Using pulsejets as combustors?

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cudabean
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Using pulsejets as combustors?

Post by cudabean » Sat Mar 18, 2006 1:15 am

I wonder if y'all have any opinions on this idea. The idea is you use a pulsjet inside a tubojet. The turbojet would compress the intake air as it normally would but in the combustion zone you could situate six or so Chinese valvless engines or a single annular pulsejet.

Positives, as I see them, are:
1. You get the advantages of higher compression via the compressor (plus even more compression takes place in the pulsejet)
2. The gas flow from the pulsejet can be much more direct into the Nozzle guide vanes leading to the turbine.
3. Pulsating combustion

Negatives:
1. Noisy, probably
2. Pulsating combustion acting on the turbine.
3. Not sure how one might start such an engine.

Marlin

hinote
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Re: Using pulsejets as combustors?

Post by hinote » Sat Mar 18, 2006 1:31 am

cudabean wrote:I wonder if y'all have any opinions on this idea. The idea is you use a pulsjet inside a tubojet. The turbojet would compress the intake air as it normally would but in the combustion zone you could situate six or so Chinese valvless engines or a single annular pulsejet.
As a not-fully-informed PJ enthusiast, I have to hand you the golden wand. You've identified one of the most useful applications for valveless pulsejets.

However....you're about 50 years (maybe more) too late!!

This concept has been touted by quite an important handful of famous names related to pulsejets. It's a well-known way to boost the performance of a turbine jet engine.

It looks like GE (and possibly others) are finally waking up to this, too; there's evidence of considerable work on applying PDE's to this concept; if successful, they would be even more effective than VPJ's at the job.

OTOH, they would also be a whole lot more complex, too.
Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

hinote
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Re: Using pulsejets as combustors?

Post by hinote » Sat Mar 18, 2006 1:56 am

cudabean wrote: Negatives:
1. Noisy, probably
2. Pulsating combustion acting on the turbine.
3. Not sure how one might start such an engine.
You're going to get other reactions to the above.

Here's my read:

1. Noise is likely to be suppressed by the multiple combustors used--and the fact that the combustors are in the "middle" of the engine. Behind the compressor and in front of the turbine.

2. Same answer as above. The combination of multiple combustors, and the use of interconnecting ducts, etc., etc-- will certainly minimize this problem.

3. Have you started a VPJ lately? Just turn on some air feed (spin the compressor), and bring the fuel up slowly. My opinion is, this ISN'T an issue!

I don't see any big problems here.
Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

cudabean
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re: Using pulsejets as combustors?

Post by cudabean » Sat Mar 18, 2006 4:35 am

Cool. Thanks for the response.

Interconnecting ducts? Back in '92 or so I was dreaming up a pulsejet design that used multiple combustors with interconnecting ducts, but for the purpose of trying to raise the compression ratio of a pulsejet. My thoughts were to borrow the cumbustion pressure from one combustor to compress the gas of a neighboring combustor.

In the embedded pulsejet what would they propose the purpose of these ducts be for?

Marlin

Bruno Ogorelec
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Re: Using pulsejets as combustors?

Post by Bruno Ogorelec » Sat Mar 18, 2006 8:24 am

cudabean wrote:I wonder if y'all have any opinions on this idea. The idea is you use a pulsjet inside a tubojet. The turbojet would compress the intake air as it normally would but in the combustion zone you could situate six or so Chinese valvless engines or a single annular pulsejet.
You mean, something like one of these?
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50_SNECMA_pulse_turbo.jpg (38.07 KiB) Viewed 4254 times

Bruno Ogorelec
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re: Using pulsejets as combustors?

Post by Bruno Ogorelec » Sat Mar 18, 2006 8:57 am

Marlin,

Noise is less of an issue here than you might think. A turbine is an amazingly effective muffler.

The issue you don't mention, but which is among the more important in such a layout, is how to maintain combustion where you want it, namely in the combustion chamber. Blowing air into a pulsejet skews the conditions there and pushes the combustion zone towards the tailpipe. Not good.

One of the answers is to avoid linear (Lockwood-style) combustors. A Chinese or a Thermojet are probably much better prospects. My tangential intake Swirl Can combustor is another layout that suits the purpose. Rossco's inverted intake combustor might or might not work in this layout; I cannot be certain, but it's a possibility.

Another answer is to provide the same ram pressure (or velocity head) to the intake and the tailpipe. This is easier said than done. But, there are ways.

Another factor creating problems is that the pulsejet intake wants to blow as well as suck, and the tailpipe likes to suck, not just blow. You have to decouple the intake and the tailpipe from the compressor and the turbine to avoid reverse flows. Again, this is easier said than done.

The pulse-combustor turbine engine has been a dream of forward-thinking developers for about a century now, but nothing about it is simple. You have to think out of the box about every single detail.

In my opinion, ultimately, the Reynst combustor will provide the answers to all the problems, but few people cherish the prospect of developing the Reynst 'pot' to work as they would like it to.

It can be cone with more conventional combustors, too, and I have entertained quite a number of layouts and shared them with some of the people in the forum. I even started a project of building one of those layouts, but was stopped by unforseen circumstances after just one combustor was built and (successfully) tested.

This is where the future of the pulsejet lies. Without a turbine, it can be pushed a fair way along, as Bill is demonstrating with his series of quite amazing machines. Eric may also have progressed quite far along that road. But there's a barrier over which the pulsejet will only be able to jump with the help of a turbine. Cross that barrier and you will be competing with the conventional turbojets and turboshafts very easily on any parameter you like.

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