Hello,
You mean to say you are getting Mach number at throat lesser than that of the inlet. The inlet conditions are provided by the user. Therefore, based on the inlet BCs, the solver will solve the equations. What is the Mach number at the inlet?
You need to understand that everytime you use converging nozzle, the velocity will not be increased. This holds good only for subsonic (Ma < 1) flows. But when you have supersonic or sonic flows (Ma >= 1), using convergent nozzle will decrease the velocity. To accelerate the sonic or supersonic flows, you need to use divergent nozzles.
I would suggest you to refer convergent-divergent nozzles for more information.
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