Manuj Dhingra

Development and Demonstration of a Stability Management System for Gas Turbine Engines

Rig and engine test processes and in-flight operation and safety for modem gas turbine engines can be greatly improved with the development of accurate on-line measurement to gauge the aerodynamic stability level for fans and compressors. This paper describes the development and application of a robust real-time algorithm for gauging fan/ compressor aerodynamic stability level using over-the-rotor dynamic pressure sensors. This real-time scheme computes a correlation measure through signal multiplication and integration. The algorithm uses the existing speed signal from the engine control for cycle synchronization. The algorithm is simple and is implemented on a portable computer to facilitate rapid real-time implementation on different experimental platforms as demonstrated both on a full-scale high-speed compressor rig and on an advanced aircraft engine. In the multistage advanced compressor rig test, the compressor was moved toward stall at constant speed by closing a discharge valve. The stability management system was able to detect an impending stall and trigger opening of the valve so as to avoid compressor surge. In the full-scale engine test, the engine was configured with a one-per-revolution distortion screen and transients were run with a significant amount of fuel enrichment to facilitate stall. Test data from a series of continuous rapid transients run in the engine test showed that in all cases, the stability management system was able to detect an impending stall and manipulated the enrichment part of the fuel schedule to provide stall-free transients.

A Stochastic Model for a Compressor Stability Measure

A stability measure rooted in the unsteady characteristics of the flow field over the compressor rotor has been previously developed. The present work explores the relationship between the stochastic properties of this measure, called the correlation measure, and the compressor stability boundary. A stochastic model has been developed to gauge the impact of the correlation measures stochastic nature on its applicability to compressor stability management. The genesis of this model is in the fundamental properties of a specific stochastic process, one that is created by the threshold crossings of a random process. The model validation utilizes data obtained on three different axial compressor facilities. These include a single-stage low-speed axial compressor, a four-stage low-speed research compressor, and an advanced technology demonstrator high-speed compressor. This paper presents details of the model development and validation, as well as closed loop experimental results to demonstrate correlation measures usefulness in coinpressor stability management.

Compressor Surge: A Limit Detection and Avoidance Problem

Compressor surge control is an important part of intelligent engines. A new perspective on this problem is presented. The problem is cast as a limit avoidance control. A single pressure sensor is used to monitor the uncertain surge limit, via an innovative method using the so called correlation measure. The stochastic nature of this measure has been analyzed and a model for the same has been developed. The model is incorporated into a digital simulation of a turbo-shaft engine. A Monte-Carlo simulation has been carried out to investigate the impact of parameters associated with the stochastic model. The limit avoidance formulation is demonstrated on a laboratory axial compressor facility.

Active Compressor Stability Management Via a Stall Margin Control Mode

An active engine control scheme for protection against compressor instabilities such as rotating stall and surge is presented. Compressor stability detection is accomplished via a parameter known as the correlation measure, which quantifies the repeatability of the pressure fluctuations in the tip region of a compressor rotor. This work investigates the integration of the correlation measure with an aircraft engine control system through the use of a stall margin control mode. The development and implementation of the stall margin mode is described. The effectiveness of the overall active control framework—an active compressor stability management system—is assessed using a computer simulation of a high-bypass, dual-spool, commercial-type turbofan engine.Copyright

Compressor stability detection on a turboshaft engine with a hybrid axi-centrifugal compressor

A compressor stability measure rooted in the unsteady characteristics of the flow field in the tip region of a compressor rotor has been previously developed. This measure, called the correlation measure, has been used to successfully control a laboratory axial compressor facility, and has also been demonstrated on a modern gas turbine engine. The present work is the first application of this measure to a hybrid axi-centrifugal compressor. Experimental studies have been carried out on a small turboshaft gas turbine (GE-T700) at NASA Glenn’s Engine Components Research Lab. The experiments conducted include steadystate compressor operation at multiple speeds and transient operation with acceleration of the compressor spool. The compressor operating line is raised by injecting external air through bleed ports downstream of the compressor. The results show that the measure can successfully detect the limit of stable compressor operation, prior to the onset of instabilities. Within the limitations of the test facility, proof-of-concept closed-loop control has demonstrated the eectiveness of fuel actuation for compressor stall avoidance. In particular, it has been determined that such a control can be implemented using existing fuel actuators.

