Numerical study of thermal and fluid characteristics of supercritical nitrogen flowing in an inclined semicircular flow channel

The thermal and hydraulic phenomena in an inclined semicircular channel through which nitrogen changes to supercritical state flows were studied through numerical analysis. The geometry of the flow channel was semicircular in cross-section with a diameter of 2 mm and a length of 350 mm. The heat fluxes applied to all sides of the semicircle were 20, 40, and 60 kW/m2; the angles of the inclined flow channel were 30°, 45°, 60°, 90°, and −90°, and one more case was analyzed under zero gravity as a reference case. Under the given conditions, nitrogen changed to a supercritical state owing to the heat flux applied while flowing through the channel, and the pseudo-critical point differed depending on the size of the heat flux. However, with a change in the inclination angle, physical properties of the heated working fluid, such as the density, viscosity, and cp, changed rapidly, and the buoyancy effect and acceleration parameter values changed accordingly. As a result, a secondary flow appeared, and the change in the heat transfer performance was explained by the heat transfer coefficient. By calculating the change in the Nusselt number and the change in each variable according to the location of the channel, we developed correlations based on the change in the inclination angle and heat flux.