R.C. Waterfall

Measurement of the solids volume fraction and velocity distributions in solids–liquid flows using dual-plane electrical resistance tomography

Abstract This paper describes a dual-plane Electrical Resistance Tomography (ERT) system for measuring the local solids volume fraction distribution and the local solids axial velocity distribution in solids–liquid flows. The paper also describes a local, intrusive conductivity probe system for providing reference measurements of the distributions obtained using the dual-plane ERT system. Experiments were performed using both the ERT system and the local probe system in vertically upward and inclined solids–water flows. Good agreement between the two techniques was observed. The local solids volume fraction distribution and the local solids axial velocity distribution obtained using the ERT system enable good estimates to be made of the mean solids volume fraction, the mean solids velocity and the solids volumetric flow rate.

Combustion imaging from electrical impedance measurements

The development of stray-immune circuits to measure very small floating capacitances has enabled the technique of electrical capacitance tomography (ECT) to be developed. Burning fuel, for example in an internal combustion engine, produces a high concentration of ions, thus modifying the dielectric constant of the combustion volume. Thus ECT can be applied to produce tomographic images inside a combustion chamber, independent of flame luminosity. However, the major effect of the increase in ion concentration is to increase the conductivity of the volume. The ECT technique has been extended to measure both the change in permittivity and change in conductivity, yielding additional information on the flame composition. Current spatial and temporal resolution is poor but developments are in hand to overcome these limitations. Alternatively, the way forward may well be to abandon images and instead use extremely simple and cheap sensors to monitor knock on a cycle-by-cycle basis. An alternative use of such a simple sensor would be to monitor cycle dispersion in a lean burn engine. Both these techniques could help minimize fuel consumption, whilst maximizing power output and keeping emissions low.

A six-electrode local probe for measuring solids velocity and volume fraction profiles in solids-water flows

This paper describes the design and construction of a local six-electrode conductivity probe which can be used in solids-water pipe flows to simultaneously measure the local solids volume fraction and the local solids axial velocity. Using finite element analysis, the probe electrode geometry was designed so that the regions of the solids-water mixture that were interrogated by the probe were optimal for measurement of the volume fraction and for cross correlation velocity measurement. The probe was used, in conjunction with a computer controlled traversing mechanism, to obtain distributions of the local solids volume fraction and the local solids axial velocity both in vertical upward and in upward inclined solids-water flows. Such distributions can be used to validate volume fraction and velocity profiles obtained using dual-plane electrical resistance tomography systems. Experimental results indicated that the six-electrode probe can be used to estimate the local solids volume fraction in vertical upward solids-water flows with a mean absolute error of approximately 0.01. Experimental results also indicated that the six-electrode probe can be used to measure the local axial solids velocity with a mean error of 2% of the reading.

Visualizing combustion using electrical impedance tomography

The techniques developed for electrical capacitance tomography (ECT) have been adapted to characterize combustion phenomena in a scaled model of an internal combustion engine. The method is able to locate flame position, measure flame size and monitor the effect of varied air/fuel ratio. Flame failure can be identified. The technique can measure flame front arrival time and reliably depict the development of the combustion process in a research model of a single cylinder internal combustion (IC) engine. Low-resolution images at over 300 frames per second have been captured.

Flame visualizations using electrical capacitance tomography (ECT)

Stray-immune circuits are available that will measure very small floating capacitances at high speed These circuits have facilitated the development of Electrical Capacitance Tomography (ECT), which uses fixed, robust metal plates as sensors, arranged non-intrusively around the periphery of the volume of interest. The phenomenon of flame ionization is well understood, with general agreement that the concentration of all the major ions maximize at the flame front. There is also strong experimental evidence to suggest that the presence of carbon particles (and their precursors) results in a large increase in the charge carrying capacity of the flame. This paper reports some initial results from using ECT to visualize the position, size, composition and movement in flames in open and closed combustion chambers.

Imaging Immiscible Liquid-Liquid Systems by Capacitance Tomography

Electrical capacitance tomography offers the opportunity to visualize the contents of a process vessel or pipeline which contains dielectric materials or where the continuous phase is a non-conducting dielectric. This work describes the application of this novel imaging technique to a range of liquid-liquid processes from a laboratory-scale mixing vessel to an industrial-scale liquid-liquid contactor. Single-plane specially-designed sensors containing arrays of either 8 or 12 measurement electrodes, with the same number of driven-guard electrodes, were attached to a bench-scale (diameter—0.15 m) mixer, a bench-scale (diameter—0.10 m) flow column, and a large (diameter—0.6 m) flow column. The capacitance measurements were processed by an image reconstruction algorithm based on linear back-projection to convert the measurements to images of the distribution of dispersed phase across the section of the equipment under investigation. Tomographic images of the distribution of dispersed phase were obtained. These images clearly show the distribution of dispersed phase within the sensing zone of these liquid-liquid systems. The potential application of ECT as a non-intrusive monitor of liquid-liquid systems in industrial-scale contactors is very promising indeed.

Calibration of capacitance tomography systems for liquid-liquid dispersions

Electrical capacitance tomography (ECT) provides information on the flow regime and concentration distribution in process vessels and pipelines containing two-phase mixtures. This is achieved by arranging sensors around the pipe or vessel to be imaged. The sensor output signals depend on the position of the phases within the sensing zone. A computer is then used to reconstruct an image of the pipe cross section from the sensor measurements. It is necessary to calibrate the ECT system by filling the sensing zone consecutively with the low permittivity material, then the high permittivity material. When the high permittivity material is introduced into a low permittivity background within the sensing zone, the capacitance between sensors will increase. This paper demonstrates that this calibration procedure coupled with the existing image reconstruction software is inappropriate for imaging liquid-liquid dispersions containing low fractions (below 0.3) of water in kerosene. An alternative calibration procedure is proposed which yields substantially improved images.

Electrical tomography techniques for multiphase flow applications

This chapter considers the application of two electrical tomographic techniques to the problem of monitoring multiphase flows. Many designs of flow meter are available but, in general, they only operate reliably over a limited range of conditions. If the flow conditions are other than those assumed, inaccurate or misleading indications of flow can be produced. In many situations, both the phase distribution and velocity profiles vary significantly, and sometimes rapidly, in time and space. Electrical techniques detect a change in dielectric constant or conductivity from electrodes surrounding a cross-sectional area of interest. From these measurements, the area distribution of the phases in the cross-section can be inferred. By taking readings from two planes of electrodes, of known separation, cross-correlation can be used to calculate the velocity of material from point to point between the two planes. Thus the volumetric flow rate of the phases can be determined. The electrical techniques described in the paper are Electrical Capacitance Tomography (ECT) and Electrical Resistance Tomography (ERT). Both techniques use non-invasive passive metal electrodes, with low amplitude electrical excitation. The techniques are rugged, low-cost, low-impact and fast. Absolute accuracy, on the other hand, is currently not better than 5%.