Talk:Combustor

Afterburner
My memory says, someone showed how to reduce emission with an afterburner (with additional fuel injection).

Fuel mixing
Most stable fuel - oxidizer mixing is coaxial. See rocket engine. nitroglycerin is also produced in this way. Mixing occurs by pure axial velocity differences. This principle holds true for liquid-liquid (rocket engine) and gas-gas (hydrogen, or (fat) turbine exhaust-fuel mix with bypass air) mixing. Mufflers are used to prevent the noise from going upstream, or through the exhaust (in the case: exhaust with bypass). In liquid-gas mixing, the gas becomes unimportant. Instead the liquid must be sprayed efficiently. To me it is not clear that all sprayers have a conical angle. There are webpages at NASA where the fuel is injected in a straight jet in cross flow. I guess this avoids having any obstacle in the laminar flow. As is also known from Diesel engines, big droplets produce soot. High pressure liquid at high temperature produces the smallest droplets, thus regenerative cooling up to the coking limit helps. In successful engines the mixing occurs before combustion. In Otto engines and in lean premix gas turbines the droplets evaporate as much as possible before evaporation cooling halts the process leading to the most homogeneous mixture. In a lean premix engine the mixture laminarly flows through a tube at a velocity higher than the flame speed. At the rear the tube is divergent and velocity reduces. At the flame front velocity the flame front stays. A low swirl is introduced to slow down the flow within the centre of the tube, this gives the flame a front a convex shape and stabilizes it. In high power combustors multiple of these diffusers aim at an angle around a flame holder. The angle leads to a strong swirl, which leads not only to a slow down of flow in the holder, but to a circulation. Since the holder is in the centre of the flame, the gas there suffers the lowest radiation cooling. Due to the centrifugal forces hot pockets in the gas and cracked molecules from intermediate steps of the reaction diffuse to the centre and recirculate. When they mix at the sharp edges of the holder with the fuel-oxidizer mix, they still trigger combustion if the flow is increased so much that the individual diffusers are blown out. The holder needs to be thin film cooled and needs a polished clean metal surface to reflect all heat radiation. Centre- flameholders are not used in rocket engines.

Ignition
(Gas turbines use as high compression as Diesel engines and so the fuel self ignites. For some reason this seems to be a too slow a way for gas turbines.) < This is incorrect, gas turbine engines DO NOT use compression to ignite their fuel. They start-up using an igniter plug (or more depending on the type of combustion chamber) and then burn continuously until the fuel flow is cut off or the engine suffers a flameout. How do you think the gas ring on your cooker, bunsen burners, lighters and flamethrowers continue to burn? They work by continuous fuel supply to the burner. Otto cycle engines do not have continuous fuelling. Brayton cycle engines do. 86.184.89.177 (talk) 14:53, 25 March 2012 (UTC)

Cooling
The cooling of the walls and of the first guide vanes are both due to having a layer of cold air between the wall and the exhaust gases. Depending on the pressure loss in the injector this air can in fact be moving faster than the exhaust and does not qualify as a boundary layer in the sense Prandle had in mind. My image tried to show this similarity and it also tried to explain how a cooling flow injected in the front of the guide vanes can turn 180° without flow separation. Shrouds on the turbine allow to extend this principle further.

Without this film the metal feels temperature peaks and due the exponential activation of chemical reaction and due to thermal stresses these harm the metal. It is then very important to avoid these peaks in the exhaust, by using lean premix, turbulence in the dilution, turbulence to diffuse the wake behind a film cooled metal surface (guide vane), by using an annular combustor.

Mechanical
If the annular combustor is placed between compressor and turbine the outer wall holds the pressure like a ballon and the axis between the compressor and the turbine has to hold this pressure, which is no problem for the axis. If the annular combuster is around the turbine, the inner wall needs to be strong and hold the pressure. Since now pressure acts on the wall it tends to crumble and the walls need to be thicker. The cannular combuster placed around compressor or turbine consists only of ballone like pressure vessels and is thus light. The axis between compressor and turbine is shortened.

