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-   -   simulate flow of rising steam beside air around (http://www.cfd-online.com/Forums/floefd-floworks-flotherm/106876-simulate-flow-rising-steam-beside-air-around.html)

unit22 September 11, 2012 09:17

simulate flow of rising steam beside air around
 
Hallo,

Is it possible to simulate rising steam from a source of 373kelvin beside normal air around the steam with FloEFD? ( In my opinion a two phase flow ) The target is to simulate a cooking situation with steam and normal air.
Has sombody expierience with this?

Thanks in advance,

Kay

Boris_M September 12, 2012 03:01

If you are just looking at the steam flow in the air and not the boiling itself, then you might want to try to use the tracers which is part of the HVAC module in FloEFD.

You can define a hot surface such as the hot water surface and a wet surface tracer condition which will stand for a wet surface like a puddle of water and you can see the evaporation of the water. If it is an inlet of a pipe with steam coming out, you can also use a boundary condition (BC) for defining a amount of water vaport tracers coming out in addition to the hot airflow.

Tracers are not a two phase flow but can simulation a certain amount of contaminant or vapor released from a vent or opening into a room or sorts. Tracers how ever only work within certain limitations, they are a post processing feature just like a particle study and therefore have no influence on the actual flow but will follow the flow and dissipate in the environment, it is an ad-mixture passive scalar.

I hope this helps,
Boris

unit22 September 13, 2012 05:47

Hi Boris,

Thanks for your reaction and informations! The idea is to simulate steam rising up out of a pan. It is not the target to simulate the boiling water itself.
What do you think about degree of realty in a scale from 1 to 10 (10 is sufficient realistic) if tracers will be used?

Regards, unit22

Boris_M September 13, 2012 07:39

To be able to determine if it gives you correct results I would need to know what the goal of the simulation is. Are you designing fume hoods?

Generally in this case I would say Tracer is the right choice but I cannot comment on the 1 to 10 question as long as I don't know what exactly you try to get from the simulation. What I mean is, do you want to see any numerical values such as mass flow rate of water that is evaporated in the pan and just how it is dissipating into the room or rather succed away by the fume hood in a distribution way, not by a certain value. I don't want to say it would be inaccurate for either way but depening on your goal it might be more or less suitable (meaning a 7 instead of a 9 or so).

Boris

unit22 September 14, 2012 06:28

The target is to simulate the interaction of vapor and a fume hood. I want to see how the vapor dissipating in the room and later sucked away by the fume hood. Values are also interesting to get an imagination if the simulation displays the effects sufficient. For me both of your examples are interesting.

The goal is also to see how much vapor flows in the hood and how much along the hood. Or how the air outlet of a recirculation hood effects the rising vapor and the suctioning behavior of the hood.

I hope you got a better vision of the goal of the wished application of FloEFD. Could you try to give your estimation about the feasibility with this additional information?

THANKS for your time and information’s up to now.
Kay

Boris_M September 14, 2012 07:53

I feel very confident that you will get very good results with this simulation using Tracer. If you define the correct boundary conditions for the Tracer study, you will be able to see how much mass flux the of H2O tracers (vapor) you have on the wet surface of the pan and the vapor will follow the airflow. So if you had previously good results with the general airflow, the Tracers are basically following the airflow and therefore go into or around the hood.
You are be able to get a surface parameter of the surface of the filter to see how much flow is going through the filter in the hood. However, for some reason the surface parameter does not show the mass flux through this surface but you can go the other way of requesting the mass fraction of the tracer and the fluid mass flow rate at the filter surface and then simply the same fraction of the mass flow rate of the fluid is the mass flux of the tracer.
I never tried this before so I will forward it to the developers to include it in future. (Thanks for that.)

So my answer for this case would be 9 for the general flow and distribution and 8 for the values as you cannot use the surface directly for the value and Tracers are not a 100% perfect solution for evaporating water but 90% I would say and I don't like giving 10 in general and since simulation is always very depending on the boundary conditions :o)

I hope this helps,
Boris

PS: it would be great if you could share your experience if you have any way of measuring the model and therefore validating the application you have.

unit22 September 18, 2012 05:11

Hi Boris,

Thanks for your ideas and consult. Is there in general a method to make the spreading of the vapor visible? What is in your opinion the best evaluation method to display the behavior of the vapor beneath the fume hood?

Regards, Kay

Boris_M September 18, 2012 07:32

Usually to see some kinf of plume like a smoke plume you would use iso surface with a certain value for the parameter you want to use. For example mass fraction of Tracer 0.05 or so, but you will have to play with the value a little to get a nice visulization or if you are interested in a certain contentration it is easier as the value is known. Other than that would be cut plots.

However, the tracer are not for a transient flow calculation so you will not be able to visualize the tracer results as it fluctuates like the flow does in a transient mode that easily. There is a kind of manual way to do so. You would have to load the single result files of a transient simulation and visualize the tracer, save the image and then the next file etc. then there are freeware tools such as Virtual Dub where you can drag and drop a range of images (should have a running number at the end of the name) and save it into a video. That way you'll get an impression in an animated way on how the vapor plume fluctuates. But since it can be quite some work to do it maually, you should make sure the settings for the visulizations are right (right angle, geometry in display, values in range etc.) for some result examples.

Is that what you are looking for?

If the question is for the parameters, usually the velocity tells a lot since the vapor is also guided mostly ba the velocity but you can also define diffusion coefficients for the tracer. So to see the tracer concentration the parameters would be mass fraction of Tracer or mass flux of tracer.

Hope this helps.

unit22 September 19, 2012 05:05

Hi Boris,

I get imagination. Thanks for you help. Now it is time to perform:).

Have a good day, Kay


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