HRRPUA value

[XANGPHEUAK INTHAVIDETH]

Dear all,

Currently, I investigated the smoke behavior ejected from the chimney. I used the HRRPUA function to generated the smoke and controlled the temperature of smoke. I attempt to vary the HRRPUA to controlled the temperature of smoke at the top of the chimney.

However, it seems strange when higher value of HRRPUA is prescribed. when the HRRPUA is high, the temperature of smoke at top of the chimney is decreased as shown in the attached ppt file. This does not make sense in the actual. The temperature of smoke at the top of the chimney should be increase when the HRRPUA is high.

(I have added the input file for reference)

In this case, could anyone help me? 

And what is maximum value of HRRPUA in FDS?

Thank you in advance.

Best regards,

Xangpheuak

[dr_jfloyd]

I commented earlier on this setup in a post of yours a while back that this is not an appropriate method to introduce smoke to represent a chimney.  An actual chimney is the exhaust for some kind of fire box where air and fuel mix and combust before entering the chimney.  You do not have a fire box and just have a VENT adding fuel into a pipe with an open top and closed bottom. Think about what you have physically specified. Imagine you have a pipe with a close end with a small hole where you insert a pipe to add natural gas. What happens if you turn on the gas just enough to light a flame and then start increasing the gas flow rate? At first with a very small flow rate you can get air to enter the top of the pipe while hot gases exit. As you increase the fuel flow rate, eventually you will reach a point where the mass flow of fuel is large enough to prevent any air from entering the pipe. At that point the top of the pipe will become the burner. This is what is happening here.  At first burning will happen close to the top of the pipe but as you continue to keep increase the flow rate you will wind up with a cold core of fuel that persists until enough air can be entrained to burn it.  

For what you are trying to model you either need to model a fire box below the chimney with enough resolution for the both the ox and the chimney to resolve the flow (many grid cells across the chimney opening) or just specify a VENT at the top of the chimney where you inject a mix of air and combustion products representative of the temperature conditions. 

Don't specify both supply and exhaust as fixed flow surfaces. Constant volume flow is not the same as constant mass flow once temperature starts changing. Also you are adding mass to the domain with your smoke source and you have not accounted for that.

[XANGPHEUAK INTHAVIDETH]

Thank you very much for your kind feedback.

I will do it accordingly.

By the way, I still do not understand that when the HRRPUA is high, the temperature of smoke at top of the chimney is decreased. In fact, the temperature of smoke at the top of the chimney should be increase when the HRRPUA is high.

Could you help me in this point?

Does the FDS program have limited of HRRPUA value set up?

Thank you in advance.

[dr_jfloyd]

There is no limit to the HRRPUA you can specify other than eventually a high enough HRRPUA will violate the low-speed flow limitation of FDS.

Fuel does not burn until it can mix with air.

If you have no fuel coming out of the burner the temperature at the center of the burner will be ambient

If you have a very small flow rate, it will fully mix with air in the first grid cell above the burner and burn there resulting in an increased temperature.

As you increase the flow rate, as long as it is still fully mixing in that first grid cell, the temperature will continue to increase.

At some point the fuel flow rate will be large enough that it no longer can burn in the first grid cell. Some of it will have to be transported further away before it burns. This excess fuel will act as a diluent and temperatures will begin to drop. 

[XANGPHEUAK INTHAVIDETH]

Thank you very much for your valuable feedback.