Overview Of Modeling Effort

In the course of this work a new convective heat transfer model was developed which determines the values of heat transfer coefficients from boundary layer theory, linking them to local effective flow velocities (Morel and Keribar, 1985). The present model attempts to account for all of the important in-cylinder motions, and introduces a differentiation between the combustion chamber surfaces to resolve heat flux differences along combustion chamber walls. It has a degree of spatial dependence, in that it divides a bowl-in-piston geometry into three flow regions (squish region above piston crown, cup volume, and region above the cup), and solves in each differential equations for swirl and turbulence. It includes the capability to treat re-entrant piston bowl shapes. An example of its resolution.is displayed in Figure 1, showing the differences between calculated heat transfer coefficients on three in-cylinder surfaces. The model is global in nature, but it incorporates much more physics than previous models and thus it requires much less adjustment from engine to engine or from one combustion chamber shape to another than previous models. In fact, the model has no adjustable constants. It has been used on a range of engines from small high swirl diesels to a locomotive size diesel, and the global, cycle averaged heat rejection levels it predicts have been found to be in good agreement with the heat rejection levels observed in test cell experiments.

Due to soot formation during diesel combustion, thermal radiation from gases to surrounding combustion chamber surfaces is a significant component of heat transfer. Its importance increases in insulated engines where convective heat transfer is reduced and radiation can be the dominant heat transfer mechanism. To predict its magnitude, a new heat radiation model was developed, which accounts for all of the key parameters governing heat radiation: soot loading of combustion gases, radiation temperature, and spatial distribution of radiation to the appropriate in-cylinder surfaces (Morel and Keribar, 1986).

Figure 1. Distribution of heat transfer coefficients on various in-cylinder surfaces.


Figure 1. Distribution of heat transfer coefficients on various in-cylinder surfaces.

Instantaneous and mean levels of heat radiation are functions of the volume and distribution of burning gas, amount of soot present in the burning gas, combustion chamber geometry, and also of surface emissivities and temperatures. The heat radiation model takes into account all of these dependencies, using a kinetics based soot.model and a zonal approach to spatial distribution of radiation. One example of predicted radiation heat transfer in an insulated diesel is shown in Figure 2, identifying the separate contributions to the total heat transfer.

Figure 2. Predicted heat transfer in an insulated heavy duty diesel engine operating at the peak torque, showing contributions of convection and radiation.

Cyclic temperature transients arising in surfaces of the combustion chamber under nominally steady-state engine conditions have been calculated.

A method was developed which aiiows differentiation in these transients from surface-to-surface, including the effects of the local gas-phase heat fluxes, local material properties and local wall temperatures (Morel et al, 1985). It was also found that the surface temperature swing has a feedback effect on the mean wall temperature, and the correct method for accounting for this has been developed. The model was applied to a parametric study of various materials and conclusions were drawn about the magnitudes and importance of the cyclic temperature "swings," which were shown to reach the magnitude of several hundred degrees K for insulating materials under realistic engine conditions (this result has been confirmed by the test data discussed below).

The three models described above can be used to provide accurate boundary conditions for the calculation of structure temperature by the finite element method (FEM). For components that are in relative motion with respect to other components, e.g., valves or piston, this must be augmented by a method accounting for the varying boundary conditions at the moving interfaces. The method developed in this project is the first one that allows taking these interactions into account in a correct, fundamentally based manner. It, therefore, permits a reliable and accurate calculation of important parameters such as the thermal loads and main heat paths through the components to the various cooling circuits, piston and piston ring temperatures, etc.

Another method, first developed in this program, was the calculation of thermal shock, which occurs in components subjected to a rapid change in engine operating conditions (Keribar and Morel, 1987). This requires the calculation of transient engine performance, fluid flows and time-variation of gas-side boundary conditions for FEM calculations. The FEM must be solved in a transient form, including the effects of thermal inertia. Among the results obtained, it was shown that maximum component stresses occur during the transient rather than at the maximum steady-state operating point, which is highly significant in the design and developmental testing of insulating components.

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