Sciencemadness Discussion Board

Propane torch exhaust conditions

Fusionfire - 22-10-2011 at 11:12

Hello folks,

I'm trying to inexpensively get a steady flow of hot air with the following properties:

1) 19% oxygen concentration (in normal air this is 21% by volume, by mass if we assume all the components are ideal gases)
2) Temperature: about 1000K
3) Water percent by mass: 4%
4) CO<sub>2</sub> percent by mass: 9%

How close is an over-the-counter propane torch for brazing/weed killing to these conditions?

At the very least I think (1) is very low for a propane torch (just blocking the air intake a bit turns the flame yellow, so it isn't a very lean burn). Temperature of a propane torch probably exceeds 1000K by a few hundred K.

Both of these can be fixed by having air inlet holes near the flame, so that fresh air is entrained in, increasing oxygen concentration and reducing temperature.

How would you modify a propane torch to get close to these conditions?

Does anyone have fuel consumption figures for a common propane torch model/brand? This would allow me to calculate (3) and (4).

Fusionfire - 23-10-2011 at 13:38

Quote: Originally posted by Fusionfire  
Hello folks,

I'm trying to inexpensively get a steady flow of hot air with the following properties:

1) 19% oxygen concentration (in normal air this is 21% by volume, by mass if we assume all the components are ideal gases)
2) Temperature: about 1000K
3) Water percent by mass: 4%
4) CO<sub>2</sub> percent by mass: 9%

How close is an over-the-counter propane torch for brazing/weed killing to these conditions?

At the very least I think (1) is very low for a propane torch (just blocking the air intake a bit turns the flame yellow, so it isn't a very lean burn). Temperature of a propane torch probably exceeds 1000K by a few hundred K.

Both of these can be fixed by having air inlet holes near the flame, so that fresh air is entrained in, increasing oxygen concentration and reducing temperature.

How would you modify a propane torch to get close to these conditions?

Does anyone have fuel consumption figures for a common propane torch model/brand? This would allow me to calculate (3) and (4).


Guys if you're wondering these are the exhaust conditions for a typical turbofan. I am trying to replicate it in the lab without having to have the engine.

watson.fawkes - 23-10-2011 at 13:51

The first thing to do is to develop mass balance equations for (1) oxygen depletion (2) H2O and CO2 introduction. These will be different for different fuels. The C:H ratio of your fuel will determine the CO2:H2O ratio in the exhaust. The molar flow rate ratio of combustion air to fuel gets you the oxygen depletion. Worry about temperature manipulation later.

How precise do you need the percentage on the target gas mix? Are you going to measure it, or just rely on the operating parameters of your burner design?

Fusionfire - 24-10-2011 at 02:48

AIUI JP8 used in turbofans is mostly kerosene with some additives. I'm assuming kerosene to be chemically represented by decane:
Quote:

Kerosene, a thin, clear liquid formed from hydrocarbons, with a density of 0.78–0.81 g/cm3, is obtained from the fractional distillation of petroleum between 150 °C and 275 °C, resulting in a mixture of carbon chains that typically contain between six and 16 carbon atoms per molecule.


http://en.wikipedia.org/wiki/Kerosene

I am favouring propane torches because they are off-the-shelf and I won't have to mess around with kerosene fuel injection systems and air compressors.

I've done all the air:fuel mass ratio calculations to get the figures mentioned in the OP.

My budget for this project to get a replica turbofan exhaust stream with boron afterburners is about £5k. This includes workshop labour, parts, etc. A few thousand grand will be spend acquiring high purity micrometic boron powder which is excluded.

Yes I know boron and borane afterburners are known technologies, but I'd still like to build them myself if I can :)

I'm fairly flexible on the consitutent percentages in the target gas mix, say +/- 25%. I will be relying on doing the sums to estimate the water:CO2:oxygen fractions.

watson.fawkes - 24-10-2011 at 05:29

Quote: Originally posted by Fusionfire  
I've done all the air:fuel mass ratio calculations to get the figures mentioned in the OP. [...]
I'm fairly flexible on the consitutent percentages in the target gas mix, say +/- 25%. I will be relying on doing the sums to estimate the water:CO2:oxygen fractions.
Hell, put your partial results out for others to read, then. No good sense in asking for help if you're leaving things out. If you've got tolerances that large, then propane probably does substitute for kerosene. That was my first question.

You'll need a pair of flow rate meters, one for propane, one for combustion air. I'd recommend using a naturally aspirated injection burner like the kind that blacksmiths have started using. Instead of using open aspiration, though, you'll want a variable-speed blower, to replace the turbine action. These torches can be built almost entirely from black iron pipe. There's need for something less reactive at the output, where the combustion front is. Folks tend to use welding nozzles as an injector, although there are other ways.

