chilled water storage simulation
 

hi everyone ,i am filling a tank (radius=10m,height=20m)with 4℃water，the tank was full of 12℃water at first.max grid interval is 20mm,max time interval is 2s. charing time is 7hours,i calculated the velocity ,but after 7hours,there are only 70% of the tank is 4℃water,although thermocline forms ，there are still about 15 of rest water is 12℃，any one can tell me where is wrong?http://vip.tu265.com/b-27611-510f6bab225ae.png
btw，the model is 2d,there are two inlet holes in the bottom ,and two outflow holes in the top .lenth is 0.015m

It should not a stiff problem.
- Have you enabled gravity?
- Have you enabled surface tension?
- Have you enabled incompressible density?
- Which multiphase model are you using?