Adiabatic Entrance Length Selection for Supercritical CO2 Flows in Tubes With Various Hydraulic Diameters

In studies of horizontal supercritical carbon dioxide (sCO2) pipe flow, it is common practice to include a fixed-length adiabatic section before the heated region to allow for hydraulic flow development. This entrance section is intended to be long enough to ensure the flow becomes hydrodynamically fully developed, thereby minimizing entrance effects on heat transfer measurements. While well-established guidelines exist for subcritical flows, the criteria for determining the appropriate adiabatic entrance length for sCO2 flows are less clearly defined. This paper investigates this issue using 2 mm diameter and 4 mm diameter round tubes, each subjected to a constant heat-flux-to-mass-flux ratio (q/G) of 60 J kg−1. The 2 mm and 4 mm diameter tubes include adiabatic entrance lengths ranging from 0 to 1000 mm (0D to 500D and 0D to 250D respectively), followed by a 500 mm (83D) cooling section where heat transfer is evaluated. The influence of the entrance length on flow development and thermal performance is assessed through several flow characteristics, including centerline pressure and velocity, axial velocity profiles, wall shear stress, and local heat transfer coefficients. Results indicate that, for the given conditions—2 and 4 mm diameter tube, cooling boundary conditions, and q/G of 60 J kg−1—a minimum adiabatic entrance length of over 55D hydraulic diameters and 59D hydraulic diameters respectively, is necessary to achieve a hydrodynamically fully developed flow state.