1. Combustor modeling in a 1D flow network tool

2. agenda Reasons for a 1-D flow network gas turbine combustor model?
1D network model approach
Advantages
Flow distribution modelling
Heat transfer process modelling
Combustion process modelling
Flownex Demonstration Model
Why Flownex?
Methodology
Results

3. Reasons for a 1-D flow network gas turbine combustor model?
Preliminary design phase or when considering modifications to existing designs it’s essential to make realistic predictions of
Mass flow splits through the various air admission holes
Total pressure losses
Liner temperatures along the length of the combustor
Although powerful, CFD solutions of combustors are specialized, time consuming processes and therefor seldom used during initial sizing of a combustor
Network tools allow more accurate initial designs, less time is spent on advanced 3D simulations and rig tests, thus reducing development time and cost

4. 1D network model approach
Model of independent sub-flows linked together to model a certain process, that can be described by overlaying a network on the system geometry comprising elements that are linked together by nodes
Network models employ conservation governing equations thus
Suitable for incompressible and compressible flow
Allow integrated prediction of
1) Flow distribution
2) Combustion
3) Heat transfer
Parameters such as gas temperatures, combustion efficiency, gas emissivity, and correlations to predict film cooling efficiencies can be obtained from the empirical formulations

5. Advantages
Capable of modelling complicated geometries effectively without difficulty, while maintaining rapid execution
Several design modifications can be investigated easily
Experimental time can be reduced to only a few verification experiments
Data obtained from 1D analysis the can be used as as boundary conditions for 3D numerical models

6. Flow distribution modelling
Predict pressure distribution and mass flow rates throughout a gas turbine combustion system
Elements define actual geometrical features e.g. orifices and duct sections in the domain of interest
Overall governing equations are solved within the nodes
While pressure drop–flow relationship is applied for every element
Typically defined using the Darcy-Weisbach or other exponential or empirically determined correlations.

7. Combustion process modelling
The combustion process has to be accounted for when a 1-d analysis is conducted
Gas temperature effects fluid density, and therefore the flow distributions and pressure loss
Accurate gas temperature profile required for wall temperature predictions
NASA CEA program incorporated into networks allows prediction of adiabatic flame temperature as a function of local air/fuel ratio and the chemical equilibrium product concentrations from reactants

8. Heat transfer process modelling
Allows prediction of liner wall temperatures by conducting heat balance along liner wall
Typically inner wall is heated by convection & radiation from the hot combustion gases & cooled on the outside by the annulus air flow through convection and radiation from the outer liner surface to the casing wall
Specialized cooling mechanisms can also be employed
Convection
Film & jet impingement cooling
Fluid & surface to surface radiation
Conduction

9. Heat transfer process modelling
Internal radiation
Typically the major contributor of heat transferred from the hot gases the liner wall
Total emitted radiation has two components
“nonluminous” radiation that emanates from certain heteropolar gases, notably carbon dioxide and water vapor
“luminous” radiation that depends on the number and size of the solid particles (mainly soot) in the flame
External radiation
Heat transferred from the liner to the casing is usually small compared the external convective heat transfer
Approximated by assuming gray surfaces with known emissivities and view factors

10. Heat transfer process modelling
Convection
In the absence of more exact data, it is reasonable to assume that some form of the classical heat-transfer relation for straight pipes (Dittus-Boelter) will hold for conditions inside a liner
For external convection Re is based on the hydraulic mean diameter of the annulus air space
Conduction
2D axisymmetric
Conduction and cross-conduction components

11. Heat transfer process modelling
Film cooling
Air injected through slots axially along the inner liner wall liner provides protective film of cooling air between wall and hot combustion gases
Film cooling correlations
Levebvre & Ballal
Wieghardt

12. Heat transfer process modelling
Impingement Cooling
Impinging jets provide an effective transfer heat and can be positioned to provide extra cooling on liner hot spots
Jet impingement correlations
Florschuetz

13. Flownex Demo Model Why Flownex?
Commercial 1-D thermal fluid network tool that employs the conservation of conservation governing equations
Mass
Momentum
Energy
User friendly GUI
Integrated CEA program calculates chemical equilibrium product concentrations from any set of reactants – ideal combustion model
Library that employs industry standard correlations for gas turbine heat transfer
Scripting capabilities make tool flexible and user definable
Solve time
Sensitivity analysis setup
Easy to link to 3rd party FEA, CFD or post processing software

14. Flownex Demo Model Engine Rolls Royce T56 turboprop gas turbine engine
Combustor design in use is old compared to modern-day combustion systems but still widely used
Can-annular type combustion section
Gouws, J. J., R. M. Morris, and J. A. Visser. "Modelling of a Gas Turbine Combustor Using a Network Solver." South African Journal of Science

15. Flownex Demo Model
Single T56 combustion chamber and description of hole layout & Operating conditions
Gouws, J. J., R. M. Morris, and J. A. Visser. "Modelling of a Gas Turbine Combustor Using a Network Solver." South African Journal of Science

16. Flownex Demo Model Methodology
Set up model to predict pressure distribution and mass flow rate distribution validated against test data
Implement heat transfer to predict gas and liner wall temperatures validated against test data
Once base case is established, investigate effect possible geometry modifications have flow and temperature predictions

17. Flownex Demo Model
General Flownex network layout for the combustion chamber

18. Flownex Demo Model
General Flownex network layout for the combustion chamber

19. Flownex Demo Model
Heat transfer and flow layout for film cooling device

20. Flownex Demo Model

21. Flownex Demo Model
Heat transfer and flow layout for film cooling device
Section 2 (distance 100mm)
Film cooling hole set Ae ~64mm^2
Liner hot side temperature = K
Section 2 (distance 100mm)
Film cooling hole set Ae ~128mm^2
Liner hot side temperature = K
Model execution time ~2 seconds