LINE CHILLDOWN USING LIQUID HYDROGEN
This validation case compares SINDA/FLUINT predictions with a 1966 test by the National Bureau of Standards (NBS, now National Institute of Standards and Technology, NIST). In the NBS tests, a pressurized 300 liter dewar containing either LN2 or LH2 was isolated from an empty 12 liter line (open to the atmosphere) by a valve. The 3/4” line (1.59cm ID, 1.9cm OD, 61m long) was vacuum-jacketed and made with an unspecified copper alloy. At time zero, the valve between the dewar and the line was opened, and cryogenic liquid was allowed to flow until the line was completely full and liquid was discharged from exhaust end of the pipe.
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A significant update has been made to the comparison that was originally performed in 1988, which itself had been updated in 2004 to make use of CRTech’s FloCAD® to facilitate model building and postprocessing.
This updated (2009) study uses both Sinaps® and FloCAD®. In the prior studies, a single test run using liquid hydrogen was made. In this more complete study, data from eleven tests using both hydrogen and nitrogen was digitized such that automated comparisons could be made. More significantly, a detailed study of uncertainties was performed, providing a demonstration of automated calibration and statistical design tools as well. The importance of such explorations is illustrated by the improved intuition that was gained in the process.
FloCAD Model Diagram
Sinaps Model Diagram
with Test Data
The transient profile below presents one result of this paper: a comparison of SINDA/FLUINT predictions with NBS test data for the case that had been published previously.
However, this is just the beginning of the story for the updated report. Differences between parahydrogen and normal hydrogen are explored, since the exact composition of the hydrogen is unknown. The importance of uncertainties in heat transfer and pressure drop correlations, copper alloy properties, dewar pressure, and even the manufacturing tolerance of the tubing is explored.
For example, a statistical analysis of the uncertainty in tube dimensions demonstrates a minimum uncertainty on the order of 10% for any predictions of cooldown time or cryogen mass required. As a further example, the plot below depicts a comparison with a nitrogen test run based on the results of an automated correction for uncertainties in supply pressure, copper specific heat, and vapor flow resistance. Other uncertainties, such as those in convection (film) coefficients, were surprisingly unimportant.
Thus, the new study demonstrates not only the validity of the predictions, but also identifies which uncertainties are important and what their cumulative effect is on an overall safety factor.
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