What is the science behind it?
Both airflow directions will be able to exchange the same volume of air. The fan will not produce a vaccuum, because there will be a flow in the opposite direction. This opposed air flow will be more turbulent. Keep simple physics in mind:
- Don't disturb the air flow of the fan - especially at it's back. The air will be soaked in.
- Hot air rises. To push hot air out of a room, it may help to place the fan on an elevated level. Outside of the room may be outdoors. The arrangement of air layers is not always like in a closed room. Then an airflow aimed inwards may produce different results.
This diminishes the general usefullness of this direction.
To answer the question: Just maximize the air flow, by not blocking it. The direction will not influence the volume of exchanged air. However try to blow the warm air at the top of the room out.
To improve the usage of a ventilator
Most benefitial for cooling is evaporation. This physics effect is based on needed energy to overcome "latent heat". Just remeber the chill when rising out of a lake after swimming at windy weather. The water droplets on your skin have the temperature of the sea. Then they evaporate and cool your skin. A bowl of water before the fan will not help much. However just wetten your skin - maybe use a wet towel or blanket during sleep.
In addition to optimizing the air flow of the ventilator, there are amazing tricks for hot summer nights:
- an ice pack in the air stream of the ventilator chills the air
- cool water in a hot water bottle