Simultaneous optical diagnostic velocity and scalar field by molecular tagging technique Stratified Flames View project

International audienc

Development of a LED-based PIV/PTV system: Characterization of the flow within a cylinder wall-array in a shallow flow

River engineeringInnovative field and laboratory instrumentatio

Lagrangian Particle Tracking at Large Reynolds Numbers

Particle tracking in turbulent flows is fundamental to the study of the transport of tracers, inertial particles or even active objects in space and time, i.e. the Lagrangian frame of reference. It provides experimental tests of theoretical predictions (e.g. for the statistics of fluid accelerations and particle dispersion) and helps to understand important natural processes where particle inertia is important (e.g. cloud microphysics). While the spatial (Eulerian) properties of turbulent flows have been studied for high, atmospheric Reynolds numbers ($R_\lambda > 10^4$), the profound difficulties in accurately tracking particles in turbulent flows have limited the Reynolds numbers in the Lagrangian reference frame to the Taylor scale Reynolds numbers $R_\lambda \lesssim 10^3$. Here we describe a setup that allowed Lagrangian particle tracking at $R_\lambda$ between 100 and 6000 in the Max Planck Variable Density Turbulence Tunnel (VDTT). We describe the imaging setup within the pressurised facility, the laser illumination, the particles and the particle dispersion mechanism. We verify that the KOBO Cellulobeads D-10 particles are suitable tracers. They carry negligible charge and their Stokes number is small over the full range of experimental conditions. We present typical data from the experiment and discuss the challenges and constraints of the setup

Shear localisation in the elastohydrodynamic lubrication regime

Understanding the shear behaviour of lubricants subject to high shear stresses and high shear rates is crucial to successfully model elastohydrodynamic friction. A method for determining this is through measuring the flow condition. Three in situ luminescence-based velocimetry techniques have been applied to a point contact with submicron film thickness. The techniques focus on quantifying through-film velocity distributions to understand the validity of flow assumptions used in modelling EHL. Nanoparticle tracking velocimetry was investigated to see whether the flow behaviour could be determined from nanoparticle velocity distributions. A protocol was developed deriving formal relations between nanoparticle velocity distributions, nanoparticle concentration distributions and through-film fluid flow velocity distributions, obtained using molecular tagging velocimetry (MTV). Nanoparticle velocity distributions were found to be pressure dependent. Results were proved quantitatively that the pressure dependency was consistent with the observed change in the flow condition; as the flow changed from Couette to partial plug flow above a critical pressure. The work presents for the first time simultaneous measurements of the velocity and temperature in elastohydrodynamic lubricant films. MTV was applied to determine through-film velocity profiles of a polyphenyl ether lubricant film operating in the EHL under high pressure and high shear. In this work, velocity profiles severely deviate from Couette shear and exhibit shear localisation. The flow behaviour is heavily dependent on the thermal conductivity of the contacting surfaces and shear heating, changing the through-film position of localised shear. A novel complementary molecular tagging thermometry technique, based on the principle of phosphorescence lifetime, was developed and applied to confirm through-film temperature gradients. At higher pressures, the flow behaviour is dominated by the normal pressure, which is observed through a strong asymmetric shape resembling a glassy partial plug. The work provides an improved understanding of the parameters leading to flow heterogeneity in elastohydrodynamic lubricant films.Open Acces

Design, construction and validation of an instrumented particle for the lagrangian characterization of flows. Application to gravity wave turbulence

The design and application of an instrumented particle for the lagrangian characterization of turbulent free surface flows is presented in this study. This instrumented particle constitutes a local measurement device capable of measuring both its instantaneous 3D translational acceleration and angular velocity components, as well as recording them on an embarked removeable memory card. A lithium ion polymer battery provides the instrumented particle with up to 8 hours of autonomous operation. Entirely composed of commercial off the shelf electronic components, it features accelerometer and gyroscope sensors with a resolution of 16 bits for each individual axis, and maximum data acquisition rates of 1 and 8 kHz, respectively, as well as several user programmable dynamic ranges. Its ABS 3D printed body takes the form of a 36 mm diameter hollow sphere, and has a total mass of (19.6 $\pm$ 0.5) g. Controlled experiments, carried out to calibrate and validate its performance showed good agreement when compared to reference techniques. In order to assess the practicality of the instrumented particle, we apply it to the statistical characterization of floater dynamics in experiments of surface wave turbulence. In this feasibility study, we focused our attention on the distribution of acceleration and angular velocity fluctuations as a function of the forcing intensity. The IP's motion is also simultaneously registered by a 3D particle tracking velocimetry (PTV) system, for the purposes of comparison. Beyond the results particular to this study case, it constitutes a proof of both the feasibility and potentiality of the IP as a tool for the experimental characterization of particle dynamics in such flows

Fluidodinámica de un reactor de lecho fluidizado de dos zonas con cambio de sección y su aplicación usando membrana permeoselectiva

