When studying about the axial compressor, I came across the term 'diffusion' being used to explain the conversion of kinetic energy to pressure, also terms like 'diffusion factor'. What is being diffused in the compressor?
An axial compressor accelerates a flow of air to high speed. All the molecules in that parcel of air have that speed superimposed on their random vibrational motion. The idea of a diffuser is to get the fast-moving air to pile up against itself, slowing it down (i.e., decreasing its kinetic energy) and causing its pressure to increase (by increasing the speed of its random molecular motion).
In other words, the kinetic energy in that common motion performs compression work on the air as it travels through the diffuser increasing the random velocity of its molecules. In an ideal diffuser, all the kinetic energy given to the fast-moving air by the axial fan is transformed into compression work, increasing the pressure of the air.