Forced Air Diesel Fuel Burner

[Steamchick]

I found this... maybe it relates to your spray nozzle?
I noted you use 8psi... but this uses 3 bar = 45psi. That's a LOT more air!
For the fuel flow you want, the jet is 2mm - What size is yours?
K2

 

[Toymaker]

Hi Toymaker, Yes there are a few answers. But these are my knowledge, and a proper burner engineer may have better answers. It does look like you have good atomisation, but I think you need more air within the cloud. (The mixture ratio is too rich).
My SIMPLE understanding is as below: (I say simple, because I am NOT a combustion engineer, just picked-up a few bits on the way, and I am probably incorrect in some of this?!)
The initial Fuel air mix must be heated to a point where the hydrocarbons begin to breakdown into ions, so the fuel ions (carbon, Hydrogen, and simple paraffins, butanes, etc.) can combine with the O2 => Oxygen ions from the air. So, because of the mix of Oxygen and Nitrogen making-up air, and the natural combinations of carbon and hydrogen in the hydrocarbons we are burning, you need about 15 times more air than fuel to balance most of the fuels we are burning here.
In flames, the hydrogen ignites and burns first, as it is so reactive, elevating the fuel and ionised gas temperature of the "Mass" of the mixture. This can be initiated in a small local zone by a spark (electrical, flint, iron, or whatever!), where the "gas temperature" to make the first ions to start the combustion is a few thousand degrees C. Once initiated, the heat released from the first few burning ions rapidly ionises the rest of the mixture nearby until a sustainable flame occurs. The heat at the flame front: where fresh mixture meets the combustion zone, keeps ionising more fuel which mixes with ionised O2 from the air to permit more combustion. This is my "simple layman's model of the flame.
When the hydrogen has been ripped off some molecules of fuel, the carbon ions mixing with Oxygen ions combust around 500degrees C or higher (?) forming CO. But there can be free carbon that is "quite big" relatively in molecular/ion size within the fuel gas mix, if there is insufficient air (oxygen) in this initial mix of fuel and air. This free carbon must be heated to glowing (over 700 degrees C?) with more air in order to burn to CO.
In a third phase, all the remaining CO and C burns with the remaining air (a darker blue flame, tinged with yellow or orange where there is free carbon). But as the flame is rapidly expanding, and, because of gas laws, that means cooling, when the temperature drops below around 700degrees C the carbon stops burning (leaving free soot), and when it drops below about 350 degrees C the CO stops burning.
A crazy thing I found is that when I get enough air into the mixture initially, the flames get smaller and more compact (actually are hotter as they burn faster). When you reach a stage close to "good" complete combustion, the "yellow feathers" reduce to an orange glow in the flame, and then disappear leaving just the 2 blue Bunsen cones we are familiar with.
So I still think you need MORE air in the jet nozzle.
I.E. use a smaller fuel jet and a higher air pressure, until you get a better mixture here.
You are pumping 14L of fuel per hour through the jet: This is 14 x 0.8 /60 = 187gms of fuel per minute.
Which means you need 15 x 187 = 2.8 kg. of air per min. = 6.172lbs of air per min.
Dry air at 0�C has a density of 12.417423770565761 cubic feet per pound. So you need 76.65CFM at atmospheric pressure. At 100 psi from your compressor, that equates to 9.82cfm.
What pressure are you delivering at the jet? What air flow rate do you have at the jet?
When we understand the Mixture is correct, then we can tackle any "flame dynamics" to resolve any issues. But I think your blower and nozzle outer giving the swirl pattern is probably good.
An idea: Can you fit a jet of maybe 3/4 the diameter (size) of your jet, and increase the air pressure to 1 1/2 times the current pressure? Then let us see what that does?
If my help is "no help", just tell me to stop. I am no expert, just a meddler in this subject.
K2

K2, you can meddle all you want,... I don't mind, in fact, I think of it as "brain storming", which often leads to new ideas.

Yes, I can buy fuel nozzles the same physical size as the one I have, but with both smaller and larger fuel volume outputs. However, I don't believe that will be necessary, as the fuel flow is adjustable from 0 to 14+ Liters per hour.

