Calculations... double checking with the pro's

[thecheat]

Hey! (boy I haven't been on here in a while!)

Ok, well, since I could never seem to get that pipe pulse-jet running I decided to switch and try a ramjet (the pipe pulsejet was a bit... ah... loud for those pets in the yard, and we didn't enjoy calling the neighbors either to tell them we were all right).

So anyways I was reading the home page for this site on the ramjets and worked out some calculations for a jet.

Ok, I started with the CC as I've got some tubing perfect for a small ramjet (don't ask how and why the empty propane bottle went missing :roll: Yes, it was safely dismantled)

so Here're the measurements I got: (all measurements in CM)

Diffuser entrance (dia) 3.84
Diffuser entrance (Area) 11.57

Diffuser Exit (dia) 7.18
Diffuser Exit (area) 40.49

Diffuser legnth 19.09

CC legnth 1.52

Nozzle entrance, same as diffuser's

Nozzle exit (dia) 4.54
Nozzle exit (area) 16.18

Nozzle legnth 2.85


Now, when preforming the diffuser measurements I used a TOTAL angle of 10 degrees, was this correct? or was it supposed to be 20 degrees, 10 for each side of the cone?

Anyways, any help is greatly appreciated!

PS: isn't there some sim software someone made to check how stuff like this would work?

[Najm]

Try this software for checking out how your ramjet works...I don't know how reliable the figures are but........ they can be tested
http://www.alt-accel.com/rjsc.zip

[thecheat]

Hey, I downloaded the program... but... I really don't have a clue how to use it... :roll:

What I really need is for someone to check my measurements and see if they're even close to right!

currently the diffuser is nearly 2x the legnth of the CC....

[thecheat]

Oh, and I'm trying to figure out what a "curved insert" is. Since some of the pictures on the main page where the ramjet info is are corrupted... it makes it a little harder.

I took a look at "steve's TLAR ramjet" thread and saw his ramjet. As far as I know I just saw a flameholder, and an empty tube (filled with flame) No curved insert, and no REALLY long diffuser, actually, I think the nozzle looked longer.


Also, when looking at diffuser legnth problems I checked again on the "ramjets without a curved insert should have a diffuser angle of no less than 10 degrees" Is this just ONE side of the diffuser, or both? Once again with a total angle of that I get a diffuser that's 19cm long, with half the angle 10 degrees I get a much more "normal" looking jet, the diffuser only being about 9.5cm (the cc is 11.5 cm long)

PS: also, what's a good angle/legnth for the nozzle? one half the legnth of the diffuser? 40 degrees total? Too short is a problem, too long is probably one too.

[Najm]

10 degrees is the total angle of two sides of the diffuser.
I dunno about the nozzle .... someone else might reply to that one , but the cone angle shouldn't be too large because then thrust will get reduced , as hot metal has more resistance than cold metal to flowing air.( or maybe it was hot air and cold air?)
Are you making your ramjet for propelling something?

[Irvine.J]

On the homepage of this site, if you click on ramjet and look at figure 4...
http://pulse-jets.com/ramjet/fig4.jpg
It says your cone angle should not exceed 10* without insert.
I guess that means 10*, but you'd better check with larry or steve on that one, I went slightly less then 10* and suffered performance wise for it I believe.

[thecheat]

Hey, but your cone was a LOT shorter than your CC (or atleast is seemed to be!) with mine I tried 10 degrees total angle (both sides of cone) and the diffuser would be approximatly 19cm, the cc being 11.5cm the nozzle (I did some adjusting) being 3.5cm long...

with only one side of the cone being 10 degrees, the diffuser is only 9.5cm long.

Also I checked the measurements of the example on the main page and it has a diffuser 6 inches long... Actually, the measurements on mine and the example one are nearly the same, the only difference being units, but then why is my diffuser so much longer? Also, the angle being split in half does kinda confuse me, even though it points to the middle, I still can't help but wonder if they just meant one side of it or not.

Najm, you're right, hot metal has more resistance to air, and, I believe hot air also has more resistance than cold air (not sure on that).

Also, I really wasn't thinking all too hard about propelling something... I've toyed about converting a scooter or something, (an electric one, no batterys though, only propane, or MAYBE kerosene. Why?

PS: my plan is to take a squirrel cage fan (perhaps from a blower) and mount it directly onto the Jet, with Venturi ducts of course, and make a simple, jet engine, powering (like I said) a scooter. The pulsejets are just too bulky... and loud. I prefer not to have cops come while I'm starting the engine. (the PJ sounded too much like gunshots when we were messing with it)

[thecheat]

Ok, work is coming along... I've just nearly completed the flame holder, but, I decided to go with something TOTALLY different from most of 'em. I scrapped the idea of the "strainer" (too much interferrance with airflow at the walls in my opinion) No cone, nor asterik (That one just seems to cut off WAY too much airflow) Instead I'm going with a doughnut shape. (see picture one)

First off, do I have enough support for the "doughnut"? I'm only using four struts, but, haven't cut further incase someone here says that's not enough blockage, not enough support, ECT.

Also, since it's curved should it be pointing towards the nozzle? or the diffuser? Also, where in the CC should I put it?

PS: sorry if I'm asking alot of questions, but, I just prefer to get stuff right before I weld it.

[Irvine.J]

Mate I like this idea alot, Infact I had thought of one very similar myself! I'm not a professional, so wait till larry / eric /steve confirm this before going ahead. First, look at the diagrams for where the flameholder should be in the ramjet pictures. It is mathematically dependant on your engine size, however it should be right at where the inlet nozzle meets the CC.

I think which side the curve should be facing is difficult to ascertain, I persume the curve should be facing the direction of air flow (As in the curve should face the front of the engine.) yet I think both directions would work, I couldn't tell you which one would work best.

