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Thermal-hydraulic performance of four CO₂ internal heat exchanger geometries under oil-film effects: experiments and system-level COP assessment. 

Author(s) : WANG Z. H., KIM D. H.

Type of article: IJR article

Summary

This study systematically investigates the heat transfer and pressure drop characteristics of four representative CO₂ internal heat exchangers (IHXs) — double-pipe, shell-and-tube, coaxial, and microchannel — under both cooling and heating modes, with particular emphasis on the influence of lubricating-oil concentration. A dedi cated experimental setup was used to evaluate the effects of mass flow rate, exchanger length, inlet conditions, and oil concentration. The results show that the microchannel IHX delivers up to twice the heat transfer capacity of conventional tubular designs while imposing roughly 1.5–2.2 times higher pressure drop on the low-pressure side. Increasing oil concentration significantly degrades heat transfer and increases pressure drop, with the microchannel geometry being the most sensitive to oil-film effects. Integrating the measurements with ther modynamic simulations quantifies system-level impacts: under representative heat-pump operation (3 kW cooling load, 2000 annual operating hours), the microchannel IHX achieves the highest COP among all con figurations, providing an estimated annual electricity saving of approximately 69 kWh and a reduction of about 35 kg CO₂ per unit compared to the baseline double-pipe IHX, corresponding to a 3.1% COP improvement. These findings provide fundamental insights into IHX design and oil-film effects, highlight the energy saving potential and environmental benefits of microchannel IHXs, and offer practical guidance for deploying high-efficiency CO₂ heat-pump systems.

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Details

  • Original title: Thermal-hydraulic performance of four CO₂ internal heat exchanger geometries under oil-film effects: experiments and system-level COP assessment. 
  • Record ID : 30034936
  • Languages: English
  • Subject: Technology
  • Source: International Journal of Refrigeration - Revue Internationale du Froid - vol. 187
  • Publication date: 2026/07
  • DOI: http://dx.doi.org/https://doi.org/10.1016/j.ijrefrig.2026.106956

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