During the summer period water storage reservoir, an important drinking water source, usually undergo thermal stratification. Consequently, warm surface water is separated from cooler bottom water by an intermediate region which inhibits the mass and heat transfer. Under this circumstances it is often found that in the lower hypolimnion region, the rate of oxygen uptake is greater than the vertical flux from the upper enriched region. This can lead to anoxic condition in hypolimnion. This also causes serious problems for drinking water supply, which include ruining aesthetic value with unpleasant odor and color and clogging filters. As a manner of destratification, aeration system, which artificially circulates water is widely adopted.
According to the previous research, dynamics of bubble plume was depended upon two dimensionless groupings; the first being a measure of the source strength $Q_{Bㆍg}$ in relation to the total pressure head over which the bubbles can act
$M_H = Q_B g/4 π α^3H u^3_s$
The second proposed dimensionless grouping is the ration of source strength to the ambient stratification
$P_N = N^3 H^4 / (Q_B g)$
However, these two variables only include Q, N, H, g, u. The aim of the present study was to elucidate the effects of bubbles size on the destratification efficiency in a stratified water. To explain the effect of bubble size the experiment was performed with three types of diffuser making various bubble sizes and plume number from 40 to 1200.
The experiment gives different flow pattern from previous research. In linear stratification the number of intrusion is more than that of previous research and also shows that in the case of bigger bubble size the number of intrusion is more than that of smaller bubble size. As a result of experiment, efficiency of Fine pore size is the highest among the three cases while efficiency of coarse pore size is lowest. This means that bubble size have great effect on the destratification. Until $P_N$ 200 the destratification efficiency was almost same for the three cases. After $P_N$ 200, however, tendency of efficiency is different. In the case of fine pore size efficiency increase with plume number and then for medium pore size efficiency is constant and for coarse pore size efficiency decrease with plume number. Efficiency of destratification with measured bubble size is nearly constant until $P_N$ 200. After $P_N$ 200 efficiency decreases with bubble size. The gradient of decline is higher at the high plume number than that at the low plume number. From present study the fact that destratification efficiency of linear stratification is higher than that of arbitrary stratification is revealed. The present study also show that at the above of 1.5mm in bubble size destratification efficiency decrease with plume number, at the below of 1.5mm in bubble size destratification efficiency increase with plume number.
