Artur J. Jaworski

Applications of electrical tomography for gas–solids and liquid–solids flows — a review

Abstract This paper presents a review of electrical tomography methods for investigating, monitoring and controlling gas–solids and liquid–solids systems. The physical laws governing the electrical measurements and issues associated with image reconstruction are described in some detail. Experimental results, obtained for a number of case studies conducted in the pilot plant scale and industrial rigs, are presented. These include circulating fluidised bed, pneumatic and hydraulic conveyor, multiphase flow metering and hydrocyclone flow. Instantaneous images, captured with the speed up to 200 frames per second, illustrate how flow patterns vary, and reveal the dynamic behaviour of two-phase systems. Application of electrical tomography for control and fault diagnosis in industrial systems is addressed — the examples include dense pneumatic conveying and hydrocyclone performance.

Application of electrical capacitance tomography for measurement of gas-solids flow characteristics in a pneumatic conveying system

Transient three-dimensional multiphase flows are a characteristic feature of many industrial processes. The experimental observations and measurements of such flows are extremely difficult, and industrial process tomography has been developed over the last decade into a reliable method for investigating these complex phenomena. Gas-solids flows, such as those in pneumatic conveying systems, exhibit many interesting features and these can be successfully investigated by using electrical capacitance tomography. This paper discusses the current state of the art in this field, advantages and limitations of the technique and required future developments. Various levels of visualization and processing of tomographic data obtained in a pilot-plant-scale pneumatic conveying system are presented. A case study outlining the principles of measuring the mass flow rate of solids in a vertical channel is shown.

Investigations of flow instabilities within the dense pneumatic conveying system

Abstract The gas–solids flows in the dense pneumatic conveying exhibit many interesting features related to their three-dimensional and unsteady nature. The dominant characteristic in this type of flows is the appearance of discrete flow instabilities commonly known as ‘slugs’ and ‘plugs’. These are investigated by two techniques: a high-speed video camera and a twin-plane electrical capacitance tomography (ECT), and the results obtained are discussed. Data extracted from tomographic measurements can be processed and presented in several different ways to enable a better understanding of the flow behaviour. Several examples are given, including body shape type information, internal structure of the flow instabilities, their propagation velocity and frequency characteristics. A short case study, focused on measurements of bidirectional flow in a vertical channel, is presented.

A capacitance probe for interface detection in oil and gas extraction plant

The design and construction of a capacitance probe for monitoring the positions of interface levels within separation equipment is described. Preliminary results from the industrial tests of the prototype device are presented.

A low-cost electricity generator for rural areas using a travelling-wave looped-tube thermoacoustic engine

Abstract This article describes the construction and preliminary testing of a pre-prototype thermoacoustic electricity generator to test the concept of a low-cost device for application in remote or rural areas of developing countries. A travelling-wave thermoacoustic engine with a configuration of a looped-tube resonator is designed and constructed to convert heat to acoustic power. Air at atmospheric pressure is used as the working gas, PVC tubing is utilized for the feedback pipe, whereas an inexpensive commercially available loudspeaker is adopted to convert the acoustic power, produced by the engine, to electricity. Preliminary experimental results are presented and discussed in detail. The results show that the approach is feasible in principle and it is possible to produce the electrical power levels in the order of 4–5 W with overall heat-to-electric efficiencies in the order of 1 per cent. Further work towards optimizing the device from the performance, manufacturing, and cost points of view is outlined.

Application of particle image velocimetry measurement techniques to study turbulence characteristics of oscillatory flows around parallel-plate structures in thermoacoustic devices

This paper describes the development of the experimental setup and measurement methodologies to study the physics of oscillatory flows in the vicinity of parallel-plate stacks by using the particle image velocimetry (PIV) techniques. Parallel-plate configurations often appear as internal structures in thermoacoustic devices and are responsible for the hydrodynamic energy transfer processes. The flow around selected stack configurations is induced by a standing acoustic wave, whose amplitude can be varied. Depending on the direction of the flow within the acoustic cycle, relative to the stack, it can be treated as an entrance flow or a wake flow. The insight into the flow behaviour, its kinematics, dynamics and scales of turbulence, is obtained using the classical Reynolds decomposition to separate the instantaneous velocity fields into ensemble-averaged mean velocity fields and fluctuations in a set of predetermined phases within an oscillation cycle. The mean velocity field and the fluctuation intensity distributions are investigated over the acoustic oscillation cycle. The velocity fluctuation is further divided into large- and small-scale fluctuations by using fast Fourier transform (FFT) spatial filtering techniques.

