GB1485750A - Method of and instrument for determination of the size of particles in a turbulently flowing fluid stream - Google Patents

Abstract

1485750 Measuring particle size electrically NATIONAL RESEARCH DEVELOPMENT CORP 29 Aug 1974 [29 May 1973] 55025/72 Heading G1N An indication of particle size or mean particle size in a turbulently flowing fluid stream containing the particles is obtained from a measurement of the instantaneous particle density at a station of the stream. It is shown that the velocity response of a particle in the stream when excited by the instantaneous velocity of the fluid is that of a first order system having a cut-off frequency given by f c = (2K 1 IID)<SP>-</SP><SP>1</SP> Hz where K 1 is a constant derived from the viscous drag coefficient and the difference in densities of the particles and fluid and D is the particle diameter. Since it is not practicable to measure the cut-off frequency directly use is made of the fact that the velocity spectrum of particle turbulence also exhibits the cut-off frequency and since in a turbulent flow the fluctuations in particle velocity are directly related to the fluctuations in instantaneous density of particles in a given volume of the stream this density may be used to obtain the cut-off frequency. Density may be measured by capacitive, tribo electric or conductive arrangements. Fig. 4. The resistance changes of the conductivity path between an electrode 1 and conducting pipe 2 due to the variations in the average number of particles near electrode 1 and being related to the velocity fluctuations of the particles is monitored by conductivity transducer 3. The output M (t) of transducer 3, whose amplitude varies with concentration, has a handwidth from f o to f c whose area varies with concentration. (See inset of Fig. 4). A.G.C. circuit 4 converts m(t) to a constant mean output amplitude Z(t) which contains all the information about the velocity of the particles but not the undesired information relating to mean concentration. The output Z(t) is then applied to respective high pass and low pass filters 5, 6 with a cross-over frequency f 1 . The mean amplitudes E 1 , E 2 of the filter output signals are measured by integrating A.C. voltmeters 7, 8 so that E 1 is equal to the area under the AGC spectrum between f 1 and f c and E 2 is equal to the area between f o and f 1 . Divider 9 then gives an output representative of the particle size. Auto-correlation Fig. 5 (not shown). Since the auto correlation function of a signal conveys the same information as the frequency spectrum of that signal and the cut-off frequency f c of the frequency spectrum is proportional to the reciprocal of the cut-off point of the characteristic auto-correlation this latter can be determined as a function of particle diameter. In practice this is done by measuring the area under the normalized auto-correlation curve. If the fluid velocity has a significant variation then the fluid velocity is measuring using a crosscorrelation method which is used to compensate the Fig. 4 arrangement, Fig. 6 (not shown).