Efficiency of elbows (90s)

[willarddaniels]

I haven't visited this site for a while...

I have just about all my parts together for my propane hybrid and I am still debating whether or not I should put 2 elbows on the chamber and turn it around just before my union, to make it just a bit shorter.

My dilemma is this: won't the two 90s slow down the flamefront to the point of being very counterproductive? I plan on inserting a shelkin spiral (don't worry- it'll be fine) to increase the performance, but I would lose all of that in the 2 elbows.

I would love to cut off 18" in total length or so by wrapping it around, but I am wary of the side effects. Any experience or websites you guys could direct my way?

Preliminary specs:
*4" x 24" chamber
*3+ barrels: 4", 3", 2", ?" don't bother telling me lengths for optimal performance, my smaller ID barrels will just be less efficient; I really don't care on those.
*Sch 80 PVC (couldn't get the CPVC cheap enough)
*remote firing
*chamber fan
*stun gun ignition source
*digital pressure gauge (for peak pressures and trends)
*chrony (not attached, duh)
*additional port for optional O2 in case I want higher mixes

The only parts I don't have are the chrony, 3" male adapter and 3" union. Oh, and John Deere paint that my wife picked out.
Hopefully I can have it all built and ready to test in the next 4-6 weeks.

EDIT: Yes, this is a hybrid cannon in the combustion discussion forum, but what we are discussing is the combustion element- so quit your flaming.

[bigbob12345]

is your hybrid chamber made of pvc
if it is dont bother making it especially in such large diameter. Get some metal it will be worth it.
and the answer to your question is no it will not slow down the flame any significant amount.

[SpudUke5]

As i mentioned in another topic, Sch 80 pvc will be fine up to 4x which is the most that has been used on a sch 80 pvc hybrid, which can be seen in this video. Note: he was supposed to use 45 psi, not 60 in air pressure.

This website should help you significantly if you havent seen it already.

Personally i wouldnt want to use pvc at all because i would want to get supersonic with my hybrid i will eventually build and also it is safer.

But its your choice.

[bigbob12345]

there have been many sch80pvc failures like the one in the launcher failure analisis which failed at 2x and that pipe was only 2in this is 4in I wouldnt be surprised if it blew on the first 4x shot.

[SpudUke5]

Well its his choice, not ours, but i do agree it should be built with metal rather than pvc.

[bigbob12345]

Im just trying to save him from a violent pvc explosion that would send shrapnel at high velocitys at him peircing his skin and sending him to the emergency room
I guess that you could use the pvc as long as you never use it above 2x

[DYI]

I agree that PVC is a really poor material to make a hybrid from. In my opinion, it just isn't worth the constant worrying, and SCH 80 PVC pipe and fittings aren't a whole lot cheaper than the equivalent in steel. If you find a friendly welder, you could probably get a decent sized steel chamber made for almost nothing.hint How much did the chamber cost you anyway? I've looked at CPVC prices, and they're ridiculous for the larger sizes.

I say that if you're willing to go to the trouble of using a Schelkin spiral to improve performance, there's no point in negating that gain by putting elbows on your chamber.

[willarddaniels]

Did you guys read my post? Thank you DYI for answering my direct question and Likimy for the website. bigbob, I did not invite flaming, so no thanks.

I am using a digital gauge to ensure I do not exceed the PSI ratings. Besides PVC is still safe up to twice the rating- but those numbers come from regulatory agencies to stop us from killing ourselves. I will only be shooting ice slugs with the hybrid so all that will be really impeding my combustion is the burst disk, which will be just strong enough to hold up to whatever mix I use. Furthermore, the entire chamber will be double layered. Honed out fittings will be glued and sealed to the continuous 4" chamber pipe. 2 plugs will be used. I am thinking about wrapping the entire housing in fiberglass and sealing that as well. Didn't I mention remote firing? etc. etc. etc.

I didn't think I would have to start going over the safety precautions, I just wanted to know what expert advise folks have about the dynamics of flamefronts in a pair of 90s. Obviously, some people just like to flame on unnecessary things or answer questions that were not asked just to get their post count up.

I am done ranting... this is one reason I stopped posting here- there are too many socially underdeveloped people.

[BigGrib]

I am done ranting... this is one reason I stopped posting here- there are too many socially underdeveloped people.

And I seriously think that you are one of those people. I enjoy posting here because every single one of these people here are concerned about everyone else's safety and there are a lot of very knowledgeable people here. As far as bigbob is concerned he did answer your question if you read his post so don't go off on him. As fas as having to go over safety precautions, I personally do it because this is dangerous information we are dispensing and if we didn't put up safety precautions in our posts and a noob were to use this advice hurt himself and didn't get safety information then we could all be in trouble. Period.

