Catastrophic failure

Post by **Mike Everman** » Wed Nov 19, 2003 2:14 am

My bad, I was scanning through an array of "Incolloy" alloys, which were all around 10; I assumed.... beggin your pardon.
My reference has Inconel 625 at 7.1 (room), 8.8 @1000F.
800HT, 825, 600 and 601 grades are all hovering around 8@room, 9-9.6@1000F.
This all begs the question: "How hot is it getting?"

Bruce · Post by **Bruce** » Wed Nov 19, 2003 2:27 am

> As I recall the Dynajet
> instructions recommend not
> running their engine static for
> more than a few seconds. As
> light as the material thickness
> is, it will nearly melt down from
> its own heat.

Actually, if the dynajet body is made from stainless then I doubt this is the actual reason.

It's simply not possible to generate a sufficiently high temperature (given the surface/area to volume ratio of the little dynajet and the flame-heat of an air/gasoline flame) to melt stainless.

The *real* reason users were warned against static running for more than a few seconds is that the aluminum valve retainer would melt.

I've static- run small 8lbs-15lbs stainless-bodied pulsejets for continuous periods of nearly 30 minutes (a full 9Kg tank of propane) at a time and the bodies, although glowing a nice bright red, have never melted or othewise failed.

However, if you're using a conventional aluminum valve retainer, more than about 30 seconds of static running will produce a melt-down.

In order to get these long static-running times I was using either the blast-ring or my latest two-layer spun-stainless retainer/injector system.

I have a nice collection of melted and fractured aluminum retainers that were used without the ring :-)

hinote · Post by **hinote** » Wed Nov 19, 2003 3:10 am

Bruce wrote:
Some of the Inconel Incoloy alloys are specifically designed to have low COTEs -- something very important in the jet-engine industry.

It MIGHT be important for us PJ guys too--don't you think?

Bill H.
Acoustic Propulsion Concepts, Inc.

hinote · Post by **hinote** » Wed Nov 19, 2003 3:13 am

Mike Everman wrote: This all begs the question: "How hot is it getting?"

Not only that, but WHERE is it getting hot?

Bill H.
Acoustic Propulsion Concepts, Inc.

hinote · Post by **hinote** » Wed Nov 19, 2003 3:21 am

hinote wrote: Not only that, but WHERE is it getting hot?

One of the things that I find interesting is the location of what I call the "hot zone"--indicating the location of the most active combustion area.

For example, the classic L-H shows its maximum color at the back of the "combustion chamber", and down into the transition cone and the smallest part of the megaphone.

OTOH the 4-tube Kentfield shows its max temp at the FRONT of the "combustion chamber", and in the flat front plate the the intake tubes are mounted on.

Does this signify a different breathing efficiency between the configs?

Bill H.
Acoustic Propulsion Concepts, Inc.

Post by **Mike Everman** » Wed Nov 19, 2003 3:43 am

for what it's worth, and assuming that the aluminum is like 6061-T6, it melts at 1080-1210F, so Bruce's valved types at least are getting that hot at the head.

bill h wrote:For example, the classic L-H shows its maximum color at the back of the "combustion chamber", and down into the transition cone and the smallest part of the megaphone.

OTOH the 4-tube Kentfield shows its max temp at the FRONT of the "combustion chamber", and in the flat front plate the the intake tubes are mounted on.

And in my extreme ignorance, I would have assumed that you guys were tuning exhaust tube and intake tube lengths so that the hot spot was centered on the length of the CC... But I'm sure now that it is much, much more than that simple.

hinote · Post by **hinote** » Wed Nov 19, 2003 4:15 am

Mike Everman wrote:
And in my extreme ignorance, I would have assumed that you guys were tuning exhaust tube and intake tube lengths so that the hot spot was centered on the length of the CC... But I'm sure now that it is much, much more than that simple.

I agree.

Actually, what I'm trying to get to here is, that I think the 4-tube front end may be more efficient--and the evidence is the forward position of the primary combustion zone.

I have a theory (and it's only THAT!) which is, the smaller tubes are feeding fresh air/fuel mixture to the boundary layer of the "combustion chamber". This allows better filling of the appropriate area and allows the returning charge coming back up the tailpipe to fill the center of the "combustion chamber" with the compression charge.

That means the new, ignited charge blows inward--creating a more efficient combustion shape and increasing efficiency.

Any opinions?

Bill H.
Acoustic Propulsion Concepts, Inc.

