Experimental results of flooding experiments in an inclined tube with liquid nitrogen and its vapor

•An apparatus for flooding observation with LN2–VN2 was built.•Different mechanisms of flooding with LN2–VN2 and water–air were stated.•A modified correlation in consideration of inclination effects was proposed.•The correlation is validated by both cases with uncertainty of 20%.

Counter-current two-phase flow behaviors of saturated liquid nitrogen and its vapor at the onset of flooding are experimentally investigated. The experiments are carried out in a vacuum-insulated 20 mm i.d. transparent tube with the inclination angles of 30°, 45° and 60° corresponding to the horizontal. The common slug flow phenomenon happened with water–air is not observed with liquid nitrogen–vapor, instead, the big interfacial wave is found to be crushed to tiny droplets. The phenomenal difference is primarily attributed to the larger viscosity of water than liquid nitrogen. Correspondingly, the sharp rise of pressure drop with water–air is largely due to the blockage of gas flow by the formed slug, while it is primarily due to the tiny droplet entrainment for the liquid nitrogen–vapor pairs. The effects of inclination angles on the incipient flooding velocity are specially emphasized and investigated. A new correlation base on Ohnesorge number and modified Froude number are presented, and the results coincide with the experimental data of both room-temperature and cryogenic fluids with the uncertainty of 20%.