Two-Phase Microchannel Heat Exchangers for Transcritical CO2 Cycles - Investigation of Heat Transfer and Pressure Drop

Abstract Supercritical and Transcritical Carbon dioxide (CO2) based cycles offer significant design and operational flexibilities for power generation and refrigeration. Transcritical CO2 cycles are particularly suited for temperate climates due to their superior thermodynamic efficiency and specific power output at lower cycle pressures. However, equipment design, particularly condensers, presents challenges due to the phase change of CO2. Accurate heat exchanger sizing and performance evaluation necessitate customized numerical models and experimental data to resolve phase change behavior and reliably predict key engineering parameters. This paper presents a numerical framework that utilizes a high-fidelity Eulerian multiphase model to resolve phase change and a one-dimensional sizing and rating model to evaluate heat exchanger sizing. A detailed investigation of heat transfer, pressure drop, and quality evolution during phase change is presented. The analysis of key non-dimensional numbers for condensation heat transfer is provided, enabling a better understanding of the model predictions. Finally, a crossflow microtube condenser is sized and rated for a 5 MW net power transcritical CO2 Rankine power cycle. The work is likely the first of its kind modeling methodology to predict heat transfer coefficients and pressure drops of CO2 in the two-phase regime.