13 Appendix D: Combustion chamber performance characteristics
13.4 LINER GEOMETRY
Appendix D: Combustion chamber performance characteristics
with
4
the total pressure upstream of the inlet hole, p j the static pressure downstream of the inlet hole.The discharge coefficient [Cd] of a hole is the relation between the smallest area of the flow and the actual area of the flow. The discharge coefficient is used to describe the inlet hole's characteristics and the flow through the hole. Only plain, circular liner inlet holes will be utilized in this study, and for non-swirling flow, the coefficient of discharge for plain circular holes is defined by Lefebvre (1 996) as
with
a
the relation of mass flow through the hole [h,] to the annulus mass flow rate[m,,,] ratio and K the ratio of the jet dynamic pressure to the annulus dynamic pressure upstream of the holes.K should vary between 2 and 6.
\ \
Figure 13.3: Illustration o f flow through liner wall.
Other factors that influence the combustor chamber's aerodynamics that will not be discussed in this study include jet trajectories and duct geometry.
the required velocity reduction in the shortest distance, with minimum loss in total pressure and uniform, stable flow conditions at the exit.
---3
- - -
Direction of air flow
Figure 13.4: Diffuser geometries.
The design parameters of a diffuser are the air ratio [AR], wall 1ength[ldfl], the inlet radius
[$,,]
the divergence angle [B].
Their relationship is defined by:The pressure loss over the diffuser [ A ] needs to be at a minimum, since any pressure loss incurred in the diffuser makes no contribution to combustion. Typical pressure loss values for diffusers with high length1 depth ratios are in the range of 0.15 to 0.45.
13.4.2 Primary zone [Pz]
The function of the primary zone is to anchor the flame and provide sufficient time, temperature and turbulence to achieve near complete combustion of the incoming airlfuel mixture as illustrated in Figure 12.4 and Figure 12.5. About 15 to 20 % of the combustion inlet air is fed through the liner around the jet and the primary inlet holes. The inlet hole position of the primary zone is in the range of 0.5 to 0.6 times the downstream liner diameter. Recirculation is created by the positions of the primary inlet holes in the liner, and the inlet hole diameter in relation to the liner diameter: 0.17 .d,,,,, = d,i,,cr if six or eight inlet holes are used around the liner.
Around 50 % of the injected air is re-circulated in the primary zone and an illustration of re- circulation can be seen in Figure 12.5. The equivalence ratio
[#]
should never be lower than 1.5 to minimize smoke and emission gas.Figure 13.5: Illustration of air flow in primary zone.
The conceptual design for development of a micro gas turbine generator.
Avwndix D: Combustion chamber wrformance characteristics 13.4.3 Secondary zone [Sz]
In order to achieve an effective burning process in the combustor, the gas leaving the primary zone should be given more air and time to combust, while more oxygen is needed to complete the intermediate chemical processes. The equivalence ratio [@]in the secondary zone is in the order of 0.8, and only I0 % of the total inlet mass flow enters the liner through the secondary zone.
Secondary zone length is set as 1.5 times the liner diameter (Lefebvre 1998).
13.4.4 Dilution zone [Dz]
The role of the dilution zone is to admit the air remaining after combustion for wall cooling requirements, to produce an exit gas stream with a temperature distribution that is acceptable for the turbine. The ideal temperature distribution pattern is one that gives minimum temperature at the turbine blade root, where stresses are the highest, and also on the blade tip, to protect seal materials. Lefebvre (1998) defined the pattern factor as follows:
Tm - T o U l Pattern factor =
- -
Dilution air is introduced by one or more rows of inlet holes in the liner wall. The inlet hole geometry and the dilution length is related to the liner width. Note: The liner length to diameter ratio
[ ]
of the dilution zone lies in the range of 1.5 to 1.8.dlirwr
The number of inlet holes in the dilution zone has to be optimized. If the total dilution-hole area is spread over a large number of small inlet holes, penetration will be inadequate, and a hot core will persist through the dilution zone. The use of a small number of large inlet holes will result in a cold core, due to over penetration, and unsatisfactory mixing. Lxfebvre (1998) provide two methods for determining the number and geometry of dilution holes; 1) The Cranfield method and 2) The NASA method, and compare them as well. In this study the Cranfield method will be used to design the dilution zone of the combustor to be used in the TCIR cycle.
13.4.5 Liner cooling slots
Removing heat from the liner wall by injecting a cold air stream in between the hot gas and the liner wall is known as film cooling. This is done by a number of cooling slots through which air is injected axially along the inner liner wall in order to protect the liner wall from the hot gas.
The film is gradually destroyed by the turbulent nature of the combustion gas, so more slots occur at regular intervals down the liner length.