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.
Real flow effects in compressors - Friction Losses
Occurs on the walls of the annulus, rotor and the stator blades.
Real flow effects in compressors - Secondary Losses
- At the gap between the tip of a blade and the wall of the annulus. The fluid can move through from the compression side of the blade to the suction side, resulting in a longitudinal vortex. Occurs in the turbine and the compressor, but the secondary flow is more prominent in the turbine as the pressure difference is greater.
- When the flow travels past a bend, vorticies are formed, and this can’t be recovered. The turning creates a centrifugal force, a large centrifugal force in the centre, but a smaller c.f at the end creating vortices at the tips of the blades.
Real flow effects in compressors - Stall losses
Essentially, separation can occur. The detatched shear layers are highly unstable and roll into vortices moving downstream. To limit this, always want the de Haller number> 0.72. The thicker the boundary layer, the more likely separation is. Separation occurs on the suction side of the blade, and the higher the incidence, the more likely separation is.
Real flow effects in compressors - Shock losses
Shocks can form when the flow upstream of the blade is supersonic. The shock itself leads to losses, but also causes separation of the B.L. The downstream the point B (the point at which the shock wave reflects off of the neighbouring blade), the stronger the shock will be. This is due to the fact that supersonic flow propagates along the convex suction surface. So, to reduce shock losses, we want to have smaller pitch.
Real flow effects in compressors - Surge
Essentially, if you are on the left of the unstable point, any decrease in the axial velocity only decreases the pressure rise, and you slow down even further. However, being on the other side gives the opposite reaction. Surge is the phenomenon of large mass flow rate oscillations.
Real flow effects in compressors - Rotating stall
If the flow coefficient is in the unstable region, rotating stall can occur w/o mass flow rate oscillations. The flow stalls in a group of neighbouring blades. Since stalled blades create a smaller pressure rise, the average axial velocity behind them becomes smaller. So, the neighbouring blades have a change in incidence, reduction on one side and an increase on the other. Therefore, the separation region will shift, and move around the annulus. This creates extra unsteady loads on the blades.
Real flow effects in compressors - Stage stacking
Essentially, we always want the mass flow rate and the Vax is all stages to be kept constant. But, if the flow coefficient is below design regime in the first stage, the pressure rise will be higher. So, the density ratio will be higher, but the Vax ratio will be less than 2. As a result, this continued until the engine is stalled.
Compressors - Fuel staging
Burning fuel rich or lean mixtures can reduce the NOx emissions, but burning at near-stoichiometric conditions at least in the primary zone is necessary for flame stability. So, we introduce a part of the fuel is heated in the pilot zone where the NOx emissions are high. The rest is added in the main zone. The pilot zone is supplied w fuel at all times, but the main zone fuel rate can be increased as necessary, w/o burning at the highest temperatures.
What is the effect of the absolute value of the pressure in the combustor on the characteristics of the combustor?
Increasing the pressure results in increased density. This means increased flame stability (increasing the density reduces the free path of the molecules - so more likely reactions). Due to increase in density, is the same amount of combustion mixture can be burned in a smaller combustor.
Explain why Mach Reflection occurs?
Mach reflection occurs because the max deflection angle becomes smaller than the turn angle required for the flow to return to the same direction in a regular reflection structure.
Explain Petrov’s formula
Regular reflection can be impossible due to a separation caused by the shock. The thin boundary layer near the wall experiences a sudden increase in pressure before and after the shock, and if this pressure rise is high enough, the BL will separate. Separation and reatachment leads to considerable losses. Petrov’s formula can be used to determine if separation will occur. They are only valid for a BL which developed under an action of zero pressure gradient over a certain distance upstream from the separation point and only for separation on a smooth wall. This is valid for the turbulent BL only.
Explain why the first stages have stalling and why the last stage have stalling.
Due to the stage-stacking phenomenon the axial velocity changes from stage to stage in the off-design regime, increasing downstream when the pressure ratio is below the design value, and decreasing downstream when it is above the design value. Because of this, the first stages are more likely to stall when the pressure ratio is below the design value, and the last stages are more likely to stall when the pressure ratio is above the design value
How does the width of the corridor change depending on the number of stages, and/or the overall compressor pressure ratio?
The more stages and/or the larger the pressure ratio, the greater is the difference in the axial velocities in the first and last stages in the off-design regime, and hence,
the less variation of the mass flow rate will be allowed within the constraints that both the first and the last stage are not stalled or choked. Hence, when the number of stages and/or the pressure ratio increases, the corridor becomes narrower.
