Use the Spark Ignition panel to configure models for spark ignition and flame propagation. Specify the Spark ignition settings, configure the number of sparks, and for each, the Spark Location, Timing, and Energy.
Spark ignition combustion is modeled as a two-stage process in Ansys Forte: the ignition kernel stage and the flame propagation stage. For the ignition kernel stage, two model options are available: one is the discrete particle ignition kernel (DPIK) model and the other is the arc channel tracking (ACT) model. You can choose which model to use by adding new sparks of the corresponding type. Once the flame kernel structure grows large enough, the ignition kernel model will transition to the flame propagation stage. The flame propagation stage can also be modeled using two different options: one is the G-equation model and the other is called the detailed chemistry direct integration (DCDI) approach. In a single simulation, all sparks must use the same ignition kernel model and flame propagation model.
The Spark Ignition Editor panel provides settings for the flame propagation models that apply to all spark events in the simulation. The G-equation model option requires the following input parameters:
Kernel Flame to G-equation Switch Constant: Enter a value for the transition from ignition kernel model to G-equation model. This transition happens when the kernel radius grows larger than this coefficient times the turbulence integral length scale. Suggested range: 1.5-2.5.. However, if the mesh surrounding the spark location is sufficiently refined, the preferred practice is to ignore this transition criterion by setting this constant to zero and use the Min. Kernel Radius for Kernel to G-equation Switch as the only transition criterion.
Min. Kernel Radius for Kernel to G-equation Switch: The spark ignition flame kernel radius must grow larger than this minimum radius before it can switch to the G-equation model. Suggested value: 0.1 cm.
Flame Development Coefficient: This coefficient controls the exponentially increasing effect of turbulence on flame propagation speed as the flame grows from laminar to fully developed turbulent flame. A larger value speeds this transition. Suggested range: 0.5-2.5.
The default settings are usually a good place to start.
When the Detailed Chemistry Direct Integration option is selected, the ignition kernel model transitions to the DCDI stage at the end of the spark duration. The DCDI approach does not involve any flame front surface tracking model. In this approach, combustion is simulated by modeling each computational cell as a well-stirred reactor and solving detailed chemical kinetics equations in it.
Next, from the Spark Ignition icon bar, use the New DPIK Spark 
or New ACT Spark 
icon to add a new spark (all sparks added in the same simulation must be
the same type). A new, named Spark node is added to the Workflow tree and its associated Editor
panel opens, as described below.
On both the DPIK Spark and the ACT Spark panels, the Spark icon bar
offers 4 icons: Rename 
, Copy 
, Paste 
, and Delete 
, which are used to manage the sparks. You can create a new spark by
copying and pasting an existing one or by using the New DPIK Spark or New ACT Spark icon on the Spark Ignition icon bar.
A Discrete Particle Ignition Kernel spark requires the following input parameters:
Location: The center of the initial spherical kernel flame (see Reference Frames).
Timing: Start and duration of the spark event can be set in terms of crank angle or simulation time with the option to specify a relative time frame (see Time Frames). For CA-based spark timing used in engine cases, you can select the Crank Angle Option as Cyclic or Global. The Cyclic option is helpful in specifying spark event that is repetitive in a multi-engine-cycle simulation. In this case, the user-supplied crank angle value will be converted to fit in the range of [0, 720) °CA (for 4-stroke engines) or [0, 360) °CA (for 2-stroke engines). The spark timing will then be treated as cyclic and repeated on a 720-degree schedule (4-stroke) or 360-degree schedule (2-stroke). You may choose to Use Global Crank Angle Limits to impose a global crank angle range for the cyclic repetition, beyond which the spark event is not active. If the Crank Angle Option is Global, no cyclic conversion is made on the user-supplied starting crank angle value.
Initial Kernel Radius: Enter the initial radius of the spark-ignition kernel.
Number of Flame Particles: Number of particles used to track the flame front location of a spark ignition kernel flame. This number remains the same for a spark throughout the simulation. Suggested value: 3000.
An Arc Channel Tracking Spark uses a string of particles to track the location of the spark arc channel. This model requires that the surface geometry contain detailed spark plug structure. This is because the motions of the two end particles are constrained to slide along the anode and cathode walls, respectively. An ACT Spark requires the following inputs:
Initial Anode End Location: Initial location of the anode-end spark arc particle. The motion of this end particle will be constrained to slide along the anode wall.
