measurement of heat flux at a point

[Vijayananda Vivek Devananda]

Dear friends,

I am trying to measure heat flux at a point through which hot gases pass. I used a gauge heat flux gas device with TMPA and 0 heat transfer coefficient. 

&DEVC ID='heat_flux_exit', QUANTITY='GAUGE HEAT FLUX GAS', XYZ=1.2,0.3,0.3/

This device is located just at the boundary of a door. This device only records a 0 value. However other such devices away from the door measures the flux. What could be the issue?

[Vijayananda Vivek Devananda]

[dr_jfloyd]

Without an input file, cannot be sure exactly what the reason is, but your input lacks an IOR..

[dr_jfloyd]

also may have been placed inside the door.  

[Vijayananda Vivek Devananda]

Dear Dr Flyod,

I have attached the input file of the case I am running to find the issue with heat flux measurement. I also tried to get the net heat flux using surface integral and I am getting zero. Also, what did you mean when you said the input lacks an IOR? Please guide me. I am trying to find the total heat flux (radiative and convective) at different points in the domain. Thanks.

[Kevin McGrattan]

That looks like a bug in PyroSim. There should be an ORIENTATION on the DEVC line

[Vijayananda Vivek Devananda]

Dear Dr. McGrattan,

there is an orientation option for the gauge heat flux gas device. But I am not sure about the net flux case. Does the gauge heat flux measure accurately if the gauge temperature is TMPA and the heat transfer coefficient is not enabled? Please let me know. 

[dr_jfloyd]

The line of input in the Pyrosim figure does not have ORIENTATION on the &DEVC line. You should still ensure that you understand how FDS works and what it expects for inputs. Pyrosim makes it easy to make an input file, but it is not a substitute for a working knowledge of how FDS works.

QUANTITY='NET HEAT FLUX' is the old name for the current output of 'TOTAL HEAT FLUX'. Per the FDS User's Guide this is a solid phase only quantity.  Within the XB volume for that DEVC there are no solid surfaces. The surface integral over zero area will be zero.

[Vijayananda Vivek Devananda]

Thank you Dr. Flyod, I understood.

The orientation part is a little confusing. I kept the device 0.01m away from the wall and it's recording flux now. 

I found no difference in the value recorded by certain devices in multiple orientation combo such as pressure, and mass fraction. Heat flux changes, however with orientation. Do you think a -1,0,0 orientation is appropriate for a heat flux towards +x direction? Also, I am not sure what to provide for gauge temperature and heat transfer coefficient. please advise. Thank you so much. 

[dr_jfloyd]

Pressure and mass fraction are not vector quantities. They do not have an orientation. A flux has a direction.

-1,0,0 is a gauge looking at -x which means it is seeing heat flux moving towards positive x.

GAUGE HEAT FLUX represents the heat flux as measured by a water-cooled gauge where the temperarture would represent the temperature of water flowing through the gauge. In an experiment this will generally be either the temperature of the local water supply or sometimes a little above ambient to reduce the potential for condensation onto the gauge.  You should pick a temperature and heat transfer coefficient (or use the default values) sutiable for your specific needs. I can't make that deciscion for you.