Simultaneous development of the hydrodynamic and thermal boundary layers of mixed convective laminar flow through a horizontal tube with a constant heat flux

Abstract

The thermal and hydrodynamic features of developing mixed convective laminar flow in a long horizontal tube were numerically and experimentally investigated. The tube was heated at different constant heat fluxes and had an inner diameter of 11.52 mm and a total length of 9.5 m. Numerical simulations were conducted using ANSYS-Fluent 19.3 with a highly structured mesh and accurate temperature-dependant thermophysical properties for water. The studied parameters consisted of the wall temperatures, Nusselt numbers, vorticity, velocity and temperature gradients, hydrodynamic and thermal boundary layer development, secondary flow, as well as significant momentum terms. By analysing the vorticity characteristics, it was found that our conventional understanding of the merging boundary layer in internal tube flows had to be modified. Subsequently, methods were proposed to determine the development of the hydrodynamic and thermal boundary layers, as well as the momentum terms. The experimental and numerical results indicated that the local mixed convective Nusselt numbers decreased near the tube inlet but then increased along the tube length as secondary flow increased significantly. Furthermore, an increase in secondary flow led to a decreased thermal entrance length but increased hydrodynamic entrance length. By analysing the simultaneous development of the hydrodynamic and thermal boundary layers, it was revealed that the entrance region of a mixed convective laminar flow could be divided into six regions: (1) hydrodynamic inlet region, (2) buoyancy development region, (3) buoyancy dominating region, (4) buoyancy settling region, (5) hydrodynamically stabilising region, and (6) fully developed region.