Scaling Up and Down

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hinote
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Scaling Up and Down

Post by hinote » Fri Oct 24, 2003 3:43 pm

I'm considering a large-scale version of the Kentfield 4-tube valveless PJ I recently started to run successfully. There are several good reasons for this--more power (thrust) obviously, but also better efficiency.

I would like to double the available thrust, so presumably the combustion chamber volume should be doubled, and the rest of the engine re-proportioned to follow suit.

It has been said that scaling of existing designs is a poor way to get a larger or smaller engine--and yet several active engine builders on this forum have done so, with a degree of success. The most obvious is Bruce Simpson, with his 200lbf Locky.

Does anybody have opinions about what is the RIGHT way to go about scaling a PJ? Should length proportions be maintained, or volume--or both?

Any comments will be appreciated.

Bill H.

Pieter van Boven
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Post by Pieter van Boven » Fri Oct 24, 2003 5:41 pm

Why don't you make a copy of the SNECMA Escopette? I don't know how big (dimensions) your engine is and how much trust it produces but the Escopette would be a good alternative, I think. It has the same kind of construction and it would be interesting to see some pictures of it running.
Seen before on the forum but also the same kind of construction/principle.
Somebody has the dimensions of this engine?

http://srl.org/yard/pulse/

pieter.
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Bruno Ogorelec
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Post by Bruno Ogorelec » Fri Oct 24, 2003 5:48 pm

Well, I would say neither.

The way to do it is to use one of the fancy computer modeling programs developed by several research teams over the past 20 years or so and run it on on a supercomputer. Look for parameters that will yield double the thrust of your existing engine without straying too far from the set length (or whatever dimension it is you want to preserve).

I don't think you can scale just one or two parameters up or down and expect predictable results.

Bruno

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Post by Bruce » Fri Oct 24, 2003 7:14 pm

Scaling a design like the Kentfield (a slightly modified Lockwood) will work but the result may not be optimized for maximum performance.

There are several issues to consider as you go up in size:

1. The reynolds numbers will increase which means (in simple terms) that the air will flow more freely and appear less viscous. This means that, if you scale the tubes at the same rate you scale the combustion chamber, the effect will be that the same as decreasing the combustion chamber volume as a ratio of the pipe diameters.

2. The surface-area to volume ratio of vessels changes as you make them larger or smaller. Once again, this surface area has a bearing on the ease with which gasses can enter or exit the combustion chamber -- tending to make it much easier as things get bigger.

I wouldn't bust a gut over trying to work out the ultra-fine details however, particularly with the Kentfield engine, which has two areas where adjustment can be made to compensate (ever so slightly) for these effects (tuning the intake and exhaust lengths).

The changes in proportions used for the 200lb LH were fairly minimal but I think I should have used a slightly larger combustion chamber -- one of the reasons it self-starts is that it breaths a little too easily for the size of CC due to the effects I outlined above. I did downsize the intake/exhaust slightly to compensate but I believe more might have been slightly better.

However, it's important to remember that as you make pulsejets bigger, the power increases beyond what you'd expect -- look at the Kentfield paper in respect to thrust versus combustion chamber cross-sectional area/diameter to see what I mean.

I still think it's a shame that so many pulsejet experimenters build tiny engines and then get disappointed when they don't work properly. Big pulsejets are *much* easier to get going, create useful levels of thrust, and are more efficient.

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Post by Bruce » Fri Oct 24, 2003 7:17 pm

In reviewing my last posting, I'll just add that you should probably start simply by scaling everything according to Kentfield's design (where all dimensions are related to D).

You can then always try placing restrictors on the intake/exhaust to see what effect you get.

Scaling will certainly produce an engine that runs -- and one that runs is much easier to tune than one that doesn't :-)

hinote
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Post by hinote » Fri Oct 24, 2003 9:06 pm

Pieter van Boven wrote:Why don't you make a copy of the SNECMA Escopette? I don't know how big (dimensions) your engine is and how much trust it produces but the Escopette would be a good alternative, I think. It has the same kind of construction and it would be interesting to see some pictures of it running.


pieter.
Nice pitch, Pieter--but I've abandoned my pursuit of the Escopette. Here are some good reasons I'm using the Kentfield 4-tube design, rather than the Escopette:

1. The Escopette is the longest (and most complex in planform) of the L-H family. The 4-tube is the shortest (aspect ratio) and least complex--lots of straight tube and flat plate.

