1 / 28

Large Eddy Simulation of Impinging Jets with Heat Transfer

Large Eddy Simulation of Impinging Jets with Heat Transfer. Thomas Hällqvist KTH / Scania CV AB. Outline. Background Project description Computational method and cases Results Summary. Background. Project initiated in year 2000 by KTH and Scania CV AB

argus
Download Presentation

Large Eddy Simulation of Impinging Jets with Heat Transfer

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Large Eddy Simulation of Impinging Jets withHeat Transfer Thomas Hällqvist KTH / Scania CV AB

  2. Outline • Background • Project description • Computational method and cases • Results • Summary

  3. Background • Project initiated in year 2000 by KTH and Scania CV AB • Industrial goal:Improve the cooling capacity of Scania heavy trucks • Increase of the engine power. • Decrease of available space.

  4. Outline • Background • Project description • Computational method and cases • Results • Summary

  5. Project description • To capture basic physical features a simplified geometry is studied • The under-hood flow is approximated by an impinging jet • Scientific goal:To enhance the understanding of the flow and dynamics of impinging jets; including • Impinging jet flow and related heat transfer. • Turbulence and its modeling for such flows. • Utilizing modern computational tools.

  6. The Impinging Jet (I) Impinging jets are common in engineering applications • Processing of metal, glass and paper. • Cooling applications: electronics, gas-turbine combustion chambers, mechanical devices. Other more indirect application areas • VTOL aircrafts, rockets (when close to the ground). • High pressure washers.

  7. The Impinging Jet (II) The impinging jet is characterized by three flow regions • The free jet region. • The stagnation region. • The wall jet region. Geometrical parameters D: Nozzle diameter H: Impingement distance W: Width Nozzle outlet conditions V0: Mean axial velocity C0: Mean concentration k0: Turbulent kinetic energy

  8. The Impinging Jet (III) Nozzle condition A Nozzle condition B Impingement wall heat transfer depends on • Nozzle conditions. • Impingement distance (H/D). For small H/D • Minimum of Nu at r/D=0. • Two maximums of Nu. For large H/D • Maximum of Nu at r/D=0. • Monotone decrease with r/D. Maximum in stagnation Nu • Depends on the nozzle conditions. • Within the range: H/D=3-8. • End of the potential core. H/D=2 H/D=4 Nusselt number (Nu) H/D=6 hypothetical impingement wall 0 r/D R

  9. Outline • Background • Project description • Computational method and cases • Results • Summary

  10. Computational method • Impinging jet simulated by large-eddy simulation (LES). • Space-filtering to reduce the number of degrees of freedom. • The effects from the unresolved scales must be modeled • Dissipation of energy. • Backscatter, structural information. • Despite the filtering LES is computationally highly expensive. • Particularly for wall-bounded flows. • As LES is an unsteady approach • Correct inflow conditions. • Flow development region. • LES must be conducted in a 3-D domain • No symmetry-planes. • Turbulence is three-dimensional. Turbulent velocity spectrum Velocity signal (): Unfiltered signal (): Filtered signal E() large scales, resolved small scales, unresolved ”SGS” cut-off, c  x

  11. Main computational cases • Paper 1 & Paper 2: Basics of impinging jets • Basic characters of an impinging circular jet using top-hat inflow velocity profile. Paper 1: flow; Paper 2: heat transfer. • Paper 3 & Paper 4:Swirling impinging jets • Swirl effects on the flow and wall heat transfer for circular and annular impinging jets. • Paper 5: Inflow profile effects • Radial distribution of the axial mean inflow velocity and from periodic forcing. • Paper 6: Parametric studies • Nozzle-to-plate spacing effects. • Reynolds number effects. • Fully developed turbulent inflow condition for circular non-swirling and swirling impinging jets. Data normalized by: Mean inflow velocity (V0), nozzle diameter (D0) and mean inflow temperature (C0). ( Re=V0D0/ )

  12. Outline • Background • Project description • Computational method and cases • Results • Summary

  13. Dynamical character From Paper 5 Instantaneous vorticity in the xy-plane 2 nozzle outlet, D Inviscid instability Roll-up and shedding of natural vortices, Stn y/D Vortex pairing Shedding of primary vortices, Stn/2 1 shed vortices Convection of primary vortices Formation of secondary vortices Separation and breakdown impingement wall 0 2 1 0 1 2 r/D

  14. Dynamical character From Paper 1 & 5 Dominant modes and energy at r/D=0.5 Spectrum at r/D=0.5, y/D=1 y/D PSD • Two dominant modes. • Sharp decrease of PSD for higher St. VP St E St • Natural mode initially dominant. • Delayed amplification of the subharmonic mode. • Vortex pairing (VP) between: E(Stn)=E(Stn/2) and max[E(Stn/2)].