Fusionfire - 24-10-2011 at 06:48

Quote: Originally posted by watson.fawkes  
Quote: Originally posted by Fusionfire  
I've done all the air:fuel mass ratio calculations to get the figures mentioned in the OP. [...]
I'm fairly flexible on the consitutent percentages in the target gas mix, say +/- 25%. I will be relying on doing the sums to estimate the water:CO2:oxygen fractions.[/rquote]Hell, put your partial results out for others to read, then. No good sense in asking for help if you're leaving things out. If you've got tolerances that large, then propane probably does substitute for kerosene. That was my first question.


If you assume an air mass flow rate through the combustion chamber of ~25kg/s, a fuel consumption of ~0.7 kg/s (assume all decane) and stoichiometric decane combustion equation of:

2C10H22 + 31O2 → 20CO2 + 22H2O

You get the OP figures. Roughly it is 800 mol/s of oxygen and 5 mol/s of decane.

[rquote]
You'll need a pair of flow rate meters, one for propane, one for combustion air. I'd recommend using a naturally aspirated injection burner like the kind that blacksmiths have started using. Instead of using open aspiration, though, you'll want a variable-speed blower, to replace the turbine action. These torches can be built almost entirely from black iron pipe. There's need for something less reactive at the output, where the combustion front is. Folks tend to use welding nozzles as an injector, although there are other ways.


So I take it a commercial-off-the-shelf (COTS) propane torch produces an exhaust that deviates too far from my design conditions?

The cheapest air compressor I have been able to find is a 12V portable car tyre inflator (the sort that go into a car's cigarette lighter jack). It's output should be sufficient for my needs if I scale down the engine. Problem is that I don't know what it's air mass flow rate is.

IME anything custom with instrumentation is expensive and best avoided unless absolutely necessary. The preferred approach is COTS, modified COTS and then custom made.

I was thinking about building a tube out of fire clay on a potter's wheel, putting aspiration holes in it (transverse to the flow/flame direction) and then firing it in a kiln. The max service temperature of a fire clay tube would be higher than an iron tube.

Another advantage of the clay tube approach is that I can modify its dimensions easily without having to take it to a workshop. The clay tube can also be moulded to fit any nozzles while soft before firing.

[Edited on 24-10-2011 by Fusionfire]

watson.fawkes - 24-10-2011 at 09:36

Quote: Originally posted by Fusionfire  
The cheapest air compressor I have been able to find is a 12V portable car tyre inflator
[...] anything custom with instrumentation is expensive and best avoided unless absolutely necessary. The preferred approach is COTS, modified COTS and then custom made.
[...] I was thinking about building a tube out of fire clay on a potter's wheel, putting aspiration holes in it (transverse to the flow/flame direction) and then firing it in a kiln.
You don't need a compressor. You need a blower, also known as a fan.

Flow rate gauges are COTS. They hook up with standard pipe threads. If you think you can get what you want without any instrumentation, you're beyond my imagination of how to build what you want, short of just purchasing a small turbine.

You don't need the mixing chamber to be made of ceramic, since that's not where combustion happens. Using a ceramic output nozzle is pretty much a necessity for what you're thinking about.

As for the question about whether propane will work, do the calculation.

Fusionfire - 24-10-2011 at 11:04

Quote: Originally posted by watson.fawkes  
You don't need a compressor. You need a blower, also known as a fan.


From Bernoulli's equation pressure and flow speed are interconvertible. Hooking up a compressor rather than a fan to the contraption has the advantage of having a small inlet port.

Quote:

Flow rate gauges are COTS. They hook up with standard pipe threads. If you think you can get what you want without any instrumentation, you're beyond my imagination of how to build what you want, short of just purchasing a small turbine.


I'm trying to reduce instrumentation where possible to save costs, not eliminate them entirely.
Quote:
As for the question about whether propane will work, do the calculation.


Unfortunately I don't have the air + fuel flow rates of a typical consumer propane torch handy. I can estimate the fuel flow rate by how long a tank of fuel mass M grams lasts, but the air flow rate is more difficult to obtain. Suggestions are welcome.

Might be worth noting that I am interested in relative augmentation of thrust by afterburning with different metal/metalloidal fuels, in a relatively aerobic + humid exhaust stream. I'm not so interested in the absolute performance of a given configuration because I know whatever engine replica I cobble together with £10k worth of money (incl. fuels) will be sub-optimal anyway.

[Edited on 24-10-2011 by Fusionfire]

Neil - 24-10-2011 at 11:53

watson.fawkes is giving you excellent information on how to build an efficient burner, which is what you described in your opening post.

If you are trying to simulate the exhaust gasses of a turbofan you are going the wrong direction;).

A burner makes a very small amount of air very hot.

