[rabbitmelon]

Hello everyone,

I am new to gas dynamics, so I am now confused by the system that I am trying to simulate, which is schematically shown in the picture below. It is a cold spraying system. A convergent-divergent nozzle (CD-nozzle) is mounted to a heating chamber. A main Nitrogen flow enters the heating chamber and is heated by the heating element in the chamber. The mass flow rate of main nitrogen flow (m0) is measured somewhere upstream of the system. At the entrance of the chamber, the temperature (T0) and pressure (p0) are also measured. Not far from the nozzle entrance, the temperature (T1) was measured again with a second thermal couple. Here comes also a small pipe connected to a powder feeder (PF) to inject powder into the nozzle. The pressure in the PF (p1) and mass flow rate into PF (m1) are also monitored.

Picture1.png

Firstly, I am trying to simulate the flow through the CD-nozzle with only the main Nitrogen flow. I have a nozzle with a throat diameter of 2.7 mm and a exit diameter of 8.5 mm, which resulted in a area ratio of Aexit/Athroat = 9.9108. I tried to get a rough estimation of the flow through the nozzle using the isentropic gas flow equations. I typed the area ratio into the calculator on the sites below and I get a pressure ratio, pexit/pentrance, of 0.007405.

http://www.dept.aoe.vt.edu/~devenpor/aoe3114/calc.html
https://www.grc.nasa.gov/www/BGH/isentrop.html

Because my system can only reach a pressure (reading of p0) as high as 40 bar. That means my pexit can be at most 0.2962 bar. Because it is much lower than the ambient pressure, so I can only get shock inside the nozzle at all the operating conditions of my system.

I am very confused that why the nozzle is designed in this way. Therefore, I think there must be something wrong with my estimation. But I could not find out what it is. Can anyone help me with that?

Thanks a lot in advance!