Experimental study of roughness effect on critical heat flux in subcooled flow boiling

Surface roughness represents a fundamental parameter governing vapor-liquid interfacial behaviors and critical heat flux (CHF) in boiling system. Separated effect of roughness on CHF in subcooled flow boiling is studied in this work, employing copper surface with controlled averaged roughness (Ra) ranging from 0.141 μm to 2.135 μm. The surface wettability is moderately influenced and surface negligible wickability is maintained. The results demonstrate a consistent non-monotonic relationship between roughness and CHF across the tested range of mass fluxes (100 to 400 kg/m²s). As Ra increases, the CHF initially rises and peaks at Ra=0.699 μm, exhibiting a ∼30% enhancement compared to the CHF at the smoothest surface (Ra=0.141 μm). Then, the CHF stabilizes or even experiences a slight decline with further increases in roughness. The impact trend is attributed to the limitedly increased nucleation site density, restricted microlayer evaporation, and increased liquid rewetting friction under high Ra. In addition, the thermal parameters effect, including mass flux, subcooling, and pressure, is studied, and the increasing trend is explained based on the visualization results of vapor-liquid dynamics. Building upon these experimental insights, a prediction model for CHF is established considering the roughness effect and thermal parameter effect, with a good agreement of ±15% errors compared to experimental data. The findings provide insights into surface fabrication on boiling heat transfer enhancement and safety design in boiling systems.