Numerical modeling for the flow and heat transfer of slush nitrogen in a horizontal pipe based on population balance equations

Slush nitrogen, the cryogenic suspension of solid nitrogen particles and liquid nitrogen, is considered to be a potential coolant for high temperature superconductive cables. In the present study, an improved two-fluid model has been built to analyze the flow and heat transfer of slush nitrogen in a horizontal pipe, where the slush effective viscosity is used to modify the drag law and the interphase heat transfer coefficient. Besides, the particle size distribution due to particle breakage is calculated by the population balance model (PBM). The numerical model is verified to have improved performance for slush nitrogen flow, compared with the previous two-fluid models. Based upon the improved model, the flow and heat transfer characteristics of slush nitrogen in a horizontal pipe with different flow velocity, solid fraction and wall heat flux have been analyzed. The calculation results for the pressure drop and the heat transfer coefficient show good agreement with the reference experimental data, with deviations within 5%. Moreover, the pressure drop reduction and heat transfer deterioration phenomena (i.e., the pressure drop or heat transfer coefficient for slurry flow can be lower than that of subcooled liquid) are discussed.