International Journal of Heat and Mass Transfer, Vol.40, No.10, 2419-2419, 1997
Buoyant Instabilities in Downward Flow in a Symmetrically Heated Vertical Channel
This study of the downward flow of nitrogen in a tall, partially heated vertical channel (upstream isothermal at T*(in), heated region isothermal at T*(s) downstream adiabatic) shows the strong effects of buoyancy even for small temperature differences. Time-dependent oscillations including periodic flow reversals occur along the channel walls. Although the flow and heat transfer are asymmetric, the temperature and axial component of velocity show symmetric reflections at two times that are half a period apart and the lateral component of velocity shows antisymmetric reflections at the two times. There is strong interaction between the downward flow in the central region of the channel and the upward how along the heated channel walls. At the top of the heated region, the upward buoyant flow turns toward the center of the channel and is incorporated into the downward flow. Along the channel centerline there are nonmonotonic variations of the axial component of velocity and temperature and a large lateral component of velocity that reverses direction periodically. Results are presented for Re = 219.7 and Gr/Re-2 = 1.83, 8.0 and 13.7. The heat transfer and the frequency of the oscillations increase and the flow and temperature fields become more complex as Gr/Re-2 increases. The results have applications to fiber drying, food processing, crystal growth, solar energy collection, cooling of electronic circuits, ventilation, etc.