The fuel nozzle works on the siphon or venture principle, and has two input ports, one for fuel and one for air pressure. Rated air pressure is between 3 and 6 psi; the higher the air pressure the larger the fuel flow. The nozzle I have is rated to atomize 14L/hour at 6 psi, but I've found the nozzle continues to work and deliver even more fuel at pressures up to and above 8 psi. Starting at a low air pressure setting of 4 psi, increasing air pressure to the fuel nozzle increases the fuel flow and I need to also increase the RPMs of the air blower, which is adjustable from zero to above max rated RPM. The blower I'm using is rated to deliver 3.6 CM/m (127 CFM) at atmospheric air pressure at full rpm. Even at very high fuel burns, using 6 psi at the nozzle, I need only a bit over half of full rpm from the air blower (127/2 = 63.5 CFM); which tracks fairly close to the 76 CFM you estimated that I would need for a fuel burn of 14 L/Hr. Isn't it nice when the math works out

At any fuel flow setting below 5.5 psi I can easily blow out the burner's flame by increasing the air blower rpm. So I don't believe insufficient air flow is the problem.

 

[Toymaker]

I found this... maybe it relates to your spray nozzle?
I noted you use 8psi... but this uses 3 bar = 45psi. That's a LOT more air!
For the fuel flow you want, the jet is 2mm - What size is yours?
K2

Apparently these nozzles are made for use with different air pressure requirements. The 4 to 6 psi rating on mine is very typical of nozzles found on torpedo burners using kerosene or diesel; the air pressure in these units is supplied by a vane pump mounted on the back of the fan motor. So, very low air pressure.

Specs for my nozzle:
Total Length: 67 mm
Oil Inlet: 1/8" NPT
Air Inlet: 1/4" NPT
Nozzle Bore Diameter: 2.0mm
Working Pressure: 4 ~ 6 PSI
14 liters/hour

 

[Toymaker]

Today's test results:

It's barely noticeable, but I reduced the diameter of the stainless plate partially blocking the exhaust.


Without the stainless steel plate:

Same fuel and air settings, but with the stainless steel plate.

Video of medium-low fuel flow rate:

I held both an 8mm aluminum tube and an 8mm copper tube directly in the exhaust flames; the aluminum quickly melted but left no trace of soot. The copper tube darkened a bit but collected no noticeable soot after being exposed to the exhaust gases for about 60 seconds.

My theory is that the steel plate somewhat slows the exhaust gasses by forcing them to make two 90 degree turns, and to very slightly collect as they impact the steel plate before they're forced to make the turn. These actions force the gases to stay inside the high temperature environment of the combustion chamber just long enough to more completely burn.

 

[Steamchick]

Sounds like you have solved your problem. The Iast check I would do, is see if the Room CO Monitor I keep in my garage would sound in the exhaust. I found one of my burners, that otherwise looked OK, was chuffing out loads of invisble, toxic gas, as just a few minutes in an open garage and it sounded. About as bad as my 1979 Moto Guzzi!
I reckon it was a " half hour to unconsciousness" rating, but explained my headache. That burner was retuned with a smaller gas jet and is clean, although lower gas power.
Keep safe, that's a big burner!
K2

 

[Toymaker]

Sounds like you have solved your problem. The Iast check I would do, is see if the Room CO Monitor I keep in my garage would sound in the exhaust. I found one of my burners, that otherwise looked OK, was chuffing out loads of invisble, toxic gas, as just a few minutes in an open garage and it sounded. About as bad as my 1979 Moto Guzzi!
I reckon it was a " half hour to unconsciousness" rating, but explained my headache. That burner was retuned with a smaller gas jet and is clean, although lower gas power.
Keep safe, that's a big burner!
K2

I will need to order a CO monitor on-line as here in the tropics of Thailand almost no one has a furnace in their home, so no stores in Thailand carry them.

As for the burner, I suspect I may learn more about the exhaust gases once I begin blowing them through the coils of boiler tubes; as the gases cool down significantly they may leave behind deposits on the tube walls. I will need to watch for this and any signs of corrosion or pitting.

 

[Steamchick]

