Turbo jet engine research project

[tinkerer1999]

Hello everyone, I am a mechanical engineering student working on a turbo jet engine as a research project. I have been working on it for about a year. I built a version one that would light but it never seemed to get going fast enough to sustain itself. I looked at it a little more and found that my combustion chamber was too small. So over my Christmas break I built a new combustion chamber following a video on youtube. After discovering this forum I noticed a lot of people talking about jetspecs so I found that and I put in my inducer diameter. I found that my flame tube should be 5" in diameter and 14" long. I made it 4" in diameter and 12" long although I did put all the holes in the right places and I made them the right size. Will the slightly smaller flame tube make that much of a difference in how well the engine runs? I'd rather not have to make a third combustion chamber.


The main problem I have run into is regarding lighting the engine. I decided to run it only on propane for simplicity's sake. I cannot get it to light with a leaf blower blowing through it. I have had some success lighting just the gas and then putting the leaf blower on it, but that scares the daylights out of me because about 3ft of flame shoots out the end. So I'm not to comfortable with that. And even then it will only burn for about 30s before going out. Also I have been just testing the combustion chamber without the turbo on it. 

[racket]

Hi Tinkerer

Have you read the guidelines here jetandturbineowners.proboards.com/thread/680/diy-turbines ??

Your 4 incher will be OK with propane .

LOL...........only a 3 foot flame, maybe cutback the propane a tad ;-) .........or have the leafblower close to the comp inlet blowing just a small amount of air through the engine when you attempt an ignition , once you have ignition increase the propane and seal the leafblower to the comp inlet and give it max power .

If testing the combustor ( atmspheric test) , you only need to supply sufficient fuel to produce normal T I Ts exiting the combustor , so you'll need your thermocouple/s mounted at the exit .

Is the engine flameing out at higher power settings , if so then its probably the fuel injector and/or your fuel supply cylinder which needs to be a large BBQ size without any safety devices or regulators, whilst using the cylinder valve as the throttle .

Cheers
John

[tinkerer1999]

Unfortunately I can't see any of the pictures in that thread. For some reason some of the threads do that for me.

I tried just a bit of air going through it to start with but that doesn't even seem to work.

What do you mean by T I T's?

It is not flaming out, it appears to just blow out. I am using a cutting torch regulator on the tank which I have set to about 5psi. I have been opening the tank valve all the way and using a needle valve right at the combustor as a throttle.

[racket]

Hi Tinkerer

You need to click on the various icons in each Section of the Link.

T I Ts is Turbine Inlet Temperatures .

Theres not much point having a regulator set at 5 psi , once the engine is producing 5 psi of compressor discharge pressure ( P2) there won't be any flow of fuel as the pressure drop across the injector will be 0 psi.

Remove the regulator and have an injector squirting the propane through a minimum of 6 small holes RADIALLY towards the flametube Primary Zone wall holes ..........NO axial injection of fuel !!

Those couple of changes should have your engine working :-)

Cheers
John

[tinkerer1999]

Okay thank you so much, I will have to try that this week

[tinkerer1999]

Here are some pictures of what I have so far:


ammonium sulphate nitrate formula

[tinkerer1999]

There are extra primary holes because one of my ideas was maybe there wasn't enough oxygen.

The turbo is an old T3/T4 turbo I got off ebay. I think its just a generic Chinese turbo but there isn't much play in the shaft

I am planning on welding up a nozzle where I will mount a thermocouple and an extension on the intake to mount a screen and an tachometer

If I put a screen over the intake will that create too much drag and limit the air flow?

[racket]

Hi Tinkerer

If you add an inlet screen make it fairly large to minimise obstruction to flow .

I noticed from your pics that you have a tangential air delivery tube at the Primary Zone , not a good idea as it can create an undesirable airflow pattern and losses as well as combustion problems if the fuel is only added to the centre of the flametube with not enough radial injection,.................we end up with the air spinning around a central core of fuel and not burning until well down the flametube , often only when the large Tertiary holes provide enough penetration to produce turbulence and mixing of the fuel and air ....................but with all that extra air from the Tertiary holes too much fuel needs to be added to create combustion resulting in excessive turbine temperatures

Cheers
John

[tinkerer1999]

So we first tried turning the regulator up to 15psi and it lit on the first try but we couldn't get it to repeat so we tried it at full pressure and it still won't light.

I see, I was under the impression that the tangential entrance would aid in the mixing. Maybe I could put a baffle or something to redirect the air. I also saw somewhere on here that it is good to have the flame tube run straight into the exit so the air can't bypass the holes. I think in the current configuration a lot of air could bypass the flame tube

[racket]

Hi Tinkerer

OK , firstly , what does your current fuel injector consist of ??

