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

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

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

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)

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