Heat transfer enhancement using CO₂ in a natural circulation loop.

Area of Science:

• Thermodynamics and Heat Transfer
• Fluid Dynamics
• Chemical Engineering

Background:

• Natural circulation loops (NCLs) offer reliable, noise-free heat transfer due to their passive operation.
• The choice of working fluid is critical for optimizing heat transfer performance in NCLs.

Purpose of the Study:

• To experimentally investigate the heat transfer performance of a natural circulation loop (NCL) using carbon dioxide (CO2) as the working fluid.
• To compare the performance of CO2 with traditional fluids like water and brine across subcritical and supercritical states.
• To analyze the effect of operating pressure on the NCL system's heat transfer efficiency.

Main Methods:

• Experimental setup of an NCL with CO2 as the working fluid.
• Operation across various states: subcooled liquid, two-phase, superheated vapor, and supercritical.
• Utilized water and methanol as external fluids for temperature control in heat exchangers.
• Varied operating pressures (35-90 bar) and temperatures for hot (5-70°C) and cold (-18 to 32°C) exchangers.

Main Results:

• CO2 demonstrated significantly higher heat transfer rates compared to water/brine systems.
• Maximum heat transfer enhancements reached 400% (subcritical vapor), 500% (subcritical liquid), 900% (two-phase), and 800% (supercritical CO2).
• Performance varied with CO2's thermodynamic state and operating pressure.

Conclusions:

• Carbon dioxide is a highly effective working fluid for enhancing heat transfer in natural circulation loops.
• The use of CO2 offers substantial performance improvements over water and brine, particularly in two-phase and supercritical regimes.
• NCLs with CO2 present a promising solution for efficient thermal management applications.