31

It is actually not that simple to ensure the proper gas speed in a gas turbine. In the compressor you want to limit the flow speed over the compressor vanes to the high subsonic range, so the inlet has to decelerate the flow down to approx. Mach 0.4 - 0.5. Less would mean less throughput and consequently less thrust. This speed, however, is far too high for ...


26

The air is compressed in the compressor section of the engine and heats up. Hot air is able to hold disproportionally more water in gaseous form than cold air, so all that water will vaporize and enter the combustion process together with the air. Since it is already vaporized, it will not absorb that evaporation enthalpy again that makes water such a good ...


19

The air in the compressor is both compressed and moving downstream towards the combustion section. The combustion does not create enough pressure to overcome all of that, and there is lower pressure as the air gets expanded through the turbine sections. When the pressure in the compressor section drops too much, the combustion flames do expand in both ...


12

Disclaimer: I may have spent several hours on Wikipedia at one point trying to answer this question for myself! Jet engines use the Brayton cycle, which is a "isobaric" process during combustion, meaning it keeps pressure constant during that phase. This is in contrast to the Otto cycle of a typical four-stroke piston engine, which is "isochoric" during ...


11

Single combustor cans are simpler to develop. To get the ignition and combustion process right in an annular combustor is quite a challenge. When the first jet engines were developed, Junkers decided for separate cans while BMW wanted to design a theoretically superior annular combustor. In the end, the troubles with the combustor meant that Junkers got ...


9

Quick answer: At the design point considered, which is assumed to be takeoff conditions, a CFM56-5C combustor raises gas temperature to 1360°C (the share of the compressor in this raise is about 600 °C). This is a common value found in modern turbine engines, a balance between fuel consumption reduction and turbine material capabilities (nickel super-alloy, ...


8

OK, this answer will definitely be trumped by better ones, and Google will serve you better, but until then.... Thrust is basically throwing a mass of stuff in the opposite direction you want to move in. The more weight &/or speed you can throw it, then more thrust. We want to throw air (because there's a lot of it around, and it's what we want to be ...


6

Modern jet engines do not use can or cannular combustors. GE made the switch to annular combustors by at least the J85, which had a first engine to test in the 1950s. The TF39 (1964) engine formed the basis of the CF6, which also uses an annular combustor. I don't know of any GE engine since the TF39 that did not use some form of annular combustors. Modern ...


6

As usual, it is a combination of measures: Flow speed at the start of the combustion chamber is the lowest in the whole engine, in the order of 30 m/s. High fuel pressure and many small fuel injector nozzles, such that the mean fuel droplet size is kept as small as possible Flame holders cause a pocket of stagnated, burning flow which helps to ignite the ...


6

In general, aircraft jet engines have diffuser section(s), that reduce the velocity of the incoming air before it enters the combustion chamber. Sample velocity profile in jet engine; image from Fundamentals of Gas turbine engines In some cases, the the diffusers are before compressors or in the stages itself- but the end effect is reduction in velocity. ...


6

Combustors usually feature recirculation zones in which the combustion takes place either completely or which at least anchor the flame. This can be done by several designs Sudden jump in cross-section in combination with vorticity Flame-holder bodies Central displacers Vorticity brake-down The blade channel of a compressor is actually divergent. ...


5

Here's an engine with old combustor cans: Here's a single annular combustor: And in cross section: And now here's a dual annular combustor (DAC) (pic from here) in cross section: See how it's like one ring (or donut) inside another ring. Wikipedia states "like an annular combustor, the DAC is a continuous ring without separate combustion zones around ...


4

The GE90-115B has a bypass ratio of 9:1. So if 1000 units [of air] enter the fan, only 100 will make it to the combustion chamber. Kerosene burns efficiently at 15:1 (air to fuel). But not all the air in the combustion chamber is burned. Only around 22 (average) of the 100 will be burned, the rest will provide cooling, flame stabilization, and dilution.* ...


4

By SidewinderX (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons Swirler The swirler is a part of the combustor that the primary air flows through as it enters the combustion zone. Its role is to generate turbulence in the flow to rapidly mix the air with fuel. Fuel injector The fuel injector is responsible for introducing fuel to ...


4

This image that TomMcW posted in the comments is a much better representation of what is going on. First, looking at axial velocity only is only half of the story, through the compressor and especially through the turbine the airflow can be more circumferential than axial. In fact the point of the stator vanes and turbine nozzles (turbine vanes) is to turn ...


4

The flow enters the compressor at approximately Mach 0.4 to 0.6 in order to ensure a high mass flow with still subsonic compressor blade speeds. Compressing the air allows to reduce flow speed, and between compressor exit and combustion chamber is a diffusor to reduce flow speed even more. Why? To ensure a high degree of combustion! The longer the reactants ...


4

The first operational jet engine contract was awarded to GE/Allison as the J35 and was later upgraded to the J47. When I was at tech school for the J47 we were told GE was given the contract because they had steam turbines of similar size and quite literally modified the steam turbines to have cans and use jet fuel! The conversion was a no brainer for the ...


3

I happened upon this question, and thought I could add some information, as I recognized the powerplant right away. This is not another full answer, but only a response to @albin and @PeterKampf as to the engine model. I cannot yet add comments, so edit this as is necessary. The engine picture you used in your answer is the power section of the Allison ...


3

How engineers are able to slow air without decompressing it Flow speed is energy, so slowing down the flow will increase its pressure. At subsonic speeds the first part happens already ahead of the intake when the airplane approaches. This pre-compression is very efficient because it happens in the free stream, and the engineers design the intake such that ...


3

If you are asking about combustor liners specifically (as opposed to the hot section in general), the ones I am familiar with are cobalt-based superalloys.


2

See this answer for the difference between a SAC and DAC combustor. The EASA type certificate for the CFM56-7 also identifies which engines have the DAC and which have the SAC. The answer depends on exactly which -7B26 version you are referring to.


2

I chose the GE90-115B because it is the largest turbofan in terms of thrust. Unfortunately, it was hard to come by specific fuel consumption figures for cruise conditions, maybe because the -115B is a relatively new version of the GE90. But I finally found enough for a preliminary answer, so here it is. From this pdf, I found that the GE90 at cruise ...


2

This is more of an engineering question than an aviation question, but since some aviators may be interested in knowing exactly what is in those turbines I will answer. There are three main types of refractory alloys commonly used for high-temperature components in turbine engines. They are: ODS (oxide dispersion strengthened superalloys) CMC (ceramic ...


2

In The Jet Engine, there is no mention of a two-material setup for the flame-holder/combustor. It's all about the design of the diffuser and combustor shape / holes. One of the fancy things they mention (p. 122, image below) is the lining of the walls in elevated ceramic-coated tiles for internal cooling. Having to deal with two materials would decrease ...


Only top voted, non community-wiki answers of a minimum length are eligible