Keith Robert Pullen

Axial-flux permanent magnet machines for micropower generation

This paper reports on the design, fabrication, and testing of an axial-flux permanent magnet electromagnetic generator. The generator comprises a polymer rotor with embedded permanent magnets sandwiched between two silicon stators with electroplated planar coils. Finite element simulations have been carried out using ANSYS to determine the effects on performance of design parameters such as the number of layers in the stator coils, and the rotor-stator gap. The effect of including soft magnetic pole pieces on the stators has also been studied. A prototype device with a diameter of 7.5 mm has been tested, and shown to deliver an output power of 1.1 mW per stator at a rotation speed of 30 000 rpm. The generator has been integrated with a microfabricated axial-flow microturbine to produce a compact power conversion device for power generation and flow sensing applications.

Measurement and CFD Prediction of Heat Transfer in Air-Cooled Disc-Type Electrical Machines

Accurate thermal analysis of axial flux permanent magnet (AFPM) machines is crucial in predicting maximum power output. Stator convective heat transfer is one of the most important and least investigated heat transfer mechanisms and is the focus of this paper. Experimental measurements were undertaken using a thin-film electrical heating method, providing radially resolved steady state heat transfer data from an experimental rotor-stator system designed as a geometric mockup of a through-flow ventilated AFPM machine. The measurements are compared with computational fluid dynamics (CFD) simulations using both 2-D axisymmetric and 3-D models. These were found to give a conservative estimate of heat transfer, with inaccuracies near the edge and in the transitional flow regime. Predicted stator heat transfer was found to be relatively insensitive to the choice of turbulence model used in the CFD simulations.

Experimental Evaluation of Turbocharger Turbine Performance Under Pulsating Flow Conditions

The steady and pulsating performance results of a turbocharger mixed-flow turbine are presented. The results are taken at an equivalent speed of 70% (42,000rpm) for a pulse frequency range of 20 to 80 Hz. All instantaneous parameters required for unsteady performance evaluation are measured and discussed. Significant improvements to the measurement of instantaneous actual power have been carried out. Large variations in the operating point of the turbine occur in each pulse cycle, a velocity ratio range of 0.43 to 1.28 is seen for a 20 Hz pulse, this range reduces as the pulse frequency increases and unsteady effects become more prominent. During periods of turbine freewheeling, negative efficiencies can arise due to momentum transfer from the turbine to the working gas, although detrimental to the efficiency the energy content in these regimes are low. The use of a modified Strouhal number (MSt.) and a pressure modified Strouhal number (PMSt.) has proved useful in assessing when the onset of unsteadiness of the flow will become significant, a value of 0.1 has been used as an appropriate limit to steadiness. The results suggest that for a typical engine speed range the rotor may be considered quasi-steady whilst the turbine stage is predominately operating in an unsteady regime. Inference from the experimental data would suggest it is adequate to capture the performance of a turbine under pulsating flow using a ‘quasi-steady’ model when the MSt. < 0.1, and a ‘filling and emptying’ code when a PMSt. < 0.1 and above this value a ‘wave action’ model is more appropriate.Copyright

The mechanical hybrid vehicle : an investigation of a flywheel-based vehicular regenerative energy capture system

Abstract Capturing braking energy by regeneration into an onboard energy storage unit offers the potential to reduce significantly the fuel consumption of vehicles. A common technique is to generate electricity in the motors of a hybrid electric vehicle when braking, and to use this to charge an onboard electrochemical battery. However, such batteries are costly, bulky, and generally not amenable to fast charging as this affects battery life and capacity. In order to overcome these problems, a mechanical energy storage system capable of accepting and delivering surges of power is proposed and investigated. A scale physical model of the system, based around a flywheel, a planetary gear set, and a brake, was built and operated in a laboratory. Tests showed that the proposed system could be used to store and provide braking energy between a flywheel and a vehicle, the latter emulated by an air-drag dynamometer. This validated the operating principle of the system and its computational model. Further, a computational analysis of a full-size vehicle incorporating the mechanical energy storage system was conducted. The results showed that the utilization of this system in a vehicle, when compared with a conventional vehicle, led to reductions in emissions and fuel consumption.

Measurement of stator heat transfer in air-cooled axial flux permanent magnet machines

Relatively little fundamental research has been undertaken on heat transfer in disc type electrical machines, a common example being the axial flux permanent magnet (AFPM) machine. Power density is usually thermally limited and therefore understanding of stator convective heat transfer is crucial in the design of AFPM machines and electrical machines in general. This paper presents direct measurements of radially resolved stator convective heat transfer in a rotor-stator system, using a thin-film electrical heater array to measure heat transfer. The heater array combines surface temperature sensing with heat flux measurement. It is found for the three gap sizes considered here that average Nusselt number (non-dimensional convective heat transfer coefficient) increases from Nu ≈ 100 at low disc speeds/rotational Reynolds numbers (Reθmin = 3.7e4) up to Nu ≈ 300–400 at higher speeds (Reθmax = 5.6e5).

