Enhanced refrigerant flow boiling heat transfer in microstructured finned surfaces

Flow boiling is used in air-conditioning and refrigeration, thermal management of electronics, distillation, chemical synthesis, desalination, power generation, and cryogenics. To enhance the flow boiling heat transfer, internally finned tubing has been widely used. In this work, we demonstrate a fabrication method to generate interal microstructures in 5.2 mm internal diameter finned aluminum tubing through crystallographic chemical etching to further increase heat transfer coefficients of low-GWP refrigerant R-515B. Enhancements in heat transfer coefficients are demonstrated across the range of vapor qualities with peak enhancements up to 172% observed at high vapor qualities with similar levels of enhancements demonstrated with R-134a. The effects of varying operating parameters such as heat flux, mass flux and saturation pressure, on performance improvement were also examined. Flow visualization studies performed through an adiabatic glass visualization section at the test-section exit allowed quantiification of film thickness and showcased lower conduction resistance for microstructured finned surfaces. The advantages of this structuring technique are further highlighted by the relatively minor increase in pressure drop ( similar to 20%). We demostrate that etching of plain aluminum tubes exceeds the performance of finned tubing under high heat flux condtions, with enhancements being more pronounced at low mass fluxes, indicating potential replacement of current extrusion methods used for the manufacture of finned tubing. Surface characterization results after mechanical expansion of the etched finned tubing demonstrated the remenance of microstructures in fin valleys, thus proving potential compatibility with heat exchanger fabrication methods.(c) 2023 Elsevier Ltd. All rights reserved.