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Convective heat transfer in an impinging synthetic jet A numerical and experimental investigation

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Description: In the present study, an idealized geometry has been created for investigating the flow and heat transfer of a purely oscillatory jet that is not affected by the manner in which it is produced, and hence has allowed us to investigate the flow and heat transfer in a generalized approach. The unsteady Navier-Stokes equations and the energy equation were numerically solved using a full unsteady, two-dimensional finite volume approach in order to capture the complex time dependent flow field produced by periodic ejection-suction at high frequencies. Moreover, laboratory experiments were performed in order to nearly replicate the idealized problem and thus allow comparison to the aforementioned numerical investigations. A detailed analysis was performed on the correlation between the complex velocity field and the observed wall heat transfer. Scaling analysis of the governing equations was utilized to identify non-dimensional groups and propose a correlation for the space and time averaged Nusselt number. A fundamental frequency, in addition to the jet forcing frequency was found, and was attributed to the coalescence of consecutive vortex pairs. This vortex pairing led to lower time averaged heat transfer, for matched Reynolds numbers. Point to point correlations showed that the instantaneous local Nusselt number strongly correlates with the vertical velocity. The dependence of the surface averaged Nusselt number to jet parameters generally agrees with the computational results. However, discrepancies found between numerical and empirical local data were attributed to heat losses that were comparable in magnitude to the convective heat transfer generated by the jet, as well as the rise of natural convection due to an impinging fluid flow that might become fairly slow. Various vortex identification methods were investigated for proper identification of the train of vortices emanating from the jet and their evolution and eventual dissipation. Intuitive definitions of vortices such as spiraling streamlines, pressure minima and isovorticity surfaces suffer from inaccuracies. In the present work, the vortex-identification criterion employed was the Q -criterion (Hunt et al. 1988), which defines vortices as connected fluid regions with positive second invariant of the velocity gradient tensor. By tracking vortices, it was found that a primary vortex advecting parallel to the target surface gave rise to a secondary vortex with opposite net vorticity. It was found that the secondary vortex was largely responsible for enhancement of the heat transfer within the wall jet region. When vortex coalescence occurs, the degradation in the heat transfer enhancement is due to the reduction in the number of vortices interacting with the surface. By understanding the mechanisms that drive the phenomenon of vortex merging, optimum conditions of operation can be achieved.
Language: English
Format: Degree Work