zeroGradient for both pressure
 

zeroGradient for both pressure and velocity on the outlet? And also, fixedValue for both pressure/velocity on inlets? Doesn't seem too consistent to me.

I'd try fixedValue velocity and zeroGradient pressure on inlets.
And fixedValue pressure and zeroGradient velocity on outlets.

p: fixedFluxBuoyantPressure f
 

p:
fixedFluxBuoyantPressure for walls, zeroGradient for inlets and fixedValue for outlet

Hi Sandeep and Matvej Thank
 

Hi Sandeep and Matvej

Thanks for your support.

I have already started my runs based on your suggestions, but i had a doubt.

I have given face at the top of the tank as outlet.
Exact pressure, that shall arise out Pressure velocity cooupling is unkown, Will atmospheric pressure work.

Hi Sandeep and Matvej I act
 

Hi Sandeep and Matvej

I acted on your suggestions but still i am getting same results