fluid dynamics - Flow Rate and Orifice Diameter

I have a electrical/electronics background and have limited knowledge of Fluid Mechanics. So, i will try to be as clear as possible.

I am currently working with brake fluid in an ABS system and to effectively construct my alternative control strategy,i have following questions:

1> Flow-rate: I just need to know how to measure the flow-rate across an orifice? I know of the formula

$$Q = CA\sqrt{\frac{2\Delta P}{d}}$$

$C$ - discharge coefficient

$\Delta P$ - Change in pressure

$d$ - density of the fluid

$A$ - Area of the orifice

Now, this formula should pretty much give me the answer, but my real question is; if i want to calculate simply the flow-rate across a circular orifice of different diameters. So, $Q1$ for $D1$,$Q2$ for $D2$ ... $Qn$ for $Dn$. I am simply interested in finding the flow rate without the consideration of pressure change or velocity etc.

Just to clarify, FLOW RATE through a PIPE of varying diameters.

I will be grateful for any and every suggestion!

Answer

OK, area $A$ is just $\pi D^2/4$. The real question is: What is $C$? It depends on the shape of the orifice (and Reynolds number). There are some quick-and-dirty approximations here. It depends on the orifice geometry, like whether its edges are rounded or sharp.