Initial Cathode End Location: Initial location of the cathode-end spark arc particle. The motion of this end particle will be constrained to slide along the cathode wall. The spark gap distance is defined as the distance between the initial anode and cathode locations.
Timing: The spark timing and spark duration of an ACT spark are specified in the same way as for DPIK spark. Note that if electric circuit discharge model is used to model spark energy discharge in the ACT spark model, the spark duration may be cut short if the energy reserve in the spark ignition system is depleted before the specified end-of-spark timing.
Spark Energy: The electrical energy discharge process can be modeled using four different options:
Electric Circuit Discharge Model: Using this option, the total amount of electrical energy stored in the secondary coil at spark timing and the energy discharge rate at a given time are modeled based on the following configuration parameters of the electric circuit:
Inductance of Primary Coil: Typical value is 0.005 H.
Peak Current in Primary Coil: Typical value is 10 A. The inductance and peak current in the primary coil determine the total amount of energy stored in the primary coil at spark timing.
Energy Transfer Efficiency from Primary to Secondary Coil: The fraction of energy transferred from the primary coil to the secondary coil at spark timing. Typical value is 0.6.
Inductance of Secondary Coil: Typical value is 50 H.
Resistance of Secondary Coil: Typical value is 3000 Ohm.
Constant Discharge Rate: Specify a constant energy discharge rate.
Time-varying Discharge Rate: Specify a time-varying profile for the energy discharge rate. Engine cases should use a crank angle-based profile. If the Repeat Profile Each Cycle box is checked, the profile will be treated as cyclic following the engine cycles, in which the crank angle values should cover the range of [0, 720] °ATDC for four-stroke engines and [0, 360] °ATDC for two-stroke engines. You can also use the Use Global Crank Angle Limits option to limit the cyclic-and-repeat treatment to be active only within a specified global crank angle range. Non-engine cases should use a time-based profile, which can also be treated as cyclic.
User Defined Function: This option allows two model components to be customized through two UDFs: one calculates the total amount of energy stored in the primary coil at spark timing; the other calculates the amount of electrical energy discharge for a specific time step. See section 5.10 (add a link here) for how to edit, compile, and link UDFs in Ansys Forte. These two functions are added to the Combustion category. Below are the description of inputs and outputs of these two UDFS:
UDF_Combustion_TotalSparkEnergy
Inputs:
sparkIndex– Index of spark plug following the order they are listed on the setup workflow tree.Outputs:
totalSparkEnergy– Total amount of spark energy stored in the secondary coil of the ignition system at spark timing [J].iError– Return 0 if there is no error, return non-zero value if there is error.UDF_Combustion_SparkEnergyDischarge
Inputs:
sparkIndex– Index of spark plug following the order they are listed on the setup workflow tree.timeStep- Time step size [s].pressure- Gas pressure in the combustion chamber [Pa].temperature- Unburned gas temperature in the combustion chamber [K].sparkGapSize- The distance between the anode and cathode ends of the initial arc channel [m].sparkArcLength- The current length of the spark arc channel [m].In-and-outputs:
storedEnergy- Energy stored in the secondary coil of the ignition system. the initial value passed in will be modified and returned [J].Outputs:
dischargeEnergy- Energy released for the current time step [J]current - Current in the secondary coil [A]. (For output reporting only)
totalVoltage- Total voltage fall [V]. (For output reporting only)arcVoltage- Voltage fall across the arc channel [V]. (For output reporting only)breakdownVoltage- Breakdown voltage [V]. (For output reporting only)triggerRestrike- Return 1 if a restrike should be triggered, return 0 otherwise.iError– Return 0 if there is no error, return non-zero value if there is error.
Energy Transfer Efficiency from Arc to Gas: Efficiency of conversion from the electrical energy discharged by the electric circuit to the thermal energy deposited into the gas mixture.
Initial Number of Arc Particles: Initial number of particles forming the arc channel. The actual number of arc particles will be adaptively adjusted to maintain optimal particle resolution during the simulation.