Internal Temperature Distributions of Vaporizing Droplets: Effect of Their Spacing

A line of mono-sized and periodically spaced droplets is moving in the diffusion flame sustained by the droplet fuel evaporation. The temperature field within the droplets is measured using the two-color laser induced fluorescence technique. Experiments are undertaken on droplets made of different fuels including acetone, ethanol, 3-pentanone, n-heptane, n-decane and n-dodecane which have very different volatility and viscosity. A simplified model of the heat transfer within the droplet is developed, taking into account both heat conduction and heat advection by the droplet internal fluid circulation. Streamlines are assumed to follow those of a spherical Hill vortex, the intensity of which can be related to the friction coefficient. Comparisons between the measurements and the simulations reveal that the heat convection within interacting droplets is strongly reduced compared to the model of the isolated droplet.</jats:p

Internal temperature distributions of interacting and vaporizing droplets

International audienceA line of mono-sized and periodically spaced droplets is moving in the diffusion flame sustained by the droplet fuel evaporation. The temperature field within the droplets is measured with the help of the two-color laser-induced fluorescence technique. Experiments are undertaken on droplets made of different fuels including acetone, ethanol, 3-pentanone, n-heptane, n-decane and n-dodecane which have different physical properties such as their volatility and their viscosity. In some cases, the isotherms appear circular and concentric suggesting that only thermal conduction occurs in the droplet. In other cases, measurements show rather significant temperature differences between the leading and the trailing edges of the moving droplets. A simplified model of the heat transfer within the droplet is developed, taking into account both heat conduction and heat advection by the droplet internal fluid circulation. Heat and mass transfer are described in a quasi-steady approach within the framework of the film theory. The internal velocity field is assumed to correspond to the spherical Hill vortex solution, so that the velocity can be related to the stress exerted on the droplet surface. Comparisons between the measurements and the simulations reveal that the heat convection inside interacting droplets is strongly reduced, compared to the model of the isolated droplet

AC dielectric barrier discharge for separation control in the case of a turbulent jet diffuser

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Experimental investigation of spray impingement hydrodynamic on a hot surface at high flow rates using phase Doppler analysis and infrared thermography

International audiencearge panels of studies have been performed in order to investigate the dynamic behavior of a droplet during the impingement onto a rigid wall. Most of them are dealing with single droplets and only fewer are devoted to sprays, for which several diagnostics were implemented. Moreover, when sprays are involved, the liquid mass flux is generally quite low (on the order of several kg/m2/s) and the case of a cold surface was mainly considered. Therefore, the present experimental work aims at investigating the impingement of sprays onto hot surfaces (up to 800 °C) with liquid mass flux up to 13 kg/m2/s. Specifically, the main objective is to characterize the secondary droplets as a function of the surface temperature, starting from the Leidenfrost regime. To that purpose, a 2D phase Doppler analyzer (PDA) is synchronized with an infrared camera in order to measure simultaneously the time evolution of the local droplets size and velocity distributions with the surface temperature. Six full cone sprays were used in order to obtain a wide range of impingement conditions (normal incident Weber and liquid mass flux). A comparison of the impingement characteristics is also performed when the surface remains at room temperature. Another important point concerns also the effect on the spray flow of the presence of the surface. Results show clearly that the incident droplet trajectories are modified by the presence of the solid wall itself and depend strongly on the temperature surface. The in-depth investigation of the impingement characteristics of the spray is mainly focused on the description of the statistical mean diameter and the mass flux after the impingement. The main results highlight that the mean diameter value of the secondary droplets does not change during the Leidenfrost regime and increases for lower surface temperature. This behavior is strongly correlated to the liquid film formation for temperatures lower than the Leidenfrost point. The expelled mass flux exhibits a similar behavior but a decrease is observed after a given temperature, which can be attributed to the deepening of the liquid, leading to a reduction of the number of expelled droplets

Contrôle de la séparation par actionneurs plasma : jet rectangulaire et profil Naca 15

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Simulation of dispersed flow film boiling in LOCA conditions considering different fuel rod blockage ratios

International audienceDuring a loss of coolant accident (LOCA), in a nuclear-pressurized water reactor, fuel assemblies can be damaged and water is injected to cool down the core. In this hypothetical situation, a dispersed flow of steam and droplets occurs downstream of the quenching front. This two-phase flow propagates through the assemblies which can be deformed due to the swelling of the fuel rods' cladding causing blocked sub-channels. This complex flow plays an important role in the initial cooling of fuel rods that are not yet immersed into water. However, blocked sub-channels cooling is degraded because of the preferential steam flow towards less blocked regions. In previous work, we presented a mechanistic model implemented in the NECTAR code, which calculates heat and mass transfer phenomena as well as droplets dynamics in a polydispersed flow film boiling. NECTAR was validated with experimental measurements using three different geometries representing the cladding ballooning at a sub-channel scale. The French Institut de Radioprotection et de Sûreté Nucléaire (IRSN) has recently performed measurements of flow redistribution in a 7×7 fuel rods bundle with 4×4 ballooned rods under different geometric conditions (blockage ratio, length, and coplanarity), as well as different flow rates, showing that the blockage ratio is the predominant factor in the amount of deviated flow. In this article, we analyze the influence of the steam deviation on the heat transfer in a blocked subchannel, with different blockage ratios (61% and 90%) and lengths (100 mm and 300 mm) for representative LOCA conditions. The internal heat dissipation is evaluated and the contributions of the different involved mechanisms are analyzed