Droplet heat transfer on micro-post arrays: Effect of droplet size on droplet thermal characteristics

Heat transfer towards a water droplet from hydrophobic micro-post array surface is considered while mimicking the environmental temperatures. Micro-post arrays are created on a silicon wafer surface via lithography technique. The textured surfaces are replicated by polydimethylsiloxane (PDMS) to achieve an optical transmittance. The droplet adhesion on micro-post array surface is presented and the influence of droplet size on the heat transfer and droplet internal flow characteristics is examined. The flow predictions are validated via the particle image velocimetry data. It is found that adhesion force between the water droplet and the micro-post arrays surface depends on the geometric size and the orientation of the micro-post arrays on the surface. Temperature and flow fields are influenced by the droplet size. The Nusselt and the Bond numbers increase with the droplet volume; however, the Bond number remains less than unity indicating that the Marangoni current dominates over the buoyancy current in the droplet. The Nusselt number attains larger values for micro-post array surface than that of the plain surface. This is because of temperature and velocity oscillations along the contact lines at the droplet bottom due to the pitches of the micro-post arrays.     

Study on imaging method for measuring droplet size in large sprays

Information of droplet size and size distribution lays the basis for investigations of atomization mechanisms and performance optimization. However, the laser diffraction and phase Doppler particle analyzers have difficulty in accurately characterizing sprays with a wide range of droplet sizes and very large droplets, especially if a large number of droplets are aspherical. A method to measure size in such large-droplet sprays based on digital imaging with backward illumination was developed, including an image acquisition system and image process programs. Calibration of the measurement system was performed using a dot calibration target with different dot sizes. An experimental setup was designed and established to characterize spray nozzles under different operation loads, as well as different nozzle arrangements. Results show that the droplet size of sprays ranges from dozens of microns to several millimeters. The superiority of wide load range for such nozzles was indicated by the size-measurement results under half-load to full-load operations. The present study revealed that the image processing technique can be effectively implemented for in-line size measurements of sprays with a wide distribution of droplet size and aspherical droplets, which would be difficult to characterize by other methods.     

Study on imaging method for measuring droplet size in large sprays

Information of droplet size and size distribution lays the basis for investigations of atomization mechanisms and performance optimization.However,the laser diffraction and phase Doppler particle analyzers have difficulty in accurately characterizing sprays with a wide range of droplet sizes and very large droplets,especially if a large number of droplets are aspherical.A method to measure size in such largedroplet sprays based on digital imaging with backward illumination was developed,including an image acquisition system and image process programs.Calibration of the measurement system was performed using a dot calibration target with different dot sizes.An experimental setup was designed and established to characterize spray nozzles under different operation loads,as well as different nozzle arrangements.Results show that the droplet size of sprays ranges from dozens of microns to several millimeters.The superiority of wide load range for such nozzles was indicated by the size-measurement results under half-load to full-load operations.The present study revealed that the image processing technique can be effectively implemented for in-line size measurements of sprays with a wide distribution of droplet size and aspherical droplets,which would be difficult to characterize by other methods.     

Using image analysis to measure two-dimensional soil deformation

A system for measuring two-dimensional deformation in a soil bin using video image analysis was developed and evaluated. Soil was placed in the bin [1.22 (wide)×9.1 m long] in 50 mm deep layers of nominally uniform bulk density (1.35 Mg m⁻³) to a total depth of 0.76 m. White polypropylene markers [6.4 (dia.)×25.4 mm] were placed in the soil such that vertical rectangular cross-sectional grids were exposed by removing selected bin modules. Video image analysis was used to determine vertical and horizontal coordinates of each marker which, in turn, were compared to measurements using a sonic digitizer which were accurate to ±0.5 mm.     

Droplet dynamics and size characterization of high-velocity airblast atomization

Airblast atomizers are especially useful and commonplace in liquid fuel combustion applications. However, the spray formation processes, the droplet dynamics and the final drop size distributions are still not sufficiently understood due to the coupled gas-liquid interactions and turbulence generation. Therefore, empirical and semi-empirical approaches are typically used to estimate the global spray parameters. To develop a physical understanding of the spray evolution, a plain-jet airblast atomizer was investigated in an atmospheric spray rig using the phase-Doppler technique. The simultaneous drop size and axial and radial velocity components were measured on radial traverses across the spray at various axial distances from the nozzle for a range of atomizing pressures. The droplet turbulent and mean kinetic energies were found to be proportional to the atomizing pressure. Hence, the scatter of the radial motion of the droplets increased with the atomizing pressure. A droplet stability analysis was performed to locate the regions characterized by ongoing secondary atomization. The volume-to-surface diameter, D₃₂, of the fully developed spray was compared with estimates provided by five published formulae. The role of liquid viscosity, hence the Ohnesorge number, was found to be negligible in the investigated regime. Three commonly used size distribution functions were fitted to the measured data to analyze their dependence on the atomizing pressure. The Gamma distribution function was found to give the best approximation to the atomization process.     

