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Old   April 29, 2003, 12:07
Default Creating the spark numerically
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I want to get information on how to create the spark numerically in combustion. Names/title of reference available?

Thank you
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Old   April 29, 2003, 12:21
Default Re: Creating the spark numerically
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I do not have any references for you at hand, but essentially you set the progress variable to go 1 or about 1 say 0.85 from 0 across a collection of cells, that's if you are doing premixed of course. The problem is not getting the numerics to blow up, and choosing the write number of cells. Sorry can't be more help, I will try to get a reference for you. Lastly, you will need a good cold flow solution be for you spark, converged or at least semi-converged for LES.


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Old   April 29, 2003, 20:09
Default Re: Creating the spark numerically
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To see how it's done with a well-known simulation tool, look at pp. 81-82 in "KIVA-II: A COMPUTER PROGRAM FOR CHEMICAL REACTIVE FLOWS WITH SPRAYS" by Amsden, A. et. al. published on May 1989 by Los Alamos National Lab, NM. Also look at p. 696 in Transactions of the ASME paper titled "EFFECT OF IC ENGINE OPERATING CONDITIONS ON COMBUSTION AND EMISSIONS CHARACTERISTICS" by Lu, Jiang et. al. The KIVA code is used widely academia, government, and industry to model in-cylinder processes of ICEs.

Basically the KIVA ignition process is modeled as a heat source in the vicinity of the spark. The specific internal energy of the computation cell at the spark plug location is increased by a factor (1.0+DT*XIGNIT) on each time step DT until the combustion is self-sustained (~ 1600 K) or if the program reaches the end of the ignition window; generally it reaches 1600K so this isn't a concern. In this program, an ignition window based on crank angles or time is specified in a manner analogous to specifying a fuel injection dwell.

The reciprocal time constant or ignition energy fraction, XIGNIT (presented 5.5E3 in most often), determines not only the total energy added to the cell, but also the rate at which ignition energy is added. In Lu's paper, the cylinder pressure trace was highly dependent on XIGNIT for XIGNIT < 6.0E3. XIGNIT is usually verified with experiments until the fuel fraction burned as a function of crank angle coincides.

I hope this helps.
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Old   May 2, 2003, 01:23
Default Re: Creating the spark numerically
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How you do the spark depends somewhat on your code and precisely what you want to do. My experience is with transient codes. For steady solutions, such as industrial burners, I just created some kind of hot spot in the initial condition that was sufficient to ignite the flame without making the code blow up. This is not a cheap approach since there is usually a long transient that is of no interest. For spark-ignition engines, the KIVA approach is on the right track, but it is too ad hoc. The biggest problem is that the amount of ignition energy is uncontrolled. I found it much better to specify an ignition energy (50 mJ is a typical value for automotive engines) and to add the energy to the zone at the spark location over a period of several computational cycles. Both approaches are as much art as science, and sometimes it takes a bit of experimentation with energy, spark duration, and the like to get a satisfactory solution. Good resolution in the spark region is important.
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