The entropy equation of the nonequilibrium state was applied to the solution of air system. An expression of the change rate of entropy production in the pipe element and throttle element were derived. And the flow direction of the fluid was judged by the nonnegativity of the entropy production. Combined with the think of modular modeling

the fluid network simulation program of the transient air system was established. The Shock tube model and double cavity model were used to verify the algorithm accuracy. The simulation results are in good agreement with the experimental results

which show that the nonequilibrium entropy production can accurately determine the flow direction in the transient air system

overcome the limitation of the total pressure judgment and accelerate the convergence of the air system.