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

Effet des rangées de perturbateurs pariétaux sur les transferts de chaleur

L’étude numérique du transfert de chaleur dans un échangeur de type HEV (High Efficiency Vortices) permet d’expliquer les mécanismes de l’intensification induits par les perturbateurs de paroi. L’effet des différentes structures générées est ainsi mis en évidence. Les performances globales du HEV montrent qu’il affiche une meilleure efficacité énergétique par rapport à d’autres échangeurs du marché

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

Turbulence length scales in a vortical flow

Laser Doppler velocimetry is used to investigate the velocity spectra and turbulence length scales in a turbulent vortical flow. The turbulent vortical flow is ensured by vorticity generators (VGs) inserted into a straight circular pipe. Each VG generates a complex flow that is mainly the combination of a steady streamwise counter-rotating vortex pair and a periodic sequence of hairpin-like structures caused by the Kelvin- Helmholtz instability in the shear layer ejected from the VG trailing edges. These primary structures induce a secondary vorticity in the wake of the VG. The aim of the study is to analyze the velocity spectra and turbulent length scales for the different coherent structures in the flow. Thus, the Kolmogorov and Taylor microscales, the Liepmann-Taylor microscale and the viscous length scale are determined in different locations in the VG streamwise direction. The evolution of the length scales with respect to the Taylor-Reynolds number is compared with theoretical trends in a variety of flows in the open literature

On the synergy field between velocity vector and temperature gradient in turbulent vortical flows

The intensity of the secondary flow induced, especially, by streamwise vorticity, which are generated in their turn by vortex generators or in flows with curved streamlines has a direct impact on the heat transfer process. Thus the understanding and quantification of the physical mechanisms underlying the heat transfer by streamwise vorticity are fundamental for practical applications such as multifunctional heat exchangers/reactors (MHER) used in chemical processing industry, cooling of electronic systems and data centers, as well as biomedical engineering. In the present study, CFD simulations are performed to investigate the synergy field in two different flows. The synergy field principle is based on the assertion that the included angles θ between the streamlines and the isotherms is related to the heat flux that arises. From the local distribution of the intersection angle in the flow cross section, it is found that in the thinning region of the thermal boundary layer where the Nusselt number is the highest, θ is minimum. By introducing a characteristic parameter defined as the volume-averaged θ, it is found that the lowest θ value corresponds to the flow configuration presenting the highest Nusselt number. This confirms that the transport phenomena are intensified in the flow where the geometry minimizes this parameter. Finally, the study discusses the use of the synergy field principle in three dimensional turbulent vortical flows, and presents a new intensified MHER which can be used in several industrial processes

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

Mixing performance in Split-And-Recombine Milli-Static Mixers—A numerical analysis

Heat recovery is the reutilization of lavished thermal energy. This paper proposes a hybrid heat recovery system that utilizes exhaust gases of a generator to heat water and produce electricity using thermoelectric generators. The system is composed of a concentric tank with a copper tube passing through it. At the inner surface of the tube, a layer of TEGs is located. The main purpose of the paper is to study the effect of changing the load of the generator on the water temperature and power generated. Knowing that 100 TEGs are utilized, results show that 47 °C hot water and 141 W are produced when load is 10 kW. It increases to 97 °C hot water and 1412 W when the generator load is 38 kW (14.12 W per TEG)

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