Heat transfer behaviour of supercritical nitrogen in the large specific heat region flowing in a vertical tube

This work investigates the heat transfer behaviour of supercritical nitrogen (SCN) for the ultimate goal of optimal design of cryogenic processes/systems. To this end, a comprehensive numerical study was carried out to evaluate the heat transfer coefficient for SCN flowing in a test section under representative conditions. This paper presents the results for nitrogen flowing vertically upward in a 2 mm diameter smooth tube. CFD simulations were conducted at two supercritical pressures (3.5 and 7 MPa) for low and high mass flux at different heat to mass flux ratios (q/G). The objective is to develop reliable prediction approaches regarding the heat transfer coefficient (HTC) in the large specific heat region using the commercially available CFD software Fluent by employing the k-ε turbulence model with enhanced wall treatment. The effects of relevant parameters such as mass flux and heat flux on heat transfer performance, and the influence of operating pressure are discussed. For example, while the working pressure is close to the critical value, i.e. 3.5 MPa, the high specific heat capacity at pseudo-critical temperature produces a peak in the heat transfer coefficient trend. On the other hand, when the pressure increases to 7 MPa the heat transfer behaviour changes due to the smooth variation of thermophysical properties and as a result the HTC trend does not show a peak even at low heat flux. It is found that the heat transfer process transfers from normal mode to deterioration mode while increasing the heat flux. Fundamentally this deterioration is caused by the variation of thermo-physical properties under high mass flux conditions and by the buoyancy forces for low flow rate.