2. The Escopette was designed for very low speed applications, like launching gliders. It breathes in a huge amount of air back up its exhaust end--which makes the tailpipe effectively a built-in augmentor. All its efficiency advantage would be lost at higher speeds.

3. The Escopette only puts out 22lbf thrust for its length of 2.9 meters; the 4-tube puts out 45lbf from its 1.55 meters of length.

4. The Escopette and 4-tube are arguably the most efficient (fuel-wise) in the L-H family; however the Escopette gets less efficient with speed increase while the 4-tube gets more efficient.

One has to choose the configuration carefully, depending on how it will be applied. For me, the 4-tube has the characteristics I'm looking for.

Bill H.

Bruce
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Post by Bruce » Fri Oct 24, 2003 11:16 pm

One thing to watch though Bill, is that the 4-tube LH (ie: Kentfield) has (by the time you take those four intake tubes and the turbulence they produce into account) a rather high cross-sectional area and is aerodynamically "dirty". This means that at high-speeds you'll be wasting a lot of power simply forcing a path through the air.

You could use a variation of the Foa flow rectifier but even that's going to have quite a high cross-sectional area.

It appears very much as if the best configuration for pulsejets which are going to be used in high-speed applications is the good old conventional valved engine.

They have a low cross-sectional area, are actually somewhat streamlined (with no unwanted protrusions into the airflow) and benefit greatly from ram-air at speed.

Did you get your Kentfield running with the flow-rectifiers yet?

hinote
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Post by hinote » Sat Oct 25, 2003 12:14 am

Bruce wrote:One thing to watch though Bill, is that the 4-tube LH (ie: Kentfield) has (by the time you take those four intake tubes and the turbulence they produce into account) a rather high cross-sectional area and is aerodynamically "dirty". This means that at high-speeds you'll be wasting a lot of power simply forcing a path through the air.

It appears very much as if the best configuration for pulsejets which are going to be used in high-speed applications is the good old conventional valved engine.

They have a low cross-sectional area, are actually somewhat streamlined (with no unwanted protrusions into the airflow) and benefit greatly from ram-air at speed.

Did you get your Kentfield running with the flow-rectifiers yet?
Taking your comments and questions in the same order:
Bruce wrote:One thing to watch though Bill, is that the 4-tube LH (ie: Kentfield) has (by the time you take those four intake tubes and the turbulence they produce into account) a rather high cross-sectional area and is aerodynamically "dirty". This means that at high-speeds you'll be wasting a lot of power simply forcing a path through the air.
Heck--that's nothing! I'm going to put 2 of them in a shroud, and try to recover some of the heat in the sheet metal too. I'm also considering an aerospike in a venturi, in front of the engines; the ability of these engines to take advantage of optimized ram-air at the intakes is a well-known factor, and a manually controlled aerospike will be a simple and efficient way to experiment with the intake airflow.
Bruce wrote:It appears very much as if the best configuration for pulsejets which are going to be used in high-speed applications is the good old conventional valved engine.
Sorry, no sale. I'm a valveless aficionado. Besides, I haven't seen a Schmidt tube produce the SFC numbers that Kentfield quotes; have you?
Bruce wrote:Did you get your Kentfield running with the flow-rectifiers yet?
Not yet. All sorts of unplanned delays--but I'm just about finished.
I hope to try and run it some time next week--on liquid fuel, too (I hope!).

Bill H.

Mark
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Post by Mark » Sat Oct 25, 2003 1:22 am

Bruce wrote:One thing to watch though Bill, is that the 4-tube LH (ie: Kentfield) has (by the time you take those four intake tubes and the turbulence they produce into account) a rather high cross-sectional area and is aerodynamically "dirty". This means that at high-speeds you'll be wasting a lot of power simply forcing a path through the air.

You could use a variation of the Foa flow rectifier but even that's going to have quite a high cross-sectional area.

It appears very much as if the best configuration for pulsejets which are going to be used in high-speed applications is the good old conventional valved engine.

They have a low cross-sectional area, are actually somewhat streamlined (with no unwanted protrusions into the airflow) and benefit greatly from ram-air at speed.

