Structural analysis and optimization of flattened tube gas cooler for transcritical CO₂ heat pump systems

The performance of gas cooler is acknowledged as a vital factor in CO₂ heat pump systems. Consequently, its optimal design can significantly impact the overall efficiency, attributed to the pronounced changes in thermophysical properties of CO₂ during pseudo-critical operating conditions. This research examines, for the first time, various configurations of flattened tubes (FTs) utilized as the conduit for CO₂ flow within the gas cooler. It is established that changing the cross-sectional configuration represents an effective approach for enhancing the thermal performance of the gas cooler, resulting in a significant increase of up to 60 % in heat transfer coefficient. All FTs exhibit superior overall performance across most CO₂ cooling conditions compared to circular tubes. Regarding the performance index, the overall improvement can be reached up to 31.8 % and 18.6 % under near-critical and trans-critical conditions, respectively. Furthermore, an optimization is conducted using BBD-RSM for a specific alternating FT due to its potential to enhance the thermal performance not only on the CO₂ side but also on the cold side of the gas cooler. The analysis highlights the significant positive influence of the minimum axis on the thermal and hydraulic specifications of gas cooler, followed by the transition length.