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FDS Source: Fix units for default TC specific heat. #15065

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drjfloyd merged 1 commit into from
Sep 5, 2025
Merged

FDS Source: Fix units for default TC specific heat. #15065

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7 changes: 4 additions & 3 deletions Manuals/FDS_User_Guide/FDS_User_Guide.tex
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Expand Up @@ -10543,7 +10543,7 @@ \subsection{Thermocouples}
\frac{D_{\rm TC}}{6} \, \rho_{\rm TC} \, c_{\rm TC} \, \frac{\d T_{\rm TC}}{\d t} = \epsilon_{\rm TC} \, (U/4 - \sigma T_{\rm TC}^4) + h(T_{\rm g} - T_{\rm TC})
\label{TC}
\ee
where $\epsilon_{\rm TC}$ is the emissivity of the thermocouple, $U$ is the integrated radiative intensity, $T_{\rm g}$ is the true gas temperature, and $h$ is the heat transfer coefficient to a small sphere, $h=k \, \NU / D_{\rm TC}$. The bead \ct{DIAMETER}, \ct{EMISSIVITY}, \ct{DENSITY}, and \ct{SPECIFIC_HEAT} are given on the associated \ct{PROP} line. To over-ride the calculated value of the heat transfer coefficient, set \ct{HEAT_TRANSFER_COEFFICIENT} on the \ct{PROP} line (\si{W/(m.K)}). The default value for the bead diameter is 0.001~m. The default emissivity is 0.85. The default values for the bead density and specific heat are that of nickel; 8908~kg/m$^3$ and 0.44~kJ/kg/K, respectively. See the discussion on heat transfer to a water droplet in the Technical Reference Guide for details of the convective heat transfer to a small sphere.
where $\epsilon_{\rm TC}$ is the emissivity of the thermocouple, $U$ is the integrated radiative intensity, $T_{\rm g}$ is the true gas temperature, and $h$ is the heat transfer coefficient to a small sphere, $h=k \, \NU / D_{\rm TC}$. The bead \ct{DIAMETER}, \ct{EMISSIVITY}, \ct{DENSITY}, and \ct{SPECIFIC_HEAT} or \ct{SPECIFIC_HEAT_RAMP} are given on the associated \ct{PROP} line. To over-ride the calculated value of the heat transfer coefficient, set \ct{HEAT_TRANSFER_COEFFICIENT} on the \ct{PROP} line (\si{W/(m.K)}). The default value for the bead diameter is 0.001~m. The default emissivity is 0.85. The default values for the bead density and specific heat are that for a Type-K thermocouple; 8700~kg/m$^3$ and a linear ramp from 0.4515~kJ/kg/K at 20\,$^\circ$C to 0.6010~kJ/kg/K at 1200\,$^\circ$C, respectively. See the discussion on heat transfer to a water droplet in the Technical Reference Guide for details of the convective heat transfer to a small sphere.

