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Copy file name to clipboardExpand all lines: Manuals/FDS_User_Guide/FDS_User_Guide.tex
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@@ -5645,10 +5645,12 @@ \subsection{Chemical Time Integration}
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\item \ct{MIN_EQUIV_RATIO}=0.0
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\item \ct{MAX_EQUIV_RATIO}=20.0
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\item \ct{DO_CHEM_LOAD_BALANCE}=T
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\item \ct{CVODE_ORDER}=0
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\end{itemize}
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The parameter \ct{FINITE_RATE_MIN_TEMP} defines the minimum temperature (in $^\circ$C) above which chemistry calculation will be performed. CVODE allows specification of relative and absolute tolerances at the species level using the \ct{ODE_REL_ERROR} and \ct{ODE_ABS_ERROR} parameters in the \ct{SPEC} input line. These tolerances can also be set globally in the \ct{COMB} input line, with species-level settings taking precedence. Currently, CVODE does not allow relative tolerance at the species level. The minimum concentration of a species is determined as the product of \ct{ODE_REL_ERROR} and \ct{ZZ_MIN_GLOBAL}. Concentrations below this threshold are treated as zero.
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Additional optional parameters include \ct{EQUIV_RATIO_CHECK}, \ct{MIN_EQUIV_RATIO}, and \ct{MAX_EQUIV_RATIO}. When \ct{EQUIV_RATIO_CHECK} is enabled (set to true), the chemistry calculation is performed only for those cells for which equivalence ratio is within the specified \ct{MIN_EQUIV_RATIO} and \ct{MAX_EQUIV_RATIO} limits, reducing computational time. Enabling \ct{DO_CHEM_LOAD_BALANCE} significantly accelerates chemistry calculations by distributing the computational load evenly across all MPI processes.
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Additional optional parameters include \ct{EQUIV_RATIO_CHECK}, \ct{MIN_EQUIV_RATIO}, and \ct{MAX_EQUIV_RATIO}. When \ct{EQUIV_RATIO_CHECK} is enabled (set to true), the chemistry calculation is performed only for those cells for which equivalence ratio is within the specified \ct{MIN_EQUIV_RATIO} and \ct{MAX_EQUIV_RATIO} limits, reducing computational time. Enabling \ct{DO_CHEM_LOAD_BALANCE} significantly accelerates chemistry calculations by distributing the computational load evenly across all MPI processes. The parameter \ct{CVODE_ORDER} controls the order of discretization when solving the ODE system. By default (0), CVODE dynamically selects an order between 1 and 5. For very stiff problems, the user may specify a lower order (1 or 2) to improve stability, at the cost of slower performance because CVODE will take much smaller internal substeps.
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User can modify the default values of any or all of these parameters as needed using the following line in the FDS input file:
The output quantity \ct{BI-DIRECTIONAL PROBE} is the velocity of a modeled bi-directional probe. A bi-directional probe uses the following equation:
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The output quantity \ct{BI-DIRECTIONAL PROBE} is the velocity of a modeled bi-directional probe. A bi-directional probe uses the following equation~\cite{McCaffrey:1976}:
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\be
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C \sqrt{\frac{2 \Delta P}{\rho}}
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\label{BDP}
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\ee
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where $C$ is a calibration constant (default value is 0.93), $\Delta P$ is the pressure difference across the probe, and $\rho$ is the gas density at the probe. In a typical experiment, the gas density is computed assuming standard pressure (101325 Pa), the molecular weight of air (28.8 g/mol), and the temperature as measured by a thermocouple near the probe.
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Bi-directional probes have biases due to both the Reynolds number (based on the probe diameter) of the flow and the angle of the flow with respect to the probe axis~\cite{McCaffrey:1976}. At low Reynolds number a probe will measure a higher effective velocity. As the angle of the flow vector with the axis increases, the effective velocity at first increases up to an angle of 30$^\circ$ due to a low pressure region forming downstream of the probe, and then decreases reaching no measured flow at an angle of 90$^\circ$. This model accounts for these sensitivities and the impact of density differences from varied molecular weight at the probe. The orientation of the probe can be specified with either \ct{IOR} or \ct{ORIENTATION} on \ct{DEVC}. A probe with \ct{IOR}=-1 would have a positive velocity output when the flow is in the -x direction. Parameters for the probe can be specified with a \ct{PROP_ID} on the \ct {DEVC}. The calibration constant (default of 0.93) and the probe diameter (default of 0.0254 m) can be set respectively with \ct{CALIBRATION_CONSTANT} and \ct{PROBE_DIAMETER} on \ct{PROP}. If the probe temperature is an aspirated thermocouple or other measurement not sensitive to the radiative environment, then set \ct{TC=F} on \ct{PROP}. Thermocouple specific properties for a bi-directional probe, see Section~\ref{info:THERMOCOUPLE}, should be set with the same \ct{PROP} as for the probe.
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Bi-directional probes have biases due to both the Reynolds number (based on the probe diameter) of the flow and the angle of the flow with respect to the probe axis~\cite{McCaffrey:1976}. At low Reynolds number a probe will measure a higher effective velocity. As the angle of the flow vector with the axis increases, the effective velocity at first increases up to an angle of 30$^\circ$ due to a low pressure region forming downstream of the probe, and then decreases reaching no measured flow at an angle of 90$^\circ$. This model accounts for these sensitivities and the impact of density differences from varied molecular weight at the probe. The orientation of the probe can be specified with either \ct{IOR} or \ct{ORIENTATION} on \ct{DEVC}. A probe with \ct{IOR}=-1 would have a positive velocity output when the flow is in the -x direction. Parameters for the probe can be specified with a \ct{PROP_ID} on the \ct {DEVC}. The calibration constant (default of 0.93) and the probe diameter (default of 0.0254 m) can be set respectively with \ct{CALIBRATION_CONSTANT} and \ct{PROBE_DIAMETER} on \ct{PROP}. If the probe temperature is an aspirated thermocouple or other measurement not sensitive to the radiative environment, then set \ct{TC=F} on \ct{PROP}. Thermocouple specific properties for a bi-directional probe, see Section~\ref{info:THERMOCOUPLE}, should be set with the same \ct{PROP} as for the probe. If a dynamic calibration constant based on the Reynolds number calibration in McCaffrey~\cite{McCaffrey:1976} was used set \ct{CALIBRATION_CONSTANT=-1}.
