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I watched lots of video related to sucking process of a fan in a turbofan engine, but I'm confused at a point.

In this link and many videos like this, I found the fan's direction wrong. This fan's blades in the video have high pressure on their top and right, low pressure on their bottom and left. So in this case, air must go left, not right.

enter image description here

I think that these red dots are high pressure, yellow ones are low pressure since the fan pushes the air up by rotating upwards.

Can you explain this rotation direction and thrust to me?

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    $\begingroup$ "..the fan pushes the air up by rotating upwards". Remember Newton's third law? If the fan moves up, the air moves down! Think of an aircraft. The aircraft moves up by deflecting air downwards. $\endgroup$
    – lWindy
    Feb 28, 2023 at 13:56
  • $\begingroup$ I agree but this is the problem. If the air pushes blades of the fan down, then it means thrust is to the left instead of the right in this geometry. $\endgroup$
    – Jawel7
    Feb 28, 2023 at 13:58
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    $\begingroup$ The HP air to the right tries to travel towards LP to the left, but it can't do so as the blades are in its way. In its attempt to travel to the left, the HP air pushes the blade to the left and that produces thrust. The air enters the intake due to the lower than ambient pressure to the left; air exits the exhaust due to the higher than ambient pressure to the right. $\endgroup$ Feb 28, 2023 at 23:35
  • $\begingroup$ As are many questions that are rooted in an incomplete understanding of things, I think this is a great question that has prompted some very instructive answers. $\endgroup$ Mar 1, 2023 at 19:48
  • $\begingroup$ I don't see any issue with the engine as depicted in the video. I don't think about pressure etc, but a rather simpler thought that if those blades, turning as depicted, were pushed into a giant puck of soft butter - would they lift butter off the surface and towards the engine's innards? And the answer is yes, they would. (I accept the limitations of this visualisation in that most turbofans turn faster than this animation, and are rarely used to gather butter.) $\endgroup$ Mar 2, 2023 at 10:28

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Starting with the marine prop example you used in the comment on John's answer. It creates high pressure behind the prop and low pressure in front of the prop, so the BOAT moves forward. The water is accelerated backward. Think about the boat being held in place. The water would go out the back of the prop.

You have to think in terms not of the air that's at the blade itself but the free stream air. The low pressure at the front side of the blade causes free stream air to accelerate toward the blade. The high pressure at the back causes it to move away from the blade. The high pressure air directly at the blade can't move forward because the blade is there.

Think of just a plain wing. Low pressure on top, high on the bottom. It would LIKE to move from bottom to top, but the wing is in the way. At the tips it does just that, but the bigger force is the free stream air being pulled toward the top of the wing and accelerating downward. So the overall motion of the air is downward. The blade is just an airfoil situated sideways.

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The video and the fixed image are correct. The blades are moving up viewed from the left side of the engine in that particular video, and the LP side (top of the airfoil) is forward, the HP side (bottom of the airfoil) is aft, so the lift force is forward, a Newtonian reaction to the air being accelerated aft by the rotation of all those little wings. You may be perceiving the blade orientation backwards somehow in the videos, but they look correct to me.

The fan is just a fixed pitch propeller, little different from the wooden propeller on a Piper Cub, except lots and lots of blades, driven by a windmill in the exhaust of the engine instead of directly via a crank and piston, and with a shroud around it. But otherwise, more or less the same thing.

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    $\begingroup$ @Jawel7, you are correct that higher pressure is on the right, because it is a compressor. As the air flows from left to right it is being compressed, which increases pressure. If the blades suddenly stopped turning this higher pressure air might briefly reverse direction to equalize with ambient, although inflight ram air would probably just push it through. But the compressor blades are powered, they are doing work to force air flow to the right., compressing it in the process. $\endgroup$ Feb 28, 2023 at 15:48
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    $\begingroup$ Air is sucked in because the fan blades are spinning because they are powered by the turbine. There are plenty of questions here, (and other material on-line) that explain how turbine engines function. A full explanation is out of scope for a comment. $\endgroup$ Feb 28, 2023 at 16:26
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    $\begingroup$ @Jawel7 I think you need to go look at an ordinary, everyday fan. Look at the angle of the fan blades, predict based on your inference which way the air will blow, then turn the fan on and feel for yourself if your prediction was correct. $\endgroup$ Feb 28, 2023 at 17:29
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    $\begingroup$ @Jawel7, your written English is very good, but from your profile it is apparently not your first language. Perhaps some concepts are lost in translation? Otherwise I'm just not seeing where the disconnect is and it's frustrating me that I can't explain it. My advice: Disregard any videos having to do with submarine propellers, (water doesn't compress) and search for an explanation of how gas turbine engines work in your native language. Hopefully that will help make the light come on. $\endgroup$ Feb 28, 2023 at 18:19
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    $\begingroup$ @Jawel7 You're right, if the fan stopped spinning then the air would move the direction you say it would. However, the fan is spinning and thus constantly beating the air backwards. It acts more like a wall, from the perspective of the high pressure half, and that pushes the fan and thus the aircraft forwards. Imagine if the marine propeller were put on the nose of the submarine! $\endgroup$
    – TomatoCo
    Mar 1, 2023 at 0:52
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have high pressure on their top and right, low pressure on their bottom and left. So in this case, air must go left

Your observation is correct. The moving fan creates a high pressure zone behind it and a low pressure zone in front of it. However your conclusion is wrong. It is not a "must" that air go to the left.

