An investigation of coolant passage heat transfer in a diesel engine cylinder head

This project is motivated by the need to better understand the fundamentals of heat transfer in diesel engine cylinder heads. At low heat fluxes, heat transfer is primarily by pure forced convection; however, at higher heat fluxes, nucleate or film boiling may occur in the exhaust valve bridge coolant passages. Nucleate boiling complicates the mechanisms of heat transfer in cylinder heads and its influence has not yet been explained. The purpose of this investigation is to identify the heat transfer regimes present in the valve bridge coolant passages of a diesel engine cylinder head. Correlations are available for predicting the heat fluxes at which nucleate and film boiling occur but these are difficult to apply to cylinder heads due to the complex geometry and the uncertain and varying nature of the fluid flow within the flow passages. Therefore, the study of heat transfer in cylinder heads lends itself to experimentation.; For this investigation experimental results were used along with a finite element model in order to identify the convection coefficients along the coolant flow passages. Experimental temperature results were obtained from tests conducted at Cummins Engine Company on a fully operational and instrumented L-10 engine. A finite element model of the cylinder head was developed using ANSYS{dollar}spcircler{dollar}. This model has two distinct groups of boundary conditions. The first group, referred to as the "fixed" boundary conditions, includes those that describe conditions at locations other than along the coolant flow passages, for example, the exhaust gas temperature. Since all these boundary conditions were not measurable, an analysis was conducted in order to assess the sensitivity of the finite element model to each of these "fixed" boundary conditions. Classical experimental design methods were used to estimate the effect of each of the boundary conditions on the temperatures at the nodal points corresponding to experimental thermocouple locations.; The second group of boundary conditions includes the convection coefficients along the coolant flow passages--the determination of which is the primary goal of this work. Since these coefficients cannot be measured or calculated directly, this analysis required an iterative procedure. Values for the convection coefficients were assumed along each section of the valve bridge coolant passages. The resulting theoretical temperature distribution was computed and compared with the experimental results. The assumed convection coefficients were then adjusted until the analytical and experimental temperature results agreed. This procedure was repeated for several sets of experimental conditions and the resulting convection coefficients were compared to pure convection estimates. In this manner, the modes of heat transfer present in the valve bridge coolant passages of a diesel engine cylinder heat were identified.