Learning Forum

[manuel.pacherres]

Good afternoon everybody.
I am working on an example of condensation with concentric tubes, where my goal is to observe this phenomenon by making cold water flow through the middle tube and steam through the outer tube, and it has the following characteristics:
- The middle tube has an external diameter of 26.67 and is through which cold water will flow at a temperature of 22.5 °C and at atmospheric pressure.
- The external tube has a diameter of 380 mm and is where the steam will flow with a temperature of 105°C and a pressure of 122,300 Pa (saturation pressure according to tables according to its temperature).
- To carry out this work I am changing all the properties of the material for liquid water such as its density, specific heat, thermal conductivity and viscosity, according to the properties of water according to the tables at 22.5 °C and placing in the temperature box of reference of the properties of the material that same temperature; the only thing that I am not changing is its molecular weight and the SSE (Ansys gives me a SSE of -2.858e+08J/kgmol) I am considering it as "zero".
- In the case of steam, I am making the same manual changes with respect to the temperature with which the steam is going to enter, which is 105°C, except that this time I am considering the difference in SSE of water in liquid state with the SSE of the water in the vapor state, which gives me anxiety and are: -2.858e+08J/kgmol and -2.418e+08J/kgmol respectively, then placing an SSE of 0.44000e+08 J/kgmol.
- I am using the mixture model, for which I am considering two phases (water in a liquid state and water in a vapor state), as well as using the evaporation/condensation model, considering a mass transfer from "liquid" to "vapor".
- Regarding the boundary conditions, I have cold water entering through the middle tube at a temperature of 22.5°C, at atmospheric pressure and at a speed of 0.52 m/s, and in the outer tube I have the steam which I want to condense, with an inlet temperature of 105°C with its saturation pressure of 122300 Pa and a velocity of 20.3 m/s.
- To solve this case I am also using a hybrid initialization and a SIMPLE pressure-velocity scheme, which I left by default.
- Carrying out all this process, I obtained outlet temperatures for cold water of 97 °C on average, and for steam an outlet temperature of 103 °C on average. My solution converges at iteration number 307.

I wanted to ask if the whole process that I am carrying out is correct or not? And how could I visualize the condensation better?
I hope someone can clear this doubt for me, thank you very much in advance.

[Rob]

Hybrid initialisation may not be the best option, have a look at the results at t=0. Do you have a coupled wall between the cold water & steam? 

If you create an iso-surface at x=middle but also select the steam side before applying/creating you'll have a surface that's just in the steam side, that may help with the model review. 

[manuel.pacherres]

 

Hi Robar, thank you very much for your time.
- My figure is simply two concentric tubes as you can see in Image 1.
- What do you mean by taking a look at t=0, how can I realize that?
- I tried to perform the problem with a hybrid initialization and my solution converged at iteration number 46, but I get a note in the console with a message saying “Reverse flow” (Image 2), but I don’t know what that message is due to.
- And I wanted to ask another question: when I go to perform the “Configuration” of my figure, I get my figure vertically as seen in Image 3, but I want to work it horizontally as in Figure 4, because once I finish with this problem, I would like to apply it to another geometry, but I wanted to know then seeing the X, Y and Z axes, according to images 3 and 4, in which axis should I place the gravity? for example is it correct to place the gravity in the “Y” axis when my figure is in the position of Image 5?

Image 1

 

Image 2

Image 3

Image 4

Image 5