Method of measuring the dispersed phase velocity field from the three-dimensional spectrum of images of the two-phase flow reconstructed from holograms

Abstract

The method proposed in [2], which is based on an analysis of a picture of tNe threedimensional spectrum of the two-phase flow reconstructed from a double-exposure hologram, makes it possible to simplify and accelerate the process of measurement of the particle velocity field. However, experience with the use of this method has shown that in the case of two-phase flows with a complex three-dimensional velocity structure it is not always possible to measure the velocity field from the spectrum reconstructed directly from the hologram. This is because the spectrum contains information on the velocity of all the particles recorded in the flow volume averaged over the direction of propagation of the beam. It was therefore considered necessary to develop the method further in relation to two-phase flows with a complex three-dimensiQnalstructure. Below we examine a method of measuring the velocity field of the dispersed phase of a two-phase flow from double-exposure holograms which is based on photographically recording the image reconstructed from the hologram and analyzing the three-dimensional spectrum of the transparency obtained using a computer. It is precisely the introduction of the photographic recording stage that makes it possible to measure the velocity field of two-phase flows with a complex three-dimensional structure. The reconstructed image is recorded by means of a microscope, whose field of vision and depth of focus delimit the measuring space. Because of the nonlinearity of the photographic recording process it is possible to carry out a preliminary threshold analysis of the image and partially eliminate the speckle structure of the background, which reduces the noise level and increases the contrast of the image and the accuracy of the velocity measurements. As experiments have shown, reversal of the negative is also advisable. The three-dimensional spectrum reconstructed from these images contains information on the local particle velocity distribution, which also makes it possible to reconstruct the three-dimensional velocity field of the dispersed phase of the two-phase flow from several photographic images. Carrying out the spectral analysis on a computer considerably accelerates the image processing and also makes it possible to obtain results more reliable than those obtainable by means of the analysis carried out in [2]. The proposed method of measuring the velocity field of the dispersed phase was experimentally investigated on a subsonic water--air jet flowing from a nozzle and having the following parameters: diameter of initial section of jet 20 mm, mean velocities of gas and liquid phases 3--50 m/sec, particle concentration up to l0 s cm -3.