Numerical Simulation of a Nano-pulsed High-voltage Discharge and Impact on Low-temperature Plasma Ignition Processes for Automotive Applications

Spark-ignition (SI) processes are facing some challenges with the SI engine research continuing to move towards extremely dilute operation. Typical response from the automotive OEMs is to increase the spark energy to hundreds of mJs. However, this approach reduces the spark-plug lifetime due to erosion. In recent years, several lowtemperature plasma (LTP) technologies (e.g. microwave, nanosecond pulsed discharge, Corona discharge) have been proposed for automotive applications as a substitute for the conventional SI process, yet no LTP ignition models are available for commercial computational fluid dynamics (CFD) codes for the evaluation and optimization of these advanced ignition systems. This paper summarizes recent efforts to model LTP generated by a nano-pulsed highvoltage discharge in a multi-dimensional fashion. Streamer discharges between two pin electrodes are modeled through 2-D computations using the non-equilibrium plasma commercial solver VizGlow. The impact of key parameters such as peak voltage and gas density on the characteristics of the streamers is evaluated. The experimental dataset is used to validate the numerical predictions in terms of thermal and chemical properties of the generated plasma at the end of the discharge. Then, the impact of the post-discharge characteristics on the LTP ignition process is evaluated in combustion simulations performed using the CFD code CONVERGE

High-fidelity Numerical Modelling of Spark Plug Erosion

Spark plug erosion is critical in determining the overall efficiency of a spark ignition engine. Over its lifetime, a spark plug is subject to millions of firings. Each spark event results in material erosion due to several mechanisms such as melting, vaporization, sputtering and oxidation. With electrode wear, the inter-electrode spacing increases and a larger voltage difference is required to initiate the spark. The probability of engine misfires also increases with electrode erosion. Once a critical gap is reached, the energy in the ignition coil is not enough to cause a spark breakdown, and the spark plug must be replaced. Due to the long relevant time scales over which erosion occurs, and the difficulty of analyzing the spark plug environment during operation, determining spark plug lifetime typically requires extensive field testing. A high fidelity commercial thermal plasma solver, VizSpark is used simulate electrode erosion due to spark events. The model preserves key arc physics such as current conservation, conjugate heat transfer, fluid flow and electrode ablation. The solution framework includes the capability of coupling high fidelity arc physics with a dynamically deforming spark-plug electrode. A phenomenological model for electrode erosion based on energy is derived from prior experimental work on single-pulse electrode erosion. The energy based electrode erodion model is validated against experimental results, and 3-D electrode erosion simulations in stationary and cross-flow were performed

Computational Studies of Magnetic Nozzle Performance

An extensive literature review of magnetic nozzle research has been performed, examining previous work, as well as a review of fundamental principles. This has allow us to catalog all basic physical mechanisms which we believe underlie the thrust generation process. Energy conversion mechanisms include the approximate conservation of the magnetic moment adiabatic invariant, generalized hall and thermoelectric acceleration, swirl acceleration, thermal energy transformation into directed kinetic energy, and Joule heating. Momentum transfer results from the interaction of the applied magnetic field with currents induced in the plasma plume., while plasma detachment mechanisms include resistive diffusion, recombination and charge exchange collisions, magnetic reconnection, loss of adiabaticity, inertial forces, current closure, and self-field detachment. We have performed a preliminary study of Hall effects on magnetic nozzle jets with weak guiding magnetic fields and weak expansions (p(sub jet) approx. = P(sub background)). The conclusion from this study is that the Hall effect creates an azimuthal rotation of the plasma jet and, more generally, creates helical structures in the induced current, velocity field, and magnetic fields. We have studied plasma jet expansion to near vacuum without a guiding magnetic field, and are presently including a guiding magnetic field using a resistive MHD solver. This research is progressing toward the implementation of a full generalized Ohm's law solver. In our paper, we will summarize the basic principle, as well as the literature survey and briefly review our previous results. Our most recent results at the time of submittal will also be included. Efforts are currently underway to construct an experiment at the University of Michigan Plasmadynamics and Electric Propulsion Laboratory (PEPL) to study magnetic nozzle physics for a RF-thruster. Our computational study will work directly with this experiment to validate the numerical model, in order to study magnetic nozzle physics and optimize magnetic nozzle design. Preliminary results from the PEPL experiment will also be presented

Simulation of condensation systems in the presence of noncondensable gases

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Investigation of a pulsed-plasma jet for shock / boundary layer control

Abstract Pulsed jets with peak exit velocities as high as 250 m/s are generated by rapidly heating the air inside a chamber with an electrical discharge. The heated pressurized gas issues from a small orifice to form the pulsed plasma jet or 'spark jet'. Pulsing frequencies as high as 5 kHz are obtained. An array of these jets, in a pitched and skewed configuration, is used to force the unsteady motion of the interaction formed by a 24° compression ramp in a Mach 3 flow. The Reynolds number of the incoming boundary layer is Re =3300. The effect of the plasma jet array on the separation shock motion is studied by using 10 kHz Schlieren imaging and fast-response wall pressure measurements. Results show that when the pulsed jet array is placed upstream of the interaction, the jets cause the separation shock to move in a quasi-periodic manner, i.e., nearly in sync with the pulsing cycle. As the jet fluid convects across the separation shock, the shock responds by moving upstream, which is primarily due to the presence of hot gas and hence the lower effective Mach number of the incoming flow. Once the hot gases pass through the interaction, the separation shock recovers by moving downstream, and this recovery velocity is approximately 1% to 3% of the free stream velocity. With forcing, the low-frequency energy content of the pressure fluctuations at a given location under the intermittent region decreases significantly. This is believed to be a result of an increase in the mean scale of the interaction under forced conditions. Pulsed-jet injection was also employed within the separation bubble, but negligible changes to the separation shock motion were observed. These results indicate that influencing the dynamics of this compression ramp interaction is much more effective by placing the actuator in the upstream boundary layer

Recent advances in the modeling and computer simulations of non-equilibrium plasma discharges

International audienceThe mathematical modeling and computer simulation of low-temperature plasmas is gradually such a level of maturity that these simulation tools can be used not just for improving scientific understanding but also as computer-aided engineering design tools in an industrial setting. These models necessarily involve the description of multiple physical phenomena occurring over a range of times and lengths, thereby complicating their numerical implementation and solution. This special issue presents 12 invited contributions that present recent developments in the field of modeling and simulation of low-temperature plasma discharges. This editorial introduces these papers by providing an overview of the context in which these papers are presented