The above discussion is appropriate for a so-called ``bare bead'' thermocouple, but often thermocouples are shielded or sheathed in various ways to mitigate the effect of thermal radiation. In such cases, there is no obvious bead diameter and there may be multiple metals and air gaps in the construction. Usually, the manufacturer provides a time constant, which is defined as the time required for the sensor to respond to 63.2~\% of its total output signal when suddenly plunged into a warm air stream flowing at 20~m/s. The analysis and testing is typically done at relatively low temperature, in which case the radiation term in Eq.~(\ref{TC}) can be neglected and the time constant, $\tau$, can be defined in terms of effective thermal properties and an effective diameter:
\be
Expand Down Expand Up @@ -13185,7 +13185,7 @@ \section{\texorpdfstring{{\tt PROP}}{PROP} (Device Properties)}
\ct{CALIBRATION_CONSTANT} & Real & Section~\ref{info:bidir_probe} & & 0.93 \\ \hline
\ct{CHARACTERISTIC_VELOCITY} & Real & Section~\ref{info:pressure_coefficient} & m/s & 1. \\ \hline
\ct{C_FACTOR} & Real & Section~\ref{info:sprinklers} & (m/s)$^{1/2}$ & 0. \\ \hline
\ct{DENSITY} & Real & Section~\ref{info:THERMOCOUPLE} & kg/m$^3$ & 8908. \\ \hline
\ct{DENSITY} & Real & Section~\ref{info:THERMOCOUPLE} & kg/m$^3$ & 8700. \\ \hline
\ct{DIAMETER} & Real & Section~\ref{info:THERMOCOUPLE} & m & 0.001 \\ \hline
\ct{EMISSIVITY} & Real & Section~\ref{info:THERMOCOUPLE} & & 0.85 \\ \hline
\ct{FED_ACTIVITY} & Integer & Section~\ref{info:FED} & & 2 \\ \hline
Expand Down Expand Up @@ -13229,7 +13229,8 @@ \section{\texorpdfstring{{\tt PROP}}{PROP} (Device Properties)}
\ct{SMOKEVIEW_PARAMETERS(:)} & Char.~Array & Section~\ref{info:SMOKEVIEW_PARAMETERS} & & \\ \hline
\ct{SPARK} & Logical & Section~\ref{info:ignition} & & \ct{F} \\ \hline
\ct{SPEC_ID} & Character & Section~\ref{info:alternative_smoke} & & \\ \hline
\ct{SPECIFIC_HEAT} & Real & Section~\ref{info:THERMOCOUPLE} & $\si{kJ/(kg.K)}$ & 0.44 \\ \hline
\ct{SPECIFIC_HEAT} & Real & Section~\ref{info:THERMOCOUPLE} & $\si{kJ/(kg.K)}$ & \\ \hline
\ct{SPECIFIC_HEAT_RAMP} & Character & Section~\ref{info:THERMOCOUPLE} & & \\ \hline
\ct{SPRAY_ANGLE(2,2)} & Real & Section~\ref{info:sprinklers} & degrees & 60.,75. \\ \hline
\ct{SPRAY_PATTERN_BETA} & Real & Section~\ref{info:sprinklers} & degrees & 5. \\ \hline
\ct{SPRAY_PATTERN_MU} & Real & Section~\ref{info:sprinklers} & degrees & 0. \\ \hline
Expand Down
8 changes: 5 additions & 3 deletions Source/read.f90
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Expand Up @@ -6602,13 +6602,14 @@ SUBROUTINE READ_PROP
PY%HEAT_TRANSFER_COEFFICIENT= HEAT_TRANSFER_COEFFICIENT
ALLOCATE(PY%SPECIFIC_HEAT(0:I_MAX_TEMP))
IF(SPECIFIC_HEAT > 0._EB) THEN
PY%SPECIFIC_HEAT = SPECIFIC_HEAT*1000._EB/TIME_SHRINK_FACTOR
PY%SPECIFIC_HEAT = SPECIFIC_HEAT*1000._EB
ELSE
! Type-K CP(20 C)=0.4515, CP(1200 C)=0.6010
! Type-K CP(20 C)=0.4515 kJ/kg, CP(1200 C)=0.6010 kJ/kg
DO J = 0,I_MAX_TEMP
PY%SPECIFIC_HEAT(J) = 0.4515_EB+0.001_EB*(REAL(MAX(20,MIN(1200,J)),EB)-20._EB)*(0.6010_EB-0.4515_EB)
PY%SPECIFIC_HEAT(J) = 451.5_EB+0.001_EB*(REAL(MAX(20,MIN(1200,J)),EB)-20._EB)*(601.0_EB-451.5_EB)
ENDDO
ENDIF
PY%SPECIFIC_HEAT = PY%SPECIFIC_HEAT/TIME_SHRINK_FACTOR
IF (SPECIFIC_HEAT_RAMP /= 'null') CALL GET_RAMP_INDEX(SPECIFIC_HEAT_RAMP,'TEMPERATURE',PY%SPECIFIC_HEAT_RAMP_INDEX)
PY%C_FACTOR = C_FACTOR
PY%CHARACTERISTIC_VELOCITY = CHARACTERISTIC_VELOCITY
Expand Down Expand Up @@ -15210,6 +15211,7 @@ SUBROUTINE PROC_DEVC
2._EB*PY%DENSITY*PY%SPECIFIC_HEAT(293)*PY%DIAMETER
PY%DIAMETER = PY%DIAMETER - F/DFDD
TOL = ABS(F/DFDD)
WRITE(*,*)'C',TRIM(PY%ID),PY%DIAMETER
ENDDO
ENDIF
ENDIF
Expand Down
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