Technical developments for computed tomography on the CENBG nanobeam line

The use of ion microbeams as probes for computedtomography has proven to be a powerful tool for the three-dimensional characterization of specimens a few tens of micrometers in size. Compared to other types of probes, the main advantage is that quantitative information about mass density and composition can be obtained directly, using specific reconstruction codes. At the Centre d’Etudes Nucléaires de Bordeaux Gradignan (CENBG), this technique was initially developed for applications in cellular biology. However, the observation of the cell ultrastructure requires a sub-micron resolution. The construction of the nanobeamline at the Applications Interdisciplinaires des Faisceaux d’Ions en Region Aquitaine (AIFIRA) irradiation facility has opened new perspectives for such applications. The implementation of computedtomography on the nanobeamline of CENBG has required a careful design of the analysis chamber, especially microscopes for precise sample visualization, and detectors for scanning transmission ion microscopy (STIM) and for particle induced X-ray emission (PIXE). The sample can be precisely positioned in the three directions X, Y, Z and a stepper motor coupled to a goniometer ensures the rotational motion. First images of 3D tomography were obtained on a reference sample containing microspheres of certified diameter, showing the good stability of the beam and the sample stage, and the precision of the motion

Towards self-sustained oscillations of multiple flexible vortex generators

Passive methods are widely used for flow control in engineering processes for heat and mass transfer enhancement. Using flexible vortex generators (FVGs) in such applications in order to destabilize the flow can be thought to achieve higher performances taking advantage of the fluid-structure interaction. In this paper, we discuss the assessment of getting self-sustained large oscillation amplitudes of multiple FVGs from an upstream confined laminar flow. The FVGs are located on the opposite channel walls in alternated positions, separated by a distance equal to their span and inclined in the upstream direction with an angle of 30° with respect to the wall. Five cases are studied which differ by the number of alternating FVGs in the system and investigations are also performed adding two co-planar FVGs upstream. The Reynolds number is held constant with a value of 2000 (based on the hydraulic diameter) for all the cases. The effect of increasing the degree of freedom of the system, on creating a large displacement flapping motion is numerically investigated. The results show that a minimum of three alternating FVGs is needed to produce a self-sustained and periodic flow instability, leading to large FVG displacement when the co-planar FVGs are not present. The introduction of upstream co-planar FVGs destabilizes the flow by producing vortices which act as periodic forces on the downstream FVGs. In this case, large displacement amplitudes are thus observed with two alternating FVGs added downstream. A phenomenon of inverted drafting is observed in all the cases: upstream FVGs display smaller drag force values than the downstream ones. Since the downstream FVGs oscillate in resonance with the incoming flow, motion amplitudes become higher. Moreover, it has been observed that for all the configurations studied here, the FVGs located at the same wall location oscillate in phase with each others and out-of-phase with the ones located on the opposite channel wall

Effect of the angle of attack of a rectangular wing on the heat transfer enhancement in channel flow at low Reynolds number

Convective heat transfer enhancement can be achieved by generating secondary flow structures that are added to the main flow to intensify the fluid exchange between hot and cold regions. One method involves the use of vortex generators to produce streamwise and transverse vortices superimposed to the main flow. This study presents numerical computation results of laminar convection heat transfer in a rectangular channel whose bottom wall is equipped with one row of rectangular wing vortex generators. The governing equations are solved using finite volume method by considering steady state, laminar regime and incompressible flow. Three-dimensional numerical simulations are performed to study the effect of the angle of attack α of the wing on heat transfer and pressure drop. Different values are taken into consideration within the range 0° < α < 30°. For all of these geometrical configurations the Reynolds number is maintained to Re = 456. To assess the effect of the angle of attack on the heat transfer enhancement, Nusselt number and the friction factor are studied on both local and global perspectives. Also, the location of the generated vortices within the channel is studied, as well as their effect on the heat transfer enhancement throughout the channel for all α values. Based on both local and global analysis, our results show that the angle of attack α has a direct impact on the heat transfer enhancement. By increasing its value, it leads to better enhancement until an optimal value is reached, beyond which the thermal performances decrease

