The Cold Jet (ducted fan/propeller)

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In summary: If you can't see why this won't work with fans rotating in the same direction (at the same speed) with no stator vanes, I think you need to learn some basic fluid mechanics of turbomachinery.
  • #1
kach22i
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I had an idea once which I'd like to go into a little. It involves how a fan or propeller draws in air verses only providing thrust.

The idea is/was to cut down on aerodynamic drag and resistance a propeller imposes once the craft is up to speed. By having a remote fan/propeller air intake like a jet or even a NACA inlet; in-line tandem fans which are located down a serpentine duct would provide thrust.

The first fan/propeller would act to drawn air into the air intake plenum feed duct; the second fan would kick it up a notch. Much like a compressor.

The second fan might rotate in the same direction or be counter-rotating; it does not matter at this point. I would also use additional duct length to straighten the flow in lieu of stator vanes AFTER THE LAST FAN.

All this together is aerodynamic and quiet if not the last word in weight control or efficiency margins.

I'm not claiming this is a new idea or invention, do a search on "cold jet" or "cold thrust", pre-WWII Italy had an example or two, and of course there was Henri COANDA in 1910.

Example the Davis Wing:
http://members.cox.net/rebid/DavisWing.html

Note: The inlet may not always face the direction of motion in my example, it could be at a right angle. This might also be used in a hovercraft, not just an aircraft.

What do I need to know or reference to pull off such a configuration?
 

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  • #2
Sounds like a more complex way of driving a turbofan.
 
  • #3
Not a turbofan; it's just a basic ducted fan. Nothing special there. The machine would probably be more efficient if that same engine power was directed through a propeller.
 
  • #4
Danger said:
Not a turbofan; it's just a basic ducted fan. Nothing special there. The machine would probably be more efficient if that same engine power was directed through a propeller.
There is a hovercraft design booklet from the 1960's which shows a diagram of one fan in front of the other. It claimed that it would be able to absorb twice as much power as a single fan, not that it would be more efficient than a single larger fan.

There are also examples in light hovercraft design in which two ducted fans are used side by side (not in tandem) in lieu of a single large fan to lower the C of G.

Many ways to skin a cat, not looking for the best here, just looking for what I need to match an engine (4-stroke automobile) to a ducted fan (in series) to another another fan. Would you call this a two stage compressor?
Many people have matched engines to fans/propellers, what do I need to match fan to fan.

The question "why" can be only answered by "because I can".

NOTE: maybe the word "quest" is similar to the word "question" for a good reason.

EDIT: This is different.
http://www.kulikovair.com/Tandem.htm
 

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  • #5
kach22i said:
The second fan might rotate in the same direction or be counter-rotating; it does not matter at this point. I would also use additional duct length to straighten the flow in lieu of stator vanes.

What do I need to know or reference to pull off such a configuration?

If you can't see why this won't work with fans rotating in the same direction (at the same speed) with no stator vanes, I think you need to learn some basic fluid mechanics of turbomachinery.

Contra-rotating two stage fans, both ducted and unducted, are nothing new of course.
 
  • #6
AlephZero said:
If you can't see why this won't work with fans rotating in the same direction (at the same speed) with no stator vanes, I think you need to learn some basic fluid mechanics of turbomachinery.
Okay, let's say we have stator vanes in between the fans.

Now what?

As I understand the concept the first fan is of shallow pitch, the second fan greater pitch. To keep it simple I want to stay away from Contra-rotating fans.

In essence the first fan is feeding the second fan higher velocity air or is it higher pressure air? Why is this a good or bad thing? The first fan in the scenario I've outlined is also acting to draw air in which is part of the work its doing.

Assume I know nothing of the basic fluid mechanics of turbomachinery (which I don't), where do we go next?

EDIT: Found this......click to see figures
http://web.mit.edu/16.unified/www/SPRING/propulsion/UnifiedPropulsion9/UnifiedPropulsion9.htm
The stator removes swirl from the flow, but it is not a moving blade row and thus cannot add any net energy to the flow. Rather, the stator rather converts the kinetic energy associated with swirl to internal energy (raising the static pressure of the flow). Thus typical velocity and pressure profiles through a multistage axial compressor look like those shown in Figure 9.5.
 
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  • #7
I have been working on this particular type of model for about------ oh my goodness--12 YEARS!------ I have developed the prototype and computations within an auto cad program----- however have yet to build the darn thing...

These Models account for many different things----- first off... TURBULENCE...

Having two fans with separate angles of attack within a tube---- and then adding the drag properties of the fan disc--- with the increase of inlet wind velocity speed (while in flight)---- you are looking at a major catastrophe within the distance separating the two fans...

You will at minimum create huge deficiencies in laminar wind velocity... and at maximum create a pressure/turbulence bubble that would cause your fans to stall or even cavitate your duct (If not make it blow up)----

Using Fans and Stators close together is the best thing to do in terms of creating a huge velocity difference (the real goal) on each side of the Fan(s) Disc

With Stators in a Jet engine... they are used to compress the air for combustion with fuel mixtures... If you reverse the airfoil in the stator and decompress the air ... You remove pressure and build velocity inversely (as stated--- the goal is velocity)...

