Etienne Robert

Cavitation bubble behavior inside a liquid jet

The growth and collapse of laser-induced vapor cavities inside axisymmetric free-falling liquid water jets have been studied. Bubbles of different size are generated at various distances from the jet axis and the effects on the jet interface are recorded by means of ultrafast cinematography. The configuration is characterized by two dimensionless parameters: the bubble to jet diameter ratio δ and the eccentricity coefficient e defined as the radius of bubble generation divided by the jet radius. For high δ and e, microjets and droplets are ejected from the liquid jet at speeds exceeding 100m∕s. The observed jet fragmentation shows similarities with experiments conducted on a liquid mercury jet hit by a pulsed proton beam, a candidate configuration for future accelerator based facilities.

Mass spectrometer calibration over wide concentration ranges in multicomponent gas mixtures

A quadrupole mass spectrometer (QMS) is used to measure the mixture composition of gas samples gathered in a novel burner configuration used to generate unstretched diffusion flames. The large variations of species concentration in the mixtures found in the burning chamber lead to accuracy problems due to the nonlinearities inherent to the instrument mode of operation. To obtain precise and real-time measurements, the sensitivity of the instrument is mapped to account for important changes in the sample composition. The implemented calibration procedure accounts for the concentration of the various species of interest in the burner (H2, He, H2O, CH4, O2, Ar, CO2) in mixtures containing up to five constituents, using up to one hundred reference mixtures. When necessary, calibration adjustments are performed using a small set of measurements to account for the effect of the drift of instrument sensitivity resulting from instrument wear or fouling. This procedure allows us to keep the relative error on the concentration of every species of interest below 5% for most of the mixtures while a classical calibration based on a limited number of reference mixtures often resulted in relative errors in excess of 50%.

Thermal shocks and magnetohydrodynamics in high power mercury jet targets

The response of mercury samples submitted to a pulsed proton beam and the magnetohydrodynamic (MHD) effects of a mercury jet injected into a 20 T magnetic field are reported. The experimental conditions differ from those of proposed neutrino factories and the purpose of these measurements is to provide benchmarks for simulation tools of a realistic free mercury jet target. These measurements were completed in June 2002. Analysis is ongoing and the presented results are preliminary.

Acoustic separation of submicron solid particles in air

The use of ultrasound to continuously separate submicron particles suspended in air is investigated in a rectangular channel with adjustable height. An electrostatic transducer is used to generate a standing wave in the 50-80 kHz frequency range and the particles experience forces within the acoustic field causing them to concentrate at the pressure nodes. To assess the effect of several key design parameters on the separation efficiency, a simple method based on light scattering is implemented to provide information on the particle concentrations as a function of position in the channel. The images acquired are processed to yield a separation efficiency metric that is used to evaluate the effect of acoustic, flow and geometrical parameters. The results show that, in qualitative agreement with theoretical models, the maximum separation efficiency increases with the pressure amplitude of the sound wave. The separation efficiency is also linearly proportional to the standing wave frequency, when it is varied between 50-80 kHz. On the other hand, the effect of the average fluid velocity is less pronounced than expected, suggesting that in our channel separation is not limited by the interaction length between the acoustic field and the suspended particles. The effect of the parallelism of the reflector relative to the transducer is also investigated.

Acoustic separation of sub-micron particles in gases

In several areas of science and technology there is a strong need for concentrating, separating and sorting small particles suspended in gaseous flows. Acoustic fields can be used to accomplish this task, an approach extensively used in liquid phase microfluidics that has great potential for aerosol treatment. This paper presents an experimental investigation of acoustophoresis for very small particles in gases, with sizes ranging from tens to hundreds of nanometers. The phenomenon is studied in a rectangular channel with variable height in which a standing acoustic field is created by a broadband electrostatic transducer operated in the 50-100 kHz range. Downstream of the separation channel, the flow is separated into enriched and depleted streams with adjustable slits for analysis. The particle number density and size distribution is measured with a Scanning Mobility Particle Sizer (SMPS) as a function of position in the standing wave pattern. From these measurements, the separation efficiency is determined as a function of the particle size and the amplitude of the acoustic field. For the very small particles used here, this yields novel information on the magnitude of acoustophoretic forces in the transition and molecular flow regimes.

