WALL TEMPERATURE

[DRAKE JONES]

Hi all, 

        I am modelling a scenario in which there is fire in the middle of the compartment. 

1. In order to measure the temperature at the wall surface, I inserted  devices on the wall surface  using QUANTITY='WALL TEMPERATURE' (using IOR) and QUANTITY='TEMPERATURE'.

2. The temperature of the gas in the vicinity of the wall is also measured by inserting a device using  QUANTITY='TEMPERATURE'. 

         The values recorded by the device  QUANTITY='TEMPERATURE' on the wall is very less than the temperature in the vicinity of the wall. I want to know what are the values  recorded by the devices QUANTITY='WALL TEMPERATURE' and QUANTITY='TEMPERATURE' which are on on the surface of the wall or why is there an change in both of the quantities. Please throw some light on this. Thank you. 

[Randy McDermott]

TEMPERATURE is the gas temperature of a gas phase cell.

WALL TEMPERATURE (use with IOR) is the surface temperature.

GAS TEMPERATURE (use with IOR) is the gas temperature of the cell adjacent to the wall cell.  If the have the DEVC positioned correctly, then TEMPERATURE and GAS TEMPERATURE will be the same.  But be careful about putting the XYZ of the DEVC directly on a face.  Put it off the face in the direction opposite the IOR just a bit.  For example, for a face normal to the +x direction, put the device just to the +x side, then use IOR=+1.  GAS TEMPERATURE and TEMPERATURE (no IOR) should be the same.

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[DRAKE JONES]

The following are the two devices placed right on the surface of the wall

&DEVC ID= 'Temperature1', XYZ=13.4665,2.03475,3.062, QUANTITY= 'TEMPERATURE',IOR=2/

&DEVC ID= 'Temperature2', XYZ=13.4665,2.03475,3.062, QUANTITY= 'WALL TEMPERATURE',IOR=2/

IOR=2 as my face normal is along +y direction. The values recorded by Device 1 is very less than recorded by Device 2. The value recorded by the Device 2 seems right as I checked the temperature in the vicinity of the wall. If Device1 is recording the  gas temperature of a gas phase cell, why is there a huge difference. 

[Randy McDermott]

First, do not put DEVC directly on the wall.  Add +0.001 (or something small but not larger than DY).  This helps avoid any epsilon errors, to be on the safe side.

Second, do not use IOR with TEMPERATURE.  I am surprised the code does not throw an error on this.  You want GAS TEMPERATURE if you use IOR.

The gas temperature and wall temperature will likely be different due to convective and radiative heat transfer, but I don't know enough about the case to say more than that.

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[DRAKE JONES]

You mean to say that the device measuring WALL TEMPERATURE should be a bit outside the wall? Or it should be a bit inside the wall? 

which is right?

[Randy McDermott]

Outside

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[Kevin McGrattan]

FDS recognizes 'WALL TEMPERATURE' as a solid phase quantity, and works backward from the XYZ to find that actual surface. When you put the XYZ slightly off the surface, it helps FDS find the right surface, even if the surface shifts to fit the gas phase grid.