Assignment6 (1).docx

DELFT UNIVERSITY OF TECHNOLOGY FACULTY OF AEROSPACE ENGINEERING TRACK FLIGHT PERFORMANCE AND PROPULSION

ASSIGNMENT 6

Aero Engine Combustor

Alexandros Koutsoukos (4856937) Frans Loekito (4776887)

Assignment for the course of Aero Engine Technology

Delft, 18/1/2019

List of Symbols Table A. Symbols Symbol Cp FN ṁ3 T3 T4 p3 FARstoich φ ṁf ηcomb V

Explanation Specific heat capacity Thrust force Air mass flow rate Compressor-exit temperature Turbine inlet temperature Combustor pressure Stoichiometric fuel-air ratio Equivalence ratio Fuel mass flow rate Combustor efficiency Combustor can volume

Unit [J/kg K] [N], [kN] [kg/s] [K] [K] [Pa] [-] [-] [kg/s] [-] [m3]

Problem Statement Nowadays, the regulation on combustion emission is getting stricter. Each aircraft engine have to comply with these regulations, such as NOx and CO2 restriction, in order to be certified, while still fulfilling the mission requirement of the corresponding aircraft. Due to this requirements, aircraft engine manufacturers are striving to optimize the combustion process.

Figure 1(a). Air distribution in the examined combustor, (b) schematics of PW4056 engine combustor [1]. Table 1. Air mass flow (m3), pressure (p3), Temperature (T3 - T4) and net thrust (FN) of the examined operating conditions.

Operating m3[kg/s] p3[Pa] T3[K] T4[K] FN[N] Conditions 40 750000 545 1000 40000 1 77 1550000 670 1250 120000 2 110 2350000 745 1450 200000 3 127 2850000 805 1600 255000 4 To do this, we can control the equivalence ratio of each zone in the combustor, to find the trend of the combustion in each zone in every flight stage (indicated by the amount of thrust produced). The aim of the report is to analyze this trend of the PW4056 engine’s combustion, to see if improvement (in the sense of emission production) is possible. Table 1 shows the 4

2

different operating conditions to be examined, while Figure 1 shows the amount of air flow to each zone in the combustor.

Assumptions The assumptions of this report are:      

The stoichiometric fuel air ratio of kerosene is (FARstoich) 0.0682. Air is considered an ideal gas with constant Cp = 1150 J/kgK. The equivalence ratio for each zone is equal to the equivalence ratio at the end of each zone. Stable combustion. Combustion efficiency ηcomb=0.99. Low Calorific Value of fuel LCV=43 MJ.

Methodology and Calculations To calculate the equivalence ratio of each zone, we first have to find the mass fuel rate for each flight stage. We use the conservation of energy between the fuel chemical energy and the air flow thermal energy: mf 

mair  C p ,g  T4  T3  ncomb  LCV

(1)

Using the values of ṁair and tempratures at each operating condition in Table 1, we can calculate the corresponding mass fuel rate. Next, we can use these values to calculate the overall combustion equivalence ratio in each operating condition.



mf mair



1 FARstoic

(2)

Aside from the combustion overall equivalence ratio, it is also important to analyze the heat density of each operation condition. The heat density correlates to the amount of energy released in the combustion process. We define heat density as: Q V



mair  C p ,g  T4  T3  V

(3)

or the amount of heat energy per unit volume (the can volume of the combustor = 0.08 m3). Lastly, it is calculated the equivalence ratio within each zone in each operating condition using Eq. (2). The air mass flow in each zone is the cumulated air mass flow up to that zone (40% for primary zone, 80% for secondary, and 100% for dillution zone).

Results-Observations Using Eq. (1), (2) and (3), the fuel mass flow rates and equivalence rations for each operating condition are calculated and presented in Table 2. It is obvious that the increase in thrust leads to an increase in mass fuel rate. This increase will also increase the equivalence ratio values, which is justified by the fact that adding fuel in the combustion process leads to a richer combustion. However, the overall equivalence ratio values are still within the lean combustion range (lower than φ = 1, stoichiometric combustion). This fact is in accordance to recent trend, 3

which emphasizes on lower NOx, soot and CO emission (compared to rich combustion). Regarding heat density, it is observed that the fuel increase leads to increase in heat density values, which was expected since more air is added to the system, and the temperature difference per condition increases. Thus, more heat is transferred into the system (higher heat density). Table 2. Calculated mass fuel rate (mf), equivalence ratio (φ) and heat density (Q/V) for the operating conditions.

Overall Heat density Equivalence (Q/V) ratio (φ) [MW/m3] 0.49 0.180 261.63 1 1.21 0.230 641.99 2 2.10 0.280 1114.78 3 2.73 0.315 1451.37 4 Observing Figure 2 and Table 2, the combustion process becomes leaner towards the dilution zone, as was expected. In recirculation zone the equivalence ration is greater than 1 (rich combustion) except for operation in the low thrust condition. Despite that, the recirculation zone is always richer in fuel than the later zones to avoid flame blowout. In primary zone, air is added through primary holes in order to anchor the flame and to ensure complete mixing of the reactants. This addition causes the equivalence ratio to drop, as shown in Table 2. In secondary zone, air is also added to ensure complete combustion of CO and unburned hydrocarbons, and to reduce the temperature (quenching the flow). Thus, equivalence ratio drops further. Finally, the dilution zone is used to further reduce the temperature achieving turbine inlet temperature requirements and to complete the oxidization of CO to CO2. Therefore, equivalence ratio reduces even further. Operating conditions

mf [kg/s]

Primary Zone Secondary Zone

Dillution Zone

Figure 2. Equivalence ratio distribution along the combustor.

These lean equivalence ratio in each zone, however, poses a problem: the flame stability. To prevent a flame blowout, rich combustion at the primary zone is more preferred. The NOx 4

emission, on the other hand, is still acceptable, according to Figure 3 (the NOx production at φ=0.8, at high thrust, is still comparable to that of rich combustion). To ensure flame stability, technique such as TAPS (Τwin Annular Premixed System, certified by GE) can be implemented. The system pre-mix the fuel and air in the pilot fuel injector (rich combustion) and additional small injectors (lean combustion). The overall equivalent ratio in the primary zone is low, but the local equivalent ratio at the exit of the pilot fuel injector is rich, ensuring flame stability. Table 3. Equivalence ratio (φ) values per operating conditions for the Recirculation, Primary, Secondary and Dilution zone of the combustor.

OPERATING RECIRCULATION CONDITIONS ZONE 0.901 1 1.149 2 1.396 3 1.575 4

PRIMARY ZONE (PZ) 0.451 0.574 0.698 0.787

SECONDARY ZONE(SZ) 0.225 0.287 0.349 0.394

DILUTION ZONE(DZ) 0.180 0.230 0.279 0.315

Figure 3. Pollutant products [2].

Conclusions      

Increase in thrust leads to an increase in mass fuel rate. Increase of equivalence ratio with increase in mass fuel rate. Overall equivalence ratio values are still within the lean combustion range. Combustion process becomes leaner towards the dilution zone. Main problem is the flame stability due to lean primary zone. Technique, such as TAPS, can be used to avoid this problem. The NOx emission of the lean PZ, on the other hand, is still acceptable.

References [1]. Assignment 6: Aero Engine Combustor. AE4238 Aero Engine Technology. [2]. Aero Engine Technology Reader. Ir. van Buijtenen, W. Visser, A. Rao. Faculty of Aerospace Engineering, TU Delft 5