CFD is very sensitive to the meshing close to the holes. And the flow out of the holes is very sensitive to how 'clean' the holes are - i.e. whether you carefully cleared all the swarf from the inside of the pipe after drilling the holes.

As for calculations, you are just as well off doing a one dimensional 'hand' (spreadsheet) calculation for flows through a manifold. This is well explained in Internal Flow Systems, 2nd edition, by D S Miller. There are probably other sources that also explain manifold or sparger design calculations.

Clearly the total hole area is determined by the velocity you want out of the holes. The general 'rule of thumb' is to keep the hole diameter not more than 25% of the main pipe diameter so the static pressure down the pipe doesn't change much. Otherwise the hole size will need to vary along the pipe to give equal flow and then the exit velocities will vary too. This may take a few iterations until you get an acceptable design.

And then ensure your holes have a good internal finish of course!

... And if you need a particular spray pattern I doubt you will manage that with just holes which will give you a coherent jet. I think you will need to add nozzles.

Thank you. I Will go through the book by D S Miller and get back with possible solution.

Miller's book is divided into two parts: Part 1 describes the origins of the pressure losses in various components and Part 2 gives the loss coefficients and calculation procedures. For manifolds section 5.7.4 describes the issues (definitely worth reading first) and section 13.13 gives the calculation procedure.