Consider a balloon (just a regular one, made of rubber and inflated by human breath). The air pressure inside the balloon is higher than the air pressure outside the balloon. Therefore by your reasoning you may think that air must move from the inside of the balloon to the outside. But this does not happen. The balloon stays inflated. Why is that? It's because there is a wall separating the inside of the balloon from the outside - the rubber skin.

Exactly the same thing happens with the fan blade. The movement of the fan creates a zone of high pressure behind it and a zone of low pressure in front of it. But the high pressure air does not flow to the low pressure zone for exactly the same reason - there is a literal wall separating them: the fan blades.

But you say, "look: there are gaps between the blades"! Yes, and some air can move through the gaps. However air has mass and therefore inertia. Air cannot instantaneously teleport from one location to another. By the time the air tries to move into that open gap the fan blade would have moved to cover it (but obviously leaving a gap behind it - then the air may try to move into that gap but by the time it can get there the fan blade from behind will move to cover that gap (leaving a gap in front of it but again it takes time for air to move and by the time it can get to that gap the fan blade would advance to cover the gap... repeat to infinity)).

Now, if this is all that's happening sooner or later the two sides will sort of equalize anyway and there would be no pressure difference. All the fan would do is create turbulence in the local space with air bleeding through to the low pressure side via gaps we can't quite cover (eg. the gap between the fan tips and the engine's duct/body). You can see this phenomena happening in blenders. However, thinking this way ignores an important fact: the fan is not the only object in this universe - the fan is operating in a whole universe that surrounds it.

One thing you are missing from your thinking is that the entire engine is surrounded by the planet's atmosphere. Yes, there is a low pressure zone in front of the fan blades but you are forgetting that there is an atmospheric pressure zone in front of that zone. Since air moves from high pressure to low pressure and since the atmospheric pressure is higher than the zone in front of the blades air will move from the outside world into the engine. This is the sucking action.

It's the same for air behind the blades: yes there is a high pressure zone behind the blade but there is an atmospheric pressure zone behind that zone. So air will move form that high pressure zone to the low pressure of the atmosphere behind it. This is the blowing action.

But how do the blades themselves create the high pressure and low pressure zones? The same way aircraft wings do it (or your cupped hand do it when you put your hand out the window of a moving car). The aerodynamic effect of the blade's airfoil coupled with the blade's angle of attack creates the pressure differential. We normally call this effect "lift" but in fans we call it "thrust".

And yes, as I mentioned earlier this lift/thrust is a transient effect (you can think of it as the blade beating down on the air). If left alone the air would move back to their original positions and thus no net air movement would be created. But the blade moves continuously, constantly creating the pressure differential and not giving the air a chance to recover back to equilibrium. It's kind of like the difference between swinging your hand and putting your hand out of a moving car. When you swing your hand you do get lift but since you stop swinging at the end you don't even have time to feel that lift. Putting your hand out of a moving car however you can feel the lift because your hand is constantly moving into fresh air and thus never allowing the high pressure zone below your hand to equalize with the low pressure zone above your hand. It is the constant movement that forces the two zones to remain separate.

If the engine were to stop spinning you will find that the high pressure zone and low pressure zone will disappear. This may sound kind of obvious but they disappear not just because the engine is off. They disappear because once the fan blades stop moving the air from the high pressure zone can equalize with the low pressure zone because there is no more moving fan blades stopping it from doing so.

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  • $\begingroup$ Amazingly detailed explanation and helpful to be honest. So let me summarize briefly; 1-) My analysis that fan blades create low pressure area in front of them, and high pressure area behind them. I said that air should intend to go out of the engine, instead of getting sucked in, which is wrong, since you say that yes there is low pressure area in front of the fan blades, but there's also another air zone which is high pressure zone relatively called "ambient". It basically means that air in the surrounding area will intend to go from higher pressure to lower pressure towards engine. $\endgroup$
    – Jawel7
    Mar 1, 2023 at 5:12
  • $\begingroup$ This is the sucking action actually. Force and intention of the air behind the fan blades to go out is less than force and intention of the air in the ambient to get in the engine. As results, air goes in, not goes out. This is also why a turbofan engine's big fan is located in a tube somewhere a bit deep, because we want blades to rotate as fast as possible by accelerating air molecules while decreasing pressure of air front, and increasing pressure of air behind. Nevertheless, there's still air outside with higher pressure than air inside in front of the blades, and this why air comes in. $\endgroup$
    – Jawel7
    Mar 1, 2023 at 5:19
  • $\begingroup$ Did I understand precisely? @slebetman $\endgroup$
    – Jawel7
    Mar 1, 2023 at 5:20
  • $\begingroup$ Basically yes. And as for the reason for the tube (and why the fan is in the tube instead of just outside).. Remember I mention that air can bleed from the high pressure at the blade tips? Have you ever seen pictures of airplanes flying through a cloud or some fog or mist? Do you notice that circular airflow from below the wings? That's the high pressure air attempting to flow to the low pressure region. We call this tip vortex. The tube is there to prevent this from happening so that we maintain more of that high pressure behind the blades... $\endgroup$
    – slebetman
    Mar 1, 2023 at 5:38
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    $\begingroup$ ... note however that there are pros and cons of having a fan inside a tube. While we do recover pressure loss from preventing tip vortex the tube itself is a source of drag. This is why regular propeller planes are still used in the real world instead of everyone converting to ducted fans - the weight of the tube plus the drag of the tube is enough to offset any advantages we gain from preventing the high pressure air from bleeding into the low pressure zone. The alternate solution to minimize the loss is to increase the size of the props which is why planes like the Tu95 have giant props $\endgroup$
    – slebetman
    Mar 1, 2023 at 5:42
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Whenever air is pushed, there is higher pressure created in the direction of the push. And this is what's happening at the fan and at propellers: they push air backwards.

