what to do when FW shows error "memeory allocation limit exceeds". how it occurs.

This is mainly occur when total mesh size exceed certain limit that flowizard has mantain. in thiS case please check the geometry in gambit or any other CAD package and clean the small faces which FW can't clean by itself.

Sometimes this error can be fixed by changing default value of maximum number of elements on single face.
Flowizard has a default vaalue of 100K
You can change this value to 500K by using following TUI command
mesh.setDefault(name="MESH.TRIMESH.MAX_FACES", numeric=500000).
This default should be set before meshing.