Falling film evaporators can reduce the charge of refrigerant systems when compared to flooded evaporators, but dryout is a limiting factor that must be overcome if the refrigerant charge is to be reduced to a minimum. Commercially enhanced tubes can improve dryout resistance compared to plain tubes. Nanostructures have been shown to improve wettability and wickability when added to surfaces, offering further opportunity to improve the dryout resistance of commercially enhanced tubes.
In this study, a set of uncoated and nanostructure coated commercially micro-enhanced tubes are comparatively tested to determine the influence of microstructures, nanostructures and combined microstructure/nanostructured surfaces on dryout resistance. Horizontal tubes were tested and their outside heat transfer coefficients measured over a range of heat fluxes with heating water being passed through the inside of the tubes under falling film boiling conditions. The test fluid is refrigerant R134a at 5°C. The inside heat transfer effects are characterized by using a modified Gnielinski correlation through the Wilson plot analysis. The film flow rate was decreased from high to low to determine the heat transfer performance and the dryout profiles of the coated and uncoated tubes.
The commercially produced Gewa KS low-finned tube caused reduced dryout resistance, while the Gewa B5 and Turbo EHPII tubes were shown to improve the dryout performance compared to a plain smooth tube under some conditions, particularly at higher heat flux. The nanocoating applied to the commercially enhanced tubes did not notably improve dryout resistance. Microstructures and nanostructures optimized for dryout improvement in refrigerants may be able to improve on these results.