Mechanical cross ventilation

andfranz · Posts: 104

I’m using DesignBuilder v. 2 with CFD.
I have two questions regarding a case like that one presented schematically in the following picture: there are a heated space and an adjacent unheated space (e.g. a sunspace), a mechanical extraction system is installed in the heated space (it creates a depression) and holes are realized between the heated space and the unheated space and between the unheated space and the external environment. Therefore the supply air has a unique direction and it passes through the unheated space.

My questions are:
1.Is the modelling of a such situation possible in DesignBuilder v. 2? And in DesignBuilder v.3?
2.Is the calculation of the temperature of the supply air in the heated space possible through the CFD module (the temperature of the supply air depends on the position of the holes for the air passage and on the air velocity)? And is the result of that calculation utilizable in order to calculate the heating requirements for the heated space?

With Regards

andfranz · Posts: 104

Concerning my first question, I think the modelling of a such situation possible only in DesignBuilder v.3:

http://www.designbuilder.co.uk/component/option,com_forum/Itemid,25/page,viewtopic/t,3001/sid,c42e51b8dae815c5bf5149a33b726114/

http://www.designbuilder.co.uk/component/option,com_forum/Itemid,25/page,viewtopic/p,9442/sid,1c85c5eb2c42d0de55381ccf8bbbee4f/

Isn't it?

With Regards

Steve Potter · Posts: 614 Location: newcastle

Hi,

DB CFD is not intended to be used for determining spatial heating requirements but more for predicting the distribution of air velocity, temperature and comfort conditions throughout a space given pre-determined boundary conditions. In this particular case, you could use EnergyPlus to carry out the thermal calculations and then use the results from the EnergyPlus calculations as boundary conditions for a CFD calculation if you then want to assess internal distribution of temperature, etc.

Regards,

Steve

andfranz · Posts: 104

Steve Potter · Posts: 614 Location: newcastle

Yes, you can use the version 3 EnergyPlus module to do the thermal calculations.

Regards,

Steve

andfranz · Posts: 104

I have created a virtual space for the air exctraction (it is called “exhaust air space”) and I have defined it as plenum in Activity → Zone Type

I have created a hole in the wall between the “exhaust air space” and the heated space. The hole is big like all the wall. 2 other holes have been created between the heated space and the sunspace and between the sunspace and the external environment.
Then I started to work with the HVAC system detailed modality. I have created a generic air loop and two zone groups. To one group the heated space is assigned, to the other one the sunspace. In the air loop I have replaced the zone mixer with an “air loop return plenum” and I have assigned to that the “exhaust air space”.
In the “Edit Air haindling unit” data I have imposed a constant air flow.
My problem is that I cannot close the loop because I cannot conect the extraction from the sunspace with the “direct air” terminal of the heated space (the cursor become green but the connection does not happen; I attach a picture).

If DB starts the simulation the errors are the following:

Building 1 - Component disconnected: Block1:HeatedSpace Direct Air 1
Building 1 - Component disconnected: Zone Group
Building 1 - Component disconnected: Zone extract
Building 1 - Component disconnected: Zone Group 1
Building 1 - Error in HVAC definition

Any advices, please?
With Regards

Steve Potter · Posts: 614 Location: newcastle

Hi,

You cannot supply air from one conditioned zone to another conditioned zone which is why the software will not allow you to connect a zone exhaust component to a zone supply component. One approach you might consider is to include the unconditioned sunspace as a supply plenum and then connect the supply plenum to the conditioned or 'heated' space as shown on the attached screenshot.

Regards,

Steve

andfranz · Posts: 104

Thank you very much!
Now the simulation is running. Moreover, I have added a hot water convector, connected to a boiler.
Unfortunately, in the simulation results I'm not able to find results concerning the ventilation, although I select “External Vent” among the data which I want to visualize. The problem exists both for the sunspace and for the heated space.

Therefore I have tried to change the ACH to observe how the temperature in the sunspace changes.

with the design ventilation 0,0117 m^3/s

with a higher ventilation: 1 m^3/s

Correctly, the maximum values are lower than in the previous situation (actually, very little), but the maximum values are higher.

With a very low ventilation 0,001 m^3/s

The maximum values are, correctly, higher, but the minimum values (during the night or in particularly cloudy days) are lower, strangely.

The temperature in the heated space is, correctly, always 20°C ( a little bit higher only in very sunny hours).

With Regards

andfranz · Posts: 104

The recirculation rate was "autosize". Part of the exhaust air from the heated space was therefore sent in the sunspace. I'm sorry... Now I have eliminated it. And I was able to see the ACH in the results...
Regards.

andfranz · Posts: 104

I have still a little problem: sometimes the set point temperature (20°C) is not achieved. I think the set-point was correctly imposed (it is always on, 20°C, without vacation periods).
I thought that the problem was that the hot water convector was not able to provide the necessary energy, but when the set-point is not achieved the zone sensible heating is not at its maximum level, at least not at the beginning, how you can see in the following picture:

With the design air flow the problem is only at the beginning of the simulation. Could the initial conditions have a role in that problem?
With Regards

andfranz · Posts: 104

I have solved the problem of the temperature below the setpoint value increasing the sizing factor of the boiler. Anyway, I don't understand why when the temperature was below the setpoint value the zone sensible heat was below its maximum.
Regards

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