Michael J. Brear

Newton-Like Extremum-Seeking for the Control of Thermoacoustic Instability

In practice, the convergence rate and stability of perturbation based extremum-seeking schemes can be very sensitive to the curvature of the plant map. An example of this can be seen in the use of extremum-seeking to reduce the amplitude of thermoacoustic oscillations in premixed, gas-turbine combustors. This sensitivity to the plant map curvature arises from the use of a gradient descent adaptation algorithm. Such extremum-seeking schemes may need to be conservatively tuned in order to maintain stability over a wide range of operating conditions, resulting in slower optimization than could be achieved for a fixed operating condition. This can severely reduce the effectiveness of perturbation based extremum-seeking schemes in some applications. In this paper, a sinusoidally perturbed extremum-seeking scheme using a Newton-like step is developed. Non-local stability results for the scheme are formulated using a Lyapunov analysis. A local analysis of the scheme is given to investigate the influence of plant dynamics and to show that the local rate of convergence is independent of the plant map curvature. The benefit of this plant map curvature independence is then experimentally demonstrated in minimizing the thermoacoustic oscillations in a model premixed combustor.

The forced response of choked nozzles and supersonic diffusers

The response of choked nozzles and supersonic diffusers to one-dimensional flow perturbations is investigated. Following previous arguments in the literature, small flow perturbations in a duct of spatially linear steady velocity distribution are determined by solution of a hyper-geometric differential equation. A set of boundary conditions is then developed that extends the existing work to a nozzle of arbitrary geometry. This analysis accommodates the motion of a plane shock wave and makes no assumption about the nozzle compactness. Numerical simulations of the unsteady, quasi-one-dimensional Euler equations are performed to validate this analysis and also to indicate the conditions under which the perturbations remain approximately linear. The nonlinear response of compact choked nozzles and supersonic diffusers is also investigated. Simple analyses are performed to determine the reflected and transmitted waveforms, as well as conditions for unchoke, ‘over-choke’ and unstart. This analysis is also supported with results from numerical simulations of the Euler equations.

Newton-like extremum-seeking part I: Theory

In practice, the convergence rate and stability of perturbation based extremum-seeking (ES) schemes can be very sensitive to the curvature of the plant map. This sensitivity arises from the use of a gradient descent adaptation algorithm. Such ES schemes may need to be conservatively tuned in order to maintain stability over a wide range of operating conditions, resulting in slower optimisation than could be achieved for a fixed operating condition. This can severely reduce the effectiveness of perturbation based ES schemes in some applications. It is proposed that by using a Newton-like step instead of a more typical gradient descent adaptation law, then the behaviour of the ES scheme near an extremum will be independent of the plant map curvature. In this paper, such a Newton-like ES scheme is developed and its stability and convergence properties are explored.

Comparison of Open and Choked Premixed Combustor Exits During Thermoacoustic Limit Cycle

This paper compares the thermoacoustic limit cycles of a premixed laboratory combustor with acoustically open and choked exits. It is shown that the form of the downstream boundary condition can have a significant effect on the combustion chamber acoustics, with both the dominant limit cycle frequencies and the acoustic mode shapes being very different for the different combustor exits. The fundamental limit cycle frequency with the choked exit in place agrees closely with that determined by the convective time scales of entropy disturbances, as argued by other authors. A novel experimental method is then developed to examine the acoustic response of an arbitrary duct termination to incident pressure and entropy perturbations, and used to measure the response of the combustor downstream boundary condition during thermoacoustic limit cycle. The reflection coefficient for the acoustically open exit matches closely the classical result for zero mean flow. It is also shown that a choked nozzle downstream of the flame generates significant sound due to the interaction of the convected entropy perturbation with the nozzle. This final result is qualitatively in keeping with an existing analytic boundary condition for a choked nozzle, even though quantitative agreement is not observed. Reasons for this discrepancy are then suggested.

Pressure surface separations in low-pressure turbines-part 1: Midspan behavior

This paper describes an investigation into the behaviour of the pressure surface separation at midspan in a linear cascade. It is f ound that the pressure surface separation can be a significant contributor to the profile loss of a thin, solid, low pressure turbine blade that is typical of current engine designs. Numerical predictions are first used to study the inviscid behaviour of the blade. These show a strong incidence dependence around the leading edge of the profile. Experiments then show clearly that all characteristics of the pressure surface separation are controlled primarily by the incidence. It is also shown that the effects of wake passing, freestream turbulence and Reynolds number are of secondary importance. A simple two-part model of the pressure surface flow is then proposed. This model suggests that the pressure surf ace separation is highly dissipative through the action of its strong turbulent shear. As the incidence is reduced, the increasing blockage of the pressure surface separation then raises the velocity in the separated shear layer to levels at which the separation can create significant loss. NOMENCLATURE Cd dissipation coefficient

Flow Separation Within the Engine Inlet of an Uninhabited Combat Air Vehicle (UCAV)

