Flow And Heat Transfer Characteristics Of Supercritical Nitrogen In Helical Pipes

Helical pipes provide large heat-transfer area per unit volume, high heat and mass-transfer flux, and small residence time distribution. The industrial processes involving curved tubes include heat exchangers, ultra-filtration, rectification and absorption, nano-fluids, chemical reactors, nuclear reactors, and piping systems. It can be seen that curved geometries have wide range of applications, varying from human organs (lungs, blood vessels, catheter, etc.) to industrial devices (cooling coils, mixers, micro-devices, etc). Fluids with the state near the critical points- supercritical fluids show many tremendously strange characteristics, such as singularities in compressibility and viscosity, diminishing difference in vapor and liquid phases which have wide range of applications in many high-tech areas. Therefore the fundamental research on heat transfer characteristics of supercritical fluid in helical pipes would be the focus of attention for researchers.Now the study on heat transfer characteristics of supercritical fluids in helical pipes is spare. The paper summarized the heat transfer characteristics of common fluid in helical pipes and that of supercritical fluid in straight pipes, then developed the experimental setup to study the heat transfer characteristics of supercritical nitrogen flowing in helical pipes. The experimental measurement and data procedure were also described. The uncertainty analysis of experimental system was conducted to ascertain the availability of data deduction.Experiment of Supercritical nitrogen flowing upward in the helical pipe with the curvature 0.057 and pipe diameter 2mm was conducted, the effects of heat flux, mass flow rate and inlet temperature on wall temperature, local heat transfer coefficient and pressure drop were investigated. Phenomenon was explained through the variation of thermal physical properties, the coupled combination of centrifugal force and buoyancy force, as well as the heat acceleration principle. Oscillate phenomena of local wall temperature is found in the experiment and analyzed.Numerical study was conducted to compare with the experimental result, and good agreements were reached on wall temperature, heat transfer coefficient and pressure drop. y+ had an important effect on numerical results. The comparisons between numerical and experimental results show that the correlations had accuracy for Nu and friction factor in some degrees. Finally, the paper summarized the flow and heat transfer characteristics of supercritical nitrogen in helical pipes.