Abstract
The testing of assemblies for use in cryogenic systems commonly includes evaluation at or near operating (therefore cryogenic) temperature. Typical assemblies include valves and pumps for use in liquid oxygen-liquid hydrogen rocket engines. One frequently specified method of cryogenic external leakage testing requires the assembly, pressurized with gaseous helium (GHe), be immersed in a bath of liquid nitrogen (LN2) and allowed to thermally stabilize. Component interfaces are then visually inspected for leakage (bubbles). Unfortunately the liquid nitrogen will be boiling under normal, bench-top, test conditions. This boiling tends to mask even significant leakage.One little known and perhaps under-utilized property of helium is the seemingly counter-intuitive thermodynamic property that when ambient temperature helium is bubbled through boiling LN2 at a temperature of −195.8°C, the temperature of the liquid nitrogen will reduce.This paper reports on the design and testing of a novel proof-of-concept helium injection control system confirming that it is possible to reduce the temperature of an LN2 bath below boiling point through the controlled injection of ambient temperature gaseous helium and then to efficiently maintain a reduced helium flow rate to maintain a stabilized liquid temperature, enabling clear visual observation of components immersed within the LN2. Helium saturation testing is performed and injection system sizing is discussed.
Highlights
This paper reports on the design and testing of a novel proof-of-concept helium injection control system confirming that it is possible to reduce the temperature of an LN2 bath below boiling point through the controlled injection of ambient temperature gaseous helium and to efficiently maintain a reduced helium flow rate to maintain a stabilized liquid temperature, enabling clear visual observation of components immersed within the LN2
In a report issued by the United States National Bureau of Standards ‘‘Suppression of bubbling in boiling refrigerants” published in Nature [1] the authors found that the presence of bubbles in a body of boiling liquid nitrogen (LN2) may be eliminated by blowing helium, hydrogen or neon gas over the surface of the liquid
It has been shown that the residence time obtained when injecting helium at a flow rate of 2 l/min through an upwardfacing orifice of 1 mm diameter with a liquid nitrogen pool depth of 10 cm is not sufficient to achieve full saturation: evaporation of nitrogen into the helium is still occurring after this residence time and so a greater pool depth would be required to produce fully saturated helium
Summary
Takayoshi et al in their technical note ‘‘The boiling suppression of liquid nitrogen” [2] referenced the 1957 Nature article to show that the cooling effect was known, but they state that at the time of writing their technical note (2009) a comprehensive quantitative study had not been performed. This is perhaps surprising as the Nature article was published in 1957 – a 32 year gap. The Takayoshi study involved injecting gases into liquid nitrogen contained in a double walled glass Dewar flask mounted on top of a laboratory scale They measured mass loss of liquid nitrogen over time at flow rates of 1.0, 1.5, 2.0 and 5.0 l/min gaseous helium (GHe). The visual clarity improvement brought about by cooling liquid nitrogen below boiling temperature through the injection of gaseous helium has not been previously investigated
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