Entrainment of fine particles from surfaces by impinging shock waves

 When a shock wave impinges on a surface, it reflects and propagates across the surface at supersonic velocity. The gas is impulsively accelerated by the passing shock wave. The resulting high-speed flow imparts sufficiently strong forces to particles on the surface to overcome strong adhesive forces and entrain the surface-bound particles into the gas. This paper describes an experimental study of the removal of fine particles from a surface by impinging shock waves. The surfaces examined in this study were glass slides on which uniformly sized (8.3 μm diameter), spherical polystyrene particles had been deposited. Shock waves were generated in a small, open-ended shock tube at various heights above and impingement angles to the surface. Particle detachment from the carefully prepared substrates was determined from images of the surfaces recorded before and after shock impingement. A single shock wave effectively cleaned a large surface area. The centerline length of the cleared region was used to characterize the efficacy of shock cleaning. A model based upon the far field solution for a point source surface shock provides a good fit to the clearance length data and yields an estimate to the threshold shock strength for particle removal. Received: 13 November 1997/Accepted: 23 April 1998     

Investigation of the acceleration of aluminum particles behind a shock wave using instantaneous Laser Doppler Velocimetry

The acceleration of aluminum particles with a 5μm diameter in the flow field behind an incident shock wave was investigated experimentally in a 10-m long and 70 mm inner diameter shock tube. By means of instantaneous Laser Doppler Velocimetry (LDV) the velocity of the particles was observed directly. The light scattered by the moving particles is Doppler shifted and sent to the laser Doppler velocimeter. The velocimeter essentially consists of a phase-stabilized Michelson interferometer used as a sensitive spectrometer. An electro-optical circuit ensures the phase stabilization that results in a voltage signal independent of the scattered light intensity and proportional to the mean velocity of the particles at the measurement point. Because of the very short response time (1μs) of the LDV system used here, the latter gives a continuous real-time signal of the particle acceleration. To avoid particle oxidation the particles were accelerated by a high-speed nitrogen gas flow. From the measured velocity the dimensionless drag coefficient was calculated. The drag coefficient is related to the fluid dynamic force exerted by the gas on the particles. The experimental data were compared to theoretical models from the literature. A significant deviation between the model and the experimental data was observed. This deviation is supposed to be induced by the shock wave, which hits the particles and breaks them into pieces of a smaller diameter. Further experiments will be carried out in the future to check the size distribution of the particles after the shock has gone past them.      

A particle image velocimetry system for microfluidics

 A micron-resolution particle image velocimetry (micro-PIV) system has been developed to measure instantaneous and ensemble-averaged flow fields in micron-scale fluidic devices. The system utilizes an epifluorescent microscope, 100–300 nm diameter seed particles, and an intensified CCD camera to record high-resolution particle-image fields. Velocity vector fields can be measured with spatial resolutions down to 6.9×6.9×1.5 μm. The vector fields are analyzed using a double-frame cross-correlation algorithm. In this technique, the spatial resolution and the accuracy of the velocity measurements is limited by the diffraction limit of the recording optics, noise in the particle image field, and the interaction of the fluid with the finite-sized seed particles. The stochastic influence of Brownian motion plays a significant role in the accuracy of instantaneous velocity measurements. The micro-PIV technique is applied to measure velocities in a Hele–Shaw flow around a 30 μm (major diameter) elliptical cylinder, with a bulk velocity of approximately 50 μm s^-1. Received: 26 November 1997/Accepted: 26 February 1998     

Head on collision of a planar shock wave with a dusty gas layer

The head-on collision of a planar shock wave with a dust-air suspension is studied numerically. In this study the suspension is placed inside a conduit adjacent to its rigid end-wall. It is shown that as a result of this collision two different types of transmitted shock waves are possible, depending on the strength of the incident shock wave and the dust loading ratio in the suspension. One possibility is a partially dispersed shock wave, the other is a compression wave. The flow fields resulting in these two options are investigated. It is shown that in both cases, at late times after the head-on reflection of the transmitted shock wave from the conduit end-wall a negative flow (away from the end-wall) is evident. The observed flow behavior may suggest a kind of dust particle lifting mechanism that could shed new light on the complex phenomenon of dust entrainment behind sliding shock waves.      

