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-   -   modeling phase change and homogeneous bubbles in many cycles (http://www.cfd-online.com/Forums/flow-3d/109920-modeling-phase-change-homogeneous-bubbles-many-cycles.html)

 dwtme November 29, 2012 22:39

modeling phase change and homogeneous bubbles in many cycles

Hi everyone,
I have some problem about modeling phase change and homogeneous bubble.
1. According to the Flow-3D tech paper (http://www.flow3d.com/pdfs/tn/FloSci-TN57.pdf) ,I have to first remove a region of fluid as a little bubble and set temperature distribution. It can model a thermal inkjet to eject a droplet. Now I want to let inkjet eject 3 droplets. I want to run simulation for 3 cycles. After the inkjet eject one droplet, can I stop the simulation and remove a region of fluid then set a temperature distribution and continue running the simulation?
How can I set the boundery condition?
2. I want to give a constant heat flux at wall boundary as heater to heat the liquid and generate bubble.It didn't work when I set the thermal condition in the wall boundary. How can I set the boundary condition?
Hope you can help me solve the problem
Thanks,
Best regards

Tim:confused:

 dmilano December 4, 2012 11:12

For (1) removing the fluid is not really necessary - it is just a convenient initial condition. You can still use the "thermal bubbles with phase change" model without this initial void region by using the "Nucleation" option on the bubble model dialog.

(2) can be handled by creating a component in the shape of your heating element and defining it as a heat source in the component properties.

 dwtme December 12, 2012 22:55

Dear dmilano,
(1)In the phase change model,there is an item "accommodation coefficient". Does accommodation coefficient affect the bubble formation? And how will it effect the bubble?
(2)When I creat a component as a heater,can I decide heating time of the heater?And what boundary condition should I set for the heater?
Thanks,
Best regards,
Tim

 dmilano December 13, 2012 11:04

The accommodation coefficient adjusts the rate at which the phase change occurs. It is a linear coefficient, so it is pretty intuitive to adjust the rate. Take a look at http://www.flow3d.com/pdfs/tn/FloSci-TN57.pdf for a more thorough explanation.

The heat source power can be defined using tabular data, so you have complete control over the input power as a function of time.

 dwtme December 14, 2012 06:34

Dear dmilano,
Thank you for your reply. I can't find the tabular data in component. Do you mean the tabular data in the boundary condition->thermal information? Can I just create a mesh and define a surface as a wall then setting the power as a heater? Or I have to create a "component" as a heater?
Thanks
Best regards,
Tim

 dmilano December 14, 2012 12:31

In version 10 the tabular data for the component is under Component #->Component properties->Solid properties->Heat source type.

You could certainly specify the thermal power at a mesh boundary but it would put some significant restrictions on the heater geometry.

 dwtme December 18, 2012 08:39

Dear dmilano,
Thanks
Best regards,
Tim

 dmilano December 19, 2012 16:34

The tabular data input isn't in the GUI, but you should be able to do this by adding the parameters tobs and pobs into the prepin.

 dwtme December 21, 2012 10:37

Dear dmilano,
Thanks for your reply. I still have some question. I want to heat the water to generate a bubble. I use thermal bubble with phase change and heat transfer model . I heat the water to 540 K but it still doesn't generate bubble. How can I let it generate a bubble?
Thanks,
Best regards
Tim

 dmilano December 21, 2012 11:03

You probably need to activate the cavitation option (ICAV=1) and set CAVRT. There's a nice discussion of what you need to do in the Model Reference->Overview->How do I model?...->Cavitation and bubble formation section of the manual.

 dwtme December 21, 2012 22:53

Dear dmilano,
Thanks for your reply. I have tried to use cavitation before, but the result didn't match with the experiment data. According to the experiment data ,the heating power is 1.8W. In the simulation, I only can set 0.006 W as heating power. If I set the power over 0.006W, it will not eject droplet properly. Is the physical meaning of the cavitation the same as the heating to generate bubble? Do they have the same mechanism?
Thanks,
Best regards,
Tim

 dmilano December 31, 2012 13:35

Not sure why that would be, but it sounds like a problem with the fluid properties.

 parisamp January 1, 2013 05:57

droplets injection

Hi everyone
i am a fluent beginner.i am simulating EVA droplets in PE.i want to inject 200 droplets in a tube and see their deformation and coalescence.i am using vof.how can i do this?please help me .thank you

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