Cryogenic quenching of rock using liquid nitrogen as a coolant: Investigation of surface effects

Cryogenic quenching experiments were conducted using sandstone rock samples and liquid nitrogen (LN2) under ambient pressure. Four different rock surface conditions were prepared to investigate the surface effects on quenching, as compared to the baseline case where rock surface was polished by 1500 grit sandpaper. A copper sample was also made to reveal the unique property of rock surface with respect to quenching. It is found that a 130 °C increase of LFP was achieved by rock surface than copper, when both were polished by 1500 grit sandpaper. The scanning electron microscope (SEM) images showed that numerous pores and cavities with hierarchical dimensions were present on rock surface. These cavities contributed to the LFP enhancement by providing massive nucleation sites for bubble formation when intermittent solid-liquid contact occurs during film boiling thereby destabilizing the vapor film. Rock surface polished by 36 grit sandpaper was found to be superhydrophilic and rougher than the baseline case but produced no enhancement on cryogenic quenching. Quenching rates were remarkably accelerated on rock surfaces with sand layer coating and with orthogonally intersected grooves. The initial room temperature was not enough to maintain stable film boiling on these two surfaces and transitional film boiling dominated the boiling regime after quenching started. Further increase of LFP was achieved on rock surface covered by crude oil, which served as a low thermal conductivity coating. The present results demonstrated that the unique surface micro structure of rocks can significantly raise LFP temperature thus shortening film boiling duration. In addition, by appropriately modifying the rock surface condition, cryogenic quenching rates and LFP can be further enhanced.