Droplet size and morphology characterization for dense sprays by image processing: application to the Diesel spray

Up to now, measurement of drop size remains difficult in dense sprays such as those encountered in Diesel applications. Commonly used diagnostics are often limited due to multi-scattering effects, high drop velocity and concentration and also nonspherical shapes. The advantage of image-based techniques on the others is its ability to describe the shape of liquid particles that are not fully atomized or relaxed. In the present study, a model is developed to correct the main drawbacks of imaging. It permits to define criteria for the correction of the apparent size of an unfocused drop and to determine a measurement volume independent of the drop size. This considerably reduces the over-estimation of large drops in the drop size distribution. Drop shapes are also characterized by four morphological parameters. The image-based granulometer is satisfactorily compared to a PDPA and a diffraction-based granulometer for measurements on an ultrasonic spray. Then, the new granulometer is applied to a diesel spray. One of the results of the analysis is that even if mean drop size distributions are stable 30 mm downstream from the nozzle outlet, the shape of the drops is still evolving towards the spherical shape. The atomization process is thus not totally established at this position in opposition to what can be deduced from the drop size distribution alone.     

Numerical methods for the simulation of a coalescence-driven droplet size distribution

The droplet size distribution in a turbulent flow field is considered and modeled by means of a population balance system. This paper studies different numerical methods for the 4D population balance equation and their impact on an output of interest, the time-space-averaged droplet size distribution at the outlet, which is known from experiments. These methods include different interpolations of the experimental data at the inlet, various discretizations in time and space, and different schemes for computing the coalescence integrals. It will be shown that noticeable changes in the output of interest might occur. In addition, the computational efficiency of the studied methods is discussed.     

A backlighted imaging technique for particle size measurements in two-phase flows

 An imaging technique that uses backlighting has been developed to measure drop sizes in annular two-phase flows with small concentrations of drops in the gas phase. Advantages over conventional photography are realized in that data collection and analysis times are shortened considerably, and consistent unbiased results can be expected. A magnification of 1.9 was used to measure drops above 50 μm. A drop size distribution was obtained for an air–water system as a superficial gas velocity of 30 m/s and a liquid flow of 20 g/s. The data are used to substantiate a theory for the rate of deposition. Received: 6 February 1997/Accepted: 3 February 1998     

Investigation of the development of laminar flow at the entrance of a two-dimensional channel,using a laser-doppler velocimeter

The results of an experimental investigation of the velocity structure of Poiseuille flow at the entrance section of a two-dimensional horizontal channel with a discontinuous velocity profile at the entrance, at Re=300 and 500, are given. The experimental facility and the technique for measuring the velocity field in this channel using a laser-Doppler velocimeter (LDV) are discussed. Velocity profiles are obtained which are concave on the channel axis, and this agrees with the results of the numerical computations of [1].     

Rheology of polymer blends with matrix-phase viscoelasticity and a narrow droplet size distribution

A Hamiltonian framework of non-equilibrium thermodynamics is adopted to construct a set of dynamical continuum equations for a polymer blend with matrix viscoelasticity and a narrow droplet size distribution that is assumed to obey a Weibull distribution function. The microstructure of the matrix is described in terms of a conformation tensor. The variable droplet distribution is described in terms of two thermodynamic variables: the droplet shape tensor and the number density of representative droplets. A Hamiltonian functional in terms of the thermodynamic variables is introduced and a set of time evolution equations for the system variables is derived. Sample calculations for homogenous flows and constant droplet distribution are compared with data of a PIB/PDMS blend and a HPC/PDMS blend with high viscoelastic contrast. For the PIB/PDMS blend, satisfactory predictions of the flow curves are obtained. Sample calculations for a blend with variable droplet distribution are performed and the effect of flow on the rheology, droplet morphology, and on the droplet distribution are discussed. It is found that deformation can increase or decrease the dispersity of the droplet morphology for the flows investigated herein.     

Droplet size and velocity characteristics of water-air impinging jet atomizer

A water-air impinging jets atomizer is investigated in this study, which consists of flow visualization using high speed photography and mean droplet size and velocity distribution measurements of the spray using Phase Doppler Anemometry (PDA). Topological structures and break up details of the generated spray in the far and near fields are presented with and without air jet and for an impinging angle of 90°. Spray angle increases with the water jet velocity, air flow rate and impinging angle. PDA results indicate that droplet size is smallest in the spray center, with minimum value of Sauter mean diameter (SMD) of 50 µm at the air flow rate of Q_m = 13.50 g/min. SMD of droplets increases towards the spray outer region gradually to about 120 µm. The mean droplet velocity component W along the air-jet axis is highest in the spray center and decreases gradually with increasing distance from the spray center. SMD normalized by the air nozzle diameter is found firstly to decrease with gas-to-liquid mass ratio (GLR) and air-to-liquid momentum ratio (ALMR) and then remain almost constant. Its increasing with aerodynamic Weber number indicates an exponential variation. The study sheds light on the performance of water-air impinging jets atomizers providing useful information for future CFD simulation works.     