Eventually, the air will be part of the atmosphere again, so where does it flow to? Not through the sidewalls of the intake. Also not back to where it came from, because there is still more air being pushed in, by a fan tightly fitting in the inlet duct. Because of the construction of the engine, it can only flow outwards on the aft side of the engine.

Consider the view from the air just after the fan: it cannot flow back to the fan, because there is higher pressure there, more air being pumped by the fan. There is a lovely low pressure atmosphere in the back of the engine, and that is where it will flow to.

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This is a very common source of confusion. The cause is that you are looking at the pressure distribution on the blade surface in isolation; and not the associated fluid velocities that are inextricably tied to that pressure distribution.

The solution to the fluid flow equilibrium equations results in both an unbalanced pressure distribution on the rotor and cowl, as well as induced flow velocities in the airstream. These are co-dependent on the system geometry. Given a pressure distribution, you can determine what the total flow field is; or given the flow field, you can determine what the pressure distribution over all the surfaces is.

Since the object is to produce thrust to the left, it makes sense that the pressure difference across the blades pushes the blade to the left. And the necessary reaction of the fluid wake is to have greater momentum to the right.

So you can't look at the pressure distribution and say what the air will do in response to that pressure distribution. There is only one thing the air can be doing that is consistent with that pressure distribution, and you can be certain that it is doing it.

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First of all a couple of corrections:

The turbofan that you see in the video is a "geared turbofan". In this kind of engines the fan is not directly connected to the same shaft of the turbine(s) but there's a gearbox between the two. This gives the fan a direction of rotation that is opposite the one of the low-pressure compressor and turbine stages. Don't get fooled by this effect.

In the video, the turbofan is in flight. Normal cruise speeds for a jetliner is Mach 0.80 something. This value is way too high for the fan and for the first stages of the compressor and this is why there's an inlet in front of it: the inlet has exactly the task of slowing down the airflow till some Mach 0.4. That means that the engine does not really actively suck air in but rather it passively ingests air.

A turbojet/fan actively sucks air in basically only when it's on ground at rest. The easiest way to see this suction is to go back to the roots and obviously it cannot be done if not with Prandtl himself (please jump to minute 1:18):

Here you see what happen when an airfoil (or a blade or a wing...) passes by (to match your turbofan video where the blades move downward, just rotate the screen so that also the Prandtl wing moves downward). From the video one can see that when the blade passes by, it makes the airflow move from the top to the belly behind it i.e., if it where a compressor's blade, from the front to the core of the compressor stage. This is the "suck in" effect you were looking for.

In the next two pictures I highlighted (in red) where the airflow was just after the passage of the airfoil and (in blue) where it moved to after some seconds:

where it was

where it is

Obviously in a fan/compressor just after the first blade another one comes and then another one and again and again: this multiple-passage multiplies the "suck in" of the airflow behind each blade.

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You can watch this same effect with a simple desk or ceiling fan.

As a fan blade rotates, air "bunches up" in front of it, and bunched up (high pressure) air is higher pressure than other air around it.

The high pressure air can't escape from front of the engine, because there's an angled fan blade hovering over it. So it instead moves into the engine.

Another way to think of it is just imagine hitting a rubber ball with a paddle angled at 45 degrees upward. The rubber ball would travel up into the air. Basically, your turbofan blades are your racket, and air molecules are the ball.

enter image description here

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  • $\begingroup$ I don't really think that racket-ball is a good analogy to use when explaining "how a turbofan sucks air" and other aerodynamic principles as such. The air is sucked due to low pressure at the intake, and exhausted due to high pressure at exhaust. A single ball cannot replicate the concept of "pressure", and so it cannot be used to illustrate those principles. $\endgroup$ Mar 2, 2023 at 0:57
  • $\begingroup$ The racket-ball example explains in a very simple and good way what happen to the airflow when it encounters a wing... unfortunately it is correct only at hypersonic speed. $\endgroup$
    – sophit
    Mar 2, 2023 at 9:19

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