Simulation of cellular irradiation with the CENBG microbeam line using GEANT4

Light-ion microbeams provide a unique opportunity to irradiate biological samples at the cellular level and to investigate radiobiological effects at low doses of high LET ionising radiation. Since 1998 a single-ion irradiation facility has been developed on the focused horizontal microbeam line of the CENBG 3.5 MV Van de Graaff accelerator. This setup delivers in air single protons and alpha particles of a few MeV onto cultured cells, with a spatial resolution of a few microns, allowing subcellular targeting. In this paper, we present results from the use of the GEANT4 toolkit to simulate cellular irradiation with the CENBG microbeam line, from the entrance to the microprobe up to the cellular medium.Comment: 6 pages, 8 figures, presented at the 2003 IEEE-NSS conference, Portland, OR, USA, October 20-24, 200

Viscosity effects on liquid-liquid dispersion in laminar flows

Efficiency of liquid/liquid dispersion is an important stake in numerous sectors, such as the chemical, food, cosmetic and environmental industries. In the present study, dispersion is achieved in an open-loop reactor consisting of simple curved pipes, either helically coiled or chaotically twisted. In both configurations, we investigate the drop breakup process of two immiscible fluids (W/O) and especially the effect of the continuous phase viscosity, which is varied by addition of different fractions of butanol in the native sunflower oil. The global Reynolds numbers vary between 40 and 240, so that the flow remains laminar while the Dean roll-cells in the bends develop significantly. Different fractions of butanol are added to the oil in each case to examine the influence of the continuous phase viscosity on the drop size distribution of the dispersed phase (water). When the butanol fraction is decreased, the dispersion process is intensified and smaller drops are created. The Sauter mean diameters obtained in the chaotic twisted pipe are compared with those in a helically coiled pipe flow. The results show that chaotic advection intensifies the droplet breakup till 20% in droplet size reduction, and also reduces polydispersity

Heat transfer and mixing enhancement by free elastic flaps oscillation

An original concept is proposed to enhance heat transfer and mixing quality performances by using flexible vortex generators (FVGs) for a static mixer configuration. The role of free elastic flaps oscillations on the mixing process and heat transfer in a two-dimensional laminar flow is numerically investigated. The computational domain consists of four distant FVGs mounted on two opposite walls. Two cases are studied depending on the Reynolds numbers (based on the bulk velocity and the channel height) set to 1000 and 1850. FVGs efficiencies are compared to the corresponding cases with rigid vortex generators (RVGs). In the flexible cases, flaps oscillations increase the velocity gradients and generate an unsteady laminar flow with complex coherent vortices detaching from the tip of the flaps. The mixing efficiency is quantified by the transport of a passive scalar through the channel. It is shown that oscillations in the elastic cases enhance the mixture quality up to 98% relative to that in the rigid cases. The heat transfer enhancement is also investigated showing up to a 96% increase in the Colburn factor, 56% increase in thermal performance factor and 134% increase in the overall heat transfer. As the FVGs oscillate freely without any additional external force other than that exerted by the flow itself, the implementation of such a technique shows a great potential for the performance enhancement of multifunctional heat exchangers/reactors

The effect of self-sustained free elastic flaps oscillation on heat transfer and mixing performances

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Heat transfer and mixing enhancement by using multiple freely oscillating flexible vortex generators

In this paper, we discuss the effect of self-sustained passive oscillations of multiple flexible vortex generators (FVG) in a two-dimensional laminar flow, on heat transfer and mixing. The FVG are located on two opposite channel walls in an alternating positions, inclined in the upstream direction with an angle of 30° with respect to the wall. The FVG oscillate freely without any external force except that provided by the flow itself. Five cases are studied and they differ by the number of alternating flaps and by the presence or absence of two co-planar flaps upstream. The Reynolds number is held constant with a value of 2000 based on the hydraulic diameter of the channel. The simulations are performed by considering a two way strongly-coupled fluid structure interaction approach. The effect of increasing the system degree of freedom, by increasing the number of flaps, resulting in a larger displacement oscillation, on heat transfer and mixing is numerically investigated. The mixing process is quantified by solving the passive scalar transport equation and calculating a mixing index. The results show that mixing is enhanced for larger flaps displacement achieving up to 99% in mixing homogeneity. Moreover, the high amplitude oscillations when compared to the results of an empty channel, show a great ability to reduce the thickness of the thermal boundary layer and to enhance heat transfer resulting in up to 275% increase in the global Nusselt number, 317% increase in the local Nusselt number and 34% increase in the thermal performance factor