I need help finding resources on the Web with regards to conical diffuser design. I am trying to find the optimum geometry for a conical diffuser for an incompressible fluid.(water)

(1). The optimum half angle of a conical diffuser is around 8~10 degree. But you still have to worry about the area ratio. (2). Researchers at the Stanford University have done some systematic studies , and there is also a fairly standard diffuser map available.(which show you the various flow domain as a function of the geometry parameters) You can also check the ASME/ Journal of Fluid Engineering.