Improved near wall treatment for ci engine cfd simulations
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Improved Near Wall Treatment for CI Engine CFD Simulations. Mika Nuutinen Helsinki University of Technology, Internal Combustion Engine Technology. Conjugate Heat Transfer in CFD . Continuous heat flux across surface

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Improved Near Wall Treatment for CI Engine CFD Simulations

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Improved Near Wall Treatment for CI Engine CFD Simulations

Mika Nuutinen

Helsinki University of Technology,

Internal Combustion Engine Technology


Conjugate Heat Transfer in CFD

  • Continuous heat flux across surface

  • Simultaneous determination of heat flow and temperature within a fluid and its adjacent solid e.g.

    • Cylinder charge and piston

    • Engine block and coolant


Why conjugate heat transfer?

  • Primarily: Designer needs accurate temperature data in/on solid part

    • Maximum temperature (melting)

    • Temperature distribution (thermal loads)

  • Secondarily: Produces transient, more accurate boundary condition for temperature

    • More accurate heat loss prediction

    • More accurate overall temperature/pressure fields


CFD problems in heat transfer

  • Inaccuracy of RANS turbulence models (k-ε, k-ω)

  • Extreme field gradients near walls

  • Standard wall treatment (wall functions) omits the effects of temperature induced density variations near walls


New wall function formalism

  • Derived similarly to standard wall functions, but with smooth turbulent viscosity transition (Mellor) and variable near wall turbulent Pr (Kays)

  • Sensitive to temperature induced density variation near the walls unlike standard wall functions

    +Improves heat transfer and temperature predictions

    +Easy to include other temperature variable effects to e.g. heat capacity, μ, k…

    -No analytical solution -> computational burden


Essential equations


Velocity wall functions (hot gas case)


Temperature wall functions (hot gas)


Wall Heat flux prediction (hot gas)


Wall function comparison, typical CI engine simulation case

  • Simulations were made with 4 combinations of turbulence models and near wall treatments:

  • 1) High Reynolds number k-e model with standard wall functions.

  • 2) High Reynolds number k-e model with the new variable density wall functions

  • 3) High Reynolds number RNG k-e model with standard wall functions.

  • 4) Low Reynolds number k-e model with hybrid wall treatment.


Spray and Combustion modeling

  • Lagrangian particle tracking

  • Transfer of mass, momentum and heat modeled

  • Droplet break up models: Reitz-Diwakar etc.

  • Turbulent dispersion, collisions, coalescence

  • EBU LaTCT (laminar and turbulent characteristic time) combustion model


Computational grid, fluid & solid zones


Cylinder pressure


Piston heat transfer


Piston peak temperature


Piston surface temperature


Concluding remarks

  • The new wall function formalism works well in practical simulations

  • Enhances the predicted wall heat transfer in CI engine simulations when the gas is hot (and vice versa)

  • Further improvements easy to implement

  • Computational burden can be minimized by selecting a smaller boundary where the heat transfer is critical


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