The fingers advance downward unceasingly and the fingers become fewer. Some larger fingers travel more or less independently with less interaction with their neighbouring fingers. These fingers undergo strong nonlinear dynamics such as shift in the feeding sites and lateral pinch-off. Fig. 4(d) corresponds to the stage when the convection has been decayed. The fingers have reached the bottom of the zone. Once the fingers touch the bottom boundary, the aquifer boundary will affect the mixing process and the interactions between fingers become less important than the bottom
boundary effect. The dissolved CO2 contour maps shown in Fig. 4 indicate strong nonlinear behaviours, which become more evident in the evolution of CO2 dissolving rates shown in Fig. 5. Fig. 5(a) illustrates the CO2 mass fluxes at the top boundary for different Rayleigh numbers together with analytical mass flux at the top boundary considerating pure iffusion only, which have been plotted in logarithmic scale. The rate of CO2 uptake at the dissolution boundary (Z = 0), per unit cross-sectional area, can be given as [3]: FðZ ¼ 0; tÞ ¼ /qX0 ffiffiffiffiffiffi
D
pt
r
ð18Þ
Eq. (16) illustrates analytically that diffusive mass flux without convection is inversely proportional to t0.5, indicating that dissolution rate declines with time. However, the relation is changed to be non-monotonic since the onset of convection and the flux with convection is increased to several orders higher than the pure diffusive flux as shown in Fig. 5(a). The temporal evolution of CO2 dissolution
rate roughly follows four periods in time: diffusive period,
modulated convective period, constant convective period and decay
convective period, as shown by the dashed vertical lines for
Ra = 4310 in Fig. 5(b). In the diffusion dominated period (Period
1), the agreement between numerical results and analytical diffusive
flux is good since diffusion is the only physical process for
the three different Rayleigh numbers in this period. With the onset
of convection, a general increase in dissolution rate is shown in the
modulated convective period (Period 2). The partial coalescing and