Hi, I am developing a convergent nozzle test for students. The isentropic solutions are calculated based on the choked condition and area ratio. The case is replicated in Fluent, which is converged well. However, there is some difference between them except for the nozzle exit. Could someone give me some ideas?
First of all I appreciate the way you have clearly given all the calculations made by you.
The catch comes in taking velocity of sound as constant. the velocity of sound you have considered is valid for a static temperature of 297 K. Since the flow is compressible in the convergent nozzle the static temperature changes appreciably along the length of the nozzle. You need to consider the local static temperature to evaluate local speed of sound and carry out the calculations accordingly. Hope it will solve the anomaly.
Hi Afroz, thanks for your answer. The difference still exists. Even though I change different methods in Fluent, inviscid, laminar or turbulent, the Mach number from Fluent is still like in the plot.
For your Entry total pressure and throat area, the mass flow rate comes equal to 0.07905 kg/s. This mass flow remains constant throughout the length of flow. While somehow your hand calculations show variation in mass flow rate. Please check this anomaly.
Also you need to evaluate the Mach number at desired cross sectional area. This is a function of total pressure and total temperature. and varies only with area .
Then evaluate the velocity using the local static temperature.
I hope this helps you, Fluent results look to be alright. The problem lies with the isentropic calculations. And believe me, the isentropic calculations are algebraically challenging! Good luck with your calculations...
Also Fluent does some manipulations. Try giving a large reservoir may be double the radius of the inlet nozzle and half the length at the entry, make a total pressure condition at this reservoir entry and everything will be alright. Or force the inlet conditions what you got through the isentropic calculations like static pressure and speed.
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