Arnero (talk) 09:14, 24 July 2008 (UTC)

Types of Combustors
This section could use a lot of additional information... anyone have some expertise to add to it? -SidewinderX (talk) 02:48, 17 August 2009 (UTC)

I think the image for annular combustor is slightly confusing. I am not sure you can depict it in an easy to understand manner with only a longitudinal view. Probably a cut away side-on or isometric view would show it better. 128.158.1.166 (talk) 22:12, 29 July 2010 (UTC)


 * I agree that an isometric view would be better -- unfortunately all those that I have found are copyrighted so I can't upload them to wikipedia. And my sketching skills aren't nearly good enough to draw it myself! -SidewinderX (talk) 19:18, 17 September 2010 (UTC)

Chirp response
I'd like to see a little more on basics, ie. where does the energy come from, before getting into details. Maybe new bit between * *

A combustor is a component or area of a gas turbine, ramjet, or scramjet engine where combustion takes place. It is also known as a burner or flame can. *Any engine needs a source of energy, and most get it by burning carbon, typically from fossil fuels, in oxygen, exploiting the exothermic reaction C + O2 -> CO2. The gas turbine differs from the internal combustion engine in burning its fuel continuously. The energy released by the chemical reaction is converted into the increased kinetic energy of the gas flow through the engine.*

Bearing in mind that, in a sense the purpose of the combustor is to produce CO2, I'd be inclined to put this first in the emissions section, then the other gases then smoke. Perhaps as follows:

Emissions
One of the driving factors in modern gas turbine design is reducing emissions, and the combustor is the primary contributor to a gas turbine's emissions. Generally speaking, there are five major types of emissions from gas turbine engines: carbon dioxide (CO2), carbon monoxide (CO), unburned hydrocarbons (UHC), nitrogen oxides (NOx) *moved* and smoke.

Carbon dioxide is a *necessary product of the combustion process; 1 kg of pure carbon burnt in oxygen produces would produce about 3.67 kg of CO2. In practice, 1 kg of jet fuel burned produces about 3.2 kg of CO2. CO2 emissions can be reduced by designing better aircraft which require less power; by reducing energy losses within the engine; or by improving combustor design so that more energy is released per kg, inevitably with more CO2 but reducing the dead weight of ineffectively burnt fuel the aircraft must carry. *

Unburned hydrocarbon ....

Carbon monoxide is an intermediate product of combustion, and it is eliminated by oxidation. CO and OH react to form CO2 and H. This process, which consumes the CO, requires a relatively long time *compared with the main energy releasing reaction*, high temperatures, and high pressures. This fact means that a low CO combustor has a long residence time (essentially the amount of time the gases are in the combustion chamber).

Nitrogen oxides (NOx) are the final major type of combustor emissions. *The nitrogen come from the air.* Like CO, it is produced in the combustion zone. However, unlike CO, it is most produced during the conditions that CO is most consumed (high temperature, high pressure, long residence time). This means that, in general, reducing CO emissions results in an increase in NOx and vice versa. This fact means that most successful emission reductions require the combination of several methods.


 * moved from top* Smoke is primarily ...

Next
Is this enough, or should the article dive into the nitty-gritty of emissions reduction?
 * I'd say enough, but what do others think? On chemicals: some folk object to using the formula, say CO, instead of the name carbon monoxide.  The Wiki has many articles on chemistry; does it have policy?

I'd also be interested to hear what others think about afterburners and scramjets - I'm still debating if these belong here.

Any way a few first thoughts; in general it looks a very useful article.TSRL (talk) 17:32, 19 January 2010 (UTC)


 * Thanks for the response! Your point at the beginning makes sense. Most of my resources devote some space to the combustion process itself (at the chemistry level). I wanted to avoid a lot of that because we have a "combustion" article to deal with that, but perhaps I avoided too much of it. I'll probably follow your suggestions and see where I can add a bit of detail without overcomplicating the article and replicating too much of the combustion article. -SidewinderX (talk) 17:41, 19 January 2010 (UTC)


 * Another point: I know the USA ploughs (plows?)its own furrow on units, but I think most folks everywhere will find the use of Rankines an unnecessary stumbling block. I'd guess that this side of the pond most engineers would use Celsius, and physicists and chemists either Celsius of Kelvin.  Lest you think I'm being parochial, the Wiki article quotes "However, throughout the scientific world and many times even in United States engineering, thermodynamic temperature is measured in kelvins.[2] The US National Institute of Standards and Technology does not recommend using degrees Rankine in NIST publications.[1]".  Cheers, TSRL (talk) 17:54, 19 January 2010 (UTC)


