It sounds as though you may need to investigate isokinetic inlets, nozzles, and vents for measuring air flow/volume.  This link may be useful to you:  http://www.levego.co.za/NACA_Mpumalanga_Pres_A.pdf 

Eric

Given the diameter, height and temperature at the base and top of the chimney, there will be a buoyancy driven flow from the direction of low density regime to high density regime. The flow rate will change if any of these parameters is changed. It is not clear what's the purpose here. 

Dear Eric Steven Hall

Thank you for the link. It sounds great to use isokinetic measurement but i'm looking for a mathematically method to predict the flow rate and after that go to experiment issue.

Dear Subhasish Mitra

You have got it correctly. There is a flow rate caused by Buoyancy phenomena. Now consider I want to use the force of wind against this flow rate. As you know Buoyancy force (Stack effect) makes a pressure and density difference at inlet and outlet of duct. Now if I want to use Bernoulli principle by taking the mentioned pressure and density ( and also in my case Humidity) into account, I'm not able to measure the applicable height of duct to have a flow rate bigger than flow rate caused by Buoyancy!

that is my objective. To find out the exact height which gives the flow rate bigger and against the flow rate of chimney.

In a natural draft chimney, the maximum flow rate, you can obtain is only due to the contribution of buoyancy which is given as Q = C x A x sqrt [2 x g x H x (Ti - To)/Ti] where constant C is discharge coefficient (0.6 - 0.7), A is stack area, g : 9.81 m/s2, H is stack height, Ti - Avg. temp in stack and To - outside ambient temp. The flow rate in principle should increase if you increase stack height or diameter.

Now coming to your purpose of "use the force of wind against this flow rate" is bit confusing. Are you anticipating that at the stack exit wind is blowing which will prevent this stack flow?

Or your purpose is to estimate a flow rate using Bernoulli principle over this stack height H?

If you are using Bernoulli principle, did you consider friction loss in stack?

Yes.

I want to know by using which method it is possible to find the flow rate over this stack height H? Consider I want to use the energy of wind velocity blowing inside the shaft and overcome to flow rate made by stack effect. as it is known wind profile according to the latitude is not uniform so in which height it is possible to over come on the flow rate made by stack? which law can held this answer?

Or better question: Is it possible to over come on the flow rate of stack effect at height H without using any active force like fan or ....?

I do not quite understand your  problem..you want to overcome  the stack flow problem or want to play around with mathematics concerning stack flow. stack flow problem can be overcome by introducing gap  your chimney  , as let atmospheric pressure do the job

Dear prof Mohamad Afifi

There is a flow rate from lower level to the upper level inside the shaft due to stack effect. 

If I have wind velocity at the shaft inlet (upper level) it is supposed to have a flow rate (against the stack effect flow rate) from upper level to the lower level. (according to Bernoulli principle)

Am I right up yo now?

If yes, I want to know how height the shaft should be? or Ho much the velocity should be to have flow rate from up to down more than flow rate from down to up due to stack.

@Eric Steven Hall

Isokinetic sampling is not the proper way to sample particles. See ANSI N13.1.

@Mahdi Hozhabri Namin

Your question appears theoretical rather than practical. A practical problem is wind over a stack can increase the upward flow of a stack. The higher velocity flow at the top of the stack will have a lower pressure than the bottom of the stack by Bernoulli's  principle.

If you are looking for practical answers may i suggest Mahdi visit a  Thermal Power Plant and ask question on Forced draught or Induced Draught...in my opinion an engineer who plays around with computer packages is not complete  you should relate to existing practical solution and then innovate for solutions