Mechanism analysis on heat transfer of supercritical LNG in horizontal U-bend tube

Abstract

A comprehensive comprehension of the intricate heat transfer mechanisms pertaining to supercritical fluids within U-bend tubes holds paramount significance in the optimization design of submerged combustion vaporizers and other water-bath heating heat exchangers. This paper employs a combined approach of experimental analysis and numerical simulation to explore the complicated heat transfer processes of supercritical liquefied natural gas in the U-bend tube. For a careful consideration of similarity and safety, the N2–Ar mixture is used as the alternative experimental media. Upon validating the model's accuracy using experimental data, this study unveils the mechanism of the coupling effect between centrifugal force and buoyancy on heat transfer based on simulation results. The results show that the internal circulation flow field structure caused by centrifugal force weakens the heat transfer on the inner side within an angle range of approximately 60°. Along the entrance region of downstream straight tubing, secondary flow continues to be predominantly governed by centrifugal forces due to inertia-driven effects. Subsequently, buoyancy-driven secondary flow impels compression upon centrifugally-induced secondary flow towards the inner side, thereby instigating significant augmentation in fluidic heat transfer resistance in both the viscous sub-layer and buffer layer of the inner side. Overall, the downstream straight tube section exhibits a special heat transfer variation pattern characterized by an initial strengthening, followed by deterioration, and ultimately recovery due to the influence of centrifugal force, with a maximum influence distance extending to 175.5d.