Application of experimental and computational fluid dynamics techniques to the design of vortex-amplifers

Abstract

Operation of gloveboxes containing radiological hazards relies on risks being broadly acceptable. The operator's protection is maintained in the event of damage to the containment wall or gloves by Vortex Amplifiers (VA's). The VA is a hybrid of the vortex diode and the proportional beam deflection amplifier; two fluidic devices relying on fluid momentum (rather than internal moving parts) to control flow. Further, environmental and fire protection is afforded by inerting the internal glovebox atmosphere utilising Nitrogen or Argon as a constant purging medium.
Over the years, ergonomic restrictions have driven VA development towards geometrically scaled-down versions of previously accepted designs. Performance of the new so called mini-VA units was found to be disappointing; investigation of high Oxygen concentrations within gloveboxes identified the mini-VA as being the source. Study of the mini-VA behaviour using smoke visualisation techniques reported here has shown a potential mechanism for leakage from the device control ports into the glovebox. In order to mitigate the leakage and ensure that gloveboxes remain inert and moisture free, increased purging flows are required. The increase in cost of Nitrogen to BNFL is estimated to be in the region of £1 000,000 per year on one plant alone.
Very little has been published in respect to back diffusion from VA's or the flow phenomena that cause it. The following work is an experimental and numerical investigation of the physics and characteristic behaviour of mini-V A's, resulting in the development of a potential retrofit mini-VA solution capable of reducing Oxygen leakage by up to 78%.
Performance measurements are presented for alternative mini-VA's geometries operating in the incompressible flow regime, with air used as the medium. VA geometry and operating methods have been designed to simulate those used at Sellafield. The effects of both control and supply port geometry on the VA characteristic and VA performance have been investigated utilising both fluid measurement and smoke visualisation techniques. Standard VA performance criteria have been adopted, based upon maximum to minimum flow and pressure ratios. Information gleaned from the physical investigations was used to inform further study using numerical techniques.
A commercial Computational Fluid Dynamic (CFD) software 'ANSYS-CFX' was used to conduct further design investigations of the mini-VA geometry. Extensive verification studies are presented demonstrating adequate resolution of the VA geometry utilising an unstructured tetrahedral mesh and detailed representation of the complex anisotropic forces that exist within the VA. Variation in geometric length scales and fluid characteristics were overcome, enabling representation of the complex boundary layers formed within the mini-VA's thin vortex chamber. Consideration has also been given to both quasi-static and dynamic flow conditions. The anomalous reverse flow in the supply ports of the mini-VXA has been captured and instabilities in the vortex core successfully reproduced. This is believed to be the first study capturing numerically the reverse supply flow phenomena in vortex amplifiers. Use of the code was validated against the results of previous physical experimental data. Results of the simulations and physical investigations were used to develop alternative prototype mini­V A geometry.
Following a series of tests carried out at Nexia Solutions BTC test facility, Sellafield, leakage characteristics for the prototype mini-VA geometries have been compared with that of the current BNFL mini-VA design geometry. Performance of the proto-type geometries is shown to be significantly better than that of the existing BNFL geometry.