What is a diffuser?

Explanation:

Diffuser:

• A diffuser is defined as "a device for reducing the velocity of a fluid passing through a system while increasing the static pressure". 
• The fluid's static pressure rise as it passes through a duct is commonly referred to as pressure recovery.

This is because the energy in the flow is conserved.

Bernoulli's principle describes this property of a steady, incompressible, and inviscid flow, which states that in this flow,
\({\frac{1}{2}\rho V^2~+~p~+~\rho gz~=~constant}\)

The contraction or enlargement of the section in a duct leads to a respective increase or decrease of velocity to conserve the mass flow. 

The section of a diffuser grows along the stream, causing the flow to slow down. 

Assuming the height is the same, the previous equation will result in (index 1 for upstream and index 2 for downstream):
\(\frac{1}{2}\rho V_1^2~+~p_1~=~\frac{1}{2}\rho V_2^2~+~p_2\)   (V1 > V2)

This implies pressure has to increase to conserve energy, i. e. p2 has to be higher than p1.

Additional Information

Nozzle:

• A nozzle is used to increase the discharge velocity while simultaneously lowering the pressure of a fluid passing through it.