A turbo fan is a by-pass turbine engine that is designed to heat a small amount of air and use that heat to drive a fan which moves a large volume of air. Most of the exhaust of a turbo fan has never been burned, they are very close to a turbo-prop engine which replaces the fan with a propeller.

To simulate a turbofan you would want to introduce the products of combustion to a very fast moving or very high pressure air stream which is then accelerated. The relevance here is that the air is being heated first by compression and then by dilution with the exhaust products to give your high oxygen lowish temperature exhaust stream.


With regard to the composition of your weed burners exhaust-
A propane torch burning with a neutral flame is stoicometrically rich in fuel. A flame that is burning fuel and oxygen 1:1 is very oxidizing and erosive, any home fired ceramic vessels will likely show considerable flame erosion in long term use. Flames that are much leaner are unstable and will blow out easily.


To simulate the exhaust of a turbo fan you need a high volume compressor as well as a flame holder as well as fuel injection and likely ignition systems.

A vacuum cleaner's compressor may give you the flow rate and air speed you would need, if you took the fans exhaust separated it into two flows and burned one of the flows then re-mixed them you would be on your way.

Once you assume two flows and re-mixing all you need to work out is flow rates for your fuel.

The standardization of MIG nozzles for home smiths building venturi burners is so that only knowing the propane's pressure and the known nozzle orifice size it is easy to approximate fuel flow.


Set up a combustor can with a known nozzle size, hook up a pressure gauge to your propane and then set your air flow and fuel levels by checking your exhaust gas temperature.


These links might give you a leg up.

http://en.wikipedia.org/wiki/Flame_holder

http://en.wikipedia.org/wiki/Combustor

http://www.ueet.nasa.gov/StudentSite/engines.html

Fusionfire - 24-10-2011 at 13:02

Interesting, thanks for all the sage advice guys ;)

My calculations took the "fan" out of turbofan and assumed a bypass ratio of 0. This is because the afterburners would be applied behind the turbine before the engine core flow mixes with the bypassed flow.

It seems like my best option is to burn fuel in a low speed flow to prevent blowout and then mix it with a larger, cooler volume of air. As Neil says, split air from a compressor, mix one with fuel and burn it, and mix the other bit with the exhaust to cool it + increase oxygen content for afterburning. And since I will be spending so much money on the plumbing it makes sense to get the fuel right and use kerosene too :)

Now if only I could get supersonic exhausts for a budget of £10k because of the direct proportionality of thrust to exhaust speed :D

watson.fawkes - 24-10-2011 at 18:32

Blower from a vacuum cleaner is a pretty good suggestion. You can generally find them cheap at thrift stores and the like.

One of the advantages of using propane is that it'll be a lot easier to meter than kerosene. It'll be a bit hydrogen-rich, about 15-20% more H, but it should be OK for a first prototype. If your first one works, you can always do a second one with kerosene fuel.

Here's a representative manufacturer page for the kind of flow meter I was thinking of. There are other types. For the bulk air flow, a Pitot tube is a good choice.

I forgot to mention it before, but you'll want separate ceramic nozzle pieces because they'll fail and need replacement.

Neil - 24-10-2011 at 18:51

Bonus bonus; do a bit of research and pick a common vacuum brand with a standardized pump unit Eg the AMETEK 4368931


Almost all of their large vacuums use that motor and so it is easy to replace them when you kill them. I've got two or three booting around and find them to be rather robust and dependable.

Dr.Bob - 25-10-2011 at 11:53

If you take a standard vacuum cleaner and try to clean up a hexane spill with it, you can get the exhaust to look a lot like a jet engine exhaust. :o And yes, someone tried it at least once many years back, but I unfortunately wasn't there to see it. It even survived, believe it or not...

You can buy a combustion product analyzer for testing furnaces and other burners for about $1000 new, or much less used, which would be useful for testing and tweaking your system. They usually have CO, O2, temp, and flow meters all in one. Most good HVAC people have one around, you might be able to borrow one from a maintenance person in your facility. Your numbers may be hard to do without preheating the excess air, if you are only using 2% of the O2 in the air.

Bob

bquirky - 25-10-2011 at 22:51

you have already said that you don't want to buy an engine but on the off chance you didnt think of it its possible to buy turbine engines for RC aircraft for about $2k

this has a few advantages

it runs out of the box and comes with all its pumps and control systems.

its a real jet that runs on jet A1 (kerro) so will have the exhaust of a real jet running kerro

and it will have some resale value. so you can probably sell it after your experiment to recoop some of its costs.

but id use a vacume cleaner a small fuel pump ;) with flow rates seperatly measured by timing the inflation of a garbage bag and a set of scales underneath the fuel tank and a stopwatch.

Fusionfire - 26-10-2011 at 02:38

Wow, I didn't know you could buy small turbine engines at that price.

This makes things a whole lot simpler :)