Norbert Roth

Refractive-index measurements for the correction of particle sizing methods

For many optical methods in liquid particle sizing the refractive index of the liquid must be known. There is no problem if the data are available in the literature. If the liquid is unknown or if the refractive index changes because of a heating process or chemical reactions, the refractive index must be measured to achieve accurate sizing. A method is presented to determine the real part of the refractive index of droplets using the shift in the position of the first rainbow. Results from the application of this method to burning droplet streams are presented.

Effect of viscosity on droplet-droplet collision outcome: Experimental study and numerical comparison

The influence of viscosity on droplet-droplet collision behavior at ambient conditions was studied experimentally and numerically. N-decane, monoethyleneglycol (MEG), diethyleneglycol (DEG), and triethyleneglycol were used as liquid phase providing viscosities in the range from 0.9to48mPas. Collision Weber numbers ranged approximately from 10 to 420. A direct numerical simulation code, based on the volume-of-fluid concept, was used for the simulations. Experimentally, observations of two droplet streams using a modified stroboscopic technique (aliasing method) were used to investigate the whole range of impact parameters during one experimental run. The experimental method has previously been verified for the water/air system [C. Gotaas et al., Phys. Fluids 19, 102105 (2007)]. In the present work, it was tested and validated for the n-decane/air system. Measured data agree well with those published in the literature. Well-defined regions of stretching separation and coalescence were identified, while refl...

The velocity change of ethanol droplets during collision with a wall analysed by image processing

Monodisperse droplet streams are used to study the droplet wall interaction of ethanol droplets in the micrometer range. Qualitative results are given for different regimes of droplet wall interaction. The phenomena observed range from complete wetting to almost elastic reflection of the droplets. Complete wetting is observed for low wall temperatures, whereas reflection occurs for wall temperatures above the Leidenfrost temperature. For high impact velocities and high wall temperatures above the Leidenfrost temperature the formation of secondary droplets can be observed. Image processing is used to obtain quantitative results for the loss of momentum during wall interaction for cases of droplet reflection without formation of secondary droplets.

New technique for investigating phase transition processes of optically levitated droplets consisting of water and sulfuric acid

Experimental techniques for studying supercooled or frozen droplets will be presented. Single particles consisting of water or of water contaminated with sulfuric acid have been levitated optically and stabilized in a vertical laser beam by radiation pressure forces. In the experiments the levitated droplet has been observed in a chamber, in which temperatures down to -60°C can be reached, in order to simulate conditions in contrails. After introduction into the chamber the droplet is supercooled rapidly. Depending on temperature and vapor pressure in the chamber, the droplet will grow or evaporate until it disappears. During this process the droplet is observed by a video camera. Furthermore the scattered laser light is evaluated at scattering angles of approximately 45° to characterize the droplet and to determine the droplet size. Simultaneously with the size, the position of the droplet along the axis of the laser beam is determined. The droplet oscillates for a fixed laser power along the axis of the laser beam, when the droplet radius changes with time. These oscillations are due to oscillations of the radiation pressure forces during monotonic decrease or increase of the radius, i.e., for evaporation or condensation. With this new technique it is possible to associate directly the frequency of these oscillations with the radius change rate. Typical experimental results for size and for the radius change rate are shown.

Size and polarization behaviour of optically levitated frozen water droplets

Experiments with single optically levitated droplets were performed in order to simulate phase transition processes during contrail formation. Freezing, sublimation, and crystal growth of levitated droplets were studied. Shadow pictures of the levitated droplets showed, that the droplets remain spherical immediately after freezing and during the sublimation process with dry ambient conditions. Under supersaturated conditions growing of ice crystals occurred on the frozen droplets. Size measurements were performed by evaluating the shadow pictures. The experiments showed, that the polarization ratio becomes non-zero at the moment of freezing. Immediately after freezing and for sublimating frozen droplets mean values of the polarization ratio ranging between 0.1 and 0.25 were measured. Measurements on frozen droplets with growing ice crystals showed increasing polarization ratios with a level off between 0.4 and 0.5.

