Numerical and experimental investigations on thermal electro-hydrodynamic convection in a cylindrical annulus with different orientation and with rotating inner cylinder

点击翻译按钮获取中文摘要

The dielectrophoretic (DEP) force acts on a dielectric fluid when it undergoes a permittivity stratification combined with an inhomogeneous electric field. If the permittivity is stratified by the application of a temperature gradient, the DEP force can be seen as a thermal buoyancy induced by an artificial gravity of electric nature. In a cylindrical annulus whose inner cylinder is heated and connected to a high frequency alternating electric potential and the outer cylinder is cooled and grounded, the dielectric fluid will be affected by a centripetal electric gravity field and thermoelectric instability may occur. This thermal electro-hydrodynamic (TEHD) instability enhances the heat transfer through the cylindrical surfaces and therefore allows for technical applications such as heat exchanger devices. Many theoretical, numerical and experimental investigations of the DEP force effect in an annulus have been carried out considering also microgravity conditions. During the actual running TEHD-project, an exhaustive numerical and experimental analysis of the combined effect of the electric gravity and of the Earth’s gravity in a vertical cylindrical annulus has been performed. Amongst other things, it could be shown that the thermoelectric instability takes the form of axially aligned stationary columnar pairs of counter-rotating vortices. These vortices consist of convective plumes with azimuthally alternating radially in/outward directed cold and warm jets which increase the heat transfer. In the framework of an optimized system for the enhancement and control of the heat and mass transfer, experiments and numerical simulations with two new configurations of the cylindrical annulus subjected to the DEP force will be carried out: (i) a horizontally lying configuration and (ii) a rotation of the inner cylinder is added in the vertically aligned cylindrical annulus. These two new experiment configurations will augment the scientific knowledge on the use of the thermoelectric buoyancy for flow and heat transport control. In addition, the control problem of optimizing heat transfer in both configurations will by solved by numerical methods.