Simcenter STAR-CCM+ Optimizing Soot Parameters for Accurate Soot Prediction in STAR-CCM+

Tuning soot parameters is essential to account for the expected physical behavior of soot particles and therefore achieve accurate soot predictions in Simcenter STAR-CCM+. This can also involve adjusting soot model parameters to align simulation results with experimental data. There are five parameters in the soot model that can be tuned to achieve the desired soot amount. This article aims to explain the significance of each soot parameter and demonstrates the effect on soot volume fraction when varying the range of each parameter from the default.

For illustration purposes, the 2D simulation from the Kent Honnary validation case was utilized. Default values for the soot model parameters were maintained as the baseline case. Each parameter was varied separately from 0.2 to 1 while keeping the default values for the rest of the parameters, demonstrating their individual influence on soot volume fraction

1. Steric Factor: 

The first parameter to tune when the desired amount of soot is not achieved compared to experiment is the steric factor. In STAR-CCM+ this value typically defaults to 0.6. 

• Purpose: The steric factor accounts for the collision efficiency between soot particles. It influences how often particles collide and stick together during the coagulation process.
• Adjustments: As illustrated in the graph below,  increasing the steric factor leads to faster coagulation and potentially larger soot particle resulting in higher soot concentrations in the flame. Decreasing slows down coagulation resulting in smaller soot particles and lower soot concentration on the flame.   

2. Surface Growth Scale: 

• Purpose: Surface growth scale determines how rapidly soot particles grow by accretion of precursor molecules on their surfaces.
• Adjustment: Increasing the surface growth scale accelerates the growth of existing soot particles, leading to larger particle sizes. Decreasing it slows down particle growth, resulting in smaller soot particles.

3. Nucleation Scale:

• Purpose: This parameter controls the rate of nucleation, the process by which initial soot particles form from precursor species.
• Adjustment: Increasing the nucleation scale enhances the rate of initial particle formation, leading to higher concentrations of soot nuclei. Decreasing it reduces nucleation rates, resulting in fewer initial soot particles.

4. Oxidation Scale:

• Purpose: The oxidation scale governs the rate at which soot particles are consumed or oxidized in the combustion environment.
• Adjustment: Increasing the oxidation scale accelerates soot oxidation, reducing the concentration of soot particles. Decreasing it slows down oxidation, leading to higher concentrations of soot particles in the combustion products.

5. Coagulation Scale:

• Purpose: This parameter determines the rate at which soot particles collide and aggregate to form larger particles
• Adjustment: Increasing the coagulation scale results in slower coagulation rates, leading to smaller aggregates and lower soot volume fractions. Conversely, decreasing the coagulation scale accelerates the coagulation process, promoting the formation of larger aggregates and potentially higher soot volume fractions as shown in the image below. 

Note:  It’s important to note that these parameters are interdependent, and changes in one can influence the behavior of others. Therefore, careful calibration against experimental data is essential for accurate soot prediction.