I think with this flameholder your defuser placement will be a big factor. Steve's TLAR had it facing the injector plate from behind, but in your case this will not work. (As there is a big hole in the centre)
If i recall correctly Larry said that the defuser nozzle should be infront of the Flameholder, injecting toward the CC (in your case there is a hole in the middle so you wont get alot of mixing). Perhaps make a nozzle with more then 1 exit point, pointing toward the FH and out towards the walls, i think with 2/4 you'd get far better mixing before combustion.

In regards to using 4 attachpoints, try it, if it fails and destroys itself, you can always make another, though i think 4 shall be sufficient.

Can't wait to see it run, make sure you have a good blower as well, the better the blower/airflow, the more thrust it will give.
When I was starting my ramjet, I gave it too much gas, and got a big bunsen, be careful, and larry told me to make sure i keep my hand on the 1/4 turn gas valve, because if the blower stops, its a huge fireball. <- This did happen to me.

Also to get it going initially, dont necissarily have the airflow at full belt, in the video PART II, i had to light the back end, push more air, and then drop the airflow... this enabled the flame to race back into the CC, and begin combustion properly (You'll know when its working properly! Mine went BOOM!) Both the bunsen effect and boom happens on the video PART II on the thread I posted after your reply to my UGLY thread :D. Best of luck, hope I've been of some help.
Best thing I think is to pioneer it, give it a go, wear flame resistant clothing, and most of all HAVE FUN! Report all your developments/findings, can't wait to see what you have achieved.
Regards,
#47 If Larry/steve/eric/bruno/anders tear my opinions apart on this one, listen to them, they have far more experience then I, and their advice has been A1 to me from the start.
This is Part II i was talking about.
http://www.metacafe.com/watch/219606/mangy_jet_part_2/

[thecheat]

Hmm... actually, I really do want larry or one of the "hard core" gang to say SOMETHING is wrong... but they're not.... soooo I think I'm going to just go ahead as planned, 10 degrees being only half the sum of both diffuser angles.

Also, I just finished completing the flameholder, and decided that since the whole peice is bowl shaped (actually it's the bottom of the propane tank...) so it's got some structural stregnth already. As for mixing... hmm... I really didn't think my shape would prevent mixing... actually, I kinda designed it so gas (mixed) would flow through the center and around the edges for a nice beutiful flame... (designed/guesstimated... same difference!) Ok, but I'm taking note of your concern, I'll make SURE to have a GOOD mixing system worked out for the propane.

Also, I've wondered about some spinning of gasses, and wether that's desirable or not... It wouldn't be hard to incorperate some vanes on the flameholder...

Last of all, can you just solder something on the tip of your feed line (it's being cooled by the cold onrushing air) or does it still get hot enough to pose a problem so only threads can be used?

[Irvine.J]

Hmm... actually, I really do want larry or one of the "hard core" gang to say SOMETHING is wrong... but they're not....
Last of all, can you just solder something on the tip of your feed line (it's being cooled by the cold onrushing air) or does it still get hot enough to pose a problem so only threads can be used?

This is possibly because NOTHING is wrong :D
Your engine looks good! They might be on holiday or something, but I'm sure the pro's will offer good advice as soon as they can.
If your fuel diffuser is outside the CC, then silver solder should be fine, My final SS joints were outside the CC, and they have held up fine. I believe they would have melted very quickly if I had any soldered joints withing the combustion chamber itself. Hope that helps.
I wouldn't worry about vanes in your ramjet just yet, some PJ's operate with spinning air, however it may not be much benifit in a ramjet...you can always add them later, but do it if you want to, I'd be interested in your findings.Goodluck.

[thecheat]

well... ok... guess it's time to go cut some metal!

[larry cottrill]

cheat -

As Ben has rightly indicated, I (and most others here) are amateurs. We are lucky to still have all our fingers and facial hair.

I think your cut-up half donut looks fine. It is unusual, but makes good sense (as does the domed asterisk design Steve used). I think the area is just about right.

If your diffuser angle is around ten degrees (the FULL angle, not the half angle), that is also good. It does seem to me that it is a bit long; i.e. the outlet-to-inlet areas are in a pretty high ratio. This would be fine if you can get a high blower speed into it (say 300 MPH or something), but if you're just using a typical leaf blower, I don't think it will quite perform the way you expect and then you'll want to lower the ratio. That can come later, after your initial tests; maybe I'm just ... wrong, and it will work great. Lowering the ratio would just be a matter of gradually cutting the front end inlet back - a simple method (UNTIL you realize you've gone too far ;-)

Anyway, I think the basic idea looks really good.

On a ramjet, ALWAYS provide a "quick cutoff" fuel valve at your propane supply, and keep one hand close to it while testing. You simply never know when an air supply failure might occur, and suddenly at that!

Good luck!

L Cottrill

[thecheat]

Finally! Heh, though you're an "amateur" I know a good deal less.... (so what would that classify me as?)

Anyways, I was just wondering, wouldn't cutting off the cone in the front cause problems as now you've got a larger opening? I guess it wouldn't matter as it's the "same" amount of air being supplied by the blower... ok, well I also made my first mistake with the cutting... trying to make a cone with a cylinder doesn't work with one slice! Fortunatly I've got a second cylinder beside me that's being marked for cutting tommorow, I plan to make eight incisions, cutting out triangles, then, weld 'em up for a cone.

PS: there any formula whatsoever or anyway to design cones, then, slice the side open and lay it flat? I may need to get some sheetmetal and knowing how to cut it would be greatly obliged.

[snakeye]

Yes, you can calculate the cone with some standart calculations...
but your way is good too I think. If you need help with the calculations let me know.