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Large Eddy Simulation of Impinging Jets with Heat Transfer

VIEWS: 9 PAGES: 28

  • pg 1
									Large Eddy Simulation
            of
   Impinging Jets
          with
    Heat Transfer

      Thomas Hällqvist
     KTH / Scania CV AB




                          1
     Outline
• Background
• Project description
• Computational method
  and cases
• Results
• Summary




                         2
                                            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.
                    900
                                                         y = 11.598x - 22585
                    800

                    700
Engine power [hp]




                    600

                    500

                    400

                    300

                    200

                    100

                     0
                     1975    1980   1985   1990   1995     2000     2005       2010   2015   2020   2025
                                                           Year




                                                                                                           3
     Outline
• Background
• Project description
• Computational method
  and cases
• Results
• Summary




                         4
           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.




                                                               5
              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.




                                                             6
             The Impinging Jet             (II)

The impinging jet is characterized by
  three flow regions
        a)   The free jet region.
        b)   The stagnation region.     Geometrical parameters
        c)   The wall jet region.       D: Nozzle diameter
                                        H: Impingement distance
                                        W: Width

                                        Nozzle outlet conditions
                                        V0: Mean axial velocity
                                        C0: Mean concentration
                                        k0: Turbulent kinetic energy




                                                                   7
                                                            The Impinging Jet                 (III)
                                Impingement wall heat transfer
                                  depends on                                           Nozzle                Nozzle
                                                                                     condition A           condition B
                                                     – Nozzle conditions.
                                                     – Impingement distance (H/D).
                                For small H/D
                                                     – Minimum of Nu at r/D=0.
                                                     – Two maximums of Nu.                         H/D=2

                                For large H/D
                                                     – Maximum of Nu at r/D=0.
                                                     – Monotone decrease with r/D.
                                Maximum in stagnation Nu
                           Nusselt number (Nu)




                                                                                                   H/D=4
                                                     – Depends on the nozzle
                                                       conditions.
                                                     – Within the range: H/D=3-8.
       hD
Nu                                                  – End of the potential core.
        k
      k (T / y ) y 0                                                                           H/D=6
h
         Tw  T0
                                                                                       hypothetical
                                                 0            r/D         R            impingement wall
                                                                                                                         8
     Outline
• Background
• Project description
• Computational method
  and cases
• Results
• Summary




                         9
             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
       Turbulent velocity spectrum       scales must be modeled
                                           Velocity signal
        – Dissipation of energy.              ():   Unfiltered signal
        – Backscatter, structural information.():   Filtered signal
E()




 •     Despite the filtering LES is computationally highly expensive.
        large scales,
        – Particularly for wall-bounded flows.
                              small scales,
       resolved             unresolved
 •     As LES is an unsteady approach
                            ”SGS”
        – Correct inflow conditions.
        – Flow development region.
 •     LES must be conducted in a 3-D domain
                           cut-off, c
        – No symmetry-planes.
        – Turbulence  three-dimensional.
                     is                                           x



                                                                         10
         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/ )
                                                                                    11
     Outline
• Background
• Project description
• Computational method
  and cases
• Results
• Summary




                         12
From Paper 5                               Dynamical character
                                 Instantaneous vorticity in the xy-plane
                         2                           nozzle outlet, D                          Inviscid
                                                                                               instability

                                                                                               Roll-up and
                                                                                               shedding of
                                                                                               natural vortices,
                                                                                               Stn
                   y/D



                                                                                               Vortex pairing
                         1                                                 shed vortices       Shedding of
                                                                                               primary vortices,
                                                                                               Stn/2

                                                                                               Convection of
                                                                                               primary vortices
                                                                                               Formation of
                                                                                               secondary vortices
                                                                                               Separation
                                                                                               and breakdown
                                                    impingement wall
                         0
     1  v u              2              1               0              1               2
z  
           
     2  x y 
                                                            r/D
               

                                                                                                                13
From Paper 1 & 5                   Dynamical character
                       Spectrum at r/D=0.5, y/D=1     Dominant modes and energy at r/D=0.5

                                                    y/D
             PSD


                                                                             VP




                                  St                              St                   E
                   •   Two dominant modes.           • Natural mode initially dominant.
                   •   Sharp decrease of PSD for     • Delayed amplification of the
                       higher St.                      subharmonic mode.
                                                     • Vortex pairing (VP) between:
                                                       E(Stn)=E(Stn/2) and max[E(Stn/2)].




