Multi-exposure method for studying compressible multiphase flows

Kurzfassung

In unsteady compressible multiphase flows it is of interest to simultaneously visualize the development in time both, of the dispersed phase and of discontinuities. In order to do so a multi-exposure method has been developed. The method is based on a combination of shadowgraphy and particle tracking. A modulated laser diode is applied as a short-pulse light source. Shadowgram images are received by a CCD-camera. Control signals for laser, camera etc are formed by a signal sequencer. The whole system is controlled by a personal computer. Both, pulse shapes and sequences of light pulses can be varied within a wide range, the duration of single pulses being as low as 20 ns and the interval between different pulses being greater than 20 ns. The exposure time of the CCD-camera is as low as 10 μs. The time delay of a signal between the signal sequencer and any device is smaller than 150 ns. The time accuracy of the signal sequencer is 1 ns. The present method is particularly well suited for investigating the behavior of small particles at high velocities and small concentrations. Furthermore, taking shadowgraphs of the gas flow in addition to particle images and thus obtaining information on gas flow and particle motion at the same time enables an easy interpretation of unsteady processes occurring in the flow. The wide variability of sequences of light pulses allows for determining not only the velocity and acceleration of particles but provides also additional information as, for example, on concentration, shape and size of particles or the shape of shock waves. The diagnostic method of multi-exposure visualization has been used for studying processes in the thin shock layer ahead of a plate in supersonic jet impingement flows. A nearly ideally expanded jet of Mach number M = 2.8, laden with metallic particles (radii: 5-50 μm, velocity: ~400 m/s), was directed against a plate. At certain nozzle-to-plate distances a recirculation bubble is repeatedly formed and destructed in the shock layer. Then, the flow ahead of the plate and the position of the bow shock oscillate strongly. The behavior of reflected particles in the cloud ahead of the plate also changes essentially. The comparatively slow motion of the shock ahead of the plate has been detected by using a sequence of 5 light pulses (duration: 100 ns) with time intervals between the pulses ranging from 2 to 10 μs (Fig. 1a). Different intervals were chosen in order to determine the direction of the shock motion. Fig. 1b shows an image of particles in the shock layer ahead of the plate. Here, a sequence of three light pulses (duration: 50 ns, time intervals between the pulses: 200 ns and 400 ns) was applied. High-velocity particles ahead of the bow shock are represented by a track of three images (marked by white circles in Fig. 1b). In the particle cloud ahead of the plate the velocity of most particles is so small that the short time intervals between the pulses do not suffice to separate the three images of individual particles. Particles having just one image are also present upstream of the shock. Typically, a bow shock can be seen ahead of such particles. These particles have been reflected off the plate. Particle velocities in the shock layer were determined by increasing the time intervals up to 4 μs. Variations in time of location of the shock and of the axial distribution of reflected particles in the unsteady shock layer are shown in Fig. 2. It can be seen that particles normally stay within the shock layer close to the plate. However, when the shock approaches the plate, reflected particles are also present upstream of the shock. A comparison of characteristic times of the oscillating shock and relaxation times of particles shows that the former is at least an order of magnitude greater than the latter. Therefore, the particle cloud has enough time to expand and fill the whole space between the upstream moving shock and the plate. When the shock moves back towards the plate the shock is very fast. Measured velocities of the shock reach up to values of uSL ≈ 100 m/s. Hence, it takes the shock about 10 μs to change its position. This time is much shorter than the relaxation time of particles. Thus, particles remain upstream of the plate shock in the supersonic flow for some time until they reach the shock layer again.