Exploding Foil Initiator (EFI) flyer layer velocities measured down the barrel of an EFI are presented. Flyer velocity was shown to be proportional to supply voltage and of a similar order to other studies previously conducted. Bridge volume ejection was shown to be proportional to capacitor voltage. Current density increased with respect to capacitor voltage up to a point of saturation between 2400 V and 3000 V (evidenced electrically). Beyond the saturation voltage, high voltages demonstrated sustained energy delivery at a reduced current. This work indicates that control of active bridge volume or electrical supply signal may enable more closely controlled EFI flyer layer ejection behavior, and it demonstrates the relevance of using current per active bridge (specific current) as a metric to describe EFI electrical performance with relevance to dynamic response of the EFI. The impulse delivered by an EFI can be modulated via manipulation of the firing circuit input signal giving rise to system behavior variation.
Analytical and numerical models, validated against published data, were developed to calculate the velocity and time of arrival duration (ToAD) of the flyer-plasma material at the top of the barrel of an exploding foil initiator (EFI), as commonly used in explosive devices. Such tools will aid system designers in the optimization of capacitor discharge circuit (CDC) or EFI bridge material properties. The analytical elements of the approach developed support the requirement for the consideration of mass ejection variation with respect to initial capacitor voltage. The numerical elements of the approach developed demonstrate that EFI design alteration to increase flyer mass is less effective in reducing ToAD than supply voltage modulation via the CDC. This finding is of particular relevance for in situ control of functional performance characteristics. This work goes on to demonstrate that such control is impracticable when using hexanitrostilbene, since the initial capacitor voltages necessary to yield appropriate ToAD for deflagration deliver insufficient energy to instigate a response from the EFI.
Highlights• Laser ignition of natural gas and air conducted in an atmospheric pressure combustion test rig.• Minimum ignition energy for a natural gas-air mixture under engine like conditions investigated.• Analysis of influence of flow velocity and temperature on ignition characteristics conducted.• Required threshold photon flux density for ignition of a natural gas-air mixture determined. AbstractLaser induced spark ignition offers the potential for greater reliability and consistency in ignition of lean air/fuel mixtures. This increased reliability is essential for the application of gas turbines as primary or secondary reserve energy sources in smart grid systems, enabling the integration of renewable energy sources whose output is prone to fluctuation over time. This work details a study into the effect of flow velocity and temperature on minimum ignition energies in laser-induced spark ignition in an atmospheric combustion test rig, representative of a sub 15 MW industrial gas turbine (Siemens Industrial Turbomachinery Ltd., Lincoln, UK). Determination of minimum ignition energies required for a range of temperatures and flow velocities is essential for establishing an operating window in which laser-induced spark ignition can operate under realistic, engine-like start conditions. Ignition of a natural gas and air mixture at atmospheric pressure was conducted using a laser ignition system utilizing a Q-switched Nd:YAG laser source operating at 532 nm wavelength and 4 ns pulse length. Analysis of the influence of flow velocity and temperature on ignition characteristics is presented in terms of required photon flux density, a useful parameter to consider during the development laser ignition systems.
To aid exploding foil initiator (EFI) design, better prediction of ejecta momentum through either mass or velocity prediction is required. A numerical model was developed to calculate the mass of material converted to plasma within the confined region of an exploding foil initiator bridge during change of state under an electrical stimulus from a discharging capacitor. Optimisation is facilitated through the increased understanding of plasma evolution in current EFI designs, including the impact of this on both current delivery to the bridge and overall unit efficiency. The plasma regions were formed in key regions within the bridge, termed PA (ground side of EFI) and PB (high-voltage side of EFI) in this work. Different regions were dominant in mass ejection for different operating voltages. A trend is identified wherein the bridge exhibits an optimum threshold between the capacitor energy being utilized for mass conversion to plasma and that used for acceleration of this mass. It is postulated that, through geometric design modification, this threshold can be adjusted to deliver the momentum threshold of the explosive for which an EFI may be designed.
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