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Improved Near Wall Treatment for CI Engine CFD Simulations

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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  1. Improved Near Wall Treatment for CI Engine CFD Simulations Mika Nuutinen Helsinki University of Technology, Internal Combustion Engine Technology

  2. 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

  3. 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

  4. 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

  5. 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

  6. Essential equations

  7. Velocity wall functions (hot gas case)

  8. Temperature wall functions (hot gas)

  9. Wall Heat flux prediction (hot gas)

  10. 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.

  11. 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

  12. Computational grid, fluid & solid zones

  13. Cylinder pressure

  14. Piston heat transfer

  15. Piston peak temperature

  16. Piston surface temperature

  17. 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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