how Aux ventilation is treated in CFD calculation

[golden eagle]

Hi ,

I am working on a school project which is designed to operate on hybrid mode ( natural ventilation +aux vent to assist overheating when needed) , the issue here , I would like to see the impact of aux vent and how the temperature in the room changes as a result of free cooling delivered by the fan when operating ,

I expect to see area where the fan operates will be cool down with relatively lower temperature ,the problem here when aux vent system is included in the apache sim calculation , the cooling effect( or negative gain) from operation of the fan has already uniformly lowered the temperature in the room , so the entire room has received the benefit due to the fan operation , however when I export the result into CFD , the boundary condition will be exported based on this lower temperature on that particular time that I would like to see on CFD , which means in temperature scale , everywhere will look the same , but what I expected to see is the only the area where fan operates receives the benefits of cooling , you can still see some degree of cooling in the location of fan , but the other area should be much hotter .

I assume I should not include the fan operation in the apache sim calculation but add the fan with the expected flow rate in CFD ,if this option is true, then can the software know the external air condition on that particular day /time /duration that I want the fan to operate ( without using aux vent apache sim, the software should consider same outside condition to get more realistic outputs , or I guess you set the temperature separately on the boundary condition of the fan, but I think you will first need to find out outdoor temperature from vista at the time of period )

[harshadj]

You should include the fan in the Apache Sim calculations.

MicroFlo is an adhoc CFD tool. That means ApacheSim/Template Manager/ApacheHVAC will not know about any changes that you make to the room in MicroFlo.

The correct procedure would be to:
1) Run Apache Sim with all appropriate settings (aux vent/nat vent etc, internal gains, set points, profile, marking the room you want cfd results for)
2) Choose an instant you want to look at in detail (how you decide the instance is up to you)
3) Export the boundary conditions for that instance.
4) Import boundary conditions and room gains
The boundary conditions are strictly the conditions on the various surfaces of the rooms in terms of temepratures. If you have openings from MacroFlo, you can also get flow conditions. But in this case of aux vent, you only need surface temperatures and room gains.

Query the VistaPro results to get the aux vent flow rate for that instance. Draw an flow inlet boundary with that flow rate and appropriate supply temperature (could be atmospheric or based on profile specified.) Draw and outlet boundary as well.

Run the sim.

[golden eagle]

Hi Harshadj

thank you for your response ,

if you include Aux Vent in the apache sim calculation before exporting boundary conditions , the room temperature in the entire room will reduce , which eventually impact the surface temperatures and the operating temperatures . IES uniformly reduce or increase the temperatures in the room , but in reality there will time lag until air floats from one location to other location , if you export the boundary conditions at that particular time that you wish to see the analysis
the lower air temperature will be exported ( due to fan operation) , however we would like to see drop in temp only in the location of the fan or supply diffusers. this seems to be adding additional cooling effect in the area of the fan on beyond and above of the apache sim room conditions.

I still think we should only add aux vent at specified temperatures and the flow rate in the CFD calculation so that we can see true impact of the fan or diffuser operation , this is based on CFD understand and correlate implication of lower supply temperature from the inlet position of the fan or the diffusers . I assume you will say I am missing something
may be I am .



the start introducing air say when tempso in , say you have a single supply diffusers at the corner of the room a

[harshadj]

What you are talking about seems like a transient analysis - 'want to see how thing change with time'

MicroFlo is a steady state CFD tool. This means all the inputs have to be steady.

To overcome this, what you can do is two steady state CFD simulations - just before fan starts and 1 hour after fan has started. This will show you how conditions have changed rather than while they are changing. The DSM will take care of the changing part of things.