Abstract
Vapor compression cycles are not an established technology for microgravity applications yet. One reason for the low technology readiness level in microgravity is the concern of liquid flooding from the evaporator, which could reduce the lifetime of the compressor. Liquid refrigerant can accumulate in the evaporator while the cycle is turned off and slug into the suction line upon the next compressor start-up. This paper describes experiments on parabolic flights, which investigated this issue. A test stand was designed to observe liquid flooding from the evaporator upon compressor start-up. The time from compressor start-up until flooding first occurred and the duration of flooding were recorded and used as quantifiers. They showed that the occurrence of liquid flooding is mainly a function of the refrigerant mass in the evaporator before start-up and that there is a mass threshold below which flooding does not occur. Test were conducted with an off the shelf and one similarly sized transparent evaporator. The hypothesis that the threshold would be significantly smaller in microgravity was not confirmed with the data collected on the parabolic flights. Instead, flooding typically occurred at the same charge level for both evaporators and both testing environments. Flow visualizations showed that mostly stratified flow during start-up in ground testing became mostly annular flow in microgravity. Both flow regimes keep liquid close to the wall so that vapor generated during the start-up process can escape through a void tunnel.
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Data Availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.
Abbreviations
- \({g}_{e}\) :
-
Earth gravity (\(\approx\) 9.8 m/s2)
- \(\Delta {T}_{start}\) :
-
Time from turning on pump till the first flooding.
- \(\Delta {T}_{flooding\; ends}\) :
-
Time from turning on the pump until flooding stops.
- \(\Delta {T}_{elapsed}\) :
-
Time from the first until the last indication of flooding.
- \(\Delta {T}_{0G}\) :
-
Time of microgravity.
- \(\Delta {T}_{70 kPa}\) :
-
Time from turning on the pump until the pressure reached 70 kPa.
- h:
-
Enthalpy [kJ/kg]
- P:
-
Pressure [kPa]
- \({P}_{in}\) :
-
Pressure at test section inlet [kPa].
- \({P}_{out}\) :
-
Pressure at test section outlet [kPa].
- VCC:
-
Vapor compression cycle
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Acknowledgements
The authors appreciate the financial support of Air Squared. The support of this work by NASA under SBIR contract 80NSSC18C0049 is gratefully acknowledged. The experiments would not have been possible without major support from the technical shop of the Ray W. Herrick Laboratories: Frank Lee, Charles Baxter, Ryan Thayer and Robert Hughes.
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Highlights
- Experimental investigation of liquid flooding from an evaporator upon compressor start-up
- Transparent evaporator that mimics behavior of real evaporator
- Liquid flooding quantifiers for microgravity similar to normal gravity
- Flow visualizations of start-up process in normal and microgravity
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Brendel, L.P., Beck, P.E., Caskey, S.L. et al. Liquid Flooding From an Evaporator Upon Compressor Start-up in Microgravity. Microgravity Sci. Technol. 34, 73 (2022). https://doi.org/10.1007/s12217-022-09978-9
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DOI: https://doi.org/10.1007/s12217-022-09978-9