Turbocharger motor-generator for improvement of transient performance in an internal combustion engine

Abstract Turbocharging of internal combustion engines is an established technology used for the purpose of increasing both power density and in some cases the cycle efficiency of diesel engines relative to naturally aspirated engines. However, one significant drawback is the inability to match the characteristics of the turbocharger to the engine under full load and also to provide sufficiently good transient response. Under many conditions this results in reduced efficiency and leads to higher exhaust emissions. The design of turbocharger components must be compromised in order to minimize these drawbacks throughout the entire operating range. However, when shaft power can be either added to or subtracted from the turbocharger shaft by means of a direct drive motor-generator, an additional degree of freedom is available to the designer to achieve a better turbocharger-engine matching. Both engine efficiency and transient response can be significantly improved by means of this method, normally described as hybrid turbocharging. This paper describes the results of a theoretical study of the benefits of hybrid turbocharging over a basic turbocharged engine in both steady state and transient operation. The new system and its benefits are described and four different engine-turbocharger systems are analysed in addition to the baseline engine. The main conclusion of the paper is that a significant increase in design point cycle efficiency can be afforded by re-matching the turbocharger components under steady state conditions while at the same time improving full throttle transient performance. Emissions are not considered in this paper.

The development of a dynamometer for torque measurement of automotive turbocharger turbines

Abstract This paper describes the development of a high-power high-speed eddy current dynamometer for turbocharger turbine research. Turbochargers are present in virtually all automotive diesel engines and it is essential to determine the characteristics of the turbine and compressor for inclusion into the engine model. Accurate measurement of the turbocharger turbine torque forms a vital part of this process. Earlier developments of dynamometers for this purpose are summarized and their limitations commented upon. The paper then describes the principle behind a new permanent magnet disc-type eddy current dynamometer including the relevant electromagnetic theory and analysis. The overall performance is given together with the results of an uncertainly analysis; a wide load range of 1.7-62.2 kW has been demonstrated at 60 000 r/min, and it allows characterization of turbine performance over the entire operating range, including the ability to measure unsteady torque.

Design Overview of a Three Kilowatt Recuperated Ceramic Turboshaft Engine

A three kilowatt turboshaft engine with a ceramic recuperator and turbine has been designed for small unmanned air vehicle (UAV) propulsion and portable power generation. Compared with internal combustion (IC) engines, gas turbines offer superior reliability, engine life, noise and. vibration characteristics, and compatibility with military fuels. However, the efficiency of miniature gas turbines must be improved substantially without severely compromising weight and cost, if they are to compete effectively with small IC engines for long-endurance UAV propulsion. This paper presents a design overview and supporting analytical results for an engine that could meet this goal. The system architecture was chosen to accommodate the limitations of mature, cost-effective ceramic materials: silicon nitride for the turbine rotors and toughened mullite for the heat exchanger and turbine stators. An engine with a cycle pressure ratio below 2:1, a multistage turbine, and a highly effective recuperator is shown to have numerous advantages in this context. A key benefit is a very low water vapor-induced surface recession rate for silicon nitride, due to an extremely low partial pressure of water in the combustion products. Others include reduced sensitivity to internal flaws, creep, and foreign object damage; an output shaft speed low enough for grease-lubricated bearings; and the potential viability of a novel premixed heat-recirculating combustor.

Design, fabrication and characterization of an axial-flow turbine for flow sensing

This paper presents a novel micro-power conversion device that combines an axial flow turbine with an axial-flux electromagnetic generator. The device has a sandwich structure, consisting of a top silicon stator, a polymer rotor and a bottom silicon stator. Axial gas flow through the device causes rotation, while permanent magnets embedded in the rotor induce output voltage in planar coils on the stators. Compared to radial flow devices the axial-flow design is more compact and can operate at lower pressure ratios. Details of the device design and fabrication process are given, and preliminary results are presented for the variation of output voltage, rotation speed, and pressure ratio with flow rate for nitrogen gas. The test results show operation over a wide dynamic range.

Design and development of a low-cost, electricity-generating cooking Score-Stove™

A design of the thermo-acoustic engine to combustion interface of an electrically generating clean cookstove is presented. Social surveys carried out by the SCORE project have indicated that adding electrical generation to a clean cookstove should increase their uptake considerably above the current 8% level (outside China) as electricity is perceived as high value and so wanted by the communities. This work discusses the development process, using elements of the formal design methodology BS 7000 to design a mass-producible, easy-to-manufacture, low-cost cooking stove that uses a thermo-acoustic engine to produce electricity whilst cooking. The iterative design process is discussed with analyses made of predicted cost and performance at each iteration and compared with the targets set from social surveys. With currently available technology and suitable investment in tooling and production facilities, estimated production costs are £100. An independent audit by engineers and cost estimators from a large international blue chip company made a prediction of £150. At this stage of the design, this is considered a quite good correlation. Proposals for bringing this cost down to £60 are made. To obtain lower costs would require more research. SCORE market evaluations indicate that at the upper-cost target of £60 (2007 prices), 60 million people would afford the stove. A wood-burning Score-Stove™2 prototype successfully developed 22.7 W of electricity based on the presented planar thermo-acoustic engine design, indicating that the new Score-Stove™2 design may have the potential, when manufactured in volume, to meet the social and cooking requirements of rural people in developing countries.