Abstract:
Falling film evaporators that operate in the nucleate boiling regime in the refrigeration industry offer a number of advantages over their flooded counterparts such as lower refrigerant charge and at times improved heat transfer. Existing literature has not characterised the influence of surface characteristics on the falling film boiling process, and they are poorly understood for the pool boiling process. The purpose of this study was therefore to experimentally measure the influence of roughness, material and nanostructures on the heat transfer of falling film boiling and pool boiling of saturated refrigerants on the outside of horizontal tubes. The critical heat flux point was measured if it occurred, and the falling film heat transfer enhancement ratio, critical dryout threshold and general dryout characteristics were investigated in the study.
The tubes tested consisted of plain copper, stainless steel and mild steel tubes that were polished and roughened with various grades of sandpaper. Furthermore, three types of nanostructured surfaces were applied to polished copper tubes, namely a layer-by-layer (LbL) coating of silica nanoparticles, a copper oxide (CuO) nanostructure coating and a commercial nanocoating process termed nanoFLUX.
The nanoFLUX tube had the highest heat transfer coefficients of tubes tested under both pool boiling and falling film conditions, with between 40 and 200% higher heat transfer coefficients than those of a polished copper tube. The nanoFLUX surface outperformed the other surfaces due to a combination of rougher microstructure and a unique heat transfer mechanism, possibly linked to capillary wicking of liquid inside the nanochannels of the porous coating.
The falling film heat transfer enhancement ratio was found to increases as surface roughness was increased on plain tubes, suggested to be as a result of enhanced microlayer evaporation from the trapped sliding bubbles in the thin flowing film.
The nanoFLUX and CuO surfaces experienced lower critical heat flux as a result of departure from nucleate boiling under pool boiling and falling film boiling conditions compared with plain surfaces.. However, the nanoFLUX and CuO tubes performed well in terms of critical dryout at lower heat fluxes. The wicking capabilities of the nanoFLUX and CuO surfaces were thought to be the cause of their improved dryout capabilities at lower heat fluxes, but increased heat fluxes possibly led to dryout of the nanostructures resulting in operation in the Cassie-Baxter state and subsequent reduced wettability.