 * Haha, no not a problem. I just changed it. I think when I took that from my source and didn't think to change it to a unit that others might more easily understand. -SidewinderX (talk) 18:45, 19 January 2010 (UTC)


 * Whitespace problem with lead, nowikied image? Nimbus (Cumulus nimbus floats by)    21:00, 9 February 2010 (UTC)

Guide vane image


This image was hidden in the Lead. This does not seem to fit in or does it some how? -Fnlayson (talk) 03:34, 16 February 2010 (UTC)


 * That was in the article before I rewrote it, and it didn't really fit with the article anymore... I didn't want to outright delete it, but it doesn't add to the article in it's current state. -SidewinderX (talk) 13:40, 16 February 2010 (UTC)

Merge Flame holder
Dose any one else think that this stub should be merged with/redirect to this article?

flame holder — Preceding unsigned comment added by Wiki edits 198 (talk • contribs) 11:54, 3 February 2011 (UTC)


 * Hmm, it's an interesting thought. On one hand, flame holder is a more general component that is used within gas turbine combustors, afterburners, and rocket engines, so I could see it has a stand alone article. On the other hand, the flame holder article is just a stub right now without that much content.
 * I think my preference would be to let flame holder be expanded and stand on its own, but I don't have the time/energy/motivation to do that myself. -SidewinderX (talk) 14:55, 4 February 2011 (UTC)

Physics
The article claims in the diffuser section, that reducing the gas velocity reduces the pressure. This assertion is wrong as it does not conserve energy (pV = power flux = constant). In a jet/turbine the outside air is accelerated to high speed and moderate pressure. The kinetic energy is then converted to potential, that is a large increase in pressure driven by a decrease in speed, as it enters the combustor. The high inlet pressure is then higher than the combustor exit pressure, keeping the air moving in one direction. If the inlet pressure should drop for some reason, via say a compressor surge, then the combustor will backfire through the compressor, and destroy it. I(The governing equation pV=const is from Bernoulli's equation.) A large amount of energy is added to the gas in the combustor, and it must exit at high speed, such that KE greatly increases but PE does not.203.213.62.123 (talk) 02:19, 22 August 2014 (UTC)

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Reverse-Flow
This article is linked to via both the Gas Turbine article & also various articles on specific engines. It seems that 'reverse-flow' can be a confusing description to apply to an engine because it sometimes has an idiosyncratic meaning specific to a particular engine (which may or may not also have reverse-flow combustors). However, 'reverse-flow combustor' is listed in the specifications of quite a lot of engines without any explanation of what that means. I'd suggest the best place for such an explanation would be here in the combustor article, that would help to integrate the content across various articles. It needs to be written by someone with more knowledge & expertise than me though... 86.159.170.103 (talk) 20:07, 19 February 2019 (UTC)
 * Did we just go through some of this at Talk:Gas turbine? Andy Dingley (talk) 20:20, 19 February 2019 (UTC)
 * Yes, we did (my ISP has changed my IP address) but from your helpful explanation there I concluded that it's chiefly this article here that's badly in need of some input on this topic. It seems to me that other uses of phrases like 'reverse flow' can only really be properly explained on the page for the individual engines concerned (e.g. Allison Model 250/Rolls-Royce M250) 86.159.170.103 (talk) 22:11, 19 February 2019 (UTC)
 * Yes, well there's another engine with something of the trombone about it. I'm not sure if I'd count it as a reverse flow combustor though - in some ways it's more of an L elbow.  The distinction would come down to the combustion liner, the sheet with the holes in it that separates inlet flow and the combustion gases (fuel is injected downstream of this liner). In a reverse flow combustor, the flow on opposite sides of this liner is in opposite directions. For the 250, the flow outside goes both ways!  (the flow from the two compressor ducts enters in the middle). The distinction probably relies on whichever has the greater mass flow, and for the 250 I presume that's the reverse direction, as most of it enters from the end with the fuel nozzle. However there's also some which blends in downstream of the entry point. Andy Dingley (talk) 22:53, 19 February 2019 (UTC)