Even when the temperature of the air around a fan and the ambient temperature are the same, the fan can still make you feel cooler due to the phenomenon known as evaporative cooling. Here's how it works:
Increased Evaporation: When the air blows from a fan, it increases the airflow around your body. This increased airflow enhances the process of evaporation of moisture from your skin, such as sweat. As the moisture evaporates, it absorbs heat energy from your skin, resulting in a cooling effect. The faster evaporation rate due to the fan's airflow makes you feel cooler than if you were in still air.
Enhanced Heat Transfer: The movement of air created by the fan helps to improve convective heat transfer. Convective heat transfer occurs when heat is transferred from your body to the surrounding air through convection. The increased airflow from the fan facilitates the heat transfer process, carrying away the heat from your body more effectively. This enhanced heat transfer helps to cool you down more efficiently than stagnant air.
Disruption of Boundary Layer: When air is stagnant around your body, a thin layer of still air called the boundary layer forms on your skin's surface. This boundary layer acts as an insulating barrier, reducing the rate of heat transfer between your body and the surrounding air. However, when a fan blows air, it disrupts and thins out this boundary layer, allowing fresh air to come into contact with your skin. This disruption promotes heat transfer and enhances the cooling effect.
It's important to note that while a fan can make you feel cooler, it does not lower the actual temperature in the room. It simply helps to increase the cooling sensation on your body by promoting evaporation and convective heat transfer.