Bruce · Post by **Bruce** » Wed Nov 19, 2003 5:06 am

Don't make the mistake of assuming that the hotest part of the engine is where the most combustion is occuring -- it's simply where the most heat is being transferred from the combustion process to the engine itself.

Look at a lockwood and you'll see the area where there is the most surface-area for a given cross-section is the narrowest section of tailpipe -- where it joins the combustion chamber.

Hot gases passing through there will end up imparting a *lot* more of their heat to the metal of the engine there than gases of exactly the same temperature would in the chamber itself.

Also remember that on the LH, the incoming fresh air charge dramatically cools the front half of the CC once per cycle -- in fact just look at how cool the intake tube runs compared to the exhaust tube and you'll see that, even when you run the engine so that actual flames are coming out the intake, it never glows red hot -- in fact the stainless doesn't even get hot enough to discolor.

I'd say that the reason the Lockwood glows more at the back than the front is a combination of these factors.

So why does the Kentfield glow at the front?

Probably because, unlike the LH, there's a very sharp transition from the intake tubes to the CC. The incoming cold air charge will probably travel far deeper into the chamber before it becomes turbulent enough to impinge on the chamber walls.

This means that the front half of the chamber wall gets very little cooling from the incoming airflow and thus remains hot.

I doubt it has anything at all to do with where combustion is actually occurring.

By the way, are you getting the rated thrust from your Kentfield? Have you measured it yet?

hinote · Post by **hinote** » Wed Nov 19, 2003 5:28 am

Bruce wrote:
So why does the Kentfield glow at the front?

Probably because, unlike the LH, there's a very sharp transition from the intake tubes to the CC. The incoming cold air charge will probably travel far deeper into the chamber before it becomes turbulent enough to impinge on the chamber walls.

EXACTLY. That's why I think it's more fuel-efficient than the standard, single-tube L-H configuration.

By the way, are you getting the rated thrust from your Kentfield? Have you measured it yet?

Haven't got that far yet. I'm (successfully) weaning the darned thing off propane. Energy level (on gasoline) is now so high that the sound pressure bounces a standard earphone-type protection right off my head.

Now, THAT'S power!

BTW, my 2nd iteration of liquid fuel delivery is considerably more refined, as well as a lot more costly. I consider fueling to be the biggest hurdle to a successful Pj project--unless you want to stick with propane, of course.

Bill H.
Acoustic Propulsion Concepts, Inc.

Bruno Ogorelec · Post by **Bruno Ogorelec** » Wed Nov 19, 2003 6:49 am

Mike Everman wrote:This all begs the question: "How hot is it getting?"

Well, here's your answer. The temperatures are in degrees Reaumur (now rarely seen). The source is a general overview of results of the legendary Project Squid.

Bruce · Post by **Bruce** » Wed Nov 19, 2003 7:20 am

Mark · Post by **Mark** » Wed Nov 19, 2003 7:38 am

brunoogorelec wrote:
Mike Everman wrote:This all begs the question: "How hot is it getting?"
Well, here's your answer. The temperatures are in degrees Reaumur (now rarely seen). The source is a general overview of results of the legendary Project Squid.

I hate it when molecules dissociate. It's bad enough dealing with metal fatigue and rupture, let alone the molecules cracking too.
Mark

Post by **Mike Everman** » Wed Nov 19, 2003 3:11 pm

I hate it when molecules dissociate. It's bad enough dealing with metal fatigue and rupture, let alone the molecules cracking too.
Mark

yeah, can't they just learn to get along?

what I tried to address in my first design was the scavenging of as much waste heat from the CC as I could, forcing it to do work by making the recuperator, while a cooling influence on the CC, expand the air thermally as well as classic augmentation. The rectangular cross-section only got me half way there, with structural problems too! :@

Axt · Post by **Axt** » Fri Nov 21, 2003 6:11 am

How heavy was the steel in the big lockwood Bruce? (got a picture?)

Mark · Post by **Mark** » Fri Nov 21, 2003 2:28 pm

Bruce wrote:> Actually, the Inconel family has
> almost exactly the same thermal
> coefficient as the 300 series
> stainless', about 9-10 ppm/degF

Where did you get that info from?

All the references I have here (and I have quite a few) claim that 300 series stainless has a COTE of around 9.4-9.9 versus Inconel 625's COTE of just 5.5.

Check the bottom of this page (the first online reference I found when searching on Google just now:

http://www.burnsstainless.com/TechArtic ... ticle.html

Grade 2 commercially pure titanium did pretty good in the low expansion realm when heated, compared to the others.
Mark