An Improved Near Wall Heat Transfer Model for Multidimensional Engine Flow Calculations

An important aspect of calculation of engine combustion chamber heat transfer with a multi-dimensional flow code is the modeling of the near wall flow. Conventional treatments of the wall layer flow employ the use of wall functions which impose the wall boundary conditions on the solution grid points adjacent to solid boundaries. However, the use of wall functions for calculating complex flows such as those which exist in engines has numerous weaknesses, including dependence on grid resolution. An alternative wall modeling approach has been developed which overcomes the limitations of the wall functions and is applicable to the calculation of in-cylinder engine flows. In this approach the wall layer flow is solved dynamically on a grid spanning a very thin boundary layer region adjacent to solid boundaries which is separate from the global grid used to solve the outer flow. On the wall layer grid, a set of boundary layer equations is solved in order to determine profiles of mean velocity, mean thermal energy and turbulent kinetic energy. The calculated wall layer solution allows calculation of wall heat flux directly from the temperature profile slope at the wall. Calculations of several test case flows have been performed. Comparisons of the new model predictions with those of conventional wall functions and experimental data show that the new model removes several of the weaknesses found in the wall function formulations, which makes it more appropriate for engine applications.