I am trying to simulate a critical flow nozzle with Fluent, but I could not reach  proper results.we know that in critical flow nozzles , the mass flow rate depends on pressure inlet. furthermore,for the real gas , the mass flow rate is less than the ideal mass flow rate (when the flow in one-dimensional and  isentropic) because of real gas effects such as being multi-dimensional and non-isentropic.

In fact, in low pressure inlet( less than 1 bar) , I could gain good results but in high pressure inlet( more than 1 bar) , the real mass flow rate becomes more than ideal value. why? How can I fix this problem?

Aslo,I used Redlich-Kwong  EOS which is defined in Fluent 15

I will appreciate if any one could help me...