An experiment on the breakup of impinging droplets on a hot surface

Breakup characteristics of liquid droplets impinging on a hot surface are investigated experimentally with the wall temperatures in the Leidenfrost temperature range of 220–330°C for n-decane fuel. Factors influencing droplet breakup are wall temperature, impinging velocity, droplet diameter and impinging angle. The 50% breakup probability shows that the impinging velocity decreases linearly with the droplet diameter increase and there exists an optimum impinging angle near 80° having the minimum value in the impinging velocity for given wall temperature and droplet size. Near the wall temperature of 250°C corresponding to the Leidenfrost temperature, a peculiar nonlinear behavior in the breakup probability is observed.This work was supported by the Turbo and Power Machinery Research Center, Seoul National University.     

Hydrodynamics and boiling phenomena of water droplets impinging on hot solid

The collision of single water droplets with a hot Inconel 625 alloy surface was investigated by a two-directional flash photography technique using two digital still cameras and three flash units. The experiments were conducted under the following conditions: the pre-impact diameters of the droplets ranged from 0.53 to 0.60 mm, the impact velocities ranged from 1.7 m/s to 4.1 m/s, and the solid surface temperatures ranged from 170 °C to 500 °C. When a droplet impacted onto the solid at a temperature of 170 °C, weak boiling was observed at the liquid/solid interface. At temperatures of 200 or 300 °C, numerous vapor bubbles were formed. Numerous secondary droplets then jetted upward from the deforming droplet due to the blowout of the vapor bubbles into the atmosphere. No secondary droplets were observed for a surface temperature of 500 °C at the low-impact Weber numbers (∼30) associated with the impact inertia of the droplets. Experiments using 2.5-mm-diameter droplets were also conducted. The dimensionless collision behaviors of large and small droplets were compared under the same Weber number conditions. At temperatures of less than or equal to 300 °C, the blowout of vapor bubbles occurred at early stages for a large droplet. At a surface temperature of 500 °C, the two dimensionless deformation behaviors of the droplets were very similar to each other.     

Hollow droplets impacting onto a solid surface

The impact process of spherical hollow droplets impinging onto a solid surface has been experimentally studied. Formation of a counter-jet in a wide range of Reynolds and Weber numbers was revealed, this jet being similar to a Worthington jet. For characterizing the regime of liquid flow in the hollow droplet, we propose using the Euler number. Theoretically, the problem was treated using a simple model of axisymmetric liquid flow. The obtained results proved to be consistent with experimental data.     

Coalescence of two droplets impacting a solid surface

Coalescence of a falling droplet with a stationary sessile droplet is studied experimentally. High-speed video images are presented to show coalescence dynamics, shape evolution and contact line movement. Emphasis is put on spread length, which is the length of two coalesced droplets along their original centers. Experimental results have shown that the spread length can be larger or smaller than the ideal spread length, which is the spread diameter of individual droplet plus the center-to-center distance between the two droplets. Three different coalescence mechanisms based on comparing the maximum and the minimum spread lengths to the ideal spread length are identified. Correlations for the maximum and the minimum spread lengths are developed, which can be combined with the coalescence domains to determine the deposition conditions for forming continuous or discontinuous lines.     

Three-dimensional numerical analysis of the deformation behavior of droplets impinging onto a solid substrate

The collision dynamics of water droplets impacting onto a solid is studied by means of three-dimensional computer simulations. The Navier–Stokes equations for unsteady, incompressible, viscous fluids in the three-dimensional Cartesian coordinate system are approximated and solved by a finite difference method. The volume-of-fluid (VOF) technique is used to track the free liquid surface. Normal and oblique collisions of droplets with the substrate are simulated at low droplet impact inertia. The effect of impact angle on the deformation behavior of droplets is investigated. The experimental observations and the numerical results are in reasonable agreement. Theoretical aspects of the physics of the collision phenomena are addressed.     

Particle velocimetry inside Newtonian and non-Newtonian droplets impacting a hydrophobic surface

A particle velocimetry technique is described which enables the measurement of the fluid velocity inside impacting drops. Using high speed photography of 2 μm fluorescent tracer particles suspended in the fluid, the velocity field was measured as a function of time and radial position. The potential of the technique is illustrated using velocimetry measurements of drops of pure water and aqueous solutions of 200 ppm poly-(ethylene oxide) (PEO). Dilute solutions of PEO have been known for some time to suppress the rebound of water from hydrophobic surfaces. The dissipation has traditionally been attributed to an increased extensional viscosity as the polymers stretch in the extensional flow of the droplet. Our results enable us to infer that the extensional viscosity of PEO drops, during both the spreading and retraction phase, is similar to that of pure water. The data suggest that the true source of dissipation lies at the droplet edge. We also show, by analysing the spreading of water drops, that the Roisman-Yarin theory for a droplet spreading on a surface is valid in the bulk of the droplet prior to the final stages of spreading.     

