Martin John Birch

Computational Fluid Dynamic Studies of Vortex Amplifier Design for the Nuclear Industry—I. Steady-State Conditions

In this paper computational fluid dynamics (CFD) techniques have been used to investigate the effect of changes to the geometry of a vortex amplifier (VXA) in the context of glovebox operations in the nuclear industry. These investigations were required because of anomalous behavior identified when, for operational reasons, a long-established VXA design was reduced in scale. The study simulates the transient aspects of two effects: back-flow into the glovebox through the VXA supply ports, and the precessing vortex core in the amplifier outlet. A temporal convergence error study indicates that there is little to be gained from reducing the time step duration below 0.1 ms. Based upon this criterion, the results of the simulation show that the percentage imbalance in the domain was well below the required figure of 1, and imbalances for momentum in all three axes were all below measurable values. Furthermore, there was no conclusive evidence of periodicity in the flow perturbations at the glovebox boundary, although good evidence of periodicity in the device itself and in the outlet pipe was seen. Under all conditions the modified geometry performed better than the control geometry with regard to aggregate reversed supply flow. The control geometry exhibited aggregate nonaxisymmetric supply port back-flow for almost all of the simulated period, unlike the alternative geometry for which the flow through the supply ports was positive, although still nonaxisymmetric, for most of the period. The simulations show how transient flow structures in the supply ports can cause flow to be reversed in individual ports, whereas aggregate flow through the device remains positive. Similar to the supply ports, flow through the outlet of the VXA under high swirl conditions is also nonaxisymmetric. A time-dependent reverse flow region was observed in both the outlet and the diffuser. It is possible that small vortices in the outlet, coupled with the larger vortex in the chamber, are responsible for the oscillations, which cause the shift in the axis of the precessing vortex core (and ultimately in the variations of mass flow in the individual supply ports). Field trials show that the modified geometry reduces the back-flow of oxygen into the glovebox by as much as 78. At purge rates of 0.65 m 3h the modified geometry was found to be less effective, the rate of leakage from the VXA increasing by 16-20. Despite this reduced performance, leakage from the modified geometry was still 63 less than the control geometry.

Variability and spatial fine structure of precipitating and trapped medium‐energy electron fluxes in the noon sector

The relationships between the precipitating and trapped components of magnetospheric electron flux for energy ranges exceeding 30 and 100 keV have been investigated using data from polar orbiting satellites, the study being restricted to a limited geographic region at auroral latitudes in the noon sector. The electron flux of these energies is the cause of auroral radio absorption. The data are analyzed at two levels of detail. Variations between different passes are studied using their median values, and variations within passes are derived from individual data points at 2 s intervals, equivalent to about 10 km in distance. Several types of behavior are recognized. Basically, the ratio of precipitating to trapped flux at energies exceeding 30 keV varies in proportion to the trapped flux, though there is a limiting upper value where the two components are approximately equal. The precipitating flux never exceeds the trapped flux by any significant amount. These types appear to be consistent with weak and strong pitch angle scatterings, respectively. The precipitation at >100 keV varies somewhat with the >100 keV trapped flux but more strongly with the >30 keV component, consistent with scattering by chorus waves produced by electrons less energetic than those being scattered. Comparison between the two energy ranges shows that the precipitating component is always softer than the trapped. The detailed relationship between the precipitating and trapped components varies from pass to pass by an amount related to the east-west component of the interplanetary magnetic field. Superimposed on the above behavior are large reductions of precipitation, spatial rather than temporal in nature, during which the trapped flux remains virtually unchanged. These reductions appear to be due to structures some tens of kilometers across, perhaps related to “ducts” within the magnetosphere. Some theoretical considerations based on the Kennel and Petscheck theory of scattering are given in an Appendix.

A Review of Vortex Amplifier Design in the Context of Sellafield Nuclear Operations

Vortex amplifiers have for over 30 years been used to ensure containment of glove-box ventilation in the event of a barrier breach, the most likely such breach being damage to the glove itself. Containment is achieved using fluidic principles to control the glove-box depression and ventilation rate under both normal and emergency conditions; in the event of such a breach vortex amplifiers can switch quickly between these two states without recourse to electrical, pneumatic or manual intervention. This paper begins by summarising the developments in vortex amplifier design used at the Sellafield site by successive companies engaged in fuel technology, reprocessing and decommissioning (British Nuclear Fuels PLC (BNFL), BNFL Engineering Limited, British Nuclear Group and Sellafield Limited). The main reasons for design changes have been practical issues of set-up, cleaning, filter and waste minimisation, and space limitations. The development culminates in the use of a smaller version of the vortex amplifier (VXA) which is a nearly exact geometrical scaling of its predecessor and which has been standard design for over a decade. Initial use of this device, the mini–VXA, led to a substantial increase in the amount of inert gas needed to maintain the required oxygen-depletion conditions within the glove-box, implying some escape of oxygen into the glove-box. The use of the mini–VXA introduced practical issues relating to (i) its control characteristics and (ii) the reverse flow of air in the supply port. Comparison with the published design specification demonstrates that the geometrical scaling process has led to a slightly hysteric characteristic. Tests conducted by the authors indicate (i) that the origin of the escaping oxygen is the control air feeding back through the supply ports and (ii) that a prototype chamber and orifice plate arrangement between the glove-box and mini–VXA significantly reduces the inert gas demand in normal usage. This prototype arrangement introduced problems in maintaining a clean environment in the chamber, so the chamber and orifice was substituted by a detachable cowl that enabled the mini–VXA to be located within the glove-box and provided access for cleaning.Copyright