Vortex flow pattern selection and temporal-spatial structures of transverse and mixed vortex rolls in mixed convection of air in a horizontal flat duct

Abstract

Combined Row visualization and temperature measurement were carried out to explore the processes for selecting the buoyancy-induced vortex flow patterns and the temporal-spatial structures of the transverse and mixed longitudinal and transverse vortex rolls in air flow through a bottom-heated horizontal flat duct. The results for 1 less than or equal to Reynolds number Re less than or equal to 50 and 1800 less than or equal to Rayleigh number Ra less than or equal to 30 000 showed that the vortex how patterns were determined from the competition between the buoyancy-induced thermal plume, which tends to generate transverse rolls and the onset of longitudinal vortices in the entry region of the duct. At 70 less than or equal to Ra/Re-2<200 and 6 less than or equal to Re less than or equal to 20, the mixed longitudinal and transverse rolls prevail, while the moving trans-verse rolls dominate for 200 less than or equal to Ra/Re-2<1900 and 1 less than or equal to Re less than or equal to 7.5. Moreover, the measured spanwise distributions of the time-averaged air temperature theta(av) indicated that when theta(av) at the midheight of the duct rises to a value very close to zero, the transverse rolls appear. This simple condition provides a convenient method with which to detect the existence of the transverse rolls. The measured time samples of the instantaneous air temperature showed that in the duct filled with the pure and regular transverse rolls the entire flow oscillates at the same frequency and amplitude. At higher buoyancy, the downstream transverse rolls deform and can even break into irregular cells when Iia is high enough. The corresponding temperature oscillation becomes nonperiodic and its amplitude increases with the downstream distance, In the mixed-roll structure, the axial growth uf the longitudinal rolls tends to bend rbs transverse rolls and causes them to become knotted. Finally, correlation equations for the oscillation frequency of the air temperature: and the wave speed in the transverse and the mixed longitudinal and trails verse vortex flows were provided.