Full-field strain measurement using a luminescent coating

In this paper we describe an optical-based technique, called strain sensitive skin (S³), for measuring in-plane strain data on structural members under static load. The technique employs a coating consisting of a luminescent dye and polymer binder that is applied to the surface of a test part via conventional aerosol techniques. Proper illumination stimulates the dye, which in turn emits higher wavelength luminescence. The excitation and emission intensities have different wavelengths; therefore, enabling optical filtering to separate the two signals. The optical strain response is intensity based. A network of randomized microcracks within the binder scatters the waveguided luminescence from the excited dye molecules. The amount of scattered luminescence is related to the changes in the microcrack openings and orientations via mechanical strain. Various calibration tests show the optical strain response to be proportional to the sum of in-plane principal strains. With this new experimental testing tool, full-field high-resolution strain measurements can be acquired. The optical strain response of this new sensor is minimally dependent on viewing and lighting directions, rendering the technique viable to imaging and determining strain fields for three-dimensional complex geometries.     

Probe for measurement of strains inside solid bodies

Trees develop high residual stresses in their stems so they can bend in the wind without being damaged. Crosscutting the stem into short cylindrical sections relieves and redistributes the residual stresses. A probe was built to measure the stress distribution near the section ends. It consists of a tube holding a series of 12 strain-gage clips which register the diametrical changes of a drillhole as the stresses are relieved by sawing the stem into short sections.     

Full 3D-3C velocity measurement inside a liquid immersion droplet

We describe a tomographic PIV system for the measurement of the internal flow in a droplet over a stagnant and a moving surface. The flow condition is representative of the flow in an immersion droplet applied in a liquid immersion lithography machine. We quantify the accuracy and reliability of the measurements and compare the shape of the reconstructed measurement volume to shape measurements by means of shadowgraphy. First results indicate the internal flow pattern near the receding contact line, showing a small recirculation region.     

Numerical simulation of convective heat transfer to a radial free surface jet impinging on a hot solid

The convective heat transfer between a circular free surface impinging jet and a solid surface has been studied numerically. The thin liquid film formed on the surface has been assumed to be in non-turbulent free surface flow. The effects of surface tension, viscosity, gravity and heat transfer between the film flow and the solid surface have been taken into account. The flow structure on a non-heated surface has been investigated first. Next, the steady-state flow structure in the liquid film as well as the heat transfer has been examined. The predicted results have been compared with experimental data for the purpose of validating the analysis. The hydrodynamics of the liquid film and the heat transfer processes have been investigated numerically to understand the physics of the phenomena. Received on 5 October 1998     

Full-field and point strain measurement via the inner surface of a rolling large lug tyre

Vehicle safety and performance can be dramatically improved if force or friction measurement of the tyre-terrain interface is known. Since the tyre-terrain interface is responsible for the majority of forces acting on the vehicle, this region has received a lot of attention in vehicle dynamics. Direct measurement of the tyre-terrain interface is difficult since it is hidden by the tyre and terrain. A lot of research has been conducted on the inside of tyre using accelerometers or strain gauges with research more focussed on passenger car tyres and very little work performed on agricultural tyres with larger lugs. This study performs strain measurements using point measurement, from strain gauges, and full field measurement, using a stereo camera measurement system, of the inside of an agricultural tyre on a drum test rig during vertical and lateral loading. Results show similar trends when compared to results on passenger car tyres, however the mounting of the strain gauge relative to the lug is shown to play a large role in the developed strain. Linear relationships between the applied tyre force and strain were obtained in different direction with R² values above 0.97.     

Breakup of a droplet at high velocity impacting a solid surface

We have studied the collision between a droplet of different liquids with high impact energy and a solid plate with varied surface roughness, which is characterized by a dimensionless Weber number (We, defined as the impact inertia of the droplet normalized by its surface force) extending up to 12,000 for water. To make such collision, a technique was developed to generate a single droplet with speed up to 42 m/s, which was initially driven by upstream air flow through a nozzle and accelerated to nearly the same velocity of the high-speed flow downstream. Via a high-speed photographing system, the various splashing mechanisms were investigated and a specific prompt splash on a smooth plate was found at sufficiently high We, which was different somehow from the conventionally defined one that was generally believed to occur only on a rough surface. The radius when multiple secondary droplets were shed out of the rim of the expanding lamella was found to scale almost invariantly with We at large values, whereas the coupled effect of liquid viscosity might affect the ultimate value.     

