Assuming that your 12706 Peltiers are the same spec as this 12706 Peltier then.
The specifications are :
Qmax: (ΔT =0) 63 W ΔT max. = 68°C
To operate two at full power you need 15.4V at 12.8A or about 200 W of power.
Qmax = 126 Watts of cooling.
Assume say 100 W of available cooling.
Water will transfer about 4.2 J/K/cc
Your 100W of cooling will cool 100/4.2 = 23.8 cc.K of water per second.
Lets say 25 cc.
ie cc x deltaT ~= 25 per second.
So 25cc per escond by 1 degree K or
1 cc per second by 25 degrees K
or 0.5CC per second by 50 degrees K.
That doesn't sound much, but 0.5cc / second = 1.8 litre/hour x 50C drop.
As dTmax = 68 C you cannot get more cooling by lowering water flow to much less than 0.5 CC/second.
The cooling is relative to the hot side.
To get cooling to under ambient the hot side should be as little above ambient as possible = lots of air flow.
Do I really need thermal paste
Thermal paste is a VERY good idea.
Without paste you waste cooling by creating temperature differentials across the joints.
I have the same peltier , and I am running it on a power supply of 45 amp but on 12 volt( it's a computer power supply). Is voltage any problem. 15 volt will not affect them as everyone said they are 12 volt operational.
See datasheet. The 45A figure is the current your supply CAN provide at 12V - NOT what the load actually takes at 12V.
Also I am running it for one hour but still no effect on temp I think cooling should be slow but there should be some on 12 volt ,do you think it is totally useless on 12 volt
Each Peltier takes about 6A at 15V and 4.5A at 12V.
The charts at the bottom of page 2 show surprisingly little difference at 4.5A or 6A.
Peltier Datasheet here
At 12V each device should draw 4 to 4.5A, and provide say 30W to 40W of cooling at 20 C differential. That's 70W for 2 or about 70% of what I assumed in my answer. Reduce flow rates to 70% of those above for same temperature drop. ie IF you get cooling with no water flow then a very slow water flow should be cooled. About 0.4 CC/second should give 25C cooling.
See datasheet - note that as Thot-Tcold differential rises the cooling Watts drop.