Hi Toymaker, E&@y has CO monitors for buildings, just less than £10 in the UK. They only sound an alarm if too much CO, not give direct readings or anything clever. But good enough to be sure the combustion is complete!
In Europe, with supposedly cheap Propane, this type of large burner is common: CHEAP and simple.
https://www.ebay.co.uk/itm/19341260...2BRGt3XahXz7bcTzM%3D|ampid:PL_CLK|clp:2047675But it follows the same principle you are trying to achieve. Complete combustion of hydrocarbon fuel with entrained air.
I have had a think about your style of burner, used for space heating. It is not quite the same in needs as a burner for a furnace or boiler.
A SPACE heater needs a LARGE amount of excess air mixed with the combustion exhaust (The external air from the blower) - to blow around the space for heating. The combustion MUST be completed with no CO. Your design appears to do this.
A burner for boiler or furnace needs to just have a stoichiometric balance of fuel and air. Heating air to blow through the boiler or furnace simply cools the top temperature and mean temperature available. So these burners take all the air needed for combustion through the mixer tube before combustion, with no EXTRA external air to complete the combustion, or cool the exhaust gases.
Suppose you want a boiler generating steam at 200degrees C.? The Exhaust leaving the boiler cannot be below 200deg.C. So you want the inlet to be as hot as possible, with no extra air, to facilitate heat flow from burned gases to metal and water/steam. Therefore I suggest you need to use a smaller fuel jet, and higher feed air pressure, to increase the air-fuel balance of injected fuel-air cloud, and minimise the outer blower air.
Hope this helps?
K2

 

[Toymaker]

Hello Steamchick, Unfortunately for me, as you've no doubt noticed, the fuel nozzle I'm using lists the amount of fuel being atomized at max rated pressure, but does not list the volume of air at any pressure. Various videos on the web show these nozzles in use burning "waste oil" and the observable flames are always yellow. As I found through using my own nozzle with kerosene, unless additional air is blown into the flame, the air supplied by the siphon nozzle alone is insufficient for complete combustion. This is also demonstrated in the design of torpedo space heaters, which we've both pointed out, supply far more air then is required for complete combustion. So the real trick, and goal, is to supply just enough additional air from the blower to achieve a stoichiometric fuel and air burn, and because I'm able to independently control both fuel flow and the volume of added air, I believe I can come very, very close to that goal.

I've compared my burner to a typical torpedo space heater, but I failed to point out the major difference.
If you look closely at the above pic, you'll notice that the combustion chamber is mounted inside a larger diameter tube. Most of the air that flows through a torpedo heater flows around the combustion chamber, cooling it and transferring the chamber's heat into the air. Only a small fraction of the total air volume flowing through a torpedo heater is actually used to burn the fuel. Most of the air is mixed with the exhaust gases to cool those gases and produce a large volume of pleasantly warm or hot air.

In contrast, all the air flowing through my burner is used in the combustion process; even the air used to cool the stainless steel shell that forms the combustion chamber is vented into the chamber to burn with the unburned fuel.

Now, because the exhaust gases from my burner aren't blue, clearly I'm not reaching the highest possible temperature,...but as I've questioned earlier, is that even necessary? For peak efficiency, the answer is certainly yes. But is what I have now perfectly acceptable to boil lots of working fluid? Again the answer is yes.

I still have a few more ideas I picked up while working in the jet engine field,...like using flame holders mounted mid-way inside the combustion chamber to induce vortices to form which will further mix and slow the flame front, thereby further aiding the combustion process. But for now, I really want to move on to the boiler, and leave fine tuning of the burner for a future day

 

[Steamchick]

Hi Toymaker, now you have introduced a different variable. The nozzle designed for waste fuel will pass much more kerosene, as it has a lower viscosity, so you need to reduce the fuel jet size! If you plan on 14l per hour (max) of waste oil as the rating of the jet at a certain air pressure, then you will be probably pumping at least twice that with kerosene?
I shall try and find a simple ratio to give you a suitable jet size to try?
Design is simple, but you do need to do the math to get it right.
K2

 

[Steamchick]

Hi again... viscosity kerosene = 10 cps.
Viscosity motor oil (typical!) = 50 cps.
Poiseulle's law says they have a linear relationship to flow. I.E. 1/5 viscosity = 5 x flow rate.
So you could expect 70l/hrs of pumped kerosene instead of the 14l/hrs of waste oil....
So you want 20% of the CSA of the jet you have, for the same air pressure as the manufacturers proposed for waste oil.
In fact a completely smaller nozzle assembly for 3l./hr of waste oil may be the simple option?
You may not need any blower air then?
Is that simple enough maths?

K2

 

[Toymaker]

Hi Toymaker, now you have introduced a different variable. The nozzle designed for waste fuel will pass much more kerosene, as it has a lower viscosity, so you need to reduce the fuel jet size! If you plan on 14l per hour (max) of waste oil as the rating of the jet at a certain air pressure, then you will be probably pumping at least twice that with kerosene?
I shall try and find a simple ratio to give you a suitable jet size to try?
Design is simple, but you do need to do the math to get it right.
K2

Many of the siphon nozzles I've seen being sold on eBay intended for use with "waste fuel" state that the fuel flow numbers posted are based on water. Even the pdf file you sent me in post 21 giving the specs for siphon fuel nozzles states at the top of page 3: Kindly be noted above data is base on water for reference

Since both kerosene and diesel have viscosities slightly higher than water, the selected nozzle should be a bit larger.

