Experimental and Numerical Analysis of Near Wall Flow at the Intake Valve and its Influence on Large-Scale Fluctuations

Transcription

1 Experimental and Numerical Analysis of Near Wall Flow at the Intake Valve and its Influence on Large-Scale Fluctuations Frank Hartmann, Stefan Buhl, Florian Gleiß, Christian Hasse Philipp Barth, Martin Schild, Sebastian Kaiser LES for Internal Combustion Engine Flows [LES4ICE] IFPEN / Rueil-Malmaison December 2014 Chair of Numerical Thermo-Fluid Dynamics TU Bergakademie Freiberg Freiberg F. Hartmann frank.hartmann@iec.tu-freiberg.de 1

2 Overview Motivation Experimental and numerical setup Results Instantaneous flow field Comparison of the mean flow field between simulation and PIV Comparison using the relevance index (RI) Investigation of the unsteady flow behavior behind the valve rod Summary and Conclusion

3 Motivation Cycle to cycle variations of internal combustion engines (ICE) Cause and Effect Chain: Fig. 1. Pressure curves of 40 consecutive cycles (grey, thin lines) and the average pressure (black, thin lines) in an SI engine. [1] Which flow phenomena influence the intrinsic cyclic variations from cold-flow (e.g Jet fluctuation)? [1] Hasse et. al, Numerical investigations of cyclic variations in gasoline engines using hybrid URANS/LES modeling approach, Computers & Fluids, 2010

4 Motivation Combined experimental and numerical investigation of the flow on a flow bench to characterize intake port flow. - 5 mm Valve Lift - Engine head with tumble intake port - Wall guided flow through a small intake valve gap - Turbulence generation at strong jet shear layer Section for 2D-2C PIV - 3D CFD Simulation - Unsteady flow separation in the wake of the valve rod - Geometrical fixed valve separation edge - Characteristic jet flapping Multiple engine relevant flow structures captured through 3D- CFD and highly-resolved 2D-2C PIV measurements

5 Overview Motivation Experimental and numerical setup Results Instantaneous flow field Comparison of the mean flow field between simulation and PIV Comparison using the relevance index (RI) Investigation of the unsteady flow behavior behind the valve rod Summary and Conclusion

6 Experimental Setup Highly-resolved 2D-2C PIV: Particles: DEHS (di-ethyl-hexyl-sebacate) Particlediameter: 0.2 µm µm Laser: dual cavity, double pulsed Nd:YAG at 532 nm with 30 mj/pulse Light Sheet Thick.: 0.5 mm 0.9 mm Camera Res.: 2560 x 2160 pixel² Repetition Rate: 15 Hz Resolution: 725 x 725 µm²

7 Experimental Setup Steady Flow Bench: Intake port Engine head Outflow pipe - Outflow pipe mounted on an engine head - Volume flow metering with a rotary blower - Temperature and pressure metering at intake port (1) and outflow pipe (2) - Inlet mass flow and pressure level at (2) as boundary conditions for CFD - Cross-checked pressure measurements at (1)

8 Numerical Setup Geometry and Mesh: Top Side Mesh Mesh Densities Elements: 105 mio. Nodes: 20 mio. Domain Volume: 3416 cm³ max : 2 mm min : 0.25 mm Nodes/BL: 10 u max : 80 m/s t: 3µs - Hybrid mesh, based on tet, prism, hex - 10 Prism Layer - Mesh Density: =0.25 mm - Extrusion1: =1mm - Extrusion2: =2mm - Time step estimation using CFL = 1, min =0.25 mm

9 Numerical Setup Implicite, node centered FEM based finte volume code (ANSYS CFX) Boundary Conditions: - Incompressible Navier Stokes equations - Spatial discretization: central differencing scheme - Time discretization: second order backward Euler scheme - Adiabatic walls - Inlet Massflow: 3 kg/min (measured) - Outlet: ambient conditions Turbulence Model: - Scale resolving DES-SST turbulence model Ensures two-equation URANS like behavior in underresolved grid regions

10 Overview Motivation Experimental and numerical setup Results Instantaneous flow field Comparison of the mean flow field between simulation and PIV Comparison using the relevance index (RI) Investigation of the unsteady flow behavior behind the valve rod Summary and Conclusion

