A diffuser is the mechanical device that is designed to control the characteristics of a fluid at the entrance to a thermodynamic open system. Diffusers are used to slow the fluid’s velocity and to enhance its mixing into the surrounding fluid. In contrast, a nozzle is often intended to increase the discharge velocity and to direct the flow in one particular direction.
Frictional effects may sometimes be important, but usually they are neglected. However, the external work transfer is always assumed to be zero. It is also assumed that changes in thermal energy are significantly greater than changes in potential energy and therefore the latter can usually be neglected for the purpose of analysis.
Diffusers can be as a shape of round, rectangular, or can be as linear slot diffusers(LSDs, e.g.: Linear slot diffusers takes the form of one or several long, narrow slots,mostly semi-concealed in a fixed or suspended ceiling.
Occasionally, diffusers are mostly used in reverse fashion, as air inlets or returns. This is especially true for Linear slot diffuser and ‘perf’ diffusers. But more commonly, grilles are used as return or exhaust air inlets.