A non-toxic, non-flammable, low-freezing heat transfer fluid is being developed for drop-in replacement within current and future heat transfer loops currently using water or alcohol-based coolants. Numerous water-soluble compounds were down-selected and screened for toxicological, physical, chemical, compatibility, thermodynamic, and heat transfer properties. Two fluids were developed, one with a freezing point near 0 °C, and one with a suppressed freezing point. Both fluids contain an additive package to improve material compatibility and microbial resistance.

The optimized sub-zero solution had a freezing point of –30 °C, and a freezing volume expansion of 10-percent of water. The toxicity of the solutions was experimentally determined as LD50 > 5g/kg. The solutions were found to produce “minimal” corrosion with materials identified by NASA as potentially existing in secondary cooling loops.

Thermal/hydrodynamic performance exceeded that of glycol-based fluids with comparable freezing points for temperatures Tf < 20°C. The additive package was demonstrated as a buffering agent to compensate for CO2 absorption, and to prevent microbial growth. The optimized solutions were determined to have physically/chemically stable shelf lives for freeze/thaw cycles and longterm test loop tests.

This work was done by J. Michael Cutbirth of Mainstream Engineering Corp. for Johnson Space Center. MSC-24547-1

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