Soot Formation in Unstrained Diffusion Flames

The formation of soot particles has been investigated in CH4/O2 diffusion flames using a unique burner design, which allows the creation of a nearly unstrained planar reaction sheet. Spatially resolved soot volume fractions were obtained using laser-induced incandescence. These soot measurements and the sooting limits were obtained as a function of bulk flow across the flame and mixture strength. Samples were collected using thermophoretic sampling and analyzed using electron microscopy, revealing a broad range of microstructures including particles with unusually large primary diameters and carbon nanotubes. A theoretical model is presented, which confirms that under certain conditions the 1D nature of the flow field of the burner and the strong adverse temperature gradient on the fuel side of the flame result in the soot particles being held in place by thermophoretic forces and allowed to grow for very long time periods. Some of these so-called super aggregates reached sizes of tens of microns and became visible to the naked eye in the soot layer.

Estimation of acoustic forces on submicron aerosol particles in a standing wave field

ABSTRACT The net acoustic force acting on submicron particles suspended in a gas and exposed to a standing wave field is investigated as a function of particle size, by measuring both the aerosol number density and size distribution in a flow-through resonator. By taking into account all contributions relevant to the net force, this experimental study provides a first estimate for the acoustic radiation force in a size range where molecular effects are expected to be significant. The experiment consists of an electrostatic transducer generating a standing wave in the 50–80 kHz frequency range, with the submicron aerosol particles concentrated at pressure antinodes located across the height of a rectangular channel. A section of the flow is sampled isokinetically and analyzed using a Scanning Mobility Particle Sizer (SMPS), while the nodal patterns are visualized simultaneously using light scattering. The net acoustic force is calculated from their measured displacement along the axis of the 1D standing wave field. The component of this force resulting from radiation pressure is estimated by subtracting contributions from other forces. The results provide the first experimental estimation of the size dependence of the acoustic contrast factor for submicron aerosol particles, demonstrating the possibility of performing acoustic separation for diameters as small as 150 nm. Copyright

Numerical Investigation of a Confined Jet Array Penetrating into a Counterflow

A closely spaced jet array penetrating into a counterflow is investigated numerically to identify the location of the stagnation points and the conditions in their vicinity. Of special interest is the spatial homogeneity of the concentration field immediately upstream of the stagnation points for the species being supplied through the jet array. This configuration is used to produce the boundary conditions required in a research burner in order to create unstretched one-dimensional diffusion flames. The aim of the work presented here is to identify the parameter space conducive to the creation of such flames. The ratio of jet velocity to that of the bulk flow is the main control parameter and is investigated in cold-flow simulations. The results are compared with a limited set of experimental measurements available for this configuration.

Acoustophoresis in gases: Effect of turbulence and geometrical parameters on separation efficiency

Advanced particle manipulation techniques based on acoustophoresis have been developed in recent years, driven by biomedical applications in liquid phase microfluidics systems. The same underlying physical phenomena are also encountered in gases and hold great potential for novel particle separation and sorting techniques aimed at industrial and scientific applications. However, considering the physical properties of gases, optimizing the performance of flow-through separators unavoidably requires an understanding of the re-mixing effect of turbulence. In the work presented here we have investigated the effect of turbulence intensity on the separation efficiency of a variable frequency acoustic particle separator featuring a rectangular cross-section with adjustable height. This allows the creation of a standing wave with a variable frequency and number of nodes. The air flow is seeded with alumina particles, 300 nm nominal diameter, and the excitation source is an electrostatic transducer operated in the...