non-ideal effects in compressible swirling flows
TRANSCRIPT
Non-Ideal Effects in
Compressible Swirling Flows
3rd International Seminar on NICFD
Delft, The Netherlands
F. Tosto, A. Giuffreβ, P. Colonna, M. Pini
Propulsion and Power, Delft University of Technology
The information in this document is the property of TU Delft and may not be
copied or communicated to a third party without prior written consent 11/11/2020
Background1
Limited knowledge of NICFD
effects on turbomachinery
β’ efficiency
β’ operability
choking instability
π’ π’ππ’π
NICFD effects: isentropic exponent πΎππ£
Dilute gas state: πΎππ£ β πΎ, with πΎ > 1
2
πΎππ£ = βπ£
π
ππ
ππ£π
CO2CO2MM
Relevance of NICFD effects on losses3
Efficiency can
differ of up to ~π%
Giuffreβ and Pini, βDesign guidelines for axial turbines operating with non-ideal compressible flowsβ, JEGTP, 2020, article in press
Problem Statement4
No existing knowledge on quantitative impact of NICFD effects on
β’ Choking conditions
β’ Flow deviation in post and pre-expansion processes
Scope of Research
β’ Assess variation of flow deviation as π(πΈππ)
β’ Assess impact of πΈππ on choking in turbomachinery
5
Methodology: 2-steps Approach6
1β’ Theoretical analysis on
Swirling Flow
2β’ Verification and
validation through CFD
7
Theoretical Analysis
Corrected mass flow per unit area (1)8
absolute
Mach number
= π cos πΌmeridional
Mach number
π ππ
ππ
Assumption: πΎππ£ = ππππ π‘
αΆπππππ βππ
1 +πΎππ£ β 1
2π2
πΎππ£+1
2(πΎππ£β1)
Corrected mass flow per unit area (2)9
absolute
Mach number
= ΰ·π’πswirl parameter
π’ π’ππ’π
Assumption: πΎππ£ = ππππ π‘
αΆπππππ β 1 +πΎππ£ β 1
2π2
π2
1 +πΎππ£ β 1
2π2
βπ’π
πΎππ£,π‘ππ‘π ππ‘
2
Investigated Processes10
πΎππ£ > πΎ πΎππ£ < πΎ
CO2MM
Influence of πΎππ£ on Choking11
Choking: ππ = 1
π πΎππ£
πΎππ£ > πΎ πΎππ£ < πΎ
Exemplary Post β Expansion in Turbine12
Horizontal line at constant αΆπππππ (A-B-C)
β’ Point B: sonic throat (π = 1)
β’ Point C: post-expansion final state (π = 1.8)
β’ Deviation angle increases if πΈππ decreases
Exemplary Expansion in Supersonic Turbine13
Vertical line at constant ππ (D-B-E)
β’ Constant flow coefficient
β’ Negligible post-expansion
β’ β αΆπππππ increases if πΈππ decreases
β’ Larger area variation to accommodate larger
volumetric flow ratio πΌ = ππ‘,ππ/πππ’π‘
14
CFD Verification
Test Cases & Numerical Setting15
3D transonic stator vanes
Spatial discretization: 2nd order
Turbulence: π β π SST (y+ β€ 1)
Fluid mesh: 5M points
Look-up table: 2.5M (iMM) - 4M (niMM) points
iMM
niMM
iMM: π½π‘π = 2.0 β 2.8 β 3.4
niMM: π½π‘π = 1.8 β 2.7 β 3.8
Results16
β’ 1D model qualitatively in line with CFD if sufficiently far from chocking
β’ Large deviation in volumetric flow ratio
- iMM: πΌπ‘π = 2.2 β 3.1 β 3.8- niMM: πΌπ‘π = 1.6 β 4.0 β 6.9
iMM
D
B
C
niMM
D
B
C
Physical Insights17
β’ πΎππ£ < πΎ β larger area variation to accommodate larger πΌ at given π½
β’ πΎππ£ < πΎ β larger flow deviation in convergent blade channels
iMM, πΌπ‘π = 3.8 niMM , πΌπ‘π = 6.9
Take-Away Messages
β’ Expansions characterized by πΈππ < πΈ lead to earlier choking and
larger flow deviation when fixing π½π‘π
β’ Turbines operating with flow πΎππ£ < πΎ (ORC) more susceptible to
reach complete choking operability/efficiency issues
β’ Research hypothesis:
choice of convergent-divergent nozzle driven by ability to control
flow deviation and extend operating range
18
Ongoing & Future Work
β’ Verify hypothesis by
- Investigate expansion in convergent-divergent nozzles
β’ Pre-expansion at compressor inlet
β’ Analysis of compression processes
19