There are some articles out there on this theory...

The Idea of a ducted fan is a great one... but must have the focus of its creator on creating high air velocities--- w/o Pressurizing the air mass... as well as creating laminar (non turbulent) flow (you need stators to accomplish this)

Its a love/hate game in efficiency----

and someday... I will get the time to put a working example together...

Congrats on your theory though... It is where I started until I progressed the Model once I became a FAA certified Mechanic and majored in aeronautics...

The way to do it----- is out there... and it is no longer imagination--- but, the person who builds it first that will reap the benifits...
 
  • #8
OH... and counter rotation of the fans is the only way to maintain efficency---

Look at Propeler efficency theory! or even look up dual rotating propeller test that have been studied by NASA
 
  • #9
kach22i said:
I had an idea once which I'd like to go into a little. It involves how a fan or propeller draws in air verses only providing thrust.

The idea is/was to cut down on aerodynamic drag and resistance a propeller imposes once the craft is up to speed. By having a remote fan/propeller air intake like a jet or even a NACA inlet; in-line tandem fans which are located down a serpentine duct would provide thrust.

The first fan/propeller would act to drawn air into the air intake plenum feed duct; the second fan would kick it up a notch. Much like a compressor.

The second fan might rotate in the same direction or be counter-rotating; it does not matter at this point. I would also use additional duct length to straighten the flow in lieu of stator vanes AFTER THE LAST FAN.

All this together is aerodynamic and quiet if not the last word in weight control or efficiency margins.

I'm not claiming this is a new idea or invention, do a search on "cold jet" or "cold thrust", pre-WWII Italy had an example or two, and of course there was Henri COANDA in 1910.

Example the Davis Wing:
http://members.cox.net/rebid/DavisWing.html

Note: The inlet may not always face the direction of motion in my example, it could be at a right angle. This might also be used in a hovercraft, not just an aircraft.

What do I need to know or reference to pull off such a configuration?
This appears to be quite similar to the drive system for jet-boat. The material being moved is quite different in density, but the principle is the same.

In either case (water or air) the intake duct introduces frictional losses that decrease the efficiency of the system, regardless of the direction of flow.

A similar application (kort nozzles) is used on slow moving watercraft to gain thrust. These are ineffective above a threshold speed as they induce more and more drag on the vessel.

http://www.navweaps.com/index_tech/tech-023.htm
 
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  • #10
xtrdouglas said:
The Idea of a ducted fan is a great one... but must have the focus of its creator on creating high air velocities--- w/o Pressurizing the air mass... as well as creating laminar (non turbulent) flow (you need stators to accomplish this)

Its a love/hate game in efficiency----

1. I thought that it was the difference in pressure which moves the body (craft) though it's medium.

2. And the greater the exiting velocity of air (or water when compared to body speed) the more losses in energy conversion (sub-sonic conditions).

3. The concept as stated mentions hovercraft because I really meant to say that I am willing to give up some efficiency for safety and noise considerations. I mean how many surface based vehicles have open spinning propeller blades? You can put guards on propellers/fans, you can put them in a duct, and you can locate them high on the roof and out of the way of curious hands. However the potential danger will always be there. By enclosing the fan/compressor/propeller deep in the body of any craft, a certain element of safety is bound to be added. At least that's what I was thinking.
 
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  • #11
Someday I would like to measure the thrust coming out the back of my two person hovercraft (6ft W x 10ft L). I also need a way of metering/measuring the mass air flow being drawn into the ducted fan.

Any ideas on how to do that?

I'm assuming both sides an equation to confirm the results would have to equal, so that I could calculate the work being done (and energy lost).

The second phase of the experiment would be to build and install a rigid foam board inlet which would allow air to be drawn from overhead (or from the sides) in lieu of straight in.

As crazy as that sounds (don't know where I would sit) several hovercraft do something similar for their lift fans already. Nobody I know of has tried it for their thrust fan/propeller for reasons all to obvious (except to nutty guys like me).
 
  • #12
kach22i said:
1. I thought that it was the difference in pressure which moves the body (craft) though it's medium.

2. And the greater the exiting velocity of air (or water when compared to body speed) the more losses in energy conversion (sub-sonic conditions).

3. The concept as stated mentions hovercraft because I really meant to say that I am willing to give up some efficiency for safety and noise considerations. I mean how many surface based vehicles have open spinning propeller blades? You can put guards on propellers/fans, you can put them in a duct, and you can locate them high on the roof and out of the way of curious hands. However the potential danger will always be there. By enclosing the fan/compressor/propeller deep in the body of any craft, a certain element of safety is bound to be added. At least that's what I was thinking.

1) the body moves forward because of the momentum the engine imparts on the air (and vice versa) Momentum is mass times velocity, so high velocity is good. In turbofans, the annular flow through the fan is high velocity, high mass flow of air to achieve the bulk of the thrust. The flow through the core is there to power the jet engine which drives the fan. As the core flow goes through the compressor, it slows down as pressure goes up. Increased pressure is desired so the air has better combustion properties. After blowing up the air with fuel, it accelerates and drives the turbine, which drives the compressor and fan.