Dynamic edge angles of wetting upon spreading of a drop over a solid surface

The hydrodynamic free-boundary problem of the axisymmetric spreading of a viscous-fluid drop over the smooth surface of a solid under the action of capillary forces and under the conditions of weak gravitation is considered. For finite inclination angles of the free surface and small capillary numbers, the problem is reduced to the simpler hydrodynamic problem in a region with known boundary by the asymptotic method. An expression for the dynamic edge angle of the drop is obtained. It is shown that in addition to the local inclination angle of the boundary near the contact line of three phases, one drop has several dynamic edge angles. These angles are calculated for small Reynolds and Bond numbers. Institute of Mechanics of Multiphase Systems, Siberian Division, Russian Academy of Sciences, Tyumen' 625000. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 1, pp. 101–107, January–February, 1999.     

Full-field bubbly flow velocity measurements using a multiframe particle tracking technique

The study is an examination of two-phase dispersed air bubble flow about a cylindrical conductor emitting a constant heat flux. The technique of Particle Image Velocimetry is utilized in order to obtain a full-field non-invasive measurement of the resulting bubbly flow velocity field. The employed approach utilizes a flow visualization technique in which the instantaneous velocity profile of a given flow field is determined by digitally recording particle or bubble images within the flow over multiple successive video frames and then conducting a completely computational analysis of the data. The use of particle tracking algorithms which perform a point-by-point matching of seed images from one frame to the next allows construction of particle or bubble pathlines and instantaneous velocity field. Results were initially obtained for a synthetically created flow field and a single phase liquid convective field seeded with flow-following tracer particles. The method was additionally extended to measurements within a gas/liquid system in which bubble rise velocities over a substantial two-dimensional flow area were determined in order to demonstrate the effectiveness of the developed digital data acquisition and analysis methodology.A version of this paper was presented at the 12th Symposium on Turbulence, University of Missouri-Rolla, 24–26 September, 1990     

Computations for a jet impinging obliquely on a flat surface

A SIMPLE-C algorithm and Jones-Launder k-ε two-equation turbulence model are used to simulate a two-dimensional jet impinging obliquely on a flat surface. Both the continuity and momentum equations for the unsteady state are cast into suitable finite difference equations. The pressure, velocity, turbulent kinetic energy and turbulent energy dissipation rate distributions are solved and show good agreement with various experimental data. The calculations show that the flow field structure of the jet impinging obliquely on a flat surface is strongly affected by the oblique impingement angle. The maximum pressure zone of the obliquely impinging jet flow field moves towards the left as the oblique impingement angle is decreased.     

Investigation of droplets impinging on a deep pool: transition from coalescence to jetting

An experimental investigation of droplets impinging vertically on a deep liquid pool of the same fluid was conducted. Coalescence and jetting as two of the main regimes were identified and studied. Five fluids, distilled water, technical ethanol, n-pentane, methanol and 1-propanol were used for providing different liquid-phase physical properties with density from 600 to 1,000 kg/m³, viscosity from 0.20 to 2.00 mPa s, and surface tension from 13.7 to 72.0 mN/m. Except for the experimental run of n-pentane, which was carried out in n-pentane saturated vapor, the ambient gas for the other experiments was air. The impact processes of micro-level (diameter below 1 mm) droplets were captured using a high-speed camera with a backlight. The observations, velocity and diameter ranges of the experimental runs were described, and based on them, the effects of the liquid-phase properties were studied. It was found that both low viscosity and low surface tension can increase the instability during impact processes. By curve-fitting, the transition from coalescence to jetting was characterized by using two models, one employing the Weber number (We) and the Ohnesorge number (Oh), and one employing the Froude number (Fr) and the Capillary number (Ca). Both models characterize the coalescence-jetting threshold well. The We-Oh model was based on a commonly used model from Cossali et al. (in Exp Fluids 22:463–472, 1997) for characterizing coalescence-splashing. For the small droplet diameters (below 1 mm) considered in this study, it was required to modify the We-Oh model with a diameter-dependent term to fit the sharp change in thresholds for fluids with relatively high viscosity. The Fr-Ca model has not previously been presented in the literature. A comparison of the two models with literature data (Rodriguez and Mesler, J Colloid Interface Sci 106(2):347–352, 1985) indicates that they are also valid for impacts of droplets with diameters above 1mm. Calculation methods to generalize the two models were proposed.