another idea

[robherc]

OK, having just read your post, then taken about 4.5 minutes to think it over before replying:

1. I am starting to see how your compressor/force-feedback idea might have some more merit than I had seen before. I'm not convinced there yet, that takes a LOT more thought, but I'm keeping an open mind, and I'm still working on chasing down energy-losses in my head, too see what I come up with.

2. I still believe that the "bent" venturi or "S" duct will gain us nothing, though I'm not 100% sure there & I'll still approach anything you use to explain its benefits to me with an open mind...but right now, I'm still pretty skeptical of it. (though of this after I typed the above...aren't the primary gains from you compressor stage, the fact that it can neutralize the pressure wave to the front, and the vacuum force left to the rear, of the energy-harvesting turbine by acting as a "one-way-valve" per se? ... thus nullifying any gains realized by changing the axis of the turbine's operation?)

3. I'd really like to discuss your views on using a counter-rotating turibne in place of the stator. So far, here's my thinking on the subject:
* Inciting "spin" to the air, through any means, has an inherent drag/entropy penalty, and an inherent energy harvest in the form of torque opposite the new wind flow.
* A fixed stator develops this thrust, and transfers it to the duct directly, where it is unwanted, and causes more weight to be necessary in the mounting equipment...to counteract the torque & hold the assembly in place.
* A counter-rotating, >= 2 stage, turbine still incites an amount of spin to the air, which still improves the efficiency of the 2nd stage, but the first stage actually transfers most of the torque it develops to the drivetrain. Thus increasing energy output, reducing necessary weight, and improving overall torque/drag ratio i.e. efficiency.
*CON: Using this design necessitates some form of mounting hardware where the stator had performed this function itself. I still believe it can be mounted without suffering losses high enough to negate the improvements listed above, but haven't developed a model to test it on yet.

5. I studied lasers in-depth about 14-16 years ago, so I'm fairly well educated in basic laser tech. This is actually what caused me to see the poss. benefit of the compressor stage to us (just had to see it as a "mirror" first), but now you're being cruel by making me dig THAT far back

[robherc]

Oh, by-the-way, it took about 23 minutes' thought, and 2.5 minutes' typing to post that reply. Now, 30 minutes later, I believe I've reached at least stage 3

[yipster]

without thinking just recalling what i've read lately...
big bended blade tips seem to give notable better result
so how bout a virtual ducted counter rotating mill ?
tv last night here showed vibrations and forces in mills

[robherc]

yipster

Yes, as I recall, the "bended" blade tips you're referring to are usually called "swept blades" on aircraft and were developed to reduce the tip speeds on near-sonic aircraft. might work really well if you reversed the sweep for our purposes...I'll have to think on that some.
Also, yes, I do think there would be significant gains realized in your "virtual ducted counter rotating mill" as opposed to a "normal" unducted turbine. I think there would be a bit more room for progress with a ducted fan, but you are probably on to something quite useful. Who knows, you may even be able to tweak that idea enough to compete with, or beat, most ducted designs. Go for it!

[MPraamsma]

robherc,

Thanks for the feedback and the observations. To reflect on what you said about the torque developed at the stator, it is not that severe a force because the stator doesn't rotate. The way I envision this type of thing in practice is always to use high speed rotating turbines with high gyroscopic and other forces in pairs. These would have opposite blades, rotations etc, to cancel any of these gyro forces. The reason I am favoring this type of thing, is simply my gut feel as an engineer, to always find the system with the least number of moving parts. There is only one moving part in this arrangement, sitting on two perfect bearings. Sort of like the turbines at Hoover Dam, running continuously for decade upon decade.

It is possible to find a combination of blades and pitch angles that completely cancels axial thrust, but that would only be at one particular speed and load combination. Since this might vary widely as conditions of speed and wind vary, it seems better to render any residual forces moot by confining them to the vertical (Z)axis. This (Z)axis is immobilized by the fact that we are dealing with either vessels or vehicles, which are free to only move freely in the X and Y planes. Turning the turbine 90 deg is the best way to prevent any residual forces affecting the motion of the vessel/vehicle. Also, any excess force could be used to help support the weight of the rotating components.

Your visualizing this as an energy mirror is the key.

[MPraamsma]

robherc,

To elaborate on your question of the compressor purpose, it is to 'pump' up the pressure to the next stage of the venturi. All venturis are essentially the same basic shape, about a 4:1 reduction in area. It is not possible to make a venturi that has 20:1 for instance, because it would stall. The (bent 4:1) inlet is part of the initial 'on ramp' to get the air moving as fast as possible using just the motion of the vehicle. Even if the vehicle goes 50 MPH groundspeed, the duct still only goes 200 mph. However, if you place a propeller in that flow, it is already going 200 mph! We want to kick it up a notch to around 500~600 mph after it squirts out of the stator exit. The stator exit is the highest stage of venturi throat in the system.

The object of the optimization is to direct the fluid (air) through the smallest possible venturi at the highest possible speed. Also, we want the blades to be as short as possible so they don't need any twist. This is best done by arranging the geometries to favor a very narrow, large diameter annular shaped prop chamber with lots of short stubby blades.

