Cryogenic Heat Transfer of Near Pseudo-Critical Nitrogen in Helically Coiled Tube : The Dual Effect of Buoyance and Centrifugal Force

This paper investigates the fundamental cryogenic heat transfer phenomena of nitrogen flowing in helically coiled tubes and under the combined influence of pseudo-critical conditions, buoyancy and curvature induced centrifugal effects. Local heat transfer coefficients along the length of the coil were evaluated peripherally across the local tube cross section. First the effects of pressure, mass flux and heat flux on the heat transfer were examined. The dual effect of buoyancy and coil curvature on heat transfer coefficients was explored and interpreted via a dimensionless number Ψ, which denotes a ratio between the two effects. The heat transfer coefficients of near pseudo-critical nitrogen in the helical coil were also compared with the heat transfer coefficients in straight tube under similar experimental conditions. Results reveal that the heat transfer coefficients increase with increased mass flux but decreased pressure and heat flux. Furthermore, the buoyancy effect dominates the heat transfer between nitrogen and helical coil wall at fluid temperatures below the pseudo-critical temperature while the coil curvature induced centrifugal effect resurges at temperatures above. In addition, the heat transfer coefficients for the helical coil were lower compared to those in straight tube at fluid temperatures below the pseudo-critical temperature but the difference in heat transfer coefficient values becomes small at temperatures above. The reason is that the benefits of coil curvature and improved turbulent mixing on heat transfer are counteracted by the buoyancy effect