Experimental study of downward facing boiling on a structured hemispherical surface

A heating system with liquid metal as the intermediate heat transfer medium was introduced into a scaled three-dimensional reactor vessel. The liquid metal was heated by heaters and then circulated inside a hemispherical vessel. Then, the outer surface of the lower head was cooled by boiling water. The objective of this study is studying the boiling regimes and heat fluxes on the outer surface of the hemispherical lower head. The boiling heat transfer was investigated on a hemispherical plain surface and on a surface with interconnected grooves with triangular cavities surface using saturated deionized water at atmospheric pressure. The critical heat flux (CHF) on the plain surface at an inclination angle of 85° was 857.3 kW/m2, with no boiling crisis observed on the structured surface up to the highest heat flux of 1366.9 kW/m2 at the inclination angle of 85°, with the liquid metal temperature higher than 400 °C. Thus, the CHF on the structured surface was more than 59% greater than on the plain surface at an inclination angle of 85° with the liquid metal temperature inside the pressure vessel reduced by 80-100 °C for the same heating power. The structured surface forms a liquid-vapor conversion path with the cavities as stable nucleation sites and the interconnected grooves as cooling water supply pathways. Thus, the structured surface significantly enhances the boiling heat transfer and the CHF.