Is it possible for cfd to model a leaking valve that contains such a small gap between valve disc and valve body? Can I restrict the cfd inlet velocity to simulate the small leakage rate of 20 cc/hr to 120 cc/hr?

(1). There is no limit on the size of the gap, there is no limit on the Reynolds number. (2). There is even a new field called "micro-fluid dynamics"- fluid dynamics in micro devices.(see ASME Journal of Fluid Engineering, recent issue? Also Journal of Tribology.)

Stephen,

I am sure CFD can do it. But if I were you I wouldn't bother to do CFD for such small flows use a annular leak equation and calculate the pressure drop (if thats what you want to find).

Q = pi*(bore dia) *(radial clearance).^3 *(delta p)/ (12*mu*length)

Anil