A Computational Investigation of Unsteady Heat Flux Through an I.C. Engine Wall Including Soot Layer Dynamics

This paper deals with the influence of a wall soot layer of varying thickness on the unsteady heat transfer between the fluid and the engine cylinder wall during a full cycle of a four-stroke Diesel engine operation. For that purpose a computational investigation has been carried out, using a one-dimensional model of a multi-layer solid wall for simulating the transient response within the confinement of the combustion chamber. The soot layer is thereby of varying thickness over time, depending on the relative rates of deposition and oxidation. Deposition is accounted for due to a thermophoretic mechanism, while oxidation is described by means of an Arrhenius type expression.

Results of the computations obtained so far show that the substrate wall temperature has a significant effect on the soot layer dynamics and thus on the wall heat flux to the combustion chamber wall. Using insulating materials an increase of the apparent heat transfer coefficient has been observed, but this enhancement is not enough to explain some experimental findings in low-heat-rejection engines to a full extent.

On the other hand a certain sensitivity of the results concerning the soot oxidation rate parameters and the deposition process itself has been demonstrated. Future computational work will include the thermal behavior of the near-wall laminar gas sub-layer, while appropriate measurements have to be designed for validation of the simulation results.