Impingement and mixing dynamics of micro-droplets on a solid surface

The hydrodynamics and mixing during the nonaxisymmetry impingement of a micro-droplet and a sessile droplet of the same fluid are investigated by many-body dissipative particle dynamics(MDPD) simulation.In this work,the range of the impingement angle(θ_(i)) between the impinging droplet and the sessile droplet is 0°-60° and the contact angle is set as 45° or 124°.The droplets impingement and mixing behavior is analyzed based on the droplet internal flow field,the concentration distribution and the time scale of the decay of the kinetic energy of the impinging droplet.The dimensionless total mixing time(τ_(m)) is calculated by a modified mixing function.With the Weber number(We) ranging from 5.65 to22.7 and the Ohnesorge number(Oh) ranging from 0.136 to 0.214,we find rm hardly changes with We and Oh.Whereas,θ_(i)and surface wettability are found to have a significant effect on τ_(m).We find that θ_(i)has no clear effect on τ_(m)on a hydrophobic surface,while on the hydrophilic surface,τ_(m)increase with the θ_(i).Thus,reducing the impinging angle is a valid method to shorten the τ_(m).

Thermographic Observation of High-Frequency Ethanol Droplet Train Impingement on Heated Aluminum and Glass Surfaces

The present study considers the impingement of a train of ethanol droplets on heated aluminum and glass surfaces.The surface temperature is allowed to vary in the interval 140℃–240℃.Impingement is considered with an inclination of 63 degrees.The droplet diameter is 0.2 mm in both aluminum and glass surface experiments.Thermal gradients are observed with a thermographic camera.It is found that in comparison to glass,the aluminum surface displays very small liquid accumulations and better evaporation performance due to its higher thermal conductivity.The relatively low thermal conductivity of glass results in higher thermal gradients on the surface.The droplet impact area on the aluminum surface is smaller than the corresponding area for the glass surface.Interestingly,the liquid accumulation area is not symmetrical.Moreover,the extension of the droplet train impact region decreases on increasing the surface temperature because higher temperature values allow greater surface energy levels that enhance significantly the evaporation rate.

HYBRID AIRFOIL DESIGN FOR FULL-SCALE ICE ACCRETION TEST

A hybrid airfoil inverse design method according to the target pressure distribution and the impingement efficiency is presented.The method is developed to design hybrid airfoils that simulate the droplet impingement and ice accretion of full-scale airfoil.Flow field and droplet impingement around the full-scale airfoil are calculated to obtain pressure distribution and impingement efficiency firstly.The Navier-Strokes(N-S)solver is used in flow field calculation to improve calculation precision.The droplet impingement and ice accretion on the airfoil are performed by FENSAP-ICE.Once the target chord or original airfoil is given,the hybrid airfoil geometries can be computed.The designed hybrid airfoil consists of full-scale leading edges and redesigned aft-section.The hybrid airfoil can be tested under full-scale conditions to produce full-scale ice accretion in the exiting icing tunnels which are too small to perform ice accretion testing of full-scale airfoils.Moreover,the ice shapes formed on the full-scale and hybrid airfoils are compared at various attack angles.The results demonstrate that ice shapes between hybrid and full-scale airfoils match well and the developed method is effective.

Characterization of cooling rate and microstructure of Cu Sn melt droplet in drop on demand process

Different sized single droplets of Cu-6%Sn alloy were prepared by drop on demand(DOD)technique.The secondarydendrite arm spacing was measured and correlated with the droplet cooling rate by a semi-empirical formula.The microstructure ofdroplets was observed by optical microscopy(OM)and electro backscatter diffraction(EBSD).The dendrite feature of singledroplets depends on solidification rate,cooling medium and flight distance.When droplets collide with each other at temperaturesbetween solidus and liquidus,the dendrites and grains are refined obviously possibly because the collision enhances the heat transfer.The cooling rate of colliding droplets is estimated to be more than4×104K/s based on a Newton’s cooling model.The dendritesgrow along the colliding direction because of the temperature gradient induced by the internal flow inside the droplets.

Analysis of Material Loss Behavior According to Long-Term Experiments on LDIE-FAC Multiple Degradation of Carbon Steel Materials

Recently, damage caused by liquid droplet impingement erosion (LDIE) in addition to flow-accelerated corrosion (FAC) has frequently occurred in the secondary side steam piping of nuclear power plants, and the damage-occurring frequency is expected to increase as their operating years’ increase. In order to scrutinize its causes, therefore, an experimental study was conducted to understand how the behavior of LDIE-FAC multiple degradation changes when the piping of nuclear power plants is operated for a long time. Experimental results show that more magnetite was formed on the surface of the carbon steel specimen than on the low-alloy steel specimen, and that the rate of magnetite formation and extinction reached equilibrium due to the complex action of liquid droplet impingement erosion and flow-accelerated corrosion after a certain period of time. Furthermore, it was confirmed at the beginning of the experiment that A106 Gr.B specimen has more mass loss than A335 P22 specimen. After a certain period of time, however, the mass loss tends to be the opposite. This is presumed to have resulted from the magnetite formed on the surface playing a role in suppressing liquid droplet impingement erosion. In addition, it was confirmed that the amount of erosion linearly increases under the conditions in which the formation and extinction of magnetite reach equilibrium.

Two-Dimensional Lattice Boltzmann Model for Droplet Impingement and Breakup in Low Density Ratio Liquids

A two-dimensional lattice Boltzmann model has been employed to simulate the impingement of a liquid drop on a dry surface.For a range of Weber number,Reynolds number and low density ratios,multiple phases leading to breakup have been obtained.An analytical solution for breakup as function of Reynolds and Weber number based on the conservation of energy is shown to match well with the simulations.At the moment breakup occurs,the spread diameter is maximum;it increases with Weber number and reaches an asymptotic value at a density ratio of 10.Droplet breakup is found to be more viable for the case when the wall is non-wetting or neutral as compared to a wetting surface.Upon breakup,the distance between the daughter droplets is much higher for the case with a non-wetting wall,which illustrates the role of the surface interactions in the outcome of the impact.