Emissivity and seasonal temperature for solar loading.

[Anonymous]

Hi,

i am currently performing my analysis on solar loading in CFD autodesk to evaluate the thermal performance of two different types of building fabric.

Till now i have performed a diurnal analysis with an external volume around my model.

I have already got some results from my analysis however i would like to have more accurate results.

I want to include seasonal temperature variation and sky emissivity in my analysis.

However, I am not sure where to apply the emissivity transient boundary condition on my model and also I am not sure where to apply the temperature boundary conditions for seasonal variation in my model.

Any kind of suggestions will be highly appreciated.

Thanking you in anticipation

Best regards,

Kaviraj

[David.Short.]

Good morning @Anonymous,

I am glad that you are making progress with your project.

So if i understand correctly you want to add further temperature variation to your simulation which currently uses the repeating Ramp Step. It may be simpler to clone the initial scenerio and change the value in the already functioning boundary condition. In this way you could have a winter scenerio, summer scenerio etc.

In terms of varying the emissivity of the sky try applying a radiation BC on the external surfaces of the air volume with transiently varying emissivity. This should hopefully overide the emissivity value assigned in the material edittor.

I hope this helps to answer some of your questions.

All the best,

David

[Anonymous]

Hii,

Thank you very much for repeated help.

In fact the ramp step boundary condition is for the sky temperature.

I need another boundary condition for the seasonal external temperature variation.

I cannot decide where this boundary condition can be applied.

Should i be applying it as transient in the air volume ?

For the emissivity i have already understood what you have said.

Thank you for your kind and usual help.

Any suggestions will be appreciated.

Best regards,

Kaviraj

[David.Short.]

Hello,

No problem at all, I am happy to help!

You can only apply one temperature boundary condition to the external air volume and you have already done this applying a so-called sky temperature (simply a transient temperature applied to the external surface). This condition causes / affects the ambient temperature throughout the simulation.

To study the effect of seasonal variation why not run alternative scenarios, as I hope you are not planning on running a year long simulation!

All the best,

David

[David.Short.]

Hi,

To add to my previous post. Can you please help me to understand the envisaged results / end goal of the analysis. What comparisons / conclusions do you hope to make?

As it sounds like you are about to embark on some pretty extreme simulations, it may be sensible to first run a best and worst case scenario, for example a summer and winter scenario. This should give you lots of usable data without running a mammoth analysis!! 

All the best,

David 

[Anonymous]

Hii again @David.Short.,

As i mentioned previously, i have two different building fabric to study.

I have to study their thermal behavior under real world conditions and find which of the two provides best human comfort(inside the building).

By real world conditions i mean;

solar loading

weather conditions

wind(which is assumed to be negligible)

Thermal buoyancy.

Is that enough clear or should i elaborate more?

Thank you.

Kaviraj

[David.Short.]

Hello hello,

Ok so it sounds like your current setup should give you sufficient data. There is no need to model the entire year as most of the data will not be of interest. Modelling the extremes throughout the year should hopefully be sufficient. 

I would suggest:

1. Clone the scenario, to get one for winter and one for summer (to start with).

2. Apply the relevant solar loading to each scenario.

3. Apply sky temperature transient BC to each accounting for the difference in season.

4. Run a one day one night analysis of each and compare.

If you ran monthly analyses of each the only variation within the month would be solar radiation, and this variation would be relatively small and probably not of much interest.

To expand the analysis create new scenarios for the different seasons.

I hope this helps. 

All the best,

David

[Anonymous]

Hi @David.Short.,

Thank you for your helpful suggestions.

Well, about seasonal variation i meant hourly weather data variation.

I will be running the simulation for summer season only.

However, i need to include hourly weather data since this will affect my overall results.

The problem is where can i apply this transient boundary condition(hourly weather data) on my model?

I have already applied sky temperature boundaries on the outer surface of the air volume.

if possibly you could suggest me.

Thank you.

Best regards ,

Kaviraj

[David.Short.]

Morning @Anonymous,

To apply hourly variation you should modify the sky temperature boundary condition (this is the only temperature input required for your analysis). You cannot apply an addition temperature boundary condition to the environment because then the model would be over constrained.

To have an hourly varying temperature boundary condition it should be possible to use a piecewise linear relation. As seen in the screen shot attached.

Thanks,

David

[Jon.Wilde]

Can I jump in here with a question?

Might it be simpler to take the best and worse case scenarios and capture only that?

I am not super clear on why the variation would be more useful, what are you looking to measure within the model? (What question are you trying to answer?)

Thanks,

Jon

[Anonymous]

Hi, @Jon.Wilde,

I'm trying to obtain the variation of the internal temperature of the building(under real world conditions) to measure its deviation from human comfort zone in two different cases.

The two different cases represent the building made up of two different building fabric.

i need to include thermal mass property of the fabric under solar loading

And i think the outside weather conditions will affect the temperature inside.

[Jon.Wilde]

I agree, the weather would play a part here.

The idea though is to just take the worst case scenario and capture just that, rather than all the data in-between where the comfort is 100% OK.

I would still suggest just running two data points (or more if necessary) but just a quick steady state analysis, coldest, hottest and done 🙂

You are likely to have just one datapoint that is the worst for whoever is inside, do you already know what that is likely to be? 

Hope that helps,

Jon

[Anonymous]

Hii @Jon.Wilde,

Thank you for your kind reply

I have adopted a new way to perform the analysis.

Instead of using a volume as environment around my model, i am using  film coefficient as boundary condition on the building facade and run the analysis with solar loading.

To cater for the hourly temperature variation, the film coefficient is set as a transient boundary condition to vary the 'ref temperature' .

Does that sounds to be good?

Thank you.

[Anonymous]

Hi @David.Short.,

I am removing the air volume which was representing the environment form my model. I will now be using transient film coefficient as boundary condition on my building facade surface. The 'ref temperature' will be varied using a piece wise linear model. This will represent the hourly temperature variation.

This sounds to be ok to me.

What are your suggestions?

Thank you very much for your kind help.

Best regards

Kaviraj

[David.Short.]

Good morning @Anonymous,

I like the sound of your new setup!! It should save on run time a lot. Using a film coefficient with varying ambient temperature is certainly a valid approach to take account for the external environment.

To apply solar loading without an external air volume you must use the solar wall or solar window materials for your structure as then the effect of the solar radiation will be felt on the building.

If you used normal materials with no external air volume the solar radiation would not be felt. 

Here is a nice link for you!

Best regards and good luck,

David

[Anonymous]

Hii again @David.Short.,

My building fabric is made up of two materials.

I have attached a top view of the building facade(without roof)

Suppose if i set a U value(solar wall) for the concrete layer only and set the air layer as variable.

Will the model be good?

as i need to know how the temperature of air varies inside the concrete fabric during a diurnal analysis.

I also need to know how the 'internal air' temperature varies during a diurnal analysis. 

Thank you in anticipation.

And thank you for your help provided so far.

Best regards,

Kaviraj

[David.Short.]

Hi @Anonymous,

That all sounds fine to me. 

If you wish to take natural convection into account yes make air variable. If you are running just heat transfer (meaning only conduction and radiation is considered) you can leave air as fixed.

Thanks,

David