Design and Optimization of Contra-Rotating Fans Oliver Velde #, Christian Friebe &, Marius Korfanty # # CFturbo GmbH & ILK Institut für Luft- und Kältetechnik ggmbh
Motivation to use Contra-rotating fans Very compact compared to a two stage fan with guiding vanes Smaller impeller diameter and/or lower speed compared to single stage fans Alternative for low pressure radial ventilators Potential for better acoustic behavior 20 Nov. 2017 2/20
Contra-rotating fan principle First impeller generates a negative pre-swirl for the second impeller I w 2 I II c 2 I u I c 1 II w 1 II u II w 1 I u I c I I 1 = c m1 w 2 II c 2 II u II I Y Euler = c u u Y I = c u2 u I II, Y II = c u2 I Y I + Y II = c u2 u I + u II 20 Nov. 2017 3/20 u II
Gear less single motor driven Contra-rotating fan Torque/Power distribution imposed by aerodynamic design of impellers Impeller mounted on rotor Impeller mounted on axis 20 Nov. 2017 4/20
Contra-rotating fan design Design point: pressure difference Δp tot, flow Q, Fluid properties Power distribution between 2 impellers defined by either of Pressure difference and speed: Δp tot,i, n I und Δp tot,ii, n II Torque and speed: T I, n I und T II, n II (with T n = Δp tot Q) Example: fan impeller I impeller II Q [m 3 /h] 540 540 540 Initial design: Δp tot [Pa] 1000 690 310 n [rpm] 8000-5000 Design Point Initial design Number of blades [-] 8 6 Profile NACA 6508 NACA 6508 Stagger angle λ [ ] 46.7.. 36.1 45.0.. 36.3 Chord length l [mm] 32.. 24 32.. 26 20 Nov. 2017 5/20
Contra-rotating fan design with CFturbo 20 Nov. 2017 6/20
Computational model Rotor-Stator Interface Mixing Plane Outlet BC: mass flow Periodic Interfaces Evaluation planes EP1 EP3 Rotor-Stator Interface Mixing Plane Rotor-Rotor Interface Mixing Plane EP1 EP2 EP3 Inlet BC: p = 1 [bar], T = 20 20 Nov. 2017 7/20
Mesh Hexahedral Mesh 2 Mio. elements ANSYS TurboGrid 20 Nov. 2017 8/20
Boundary conditions & solver settings INLET Pressure: 1 [bar] OUTLET Volumetric flow: 400 650 [m 3 /h] Rotational speed: 8000 rpm for impeller I -5000 rpm for impeller II Fluid properties: Air (Perfect Gas) ANSYS-CFX v18.0 Steady State Simulation Heat Transport (Total Energy) Mixing-Plane Interface SST-Turbulence model High Resolution differencing scheme 20 Nov. 2017 9/20
Δp tot [Pa] Simulation of initial design, results 1400 1200 1000 800 600 fan 400 impeller I 200 impeller II 0 350 400 450 500 550 600 650 700 Volumetric flow [m 3 /h] 20 Nov. 2017 10/20
η [%] Simulation of initial design, results 100% 95% 90% impeller I 85% fan 80% 75% 70% impeller II 65% 60% Optimization goal: Improving of impeller II efficiency 55% 350 400 450 500 550 600 650 700 Volumetric flow [m 3 /h] 20 Nov. 2017 11/20
Optimization of initial design Optimization process: Control unit for complete process Parametric CAD model Meshing, simulation, analysis 20 Nov. 2017 12/20
Optimization of initial design Objective: Restriction: improve second impeller with respect to efficiency match design point Parameters: only impeller II i β B1 β 1 w 1 l / t Solidity ϕ w 2 γ = α + β stagger angle β 2 β B2 δ t = π d/z Pitch Parameters range Number of blades z [-] 4.. 10 Stagger angle λ hub [ ] 20.. 60 Stagger angle λ tip [ ] 20.. 60 Chord length l hub [mm] 32 (const.) Chord length l tip [mm] 20.. 32 Leading edge position Δz hub [mm] 1.. 8 5 parameters for Optimization u 1 u 2 Δz hub 20 Nov. 2017 13/20
Optimization process Heeds CFturbo: Parametric CAD model ANSYS TurboGrid: Mesh generation ANSYS CFX: Simulation & Postprocessing 144 Designs 70min/Design / 8 Processors 20 Nov. 2017 14/20
Optimization results impeller II Initial design impeller II Total pressure difference Δp tot [Pa] 303 319 Efficiency η [%] 78 81 Initial design impeller II Number of blades z [-] 6 7 Stagger angle λ hub [ ] 45.0 50.0 Stagger angle λ tip [ ] 36.1 30.4 Chord length l hub [mm] 32 32 Chord length l tip [mm] 26 26 Leading edge position Δz hub [mm] 1 2.6 Optimized design impeller II Optimized design impeller II 20 Nov. 2017 15/20
Δp tot [Pa] Performance curve 1400 1200 1000 Initial design Optimized fan 800 600 impeller I 400 200 impeller II 0 350 400 450 500 550 600 650 700 Volumetric flow [m 3 /h] 20 Nov. 2017 16/20
η [%] Efficiency 95% 90% impeller I 85% fan 80% 75% 70% impeller II 65% Initial design 60% Optimized 55% 350 400 450 500 550 600 650 700 Volumetric flow [m 3 /h] 20 Nov. 2017 17/20
Pressure distribution mid span Initial design Optimized 20 Nov. 2017 18/20
Relative velocity distribution mid span Initial design Optimized 20 Nov. 2017 19/20
Summary Automated process for design and optimization for Contra-rotating fans Reasonable initial design for both impellers using CFturbo software Optimization of second impeller only Simple method: passage only, steady state simulation Computational resources Validation in progress Transient flow simulations/360 More detailed geometry modeling Experiments Acoustic investigations planned Initial design impeller II Optimized impeller II 20 Nov. 2017 20/20