Particle tracer response across shocks measured by PIV

The experimental approach used for the evaluation of the particle response time across a stationary shock wave is assessed by means of PIV measurements. The study focuses on the experimental requirements for a reliable and unbiased measurement of the particle response time τ _p and length ξ _p based on a single-exponent decaying law. A numerical simulation of the particle response experiment returns the parameters governing the measurement: namely the normalized spatial and temporal resolution, shock strength, and digital resolution. Representing the velocity decay in logarithmic coordinates it is shown that measurements performed with laser pulse separation time up to τ _p and interrogation window up to ξ _p still yield unbiased results for the particle response. A set of experiments on the particle response across a planar oblique shock wave was conducted to verify the results from the numerical assessment. Liquid droplets of DEHS and solid tracer particles of silicon and titanium dioxide with different primary crystal size are compared. The resulting temporal response ranges from 2 to 3 μs, corresponding to values commonly reported in literature, to almost 0.3 μs when particles are properly dehydrated and a filter is applied before injection into the wind tunnel. It is the first experimental evidence of particle tracers with a measured response time lower than 0.4 μs. The same procedure is applied to attempt the measurement of individual particle tracers by particle tracking velocimetry to estimate the spread in the distribution of tracer time response. The latter analysis is limited by the particle image tracking precision error, which biases the results introducing a wider broadening of the particle velocity distribution.     

Rheological properties of peanut butter

The rheological properties of two types of commercial peanut butter have been studied. Both products are concentrated suspensions, and differ by the presence of additives. The first type, referred to as “100% peanuts,” is an unstabilized suspension consisting of solid peanut particles in peanut oil which is a Newtonian fluid. The second type, referred to as “smooth,” consists of the same suspension stabilized with a vegetable oil and contains other ingredients such as salt and sugar in very small quantities. A mean volume particle diameter of 6.6 μm has been determined, the particle diameter distribution was found to be narrow, and the solids volume fraction was estimated to be 0.6. Slip encountered in rheometry was greatly reduced by gluing sandpaper to the parallel plates of the rheometer. Both samples behaved like plastic materials and apparent yield stresses of 24 Pa and 370 Pa have been determined for the unstabilized and the stabilized suspensions, respectively. No linear domain was found for both suspensions and the non-linearity was confirmed by deformed Lissajous curves and higher odd harmonics in the output signal of small amplitude oscillatory shear experiments. The stabilized suspension behaved more like a solid, the elastic modulus being larger than the loss modulus and almost independent of the frequency. This solid-like behavior is supposedly caused by strong repulsive (steric) forces induced by the stabilizing agent. Received: 29 September 1999 Accepted: 9 August 2000     

Measurement of turbulent spectra of particle-laden flows in a horizontal channel

Frequency spectra of air turbulence of particle-laden flows were investigated by use of a laser-Doppler velocimeter to discover the eddy-length scales that are influenced by the transported particles. The influence of glass and steel particles of 100–1,000 μm diameter was measured in a horizontal channel and a horizontal pipe for the streamwise and transverse components of the velocity vector. Particles that were small compared with the integral length scale of the particle-laden flow decrease the turbulent power density of the greatest eddies in varying degrees, depending on mass loading and distance from the wall. All fractions create turbulence in their wakes, the size of which depends on loading and slip velocity. These results support the hypothesis that the particles consume energy by following the large eddies that are much greater than the particle diameters, and in so doing, turbulence is created by this energy. Received: 28 September 2000/Accepted: 9 April 2001     

Quantum nanospheres for sub-micron particle image velocimetry

Quantum Nanospheres™ (QNs) have been developed as a new type of flow-tracing particle for micron resolution particle image velocimetry (PIV). The 70 nm diameter QNs were created by conjugating quantum dots to polystyrene beads. The fluorescent QNs have a large Stokes’ shift and are impervious to photobleaching. The use of QNs as flow-tracing particles for micro-PIV was demonstrated by measuring fluid motion in a 30 × 300 μm channel. Using an interrogation region of 1 × 1,024 pixels and ensemble averaging 1,800 image pairs, the physical volume of the interrogation region was 117 μm × 117 μm × 2 μm.     

