Two-phase flow boiling frictional pressure drop of liquid nitrogen in horizontal circular mini-tubes: Experimental investigation and comparison with correlations

The two-phase flow boiling characteristics of liquid nitrogen (LN₂) in horizontal circular mini-tubes were experimentally studied. Experiments were performed in a wide range of flow conditions, e.g. inlet pressure from 0.17 to 0.35 MPa, mass flux from 140 to 330 kg/m² s, heat flux from 0.5 to 69.4 kW/m² and tube diameters of 2.92 mm and 3.96 mm. The influences of mass flux, heat flux, and inlet pressure on the pressure drop were discussed. The results indicated that the pressure drop increases with the increasing mass flux and heat flux but decreases with the increasing inlet pressure. But the influence of heat flux on the frictional pressure drop of LN₂ was weaker than mass flux and inlet pressure. The frictional pressure drop of two-phase flow of LN₂ was compared with homogeneous model and several semi-empirical correlations. An improved correlation based on the Lockhart-Martinelli model, which used coefficient C as a function of Reynolds number and Weber number was proposed.