                                                                                             14
From Paper 2                    Unsteady heat transfer
                         Instantaneous vorticity in the xy-plane, Nu and Cf plots
                                                nozzle outlet, D

                                                                        Attached
                                                                        vortices



                                                                     PV                 A: mean flow
                                         V0
                                                                                        convection

                                                                                        B: coherent heat
                                                                                        transfer

                                                                                        C: incoherent
                                                                                        heat transfer
PV : Primary vortex
SV : Secondary vortex
Conv. vel. Uc  V0 / 2

     (—): Cf                                    impingement wall

     (—): Nu             C           B                A             B              C
                             Stagnation point                               SV, separation
                                                                                                        15
From Paper 2         Unsteady heat transfer
               Instantaneous vorticity in the xy-plane, Nu and Cf plots




    (—): Cf
    (—): Nu
                                               separation point   reattachment point
                                                                                       16
From Paper 2                Unsteady heat transfer
                 Vorticity,   z                          Velocity vectors

                               PV




                                       hot fluid
                    SV




               Separation
                                        Reattachment
               point
                                        point


                         PV: counter-clockwise rotating               1  v u 
                                                                  z    
                         SV: clockwise rotating                       2  x y 
                                                                               
                                                                   (---): Cf

                                                                                    17
From Paper 2                 Unsteady wall heat transfer

                       Wall friction                               Wall heat transfer




                                   convective wave                                 convective spot
               Red color: high wall friction               Red color: high wall heat transfer
               Blue color: low wall friction, separation   Blue color: low wall heat transfer



                                                                                                     18
From Paper 6           Mean inflow profile effects
               Instantaneous temperature distribution in the xy-plane (H/D=4)




                       top-hat     ”fully turbulent” (ref. case)   mollified

                   •    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.




                                                                                19
From Paper 1 & 6                          Mean flow character
                                                                                                      Inflow:
                                                                                                    ”Fully developed ”
                              Mean axial velocity decay          Radial velocity at r/D=1           turb. pipe flow.

                                                                             ():   LES (pipe)      Top-hat mean
                                                                             ():   LES (top-hat)   velocity profile.
                                                                             (О):   Cooper et al.
                                                                             ():   Geers et al.
                   y/D
                                                                                                    Fully developed
                                                                                                    turb. pipe flow.




                                          V/VCL                                 U
                         •   Potential core extends to       •    Top-hat: low axial momentum
                             y/D≈1.                                low peak velocity.
                         •   Top-hat: earlier decay.         •    Pipe: stronger wall shear-layer,
                         •   Pipe: later decay  high             high peak velocity.
                             correlation with Geers et al.   •    High correlation with Cooper et
                                                                  al.
                                                             •    Experimental discrepancy:
                                                                   –   Measurement technique.
                                                                   –   Nozzle conditions.

                                                                                                                 20
From Paper 1 & 6                          Turbulence statistics
                        urms (vrms) at r/D=0           Production of k at r/D=0        urms at r/D=1
                                                       ():   LES (pipe)
                                    (vrms)             ():   LES (top-hat)
              y/D
                                                       (О):   Cooper et al.
                                                       ():   Geers et al.




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

                                                                                                                  21
From Paper 3 & 6                              Effect from swirl
                       Mean axial velocity decay          k at y/D=0.15                  Nusselt number

                                                                                                            S=Ut/V0
              y/D                                    k




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

                                                                                                                         22
     Outline
• Background
• Project description
• Computational method
  and cases
• Results
• Summary




                         23
 Summary
1. The inflow boundary conditions is of significant importance for
   the development of the flow and scalar fields.

2. The underlying mechanisms of impinging jet heat transfer have
   been identified, discussed and visualized.

3. 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.

4. The LES approach provides accurate results in an efficient
   manner. The simulation method is not problem dependent.




                                                                     24
              Possible extensions
1. Study and explore (new) SGS models for the near-wall
   region.

2. Determine quantitatively the sensitivity of the Nusselt
   number from inflow condition uncertainties.

3. Study the effects of blade generated flow.

4. Determine the flow due to wall porosity.

5. Flow induces acoustics.




     Instantaneous velocity field   Acoustics source distribution
                                                                    25
Thank you!




             26
27
From Paper 2 & 5                      Summary: wall heat transfer
                               Trends of: mean Nu, Cf , k,               Correlation between Nu and Cf

                                                                                       III
               Nu, Cf , k,                                               I
                                                                    Ruc                            IV
                                     ():   Nu
                                     ():
                                     ():
                                            Cf
                                            k
                                                                                II
                                     ():   




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

                                                                                                                   28

								
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