Thermal plume turbulent enhancement, reverse transition, and relaminerization in stably stratified enclosure

For a thermally buoyant plume in stably stratified ambient air at the heati ng rate Q = 0.32-27.8 W/m, Row patterns are visualized experimentally, the time-dependent temperature is measured, and the power spectrum density (PSD ) is obtained. The PSDs have gradients with -9/2 and -8.0 in the laminar st ate and -5/3 and -3.0 in the turbulent state. In addition, the frequency ba nd of turbulence is higher than that of the swaying motion, where stable st ratification increases the swaying frequency by 37% at the stratification d egree sf = 0.3 K/cm, Flow regions of the laminar, transitional, and turbule nt states at any location are determined. When the Row regions are plotted on visualization photographs, the turbulent transition, reverse transition, and relaminarization are specified and compared with unstratified results, As a result, stable stratification enhances the turbulence generation belo w the plume front, suppresses turbulence near and above the plume front, an d Leads to reverse transition and relaminarization, which never occur in an unstratified ambient and characterize the plume in a stratified ambient. T he Grashof number Gr(s)( = 1.32 x 10(8)Ra(B)(1.62)) for the beginning of th e turbulent transition increases with an increase of Q, where Rao is the Ra yleigh number proportional to Q, The transitional or the turbulent state oc curs at larger than the critical Rayleigh number Ra-Bs(= 4.28 x 10(8)-9.15 x 10(8)) or Ra-Be(= 1.68 x 10(9)-2.87 x 10(9)) on the midplane.