  15. Unsteady heat transfer From Paper 2 Instantaneous vorticity in the xy-plane, Nu and Cf plots nozzle outlet, D Attached vortices PV A: mean flow convection B: coherent heat transfer C:incoherent heat transfer V0 PV:Primary vortex SV :Secondary vortex Conv. vel. Uc V0 / 2 (—): Cf (—): Nu impingement wall C B A B C Stagnation point SV, separation

  16. Unsteady heat transfer From Paper 2 Instantaneous vorticity in the xy-plane, Nu and Cf plots (—): Cf (—): Nu separation point reattachment point

  17. Unsteady heat transfer From Paper 2 Vorticity, z Velocity vectors PV hot fluid SV Separation point Reattachment point PV: counter-clockwise rotating SV: clockwise rotating (---): Cf

  18. Unsteady wall heat transfer From Paper 2 Wall friction Wall heat transfer convective wave convective spot Red color: high wall friction Blue color: low wall friction, separation Red color: high wall heat transfer Blue color: low wall heat transfer

  19. Mean inflow profile effects From Paper 6 Instantaneous temperature distribution in the xy-plane (H/D=4) • Top-hat: irregular flow character, coherent structures only close to the nozzle outlet. Qualitatively similar to the reference case. • Mollified: distinct axisymmetric ring vortices  large-scale mixing, delayed transition. top-hat ”fully turbulent” (ref. case) mollified

  20. Mean flow character From Paper 1 & 6 Inflow: ”Fully developed ” turb. pipe flow. Top-hat mean velocity profile. Fully developed turb. pipe flow. Mean axial velocity decay Radial velocity at r/D=1 (): LES (pipe) (): LES (top-hat) (О): Cooper et al. (): Geers et al. • Potential core extends to y/D≈1. • Top-hat: earlier decay. • Pipe: later decay  high correlation with Geers et al. y/D V/VCL U • Top-hat: low axial momentum  low peak velocity. • Pipe: stronger wall shear-layer, high peak velocity. • High correlation with Cooper et al. • Experimental discrepancy: • Measurement technique. • Nozzle conditions.

  21. Turbulence statistics From Paper 1 & 6 urms(vrms) at r/D=0 Production of k at r/D=0 urms at r/D=1 (): LES (pipe) (): LES (top-hat) (О): Cooper et al. (): Geers et al. (vrms) y/D • Pk=0 for y/D>1. • Pipe: as the gradient increases so does Pk. • Close to the wall Pk<0 as Pk (vrms2 - urms2). • Top-hat: overall zero production. urms urms Pk • As r/D increases: inflow conditions less important. • Pipe: clear near-wall peak of Urms. • Overall good agreement with experiments (within tolerance for the two exp. setups). • Top-hat: weaker wall-shear  no distinct near-wall peak. • Top-hat: negligible level of fluctuations. • Pipe: Urms≈0.04, sharp increase close to the wall. • High correlation with Geers et al.

  22. Effect from swirl From Paper 3 & 6 Mean axial velocity decay k at y/D=0.15 Nusselt number S=Ut/V0 k y/D Nu V/VCL r/D r/D • Pipe: high level of k high Nu. • Top-hat:Nu is low, despite high level of k. • Negligible rate of mean flow convection. • Jet spreading increases with swirl. • Top-hat: significant increase  recirculation bubble. • The bubble reaches downstream to r/D≈1. • At small r/D k is strongly influenced by swirl. • Less influence at larger radius. (  ): LES S=0 (pipe) (- - -): LES S=1(pipe) (  ): LES S=0 (top-hat) (- - -): LES S=1(top-hat) (): Geers et al. top-hat case, S=1 (  ): LES S=0 (pipe) (- - -): LES S=1(pipe) (  ): LES S=0 (top-hat) (- - -): LES S=1(top-hat) (): Geers et al. • Significant influence from the character of the inflow • Radial distribution of the axial and azimuthal velocity components. • Swirl generator structures.

  23. Outline • Background • Project description • Computational method and cases • Results • Summary

  24. Summary • The inflow boundary conditions is of significant importance for the development of the flow and scalar fields. • The underlying mechanisms of impinging jet heat transfer have been identified, discussed and visualized. • The dynamics of non-swirling and swirling impinging jets have been studied in some detail. Swirl has large effect on the wall heat transfer. The swirl generating method is crucial. • The LES approach provides accurate results in an efficient manner. The simulation method is not problem dependent.

  25. Possible extensions • Study and explore (new) SGS models for the near-wall region. • Determine quantitatively the sensitivity of the Nusselt number from inflow condition uncertainties. • Study the effects of blade generated flow. • Determine the flow due to wall porosity. • Flow induces acoustics. Acoustics source distribution Instantaneous velocity field

  26. Thank you!

  27. Summary: wall heat transfer From Paper 2 & 5 Correlation between Nu and Cf Trends of: mean Nu, Cf , k,  III I IV Ruc • Nu: Local peak at r/D≈0.6. • Cf: Strong accelerating wall jet, local peak at r/D≈0.7. • k: Zero in the core region, local peak at r/D≈1.75. • : Indicates formation of counter rotating secondary vortices  high k and local increase of Nu. Nu,Cf ,k, (): Nu (): Cf (): k ():  II r/D r/D • I: Low level of k, laminar-like wall jet  high Ruc. • II: Vortical structures penetrates the wall boundary layer  low Ruc. • III:Convective structured primary vortices  high Ruc. • IV: Influence from secondary vortices and increasing level of irregular structures, i.e. eddies  low Ruc.

More Related