Hello,
I am trying to use the Microflo package to simulate a perimeter office area in India during peak summer conditions. The purpose of my simulation is to verify comfort in this area with regards to temperature and air velocity. I have run the apache simulation and exported the boundary conditions to Microflo, all that is exported is surface temperatures (which are in the region of 26 degrees). I have had to manually change the glazing surface temperature to 60degrees to achieve any effect (I would expect 35-40).
Is there a method to model solar gain and it's related impact with Microflo? I would expect a much larger impact near the perimeter zone due to solar impact than just the temperature of surfaces…
Regards
Ray
Accounting for solar gain?
Re: Accounting for solar gain?
The Microflo module is designed to model a steady state 'snapshot' of the flow inside a zone or zones within your model resulting from the prediction of the dynamic thermal model (DTM), apacheSim.
Microflo itself doesn't have a solar radiation model, or any radiation model. The effect of solar radiation is accounted for in the DTM, the vast majority of which goes into heating the surface(s) that it lands on, although there is a small proportion that is absorbed by the air itself. When you export the boundary condtions from VE-Vista the temperature of the walls/ceiling/floor are exported so the effect of the solar gain is incorporated into the CFD model even though it doesn't itself model radiation.
The difficulty in analysing the effect of a solar patch in Microflo is that it imports the boundary conditions from a coarse grain model, i.e. each surface in the DTM only has one temperature associated with it and therefore the effect of the solar gain is spread across the whole floor. The only way to get around this is to break your DTM model down further to put perimeter zones next to the windows. The floors in these perimeter zones will then have a different temperature node than the main room node so when suncast predicts that the sun will fall mainly on the perimeter zones the temperatures will be higher there and so will be in the CFD model as well once you import the boundary conditions.
Q> I have run the apache simulation and exported the boundary conditions to Microflo, all that is exported is surface temperatures (which are in the region of 26 degrees). I have had to manually change the glazing surface temperature to 60degrees to achieve any effect (I would expect 35-40).
If you have just imported the boundary conditions there are only some things that get set up:
1) the temperatures of all the surfaces: ceilings, floors, walls, windows and doors.
2) the internal gains for people, lighting etc for each zone
3) Macroflo flows through openings in a zone, internal and external, cracks etc. The flow rates and the air temperature of the adjacency.
(3) comes from the Macroflo predictions. If you haven't run Macroflo these will not be imported.
Stuff that doesn't get imported:
4) room conditioning/system stuff from the building template manager
5) air exchanges specified in the air exchanges tab in the building template manger.
These two extra items have to be set up manually. If you are using Macroflo then there might only be infiltration for (5) which will have only a small effect. (4) is very important to set up correctly. You can use the "convective plant load" from the VISTA results to work out how much heat or cooling is required to set up your CFD model to match the thermal model. Use the convective portion as the radiative portion goes into heating the room surfaces (well the vast majority anyway) which are already imported.
The reason why nothing much is happening in your room maybe that there isn't much driving the airflow. If you have just imported the boundary conditions and not added any other boundary conditions to model the system, and your DTM didn't employ Macroflo, then it is only the different surface temperatures that are driving the flow. Then it becomes just a buoyancy driven flow which takes a long time to solve. In fact if it is just buoyancy driving the flow the solution may terminate before you achieve a converged solution. This is because the flow field will evolve quickly and the residuals will go to the convergence criteria before the temperature field has finished evolving. In this case you may only get a converged solution if you change the terminal residuals to a very low level, 10-9, just so it prevents the solution from stopping prematurely, or I guess you could increase the false time step. Note only play with the convergence criteria or false time stepping if there is nothing driving the flow except for surfaces at different temps.
Microflo itself doesn't have a solar radiation model, or any radiation model. The effect of solar radiation is accounted for in the DTM, the vast majority of which goes into heating the surface(s) that it lands on, although there is a small proportion that is absorbed by the air itself. When you export the boundary condtions from VE-Vista the temperature of the walls/ceiling/floor are exported so the effect of the solar gain is incorporated into the CFD model even though it doesn't itself model radiation.
The difficulty in analysing the effect of a solar patch in Microflo is that it imports the boundary conditions from a coarse grain model, i.e. each surface in the DTM only has one temperature associated with it and therefore the effect of the solar gain is spread across the whole floor. The only way to get around this is to break your DTM model down further to put perimeter zones next to the windows. The floors in these perimeter zones will then have a different temperature node than the main room node so when suncast predicts that the sun will fall mainly on the perimeter zones the temperatures will be higher there and so will be in the CFD model as well once you import the boundary conditions.
Q> I have run the apache simulation and exported the boundary conditions to Microflo, all that is exported is surface temperatures (which are in the region of 26 degrees). I have had to manually change the glazing surface temperature to 60degrees to achieve any effect (I would expect 35-40).
If you have just imported the boundary conditions there are only some things that get set up:
1) the temperatures of all the surfaces: ceilings, floors, walls, windows and doors.
2) the internal gains for people, lighting etc for each zone
3) Macroflo flows through openings in a zone, internal and external, cracks etc. The flow rates and the air temperature of the adjacency.
(3) comes from the Macroflo predictions. If you haven't run Macroflo these will not be imported.
Stuff that doesn't get imported:
4) room conditioning/system stuff from the building template manager
5) air exchanges specified in the air exchanges tab in the building template manger.
These two extra items have to be set up manually. If you are using Macroflo then there might only be infiltration for (5) which will have only a small effect. (4) is very important to set up correctly. You can use the "convective plant load" from the VISTA results to work out how much heat or cooling is required to set up your CFD model to match the thermal model. Use the convective portion as the radiative portion goes into heating the room surfaces (well the vast majority anyway) which are already imported.
The reason why nothing much is happening in your room maybe that there isn't much driving the airflow. If you have just imported the boundary conditions and not added any other boundary conditions to model the system, and your DTM didn't employ Macroflo, then it is only the different surface temperatures that are driving the flow. Then it becomes just a buoyancy driven flow which takes a long time to solve. In fact if it is just buoyancy driving the flow the solution may terminate before you achieve a converged solution. This is because the flow field will evolve quickly and the residuals will go to the convergence criteria before the temperature field has finished evolving. In this case you may only get a converged solution if you change the terminal residuals to a very low level, 10-9, just so it prevents the solution from stopping prematurely, or I guess you could increase the false time step. Note only play with the convergence criteria or false time stepping if there is nothing driving the flow except for surfaces at different temps.