Numerical analyses of different mixing enhancement techniques in micro-devices

Zusammenfassung

In this work, 3D numerical analyses are carried out to study different heat transfer and mixing enhancement techniques in confined flows inside micro-devices, where the flow commonly moves at low/moderate Reynolds numbers. The objective is to understand, from basic principles and for very simple geometrical configurations, the different flow topology and mechanisms that are generated, and find possible paths of optimization and applications in the industry. Given the laminar nature of the flow generated in this type of devices, they usually are poorly efficient in terms of transferring heat between the fluid and the walls of the devices. However, certain fluid structures, such as Kármán vortex streets, or loops inside the flow, can be created, enhancing the mixing and increasing its heat exchanging capability and, therefore, rising their efficiency to dissipate heat from other surfaces. First, a passive enhancement method is studied. The method consisted in varying the geometry of a square channel-based heat sink device, adding some tip clearance at the top part of the device, to create an additional connection between two stagnation chambers. This tip clearance generates a balance between heat transfer and pressure drop in the device, while generating some new flow topologies that do not appear in a simple finned heat sink. The results obtained are compared and validated through experimental values. Second, flow pulsation is evaluated as an active heat transfer enhancement method. Prescribed sinusoidal flow velocity pulsations at different frequencies are forced at the inlet of a flow passing through a square section channel. Two geometry configurations are studied: a clean channel and a channel with a (high blockage) square vortex promoter inside. Channel walls are used as heat source and both heat transfer and pressure drop are analyzed. The natural shedding frequency of the Kármán vortex street that appears past the vortex promoter is used as base frequency for the pulsation studies. Multiples of this frequency are analyzed to observe if the heat exchange between fluid and channel walls can be favored. Last, another active mixing enhancement method, the mechanical stirring of a vortex promoter, is analyzed. In this case, a prescribed sinusoidal movement is applied to a square vortex promoter (prism) inside a square section channel. Three different frequencies for the prism motion are tested, using again, as base frequency, the dominant frequency of the Kármán vortex street generated past the non-moving prism. Virtual particles were seeded and the standard deviation of their distribution past the prism were computed to quantify the mixing. As in the previous cases, pressure drop was also computed in order to look for an optimal configuration in terms of mixing/power draw. Experimental results (obtained with Particle Image Velocimetry techniques) where used to validate the numerical results. All the numerical studies have been carried out using OpenFOAM, a free open source CFD software (developed by OpenCFD Ltd and ESI Group and programmed on C++) which solves the problem using Finite Volume Methods for the integral flow conservation equations. All solvers and utilities can be edited, modified and compiled by the user. The CFD analyses contained in this work have been validated with experimental results thanks to the collaboration with the School of Aeronautical and Space Engineering, of the Technical University of Madrid (UPM).