Subsonic Ramjet Design Project

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SlapHazard
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Subsonic Ramjet Design Project

Post by SlapHazard » Fri Aug 17, 2018 4:01 am

Hi!
For the past few months, I've been developing a low-speed ramjet engine design. The design itself is mostly original- it isn't by any means a typical "coffee mug" style engine, as you can see in the image below.
RJ2-Views.png
Now, before you tell me I've got scramjets on the brain, let me explain the simple reason for its shape- I don't have the tools necessary to create metal tubes from sheets. This issue had me abandon the project for a while, until I came to the realisation that if I can't make a tube, I might as well make a square. The fuel sprayers are situated at the "squeeze point" between the inlet and diffuser, the flame holder is visible in the wireframe figure and the spark plug is situated not far behind it.

I'm looking for some thoughts and criticism on the design from people who know more about this than me. At the moment, one thing in particular has me at a bit of an impasse. I've been revising the inlet to improve the effectiveness of the diffuser, and I can see two options here- either I remove the inlet scoop and slightly increase the area of the squeeze point that is now station 1, or I decrease the size of both the inlet and the squeeze point.
You can see what the "scoopless" design without the flared intake would look like below.
Scoopless.PNG
Scoopless.PNG (9.51 KiB) Viewed 245 times
Advice on anything else in the design that needs changing (I'm sure a lot of it does) is very much appreciated.
Cheers!

=======EDIT=======
This is what one of the above options looks like:
front:
front.PNG
front.PNG (1.98 KiB) Viewed 245 times
side:
side.PNG
(7.74 KiB) Not downloaded yet

SlapHazard
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Re: Subsonic Ramjet Design Project

Post by SlapHazard » Sat Aug 18, 2018 9:28 am

diagram.png
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Here is a cut-away diagrammatic view of the engine's latest iteration, with the fuel injection, flame holder, spark plug and cooling duct visible.

SlapHazard
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Re: Subsonic Ramjet Design Project

Post by SlapHazard » Thu Aug 23, 2018 5:35 am

diffuser2.PNG
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This is the latest iteration of the design.

Mike Everman
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Re: Subsonic Ramjet Design Project

Post by Mike Everman » Thu Aug 23, 2018 2:10 pm

Better. That last one I thought had too steep a diffuser.
Mike
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SlapHazard
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Re: Subsonic Ramjet Design Project

Post by SlapHazard » Fri Aug 24, 2018 11:11 am

What else would you suggest I change?

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Re: Subsonic Ramjet Design Project

Post by Mike Everman » Fri Aug 24, 2018 11:47 pm

I don't know much about them, though I do know that getting a flow to stick to a wall that diverges more than 30 half angle would be a problem. Flat walls will need to be a honeycomb panel or otherwise greatly stiffened. You will be fighting drum head modes of vibration in flat walls.
Mike
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SlapHazard
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Re: Subsonic Ramjet Design Project

Post by SlapHazard » Mon Aug 27, 2018 1:19 pm

Thanks for the tip on that. What do you think I should change about the shape of the engine duct itself?

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Re: Subsonic Ramjet Design Project

Post by Mike Everman » Mon Aug 27, 2018 1:55 pm

IDK. I know it’s not a guessing or cut and try game though. There are governing gas dynamic principles you need to observe. Ones I do not know and have not researched.
Mike
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SlapHazard
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Re: Subsonic Ramjet Design Project

Post by SlapHazard » Tue Aug 28, 2018 12:40 am

I should mention that there are some minor changes I have made which aren't shown in the recent diagrams- the flameholder gutters are now, from a top-down plan view, 1cm*1cm and have an angle of 90 degrees.
fh.PNG
fh.PNG (3.71 KiB) Viewed 197 times
Does this seem alright to you?

SlapHazard
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Re: Subsonic Ramjet Design Project

Post by SlapHazard » Sat Sep 01, 2018 8:09 am

Also, I should point out that I'm planning to use 1mm thick mild steel for construction, the engine is about 10cm tall and 62cm long. I've known the material to be very stiff when cut into dimensions like this, even more so when welded to other parts. Do you think this would mitigate the issue of drum head vibration?

