Experimental Analysis of Heat Transfer Coefficients in CO2 and CO2 + Isobutane Mixtures for Organic Rankine Cycle Engines

This study provides a comprehensive investigation into the heat transfer performance of supercritical CO₂ and a CO₂ + isobutane mixture under subcritical conditions. A laboratory scale ORC cycle with a double-pipe heat exchanger was developed and set up to explore the effects of various operating parameters on heat transfer performance over a wide temperature range (288 K – 353 K) at constant pressure (100 bar). Heat flux of the subcritical fluids was measured in the inner pipe in counter-current with water in the outer pipe. It was found that the CO₂ + isobutane mixture can be satisfactorily described by correlations available in the literature for Reynolds numbers below 1000. However, at higher Reynolds numbers, deviations were observed, indicating that further optimization may be required. Results obtained in this work are relevant for the design of heat exchangers and Organic Rankine Cycles (ORC) applied to waste heat recovery and energy efficiency improvement in industrial processes