Dielectrophoretic flow control of thermal convection in cylindrical geometries (English shop)

Preview

Table of Contents, PDF (25 KB)
Extract, PDF (300 KB)

Studies of thermal convection in cylindrical annulus geometries cover a wide range of physical and engineering problems. The focus of this work is the experimental investigation of thermal convection in a cylindrical annulus under the influence of a high-voltage electric field. The aim is to demonstrate possibilities for improving heat transport by these means. This effect can be exploited, for example, for more energy-efficient heat exchangers. For this purpose, two experimental cells were built, each having a heated inner cylinder and a cooled outer cylinder. The temperature difference generates thermal convection in the test volume between the two vertically aligned cylinders. The test volume is filled with a dielectric liquid. To generate the high-voltage field, the outer cylinder is connected to an alternating voltage and the inner cylinder is grounded. The two experimental cells differ in their radius ratio. The gap width and the height of the test volume are identical in both experimental cells, as is the aspect ratio. Within the framework of three parabolic flight campaigns, various further developments were implemented. These have to take into account the use of particles in a high-voltage electric field and full automation. A measuring device for recording heat transport was also developed and implemented. The key parameters of the experiment are the radius ratio and the aspect ratio, which describe the geometry. The properties of the fluids used are summarised in the Prandtl number. The driving force of the thermal flow is described by the Rayleigh number. Heat transport is evaluated by means of the Nusselt number.

The scientific results can be divided into two classes. The first class (laboratory experiment) comprises natural convection in a cylindrical annulus. It further comprises the superposition of natural convection with an artificial force field. The results show that a stable convection cell can be disturbed by an artificial force field. This disturbance induces an improvement in heat transport. The second class (parabolic flight experiment) describes thermal convection in a purely radial buoyancy field, exploiting the dielectrophoretic effect and without the influence of natural gravity. The observations make clear that the electrically driven buoyancy induces convection under microgravity conditions. Owing to the short microgravity phase, the flow remains in a transient state. The results presented show that the action of the dielectrophoretic effect can be used to increase heat transport.