GB2204678A - Size and velocity measuring instrument for multiphase flows - Google Patents

Abstract

A laser source 1, 2 is split at 3, 4 into two or more beams 5 whose intensities are varied with time. The beams illuminate a scattering field 6 e.g. of droplets, which scatter light. This scattered light is collected on a multi-element array detector 6, which is used to measure the angular distribution of the scattered light, and the distribution information is read by an electronics module 9. Analysis of the time and space dependent scattered light pattern enables simultaneous size and velocity determination of the scattering objects. <IMAGE>

Description

SIZE AND VELOCITY MEASURING INSTRUMENT FOR MULTI-PHASE FLOWS A knowledge of the size of particulates or droplets or bubbles is of importance in many industrial processes eg. spray drying, fuel sprays, bulk powder production etc. and in recent years greater demand has arisen for the provision of real time measurement of this property.

In many of these processes, a greater understanding of the interaction between different phases can be obtained by a simultaneous knowledge of the particle, droplet or bubble velocity. In addition it is greatly preferred if the different measured velocities can be associated with their respective size groups.

The following invention sets out to provide real time measurements of the size and velocity of particulates, droplets or bubbles. The description of how this is achieved is given with reference to Figure 1.

A laser (1) provides a collimated monochromatic source (2). This is passed through a beam splitter and modulator combination (3) which generates a double beam and provides a time varying spatial modulation to the emerging wavefront.

This amplitude modulation may be produced mechanically eg. by a rotating mask or electro-optically eg. by a Bragg cell or Pockels cell in combination with polarising elements. This time varying spatial modulation in effect creates two parallel beams (5) whose intensity profiles vary in the same way with time but which are out of phase. The modulator is configured in such a way that the front elements (4) can be rotated around the beam axis to change the plane which contains the two beams thus providing direction sensitivity for velocity measurement.

Any atomised or particulate material passing through such a beam geometry will scatter light in all directions. By collecting the light scattered around the forward direction, an analysis can be carried out on the angular distribution of light energy to obtain detailed information about the size of scattering objects. In addition, when electronic signal processing is carried out on the received scattered light signal from the two out of phase incident beams, the velocity can be derived in the manner described by the following.

To obtain the distribution of scattered light energy with different angles, a combination of collection lens (7) and concentric ring detector (8) is used.

The detector consists of up to 50 concentric photodiode annuli of varying width, being narrow near the centre and wider at the outer edge. By placing this detector at the back focal plane of the lens, at a distance F, it is well known the angular distribution of light scattered becomes related to the radial distribution of light in the focal plane. This radial distribution is strongly characteristic of the size of the scattering object and when the annular detector elements and associated electronics are so arranged to measure light energy rather than intensity, it is further the case that a given size of scatterer provides a maximum of received energy on one of the annuli of the detector. Different annuli therefore become predominantly characteristic of particular scattering size groups.

Any collection of scattering centres will consist of inhomogenieties either due to the scattering centres themselves or their distribution in space.

The received optical signal on any of the annular detectors is therefore not constant but varies in a manner determined by the velocity of these inhomogenieties through the double beam system. By carrying out electronic signal processing, for example cross correlation, of the received optical signals at a particular annulus due to scattering from the two out of phase sampling beams, it is thus possible to estimate the velocity of the inhomogeniety in the measuring space (6). Furthermore, this velocity is predominantly associated with a particular size of scatterer due to the relationship between size and radius of the annulus.

The drive circuitry for the photodiode array is contained within a separate control box (9) which may also perform signal analysis for velocity estimation electronically. Alternatively, the timing information for velocity along with the radial energy information may be passed to a microcomputer for digital analysis of size and velocity (10).

Claims (3)

1. PATENT APPLICATION NUMBER 8707195

   SIZE AND VELOCITY MEASURING INSTRUMENT FOR MULTIPHASE FLOWS - CLAIMS 1) An optical and electronic instument to enable the simultaneous measurement of size and velocity for droplets, particulates and bubbles, and consisting of an illuminating laser source, means for splitting the source into two or more beams and imposing a time dependendence on their intensities, a multi-element detector situated to measure the far field diffraction pattern of the scattering objects, an electronic module which can store signal information from the detector and a means of analysing the signals to calculate velocity and size of the scattering objects.
2. 2) An electronic instrument as claimed in Claim 1 in which means is provided to synchronise the time variation of the illuminating beam intensities and the collection of electrical signal information from the multi-element detector.
3. 3) An electronic instrument as claimed in Claims 1 and 2 in which various optical elements, spatial and intensit filters are used in the beam paths to modify the light distribution in the far field diffraction pattern.