hey everyone, in this video i'm going to be talking about afterburners i'm not going to be going into any math or equations other than this one right here but will still be going into the details
ge 3d printed jet engine, of the operation. if you just want to see a high-level overview of what afterburners are check out my "in a nutshell" video. so where do we usually
see afterburners and what are their purpose? they're usually used during takeoff, for periods of acceleration, and for supersonic flight. because of these factors they're usually only used in military aircraft although there are a couple of civilian designs that have included them. in general terms let's see what an afterburner actually does. so
take a look at this engine here. first we've taken some air, passed it through a compressor, add some fuel and ignite it, extract enough energy from the turbine to power the compressor, and then with the left over energy we turn it into kinetic energy to spit out the back at a certain velocity u_e and that gives us our thrust. the thrust of an engine
essentially boils down to the change in momentum of the gases going through the engine as you can see in the simplified thrust equation right here. so in a normal engine the gases coming out the back of an engine with no afterburner still have a pretty high exit velocity, but let's say we wanted to increase the thrust by increasing the exit velocity
even further. because engines operate fuel lean due to temperature limitations of materials using the turbine there's excess air left over that wasn't used for combustion and we can take advantage of this excess air by adding some more fuel in the afterburner, igniting it, and extracting more energy and with that energy we can now increase the kinetic
energy of the exhaust gases which increases the velocity of the exhaust gases which thus increases the thrust. you might be asking yourself why we don't just design the engine initially for that high thrust, and then just ignore the afterburner all together. well it makes more sense in terms of efficiency to design the engine for a lower dry
thrust and then have an afterburner for a period when you need the increased extra thrust. the weight reduction of only having to add that hollow jet pipe at the end of the engine outweighs the increase in fuel consumption and a flipping simply decreases in stagnation temperatures and stagnation pressures through the engine are indicative of
losses which are bad by adding a jet pipe the afterburner you end up getting stagnation pressure losses due to the drag from the flame holders friction from the pipe and when the afterburners on heat addition at a finite flow speed or mach number however these losses again are a better alternative than increasing the dry thrust of the engine
make using you can only have an afterburner a turbojet engines but you can actually have them in turbofan engines as well and in fact most military engines now are actually turbofan engines just with very low bypass ratios this comes with its own set of problems because before we can burn a few
evaporate it an evaporation is harder at lower temperatures the bypass air is at a fairly low temperature compared to the core stream so it can be hard to ignite so to sum up the overview afterburners take the fuel-air mixture coming out of the turbine at specifically metered fuel to it ignite it increase the energy in the jet pipe here and send it out the
propelling nozzle to increase the exit velocity of the gas coming at the back end thus increasing with the rest of the engine you do get a little bit of extra thrust coming in from the mass fuel flow rate that's going into the engine it ends up being dwarfed by the addition of the energy from the combustion of the fuel air mixture now that we've gone
through an overview of the afterburner let's go into more detail in the components and we're going to start with the entry to the afterburner called the diffuser the afterburner jet pipe is connected to the exit of the turbine by something called a diffuser and while you may not even see the diffuse when you're looking at a turbojet engine or a
turbofan engine or even think about it when you thinking about afterburners it's a super important component because it needs to slow down the flow coming from the turbine before enters the afterburner there's a couple of reasons that we want this below velocity coming into the afterburner the first is that flame stabilization is harder at higher
flow velocities and the second reason is that the amount of heat that you can add to the flow was governed by the mach number and if you add too much each of the flow you can actually choke the flow if you want to learn more about this check out rayleigh flow also from rayleigh flow the stagnation pressure loss for the duct is higher at higher
mach numbers and recall we want to reduce this stagnation pressure loss for the engine since the flow coming out of the turbine is subsonic to slow down even more we need to have an increasing area but we don't want the area to increase too much because we want to keep the diameter of the afterburner pipe about the same as the components in
as the other components in the engine so we don't have any problems with installation on aircraft now that we've slowed the flow down we need to inject the fuel into the air stream the mainland is accomplished is by having radial tubes that are perpendicular to the engines access you can see this is a front or back for you whatever you want
to call it of the engine so i'm just taking a cross-section view here you can see that these red lines are these spray bars and this also is just zooming in on one of those single spray bars and with the flow coming towards the camera towards you and so you can see that the spray bars shoot the fuel out sideways which ends up being perpendicular to the
gas stream to ignite the fuel we have to evaporate the fuel droplets so that's why we have the spray bars injecting fuel perpendicular to the gas stream because the gas stream then tears apart these drop with into smaller droplets and heat transfer from these hot gases still hot gases coming from the turbine heat transfer from that hot gas to the
fuel droplets evaporates them and once they're a bath rated we can ignite them one of the main factors for droplet evaporation is the initial droplet diameter so it beneficial to have some of the smallest droplet dimers and you can have it's also beneficial to have high pressures here because as the pressure drops the droplet diameters
increase we need to a higher evaporation time so droplet evaporation times are higher for higher temperatures so if you have a turbo fan that has a bypass stream that's kool the fuel injection the bat stream will take longer to evaporate than from the core stream so we've injected fuel into the stream that's been evaporated and this fuel-air
