Propulsion and Turbomachinery Flashcards
What are the 3 types of combustors?
- Can Combustor.
- Cannular Combustors.
- Annular Combustors.
Explain can combustors
Each can has its own outer separate casing. So there are 8 outer casings, inside of each chamber is a flame tube.
Explain cannular combustors
These are better than can combustors because they tend to have less space in between the cans. Instead of having separate outer casing for each can, there is simply 1 outer casing for them all.
Explain annular combustors
It utilises all the space available in an axi-symmetric method, and is the most efficient method.
What are the different zones of zonal combustion?
Primary zone where about 15-20%
of air is introduced, so that combustion is near the stoichiometric ratio, the more stable. However, the high temperatures cause dissociation, and the combustion is incomplete.
Secondary zone where another 30%
of air is added gradually (to avoid over-cooling) to achieve complete combustion.
Tertiary, or dilution zone, where the temperature is reduced to allowable limits by thorough mixing with the remaining air.
Why is zonal combustion needed?
Generally, we want to work at the stoichiometric mixture, as that gives the most stable (most self-sustainable) flame. However, this would have a temperature greater than the maximum allowable inlet temperature. But as the stoichiometric mixture doesn’t burn completely due to dissociation, the 3 zones are used instead. As if the combustion products are cooled too quickly, the recombination will not have enough time to occur and the combustion will be incomplete.
Define dissociation
At high temperatures, molecules collide at such velocity that they break down from the impact. After combustion, stoichiometric mixture still contains many broken molecules which constantly react again, and are broken again. If combustion products are cooled too quickly, then the reaction again will not have enough time and the combustion will be incomplete.
What are the reasons for the primary combustion zones?
Essentially, s the flame propagation velocity is much less than the air velocity, we have to cerate an organised flow so that the hot burning mixture has to return back to the incoming flow and air.
What are the 3 primary combustion zones?
Swirl, upstream blowing and vaporiser.
Explain the swirl primary combustion zone
In swirl combustors the air is being given a rotation by swirl vanes, and
also by tangential blowing through holes in periphery. This results in low
pressure on the axis of rotation. Somewhat further downstream rotation is
weakened by friction, and the pressure on the axis is higher: as a result, reversed flow appears.
Explain upstream blowing in primary combustion zones.
With this configuration it is difficult to prevent overheating of the injector. For this reason
upstream blowing is mostly used in afterburners, which switch on only for a
limited time.
Explain the vaporiser method in primary zone designs.
A jet flow fuel and turns, which causes the fuel to vaporise.
However, it doesn’t work when it’s not hot enough.
Fuel injectors and atomisers - information and what are the 2 types of fuel injectors?
The pressure drop is proportional to the fluid droplet size.
Too small droplets means they might not penetrate far enough into the primary zone.
Too big means they may not evaporate fast enough.
So the pressure drop is controlling the droplet size, and not the fuel flow rate. To keep the drop size near optimal, the
pressure drop should not be varied extensively
2 types are:
Duplex and the spill burner.
Explain Duplex atomisers
There are 2 conical channels and only the outer one is switched on at high loads.
Explain the spill burner atomisers
They have an extra outlet, form which the extra fuel can be spilled out.
Air injection prevents formation of carbon deposits. And the amount of fuel out is controlled.
Explain stability loop.
It is a function of the pressure in the chamber. The reaction is more likely to happen if the molecules collide at a higher speed.
In the primary zone, the temperature is the max temperature corresponding to the stoichiometric mixture.
Pressure Losses and combustion intensity
Explain why heat addition results in pressure loss in the combustor.
Heat addition results in an increase in temperature, which in turn causes a decrease
in density. To keep the same mass flow rate, therefore, velocity has to increase in the downstream direction. This acceleration requires a pressure drop, known as fundamental loss
Explain skin friction and large scale turbulence resulting in the pressure loss in a combustor
Skin friction is controlled by small-scale turbulence in the wall boundary layers - to ensure good mixing. This leads to extra losses. The pressure drop in a combustor scales with the square of the mean velocity.
What is the combustion intensity?
Heat release rate /( combustor volume x pressure)
Emissions in Combustors.
Co2 emissions are strictly proportional to the fuel burnt. The main factor controlling NOx is the flame temperature. NOx production rate grows exponentially with flame temperature, and is near mx for stoichiometric conditions. But we have to use stoichiometric conditions for at least the primary zone. - So fuel staging in the solution.
Real flow effects in compressors - Blockage
Due to friction on the walls of the compressor annulus, the axial velocity varies along the span of the blade, tending to 0 at the walls. This leads to a non-linearity of the axial velocity- increasing for all stages downstream. The loss of work can be accounted using a work done factor. Usually 0.95 in the first stage, dropping to 0.85 by the 8th stage.
Real flow effects in compressors - Explain the physics behind blockage.
With a given mass flow rate through a stage, the blockage effect increases the Vax through that stage, leading to a variation in α2. As α’2 is fixed by the geometry of the blade, an increase in Vax means there is a reduction of the total temperature rise in the stage.
Real flow effects in compressors - Deviation.
In real flow, the air outlet angle isn’t equal to alpha’2. If the pitch is increased, the mass flow rate per blade increases. And if Fb is fixed, the flow deflection angle decreases. Eventually, this means increasing the angle of attack, approaching stall. Overall, increasing solidity (omega > 1.5) deviation results in an increase of friction losses. Low solidity means a lot of space between each blade.