Net heat flux vent and Q_PRES

[Dushyant Chaudhari]

Hi,

I am trying to simulate a constant heat flux of 5kW/m2 on all sides of a room to see its effect on temperature due to conduction inside the room. So, I used a constant 'net heat flux' Boundary condition on all sides and the rest area of the boundary as inert. The walls are at ambient temperature initially. I assume this would give constant incident radiative flux as I desire, and it does until I encountered the scenario I am about to describe.

I was not able to understand/need clarification on few things in the model,

1) The convection around the room leads to small portion of incident flux and reduces the radiative flux. So, I increased the heat flux above 7 kW/m2 to account for the convective flux. The BNDF radiative flux now shows a negative radiative flux from the central region of the each exposed wall (meaning the wall is emitting heat). I am also checking the radiative flux from the source which shows negligible value as expected and the temperature of the wall is definitely less than the gas temperature around. If radiation measured by the radiometer is negligible, why is the BNDF radiative flux showing -4.5kw/m2. I am not able to understand why negative radiative flux is recorded only after increasing the vent flux above 7kw/m2. I would be increasing the heat flux eventually and thus am looking to understand this phenomenon.

2) The Q_PRES in the hrr output increases steadily with time. Since it is a closed domain, I would expect the pressure to increase, but I need some guidance on how this contributes to the incident heat and the physical meaning of what is happening and if increase of pressure to 2*E4 is reasonable.

I would be glad to have any advice on this.

Thank you,

Regards,

Dushyant.

[Randy McDermott]

You can add DEVC of 'GAS TEMPERATURE' along the wall.  Also, you can add a DEVC of 'WALL TEMPERATURE', and 'HEAT TRANSFER COEFFICIENT'.  Also, you can add 'CONVECTIVE HEAT FLUX' and 'RADIATIVE HEAT FLUX'.  Using all this, you can work out why you are getting a negative RADIATIVE HEAT FLUX and confirm that the NET HEAT FLUX you specified is achieved.

Regarding Q_PRES, see Eq. (16.3) of the user guide.  With a sealed box, you can integrate the terms around the surface and most will be zero.  Your NET HEAT FLUX will be balanced by Q_ENTH and Q_PRES.

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[Dushyant Chaudhari]

Hello,

Thank you for the suggestion. I followed what you suggested.

However, I turned off any convective flux by using HEAT_TRANSFER_COEFFICIENT as zero for all surfaces. Several devices measure incident flux, radiative flux, wall temperature, convective flux at the center of the roof (the visible green dot in the snapshot).  

I calculated the incident flux using the relation given in the user manual, which is ((radiative flux/emissivity) + Stefan-Boltzmann constant*wall_temp^4). This calculated incident flux does not give a zero incident flux and matches exactly with what incident heat flux device calculates. 

Attached are the graphs for your reference and the BNDF snapshot which shows zero incident flux.

Please advice.

Thank you,

Dushyant.

[Randy McDermott]

Simplify your case as much as possible and submit to the Issue tracker.  Include your input file (change extension to .txt for GitHub).  Thanks.

To view this discussion on the web visit https://groups.google.com/d/msgid/fds-smv/3153c91f-19fa-4e79-addb-8e78f8447a7a%40googlegroups.com.

[Dushyant Chaudhari]

Hello,

Until now, I was running the smokeview on remote desktop. However, after looking at the smokeview file on the physical machine, the values do not drop to zero unexpectedly.

Also, I would be glad if you could assist me with this. The Q_TOTAL drops down to a constant 10 to 15 kW lower than Q_ENTH (which settles to zero kW). I am wondering if this is wrong. I would be glad if you can suggest why this would be happening.

Thank you so much for your patience.

Dushyant.

[dr_jfloyd]

Simplify your case as much as possible and submit to the Issue tracker.  Include your input file (change extension to .txt for GitHub).  Thanks.