Juliusz B. Gajewski

Monitoring electrostatic flow noise for mass flow and mean velocity measurement in pneumatic transport

Abstract This paper presents experimental results of research on the electrostatic, non-contact measurement of the mass flow rate and mean flow velocity of solids in pneumatic transport lines. We also describe a microprocessor-based system, based on the measurement method, that had been built to be installed in industrial installations. The system permits rapid mass flow rate and mean flow velocity measurements in quasi-real time. The method is based on the phenomenon of electrostatic induction - the effect of the flux of charged solid particles flowing in a pipe on measuring ring probes (inductive transducers). The research, described in this paper, provides experimental verification of the measuring system. Results obtained for two different materials (aluminosilicate and granulated polypropylene) show the relationships between the effective values (RMS) and mean values of rectified electric signals induced in the measuring probes by the flow of charged solid particles and the mass flow rate of those particles for different mean flow velocities.

Piezoelectric properties of nonuniform electrets

Abstract It is generally well known that the piezoelectric effect in solids is tightly connected with the molecular structure of a solid dielectric. However, an analysis of a simple model of this phenomenon provides a conclusion that piezoelectricity can also appear in materials containing the fixed space charge and spatially dependent elasticity. In such a situation the changes of the space charge distribution under the external stress will not follow the changes in the molecular structure. The resulting changes in the effective surface charge densities on both electrodes are the source of a piezo-signal. To demonstrate a simple model supporting the above hypothesis, a thin double-layer electret was prepared. The layers were made of polymers with significantly different mechanical properties (coefficients of elasticity). The most interesting results of this work offer, as is believed, the possibilities of the preparation of the technologically new high-sensitive and low-cost piezoelements, and the more widespread use of materials suitable for the production of a wide variety of sensors.

Electrostatic Nonintrusive Method for Measuring the Electric Charge, Mass Flow Rate, and Velocity of Particulates in the Two-Phase Gas–Solid Pipe Flows—Its Only or as Many as 50 Years of Historical Evolution

There are a lot of different noncontact methods for measuring the electric charges carried on solid particles, the mass flow rate, concentration, volume loading, mean flow velocity, and other flow electrical and mechanical parameters in the two- and multiphase gas-solid flows as those in pneumatic conveyance, in the air, etc. One of the methods is that based on the phenomenon of electrostatic induction. The nonintrusive probes, sensors, detectors, and transducers of different shapes are used as the first parts of measuring systems built to measure the aforementioned quantities, which are strictly related to the flow of charged particles, without any disturbance to them. The method presented here employs a ring-shaped metal electrode called a probe. The flow itself produces the electrostatic flow noise that is induced on the inner surfaces of sensing devices in the form of charge or potential or both whose values are proportional to the quantities measured. The signals obtained at the output of the sensing devices are a source of information about the flow of particulates. The very first information about the method, its metrological description, and practical application to the charge measurement was published for the first time 50 years ago, and this is the reason why the author has decided to present the methods evolution and historical development, along with its many different aspects over that period.

Effect of the particle diameter and corona electrode geometry on the particle migration velocity in electrostatic precipitators

Abstract The paper presents the results of research on the movement of fly ash particles in the model of an electrostatic precipitator (ESP) with different corona electrodes: barbed plate, barbed tube, wire, and spiked band. The experimental set-up and the photographic method for measuring the particle velocity using laser light are described. The photographs showed the trajectory of particle movement in the inter-electrode space and enabled the determination of the migration velocity of particles. The experiments were performed with the use of one type of fly ash with different diameters, and for given supply voltages and a constant inlet gas velocity. The results obtained enabled one to determine the effect of particle diameter and supply voltage on the migration velocity of particles in four corona electrode geometries in the ESP model.

