Time resolved digital holography applied to droplets fragmentation by shockwave

International audienceDroplets atomization by shockwave can occur in different issues commonly encountered in the industry such as leak, tank leakage or triple aggression (high speed impact, rupture and surrounded secondary explosion) of tanks. For the last case, shockwave can interact with liquid jets of drops and propagates the liquid far away from the container zone. Very fine secondary droplets can be produced in the worst case of atomization. These small particles can generate secondary effects like explosion in case of petrol derivatives in fire or toxic effects in case of direct breathing. High speed imaging is well suited to study transient phenomenon like explosions and shockwave. A dedicated shockwave generator has been designed to cope with interferometric measurement on holographic bench. This demonstrator is made of thick plastic tubes. The high pressure chamber is isolated from the guiding tube by domestic aluminum foils, the thickness and number of which drive the pressure rupture. Previous works have been carried on by time resolved shadowgraphy to characterize generated shockwave at the guiding tube outlet. This paper deals with time resolved digital holography to perform higher accuracy measurements and of course to reach 3D reconstruction of the whole phenomenon. Lensless in-line digital holography is carried on to improve the stability of the holographic set-up. Different Phantom high speed cameras have been tested as recording sensors, following pixel pitch, pixel size and of course the maximal throughput, from 7kfps (frame per second) up to 26kfps at full 1Mpixel resolution. Different regimes of droplet trains and droplet sizes have been tested. This has also been carried on for different liquids to show the effect of the physico-chemical properties of the liquid subjected to shockwave

Secondary Atomization: Drop Breakup in a Continuous Air Jet

Understanding drop breakup will optimize aircraft engine performance, reduce agro-chemical overspray, and improve pharmaceutical tablet efficacy. Large fuel fragments in engines lead to lowered fuel economy and higher pollutant emissions, while small drops yield more agro-spray drift into surrounding residential and environmental zones. Better pharmaceutical tablets will improve drug uptake and patient comfort. Engineers and scientists are currently unable to predict the number, size, and velocity of fragments formed during important drop breakup processes. Therefore, we are required to measure these quantities. We use digital inline holography (DIH) to record three-dimensional diameter and position data for fragments formed during multi-mode breakup. DIH provides 3D images at framing rates 300 times faster than in an IMAX theater. A laser is used as the light source and a high speed camera records the breakup events to video files. A MATLAB script is used to extract the diameters and positions of all fragments in the spray. The data is sorted into bins and histograms are produced which describe the probability of observing a fragment of any particular size and speed. Results show size histograms with more than one peak, a finding in direct contradiction to the last 40 years of spray research. Multiple peaks are indicative of fragmentation processes that occur due to multiple breakup mechanisms, with the number of histogram peaks corresponding to the number of mechanisms (some combination of bag, rim, and/or stamen breakup modes). The histograms will be useful to those modeling sprays in gas turbine engines and industrial sprayers

Cortical and trabecular bone benefits of mechanical loading are maintained long term in mice independent of ovariectomy.

Skeletal loading enhances cortical and trabecular bone properties. How long these benefits last after loading cessation remains an unresolved, clinically relevant question. This study investigated long-term maintenance of loading-induced cortical and trabecular bone benefits in female C57BL/6 mice and the influence of a surgically induced menopause on the maintenance. Sixteen-week-old animals had their right tibia extrinsically loaded 3 days/week for 4 weeks using the mouse tibial axial compression loading model. Left tibias were not loaded and served as internal controls. Animals were subsequently detrained (restricted to cage activities) for 0, 4, 8, 26, or 52 weeks, with ovariectomy (OVX) or sham-OVX surgery being performed at 0 weeks detraining. Loading increased midshaft tibia cortical bone mass, size, and strength, and proximal tibia bone volume fraction. The cortical bone mass, area, and thickness benefits of loading were lost by 26 weeks of detraining because of heightened medullary expansion. However, loading-induced benefits on bone total area and strength were maintained at each detraining time point. Similarly, the benefits of loading on bone volume fraction persisted at all detraining time points. The long-term benefits of loading on both cortical and trabecular bone were not influenced by a surgically induced menopause because there were no interactions between loading and surgery. However, OVX had independent effects on cortical bone properties at early (4 and 8 weeks) detraining time points and trabecular bone properties at all detraining time points. These cumulative data indicate loading has long-term benefits on cortical bone size and strength (but not mass) and trabecular bone morphology, which are not influenced by a surgically induced menopause. This suggests skeletal loading associated with physical activity may provide long-term benefits by preparing the skeleton to offset both the cortical and trabecular bone changes associated with aging and menopause

Determination of the aerodynamic droplet breakup boundaries based on a total force approach

The determination of the critical Weg number separating the different breakup regimes has been extensively studied in several experimental and numerical works, while empirical and semi-analytical approaches have been proposed to relate the critical Weg number with the Ohl number. Nevertheless, under certain conditions, the Reg number and the density ratio ε may become important. The present work provides a simple but reliable enough methodology to determine the critical Weg number as a function of the aforementioned parameters in an effort to fill this gap in knowledge. It considers the main forces acting on the droplet (aerodynamic, surface tension and viscous) and provides a general criterion for breakup to occur but also for the transition among the different breakup regimes. In this light, the present work proposes the introduction of a new set of parameters named as Weg,eff and Cal monitored in a new breakup plane. This plane provides a direct relation between gas inertia and liquid viscosity forces, while the secondary effects of Reg number and density ratio have been embedded inside the effective Weg number (Weg,eff

