What is the function of a car fan?

In the automotive context, the shroud assists in channeling the airflow created by the fan, therefore increasing it's cooling potential. It works on the same principle as a wind tunnel. Dr. Raspet studied ways to improve the STOL (Short Takeoff and Landing) aspects of aircraft at Mississippi State University in 1960's. He took a standard aircraft and made small changes to reduce drag. For some pusher aircraft (engines and propellor mounted in rear), he added a shroud around the propellor. This increases the efficiency of the propellor, thereby developing more thrust for given engine speed, thereby increasing fuel consumption.