The Design of A Freezing Point Measurement System
This expert is experienced with numerical models and experiments that were used to assist in the evaluation of design concepts for a freezing point measurement system (FPMS). The FPMS was to be used to non-intrusively identify the chemical agent contained within a chemical munition. Several design concepts relying on thermal techniques had been proposed. Numerical models and experiments were used to quickly evaluate several of the design concepts.
The Idaho National Engineering Laboratory (INEL) was tasked with the development of a field unit to identify chemical agents inside non-stockpile munitions without endangering personnel. This necessitated a technique that did not require opening the munition. The chemical agents are contained in munitions with thick steel walls. Techniques relying on the thermal properties of the chemical agent depended on detecting the phase change point of the material within the munition. The intent of the experiments and the numerical modeling was to determine if changes in the slope of temperature-time curves could be detected externally. A change in the temperature-time curve occurs as a substance undergoes phase change. The sensitivity of the system need only be plus or minus 3 to 5 oC. This temperature variation relates to differences in the solidification point of the chemicals of interest.
Temperature measurements made during the cooling process were intended to be used to estimate / detect the solidification point. The goal of the modeling and the experiments was to determine which of the design concepts would best detect the freezing point. The role of the numerical modeling was to support the experimental program and to assist in the selection of experiments.
Samples of the detailed finite element modeling are shown in the color temperature contour plots for two of the configurations analyzed. The table shown below also outlines the advantages of using the modeling in conjunction with simple experiments.
Table 1 - Benefits of Using Numerical Modeling and Experiments to Evaluate Proposed Design Concepts
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Cooling Mechanism
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Model Results
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Interaction with Experiments
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Conductive Cooling Through Base
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Hand calculations indicate inadequate cooling
Finite element model indicates inability to detect phase change
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Only one experiment conducted to verify trends indicated by numerical models
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“Cold-Finger” Concept
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Unable to detect phase change of material
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No experiments conducted
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Vertical Convective Cooling
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Cooling trends predicted
Temperature gradients estimated and compared to experts
Increased heat transfer would make detection of phase change more difficult
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Experiments used to validate trends in numerical models
Improvements in the thermocouple technique
No experiments conducted with fan for increased heat transfer
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Horizontal Convective Cooling
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Indicated the sharpest break in temperature--time curve indicated shorter cooling times
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modeling results used as basis for proof-of-concept testing
Experimental results confirm trends predicted by model
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Figure 1 - "Break" in the temperature time curve for phase change.
Figure 2 - Color contours for the cooling of a fluid within a representative chemical munition (side view and symmetry section).
Figure 3 - Color temperature contours for the cooling of fluid within a munition positioned vertically (symmetry section).
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