Secondly , all air MUST pass through the flametube wall holes , excepting some minor leaks at the slipjoint/s , from your comments it appears as though your flametube doesn't go "full length" and theres an annular gap at the bottom outlet region, is this the case ??

Thirdly , the junction of the air delivery tube and outer can needs to provide good flow characteristics .

Cheers
John

[tinkerer1999]

My current fuel injector is a 1/4npt hex pipe plug with a hole in each face.

Yes there is a gap at the end of the flame tube. I could make a 4" to 2.5" reducer to weld on the end of the flame tube to fix that.

Do you have any suggestions how to fix my entrance tube?

[racket]

How large are the 6 holes in the hex plug ??

There must be a smooth transition from the flametube cross section to the turbine scroll inlet , any "steps" will create turbulence and reduce flow , potentially preventing the engine from operating .

All I can suggest with your entrance tube is to install a "funnel" so as to spread the air delivery over a greater area and to minimise the tangential delivery of air , with highly swirling air its difficult for the air to enter the flametube holes .

Another alternative, and one which I'd prefer, is for you to remove the tangential delivery and blank off the hole, then fit the delivery tube and diffusing funnel to the "bottom" of the flametube where it can dump "straight in" into the plenum surrounding the transition funnel between flametube and scroll jetandturbineowners.proboards.com/attachment/download/216 , having the air delivery at the " bottom" allows time for the air flow to sort itself out as it travels up the side of the flametube to the Primary holes where hopefully it will enter evenly through all holes , at least better than the current setup .

And one other thing , you mentioned extra primary hole area , you may need to blank some off to bring the area back to Jetspecs recommendation.

Cheers
John

[finiteparts]

The problem with the tangential inlets is that the flow enters the annular space with a high component of tangential velocity.  Tangential velocity is forced to turn as it flows trough the annular space and from basic physics we know that mass wants to goes straight unless acted on by an outside force.  The presence of the outer casing imposes the force on the flow, causing the flow curvature and when a flow curves, there is an increasing pressure gradient away from the center of radius or more importantly, there is a reduction in static pressure in the inner flows.

The driving force for the flow through the liner holes is the upstream static pressure (unless the flow is entering the hole straight on, then you can take advantage of the dynamic pressure) and this is the reason that the annular spacing is an important design parameter.  If the flow velocity in the annular space is too high, the reduced static pressure causes weak jets (low velocity, low penetration and low kinetic energy). If these are the primary jets, this contributes to weak mixing and poor primary zone combustion efficiency.  It is the lack of driving pressure through the holes that causes the maldistribution of the primary zone flows.  You can have similar issues even if you have the flow entering axially, but due to the radial pressure gradient of the tangential flow, it is more likely for this style of air entry.

One thing you can try is to put some air damns or scoops on the downstream sides of the primary holes.  There are some old combustors that had to go to these style of features to fix a poorly designed annulus airflow.  The Spey combustor had some issues that swirl brakes and air damns helped to fix.  These devices help to recover the dynamic head of the flow and thus feed the liner jets with a driving pressure closer to the annulus total pressure.  An example of these would be a 90 degree plate to the liner surface, that covers 3/4  or one-half of the annulus height, or a more complicated feature that has been used in the past which was a small scoop over the hole aligned to the flow. 

Good luck,

Chris

[finiteparts]

As John mentioned, another approach is to try to diffuse the flow before entering the combustor annulus space. The only problem that I have with this approach is that diffusers are very hard to design and get right. If you design a bad diffuser, the flow separates into a wake region and a high velocity, unstable flow region that sweeps around the annulus entry. So the problem that you were trying to fix is now a more complicated problem. Diffusers usually are limited to a 7 degree included angle and this would mean a very long diffuser length.

Good luck,

Chris

[finiteparts]

As with everything in engineering, there is a trade off on placement of the air entry to the combustor. As John suggested, the placement at the bottom is a nice, clean install. It allows the flow speed to reduce by the time it reaches the headend and thus provides stronger driving pressure across the PZ jets. This is usually not a big deal because you can adjust the hole size and count to get the jet velocity and penetration needed, but if you are pressure ratio limited due to local annulus velocity, this might be a consideration.

The downside is that the reduced flow on the backside of the liner has a more limited cooling capacity and will lead to a hotter metal temperature in the hottest region of the combustor. Our low PR engines do not usually suffer from overheated liners, but if longer liner life or local overheating was an issue, this might be a concern.

If you want to remake your combustor, then I would agree with John and try to design a lower entry. But I think that fixing your current design is also an option.

Good luck!

Chris