20.1.1 Using the Spark Model

To activate the spark model, perform the following steps:

1.   Select Transient from the Time list in the General task page.

2.   Select an appropriate reaction model in the Species Model dialog box.

Models Species Edit...

3.   Select Species Transport under Model in the Species Model dialog box and enable Volumetric under Reactions.

4.   The Spark Ignition model will now appear in the Models task page. Select the Spark Ignition model and click Edit.... This will open the Spark Ignition Model dialog box .

Models Spark Ignition Edit...

Figure 20.1.1: The Spark Ignition Model Dialog Box

5.   Define the spark model as either Fixed Spark Size or Time-Varying Spark Radius.

When the Fixed Spark Size is enabled, the dialog box expands to include the main spark model inputs (Figure  20.1.2). The shape of the spark can be spherical, cylindrical or hexahedral in three dimensional simulations, or circular or quadrilateral in two dimensional simulations. Depending on the shape selected, appropriate inputs are highlighted or grayed out.

Figure 20.1.2: The Expanded Spark Ignition Model Dialog Box

• To define a spherical spark, the shape parameters can be selected by clicking the Select Points with Mouse button at the bottom of the dialog box, and highlighting the center and radius of the spark kernel.

• Enter Energy, Start Time, Duration, and Diffusion Time in the Spark Ignition Model dialog box.

  When the in-cylinder model is turned on, the Start Time is entered in crank angle degrees instead of seconds (as shown in Figure  20.1.2), while the spark Duration is still in seconds.

• While the Energy input is in Joules by default, you can redefine the units as needed. The rate of energy input into the domain is constant so that the total energy will be evenly distributed over the duration that you set. The Energy input in the spark model should result in an appropriate temperature rise in the cell that is high enough to initiate combustion. The Energy input is only a model parameter and does not reflect energy input in actual automotive ignition systems, which typically range between 50 and 150 millijoules.

If you select the Time-Varying Spark Radius option, you will need to specify the Energy, Start Time, Duration, Diffusion Time, and Time Exponent. The spark is assumed spherical and will grow from an Initial Radius, $r_0$, to a Final Radius, $r_f$, over the spark Duration, with a cube root dependence on time so that the radius will grow faster at the beginning and more slowly near the end. This time-dependent behavior is consistent with experimental findings [ 31]. The Time-Varying Spark Radius option is recommended as it has been found to be less sensitive to model parameters.