Electrostatic, inductive ring probe bandwidth

Electrostatic, inductive, non-intrusive probes are employed for measuring the electric charge as well as the mass flow rate, volume loading, and mean flow velocity of solid particles especially while travelling through pipes of pneumatic transport or in ambient air when a flux of those particles forms a rather compact column-like jet of a nearly constant cross sectional area. The bandwidth of such measuring devices is a very important factor which should be determined carefully, or even planned in advance, before designing the probe itself intended for the use under well defined conditions. In the paper the bandwidth of the inductive ring probes is discussed. To find the probe bandwidth the derivation of a general formula is presented along with the accepted assumptions and an original way of formulation and consideration of that problem. The general formula obtained is an inductive ring probe frequency response function and permits the bandwidth of any inductive, non-intrusive probe, sensor, transducer, etc, to be easily determined for given mean flow velocities of particles and probe widths.

Mathematical model of non-contact measurements of charges while moving

Abstract A mathematical model is derived from an analysis of a phenomenon arising from moving charged particles of various solids exerting an influence on a metal ring placed in an electric field. The charges flow rectilinearly in air along the geometrical axis of the ring. Appropriate equations describing the influence of the charges on the entire measuring system (ring plus measuring device) are derived for both general and specific cases.

Double-layer electret transducer

Abstract A simplified analysis of the double-layer electret with a frozen space charge leads to the conclusion that such a structure can show a piezoelectric effect when the elasticity coefficients of the layers differ. Results of experiments carried out on the PTFE foil-PP non-woven fabric sandwich confirm the possibility of construction of a simple and relatively sensitive transducer using this effect.

NON-INTRUSIVE SOLIDS CHARGE AND MASS FLOW MEASUREMENTS WITH AN ELECTROSTATIC FLOW PROBE

Abstract An electrostatic flow probe for measurement of the electric charge, mass flow rate, volume loading, or concentration of solid particles travelling along a pneumatic transport pipe is mounted on a dielectric pipe section. The particles contact the inner wall surface of the dielectric pipe and cause it to be charged. The charging of this wall can be a serious metrological problem because of its influence on any non-contact measuring system based upon the electrostatic (inductive) flow probe which is a non-contact, non-intrusive device. The objective of this paper is to prove mathematically that there does not exist a problem of the dielectric pipe wall tribocharging from the metrological point of view when certain conditions are fulfilled. For the sake of illustration a sample measurement result obtained in a series of experiments performed with different powdered and granulated materials is shown to retain the validity of the mathematical and physical models. Also there have been carried out computer simulation and numerical analyses of an equivalent circuit of the probe and preamplifier to show the time response of the system to the charging of the dielectric pipe inner wall and its influence on the readings.

ELECTROSTATIC FLOW PROBE AND MEASURING SYSTEM CALIBRATION FOR SOLIDS MASS FLOW RATE MEASUREMENT

Abstract This paper presents an original system and procedure for calibrating, verifying, and testing any electrostatic (inductive), non-contact probe, sensor, or transducer, or the whole measuring system: a measuring head with a probe in its inside and an analog or digital measuring instrument connected with the head. The calibration system has been designed on the basis of the well-known method in which the Faraday pail is used for measuring the net, total electric charge as carried by charged solid particles. The method and system are described, and the results of preliminary tests are presented for granulated polypropylene and for different mass flow rates.

STATIC CHARACTERISTICS OF AN ELECTROSTATIC FLOW PROBE

Abstract This paper is a subsequent one in a series of papers on the metrological properties and characteristics of an electrostatic (inductive), non-contact probe or the whole measuring system consisting of a measuring head with this probe in its inside and an analog or digital measuring instrument connected with the probe [Gajewski, Meas. Sci. Technol. 7 (1996) 1766; J. Electrostat. 40&41 (1997) 437; J. Electrostat. 45 (1999) 255; J. Electrostat. 46 (1999) 271]. The probe and measuring system are intended for the measurement of the mass flow rate, volume concentration (or solids loading), and the net electric charge, as carried on solid particles during pneumatic conveying. The static characteristics of the probe are shown, which were obtained experimentally using a certain simulation experiment on a real measuring head and the probe, and numerically for an equivalent electric circuit of the probe after computer simulation had been done. The results are presented and they reveal that the experimental ones are in good agreement with those of computer simulation of a real metrological process and instrumentation.