Flame kernel

The answer is "NO." In the combustible mixture, an electric spark produces a flame kernel. Initially, its shape is elliptical (like an American football), and then becomes a torus (like an American doughnut). Afterwards, it changes into almost spherical shape, and propagates spherically in the unbumed mixture. This process is formed by the existence of spark electrodes, which is necessary for spark discharge. Spark electrodes lead not only to heat loss from the flame kernel but also a change in the kernel shape. Both affect the minimum ignition energy. 

Figure 3.2.1 shows flame kernels of the schlieren photograph taken by a high-speed camera. These photographs can be compared with the calculated temperature distribution in Figure 3.2.4. As can be seen, both of them bear a close resemblance. From this result, the authors firmly believe that the numerical simulation is a significant tool for grasping the mechanism of spark ignition. Of course, the experimental work should also be of importance to verify the results obtained by numerical simulations. In this work, the authors mainly introduce the results of numerical simulations that have been obtained xmtil then in their laboratory.

Schlieren photograph of flame kernels by a high-speed camera. Time is given from the onset of spark discharge in microseconds. Spark electrode diameter 0.2 mm spark gap width 1mm. 

The boundary conditions are as follows In Figure 3.2.2, z-axis component and r-axis component velocities are zero for (1) and (2), respectively. The gradients of other variables are zero for both the boundaries. The gradients of all variables are zero for (3) and (4). No slip condition and heat transfer from the flame kernel to the spark electrode are assumed for (5) and (6), at the surface of spark electrode. 

For simplicity of the model, it is assumed that the natural convection, radiation, and ionic wind effect are ignored. The ignorance of the radiation loss from the spark channel during the discharge may be reasonable, because the radiation heat loss is found to be negligibly small in the previous studies [5,6]. The amount of heat transfer from the flame kernel to the spark electrodes, whose temperature is 300 K, is estimated by Fourier s law between the electrode surface and an adjacent cell. 

Kono, M., Niu, K., Tsukamoto, T., and Ujiie, Y, Mechanism of flame kernel formation produced by short duration sparks, Proc. Combust. Inst., 22,1643, 1988. 

Nakaya, S., et al., A numerical study on early stage of flame kernel development in spark ignition process for methane/air combustible mixtures, Trans. Jpn. Soc. Mech. Eng.(B), 73-732, 1745,2007 (in Japanese). 

Chomiak, J., Gorczakowski, A., Parra, T., and Jarosinski, J., Flame kernel growth in a rotating gas. Combustion Science and Technology, 180,391-399,2008. 

Alger, T., B. Mangold, D. Mehta, and C. Roberts, The Effect of Sparkplug Design on Initial Flame Kernel Development and Sparkplug Performance. SAE, 2006-01-0224,2006. 

In Chapter 3.2, M. Kono and M. Tsue examine the mechanism of flame development from a flame kernel produced by an electric spark. They discuss results of numerical simulations performed in their laboratory in confrontation with experimental observations and confirm numerical simulation as a significant tool for elucidating the mechanism of spark ignition. 

Wilson, K. J., E. Gutmark, and K. C. Schadow. 1992. Flame kernel pulse actuator for active combustion control. In Active control of noise and vibration. ASME DSC-38 75-81. 

Pischinger, S., and Heywood, J.B., How Heat Losses to the Spark Plug Electrodes Affect Flame Kernel Development in a Si-Engine, SAE Technical Paper 900021, SAE Int. Congress Expo, Detroit, MI, Feb. 26-Mar 2, 1990. 

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