Dear all

I had conducted CFD simulation to investigate the CO2 distribution of an indoor space under four different exhaust air position. Relative ventilation efficiency: [CO2(exhaust)-CO2(supply)]/[CO2(occuiped zone)-CO2(supply)] is used to assess the ventilation performance. Obviouly, various ventilation efficiency were found for different exhaust air position.

When I increase the generation rate of CO2 source by 10 times, the simulated ventilation efficiency almost remain unchanged. Is it true that the ventilation efficiency dependent on supply/exhaust air position, air flow pattern, etc. but independent of the generation strength of the CO2 source?

Thanks a lot! Have a nice weekend!

Apple Chan 17 July 99

Hi, Apple,

>Is it true that the ventilation efficiency dependent on supply/exhaust air position, air flow pattern, etc. but independent of the generation strength of the CO2 source?

If the CO2 source does not affect the velocity distribution, then yes. Indeed, CO2 concentration is governed by a linear equation, something like this

dc/dt + u\nabla c =\nabla(diffusion coeff)\nabla c + source

If you increase 'source' n times, you can easily see that, if 'u' is the same, new 'c' will be n times the old 'c'. Generally, source can also be in boundary and initial conditions.

Look now at you definition of efficiency: if you multiply all quantities entering it by the same number, the fraction will not change.

I am also somewhat surprised by this definition. Ventilation is usually expected to be working for a long time so that the process becomes steady. In that case, since CO2 cannot be accumulated, CO2(supply)=CO2(exhaust) and efficiency, as you defined it, is zero. Something similar I encountered when I was working on centrifuges cleaning oil in car engine. We defined efficiency to be proportional to something which in your case would look like

CO2(content in the room)/CO2(supply) rate,

with other quantities entering the formula to make it nondimensional.

Hope this helps,

Sergei

Hi Sergei

Thank you for your response.

First of all I have to apologize that I have not described my simulation in detail. Actually a CFD simulation for indoor air flow field was done before, i.e. a steady state of air flow field had been obtained. Then another CFD simulation was conducted using the aforesaid simulated flow field and adding CO2 generation source. Therefore the CO2 source does not affect the velocity distribution, as you mentioned.

For the ventilation efficiency: [CO2(exhaust)-CO2(supply)]/[CO2(occuiped zone)-CO2(supply)], I think that I had not explained the meaning of each term clearly. In the room, fresh air with CO2 concentration: 400ppm [CO2(supply)] is supplied from air inlet. CO2 generation sources are located in the occupied zone. After well mixing from fresh air, CO2 concentration in the occupied zone will be simulated [CO2(occuiped zone)]. Then the air is exhausted through air exhaust outlet [CO2(exhaust)]. If this ventilation efficiency is greater than 1, that means it has good ventilation performance since most CO2 can be removed from the occupied zone and exhausted. If the ventilation efficiency is smaller than 1, that means "short circuit" occurs and most fresh air is exhausted through the air outlet directly without well-mixing with the air in the occupied zone.

Could I have your comment please?

Thank you very much!

Have a good begining in this week!

Apple Chan 18 July 99

(1). In a room with a source, the source will stay at the same location if there is no convection and diffusion of the source. (2). When there is only diffusion in a room, the source will eventually fill the whole room. (3). The only way to remove the effect of the source is to use convection. (4). If you position a duct right in front of the source and apply the suction (low pressure) at the other end of the duct, then by convection, the source will be carried out through the duct and out of the room. The operation is just like the operation of a vacuum cleaner. You apply the suction on the source and remove it through the duct. (5). The operation is also similar to the operation of the kitchen range hood suction fan. In this case, there is a distance between the surface of the cooking surface and the entrance to the suction fan. Some diffusion effect will occur in this case. (6). In general, it is not a good idea to let the source to diffuse first because then you have a much larger volume to handle through the convection process. (7). I have studied and simulated 3-D flow field of various range hood design for its effectiveness. And I hope that this will provide some help to your Co2 removal problem.