A technique to measure fatigue crack growth threshold

An experimental technique for determining fatigue crack growth threshold is presented. This experimental technique uses an increasing ΔK step loading procedure to determine threshold going from a no-growth to growth status. Stress relief annealing of the Ti-6AI-4V test specimens eliminates load history effects normally associated with the precrack, providing a measurement equivalent to what is achieved by a standard ASTM load shed test. In addition to measuring load history free thresholds, this increasing ΔK technique can be used to investigate different load history effects on threshold by using the threshold step measurement with different precrack histories and without the subsequent annealing process. Verification of the threshold step measurement is demonstrated by comparing measurements with standard ATSM load shed testing results.     

Sweep technique to measure the relative performance of dampers

The sweep technique described in this paper represents a new method for measuring the relative performance of different dampers. Based on laboratory measurements, the technique is easy to use and keeps test execution time to a minimum. It also allows measurement of the relative performance of nonlinear dampers for which it is difficult to derive the analytical expression for their mechanical impedance. In addition to describing the test setup, the paper also presents results obtained from the application of this technique to four types of commercial dampers.     

Linear and nonlinear inversion schemes to retrieve collision kernel values from droplet size distribution change

This study presents an attempt to retrieve collision kernel values from changes in the droplet size distribution due to collision growth. Original linear and nonlinear inversion schemes are presented, which use the simple a priori assumption that the total collision rate is given by the sum of the gravitational and turbulent contributions. Our schemes directly handle binned (discretized) size distributions and, therefore, do not require any assumptions on distribution functional forms, such as the self-similarity assumption. To validate the schemes, three-dimensional direct numerical simulation (DNS) of colliding droplets in steady isotropic turbulence is performed. In the DNS, air turbulence is calculated using a pseudo-spectral method, while droplet motions are tracked by the Lagrangian method. Comparison between the retrieved collision kernels and the collision kernels obtained directly from the DNS show that for low Reynolds number flows both the linear and nonlinear inversion schemes give good accuracy. However, for higher Reynolds number flows the linear inversion scheme gives significantly larger retrieval errors, while the errors for the nonlinear scheme remain small.     

Experimental determination of droplet size and density field in condensing flows

 We report a detailed experimental characterization of the process of homogeneous condensation in supersonic expanding flow. In our experiments, the supersaturated mixture expands in a Laval nozzle, where, depending on the initial conditions, a steady or periodically oscillating flow may evolve due to the non-linear interaction of nucleation and droplet growth rate with the flow field. Two experimental techniques are utilized: holographic interferometry for the determination of the density field and a time-resolved white-light extinction method. The latter is employed to derive the evolution in time of the droplet cloud (i.e., modal radius, number density, and relative width) and to measure the frequency of oscillations. In combination with the wide-field density data, droplet size measurements provide additional physical insights in the mechanism of interaction in condensing flows and serve as an excellent test case for the critical assessment of nucleation and droplet growth theories. To this purpose, the accuracy of the measurements is carefully reviewed due to the difficulties of characterizing dense sub-micron droplet clouds by means of light-scattering techniques. An important byproduct of this analysis is an evaluation of the applicability of single-scattering approximations, i.e., Lambert-Beer law, for a variety of experimental configurations. Received: 24 April 2001 / Accepted: 29 August 2001     

爆炸驱动液体分层现象和液滴尺寸模拟

爆炸驱动液体介质外界面的分散和破碎是气溶胶云团形成的重要过程。采用基于维数分裂的欧拉程序和Youngs混合界面处理方法,对中心药爆炸驱动甘油和水介质流场的液体分层现象进行了数值模拟。结合试验结果推断提出了液滴形成过程的三种并存机制:外层射流破碎、内层R-T失稳和中间液层"空化"破碎,分别建立了不同液层破碎液滴的尺寸模拟方法。对比给出抛撒甘油和水装置初级液滴的尺寸分布及最外层理论射流量。     

A cryomechanics technique to measure dissipated energies of 10 nJ

A new cryomechanics measurement technique has been developed to measure fracture-induced dissipated energies as small as 10 nJ (10×10⁻⁹ J) at temperatures near 4.2 K. The technique, with much less stringent instrumentation requirements than those used for measurement of ∼10 nJ energies, was applied to an induced fracture experiment where dissipation energies were of the order of ∼100 μJ. Fracture of 0.5-mm diameter pencil leads of two different hardnesses gave rise to measured energies of 65 ∼ 110 μJ. A two-dimensional finite-element analysis was used to interpret the experimental measurements. Based on the analysis, approximately 50 μJ of 65 ∼ 110 μJ measured is estimated to be the dissipated energy associated with crack formation and propagation.