Conjugate Heat Transfer in CI Engine CFD Simulations

The development of new high power diesel engines is continually going for increased mean effective pressures and consequently increased thermal loads on combustion chamber walls close to the limits of endurance. Therefore accurate CFD simulation of conjugate heat transfer on the walls becomes a very important part of the development. In this study the heat transfer and temperature on piston surface was studied using conjugate heat transfer model along with a variety of near wall treatments for turbulence. New wall functions that account for variable density were implemented and tested against standard wall functions and against the hybrid near wall treatment readily available in a CFD software Star-CD. INTRODUCTION Accurate prediction of the piston surface temperature and heat flux is difficult mainly because of the very large temperature and velocity gradients of highly turbulent flow near the wall and also because of the transient interaction between the gas phase and solid wall temperatures. Instead of resolving the temperature and velocity profiles down to the wall, wall functions based on some simplifying assumptions of the near wall turbulence are used. Standard wall functions and hybrid near wall treatment in the form they are usually implemented in commercial CFD codes assume incompressible flow. Therefore straightforward adoption of them into engine simulations might not be appropriate, since the gas in the cylinder is highly compressible indeed and large density variations are likely to appear near the walls. To tackle this problem momentum and energy equations near the wall are integrated in their compressible form to new modified wall functions that are sensitive to density variation. Also variable turbulent Prandtl number near the walls is included in the modified wall functions. Heat transfer in combustion engines has bee widely studied, e.g. by Schubert, Wimmer and Chamela [1], who developed quasi-dimensional heat transfer models for combustion ignition engines, by Han and Reitz [2], who studied the effect of density variations on convective heat transfer, by Urip, Liew, Yang and Arici [3], who studied the effect of unsteady thermal boundary conditions, by Urip, Yang and Arici [4], who studied conjugate heat transfer in actual internal combustion engine CFD simulations, by Tiainen, Kallio, Leino and Turunen [5], who studied heat transfer in diesel engines with density dependent wall functions in CFD and combining the obtained average heat transfer coefficients to FEM calculations of the heat transfer in solid piston material and also by Huuhilo [6], who studied and developed a FEM code for transient heat transfer in the piston surface. A good motivation for this work is figure 1 from Huuhilo’s Master’s Thesis that shows the effect of the piston surface material on the transient maximum piston surface temperature. Fig. 1. The simulated maximum temperature of the piston surface layer fabricated from steel, aluminium or zirconium oxide [6]. VARIABLE DENSITY WALL FUNCTIONS In deriving the wall functions following assumptions are made: 1) wall normal derivatives are much larger than tangential ones, so the tangential derivatives are neglected. 2) near wall fluid flow is tangential to wall. 3) near wall pressure gradient can be neglected. 4) near wall heat flux consists only of laminar and turbulent conduction. 5) gas obeys ideal gas law 6) near wall specific heat capacity is constant 7) near wall mass fractions of mixture components are constant. 8) near wall shear stress and heat flux are constant. Now the simplified near wall momentum and heat equations (1) and (2) can be written as