Nanoscale surface morphology modifications for next-generation supercritical CO2 heat exchangers: Review and perspective

Carbon nanotube (CNT) pin fin arrays, CNT, or nanoparticle coating have emerged in recent decades as novel techniques for enhancing heat transfer and reducing drag. In this paper, the suitability of these techniques is reviewed specifically for heat transfer enhancement and drag reduction in the gas side of supercritical carbon dioxide (SCO2) counterflow tube-in-tube or plate type heat exchangers for applications in CO2 refrigeration systems. The methodology and the applicability of various approaches for predicting the heat transfer rates and the frictional pressure drop associated with flow over a nanocoated surface have been reviewed. Findings from both experimental and numerical studies highlight critical limitations, including a lack of fundamental knowledge about flow over superhydrophobic surfaces and the absence of experimental data for crucial parameters such as temperature jump at the wall, velocity and temperature shifts, and the Reynolds analogy factor for nanotube or nanoparticle coatings in turbulent flow regimes. These limitations significantly hamper the predictive capabilities of both single-scale and multi-scale models for frictional pressure drops and heat transfer coefficients. To enhance the accuracy of these models, it is essential to consider surface parameters such as arithmetic mean roughness ( Ra ), root-mean-square roughness ( Rq ), effective slope ( Es ), skewness ( Sk ), and kurtosis ( ku ).