Predicting droplet deformation and breakup for moderate Weber numbers

The present work examines numerically the deformation and breakup of free falling droplets subjected to a continuous cross flow. The model is based on the solution of the Navier–Stokes equations coupled with the Volume of Fluid (VOF) methodology utilized for tracking the droplet-air interface; an adaptive local grid refinement is implemented in order to decrease the required computational cost. Neglecting initially the effect of the vertical droplet motion, a 2D axisymmetric approximation is adopted to shed light on influential numerical parameters. Following that, 3D simulations are performed which include inertial, surface and gravitational forces. The model performance is assessed by comparing the results against published experimental data for the bag breakup and the sheet thinning breakup regimes. Furthermore, a parametric study reveals the model capabilities for a wider range of Weber numbers. It is proved that the model is capable of capturing qualitatively the breakup process, while the numerical parameters that best predict the experimental data are identified

Numerical investigation of the aerodynamic breakup of a parallel moving droplet cluster

The present work examines numerically the aerodynamic breakup of a cluster of Diesel droplets moving in parallel with respect to the gas flow. Two- and three-dimensional simulations of the incompressible Navier-Stokes equations together with the VOF method are performed for Weber (We) numbers in the range of 5 up to 60 and non-dimensional distance between the droplets (H/D0) ranging from 1.25 to 20. The numerical results indicate that the proximity of droplets affects their breakup for distances H/D0≤5. For low droplet proximity distances (H/D0≤2.5), the droplets experience the so-called shuttlecock breakup mode, which has been also identified for droplets in tandem formations in a previous authors’ work and is characterized by an oblique peripheral stretching of the droplet. With decreasing H/D0 the breakup initiation time decreases, while the drag coefficient increases relative to that of isolated droplets. When the distance between the droplets is low enough (H/D0<1.5), this can result in critical We number, i.e. minimum We number leading to breakup, lower than that of an isolated droplet at the same conditions

A simple model for breakup time prediction of Water-Heavy Fuel Oil emulsion droplets

Immiscible heavy fuel-water (W/HFO) emulsion droplets inside combustion chambers are subjected to explosive boiling and fragmentation due to the different boiling point between the water and the surrounding host fuel. These processes, termed as either puffing or micro-explosion, are investigated with the aid of a CFD model that solves the Navier-Stokes and energy conservation equations alongside with three sets of VoF transport equations resolving the formed interfaces. The model is applied in 2-D axisymmetric configuration and it is valid up to the time instant of HFO droplet initiation of disintegration, referred to as breakup time. Model predictions are obtained for a wide range of pressure, temperature, water droplet surface depth and Weber number; these are then used to calibrate the parameters of a fitting model estimating the initiation breakup time of the W/HFO droplet emulsion with a single embedded water droplet. The model assumes that the breakup time can be split in two distinct temporal stages. The first one is defined by the time needed for the embedded water droplet to heat up and reach a predefined superheat temperature and a vapor bubble to form; while the succeeding stage accounts for the time period of vapor bubble growth, leading eventually to emulsion droplet break up. It is found that the fitting parameters are ±10% accurate in the examined range o

Numerical investigation of aerodynamic droplet breakup in a high temperature gas environment

The Navier-Stokes equations, energy and vapor transport equations coupled with the VOF methodology and a vaporization rate model are numerically solved to predict aerodynamic droplet breakup in a high temperature gas environment. The numerical model accounts for variable properties and uses an adaptive local grid refinement technique on the gas-liquid interface to enhance the accuracy of the computations. The parameters examined include Weber (We) numbers in the range 15-90 and gas phase temperatures in the range 400-1000 K for a volatile n-heptane droplet. Initially isothermal flow conditions are examined in order to assess the effect of Weber (We) and Reynolds (Re) number. The latter was altered by varying the gas phase properties in the aforementioned temperature range. It is verified that the We number is the controlling parameter, while the Re number affects the droplet breakup at low We number conditions. The inclusion of droplet heating and evaporation mechanisms has revealed that heating effects have generally a small impact on the phenomenon due to its short duration except for low We number cases. Droplet deformation enhances heat transfer and droplet evaporation. An improved 0-D model is proposed, able to predict the droplet heating and vaporization of highly deformed droplets

Developments in digital in-line holography enable validated measurement of 3D particle field dynamics.

Digital in-line holography is an optical technique which can be applied to measure the size, three-dimensional position, and three-component velocity of disperse particle fields. This work summarizes recent developments at Sandia National Laboratories focused on improvement in measurement accuracy, experimental validation, and applications to multiphase flows. New routines are presented which reduce the uncertainty in measured position along the optical axis to a fraction of the particle diameter. Furthermore, application to liquid atomization highlights the ability to measure complex, three-dimensional structures. Finally, investigation of particles traveling at near sonic conditions prove accuracy despite significant experimental noise due to shock-waves