mixture has a lower flame propagation velocity and then the velocity of the flow stream so what i mean by this is imagine i take a little fuel air mixture droplet thing and ignite it and throw it away from you as fast as i can if i throw it away faster than the flame can propagate back towards me it won't get back to me and this is what happens in
an engine where if you ignite some piece of fuel mixture here it will propagate down the tube faster than the flame will propagate back up to here this is important because after the fueler mixtures ignited here won't be able to keep the new incoming fuel-air mixture ignited because it will have blown out the back of the engine and this is
called blowout so one way to set up a constant ignition source then is to use these bluff bodies also called flame holders which set up a recirculation zone behind them which keeps the gas is recirculating and igniting the new fuel air mixture coming past them the ignition process just needs to start to stabilize flame and then you can turn
off the ignition source the spark or whatever they're using although some engines i've only do use a constant ignition source as well so another question might be then if you have an ignition source let's say an ignition source down here at the bottom of these flame holders this is me showing a view from the back engine looking in at these
flame holders you see there's one of them here one of them here and they have these connecting loops here so if you have ignition starting at the bottom here it's been shown that once a little portion down here ignites it'll spread around these gutters and then through these little passages as well and can ignite all of the fuel air mixture
that's recirculating in those zones all around the flame holders different engines have different flame holder orientations some of them have some cluster here and they can be staggered and also in the bypass stream and it's very engine dependent engines also stagger the ignition of these different of the
different flame holders to avoid pressure fluctuations in the engine that are undesired like i mentioned before the bypass stream is colder so it's harder to ignite there because the temperatures are lower and so what you can do then is first ignite the core stream have that set up a nice stabilized recirculating flame and then
try to ignite the bypass streams the size of the flame holders of the trade-off because figure bluff bodies will give a better recirculation zone however they will also increase the drag in the engine now we have the fuel evaporated ignited and stabilized in these flame holders we can make some interesting observations if we assume
that the heat is added uniformly in a 1d channel and of course the afterburners of 3d object but making this assumption can give us some interesting insights for subsonic flow the gas will speed up with heat addition but this can only happen up to a certain point at which the mach number will then be one so the flow will be sonic and then the duct
will be choked i've mentioned before this one the heat addition is called rayleigh flow similar results can be found for friction in a pipe and this is called fanno flow and so the friction in this pipe will again increase the mach number but only up to a certain point where the mach number of n equals one and the slowest sonic and then you get
choked flow again the length of the jet pipe from here here is also important because we need to make sure that there's enough time for the chemical reactions to fully take place to release the total amount of energy that we can however with longer jet pipes friction also increases and so does the weight of the actual structure of the jet pipe
aside from just a choking phenomena of the rayleigh flow and the fanno flow so the heat addition and the friction you also get stagnation pressure losses which we mentioned was not good for engines using the lower mach number coming in will decrease those stagnation pressure losses but you'll always get it when you add heat and have friction in a
finite mach number flow now large amplitude pressure oscillations accompanied the combustion process we'd like to damp out these high pressure oscillations before any structural damage is done so we have these resonators that are built into the afterburner walls so down here you can see a schematic of the liner and then
the outer case and then the liner we have these holes that are placed strategically along it so the first benefit of this liner is that the process of pushing and sucking air backs out through these holes takes energy out of the acoustic modem puts it into the kinetic mode for acoustic suppression these liners work best at frequencies of
but kilohertz and a second reason for these liners is that we'll have cooler air passing between the liner and the outer case and that will help keep the outer case at a reasonable temperature so now we've burned all the fuel to air we need to intelligently push all this gas out the exits recall from before that said the most efficient operation
of the engine is when the gases are expanded at the exit of the nozzle to the atmospheric exit pressure the control system of the ass burner wants to meter the fuel to keep the pressure level in the afterburner at the right level by increasing or decreasing the novel throat area engines have used both open loop and closed-loop systems in an
open loop system the pilot requests a certain throttle and based off of a predetermined schedule the engine delivers a certain fuel flow rate and a certain throat and exit area for the nozzle closed-loop systems have more feedbacks that allow you to more accurately set the fuel flow rate and the nozzle exit area but they include
more sensors in the design which are not actually trivial to add if you've invested the effort into using an afterburner in your engine you're most likely going to use a converging diverging nozzle to expand the gases in your engine converging nozzles are cheaper and they weigh less but converging diverging novels are better
at a sufficiently expanding out the gases and thus increasing the exit velocity and the thrust there's two areas to the nozzle one is the throat which is the minimum area of the nozzle and the second is the exit area the throat of the nozzle 6 is the mass flow rate through the engine while the exit area the ratio of the eggs area to the
throat area fixes the expansion of the gases through the nozzle so the culmination of the prophecy is discussed previously it's pretty much just a super high exit velocity coming out of your engine which increases the thrust of the engine at increased rates of fuel flow well at a quick glance the afterburners seem like a pretty simple part of a jet
engine they're in fact just as complex as the other parts of the jet engine i hope this video give you a little bit
more insight into their operation and if you want to check out more check out the references in the video description thanks for watching