As far as your double layer deal, you probably spent more on all that PVC when you could have done it right and went with metal. Wrapping the thing in fiberglass isn't gonna do a damn bit of good except make your life that much worse when the thing explodes on you. Ever had fiberglass in an open wound??

Anyway go have fun with it let us know how it works.

[DYI]

I've seen willard's posts from when he used to come here, and he knows what he's doing. That's why I wasn't b*tching about safety, just like I ask people not to do when I start ranting about detonation guns and such.

I think that fiberglass really could help if done properly, or maybe carbon fiber. They do use it on commercial pressure tanks, so it's obviously possible.

I have to ask, how have you calculated the size and shape of the Schelkin spiral so that it causes a useful increase in flame front propagation rate without creating detonations? And how much did the digital gauge cost you? I'd always thought that the analog gauges with another needle to record maximum system pressure would be more economical. Or would that not give an accurate response for the short pressure spike?

@willarddaniels: I think some of the newer members might have mistaken you for some n00b about to blow himself up because of your low post count. Hell, it even took me a while to remember who you were. I was looking into oxygen enriched launchers right around the same time you were, and was reading some of your postings about them. Seems you were gettting unneccesarily flamed just as much then as you are now...

[SpudUke5]

I remember you quite well becuase of the avatar, and i last rememebr you from a how to on casting lead.

Even though i knew you were here before me, i read your posts so i knew who you where already.

I dont post too often here because i really dont have to much to say because its already been said. I understand you reason for ranting, but hey we all make mistakes.

But if you really want power then you know you should stick with steel. Personally i want power and to exceed the speed of sound so im going to build a metal one.

[cdheller]

I plan on inserting a shelkin spiral (don't worry- it'll be fine) to increase the performance, but I would lose all of that in the 2 elbows.

I would love to cut off 18" in total length or so by wrapping it around, but I am wary of the side effects. Any experience or websites you guys could direct my way?
.

I don't know what it does to combustion.But I asked a mechanical (A.C.) guy that I was working with one time what effect 90's had on figuring refrigeration lines,and sizes.
He said ! long radius () had the same friction lose as 100 Feet on the vapor (bigger) side.
I didn't ask about liquid side or regular 90's
I don't have any reason to doupht(sp) the guy
I don't know if this helps or not
so take it for what it worth

[jimmy101]

The affects of the 90's is going to be pretty tough to estimate.

If the volume of the 90's is small relative to the chamber, and there is little volume between the 90's and the burst disk, then I wouldn't expect the 90's to have too much of an affect.

You might get a little more heat loss as the gases go around the turn but the decrease in performance is probably going to be pretty small.

If you have a large volume between the 90's and the burst disk then getting the fuel to mix around the bend might be a problem.

Overall, I would think the affect of the 90's would be about the same as they are in a pneumatic, a small but measurable decrease in performance.

I would be more worried about the loading on the 90's and the recoil not being aligned with the chamber. Definetely want to well brace the barrel to the chamber.

[willarddaniels]

Thanks for the further post gents, I really appreciate your willingness to share your expertise as well as your precautions. I will not use the 90s, for the large factors of jimmy's point of unbalanced load as well as a slightly weaker area of combustion chamber.

To answer the recurring theme of "why not metal?" I got the pvc really cheap and I am not as advanced a welder to try my hand at it yet, although I do have the equipment. Maybe in the future... I also chose PVC to make it more mobile, thus the want for a shorter yet large-bore cannon...

Digital gauge: $125 on eBay... originally a $950 unit. I have been watching eBay for over a year and have checked local and even international companies. The cheapest new one I could find is $225, most are $260 to start.

Shelkin spiral: because of the many variables, they are not exactly consistent in how they much they affect the flamefront propagation rate, so I will be testing that out with help of the digital gauge and chrony. I cannot find much literature on the topic due to its limited uses and I cannot think anyone has ever used one while NOT trying to get a DTD. Because I will be using a smaller fan to help circulate the fuel/air mix better, the placement of the fan may influence the pressure spikes in addition to the diameter and frequency of the Shelkin spiral. Experimentation... starting with 1x mixes and at a safe distance. That and other experiments with a Shelkin spiral in a pulse jet and force gauge will better give me a good idea. I hope I did not bore anybody with that.

[bigbob12345]

I did not mean to 'flame' you I was just concerned about safety
I did assume you were some n00b that I just havnt seen before that was going to blow himself up. But if you think you can use pvc safely than alright do it.