Final answer:
The fan's purpose in a constant-diameter duct is to increase air pressure, which aids in improving air flow and makes the environment feel cooler through convection.
Step-by-step explanation:
The purpose of a fan in a constant-diameter duct is to increase the flow of air by increasing its pressure. This movement of air also aids in heat transfer through convection, such that moving air due to the operation of a fan can make the environment feel cooler than still air. This is because the fan displaces warm air near the body with cooler air from elsewhere. In the context of air ducts and HVAC systems, another objective is to provide better air circulation and maintain air quality within a space.