"Fat Boy" 12/118 thrust engine

[Johansson]

Great to see some progress in this thread again, reprofiling the compressor wheel will certainly make a difference and with some starting air to help it up to revs I really hope you´ll get her over the "hump".

[racket]

Hi Guys

Whilst waiting for the new 1/2" evap tubing to arrive I setup a rig to test designs , I hooked up the outlet from the workshop vac , it produces ~1 psi of "total" pressure , to some fittings that allowed an evap tube to be screwed up against another fitting that included one of my bent tip syringe injectors which was supplied with water/"fuel" from a small garden sprayer that can be pumped up to increase flow rates/pressures.


With one of my current evap tubes installed , all I got from the evap outlet was a very fine mist spray with only a small amount of "dribbles" from the inside wall, it would appear that the current setup isn't allowing evaporation of the fuel once P2 were high enough to produce high velocity air through the evap .

If we assume a 5% pressure drop across the FT , then my vacs 1 psi would sorta replicate a 20 psi P2 ............or getting close to my 2.5 :1 PR where the problems begin .

Even with a plain walled evap without the "waist" or side wall indentations the fine mist just blows straight through.



I've started experimenting with a coil of 1/16" dia Inco wire fitted to the evap ID , it produces plenty of dribbles from the wall with no fine mist , just a case of making different designs of "spring" to get the best outcome , heres a pic of #18 test piece , its getting close :-)



Cheers
John

[madrocketscientist]

Hi John,

Have you tried heating the outside of the tube with a blowtorch at the same time? I wonder if the fuel boiling as it runs along the evaporator would push more fuel into the center of the tube than intended? Thinking along the lines of when water is dribbled on a red hot plate.

If you really wanted a fireball you could try the test with fuel (or maybe don't try this idea )

Shannon.

[racket]

Hi Shannon

With the "rifling" in the evap tube theres a good swirl action taking place , the spray of droplets exiting tube do so in a 60 - 80 angle , this air swirl should also aid mixing of vapourised fuel and air coming in through the flametube wall holes .

The old system with fine "atomised" droplets from the bent tip injector being discharged axially from the evap isn't very good as they impact the flametube front wall and end up dribbling down it in large droplets exacerbating evaporation.

I was surprised by the ability of a 1 psi airflow being able to prevent the fan spray from the injector supplied with 20 plus psi of "fuel" pressure , from impacting the ID of the evap only 3 or 4 mm away from the injector , with the evap tube removed but with the air and water still flowing , the fuel spray was contained into only a few millimeter sized area of highly atomised droplets which were simply being blown straight down the middle of the tube, so even if the injector discharged directly against the wall , I doubt that the fluid would spread circumferentially very far before exiting the tube .

With the "rifling" in place , there is sufficient swirl produced in the airflow within the tube for it to centrifuge the atomised fluid against the wall .

LOL, I could replace the water with LPG for some flamethrower type pyrotechnics ;-)

Cheers
John

[enginewhisperer]

hmmm, that's interesting and could explain some of the issues with the engine.

I wonder if this happens a bit in other engines' evaporator setups too, limiting their power and efficiency a bit.


Could you make the start of the evaporator tube have a step like an afterburner, and inject the fuel right in the corner near the inlet, or even have a short section with a double wall, so the fuel can be pumped into the gap between walls - hopefully staying liquid and spreading out along the outer wall for a decent length?

[racket]

Hi Andrew

Yep , I'm certainly not finished with experimenting, it really was a shock to see all that nice finely atomised spray getting blown straight out of the tube without contacting the wall , perhaps with the smaller diameter evaps its not such a problem , but its certainly made me rethink a few things as well as question some of the established practices.

The smaller diameters ones often have a bend which would then force the fuel to the wall

My original thoughts with the bent tip injector was that it would make the fuel hit the wall more at a right angle and combined with the fan shape it would cover more of the inner surface quicker, but its probably only doing that at modest pressure ratios , once the pressure drop across the flametube increases and the air gets denser it has more energy to disrupt the fuel delivery and evaporation rate.

Lots to think about, but it gives me hope that the aberrations that were occurring might just have been caused by lack of fuel vapourisation

Cheers
John

[Johansson]

Very interesting find about the syringe spray pattern, I have more or less the same design in my JU-02 but so far I haven´t seen any signs of temps running away at higher P2´s. The next couple of tests should bring the engine up higher in revs which should show if my fuel system suffer from the same problem as yours.

Keep up the good work John!

[ripp]

Feb 6, 2018 20:29:08 GMT -5 racket said:

Hi Guys

Whilst waiting for the new 1/2" evap tubing to arrive I setup a rig to test designs , I hooked up the outlet from the workshop vac , it produces ~1 psi of "total" pressure , to some fittings that allowed an evap tube to be screwed up against another fitting that included one of my bent tip syringe injectors which was supplied with water/"fuel" from a small garden sprayer that can be pumped up to increase flow rates/pressures.


With one of my current evap tubes installed , all I got from the evap outlet was a very fine mist spray with only a small amount of "dribbles" from the inside wall, it would appear that the current setup isn't allowing evaporation of the fuel once P2 were high enough to produce high velocity air through the evap .