2) This is like saying the faster you go the more drag you feel.

3) Since the success of a new technology depends on whether or not it meets the needs of the customers, I think it would be good to get some numbers on the safety issue. Ex- how many people are injured every year due the engines? How can you quantify the improved safety? You need the baseline data. Also the noise issue is important, and any improvements in this area would be well received. They key is to show that your up front design requirements (related to safety and noise) are actually desired by the aerospace industry.
 
  • #13
rbeale98 said:
1) the body moves forward because of the momentum the engine imparts on the air (and vice versa) Momentum is mass times velocity, so high velocity is good.
You are partly right.
You get the same momentum if you have a high mass and low velocity or low mass and high velocity but that does not tell the story.
The conversion of fuel (chemical energy) into kinetic energy determines the thermal or internal efficiency of engine, while the conversion of the KE to propulsive work determines the propulsive or external efficiency. This depends on the amount of KE wasted by the propelling mechanism, which in turn depends on the mass airflow multiplied by the velocity squared. A propeller moves a larger mass at lower velocity that a turbojet (especially the first generation), which uses a low mass at high velocity. From this we can see that the high-velocity, relatively low weight jet exhaust wastes considerably more energy than a propeller with its low-velocity, high mass airflow.

The turbofan is more fuel efficiency, because, with the same rated thrust, it wastes less KE and hence has more propulsive efficiency. Average fan/core exhaust velocity (V2) is closer to aircraft speed (V1). Amount of KE left in atmosphere is less. Here is an example:
Turbine expels 10 mass units of air at 1000 ft/sec KE = .5*10*1000*1000 = 5 million ft lbs of waste energy
If you double the velocity you get .5 * 10 * 2000*2000 = 20 million ft lbs of waste energy.
If you design the engine to double the mass and have original velocity then KE = .5 * 20 * 1000 * 1000 = .10 million ft-lbs of waste energy. This is a factor of two compared to four for same momentum.

The max thrust for the least fuel flow can be obtained by giving the smallest acceleration to the largest possible mass airflow. Fanjets are better than turbojets and the propeller is better, but it is limited to lower speeds due to prop inefficiency.
 
  • #14
nucleus said:
rbeale98 said:
The max thrust for the least fuel flow can be obtained by giving the smallest acceleration to the largest possible mass airflow.
I have read several papers/articles which say the same. Thank you for explaining it so clearly.

I have to admit that many of the articles/papers were not 100% absorbed on a first read, and over the last few years just some of the basics remain in my head, and kind of fuzzy at that.
 
  • #15
Yes! There are still people who are talking here!

Ok, for those who don't know... I trying to make something similar to a cold jet. Problem is, I haven't an inkling about physics.

Can anyone help or show me to look for a calculation? I need to find a out what size fan would be efficient with a motor with 306 g/cm of torque.
 
  • #16
kach22i said:
Someday I would like to measure the thrust coming out the back of my two person hovercraft (6ft W x 10ft L). I also need a way of metering/measuring the mass air flow being drawn into the ducted fan.

Any ideas on how to do that?

Sure.
1. get a bathroom scale and mount it to a wall at the ride height of the hovercraft.
2. hold a stick
3. fire up the hovercraft vertical thrust
4. push the stick into the bathroom scale
5. throttle the horizontal thrust.
6. read the scale...pounds thrust
 

1. What is a Cold Jet (ducted fan/propeller)?

A Cold Jet, also known as a ducted fan or propeller, is a type of fan that is used to produce airflow in a specific direction. It consists of a rotating propeller that is enclosed in a duct, which helps to increase its efficiency and control the direction of the airflow.

2. How does a Cold Jet (ducted fan/propeller) work?

A Cold Jet works by using the rotation of the propeller to move air through the duct. The duct helps to compress and accelerate the air, resulting in a high-velocity jet of air that is directed towards a specific area. This creates a strong and focused airflow, making it useful for various applications.

3. What are the benefits of using a Cold Jet (ducted fan/propeller)?

There are several benefits of using a Cold Jet. First, it is highly efficient, as the duct helps to increase the airflow velocity, resulting in a stronger and more focused jet of air. Additionally, it is a versatile tool that can be used for various purposes, such as cooling, ventilation, and propulsion. It also has a smaller footprint compared to other types of fans, making it suitable for compact spaces.

4. What are some common applications of a Cold Jet (ducted fan/propeller)?

Cold Jets can be used in a variety of applications, including HVAC systems, cooling electronic equipment, ventilation in buildings, and even in some aircraft engines. They are also commonly used in wind tunnels for aerodynamic testing. Additionally, Cold Jets are used in some industrial processes, such as drying and cooling products on a conveyor belt.

5. Are there any safety considerations when using a Cold Jet (ducted fan/propeller)?

Yes, there are some safety considerations when using a Cold Jet. The high-velocity airflow produced by the fan can be dangerous if not used properly. It is essential to follow all safety precautions and instructions provided by the manufacturer. Additionally, the rotating propeller can be a hazard, so proper precautions should be taken when handling or maintaining the fan.

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