Hi Mengyuan, welcome to the MFIX forum.
When seeking help here, please do the following:
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Search the forum for relevant posts, your problem may already have been solved.
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Attach your project files, so we can run the simulation here. Under the main menu in MFiX is an item “Submit bug report” which creates a .zip file with all relevant input files. Then you post that zip file here.
In your cse, searching for “PS_VOLUME” turns up a number of threads, including
https://mfix.netl.doe.gov/forum/t/problem-of-creating-of-point-sources/2978
If this does not help, please attach your zip file so we can have a look. Thanks.
– Charles