Turbulent Transport Phenomena across the Stable Thermal Stratified Layer Formed in a Circular Pipe

The behavior of a thermally stratified mixing layer formed in a circular pipe has been experimentally investigated in order to estimate the thermal stress of the horizontal pipe wall caused by the emergency cooling of power plants. Two component velocities and the temperature of fluid are simultaneously measured by fiber LDVs and a hot film thermometer. Experiments were performed at the bulk Reynolds number of 7350 where the overall Richardson numbers ranging up to 2.46. Flow visualization at the cross section of the pipe and measurements of the turbulence characteristics in the fields of velocity and temperature showed the difference of flow structure between isothermal and thermally stratified conditions. Upper and lower layers mixed well in the isothermal condition, while in the thermally stratified conditions, there obviously existed a thermally stratified mixing layer where turbulent mixing was suppressed by the buoyancy effect. This thermally stratified mixing layer spread in the horizontal direction with increasing Ri number and generated secondary flow between the wall and this mixing layer.