Investigation of the effect of electrode geometry on spark ignition

Minimum ignition energy of hybrid mixtures of combustible dusts and gases

Determining the risk of accidental ignition of flammable mixtures is a topic of tremendous importance in industry and aviation safety. The concept of minimum ignition energy (MIE) has traditionally formed the basis for studying ignition hazards of fuels. However, in recent years, particularly in the aviation safety industry, the viewpoint has changed to one where ignition is statistical in nature. Approaching ignition as statistical rather than a threshold phenomenon appears to be more consistent with the inherent variability in the engineering test data. Ignition tests were performed in lean hydrogen-based aviation test mixtures and in two hexane-air mixtures using low-energy capacitive spark ignition systems. Tests were carried out using both short, fixed sparks (1 to 2 mm) and variable length sparks up to 10 mm. The results were analyzed using statistical tools to obtain probability distributions for ignition versus spark energy and spark energy density (energy per unit spark length). Results show that a single threshold MIE value does not exist, and that the energy per unit length may be a more appropriate parameter for quantifying the risk of ignition than only the energy. The probability of ignition versus spark charge was also investigated, and the statistical results for the spark charge and spark energy density were compared. It was found that the test results were less variable with respect to the spark charge than the energy density. However, variability was still present due to phenomena such as plasma instabilities and cathode effects that are caused by the electrodynamics. Work was also done to develop a two-dimensional numerical model of spark ignition that accurately simulates all physical scales of the fluid mechanics and chemistry. In this work a two-dimensional model of spark discharge in air and spark ignition was developed using the non-reactive and reactive Navier-Stokes equations. One-step chemistry models were used to allow for highly resolved simulations, and methods for calculating effective one-step parameters were developed using constant pressure explosion theory. The one-step model was tuned to accurately simulate the flame speed, temperature, and straining behavior using one-dimensional flame computations. The simulations were performed with three different electrode geometries to investigate the effect of the geometry on the fluid mechanics of the evolving spark kernel and on flame formation. The computational results were compared with high-speed schlieren visualization of spark and ignition kernels. It was found that the electrode geometry had a significant effect on the fluid motion following spark discharge and hence influences the ignition process.

Spark ignition : experimental and numerical investigation with application to aviation safety

Cooling of the flow induced by a spark plasma discharge is found to occur at two different rates: an initially fast cooling regime followed by a slow cooling regime. Convective cooling in the fast regime contributes to 30--50% of the total cooling and occurs within the first millisecond of the induced flow.

Two regime cooling in flow induced by a spark discharge

We investigate the local flow field induced by a spark plasma discharge in quiescent air under atmospheric conditions. Particle image velocimetry (PIV) is used to quantify the induced flow field between two conical-shaped electrodes for three different electrode gaps: 8 mm, 5 mm and 2 mm. The results show the existence of three distinct stages of flow. In the first stage of flow, distinct coherent structures are observed near the tips of the electrodes. In the second stage, ambient fluid is entrained along the electrodes to produce axial jets in the electrode gap. The final flow stage is characterized by collision of the jets and decay of vorticity. The flow parameters are all scaled by the induced velocity behind the shock wave produced by the spark and the electrode gap distance. The scaling results in comparable magnitudes of velocity and vorticity induced in the 5 mm and 8 mm electrode gaps with the onset of each of the flow stages occurring at approximately the same time scale. The magnitude of axial velocity dominates the latter two stages of the flow. Significant correlation is found between circulation and entrainment calculated for the 5 mm and 8 mm electrode gaps at different time steps, where the decay rates of circulation and that of entrainment are found to be $$\sim 1/(t/{\tau )^{0.75}}$$
 and $$\sim 1/(t/{\tau) ^{0.88}}$$
 , respectively. The stages of flow development for different electrode gaps at times > 100 µs after the discharge are discussed comprehensively and the choice of scaling based on energy deposited in the electrode gap and gap distance is found to permit direct comparison of the two electrode gaps.

R. Haley

Papers

The influence of the igniter-induced blast wave upon the initial volume and expansion of the flame kernel