 

[Steamchick]

Ok. So it won't pump the same kerosene as the spec for water. But it will pump typically 5 x more kerosene than oil, for the SAME air. So if it was designed for waste oil, it will be very rich on kerosene.
A simple rule of life is that anything designed for one material, is unlikely to be right for another material. So it needs retuning.
E.g. My 1970s motor cycle is not happy with today's petrol and needed tuning, as petrol is not like it used to be.
I hope some of this is of some help?
K2

 

[Toymaker]

Ok. So it won't pump the same kerosene as the spec for water. But it will pump typically 5 x more kerosene than oil, for the SAME air. So if it was designed for waste oil, it will be very rich on kerosene.
A simple rule of life is that anything designed for one material, is unlikely to be right for another material. So it needs retuning.
E.g. My 1970s motor cycle is not happy with today's petrol and needed tuning, as petrol is not like it used to be.
I hope some of this is of some help?
K2

I don't believe these nozzles were designed to provide a perfect fuel-air mixture from the nozzle alone, after all, since the nozzles are meant to use waste oil, the manufacturer has no way of knowing what oil will be atomized and how much air to provide for clean burn. The uncertainty of what fuel, or oil will be used is likely the reason the manufacturer provides the specs based on water,...as water has a known constant viscosity. These nozzles were designed to need additional air flow,...it's up to the user to determine how much more air is required for their specific application.

 

[Steamchick]

Hi Toymaker, Appreciate the comments, but my point is that for the "best" combustion, balancing the fuel and air correctly at the nozzle gives the best mix for full combustion without excess air. External air gives a lot of extra expansion and loss of temperature (Boyles law? PV/T = R) to the gases, which in turn is a loss of efficiency in the boiler/furnace where high temperature heat exchange to a medium other than the air is wanted: (but not in a Space heater, where all you want is "hot air"..)...
Cheers.
K2

 

[Toymaker]

Hi Toymaker, Appreciate the comments, but my point is that for the "best" combustion, balancing the fuel and air correctly at the nozzle gives the best mix for full combustion without excess air. External air gives a lot of extra expansion and loss of temperature (Boyles law? PV/T = R) to the gases, which in turn is a loss of efficiency in the boiler/furnace where high temperature heat exchange to a medium other than the air is wanted: (but not in a Space heater, where all you want is "hot air"..)...
Cheers.
K2

Of course the ideal place to mix the fuel and air is at (inside) the nozzle, and if you can find a nozzle designed to mix a variable amount of between 3 to 14 L/Hr of Diesel with a stoichiometric balance of air please let me know.

 

[Steamchick]

Quite a simple device, a needle jet. Where the air whistles past the jet, developing the low pressure to withdraw the fuel and do the atomising. But at low pressure air the jet has a needle blocking most of the jet, whereas at high pressure air when more fuel is sucked from the jet, the needle (tapered) is partly withdrawn to permit higher fuel flow. In post #9 you mention that you will be supplying air from a compressor, driven by the steam turbine. Will this be variable pressure to drive the fuel jet? Or will you use a regulator or something from your small compressor? You obviously have some boiler firing control planned that I don't appreciate yet. Maybe a needle in the jet will be an adequate control on its own? Try it and see?
Similar to a carburettor - but a forced air device.
I knew a guy who drag-raced a motorcycle, using exhaust pressure gas as the gas jet to draw fuel up the fuel jet. All he did for tuning was swap different tapered needles until he got the best option. Although it was a totally different application to yours, the principle is the same. I.E. A jet of gas draws fuel through a jet, with mixture metered by a needle as the gas pressure varies.
Any use?
K2

 

[Toymaker]

Quite a simple device, a needle jet. Where the air whistles past the jet, developing the low pressure to withdraw the fuel and do the atomising. But at low pressure air the jet has a needle blocking most of the jet, whereas at high pressure air when more fuel is sucked from the jet, the needle (tapered) is partly withdrawn to permit higher fuel flow.
K2

The spray nozzle you've described, that uses a tapered needle blocking an orifice, is a very old design,... they do work, but they don't produce the super-fine droplets (see photo in post #18) that modern siphon nozzles produce. The below photo shows a modern nozzle, of the type I'm using; they take advantage of the venturi effect and create supersonic air velocities inside a chamber within the nozzle. There is no needle inside these nozzles, just a very well designed internal structure capable of atomizing a liquid into a fog-like cloud.

