Instability during transition to turbulence of supercritical pressure CO2 in a vertical heated serpentine tube

Abstract

Convection heat transfer of supercritical pressure CO2 in various channels has extensive applications in many industries. In some applications, the channel may not be straight, but rather serpentine. Researchers have found that the transition to turbulence of constant-property fluid is delayed in a constant-curvature pipe, whereas it is promoted in a serpentine tube with alternating curvature. In supercritical pressure CO2 flow in a serpentine tube, the buoyancy and centrifugal forces both influence the transition process, which needs to be studied thoroughly. This study investigates the instability during transition from laminar to turbulent flow both numerically and experimentally. The wall temperature oscillations are measured by experiments. The effects of buoyancy and centrifugal force on the flow and heat transfer are analyzed through comparisons of unheated cases, to heated cases of forced flow (no buoyancy), upward flow and downward flow by simulations. The flow and heat transfer instabilities along the flow and across the cross sections and the secondary flow pattern development are studied. The location (distance from the inlet) at which the flow becomes unstable is least for downward flow, intermediate for upward flow and largest for pure forced flow. It is found that the earlier occurrence of the instability is caused by the adverse pressure gradient in downward flow, and the enhancing effect of the buoyancy on the centrifugal force in upward flow. These findings are important for understanding the flow and heat transfer during transition to turbulence of supercritical pressure CO2 flow in vertical serpentine tubes.