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Figure~\ref{bi_dir_fig} shows the results of a bi-directional probe with varying angle to a 1~m/s flow and varying flow speed.
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\begin{figure}[ht]
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A common exercise in atmospheric dispersion modeling is predicting the plume rise height of stack emissions. Stull~\cite{Stull:2000} presents an empirical correlation for plume rise height from a smoke stack in a stable atmospheric boundary layer. Details of the correlation and simulation are found in Sec.~\ref{Plume_Height_Discussion}.
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A common exercise in atmospheric dispersion modeling is predicting the plume rise height of stack emissions. Stull~\cite{Stull:2000} presents an empirical correlation for plume rise height from a smoke stack in a stable atmospheric boundary layer. Details of the correlation and simulation are found in Sec.~\ref{PlumeHeight}.
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\section{NIST Pool Fire Experiments}
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\label{NIST_Pool_Fires_Description}
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The NIST Pool Fire Experiments include temperature, species concentration, velocity, and heat flux measurements of 30~cm and 100~cm diameter circular liquid fuel fires, and 37~cm gaseous burner fires.
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The NIST Pool Fire Experiments include temperature, species concentration, velocity, and heat flux measurements of 30~cm and 100~cm diameter circular liquid fuel fires, and 37~cm gaseous burner fires. The 30~cm and 37~cm fires are documented in Ref.~\cite{Sung:TN2162r1}.
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The 30~cm burner is 15~cm deep and has a wall thickness of 1.6~mm. The burner is fitted with legs such that the burner rim is positioned 30~cm above the floor. The bottom of the burner is maintained at a constant temperature by flowing tap water (nominally 20~$^\circ$C) through a 3~cm section on the bottom of the fuel pan. The dimensions of the circular burner are similar to Weckman's methanol experiment described in Sec.~\ref{Waterloo_Methanol_Description}.
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The 100~cm burner is also 15~cm deep, has a wall thickness of 1.6~mm, and is water-cooled.
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The 37~cm burner is actually 38~cm in diameter with an effective diameter of 37~cm. It is watercooled, and the surface temperature is maintained at approximately 40~$^\circ$C. The measured fuel flow rate for the methane fire was 0.69~g/s and its estimated HRR was 34.5~kW. The heat release rates of the three propane fires were 20~kW, 34~kW, and 50~kW.
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The 37~cm burner is actually 38~cm in diameter with an effective diameter of 37~cm. It is water cooled, and the surface temperature is maintained at approximately 40~$^\circ$C. The measured fuel flow rate for the methane fire was 0.69~g/s and its estimated HRR was 34.5~kW. The heat release rates of the two propane fires were 20~kW and 34~kW.
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The 100~cm burner is also 15~cm deep, has a wall thickness of 1.6~mm, and is water-cooled.
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Details and references with regard to the plume temperature measurements are given in Sec.~\ref{NIST_Pool_Fires_Plume_Temps}. Details on the heat flux measurements are given in Sec.~\ref{NIST_Pool_Fires_Heat_Flux_Results}. Details on the gas species measurements is given in Sec.~\ref{sec:NIST_Pool_Fires}. Details on the velocity measurements is given in Sec.~\ref{NIST_Pool_Fires_Velocity}.
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\subsubsection{Modeling Notes}
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The 30~cm pool fires are modeled at three different grid resolutions---2~cm, 1~cm, and 0.5~cm. The 100~cm pool fires are modeled at 4~cm, 2~cm, and 1~cm resolution. The mass loss rate of the fuel is specified.
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The 30~cm and 37~cm fires are modeled at three different grid resolutions---2~cm, 1~cm, and 0.5~cm. The 100~cm methanol pool fire is modeled at 4~cm, 2~cm, and 1~cm resolution. The mass loss rate of the fuel is specified for the 30~cm and 37~cm fires, and both predicted and specified for the 100~cm fire.
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A two-step reaction mechanism is implemented. In the first reaction, fuel is converted to CO, soot, H$_2$, and H$_2$O. In the second reaction, the CO, soot, and H$_2$ are converted to CO$_2$ and H$_2$O. Both reactions employ fast kinetics, but proceed in series, not in parallel. The relative amounts of CO, soot, and H$_2$ produced in the first step are still subjects of study, and for the moment have been estimated based on measured results. The fractions of carbon atoms converted to CO in the first step are as follows---0.85 for acetone; 0.95 for ethanol; 0.97 for methane; 1.0 for methanol; 0.85 for propane. For all fuels, one half of the hydrogen atoms are converted to H$_2$ in the first step.
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The radiative fractions are specified based on measured values---0.31 for acetone; 0.26 for ethanol; 0.15 for methane; 0.21 for 1~m methanol; 0.22 for 30~cm methanol; 0.22 for propane.
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The radiative fractions are specified based on reported values from Ref.~\cite{Sung:TN2162r1}---0.31 for acetone; 0.26 for ethanol; 0.35 for heptane; 0.21 for methane; 0.21 for 1~m methanol; 0.23 for 30~cm methanol; 0.23, 0.30, and 0.33 for the 20~kW, 34~kW, and 50~kW propane fires, respectively.
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