Learning Forum

[diksha]

Hello, grateful if you could guide me. I am running simulations on the condensation of R134a in a horizontal pipe. I am plotting my heat transfer coefficient (HTC) against time to ensure steady-state. My simulations have reached steady-state and my HTC matches the experimental one. The problem is with my pressure drop. It does not converge to a value, but fluctuates all the time, please see the figure attached. I have recorded the total pressure at inlet and outlet in order to calculate the pressure drop. I am using SST k-omega model. My inlet BC is mass flow rates of vapour and liquid phases of R134a. At outlet, I specify a pressure and the a heat flux is imposed at the wall. Please advise me on how to proceed with the pressure drop. I thank you in advance.

[peteroznewman]

Please insert images directly into your post because ANSYS staff are not permitted to open attachments.

[Rob]

If you plot a monitor of mass fraction of liquid how stable is that?  Monitor points and not an area or volume average. 

[diksha]

Hello, thank you for your replies.

The plot of volume fraction at a particular point (0.744, -0.002,-0.002) over time is as such:

My tube is of length 1.488 m  and diameter 8.38 mm.

 

[Vladimir]

Hello, grateful if you could guide me. I am running simulations on the condensation of R134a in a horizontal pipe. I am plotting my heat transfer coefficient (HTC) against time to ensure steady-state. My simulations have reached steady-state and my HTC matches the experimental one. The problem is with my pressure drop. It does not converge to a value, but fluctuates all the time, please see the figure attached. I have recorded the total pressure at inlet and outlet in order to calculate the pressure drop. I am using SST k-omega model. My inlet BC is mass flow rates of vapour and liquid phases of R134a. At outlet, I specify a pressure and the a heat flux is imposed at the wall. Please advise me on how to proceed with the pressure drop. I thank you in advance.

 

   Hi , I think your problem problem lies that you have too high flow velocity. In my experience that's what happens when you have velocities more than 2 Mach. In this cases I know that Mass Flow Rate BC does not work. If you define Mass Flow rate BC at the outlet or outlet eqvivalent critical mass flow rate the outlet pressure will be higher than real value, if you define mass flow rate value more than critical the outlet pressure will decreas to zero and after that you get a error in the solver manager.

You can try use Exit Mass Flow Corrected BC (to do this you have generate your mixture on water ideal gas). According the reference pressure and temperature in this BC you will have different pressures at the outlet. I don't know Is it physically correct or not , but i know that you will get different values of the pressure.

[diksha]

Hello Vladimir,

Thanks for your input.

I have calculated the Mach number, and they are below 1 for all my flow conditions. 

I still do not know why my pressure drop behaves like this.

 

[Vladimir]

Hello Vladimir,

Thanks for your input.

I have calculated the Mach number, and they are below 1 for all my flow conditions. 

I still do not know why my pressure drop behaves like this. 

If this , I think your problem in your mixture. Best way it is find the standard working fluid from ANSYS like your. Also try use conservation target option. 

[Rob]

It looks like the liquid is level isn't constant: waves may cause a change in the pressure field. Hard to know without the case & data and we're not allowed to download them. 

[diksha]

Hi, thank you for your reply.

Indeed, the liquid level is not constant. It is a stratified flow. Near the entrance, the flow is not stratified and there are droplets. Then, there is the stratified flow forming. Then, the major part of the pipe is the stratified flow.

I have tried changing my pressure outlet to outflow, but it does not help. The pressure drop becomes negative and continues to fluctuate. 

I have also changed the mass flow rate BC at the inlet to a velocity BC, with a fully developed velocity profile. It does not predict it correctly.

Maybe, I should mention that my operating pressure is the saturation pressure. Maybe, there is something that I am missing, but I don't really know what.

[Rob]

Outflow is an old boundary and shouldn't be used in this case (or in most cases for that matter).  If you monitor the free surface level once it's settled is it flat, or are you seeing waves?  Are you considering the phase change? Which multiphase model?

[diksha]

The waves appear only at the beginning of the pipe. After that, it is flat.

Yes, I am using the Lee phase change model for the condensation and VOF (GeoReconstruct) for two-phase flow. The properties of both the liquid and vapour phase are temperature dependent. 

Do you think the reference pressure location has something to do with it? I have not defined any location in my simulation.

 

 

[Rob]

Possibly, put the reference pressure in the gas space. 

I've just flagged the error: it's not just you. 

[diksha]

Ok, I will try that and let you know. Thanks again..

[Rob]

The insert images should work now too, we had an issue server side. 

[diksha]

This is the liquid phase volume fraction near the inlet of the pipe.

This is the stratified flow along the pipe, showing it is flat.

I have also input the reference pressure location, but the problem still persists. The pressure drop fluctuates and becomes negative, however when I check the velocity vectors at the outlet, there is no reverse flow.

[Rob]

Which multiphase model are you using?

[diksha]

The volume of fluid method with Geo-Reconstruct scheme.

[Rob]

OK, and how is flow entering the domain? Ie what volume fraction did you use on the inlet. 

[diksha]

The mixture enters the pipe normal to the boundary. The vapour quality that I considered is 0.25. From the total mass flow rate (in experiment), I calculated and imposed the mass flow rates of liquid and vapour phases of R134a.