OSCILLATORY NATURAL-CONVECTION OF WATER NEAR THE DENSITY EXTREMUM AT HIGH RAYLEIGH NUMBERS

An experimental investigation has been conducted with regard to the na tural convection of water near the density extremum in rectangular enc losures with the vertical walls held al different temperatures. The wa ter in the enclosure is initially set at a uniform temperature and the n the temperatures of the cold and hot walls are suddenly changed to 0 and 8 degrees C, respectively. The flow patterns of the steady state are presented for the initial temperature range of 2-6 degrees C at hi gh Rayleigh numbers, not considered previously. Al an initial temperat ure of 4 degrees C, the convective flow field consists of a symmetric double-cell circulation Forming a downward flow, called a sinking jet in the interior of the enclosure. For Ra >9 x 10(6), the interior sink ing jet shows an oscillatory behavior due to free-shear instability, w hich enhances the heat transfer rate between vertical walls. However, al an initial temperature other than 4 degrees C, an asymmetric double -cell circulation is revealed. Oscillatory flow is suppressed near the vertical walls. These phenomena are satisfactorily explained by a num erical analysis. (C) 1997 Elsevier Science Ltd.