@mariam.yehia33 - Have you seen this document?
https://mfix.netl.doe.gov/doc/mfix/21.1.4/html/reference/faq.html#what-do-i-do-if-a-run-does-not-converge
Another suggestion that came up recently on the user forum is to use the ideal gas law for the fluid phase density, as remarked here Simulation is not running showing some meshing error! - #6 by jeff.dietiker
– Charles