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 DudaAPD April 30, 2014 16:38

Non-physical Temperature raising in pipe

Hi,

I am simulating a two-phase bubbly flow case (liquid-gas) inside a long and straight (small diameter) pipe with the following:

1) 2D axisymmetric
2) bubble diameter constant equal 0,5mm
3) Gravity (pipe at the vertical)
4) Eulerian model implicit
5) Energy is on
6) k-ep realizable with standard wall function
7) Boundary conditions: mass-flow-inlet, pressure-outlet and wall thermal condition of convection, with a small U global, i.e. the pipe is well isolated and there is a good temperature difference between the inside and outside.
8) Operating pressure around 10^7

The gas phase (secondary) is ideal gas or Peng-Robinson real gas and liquid phase has a constant density.

Fluent send a warning telling that for compressible ideal or real gas models with buoyancy, it is recommended that the operating density value should be zero. What is happening is that when I do this, I have a temperature raise along the pipe which is not expected. When I use the operating density with a mixture density (high enough) this doen't happen.

Any ideas? Thank you in advance! :)

Best regards,

Duda

 SJSW October 7, 2015 23:37

2003
http://jullio.pe.kr/fluent6.1/help/html/ug/node733.htm
It's not allowed to use compressible flow with Eulerian model.
Inviscid flow is not allowed as well.

2006
https://www.sharcnet.ca/Software/Flu...ug/node950.htm
You can define only one compressible fluid phase.
Another compressible fluid phase could be achieved by UDF.
Inviscid flow is still not allowed.

So it is now OK to apply one compressible fluid with Eulerian model.
Another one compressible fluid could be achieved by UDF.

However, I can't succeed simulating the bubble injected from a mass-flow inlet at the bottom of a half-filled tank using ANSYS14. The bubble is phase-2 and compressible.
The temperature near the outlet is at the default limit 5000K when the zell zone was not patched as air (i.e. the tank is full of water).

1) 3D cylinder, height 2m, diameter 1m, water depth 1.5m.
2) air inlet at the bottom with a diameter 1cm
3) Gravity (pipe at the vertical)
4) Eulerian model, implicit,
5) Energy is on
6) Standard k-ep with standard wall function
6) Turbulence Multiphase Model: per phase
7) Boundary conditions: mass-flow-inlet or velocity-inlet, pressure-outlet and adiabatic wall, with a small U global. Inlet total temperature was 600K, and other settings are 300K.
8) Operating pressure 101325 Pa located 10cm below the outlet.

It seems that some tricks should be adopted.

Then, I looked up the chapters introducing Multiphase Models of tutorial .
In the chapter "Tutorial Guide/Using the Mixture and Eulerian Multiphase Models," Mixture Model and Eulerian model are using velocity inlet and outflow boundaries for steady simulation.
However, in the chapter " User's Guide/Cell Zone and Boundary Conditions/Boundary Conditions/Outflow Boundary Conditions," it said that
"
outflow boundaries cannot be used in the following cases:
1.If a problem includes pressure inlet boundaries; use pressure outlet boundary conditions (see Pressure Outlet Boundary Conditions) instead.
2.If you are modeling compressible flow.
3.If you are modeling unsteady flows with varying density, even if the flow is incompressible.
4.With the multiphase models (Eulerian, mixture, and VOF (except when modeling open channel flow, as described in Open Channel Flow n the Theory Guide).
"

I don't understand. They are opposite.
What are the principles when using Eulerian model and outflow boundary?