Unsteady convective heat transfer modeling and application to internal combustion engines.

摘要

Steady convective heat transfer correlations are widely used to predict heat transfer in the cylinders and manifolds of internal combustion engines. However, previous studies by Overbye et al. (1961), Annand and Pinfold (1980), Kornhauser and Smith (1994), and Bauer et al. (1998) showed that the heat transfer rates are out of phase with gas-wall temperature difference and gas velocity in engine cylinder and manifold.; In this study, a dimensional analysis of the unsteady boundary layer governing equations was performed, and two new dimensionless variables were identified. They are related to the changing rates of velocity and temperature. A new concept of dynamic variable that contains information about both the instantaneous value and it changing rate has been introduced to use the dimensionless variables to extend the existing steady heat transfer models into unsteady heat transfer models.; From this study, it is found that the heat transfer rate has a phase delay with respect to the fluid velocity variation due to the delay of turbulent intensity from mean flow velocity. This phase delay can be captured by the unsteady heat transfer model for the unsteady velocity correction.; It is also found that in a motored engine the heat transfer rates on both sides of the thermal boundary layer have a phase shift. This phase shift can be simulated by the unsteady heat transfer model for the unsteady temperature correction.; Furthermore, it is found that when the velocity decreases too fast, turbulent intensity will not follow the velocity's decrease process; rather it starts its own decay process. The heat transfer in the turbulent decay process is the primary contributor to the heat transfer increase from unsteady velocity variation. A criterion has been found for the onset of the turbulent decay process and a model has been developed to predict the turbulent intensity associated with the turbulent decay process.; The unsteady heat transfer models developed in this study are validated in a turbulent pipe heat transfer system, an engine intake manifold, and a motored diesel engine. The validation results confirmed the model's ability to capture the unsteady effects of velocity and temperature variations.