Experimental and numerical investigation of CO2 condensation in a printed circuit heat exchanger for transcritical power cycles

Printed Circuit Heat Exchangers (PCHEs) are crucial components in transcritical CO2 power cycles due to their high thermal efficiency, compactness, and ability to withstand high pressures. This study presents a combined experimental and numerical investigation of near-critical CO2 condensation in an additively manufactured PCHE, carried out within the Horizon 2020 DESOLINATION project. Experiments were conducted using the transcritical CO2 test facility at LUT University to evaluate the thermal-hydraulic performance of vertically downward CO2 flows within the PCHE and to validate the numerical model. To improve physical accuracy and predictive capability of the simulations, a Homogeneous Relaxation Model was implemented to model phase change, capturing the finite time associated with metastable vapor condensation. This model was integrated into a commercial Computational Fluid Dynamics (CFD) solver via user-defined functions and validated against the experimental results. The validated model was used to simulate CO2 condensation in PCHE minichannels across a wide range of operating conditions, including saturation temperatures from 15 to 27 degrees C, mass fluxes from 1675 to 4272 kg/m2s, and heat fluxes from 60 to 200 kW/m2. Based on the simulation results, new correlations were developed for the two-phase heat transfer coefficient and Darcy friction factor, with deviations of +/- 7 % and +/- 3 %, respectively. This work provides a high-fidelity numerical framework and new experimental data for predicting near-critical CO2 condensation in PCHEs, supporting the development of more compact, efficient, and cost-effective heat exchangers for next-generation transcritical CO2 energy systems.