is it ok to use outflow boundary condition for multiphase flow, with air coming thru nozzle? outflow can't be used when density is varying in unstaedy calculations..now if pressure is changing inside the capillary, then density can vary. Under this case, wat should be outlet boundary condition?

use a pressure outlet. If M<0.3 you can assume the flow is incompressible and use the pressure based solver, if not use the density based solver.