Passive Techniques for the Enhancement of Convective Heat Transfer in Duct Flow for Highly Viscous Fluids: the Effect of Wall Curvature and of Wall Corrugation

摘要

In this Thesis work two different passive techniques for the enhancement of the convective heat transfer in single phase duct flow are studied: the heat transfer performance of coiled tubes having smooth and corrugated wall is experimentally investigated. The curvature heat transfer augmentation effect is due to the fact that the fluid flowing inside the pipe experiences the centrifugal force that promotes a secondary flow: local maxima in the velocity distribution that locally increases the temperature gradients at the wall by then maximizing the heat transfer, are induced. The enhancing effect in corrugated pipes is mainly due to the macroscopic mixing of the fluid, activated by the destabilization of the flow which leads to the early onset of the transfer mechanism associated to the transitional regime. This issue becomes particularly important in industrial applications in which the thermal processing of medium and high viscosity fluids is required, such as in the food, chemical, pharmaceutical and cosmetics industries where the momentum transfer mechanism is necessarily laminar and therefore the efficiency of the heat transfer apparatuses in which the fluids are conveyed is inevitably penalized. The first part of this work concerns the estimation of the average heat transfer performances along the heated length of the coiled pipes in terms of Nusselt number circumferentially averaged along the cross section and computed at different axial position. In addition the heat transfer augmentation is compared to the correlated pressure drop penalties. In the second part of this work, with the aim to deeply investigate the mechanisms that govern the heat transfer in coiled pipes, the solution of the IHCP in the wall, starting from the temperature distribution acquired on the external wall surface, is employed to estimate the local convective heat-transfer coefficient on the interior wall surface along the cross section. Three different solution procedures, based on Tikhonov Regularization Method (TRM), Gaussian Filter Technique (GFT) and Quadrupole method (QM) are presented, implemented and optimized for the problem under study. The different approaches are validated, throughout their application to synthetic data and then applied to the experimental data regarding laminar fully developed region in the smooth coiled tubes. Also an estimation procedure, based on Gaussian Filter, is proposed and applied to the experimental data for corrugated wall coiled tubes and preliminary results are presented and discussed.