Thermal performance improvement for a two-phase loop thermosyphon (TPLT) with non-condensable gas (NCG) was attempted by fitting twin spiky ribbed twisted tapes (SRTTs) in the evaporator. The startup process and the steady-state boiling flow structures, boiling heat transfer rates, thermal resistance networks at various heater powers, and condenser thermal resistances were examined with 40% molar fraction of air in the TPLT. With treated water as the working fluid at an evaporator volumetric filling ratio of 50%, all tests were performed at subatmospheric pressures. The thermal performance improvement achieved by using SRTTs in the evaporator was determined by comparing the test results measured from the TPLT of the plain evaporator without NCG and the TPLT with NCG and SRTTs in the evaporator. Axial swirls generated in the evaporator suppressed the oscillatory interfacial structures caused by shooting intermittent vapor slugs, which led to improved boiling stability and reduced the overall thermal resistance. Correlations for evaluating the boiling Nusselt numbers over pool boiling, intermittent, and vapor regions as well as the dimensionless overall thermal resistance were devised for the present enhanced TPLT with NCG to represent all generated data.

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