Afterburner Design

[Yoshi Heffernan]

Icame across a NACA summary paper on the past 5 years of afterburner development and started reading cause it seemed interesting. However, a couple of pages in I scrolled back up and realized that it was written in 1956. How much have afterburners changed since then and would the information contained in the article still be relevant?

One main area of extensive research that is still going on nowadays and might have changed the most along with materials is the combustion efficiency and effective mixing of the fuel and air while keeping short afterburner lengths. Also combustion and flame stability have been important issues for which much research have been carried out.

For a fixed airspeed , this means that the level of thrust depends on both the exit jet velocity of the gases and the mass of air flowing through the engine per second. So to produce high levels of thrust we can either accelerate the exhaust gases to a greater velocity, or just increase the amount of air that is being sucked into the engine. Early turbojets attempted to maximise the exit jet velocity in order to create more and more thrust. The downside of this approach is that it decreases the efficiency of the engine. The propulsive or Froude efficiency of a jet engine is defined by the power output divided by the rate of change of kinetic energy of the air. The kinetic energy of the air represents the power input to the system. The power output P is the product of force output i.e. the thrust F and the resulting air speed . Although this is an approximation, this equation summarises the essential terms that define aircraft propulsion. So, power output is

This means that for a fixed airspeed , the efficiency can be increased by reducing the jet exit velocity . However, decreasing the jet exit velocity decreases the thrust unless the mass flow rate is increased as well. Note that the advantage of increasing the mass flow rate is that it does not have an effect on the propulsive efficiency.

The hot jet from the turbine flows into the jet pipe at a velocity of around 250 m/s to 400 m/s, and this velocity is far too high to guarantee stable combustion in the jet pipe. Just prior to the jet pipe, the cross-sectional area of the exit portion to the turbine increases to diffuse the flow to lower velocities. However, because the standard injection rate of kerosene at a good fuel-to-oxygen mixture is only around 1-2 m/s, the kerosene would be rapidly blown away even by the diffused jet stream. To prevent this a vapour gutter is placed just prior to the fuel injection nozzles that spins the jet into turbulent eddie currents, thereby further slowing down the hot turbine exhaust gases and allowing for a better mixture of fuel and jet stream. A common misconception is that due to the high temperature of the gases exiting the turbine (around 700C), the fuel-oxygen mixture in the jet pipe would combust spontaneously. Cooler combustion flames can develop at these temperatures, but because of the atmospheric pressure differences between ground level and altitude, a design that spontaneously combusts at ground level would never do so at altitude. To guarantee a stable and smooth reaction over a wide range of mixture ratios and flying altitudes, a high-intensity spark is needed.

The increase in thrust is a function of the increase in jet pipe temperature as a result of afterburning. For a perfectly efficient system, the relationship between the temperature ratio before and after fuel is burnt, and the thrust increase is nearly linear in the typical operating range with temperature ratios of 1.4 to 2.2. Within this range we can expect a 40% increase in thrust for a doubling of the temperature in the jet pipe. Thus, if afterburning raises the jet pipe temperature from 700C (973 K) to 1500C (1773 K) this results in a thrust increase of around 36%.

In a static test bed, thrust increases of up to 70% can be obtained at the top end, and at high forward speeds, several times this can be achieved. The lower the temperature exiting the turbine and the greater the extent of uncombusted oxygen, the greater the temperature increase in the jet pipe due to afterburning.

As is to be expected, afterburning naturally incurs a fuel consumption penalty, and this is why afterburning is typically constrained to short bursts. The aim of the compressor in a classic jet engine is to raise the pressure of the incoming air to the optimal pressure for efficient combustion. After expansion by the turbine stage, the gases are at a lower degree of compression, and therefore the fuel is not burnt as efficiently as in the combustion chamber between compressor and turbine. For a 70% increase in thrust the fuel consumption can easily double, but of course this increased fuel consumption is balanced by an improved performance in terms of take-off and climb. This means that the increased fuel consumption is balanced by the time saved to cover a desired distance or operating manoeuvre.

The inspiration of this post and the diagrams have all been taken or inspired by [1] Rolls-Royce (1996). The Jet Engine. Rolls Royce Technical Publications; 5th ed. edition (Amazon link). For anyone interested in jet engine design this is a beautiful book, describing lots of intricate details about jet engine design and presenting the information in an intuitive and visually pleasing manner using diagrams as used throughout this post. I can not recommend this book enough.

VORON Design is a group of volunteers passionate about creating cool parts and machines that others can build. VORON Design does not sell anything but releases all of their designs under the GPLv3 open source license.

The Voron Forum has chosen to be supported by selected banner ads from our trusted vendors. We find it sneaky to rewrite links for profit, so we don't do it and don't allow others to either. That banner is the only advertisement that will ever appear.

I've recently become infatuated with the Voron Afterburner and really want to see it any or all of my 14 Prusa MK3s'. I've never designed my own extruder or modded one, so I'm wondering, before I jump into it, has anyone created an adapter to allow the Afterburner to work with a Prusa?

Secondly, I would LOVE if PR added QOL enhancements to the stock MK3 design similar to those the Afterburner has. These are including, but not limited to: swappable toolheads, flip-up hotend fan, more secure PINDA probe, and 2 sided parts fan.

Just my opinion/hope, but since not as much modding stuff seems to be getting designed first for the Prusa these days, I think the best hope is that a common platform like the wam bam mutant takes off. That way, if anyone designs a Voron Afterburner for that platform, it can be readily attached to *any* printer, including the Prusa. I think it will catalyze innovation by tapping a broader community of modders in a way that we haven't seen for a while now.

To move an airplane through the air,thrust is generated by some kind ofpropulsion system. Most modern fighteraircraft employ an afterburner on either a low bypass turbofanor a turbojet. On this page we will discuss some of the fundamentalsof anafterburning turbojet.

In order for fighter planes to flysupersonically ,they have to overcome a sharp rise indragnear thespeed of sound. Asimple way to get the necessary thrust is to add an afterburner to acore turbojet. In a basicturbojet, some of the energy of the exhaust from the burner isused to turn the turbine. The afterburner is used to put back someenergy by injecting fuel directly into the hot exhaust. On theschematic, you'll notice that thenozzleof the basic turbojet has been extended and there is now a ring of flameholders, colored yellow, in the nozzle. When the afterburner isturned on, additional fuel is injected through the hoops and into thehot exhaust stream of the turbojet. The fuel burns and producesadditional thrust, but it doesn't burn as efficiently as it does inthe combustion section of the turbojet. You get more thrust, but youburn much more fuel. With the increased temperatureof the exhaust, the flow area of the nozzle has to be increased topass the samemass flow.Therefore, afterburning nozzles must be designed with variable geometryand are heavier and more complex than simple turbojet nozzles.When the afterburner isturned off, the engine performs like a basic turbojet.You can investigate nozzle operation with our interactive nozzlesimulator.

The nozzle of a turbojet is usually designed to take the exhaust pressureback to free stream pressure.The thrust equation for an afterburning turbojet is then given by the generalthrust equationwith the pressure-area term set to zero. If the free stream conditionsare denoted by a "0" subscript and the exit conditions by an "e" subscript,the thrust F is equal to the mass flow rate m dottimes the velocity V at the exit minus the free stream mass flow ratetimes the velocity.

3a8082e126