I'm trying to model a microfluidic device that has miroparticles flowing in it with water. I'm using CFD-ACE GUI but I don't know which module to use for this. Any help would be greatly appreciated.

It depends on what you are trying to do. When I did something similar (low RE mixing in a MEMS device), I used the Chemistry module to define the particle distribution as a simple scalar. This made it easier for me to do all the rheology stuff I was interested in (variations in viscosity, density, diffusivity etc). I could of used the User Scalar module, only this wasn't compatible with the free surface flow module I needed for my work (you don't mention an air/water interface so you may not have this problem). FYI, the user scalar module allows for anisotropic diffusion whereas the chemistry module doesn't. If you want to go for full-on particle tracking, then you need the Spray module.

I'm trying to use the Spray module but I'm not sure how to use it. I just have particles in water flowing though a channel and I want to see their flow pattern.