TYPE=ORIFICE CD1 or just TYPE=ORIFICE: Cd = 1

1

Hi,

I’m in need to compute an orifice dimension in a network to induce a desired pressure lost in the line.
I’m finding very helpful Mr.kraska example linearnet.inp which I’m using as reference, but I have some doubts.

I’m posting the file itself which I have follow in detail with my notes on it .
Most of my initial doubts have been clear now with the assistance of the ccx manual but I still need some help.

I have add some ?¿? at those points I have a doubt.
Thanks in advance.

**
**   Structure: gas network.
**   Test objective: orifice element, flux given.
**

**Node zero that is later used is not defined.

*NODE,NSET=NALL
1, 1, 0, 0
2, 2, 0, 0
3, 3, 0, 0
4, 4, 0, 0
5, 5, 0, 0
6, 6, 0, 0
7, 7, 0, 0

**Seems the test uses two consecutive Orifice elements.

*ELEMENT, TYPE=D,ELSET=EGAS
2, 2, 3, 4
3, 4, 5, 6

*ELEMENT, TYPE=D,ELSET=EIO

** a dummy network entrance element expressing that liquid is entering the 
** network (element 1). It is characterized by a node number 0 as first node. 
1, 0, 1, 2

** a dummy network exit element expressing that liquid is leaving the network (element 4)

4, 6, 7, 0

** First and last node is the same (node number 0) meaning Closed loop system. Should I expect results at entrance and exit to be the same.?¿?


**That was not in the Example file. Isn't ABSOLUTE ZERO=0 and others like conductivity definition will be required if heat transfer is involved. 
**PHYSICAL CONSTANTS,ABSOLUTE ZERO=0.

*MATERIAL,NAME=GAS

** Originally DOUBLE PRECISION. Pending to check later if needed.

*FLUID CONSTANTS
 0.1002353938E+4, 0.1711000000E-04, 0.27315E+03
 0.1002353938E+4, 0.1949281697E-04, 0.32315E+03
 0.1002353938E+4, 0.2169996934E-04, 0.37315E+03
 0.1002353938E+4, 0.2376071192E-04, 0.42315E+03
 0.1002353938E+4, 0.2569766947E-04, 0.47315E+03
 0.1002353938E+4, 0.2752859550E-04, 0.52315E+03
 0.1002353938E+4, 0.2926763423E-04, 0.57315E+03
 0.1002353938E+4, 0.3092621879E-04, 0.62315E+03
 0.1002353938E+4, 0.3251371525E-04, 0.67315E+03
 0.1002353938E+4, 0.3403789021E-04, 0.72315E+03
 0.1002353938E+4, 0.3550525531E-04, 0.77315E+03
 0.1002353938E+4, 0.3692132461E-04, 0.82315E+03
 0.1002353938E+4, 0.3829080968E-04, 0.87315E+03
 0.1002353938E+4, 0.3961776952E-04, 0.92315E+03
 0.1002353938E+4, 0.4090572698E-04, 0.97315E+03
 0.1002353938E+4, 0.4215776043E-04, 0.10231E+04
 0.1002353938E+4, 0.4337657658E-04, 0.10732E+04
 0.1002353938E+4, 0.4456456897E-04, 0.11232E+04
 0.1002353938E+4, 0.4572386531E-04, 0.11732E+04
 0.1002353938E+4, 0.4685636617E-04, 0.12232E+04
 
**That was not in the original example file.Not mandatory without heat transfers. 
**CONDUCTIVITY,TYPE=ISO
**24.34,373.
**44.5,573.

** [m2/K s2 ] units seems involved

*SPECIFIC GAS CONSTANT
287.