Did you get your Kentfield running with the flow-rectifiers yet?
Simplicity is often best, perhaps if you wanted to shorten a pulsejet you could introduce more compression/drag before the combustion escapes with pipettes, or if you wanted to fly in circles, perhaps a Logan would be the way to go. It throws out a modicum of thrust out a 90 degree side port, and according to Foa, no better method of mixing air and fuel has been found than that of a side-ported Logan. Efficient mixing of fuel and air is key, you'd want your fuel to burn like dynamite if you could, machine gun style. I wonder if instead of a Lockwood 180, if you couldn't 90 degree a thrust vectoring elbow angled backward on a Logan. My little tiny Logan is remarkable to me for how simple it is to make, "albeit" I haven't a fuel injector for it, it loves to rev up with only a film of alcohol coated on the inside walls. It would make for an interesting torch nozzle if I could adapt it to screw on one of those disposable propylene canisters.
Still, mixing fuel and air charges is tricky stuff, just imagine the little Dynajet doing it 220 times a second.
I finally located another supplier of methanol in bulk, thank goodness!, it's only $2.75 a gallon if I buy/fill my 5 gallon HDPE, high density polypropylene, racing gas can. I got tired of buy 12 ounces from Walmart for 79 cents with a milkyanti corrosion residue additive or driving an hour plus to Milton, Florida. Very dry 100% methanol is fun stuff. I suppose I could buy a 55 gallon drum for less than $100 dollars, but I have to watch how much noise I make because my backyard fence divides me and the Navy's crypto top secret air station, I can just see them wondering where all the noise is coming from when discussing spy stuff. I get tired of hearing their marching music and national anthem though, so I guess we are even, imagine hearing the theme to Monty Python's Flying Circus every weekday in the distance, what nationalism! I'm glad weekends are silent. I like the Navy as a whole but not their music bombarding my house.
Mark

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Post by Bruce » Sat Oct 25, 2003 1:27 am

I'm always a little skeptical of the TSFCs I see published for a particular design until I've verified them for myself.

Take Lockwood's claims for example - a TSFC of 0.85??? Who believes that?

However, I can see how such numbers are obtained -- the use of massive amounts of augmentation can produce dramatic increases in static thrust levels without the use of additional fuel. This does dramatically improve the TSFC -- but only while the engine is at rest.

As soon as you start moving, the effect of the augmentors rapidly reduces and the TSFC rises accordingly. At reasonable speeds (for a pulsejet) the augmentors offer no improvement in thrust or TSFC over the core engine -- which is probably why I've yet to see an augmentor used on *any* pulsejet-powered aircraft.

I seem to recall also that when the NACA did work on the Argus after WW2, they were making all sorts of claims about getting the TSFC down to around 2.0 or maybe a little less.

I know that my big LH sucks so much fuel that it now triggers the excess-flow valve in the liquid-draw propane tank! I haven't measured the actual thrust yet but it appears as if it's consuming around about 10lbs of fuel per minute which, if it's generating the estimated 200lbs of thrust, comes to the almost benchmark TSFC of 3.0

In practice, it seems awfully hard for conventional pulsejet engines to get below that figure and I have a feeling that it's very much linked to the relatively fixed rate of deflagration in a stoichiometric air/fuel mixture that is under minimal compression.

Have you measured the TSFC of your bare Kentfield engine? Even if you can't measure the thrust because you haven't fitted the flow-rectifiers, a fuel-burn rate would be useful because you could relate that to the theoretical thrust for that particular engine.

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Post by hinote » Mon Oct 27, 2003 3:46 pm

Bruce wrote: Have you measured the TSFC of your bare Kentfield engine? Even if you can't measure the thrust because you haven't fitted the flow-rectifiers, a fuel-burn rate would be useful because you could relate that to the theoretical thrust for that particular engine.
I haven't been to do a whole lot with pulsejets lately--life's been getting in the way!

After the last test runs I decided that a lot of things were going to need changing, so I might as well do them all at once. For example, the flat plate at the front of the engine isn't flat anymore--it's bowed out from the pressure and heat created by the running engine; so, I replaced it with much heavier material.

While things are apart I decided to add the liquid fuel delivery system. And, I had time to build the recuperators--and to build a low-friction thrust trolley.

I should be able to do accurate thrust tests soon, and correlate the fuel flow to get TSFC numbers.

Bill H.

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