11 Results Instantaneous Velocity Field: DES-SST 2D-2C PIV Video! More visible fluctuations of PIV due to 15Hz sampling frequency vs. numerical sampling frequency of 300Hz

12 Results Normalized velocity distribution of all instantaneous velocities: PIV snapshots Investigated section SIM snapshots of the experimental investigated section - Velocity distribution normalized by number of snapshots Most frequent velocity between 15 m/s and 18 m/s Notable difference for velocities above 60 m/s (tail of PDF)

13 Overview Motivation Experimental and numerical setup Results Instantaneous flow field Comparison of the mean flow field between simulation and PIV Comparison using the relevance index (RI) Investigation of the unsteady flow behavior behind the valve rod Summary and Conclusion

14 Results Comparison between mean flow fields: L1 PIV PIV SIM SIM Overall agreement of flow behavior between simulation and PIV Differences can be observed for the jet orientation

15 Results Velocity magnitude and RMS velocities at L1: PIV PIV SIM SIM General flow behavior of mean velocity represented higher variations about mean velocity and rms for PIV Distinctive deviations between SIM and PIV

16 Overview Motivation Experimental and numerical setup Results Instantaneous flow field Comparison of the mean flow field between simulation and PIV Comparison using the relevance index (RI) Investigation of the unsteady flow behavior behind the valve rod Summary and Conclusion

17 Results Relevance Index(RI): - Introduced by Liu and Haworth [1] to compare two velocity fields (e.g. simulation and experiment) - RI describes an angle between two vectors - Direct comparison of velocity direction between simulation and PIV in the chosen section RI = 1 parallel vectors RI = -1 antiparallel vectors RI = 0 orthogonal vectors RRRR = uu PPPPPP uu SSSSSS uu PPPPPP uu SSSSSS Overall good agreement for the velocity fields However, difference in the position of recirculation region highlighted by RI [1] Liu et. al, Development and assessment of POD for analysis of turbulent flow in piston engines, SAE Technical Paper, 2011

18 Overview Motivation Experimental and numerical setup Results Instantaneous flow field Comparison of the mean flow field between simulation and PIV Comparison using the relevance index (RI) Investigation of the unsteady flow behavior behind the valve rod Summary and Conclusion

19 Results Unsteady flow behavior behind the intake valve rod: Strouhal Number: ff dd SSSS = uu u = 45 m/s d = 3.53 mm Sr ~ 0.2 f = 2550 Hz Characteristic Jet Flapping (left) Vortex-shedding in the wake of the valve rod (right) Influence the vortex shedding frequency the characteristic Jet Flapping?

20 Results Investigate the flow behavior behind the intake valve rod using Fourier analysis: Discrete Fourier Transformation: Decompose discrete time series into a frequency spectrum - Equidistant time series given ( t=3µs) - point matrix GS to identify numerical frequency - point matrix GVG and GV proof, wether the frequency survives acceleration through valve gap

21 Results Investigate the flow behavior behind the intake valve rod using Fourier analysis: - Mean velocity fluctuation horizontal to the main flow direction (u ) on selected points Dominant frequency at 2860 Hz could be identified in all selected point matrices Frequency can be found up to the combustion chamber in point matrix GV 2860 Hz is a relevant frequency during intake stroke for real engines Influence of flow separation behind the valve rod during intake stroke possible

22 Overview Motivation Experimental and numerical setup Results Instantaneous flow field Comparison of the mean flow field between simulation and PIV Comparison using the relevance index (RI) Investigation of the unsteady flow behavior behind the valve rod Summary and Conclusion

23 Summary and Conclusion - Experimental and numerical investigations of an unsteady flow bench at 5 mm valve lift - 2D-2C PIV compared to simulations shows deviations in the jet orientation - Further investigations of the numerical model and the calibration of the experimental setup will increase the comparability

24 Summary and Conclusion - Identification of vortex shedding in the wake of the intake valve rod - Identification of a significant frequency for engines using fourier analysis - Frequency of 2860 Hz could be found in the valve gap up to the combustion chamber