Study of buoyancy effects on supercritical CO₂ heat transfer in circular pipes

Supercritical CO₂ (sCO₂ ) can be utilized as a working fluid in various systems including high scale power cycle, portable power production unit, centralized cooling system and standalone cooling device. Lack of accurate predication tools such as heat transfer coefficient correlations and insufficient knowledge behind fundamental heat transfer processes can hinder its practical realization in key energy and cooling systems. The overall objective of the proposed study is to extend fundamental knowledge about heat transfer and fluid flow processes in conduits pertinent to sCO₂ power cycle with an emphasis on buoyancy effects. Operational requirement of high pressures and temperatures for intended applications put a significant amount of constraints on measurement strategy and instrumentation. For this paper, experiments were conducted with uniform volumetric heat generation within pipe wall, for a single Reynolds number of 16,600 at test section inlet. The designed test apparatus and data reduction process are validated with high pressure air experiments, complemented by companion computations. Nusselt number was found to be within 10% of conventional correlations. For the test parameters and pipe size selected, factors of 2 to 4 variations in circumferential Nusselt number distributions are observed in sCO₂ flow. Richardson number and other similar parameters to indicate importance of buoyancy-driven flow phenomena suggest that buoyancy forces caused by large density variation of sCO₂ in flow cross-sections may cause the observed circumferential variations in Nusselt number.