HCZ high density ratio


I am currently working on a high density ratio multiphase model.
I have some complication to replicate bubble rising dynamics.
Indeed I have a negative values that appear in my composition mix (phi) and that created instabillity in the interface. I try several condition to impose a mix > 0 but it’s clearly not working.
I am using the Lee and Lin model modified by Amaya Bower.
Someone can help me ?