Klaus Anders

A new light-scattering technique to measure the diameter of periodically generated moving droplets

Abstract A new sensitive light-scattering technique for measuring the diameter of transparent droplets is described. In contrast to the usual method of average intensity measurements here the pattern of scattered light is used to determine the droplet size. The intensity distribution is recorded in the forward scattering region over an angle of approximately ten degrees using a linear CCD-array. The evaluation of scattered light, based on the theory of ray optics, is done on-line by a microcomputer. With this technique it is possible to determine the droplet diameter with an accuracy of approximately 2%. In the present paper the technique is used to measure the size of moving droplets. With a vibrating orifice droplet generator monodisperse droplets can be produced in a diameter range from 10 to 300 μm. For a constant excitation frequency all droplets have exactly the same size; therefore a quasi-stationary scattering pattern is obtained. It is shown that the new light-scattering technique can be used to measure droplet evaporation rates as a function of Reynolds number and Knudsen number.

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.

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.

Experimental investigation of Gaussian beam effects on the accuracy of a droplet sizing method

A special sizing technique is applied to measuring the diameter of monosized droplet streams that are used for investigation of fuel droplets in enginelike conditions. For these experiments the droplet diameter must be known precisely. The sizing technique used is based on the evaluation of the fringe spacing of scattered light in the forward direction. This technique is independent of the intensity of the incident light. No absolute intensities need to be measured. The droplets are exposed to a focused laser beam. Therefore the frequently used assumption of plane wave fronts is not fulfilled. Elaborate experiments have been carried out to study the influence of a Gaussian intensity distribution of the laser beam on the accuracy of the sizing technique. It has been shown that the droplet diameter can be measured to an accuracy of better than 2% even if the droplet is illuminated by a Gaussian beam for a droplet diameter that is smaller than the beam diameter.

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.

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.

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.

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.