How to use UDF/DPM to conduct combustion of boron particles?
 

I'm studying the combustion of boron particles in a combustion chamber. Because the surface of boron particles is covered with boron oxide, the combustion of boron particles is divided into two stages, ignition stage and combustion stage. The removal of boron oxide is realized in the ignition stage, and the combustion of pure boron is realized in the combustion stage. Therefore, I need to write a DPM UDF code to define the thickness and material of boron particle oxide layer.
To do this, I drew a flowchart in the attachment.. My assumption is as follows: the material physical and chemical properties of B2O3 and B are set in fluent material library (as shown on the left of the figure), instead of being defined by UDF. Then I defined reactions in fluent's Species model→reactions (shown on the right). Finally, the function I realized with UDF is as follows: I told fluent solver that material is B when particle radius is less than the initial radius of 0.99. Here the thickness of the oxide layer (B2O3) is 0.01 initial radius.
I came up with this strategy because I'm not good at UDFs. The advantage of this strategy is that the UDF programming code is very short (as shown in the figure).
The good news is that after my study of UDF in recent days, I have gradually got familiar with it. The method I've come up with so far is to use DEFINE_PR_RATE. When the particle size is greater than 0.99R0, the combustion speed of particle surface is slower. Otherwise, the particle surface burns faster.

if (TP_MASS(tp)>TP_INIT_MASS(tp)*0.99)
{
*rr=-0.1*rate;
}
else
{
*rr=-rate;
}
Here, rate refers to the surface combustion rate of pure boron particles.
So, back to my original conundrum. The problem of how to achieve surface combustion of boron in two stages remains unsolved. That is, how to define the material and thickness of multilayer partile using UDP and return the result to the fluent solver. Could someone help me?

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