|
[Sponsors] |
April 8, 2017, 05:40 |
Shock in CD nozzle?
|
#1 |
New Member
Join Date: May 2013
Posts: 22
Rep Power: 12 |
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! |
|
April 8, 2017, 09:42 |
|
#2 |
Senior Member
Join Date: Jun 2009
Posts: 174
Rep Power: 16 |
CD nozzle can experience shocks (oblique) in the divergent part depending on the area schedule and ratio.
|
|
April 8, 2017, 11:36 |
|
#3 |
New Member
Join Date: May 2013
Posts: 22
Rep Power: 12 |
Thanks for your reply, turbo.
The system is designed to accelerate injected particles to highest possible velocity. If shocks will always be present in the nozzle due to the pressure limitation and nozzle design, it is not efficient for this purpose. I have also seen one paper about CFD simulation of this nozzle. It claimed that shocks are outside the nozzle with a nozzle inlet pressure of 14 bar. Therefore, I am afraid that there is something wrong with my estimation about the nozzle exit pressure using the area ratio and isentropic flow caculator. Could you tell me if I am wrong in the estimation and where I have done it wrong? |
|
April 8, 2017, 13:12 |
|
#4 |
Senior Member
Join Date: Jun 2009
Posts: 174
Rep Power: 16 |
If you want to get supersonic all the way to the exit without shocks, you need to find the right p-static-exit for a given p-total-inlet by trial-and-error. Above all, you should be choked at the throat beforehand, otherwise you will return to subsonic. This is the so-called rocket science.
|
|
April 8, 2017, 13:50 |
|
#5 | |
New Member
Join Date: May 2013
Posts: 22
Rep Power: 12 |
Quote:
|
||
April 9, 2017, 08:26 |
|
#6 |
Senior Member
Join Date: Jun 2009
Posts: 174
Rep Power: 16 |
Post your CFD Mach contours at your target mass flow.
|
|
April 9, 2017, 17:18 |
|
#7 |
New Member
Join Date: May 2013
Posts: 22
Rep Power: 12 |
mach number sym1.png
Above is the Mach number contour. I used a 5° slice of the nozzle and the cylindric outer space to run a 2D simulation. The mass flow rate is 2.9282e-4 kg s^-1 for this 5° slice, which corresponds to a volume flow of 1100 standard liter per minute for the whole 360° nozzle. |
|
April 10, 2017, 12:13 |
|
#8 |
Senior Member
Join Date: Jun 2009
Posts: 174
Rep Power: 16 |
Your nozzle looks not too bad because you have a gradual supersonic flow in the divergent section. Problem is you have a shock at discharge leading to subsonic again afterwards. If you carefully reduce p-static-exit in CFD, you will be able to be supersonic without shocks. However, the very p-static-exit cannot be realized in the current hardware, as you described.
|
|
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
how can make normal shock in the nozzle at inlet B.C : Pressure Far field | SonicGhoul | FLUENT | 2 | May 8, 2016 06:16 |
Looking for an explanation for two weak oblique shock waves inside de Laval nozzle | heksel8i | Main CFD Forum | 2 | December 5, 2013 03:23 |
Con-Div Nozzle Shock structure problem | padmanathan | FLUENT | 2 | November 16, 2010 04:37 |
Shock in nozzle - dangerous? | Christian | Main CFD Forum | 1 | May 8, 2006 13:48 |
compressible flow in a counterflow nozzle | d.vamsidhar | FLUENT | 0 | November 24, 2005 02:45 |