**Type=Orifice  (Only Area =A seems required) What are those last parameters.page 142 ccx 2.21 manual ?¿?

*FLUID SECTION,ELSET=EGAS,TYPE=ORIFICE,MATERIAL=GAS
3.1415e-4,.2e-1,.4e-1

*FLUID SECTION,ELSET=EIO,TYPE=INOUT,MATERIAL=GAS

**TYPE=TOTAL. PRESSURE on side nodes.

*INITIAL CONDITIONS, TYPE=TOTAL PRESSURE

6,0.06e6
4,0.07e6

**Isn't inhomogeneous boundary conditions imposed inside the step block.?¿?

**Mass flow rate goes to midside node degree of freedom 1.
**Temperature goes to side nodes degree of freedom 0.
**Pressure goes to side nodes like temperature degree of freedom 2

*BOUNDARY,MASS FLOW
1,1,1,0.03

**BOUNDARY
**Isn't needed here an exclusive *BOUNDARY card for temperatures and pressures ?¿?

**temperatures
2,0,0,500
6,0,00,500

**Pressures
2,2,2,0.08e6


**Is there a Steady state solution for this problem ?¿?.
**Reducing initial time step to 0.1 doesn't solve and give error.


*STEP,INC=100
*HEAT TRANSFER,STEADY STATE,SOLVER=PARDISO
1.,1.

*NODE FILE
PT,TT

*NODE PRINT,NSET=NALL
U,NT

*END STEP
2

Any idea what those constants mean. They don’t seem to be right acording to the manual considering Cd=1 for TYPE=ORIFICE.?

*FLUID SECTION,ELSET=EGAS,TYPE=ORIFICE,MATERIAL=GAS
3.1415e-4,.2e-1,.4e-1

The orifice element is characterized by the following constants (to be speci-
fied in that order on the line beneath the *FLUID SECTION card):
• the cross section A.
• the orifice diameter d (not needed for Cd = 1).
• the length L (not needed for Cd = 1).
• the inlet corner radius r (mutually exclusive with α; not needed for Cd =1).
• Cthe inlet corner angle α in ◦ (mutually exclusive with r; not needed for Cd = 1).
• the orifice-to-upstream pipe diameter ratio β = d/D (only for TYPE=ORIFICE PK MS).
• the rotational velocity v, if the orifice is part of a rotating structure (not
needed for Cd = 1).
• a reference network element (not needed for Cd = 1).

3

@Disla , I doesn’t really have it present, but when Cd = 1 or very closed to 1 and only the cross section area is needed, then I would consider it more like an aperture in a photo objective than an orifice.
So the other way around when the length is infinitely small, then will Cd also be 1 or close to 1, and corner will be infinitely sharp.
I cannot image other than the upstream pipe diameter ratio also should be needed, but in fact I don’t know how this TYPE=ORIFICE PK MS should look like since I can’t find it in the manual. It will probably require a look in some of the references.
In fact I could risk to be more confused than you.

4

Thank you fgr.

I planned to start with the simplest option and from a given example before moving to another orifice more elaborated.

I have solved the problem by other means by the moment but there is a big potential to explode those network elements if I manage to understand them better.

This time has been too precipitated. Let’s see if I find some more examples out there.

Thank you again. :+1:

5

How ? Using hand calcs or regular CFD ? That’s how I would approach such problems in most cases. Fluid elements in CalculiX are quite specific. They are closer to Simscape/Simulink than to typical FEA.

6

One can calculate pressure drops of gases with the same formula as liquids if the relative change of density is low (change of density/density < 0.02)

I 'm still interested to make it work and solve it in a more appropriate way.

7

Why are Mass Flow , Total pressure and Temperature shown under the displacements and velocitiy tags? It’s really confusing.

displacements (vx,vy,vz) for set NALL and time 0.1000000E+01

     1  1.290000E+01  0.000000E+00  0.000000E+00
     2  1.290000E+01  7.391503E+04  3.430000E+02
     3  1.290000E+01  0.000000E+00  0.000000E+00
     4  1.290000E+01  0.000000E+00  3.430000E+02
     5  1.290000E+01  0.000000E+00  0.000000E+00