A multi-electrode capacitance probe for phase detection in oil-water separation processes : design, modelling and validation

This paper focuses on the development of a multi-electrode capacitance probe for interface measurement and phase detection within industrial three-phase separators, used in oil and gas extraction and oil refining processes. The sensors are constructed using printed circuit board (PCB) technology and are embedded within stainless steel casings forming the structure of the probe. The design process was aided by the finite element solutions of the three-dimensional electrostatic problem. A number of solutions were obtained which predict the probe readings for various configurations of phases. Validation of the probe performance was performed using a combination of laboratory and industrial tests in a real scale separator, both with representative process media. A good agreement between the experiments and modelling is shown.

Development of thermoacoustic devices for power generation and refrigeration

This paper is intended as a technical overview of the research and development work initially undertaken at the University of Manchester and subsequently transferred to the University of Leicester as part of the EPSRC-funded SCORE project (Stove for Cooking, Refrigeration and Electricity supply). The objectives of the work were twofold: Firstly, to develop an early demonstrator of a low-power electricity generator (to deliver approximately 10–20 W of electricity). This was to be based on the concept of using low-cost materials, working fluids and linear alternators suitable for deployment in rural areas of developing countries. The issues of concern here were the development of a suitable thermoacoustic engine topology and control measures; design of suitable heat exchanger configurations from initial use of electrical heaters to heat input through propane combustion; and characterisation of commercial loudspeakers to work as linear alternators and subsequent incorporation of selected models for engine prototyping purposes. These matters will be illustrated by a number of demonstrators and their testing in the laboratory environment. Secondly, to develop a demonstrator of a combustion driven thermoacoustic cooler for storage of vital medical supplies in remote and rural areas where there is no access to electricity grid. To this end, the paper will describe the design, construction and test results of an electrically driven demonstrator of a standing wave thermoacoustic engine coupled to a travelling wave thermoacoustic cooler. The final part of the paper will summarise the achievements to date and outline future work that has spun out from the original SCORE project. This will in particular include the current work on a scaled up version of electricity generator designed to deliver 100 W of electricity by using a two-stage engine configuration and the issues of integration of the thermoacoustic electricity generator and thermoacoustic cooler into one system.

Optimization of thermoacoustic stacks for low onset temperature engines

Abstract Standing-wave thermoacoustic engines are typically optimized in order to obtain high system efficiency. However, in applications targeting the utilization of waste heat, it may be necessary to optimize them for a low onset temperature difference instead, so as to enable the engines self-oscillation using low-grade energy sources. This article focuses on theoretical investigations of the critical temperature gradient in stacks, based on the assumptions of a short stack in a standing-wave acoustic field and an ideal gas. A dimensionless critical temperature gradient factor is obtained on the basis of the linear thermoacoustic theory and the analysis of the viscous and thermal relaxation losses for selected stack geometries. With a simple form, it reveals the effects of the stack geometry, the characteristic channel dimension, and the local acoustic impedance on the critical temperature gradient of the stack. In particular, it is shown that the impedance determines the proportion between the viscous loss, heat relaxation losses, and the power production from the heat energy. Numerical analysis based on this dimensionless factor clearly shows that there is an optimum channel dimension for each given stack location in the acoustic field. There exists a possible optimum combination of these parameters, which leads to the lowest critical temperature gradient.

PIV Measurement of Coherent Structures and Turbulence Created by an Oscillating Flow at the End of a Thermoacoustic Stack

In thermoacoustic devices, an acoustic wave interacts with a stack of tightly spaced plates either to produce acoustic power, induced by a temperature gradient, or to obtain a refrigeration effect, induced by an acoustic wave. This is based on the thermoacoustic effect whereby appropriately phased pressure and velocity oscillations enable the compressible fluid to undergo a thermodynamic cycle in the vicinity of a solid body. The interaction of the acoustic field with the solid boundary is governed by complex thermo-fluid processes, which are not fully understood, especially for large acoustic excitations (drive ratios Dr>1%, where Dr is defined as the max acoustic pressure divided by the mean pressure) with large fluid displacements [1]. This work is driven by research into the fluid-mechanical aspects of thermoacoustic systems, in particular the identification of the flow morphology and turbulence characteristics in the vicinity of the parallel-plate thermoacoustic stack.