Thermal diffusion interface inside a horizontal pipe flow compared between laminar and turbulent flow regimes

A visualization of thermal diffusion in a horizontal pipe flow is conducted to elucidate the axial diffusion characteristics of two fluids of differing temperatures, particularly those flows between the laminar and turbulent flow regimes. Tracer particles are used to detect the thermal diffusion interface as well as to measure streamwise velocity profiles. We found that the Hagen–Poiseuille parabolic velocity profile established a long axial diffusion length that drives thermal convection in the pipe cross-sectional plane. This situation results in a 3D interface at Reynolds number, Re = 600, and a vertically stratified interface at Re = 1900. In turbulent flows at Re = 5700, an anti-diffusion spot appears in the front section before undergoing rapid mixing under turbulence. From the visualized particle images, the apparent axial coefficients of diffusion were measured and compared with the friction properties of the pipe. Furthermore, particle image velocimetry (PIV) explored the two-way coupling mechanism of between the temperature and the velocity within the thermal diffusion process.