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Re: Subsonic Ramjet Design Project

Post by Mike Everman » Thu Sep 06, 2018 2:20 pm

Wouldn’t hurt to put an X rib on any big areas. You will wish you’d spent just a bit more on 304 or 321. I llok at some of my prototypes and wish I’d let go of mild steel much earlier.

I’ve given the rectangular architecture a lot of thought and modeling, but no gas dynamic, stress or fatigue work, some of which is beyond me.

Check some of my early posts re: javfe to see my rectangular pulsejet work.
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roland2001
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Re: Subsonic Ramjet Design Project

Post by roland2001 » Mon Sep 10, 2018 2:44 pm

Slap Hazard, am going to chime in here with just a few questions and comments....
1 what are the areas of the inlet and the nozzle?
2 what is the area of your flame holder?
3 where and how are you injecting you fuel?
4 what are you going to use for fuel?
5 as Mike pointed out for a rectagular design you are going to require some support structure...why not use a square? (I think it would require less support.

comments
1 for a subsonic design "normal shock inlet", the angle of the diffuser should be 10 degrees or less
2 you'll what to know what your fuel/air ratio is, and howmuch volume is required to burn it.

roland2001 aka Kevin feel free to contact me by email kylebear2000@hotmail.com

SlapHazard
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Re: Subsonic Ramjet Design Project

Post by SlapHazard » Tue Sep 11, 2018 5:40 am

Hi, thanks for your interest!
As for your questions, I can tell you that:
  • The area of the inlet is 19.6 cm^2 and the area of the nozzle is 28.9 cm^2
  • I'm a little unsure as to what you mean here, but the total area taken up by the gutters (including the empty space behind them) is about 6.9 cm^3- each gutter is 1.4 cm wide with an angle of 90 degrees, and has the same height as the combustion chamber (8.5 cm)
  • Refer to one of the earlier diagrams for the injector locations- they're a little bit past the "bend" immediately after the inlet
  • I was originally planning to use butane for the fuel, as this would be cheap, light, efficient, low-emission and would eliminate the need for a fuel pump, but I'm thinking kerosene may be better here. For this design, a big part of the choice comes down to the combustion temperature.
  • Again, I'm not quite sure what you mean by this. The reason the engine is 10 cm tall and about 8.5 cm wide is because of the second duct above the combustion chamber- by choosing these measurements, I have ensured that the combustion chamber is as wide as it is tall. If this isn't the answer you're after, what do you mean by "using a square"?
As for your comments:
  • By diffuser angle, are you referring to the slope angle of the walls (assuming the diffuser is vertically symmetrical) or the angle made by the two walls? As you can see from the latest diagram, the "bend" occurs a bit behind the inlet. I'm thinking it might be a bit easier to calculate and adjust the angle if I move it forward so it's in line with the inlet. How do you think this would impact the inlet's performance?
  • I'll be able to work this out once I know which fuel I'm using.

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Re: Subsonic Ramjet Design Project

Post by SlapHazard » Fri Sep 14, 2018 5:16 am

Aside from what you can see in the attached image, I have also altered the flameholder by slightly increasing the width of each gutter and increasing the length to equal the width. The angle of each gutter is now 45 degrees. The cooling duct, doubling as a means by which to attach the engine to a pylon, now has an open forward intake. The angle of the diffuser is approximately 9.62 degrees.
v7.PNG
v7.PNG (10.17 KiB) Viewed 101 times
v7-side.PNG
v7-side.PNG (7.32 KiB) Viewed 101 times
v7-front.PNG
v7-front.PNG (2.97 KiB) Viewed 101 times
How should I go about determining the length of the nozzle and the combustion chamber?

SlapHazard
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Re: Subsonic Ramjet Design Project

Post by SlapHazard » Mon Sep 17, 2018 2:30 am

Quick update: according to the resource on ramjets, the length of the combustion chamber should be about 3 times the diameter of the inlet. From its area I calculated the inlet's effective diameter (the diameter of a circle with the same area) and set the combustion chamber's length to be 3 times this, as you can see below:
v8side.PNG
v8side.PNG (5.49 KiB) Viewed 67 times
My only concern is that the angle of the nozzle may be too great. Advice on these two factors would be greatly appreciated

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