Thank you for your reply . If I want to rephrase my question, I can put it this way in two possible scenarios:
What is wrong with filling 80% of the vessel with water, and then inject powders to fill the remaining 20%? In this scenario, the problem would be that, since injection does not add to the volume of the slurry, only 80% of the vessel will be involved in the simulation, while in reality the vessel is full.
What is the problem with filling up (i.e. 100%) the vessel with water and then inject powders? I would say this does make better sense because at least we will have the entire volume of the vessel involved, just like that in real world. However, it is not in line with reality either, because in reality 80% is water, not 100% as we initially assumed in this second scenario.

Thank you