I don't posse sufficient knowledge of fluid dynamics required to design and fabricate a similar nozzle specifically engineered to use diesel fuel and deliver a stoichiometric air-fuel mix. So I will continue to use the nozzle I have.

 

[Toymaker]

In post #9 you mention that you will be supplying air from a compressor, driven by the steam turbine. Will this be variable pressure to drive the fuel jet? Or will you use a regulator or something from your small compressor? You obviously have some boiler firing control planned that I don't appreciate yet. Maybe a needle in the jet will be an adequate control on its own? Try it and see?

K2

I think you've either miss-interpreted post #9, or you meant to cite a different post?? Post 9 simply states the steam turbine will be used to spin a centrifugal air compressor, there's nothing about using the compressed air for anything fuel related. However, I may have posted something about my future plans somewhere else on this forum.

OK, from the beginning: air pressure from my wobble plate compressor delivers compressed air, between 0 and 8+ psi, into the fuel nozzle. I have control over the full range of air pressure by varying the rpm of the electric motor turning the compressor. This low pressure compressed air feeds the siphon fuel nozzle, which delivers fuel and a small amount of air, into the burner in the range of 0 to 14+ liters/Hr of fuel; the amount of air from the fuel nozzle is an unknown. A small leaf blower blows additional air into the burner assembly; I control the amount of additional air blown into the burner by controlling the RPM of the leaf blower. During start-up, both fuel flow and additional air are set to very low levels. A high voltage continuous spark is used to ignite the fuel-air mix and is turned on seconds before the fuel and added air are introduced into the burner assembly. Once the fuel-air mixture is ignited I can increase both the fuel flow and additional air flow, independently, to any desired level, and find and use the best possible fuel-air mixture for this particular burner.

In the future, once this project has reached the point where I'm using the steam turbine to spin the large centrifugal air compressor which is pumping out large volumes of compressed air at between 10 to 60 psi, I will use a solenoid to switch the fuel nozzle's air source from the wobble plate compressor over to the large centrifugal compressor and the wobble plate compressor can be turned off. At that point, air pressure into the fuel nozzle will be regulated by the Servo Controlled Pressure Regulator I spoke about earlier in my Ambitious ORC Turbine thread; in that thread, I explain how all functions of my steam turbine will be controlled by a microcontroller (small computer)

 

[GreenTwin]

Siphon nozzle burners will work over a wide range of fuel flows; typically achieving good atomization over a 4:1 range.
You can get siphon nozzles in different flow rates, with the intent being to be able to find the one that will work best with a range of packaged commercial heating units.

One does not have to get too picky about the exact siphon nozzle listed flow rates.
I have operated a Delavan 1 gal/hr nominal nozzle at 10 gal/hr with no problems.
The nominal 1 gal/hr fuel flow rate for my Delavan nozzle is just a benchmark achieved using a given compressed air pressure, but that flow rate is not limited to 1 gal/hr, and many people make this false assumption (that a 1 gal/hr Delavan nozzle must be operated at 1 gal/hr).

As I recall, the fuel flow rate varies with the compressed air pressure.

You can compensate for fuel viscosity by increasing the compressed air pressure, or by pressurizing the fuel tank to about 10 psi, and controlling fuel flow to the nozzle via a needle valve.

I calibrate my fuel flow with the burner off, and no compressed air or combustion air, and measure the amount discharged into a measuring cup in 60 seconds.

I have seen one individual install a flow meter in his fuel line, and that is not necessary for my foundry use, but would be handy to have so you could see the actual fuel flow rate.

Delavan also makes a pressure nozzle, and it uses the pressure of the fuel (about 100 psi via a gear pump) to atomize the fuel.
Pressure nozzles do not need compressed air in order to operate.

I use a Delavan 30609-11 with my foundry burner, operating at 2.7 gal/hr., and about 20 psi of compressed air, and 10 psi on the fuel tank.
This nozzle screws into a Delavan DLN 17174 adapter, and the fuel line enters the side of the adapter.
The compressed air enters the end of the adapter.


.

 

[GreenTwin]

A higher viscosity fuel will have a higher flow rate than a lower viscosity fuel, when using a pressure nozzle.

I am not sure if this is true with a siphon nozzle.

.