We discuss the structure of the flow within the engine inlet of an uninhabited combat air vehicle (UCAV). The UCAV features a top-mounted, serpentine inlet leading to an engine buried within the fuselage. The performance of the inlet is found to depend strongly on a flow separation that occurs within the inlet. Both the time-averaged and the unsteady structure of this separation is studied, and an argument relating the inlet performance to the behavior of this separation is suggested. The results also suggest that there are considerable aerodynamic limitations to further shortening or narrowing of the inlet. Since there are substantial, system level benefits from using a smaller inlet, the case for separated flow control therefore appears clear

Design and Analysis of a Modified CFR Engine for the Octane Rating of Liquefied Petroleum Gases (LPG)

This paper presents a combined experimental and numerical study of a modified Cooperative Fuel Research (CFR) engine that allows both the Research and Motor octane numbers (RON and MON) of any arbitrary Liquefied Petroleum Gas (LPG) mixture to be determined. The design of the modified engine incorporates modern hardware that enables accurate metering of different LPG mixtures, together with measurement of the in-cylinder pressure, the air-fuel ratio and the engine-out emissions. The modified CFR engine is first used to measure the octane numbers of dif ferent LPG mixtures. The measured octane numbers are shown to be similar to the limited data acquired using the now withdrawn Motor (LP) test method (ASTM D2623). The volumetric efficiency, engine-out emissions and combustion efficiency for twelve alternative LPG mixtures are then compared with equivalent data acquired with the standard CFR engine operating on a liquid fuel. Finally, the modified CFR engine is modelled using GT-Power. The full engine model contains empirical sub-models of the intake and exhaust systems, the gas exchange processes, the flame propagation and the in-cylinder heat transfer . The calibrated combustion models are used to determine the residual gas fraction and crank angle resolved mass fraction burned histories during octane rating for both gaseous and liquid fuels. Overall, this analysis suggests that the performance of the modified CFR engine is consistent with that of the standard engine operating on a conventional, liquid fuel.

An Economic and In-Service Emissions Analysis of Conventional, Hybrid and Electric Vehicles for Australian Driving Conditions

Hybrid and fully electric vehicles are becoming more common as a response to rising fuel prices and greenhouse considerations. While the benefits of electrification on urban air quality have been studied quite widely, financial assessments of the various alternative vehicle forms are less common, particularly for Australian driving conditions. The aim of this paper is therefore to identify the scenarios under which different vehicle configurations are attractive to the vehicle owner. A Class-E conventional vehicle is compared with full-electric, plug-in hybrid, parallel hybrid, series hybrid and mild hybrid electric vehicle configurations. A simulation model of a conventional internal combustion engine based large sized car is developed and validated against experimental data. The conventional vehicle model is then systematically altered to obtain its increasingly electric variants. The fuel consumption and greenhouse gas emissions are simulated on the legislative NEDC drive cycle and the more representative Australian Urban Drive Cycle (AUDC). The outcomes of these tests are used to estimate the total cost of ownership and in-service emissions, thus allowing the cost of emissions mitigation to be approximated for the different vehicles. Different scenarios are considered for the pricing of energy and major powertrain components. This provides a baseline assessment based on current prices and projections, as well as ‘electrification favorable’ and ‘electrification unfavorable’ scenarios. The impact on vehicle emissions of significant penetration of renewable energy into the Australian electricity grid is also considered.

Economic Model Predictive Control and Applications for Diesel Generators

When developing control systems for diesel generators, tuning of the controller’s parameters to achieve acceptable performance is a significant challenge, particularly while satisfying input, emission, and safety constraints in the face of unknown system disturbances. Robust economic model predictive control (EMPC) can simplify this process by directly addressing the generator’s objectives, while systematically handling constraints in a robust way. This paper details how robust EMPC can be implemented as the control solution for diesel generators. To illustrate the process, two distinct generator applications are considered. The first application is a power tracking diesel generator, operating under emissions constraints. Such an application is found in series hybrid electric vehicles. The second application concerns diesel generators onboard submarines. In this application, engine speed and exhaust temperatures must be kept constant, despite significant system disturbances. An experimental study highlights the effectiveness of the EMPC as a solution for both applications.

Pressure Surface Separations in Low-Pressure Turbines—Part 2: Interactions With the Secondary Flow

This paper describes a study of the interaction between the pressure surface separation and the sec ondary flow on low pressure turbine blades. It is found that this interaction can sign ificantly affect the strength of the secondary flow and the loss that it creates. Experimental and numerical techniques are used to study the secondary flow in a family of four low pressure turbine blades in linear cascade. These blades are typical of current designs, share the same suction surface and pitch, but have differing pressure surfaces. A mechanism for the interaction between the pressure surface separation and the secondary flow is proposed and is used to explain the variations in the secondary flows of the four blades. This mechanism is based on simple dynamical secondary flow concepts and is similar to the aft-loading argument commonly used in modern turbine design. NOMENCLATURE Cd dissipation coefficient CX axial chord (m) () X P L C