If we assume a 5% pressure drop across the FT , then my vacs 1 psi would sorta replicate a 20 psi P2 ............or getting close to my 2.5 :1 PR where the problems begin .

Even with a plain walled evap without the "waist" or side wall indentations the fine mist just blows straight through.



I've started experimenting with a coil of 1/16" dia Inco wire fitted to the evap ID , it produces plenty of dribbles from the wall with no fine mist , just a case of making different designs of "spring" to get the best outcome , heres a pic of #18 test piece , its getting close :-)



Cheers
John

Hi John,

These Sticks seem to work very well





www.youtube.com/watch?v=gNHb2ZvhebE

Cheers

Ralph

translate.google.com

[racket]

Hi Ralph

How did you make those ?

My engine really needed an inbetween diameter , the 3/8" dia tube was too small and the 1/2" is too big , but I feel with the wire spring inside it brings the flow area back to a reasonable size.

Theres certainly a need for something "extra" rather than just a plain tubing .

Cheers
John

[racket]

Hi Anders

I think with your 3/8" dia tubes and your fairly deep indentations , theres sufficient turbulence within the tube for the fuel spray to impact the wall .

With my 1/2" tubing , I had to put a waist in it to reduce flow area a bit , that waist then discharged a "core" of high speed air that didn't get affected by my "smaller" indentations to the same degree as yours , the addition of the "rifling" wire reduces the "see through" bore to about what the waist produced , but the incoming air flows in through the full inside diameter of the tube and is forced to swirl by the wire spring , the swirl is then centrifuging the fuel to the wall ..............heh heh , thats my theory ;-)

Its starting to make sense now , why I suddenly needed a lot more fuel pressure/flow as I got passed that 1 bar P2 , it simply wasn't being evaporated and burnt efficiently , probably a fair percentage dribbled to the bottom of the flametube and was what caused the very large differences in temperature between my two thermocouples , the "hotter" one was probably picking up the temp from that dribbled fuel being burnt .

More experiments today

Cheers
John

[Johansson]

Feb 7, 2018 16:27:53 GMT -5 racket said:

Hi Anders

I think with your 3/8" dia tubes and your fairly deep indentations , theres sufficient turbulence within the tube for the fuel spray to impact the wall .

With my 1/2" tubing , I had to put a waist in it to reduce flow area a bit , that waist then discharged a "core" of high speed air that didn't get affected by my "smaller" indentations to the same degree as yours , the addition of the "rifling" wire reduces the "see through" bore to about what the waist produced , but the incoming air flows in through the full inside diameter of the tube and is forced to swirl by the wire spring , the swirl is then centrifuging the fuel to the wall ..............heh heh , thats my theory ;-)

Its starting to make sense now , why I suddenly needed a lot more fuel pressure/flow as I got passed that 1 bar P2 , it simply wasn't being evaporated and burnt efficiently , probably a fair percentage dribbled to the bottom of the flametube and was what caused the very large differences in temperature between my two thermocouples , the "hotter" one was probably picking up the temp from that dribbled fuel being burnt .

More experiments today

Cheers
John

That sounds like a very plausible explanation, if the evaps suddenly stop working the engine behavior suddenly makes sense.

[racket]

Hi Anders

Latest developments .................went back to my original design of a roughly 20 mm pitch on the spring , so around 5 turns going down the evap tube , this gave best air swirl exiting the tube but not the best distribution of fuel .

So started thinking of ways to "leak" fuel under the spring coils at regular intervals ,........... grinding flats on the OD of the spring sorta gave what I wanted but it caused "fragility" of the spring , the last thing I need to worry about is bits breaking off and ending up in the turb wheel .

Back to sitting in the chair for a rethink , finally came up with twisting the Inco wire around a length of 6mm key steel instead of the 8 mm dia rod I was using , the key steel produced nice big flow passageways under the spring , but looks like it might reduce the swirl effectiveness, I'll do some more testing with the vac and water injection tomorrow to check whats actually happening , if swirl is reduced I'll try bending the wire around a hexagonal section to reduce the size of the leakage path but improve the swirl .

I'm getting closer to a solution I feel .................If it works , I'll patent it ;-)

Cheers
John

[ripp]

Hi John,

www.rc-network.de/forum/showthread.php/47424-unerkl%C3%A4rliche-Hotspots/page3

www.rc-network.de/forum/showthread.php/47424-unerkl%C3%A4rliche-Hotspots/page4

necessarily translate and read!

hope this helps
Ralph

translate.google.at

[racket]

Hi Ralph

Thank you for the Link , yes a big help.

Cheers
John

[racket]

Hi Guys

Well I've settled on a design after doing some more testing with the "square spring" , the initial one bent around a length of 6 mm square key steel gave good swirl , but I didn't like having to use the sharp edge of the key steel to bend over , so opted for length of 1/4" key steel and filed the sharp edges to produce the required 8 mm across the diagonal which when the spring is made produces a nice snug fit in the tube ID .

The newest design produces a slightly smaller gap under the wire but a nicer bend on the spring .

I tested the setup using both the vac cleaner air with syringe injection and with the unit fitted to the garden hose for water only to test swirl discharge and its effects, both were satisfactory.

Just gotta wait for the new tubing to arrive

Cheers
John