Heat transfer affects the performance and phasing of internal combustion engines. Correlations and equilibrium wallfunction models are typically employed in engine simulations to predict heat transfer. However, many studies have shown that significant errors are expected, owing to the failure of fundamental assumptions in deriving equilibrium wall-function models. Non-equilibrium wall models provide a more accurate way of describing the near-wall region of in-cylinder flows. In this study, simultaneous high-speed high-resolution particle image velocimetry and heat-flux measurements are conducted in an optically accessible engine. The experiments are performed under both motored and fired conditions at two different engine speeds. The experimental data are utilized to assess the performance of different models in predicting the thermoviscous boundary layer. These models include commonly used heat transfer correlations, equilibrium and modified wall-function models, and a recently developed non-equilibrium wall model. It is found that the equilibrium wall-function model significantly underpredicts the heat flux under both motored and fired conditions. By considering heat release effects in the boundary layer, the non-equilibrium wall model is shown to be able to adequately capture the structure and dynamics of both momentum and thermal boundary layers in comparison with experimental measurements, demonstrating its improved performance over previously employed correlation functions and the equilibrium model.
Progress in understanding turbulent combustion requires access to information that is derived from instantaneous 3D images showing the contours of the flame front, the flow field, and other quantities. The experimental effort to acquire such information is typically extraordinarily high and therefore not as widely used as needed. The introduction of commercially available plenoptic cameras now provides a means to simplify some of these measurements as is described in this study. A single 29 mega-pixel plenoptic camera was used to record the Mie scattering signals from volumetrically illuminated fuel sprays. Instantaneous 3D images were acquired from within the cylinder of an optically accessible engine for three automotive fuel injectors of differing nominal spray angles. Current reconstruction algorithms do not yet take translucent imaging conditions into account and therefore a quantitative analysis of the full 3D structure of the sprays was not possible. However, the plenoptic system did prove capable of accurately reconstructing bulk spray features such as spray angles. The 3D spray images were validated with data from a conventional 2D imaging system. This study demonstrates the feasibility of using a single plenoptic camera to view spray geometry in 3D, even from within the challenging in-cylinder engine environment.
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