The influence of single-pulse and tandem shock waves on bacteria

Tandem shock waves have shown to enhance kidney stone fragmentation during in vitro and in vivo extracorporeal shock wave lithotripsy (SWL). The purpose of this research was to study the influence of shock waves on the viability of two strains of bacteria in solution, and to verify if tandem shock waves increase microorganism death. A piezoelectric shock wave generator was modified to generate either standard (single-pulse) or tandem (dual-pulse) shock waves. E. coli and Listeria monocytogenes were exposed in vitro to thousands of standard shock waves. Another group was subjected to the same number of tandem shock waves with a delay of 450 μs. A third group was exposed to tandem shock waves having a 900-μs delay. No inactivation was observed for both microorganisms at up to 8,000 standard shock waves. About 40% of L. monocytogenes and 50% of E. coli were inactivated after treatment with tandem waves at a delay of 900 μs. Inactivation was less efficient for a delay of 400 μs. Our results could be useful in medicine, because infection stones are still a significant cause of morbidity and mortality after SWL. The use of tandem shock waves to treat persistent localized infections or as a novel non-thermal food-preservation method also might be possible.      

Aluminum Particles–air Detonation at Elevated Pressures

The effect of initial pressure on aluminum particles–air detonation was experimentally investigated in a 13 m long, 80 mm diameter tube for 100 nm and 2 μm spherical particles. While the 100 nm Al–air detonation propagates at 1 atm initial pressure in the tube, transition to the 2 μm aluminum–air detonation occurs only when the initial pressure is increased to 2.5 atm. The detonation wave manifests itself in a spinning wave structure. An increase in initial pressure increases the detonation sensitivity and reduces the detonation transition distance. Global analysis suggests that the tube diameter for single-head spinning detonation or characteristic detonation cell size would be proportional to (d ₀: aluminum particle size, p ₀: initial pressure). Its application to the experimental data results in m ~ O(1) and n ~ O(1) for 1 to 2 μm aluminum–air detonation, thus indicating a strong dependence on initial pressure and gas-phase kinetics for the aluminum reaction mechanism in detonation. Hence, combustion models based on the fuel droplet diffusion theory may not be adequate in describing micrometric aluminum–air detonation initiation, transition and propagation. For 2 μm aluminum–air mixtures at 2 atm initial pressure and below, experiments show a transition to a “dust quasi-detonation” that propagates quasi-steadily with a shock velocity deficit nearly 40% with respect to the theoretical C–J detonation value. The dust quasi- detonation wave can propagate in a tube with a diameter less than 0.4–0.5 times the diameter required for a spinning detonation wave.     

Holographic particle image velocimetry system for measurements of hairpin vortices in air channel flow

A holographic particle image velocimetry system for investigating hairpin vortices, artificially generated in a subcritical plane Poiseuille air flow, is presented. The optical setup is a modified version of the hybrid scheme, previously employed in turbulent water flows. Accordingly, separate reconstruction of holograms, successively recorded on the same photoplate, is provided by using two reference beams. The positioning of the photoplate within the image of the sample volume accompanied by special alignment procedures, minimizes the apparent displacement caused by the misalignment of the reconstruction waves. A novel method is employed for detecting in-focus particles. Testing the system with a fixed 5 μm diameter wire, results in a corresponding 3D wire image having a diameter of ≈25 μm. Finally, the instantaneous topology and 3D distribution of the two velocity components associated with the hairpin vortex are presented.     

