Head on collision of a vortex ring and a heated wall

Gelderblom, G. (2012) Head on collision of a vortex ring and a heated wall.

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Abstract:General: During the internship I did experimental research on the behavior of a vortex ring generated in water impinging a heated wall. This research is performed during an internship, part of the Thermal Engineering track of the master program of Mechanical Engineering at the University of Twente. In the experiment, vortex rings were generated pushing a volume of water out of a tube into a basin. The resulting ‘flow entity’ is comparable to the type of ring which can be seen when people push some smoke out of an o-shaped mouth. The vortex ring moves through the basin, ultimately colliding with a heated wall. The behavior was qualitatively studied using ink-visualizations and quantitatively investigated using PIV-measurements. The research is most specific performed to contribute to the scientific field of heat transfer, but is also of interest for some practical situations with vortex rings like in the field of rotor dynamics. Vortex generation: The tube diameter has a diameter of 19.8mm. The piston was driven with a current of [40 60 80]V for short periods of roughly 100-1000 ms, leading to a ratio between stroke and diameter of R=[1 2 3 4]. The piston velocity is calculated as the stroke divided by the time interval, and was determined to be between 7 and 23 cm⁄s. This is an average, the maximum is higher. Boundary layer: The wall is heated using a thermal bath and a pump. Heating of the wall leads, due to buoyancy effects, to the formation of a natural convection flow along the wall. The experiment was performed for isothermal conditions and a wall temperature of T=[0 40 55 70]°C. The Nusselt number for the heated cases varied between 150 and 210, indicating that convective heat transfer is dominant. The Prandtl number for the isothermal case is 6.22 while for a temperature of 70°C the Prandtl number is 4.17. The Grashof number is for all temperatures in the order of 〖10〗^8, indicating that the convection is still laminar, which is confirmed by the experiments. The Rayleigh number finally is in the order of 〖10〗^9, also indicating the dominance of convection. Collision: For the isothermal case, it appeared that a transition in behavior appears for higher stroke ratio’s (and circulation). For a stroke ratio up to R=1, the ring diameter just increases. For a higher stroke ratio, boundary layer separation occurs, leading to the formation of a secondary or even tertiary ring. This ring moves around the primary ring. When it moves into the center of the primary ring, instability occurs, leading to break-up and reconnection into smaller rings. For the heated cases, it appeared that the influence of the convection decreases for higher Reynolds numbers. The research focuses on the cases without boundary layer separation, in terms of conditions; the cases for R=1. It is found that for this case, the top part of the ring moves closer to the wall than the bottom part. The radius of the top part decreases, while it increases for the bottom part. In contradiction to that, for the top part the circulation seems to ‘dissipate’ slower. The convection imposes swirl to the vertical side parts. These parts move up even faster than the horizontal top part, leading to a typical ‘cat-head’ shape.
Item Type:Internship Report (Master)
Clients:
Universidad Nacional Autónoma de Mexico, Mexico D.F., Mexico
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
Link to this item:http://purl.utwente.nl/essays/62581
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