ORIGINAL: mr_matt
ORIGINAL HAAM
The fan uses all the external input to generate higher pressure output at the back, lower pressure output at the front doesn't cost any external input of the fan.
Why do you believe none of the external input (electricity) to a ducted fan motor goes to creating the low pressure in front of the fan?
Thank you for your reply.
Let us take this in an easy and simple way. You can test and study it by yourself. Put one of your hands in an empty water container. Now fill the container with water until your hand covered completely. Now take your hand out of the water. Think deeply about what happened. You used an
INPUT to raise your hand. With the use of these
INPUT you generated
TWO OUTPUTS. One of the
OUTPUTS is raising your hand and the other
OUTPUT is the flow of water to fill the space your hand left inside water.
The OUTPUT of the flow of water to fill the space your hand left inside water, has nothing to do with the INPUT you usedto raise your hand. It didn't take any input from you. It didn't cost any input. This is because water is liquid and flows, and every molecules of water is free and independent and has a POTENTIAL ENERGY. If nothing stops it it will fall. When you poured water in the container, every molecules of water fell until the water leveled. But after you took your hand out you left a gap inside water. Nothing left to hold molecules in equilibrium. Each molecule was nearer to the gap fell inside it under pressure from neighbouring molecules.
There are many other examples like that. For example if you throw something upward it will move upward for a while and then falls to the ground. Now you used input to generate the output of forcing it to go upward. But the output of coming down, didn't take any input from you it came down because it had potential energy, which is the pull of gravity.
A fan acts on air the same way your hand acted on the water inside container. Every molecules of air is free and independent and has a potential energy. When a gap created inside air all molecules rush to fill the gap. When a fan operates it pumps out air molecules between its blades backward. This creates air gap between fan blades. When the gaps created between the blades, air molecules rush from the front to fill the gaps. In this case the fan used all the input to pump out air molecules between its blades. But the act of air molecules coming back between blades from the front didn't take any input from the fan.