Application of the PIV technique to measurements around and inside a forming drop in a liquid–liquid system

A particle image velocimetry (PIV) method has been developed to measure the velocity field inside and around a forming drop with a final diameter of 1 mm. The system, including a microscope, was used to image silicon oil drops forming in a continuous phase of water and glycerol. Fluorescent particles with a diameter of 1 μm were used as seeding particles. The oil was forced through a 200 μm diameter glass capillary into a laminar cross-flow in a rectangular channel. The velocity field was computed with a double-frame cross-correlation function down to a spatial resolution of 21 × 21 μm. The method can be used to calculate the shear stress induced at the interface by the cross-flow of the continuous phase and the main forces involved in the drop formation process.     

A high-resolution laser Doppler anemometer: design, qualification, and uncertainty

A new high-resolution laser Doppler anemometer (LDA) has been developed with a working distance of 350 mm, allowing operation in lab-scale wind tunnels. The measurement volume size is 35 μm in diameter by 60 μm in length, allowing resolution of the smallest turbulence scales even at fairly high Reynolds numbers. The controversial question of velocity and validation bias in LDA data is resolved with an experimental method for measuring and removing those effects. Uncertainty estimates are also derived for all the mean and Reynolds stress measurements. Received: 27 June 1999/Accepted: 30 August 2000     

Explosive dispersal of solid particles

Abstract. The rapid dispersal of inert solid particles due to the detonation of a heterogeneous explosive, consisting of a packed bed of steel beads saturated with a liquid explosive, has been investigated experimentally and numerically. Detonation of the spherical charge generates a blast wave followed by a complex supersonic gas-solid flow in which, in some cases, the beads catch up to and penetrate the leading shock front. The interplay between the particle dynamics and the blast wave propagation was investigated experimentally as a function of the particle size (100–925 m) and charge diameter (8.9–21.2 cm) with flash X-ray radiography and blast wave instrumentation. The flow topology during the dispersal process ranges from a dense granular flow to a dilute gas-solid flow. Difficulties in the modeling of the high-speed gas-solid flow are discussed, and a heuristic model for the equation of state for the solid flow is developed. This model is incorporated into the Eulerian two-phase fluid model of Baer and Nunziato (1986) and simulations are carried out. The results of this investigation indicate that the crossing of the particles through the shock front strongly depends on the charge geometry, the charge size and the material density of the particles. Moreover, there exists a particle size limit below which the particles cannot penetrate the shock for the range of charge sizes considered. Above this limit, the distance required for the particles to overtake the shock is not very sensitive to the particle size but remains sensitive to the particle material density. Overall, excellent agreement was observed between the experimental and computational results. Received 16 August 1999 / Accepted 26 June 2000     

Modelling of dust lifting using the Lagrangian approach

The subject of this paper is dust lifting behind shock waves, a process that is important for the formation of explosive dust clouds in air. While Eulerian–Eulerian has been the standard numerical technique for such simulations, the Eulerian–Lagrangian technique has been used in this paper, making it possible to take into account more physical phenomena, such as particle–particle and particle–wall collisions. The results of the simulations are shown mainly graphically, as snapshots of particle positions at given times after the passing of the shock wave. The results show that the collisions, and the coefficient of restitution assumed for them, is important in determining the mobility and lifting of dust behind shock waves. The results also show that the idea of a horizontally travelling shock wave is an oversimplification: the strong pressure gradient at the surface results in a series of reflected waves generated at the surface and travelling into the gas phase.     

Study of high Deborah number flow of polyisobutylene in square die

A large capacity RAM extruder was designed which provides the opportunity to study high Deborah number (D) flows, with D < 1,000. A modified version of particle image velocimetry was developed to enable the measurement of the velocity field in dies of arbitrary cross section. During the measurement process, tracer particles were simultaneously illuminated by both a focused laser beam locally and a lamp globally. Only those particles that passed through the laser beam were taken into account. The beam was scanned to achieve full field measurements. This method of measurement allowed us to find the location of a particle in the direction of the optical axis of the lens, i.e. that which makes the particle image on the CCD detector of the video camera. A device employing this method was designed and used to measure velocity profiles. The results of these measurements in two cross sections of the square die, at three values of pressure, are presented. The velocity was defined as the ratio of displacement to the elapsed time during which this displacement occurred. Errors in measurements of the coordinates and the observation time of particles were estimated as ±20 μm and less than 0.1%, respectively. A large plateau in the axial velocity profile was found at relatively small Deborah numbers, e.g. D ≈ 28. In flows with higher Deborah numbers, e.g. D ≈ 766, an almost flat velocity profile was detected. Two components of velocity, one longitudinal and one transversal, were measured simultaneously. However, the transversal component appeared to be less than the error of measurements and less than 1% of the axial velocity. Received: 4 August 1998 Accepted: 5 April 1999     

