simulation of a thermosyphon heat pipe

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  • Last Post 23 November 2018
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nagesh56 posted this 28 October 2018

how to simulate a thermosyphon heat pipe involving evaporation and condensation of working fluid, is it possible to do this process by lee model of evaporation and condensation or do we need to write a separate udf for that.please can anyone describe the whole process.

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kkanade posted this 29 October 2018

I think you can use vof model with evaporation and condensation. With Lee model you may need to fine tune the parameters. It would be a good start with this and check how it goes. Then you can think of any udf if required. 

Regards,

Keyur

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nagesh56 posted this 29 October 2018

@kkanade

   Thank you for the quick reply, but your answer didn't clear my confusion. I have a lot of doubts about this topic which I can elaborate here

I am using a 2D model of a thermosyphon heat pipe of height 8cm and it is divided into three section. Top section 3cm is called condenser section and middle 2cm is adiabatic and bottom 3cm is heater section. condenser and heater are at a constant temperature of 340K and 300K respectively .toatl mesh elements 57245 of a quadrilateral. gravity enabled, implicit body force .using VOF model two phases water and vapour, the condensed liquid falls back due to gravity.inorder to reduce the saturation temp of water am operating at 20kpa pressure. corresponding saturation temp is 60deg.during patching heater section is filled with liquid and remaining is vapour.iam using transient case with dt=0.0005 .A SIMPLE scheme 

1.if I use lee model for above case then

what is the value I should give in standard state enthalpy for vapour and liquid ?, should I leave it as default or how do I calculate it? 

What is the value of pressure I should give in patching section for both liquid and vapour? And

how to tune the evaporation and condensation frequency values?

Is lee model will include the latent heat of evaporation and condensation in the energy equation during the calculation?

what is the operating density value should I give?

2. Is it required to write the UDF for the above case, if yes then how to write the UDF?

 

kkanade posted this 30 October 2018

Hello, 

Few best practices are as follows. 

Always specify the mass transfer from liquid to vapor.

Specifying latent heat is a bit tricky here. Please take care of following:

Latent heat is taken care of through formation (standard state) enthalpy specified in the

materials panel.To correctly specify the latent heat, please specify following:

 For liquid and vapor, use reference temperature as saturation temperature.

 Specify the same molecular weight for liquid and vapor phase.

 For liquid specify standard state enthalpy = 0, and

 for vapor specify standard state enthapy = latent heat (in j/kg-mol units)

 If you have latent heat data in j/kg then multiply it with molecular weight

(unit kg/kg-mol). This will give the latent heat value in required units.

 This specification will ensure appropriate latent heat removal/addition.

Standard state entropy has no role to play. It can be left to default value.

 

Hope this helps. 

Regards,

Keyur

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nagesh56 posted this 30 October 2018

Thank you 

 This information much useful to me. But, what about the condensation and evaporation frequencies and how to tune it?.

sorry question not yet solved, by mistake I click on the option.

and I am operating this at pressure 20kpa.so what value should I enter in operating density?

and what pressure value should be given while patching the sections?

Still much more help is needed from you. 

kkanade posted this 30 October 2018

Hello, 

If more evaporation is required, increase the evaporation frequency and reduce the condensation frequency. If more condensation is required, increase the condensation frequency and reduce the evaporation frequency. Though default values are set to 0.1 & 0.1 for evaporation and condensation frequencies respectively, it is observed that they may vary from 1e-3 to 1.0e5. You may need to do few iterations with your model to come up with correct frequencies. 

Regards,

Keyur

nagesh56 posted this 30 October 2018

I am operating this at pressure 20kpa.so what value should I enter in operating density?

and what pressure value should be given while patching the sections?

below is the image of my geometry and patching of section with liquid and vapour iam not considering about any non condesable gases.

bottom is heater ,middle adiabatic section and top is condensor .heater and condensor are at constant constant temperature of 340K and 300K respectively. 

kkanade posted this 30 October 2018

Hi, 

I think you can use density of lighter phase at 20kpa. 

You may also want to look at following thread. 

https://studentcommunity.ansys.com/thread/operating-density-in-fluent/

Regards,

Keyur

nagesh56 posted this 30 October 2018

what pressure value should be given while patching the sections?

is it same as operating pressure value or not?

mossaied2 posted this 21 November 2018

 

This is really good question

mossaied2 posted this 22 November 2018

 

 

The way of calculating the enthalpy is not clear yet! it is defined in ansys as j/kg.mol , but multiplyuing latent heat in (j/kg) by molecular weight which it is (kg/k.mol) will give us some number in j/k.mol not j/kg.mol 

kkanade posted this 23 November 2018

Question from mossaied2  is answered in different thread. https://studentcommunity.ansys.com/thread/standard-state-of-enthalpy/

Please create new thread for new question. Try to avoid posting in solved threads. You can give reference of old thread in new thread. 

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