i want to design a diffuser with inlet dia of about 4 to 5 mm and incoming air pressure of about 100 bar. The purpose is to lower the pressure of the vent port in a pneumatic valve. I want to know how to design the interior surface of the diffuser.
There are a couple of things that are confusing me about your last statement. I'm probably just misunderstanding what you are trying to achieve.
Are you wanting to vent to sub-atmospheric pressures? What is your vent currently venting to?
The purpose of a diffuser is to increase pressure by trading off velocity pressure for static presssure. What is your expected configuration in the valve to have the diffuser do what you stated it is needed for? Are you thinking along the lines of an ejector perhaps?