Vortex flow patterns near critical state for onset of convection in air flow through a bottom heated horizontal flat duct

In this paper, an experiment is carried out to investigate the buoyancy dri ven vortex how at slightly subcritical and supercritical Rayleigh numbers i n a mixed convective air flow through a bottom heated horizontal flat duct. Particular attention is paid to the flow at a very low Reynolds number for 3.0 less than or equal to Re less than or equal to 5.0. Results from the f low visualization have revealed two new vortex flow patterns in addition to those often seen in literature such as longitudinal rolls (L rolls), movin g transverse rolls (T rolls), and mixed longitudinal:transverse rolls (M ro lls). The newly observed vortex flow patterns include the stable longitudin al rolls near the duct sides along with nonperiodic traversing transverse w aves in the duct core, and the mixed longitudinal and transverse rolls as w ell as irregular cells. Moreover, steady longitudinal rolls and nonperiodic traversing transverse waves are noted even at subcritical Rayleigh numbers . A flow regime map delineating various vortex flow patterns is given. Temp oral characteristics of the flow are also inspected. Furthermore, a correla tion is given to estimate the local onset locations of the longitudinal rol ls. Meanwhile, the oscillation frequency and convection speeds of the trans verse rolls are correlated from the present data. Finally, many complicate processes during the vortex flow formation are noted, such as the generatio n of the L and T rolls and transverse waves, splitting of rolls into cells and the reverse process of cell integration into rolls, aside from the movi ng and bending of T rolls. (C) 2001 Elsevier Science Ltd. Ail rights reserv ed.