Near-wall imaging using toluene-based planar laser-induced fluorescence in shock tube flow

A quantitative thermometry technique, based on planar laser-induced fluorescence (PLIF), was applied to image temperature fields immediately next to walls in shock tube flows. Two types of near-wall flows were considered: the side wall thermal boundary layer behind an incident shock wave, and the end wall thermal layer behind a reflected shock wave. These thin layers are imaged with high spatial resolution (15μm/pixel) in conjunction with fused silica walls and near-UV bandpass filters to accurately measure fluorescence signal levels with minimal interferences from scatter and reflection at the wall surface. Nitrogen, hydrogen or argon gas were premixed with 1–12% toluene, the LIF tracer, and tested under various shock flow conditions. The measured pressures and temperatures ranged between 0.01 and 0.8 bar and 293 and 600 K, respectively. Temperature field measurements were found to be in good agreement with theoretical values calculated using 2-D laminar boundary layer and 1-D heat diffusion equations, respectively. In addition, PLIF images were taken at various time delays behind incident and reflected shock waves to observe the development of the side wall and end wall layers, respectively. The demonstrated diagnostic strategy can be used to accurately measure temperature to about 60 μm from the wall.     

Aerodynamic characteristics of solid particles�� acceleration by shock waves

In this study, the interaction of a planar shock wave with a group of particles has been investigated using high-speed photography and dynamic pressure measurements. Experiments were carried out in a horizontal circular shock tube. The influence of the particle loading ratio, particle diameter, driving gas and shock wave Mach number on the acceleration was studied. It was found that the higher the particle loading ratio, the greater was the particle velocity. This is due to the higher driving pressure. Helium and nitrogen gases play quite different roles in acceleration. Pressure multiplication during shock wave interaction with particles also appears. Based on the experimental results, the discussion regarding partial quantitative velocities and accelerations of particle groups, as well as the attenuation factors when shock waves pass through the particles, is given.     

悬浮RDX炸药和铝颗粒混合粉尘爆轰的数值模拟

采用两相流方法对炸药颗粒直径为20.0 m时与铝颗粒混合物的爆轰波的发展与传播过程及爆轰波参数进行了数值计算。结果表明,在炸药粉尘中加入铝颗粒,可以大大提高爆轰波参数。当铝颗粒直径为3.4 m时,尽管铝颗粒的直径较炸药颗粒直径小,但由于炸药颗粒的点火温度低,二者的点火时间相差不多。如果铝颗粒的直径为7.0 m,由于铝颗粒的点火滞后于炸药颗粒的点火,混合颗粒粉尘中可能形成双波阵面的爆轰波。     

Accurate measurement of hydrodynamic interactions between a particle and walls

A new experimental setup allows the measurement of hydrodynamic interactions between the walls of a vessel and a particle moving along a three-dimensional trajectory in a viscous fluid. The vertical motion of the particle is measured with an accuracy of 50 nm using laser interferometry, while its horizontal movements are controlled with an accuracy of 20 μm by displacing the vessel in the horizontal direction so as to keep the sphere in the fixed vertical laser beam. Three axisymmetric closed containers are used as examples: two vertical cylinders (with flat and convex lens-shaped lower walls) and a cone. Various effects of combined creeping flow hydrodynamic interactions between the particle and walls are observed. Received: 12 March 1999 / Accepted: 7 June 2001 Published online: 29 November 2001