Flame propagation in planar droplet arrays and interaction phenomena between neighboring droplet streams

Well-defined burning droplet systems consisting of three or five parallel monosized droplet streams located in a plane and forming planar droplet arrays were examined. The droplet streams produced with a vibrating orifice droplet generator were directed vertically upward. Sizes were measured in order to characterize the burning droplet arrays and to study the interaction between the droplets. To get qualitative information on the development of droplet heating along the droplet streams, the position of the first rainbow was determined. This rainbow position gives the droplet temperature if the temperature is uniform. During droplet heating with nonuniform temperature qualitative results may be obtained. The droplet streams were ignited by a heating coil. Measurements were performed both with ignition of all streams at the same location above the droplet generator and with ignition of one stream at the edge of the configuration. The latter type of ignition allows investigation of the propagation of the flame across the droplet array due to the flame spread from one droplet stream to its neighboring stream. Because the flame is quasistationary, the velocity of flame propagation is determined from the ignition points in the different droplet streams. The velocity of flame propagation is defined as the velocity of flame spread from droplet stream to droplet stream. The influence of initial droplet temperature, spacing between the droplet streams, droplet size, and volume flux were studied at constant droplet velocity. An increasing velocity of the flame propagation was found for increasing initial droplet temperature. At constant volume flux, no influence of droplet size on the velocity of flame propagation could be detected. However, for increasing volume flux at constant droplet velocity, an increasing velocity of flame propagation was found in most cases of the present experiment.

Parallel Computation of the Time Dependent Velocity Evolution for Strongly Deformed Droplets

A fully three-dimensional numerical procedure has been used to predict the behavior of spherical and deformed droplets in a gas flow. The computational grid is moving with the droplet to minimize grid size and computation time. Numerical results of drag coefficients for spherical droplets show good agreement with literature data. The behavior of droplets with initially cylindrical or disk shapes has been compared with corresponding spherical droplets for different viscosities of the droplet liquid. For low viscosities the droplets are oscillating. For higher viscosities the initially strongly deformed droplets approach a spherical shape asymptotically. The influence of the strong initial deformation is shown. The simulation has been run on the Cray T3E/512-900 at the HLRS.

Examination of the Rainbow Position of Optically Levitated Droplets for the Determination of Evaporation Rates of Droplets

In the present paper theoretical and experimental studies of the radiation pressure and of the intensity distribution in the rainbow region are presented. It has been shown that both radiation pressure and rainbow position show an oscillatory behaviour in their dependence on droplet radius. This property has been used to detect changes in radius of optically levitated droplets. From the theoretical calculations it follows that the period of these oscillations is practically independent of size and refractive index for a wide range of both quantities. Furthermore, it could be shown theoretically and experimentally that the oscillations of radiation pressure and rainbow position are strongly correlated and have the same period.

Experiments to Study Phase Transition Processes

Processes with droplets in technical and in natural systems involve different phase transitions. The evaporation of fuel droplets is essential for heterogeneous combustion. Freezing and melting as well as condensation and evaporation are important for cloud droplets in the atmosphere of the Earth. Sublimation, the phase transition between solid and vapor, is observed for instance with frozen water droplets, or when ice crystals grow on frozen droplets. Another kind of phase transition may occur in the case of solutions. If a droplet consisting of a solution evaporates a solid particle or in many cases a crystal remains at last.

Experimental Investigation of the Reduction of Burning Rate Due to Finite Spacing Between Droplets

Many combustors in technical applications are supplied with liquid fuel. The fuel is atomized in order to improve the evaporation process. In such fuel sprays the droplet diameter ranges typically from 10 µm up to 500 µm. If the distances between the droplets are small, the mutual interactions of the droplets have to be taken into account for the description of the physical and chemical processes in the combustor. Usually it is very difficult to investigate these processes quantitatively in real sprays in a detailed manner. Often only mean or global quantities can be examined. To overcome these difficulties monodisperse equally spaced droplets may be applied as a useful tool for studying basic droplet phenomena as for instance the mutual interaction in spray combustion.