements of generic low flow rate artificial lift system for an effectivetransport of the oil mixture to the surface for a representative oil production site. The research framework contemplated would include computations, modeling and experiments on generic fluid-pumping system representative of an unconventional fractured horizontal producing well as shown below.

 


     

Centrifugal Compressor Science and Technology: Multi-parameter Control for Compressor Performance Optimization

Sebastien Mannai
Advisor: Dr. Tan

Centrifugal compressor systems deployed in the industry are required to operate 24/7 with minimum down time. As such their operation at high thermodynamic efficiency across a wide operating range is of paramount importance to the customers.

Extending the compressor limits to meet the needs of a specific engineering mission is one of the most important aspects of compressor engineering. Strategies to quantitatively assess the potential for extending compressor performance and operating range must be developed. This can be done by determining the drivers that set the requirements for the broadest operable range with high efficiency retention.

The goal of this research is to first identify what are the parameters of high leverage that affect centrifugal compressor performance followed by establishing potential means of achieving near matching of centrifugal compressor components at all desirable operating points required for its mission. Some of the key parameters that are thought to have a high leverage on compressor performance characteristics are compressor speed, guide vane setting and diffuser vane angle setting; however there could be others that are to be identified during the course of the research. In light of this, formulating an effective control strategy (passive, active or a combination of both) for achieving desirable compressor performance requirements for its specified mission (as alluded to above) at an optimal cost would also be another goal for this research.
The general approach consists of leveraging on the technical capability and thinking at the MIT Gas Turbine Laboratory and of working collaboratively with Siemens Technologists. It will also be necessary to define physical experiments at Siemens Facility in Germany or at MIT Gas Turbine Laboratory for assessing ideas and concepts formulated during the course of the research.

 

 

 

Secondary Air Interactions with Main Flow in Axial Turbines

Penghao Duan, Visiting Student - ETH
Advisor: Dr. Tan

In the past decade, industrial gas turbines have by far become the most popular type of plant for power generation due to their compactness, low emissions and potential for power-heat cogeneration. In the effort to increase energy conversion efficiency, engineers have raised turbine inlet temperatures to well above the metal melting point. Turbine blades are generally protected by expensive thermal barrier coatings and various forms of internal and film cooling. However, in order to prevent hot gasses from being ingested into the unprotected cavities between rotating and stationary components, cool air bled from the compressor is used to purge the gaps at the endwalls. MIT, in collaboration with Siemens Energy and Siemens Corporate Research, is developing a computational approach to identify and understand loss generating flow processes of purge air interacting with mainstream flow in axial turbines.

Contours of change in volumetric entropy generation rate relative to a baseline case with no purge flow bring out the regions in a rotor blade passage that have modified losses as a consequence of purge flow injection from the hub gap upstream of the rotor. We have identified a number of effects that result in these changes: mixing out of the velocity difference between purge and mainstream flows, the generation of radial velocity gradients as a consequence of purge flow interacting with the passage vortex structures, and increased wetted and tip clearance flow losses due to a change of reaction. There is also a positive effect of reduced tip clearance losses when purge flow is injected from the shroud. These effects have been rigorously quantified, and their drivers have been pinpointed. This new knowledge provides clear guidelines for better turbine designs.


Turbine Tip Clearance Loss Mechanisms

Steven Mazur
Advisors: Prof. Greitzer, Dr. Tan

 

 

One of the large loss sources in a turbine stage arises from the flow through the gap between the rotor tip and the shroud. The pressure difference across the tip drives the flow through the gap, and this leakage flow subsequently rolls up into a vortex on the suction side of the blade and convects downstream. As the vortex mixes out and decays, entropy is generated. Previous work by Arthur Huang has identified the pressure gradient external to the vortex as a major mechanism for determining the loss generated by the tip vortex. The current project aims to consider new influencing factors on the vortex evolution and associated loss. 3D computational simulations are being used to study the influence of several classes of effects. Downstream influence of the transition duct at the exit of the high pressure turbine can have an impact on the external conditions the tip leakage vortex is subjected to. A parametric study is underway to illustrate how the governing design parameters influence the tip clearance loss. Future project goals are to discover how upstream and unsteady effects change the loss created by turbine tip gap flows.