[robherc]

Mike,

I understand your strategy of going for the fewest possible moving parts. i can very well appreciate that as MANY, MANY of my own designs/improvements I've come up with were created with "simplify" as my watchword; it does VERY MUCH improve the reliability of many end products. In this case, though, I think there is a statistically significant amount of energy to be harvested here, which could counter-balance the losses due to increased number of moving parts. I don't know here ,but I think I'll pursue the contra-rotating turbines a bit further myself before giving up on it.

As far as the Z-axis turbine concept of yours, I still have yet to be convinced that it will achieve what you're expecting of it, due to the reflection of the forces in the duct (I'm looking at the duct as a mirror as well) "bending" the force back onto the X or Y axis. I'm not sure of this, it is just my theory, as the opposite is yours, so I won't attempt to argue you further on this point until one of us has experimental results to support our case (since we're obviously not going to convince each other any other way).

I understand fairly wall what you're saying about the compressor helping to prevent the venturi from stalling. I'm not completely sure that you'll end up gaining any useful amount of power this way, but I don't see any real flaws with your principle there...I'm just not sure of what the ratio of losses would be. This DOES seem like a good way to enable yourself to reduce the size & weight of your turbine though, so to that end I believe you're definitely on to something.
My one nagging question immediately after reading your last post, however, is does the stall-ratio of the veturi depend on speed? Or is 4:1 fairly well universal? In my mind, it seems that it would remain fairly fixed, but I'm sure I've seen aircraft that successfully used as high as 8:1 ratios before the first compressor stage, without stalling their ducts...is there something I missed there, or do I just need to recalculate my area measurements & see that they're really at 4:1 & just LOOK to be running higher ratios?

[MPraamsma]

robherc,

Yes, the stall point varies depending on the pressure, temp, speed and venturi design. If you have tons of pressure on one side and a vacuum on the other, it would be possible to make really high ratio venturis, but at atmospheric pressure and temp, and normal speeds of vehicles, we can say that a naturally aspirated duct won't go over about 4:1. On jet aircraft, the inlets are very modest, around 1.5:1 to 2:1 because they are already flying near the speed of sound, and shocks will form if the flow reaches supersonic speeds. After each row of blades, the pressure increases, and the duct gets a little smaller, until at the end the ratio could be very high. Each time the flow passes a row of blades it is still travelling at the local speed of sound, which is changing as the temp and pressure increase. By the time it reaches the combustion chambers it is almost (relatively speaking) standing still, and is at a very high temp and pressure. (That's probably because blasting air at 50 degress below zero going at 600 mph into a combustion chamber doesn't seem to work very well, ...lol.).

In nature, air can only expand, it cannot compress and raise in temperature unless power (heat, mechanical input) is transmitted to it. Nature only has venturis. When heat (thermal or mechanical) is added to air, the velocity of the molecules increases, and so does the speed of sound. Our feedback of power from the output accomplishes this, and boosts the flow into a more efficient flight regime.

In our case we want to keep in mind that the mass flow is the same at each point (section) in the flow once it reaches a steady state. In other words, the same volume of air goes in the front as comes out the back. In the perfect venturi, the flow is adiabatic, meaning no energy is added or extracted, and the molecules do not change their speed, just their direction. Once you introduce a compressor element, that all changes, because now you can introduce additional energy into the flow temporarily, and retrieve it later. This pushes the flight regime higher while it is acting on the turbine.

After the turbine, all we need to do is return the flow to the ambient stream at a slightly lower temperature, but the same pressure. This is a matter of the inlet and outlet ratio, which don't have to be exactly the same. In fact, a variable exit geometry seems like the answer here, like flaps to control the discharge flow.

[MPraamsma]

robherc,

I made another little diagram of how the ducts can be arranged side by side to make a payload space.

[robherc]

Aha! So you're not planing on putting these turbines on a tower up in the wind at all, but more having them BE the gunwhale of your boat? Wouldn't that cause issues with water aspiration into your turbines (from waves)? So, are you trying to design your venturis to reject the water, or your turbines to be sturdy enough to handle the water w/o damage?

Or did I mislead myself, and you're still planning on mounting the ducts on a tower, just with payload space between them?

[MPraamsma]

robherc,

Before I even consider that aspect of the final configuration, I want to try and get the ideal generator concept. Maybe there is a way to integrate the duct into the entire hull in some fashion. The payload space is already sort of boat shaped. I think even some sort of hydrofoil arrangement might help keep everything out of the water. I am still looking at it as a vehicle too.

[robherc]

You could prob. use some form of counter-weighted louvers to allow MOST of the water to escape, if you put the ascending portion of your bent-venturi forward. That should allow MOST, if not all, of the water to escape (and it'd solve our disagreement about the bent venturi improving efficiency...it'd no longer matter as the bent venturi is now necessary to prevent aspiration...lol)

I'll work on an illustration to show my idea...post it in a couple minutes once I've completed it.

[robherc]

Ok, sorry about hijacking your diagram, but it made my job a bit easier

[MPraamsma]

Robherc,

Actually, the duct needs to be hermetical sealed all along it's length. The pressure at the end of the first venturi section is about .3 atmosphere at the throat. If there are holes, air will leak in and spoil the flow. The duct is subject to an outside crushing force of about 9 lbs./sq. in. near the throat.

[robherc]

Have these pressures been measured, of are they only theoretical? I'm sorry, I'm not completely seeing the physics behind the reduced lateral pressures at the throat of the venturi.