Impact of Inter-Stage Dynamics on Stalling Stage Identification

A key objective of compressor rig tests is the identification of compressor stall boundary. A complementary goal is the identification of the stalling stage based on test data. This serves two purposes: 1) Validate the pre-test prediction of the stage loading distribution, and 2) identify the weak stages, should improvements in operating range be desired in subsequent design iterations. Typically the pertinent test data is in the form of static pressure measurements. Many engineers believe that a stalling stage is accompanied by a transient upstream pressure rise coupled with a downstream pressure loss. However, inter-stage dynamics may cloud the identification of the stalling stage. To this end, an analysis of inter-stage dynamics, immediately preceding the stall event, could provide an alternate assessment of the stalling stage. This work reviews existing stall models for studying compressor dynamics. The main focus of this work is to develop ability to capture inter-stage dynamics. A 3-state equation lumped Moore-Greitzer (MG3) model is widely used to study the dynamic compressor response during surge and rotating stall transients. However the evolution of MG3 model may not provide a suitable framework for the investigation of inter-stage dynamics. On the other hand, an unsteady time marching 1-D fluid dynamic model (e.g. similar to the DynTECC formulation which includes body forces), while unable to capture the rotating stall dynamics, is sufficient for this purpose. A numerical simulation has been developed to investigate the impact of stage characteristics, as well as load distribution on the compression and expansion waves that develop prior to a surge event. Through a controlled weakening of selected stages, the time evolution of these waves is related back to the stalling stage. It is found that the weakened stage is not necessarily the stalling stage as identified via the pressure rise and downstream pressure drop pattern.Copyright

Experimental Analysis of a Waveguide Pressure Measuring System

An infinite-line probe is commonly used to measure unsteady pressure in high-temperature environments while protecting the pressure transducer. In this study, an existing theoretical model is used to derive the response of a waveguide pressure measuring system. An ambient temperature centrifugal compressor rig acts as an experimental source of fluctuating pressure. The compressor is operated at different discrete rotational speeds, and the blade-passing frequencies are used to obtain frequency response data. In the experiments, pressure waves attenuated at a rate faster than that predicted by the theoretical model for a 0.322 m (12 in.) sensor offset. Furthermore, the decay in the magnitude of the pressure oscillations accelerated at blade-passing frequencies above 9 kHz. A unique contribution of this study is to show that whereas the experimentally observed overall attenuation is broadly consistent with the theoretical predictions, pressure oscillations corresponding to individual blade passages may be disproportionally attenuated.

Benefits of Active Compressor Stability Management on Turbofan Engine Operability

Active compressor stability management can play a significant role in the intelligent control of gas turbine engines. The present work utilizes a computer simulation to illustrate the potential operability benefits of compressor stability management when actively controlling a turbofan engine. The simulation, called the modular aeropropulsion system simulation (MAPSS) and developed at NASA Glenn, models the actuation, sensor, controller, and engine dynamics of a twin-spool, low-bypass turbofan engine. The stability management system is built around a previously developed stability measure called the correlation measure. The correlation measure quantifies the repeatability of the pressure signature of a compressor rotor. Earlier work has used laboratory compressor and engine rig data to develop a relationship between a compressors stability boundary and its correlation measure. Specifically, correlation measure threshold crossing events increase in magnitude and number as the compressor approaches the limit of stable operation. To simulate the experimentally observed behavior of these events, a stochastic model based on level-crossings of an exponentially distributed pseudorandom process has been implemented in the MAPSS environment. Three different methods of integrating active stability management within the existing engine control architecture have been explored. The results show that significant improvements in the engine emergency response can be obtained while maintaining instability-free compressor operation via any of the methods studied. Two of the active control schemes investigated utilize existing scheduler and controller parameters and require minimal additional control logic for implementation. The third method, while introducing additional logic, emphasizes the need for as well as the benefits of a more integrated stability management system.

Active Compressor Stability Management and Impact on Engine Operability

A new, innovative compressor stability management system is described. The system combines surge precursor detection using a single pressure sensor with a control scheme utilizing existing actuators. A digital simulation of a small turbo-shaft engine is used to demonstrate the benefits of this system towards engine operability. Three dierent actuators are evaluated. Both fuel and bleed actuation are found to be suitable for surge avoidance. In contrast, guide vanes, which could be used for anticipatory surge margin enhancement, are found to be ineective when used actively.

An Analytical Approach to Gas Turbine Engine Model Linearization

This paper presents analytical linearization schemes of a reduced order aero-thermodynamic model of the generic back end of a turbofan engine. The proposed linearization scheme has advantages of flexibility and reusability over the commonly used linearization method based on the numerical perturbation scheme. Also, a blending algorithm employing the distance to the boundary as the weight has been incorporated into the linearization scheme to capture the change of the flow behaviour near bifurcating boundaries. The proposed linearization scheme is developed and applied to a back end model of a generic turbofan engine with bifurcations corresponding to choked/unchoked boundaries. This model is also used for proof of concept validation test.Copyright