 

Flow and Heat Transfer in Modern Turbine Rim Seal Cavity

Peter Catalfamo, and Rachel Berg
Advisor: Dr. Tan Sponsor: General Electric

Ingestion of hot gas from the flowpath into the gaps between the rotor and stator can cause turbine components to overheat and lead to deterioration in component life. To prevent this, modern gas turbines, both industrial and aerospace, use compressor bleed air to provide positive outflow through the rim seal (known as “purge” flow). This purge flow can be a substantial fraction of the total flow bled off of the compressor and as such it represents a substantial performance penalty. Past efforts have focused primarily on generating correlative orifice models using experimental data. These results are limited in their applicability by the geometry and conditions tested. In this research MIT, in collaboration with GE Energy and GE Aviation, seeks to investigate the fundamental flow physics in the turbine rim cavity region. Of particular interest is the response of the wheelspace and rim cavity to external stimuli set up by the main annulus flow such as flow unsteadiness due to rotor stator interactions. Rig data being collected by GE will be used to assess the analysis and to guide the investigation. Understanding these mechanisms is fundamental to optimizing seal design and minimizing the purge flow requirements, thus minimizing the associated performance penalty.

 

Characterization of Performance Limiting Flow Mechanisms in a Centrifugal Compressor Stage

David Erickson
Advisor: Dr. Tan, Sponsor:  GE Aviation

High performance centrifugal compressors featuring vaned or pipe diffusers are commonly utilized in gas turbine engines for today’s small aerospace applications, such as turboprops, rotorcraft, and small business jets.  With advanced aerospace engines trending towards increased overall pressure ratios and decreased core airflow, centrifugal compressors have the potential to see even more widespread use in the future.  However, the physical relationships between centrifugal compressor geometry, the resulting flow behavior, and ultimately the compressor’s performance are not as well understood as they are for axial compressors.  This makes the preliminary design of centrifugal compressors a challenging task, with reliance on empirical models of limited applicability.

This project aims to identify performance limiting flow mechanisms in a centrifugal compressor stage, with a special interest in the diffuser and its influence upstream and downstream, and to describe the causal relationships between these mechanisms and compressor performance (efficiency, pressure rise capability, and operable range).  GE Aviation is providing aerodynamic data measured with high spatial and temporal resolution on a highly instrumented centrifugal compressor utilizing various diffuser geometries and subjected to a wide range of operating parameters.  This data, along with targeted CFD experiments, will be interrogated to identify the flow mechanisms present, and ultimately to explain physically how they give rise to the measured performance trends.

 

Aerothermodynamics and Operation of Turbine System under Unsteady Pulsating Flow

Jinwook Lee
Advisor: Dr. Tan

Turbine system representative of that used in turbochargers and pulse detonation engines operate under highly pulsating flow environment. As the time scale characterizing the response of the system or its individual components (e.g. sub-volutes, diffuser et al.) can be comparable to that of the flow pulsation at the system inlet (i.e. a reduced frequency  ), the turbine system operation may have to be defined on an unsteady flow basis. The overall goal is thus to first determine the efficiency limiting flow processes in the unsteady turbine system followed by defining possible paths toward improving system performance. The approach taken consists of calculating unsteady three-dimensional flow in the turbine system followed by in-depth interrogation complemented with flow modeling on a technically adequate basis. One of the preliminary results thus far indicates that the flow response in turbine wheel, diffuser and wastegates appears locally quasi-steady. An implication of the finding is that with adequate modeling of unsteady effects in the sub-volutes, the operation of turbine system can potentially be determined based on a series of steady calculations subjected to varying inlet conditions reflecting the inlet flow pulsation.

 

 

Flow Visualization of Compressor Leading Edge Vortex Shedding in Spike-Type Rotating Stall Inception

Andras Kiss
Advisor: Prof. Spakovszky

 

Compressor rotating stall and surge limit the
stable operating range of gas turbine engines and
can result in dramatically reduced performance
and possibly catastrophic engine failure. The
formation and growth of spike-type stall precursors
is one route to rotating stall that is currently not well
understood. While spike-type stall precursors have
historically been attributed to compressor blade tip
leakage flow, experiments have shown this not to be
the case. Recently, vortex shedding at the rotor tip
leading edge has been hypothesized as the causal
mechanism for spike-type stall precursors.

 

This work seeks to assess this hypothesis through
a combined numerical and experimental approach.
A linear rotor blade cascade was constructed to
re-create the flowfield of the MIT single stage
compressor during spike-type rotating stall.
Measurements from high-response pressure
transducers installed and a hotwire anemometer
confirmed that the spike precursor was captured
in the cascade environment. Further, smoke
injection at the compressor blade tip visualized
the formation and propagation of the spike-type
stall precursor.  3D URANS calculations of both
the compressor and the cascade, which are in
agreement with the flow visualization results,
confirm leading edge vortex shedding as the
mechanism for spike formation in both
environments.

 

A remaining unknown in spike-type rotating stall
inception is the mechanisms for spike propagation
and growth. Current work is focusing on characterizing
the relative importance of vortex line stretching, viscous
diffusion of vorticity, and the shedding of new vorticity
from adjacent blades on the growth of the spike
precursor. A simplified vortex model is under
development to capture the dynamics of spike
propagation and growth.