El reactor de lecho fluidizado de dos zonas, recientemente patentado por la Universidad de Zaragoza (PCT/ES2009/070241), ha sido propuesto como una solución efectiva al problema de la desactivación catalítica que tiene lugar en procesos gas-sólido catalíticos en los que intervienen hidrocarburos gaseosos. El fundamento del reactor radica en la alimentación fraccionada de gas en el lecho. Por un punto intermedio de este se introduce una corriente de gas reactivo, mientras que por la parte inferior del mismo se alimenta una corriente de gas oxidante. De este modo se inducen dos zonas con atmósferas diferentes en un mismo lecho fluidizado (reductora y oxidante) y la circulación de partículas entre ambas zonas del lecho permite mantener una actividad catalítica constante a lo largo del tiempo. En la zona superior del lecho tiene lugar la reacción catalítica que genera, como subproducto, un depósito carbonoso (coque) sobre la superficie activa del catalizador mientras que en la zona inferior del lecho se produce la combustión de dicho coque obteniéndose partículas de catalizador regeneradas y nuevamente activas para desarrollar la actividad catalítica en la zona superior. A fin de ganar versatilidad y poder mantener el régimen de fluidización (la velocidad del gas) entre ambas zonas del lecho, aún trabajando con caudales muy diferentes de corriente reactiva y de regeneración, el diseño original del RFLDZ se modificó añadiendo un cambio de sección (RLFDZ-CS). Adicionalmente, se propuso la inclusión de membranas en la zona de superior del lecho para retirar selectivamente los productos de reacción (RLFDZ-CS+MB) con el objetivo de desplazar el equilibrio termodinámico en reacciones limitadas por éste. La presente tesis abarca la caracterización fluidodinámica del novedoso RLFDZ-CS+MB así como su demostración experimental en base al proceso productivo de propileno a partir de la deshidrogenación catalítica de propano. Las limitaciones de este proceso (bajas conversiones de equilibrio, elevada endotermicidad y rápida desactivación catalítica) pretenden ser mitigadas con el reactor multifuncional descrito. El estudio fluidodinámico del RLFDZ-CS+MB incluye la medición experimental del movimiento de las partículas catalíticas en el lecho, la caracterización del régimen de burbujeo en función de las condiciones de operación, el desarrollo de modelos matemáticos para predecir el grado de mezcla axial de sólidos y las propiedades de burbuja y la validación de códigos de fluidodinámica computacional, CFD, para simular el comportamiento del lecho. En concreto, se han utilizado técnicas de tratamiento digital de imagen y velocimetría de partículas para caracterizar el movimiento de sólidos y burbujas en el lecho, haciendo uso de trazadores ópticos fosforescentes para realizar el seguimiento del grado de mezcla axial entre las zonas superior e inferior del lecho catalítico. Se han desarrollado correlaciones hidrodinámicas para estimar la variación del tamaño y velocidad de burbujas de gas con la posición vertical en el lecho y se ha implementado el modelo de retromezcla a contracorriente (Counter-current Back-Mixing, CCBM) para predecir la evolución temporal del grado de mezcla axial. Se ha simulado el comportamiento fluidodinámico del RLFDZ-CS+MB mediante los códigos CFD comerciales Ansys CFX y Fluent, validando experimentalmente los resultados obtenidos por dichos modelos. Esencialmente, se ha evaluado el efecto del tipo de partícula, velocidad del gas, ángulo de cambio de sección y posición relativa de la alimentación superior en el comportamiento fluidodinámico del reactor. Adicionalmente se ha estudiado la influencia del cambio de escala, el uso y disposición de membranas en el lecho y la inclusión de obstáculos (internals) en el régimen de burbujeo y mezcla de sólidos. Se han analizado las limitaciones operacionales del sistema relativas la aparición de zonas muertas, by-pass de gas y regímenes de slugging y se ha descrito una ventana de operación para el correcto funcionamiento fluidodinámico del reactor. La demostración experimental del funcionamiento del RLFDZ-CS+MB, en base a la información recogida en la caracterización fluidodinámica, se ha llevado a cabo estudiando la deshidrogenación catalítica de propano en presencia de un catalizador activo y selectivo a la deshidrogenación (Pt-Sn/MgAl2O4) y de membranas densas basadas en paladio para la extracción selectiva de hidrógeno de los gases de reacción. Se ha establecido un rango de temperaturas de operación y fracciones de agente oxidante en la alimentación óptimos para maximizar la producción estacionaria de propileno. La intensificación de procesos llevada a cabo en el RLFDZ-CS+MB multifuncional ha mejorado las tasas de rendimiento a propileno de procesos comerciales, obteniéndose los mejores resultados (SC3H6 = 92%, YC3H6 = 58%) entre los reportados en literatura

Optical Techniques for Experimental Tests in Microfluidics

This PhD dissertation deals with the use of optical, non-invasive measurement techniques for the investigation of single and two-phase flows in microchannels. Different experimental techniques are presented and the achieved results are critically discussed. Firstly, the inverse use of the micro Particle Image Velocimetry technique for the detection of the real shape of the inner cross-section of an optical accessible microchannel is shown by putting in evidence the capability of this technique to individuate the presence of singularities along the wetted perimeter of the microchannel. Then, the experimental measurement of the local fluid temperature using non-encapsulated Thermochromic Liquid Crystal particles is discussed. A deep analysis of the stability of the color of these particles when exposed to different levels of shear stress has been conducted by demonstrating that these particles can be used for simultaneous measurements of velocity and temperature in water laminar flows characterized by low Reynolds numbers (Re < 10). A preliminary experiment where the TLC thermography is coupled to the APTV method for the simultaneous measurement of the three-dimensional velocity and temperature distribution in a microchannel is shown. Finally, an experimental analysis of the different flow patterns observed for an adiabatic air-water mixture generated by means of a micro T-junction is discussed. The main air-water mixture features have been deeply observed in 195 different experimental conditions in which values of superficial velocity ranging between 0.01 m/s and 0.15 m/s for both the inlet flows (air and water) are imposed. The flow patterns of the air-water mixture are strongly influenced by the value of the water superficial velocity; on the contrary, the air superficial velocity plays a secondary role for the determination of the characteristics of the bubbles (i.e. length)