If X-Z axis is in horizontal plane and +Y axis is upwards( gravity is in -Y axis). Now if the flow is in -Y direction what shud be the value of 'g'?? Is it -9.81 or +9.81??? Does the value of gravity depends on the direction of flow or it depends on the orientation of the coorodinate axis??

Gravity depends only on the orientation of the coordinate axis.

If my model is drawn such that flow is in +Y direction and hence g =-9.81m/s^2. Now I rotate the model and the axis by 180 degrees such that the flow is again in +Y axis but the difference is that +Y axis is now pointing down in the direction of gravity. Would g=+9.81 in this case?????

yes I guess,

it should be +9.81, in case you rotate the model about 180 degrees, since the flow is now towards the -Y direction which adds aditionally the gravitational force to the original acceleration of the flow.

-mp