N Terzija

Use of electromagnetic induction tomography for monitoring liquid metal/gas flow regimes on a model of an industrial steel caster

Monitoring of the steel flow through the submerged entry nozzle (SEN) during continuous casting presents a challenge for the instrumentation system because of the high temperature environment and the limited access to the nozzle in between the tundish and the mould. Electromagnetic inductance tomography (EMT) presents an attractive tool to visualize the steel flow profile within the SEN. In this paper, we investigate various flow regimes over a range of stopper positions and gas volume flow rates on a model of a submerged entry nozzle. A scaled (approximately 10:1) experimental rig consisting of a tundish, stopper rod, nozzle and mould was used. Argon gas was injected through the centre of the stopper rod and the behaviour of the two-phase GaInSn/argon flow was studied. The experiments were performed with GaInSn as an analogue for liquid steel, because it has similar conductive properties as molten steel and allows measurements at room temperature. The electromagnetic system used in our experiments to monitor the behaviour of the two-phase GaInSn/argon flow consisted of an array of eight equally spaced induction coils arranged around the object, a data acquisition system and a host computer. The present system operates with a sinusoidal excitation waveform with a frequency of 40 kHz and the system has a capture rate of 40 frames per second. The results show the ability of the system to distinguish the different flow regimes and to detect the individual bubbles. Sample tomographic images given in the paper clearly illustrate the different flow regimes.

New Static “AirArc” EMTP Model of Long Arc in Free Air

In this paper, long arcs in free air are investigated. A long arc was induced in the high-power test laboratory at FGH-Mannheim, Germany. From the recorded arc data, the features of the arc were derived and analyzed, including elongation effects associated with these arcs. Based on this analysis of a real arc, a new “AirArc” static arc model, including the length variations, was derived and incorporated into the Alternate Transients Program-Electromagnetic Transients Program software package in which it was simulated. The arc is modeled as a current-dependent voltage source with a characteristic-distorted rectangular voltage that elongates nonlinearly over time. Fault arcs are a significant source of harmonics that distort other voltages and currents in the network. The time-domain and spectral-domain features of the simulated arc are compared with those of the real arc. The assessment of transient processes during arcing faults was carried out using the discrete wavelet transform.

Wavelet-Based Image Reconstruction for Hard-Field Tomography With Severely Limited Data

We introduce a new wavelet-based hard-field image reconstruction method that is well suited for data inversion of limited path-integral data obtained from a geometrically sparse sensor array. It is applied to a chemical species tomography system based on near-IR spectroscopic absorption measurements along an irregular array of only 27 paths. This system can be classified as producing severely limited data, where both the number of viewing angles and the number of measurements are small. As shown in our previous work, the Landweber iteration method allows stable solution of this tomography problem by incorporating suitable a priori information. In the new method, a 2-D discrete wavelet transform has been used as a smoothing function. We present a method of designing the optimal wavelet-based smoothing function, depending on a priori knowledge of the subject. The significance of the particular wavelet filter selected is considered in terms of the accuracy of reconstruction of the spatial location and shape of the gas distribution. Results are presented for simulated phantoms using different sensor arrays and for experiments with propane plumes, showing excellent spatial localization and quantification. The computational time of the iterative algorithm is significantly reduced by applying the wavelet transform method. Some of our conclusions are applicable to other hard-field tomographic modalities in applications where similar constraints may be encountered.

Tomographic imaging of the liquid and vapour fuel distributions in a single-cylinder direct-injection gasoline engine

This article reports the application of optical tomography and chemical species tomography to the characterisation of the in-cylinder mixture preparation process in a gasoline, direct-injection, single-cylinder, motored research engine. An array of 32 near-infrared beams is transmitted in a horizontal plane across the cylinder bore near the top of the cylinder, through a circular quartz annulus. A novel approach to enable the optical alignment of the transmitting and receiving optics is utilised. The engine is operated at a stoichiometric condition at 1200 r/min, with negative valve overlap timing. Two tomographic measurement schemes (optical attenuation and chemically specific absorption) were used to acquire data on the spatial and temporal distribution of fuel throughout the engine cycle. Optimised data pre-processing methods are described for maximal beam count and data reliability. The presence of fuel during the intake stroke was detected by the optical beam attenuation due to scattering from the liquid gasoline droplets. Optical tomographic reconstruction of the spatial distribution of these droplets was achieved at an imaging rate of 7200 frames per second, revealing rapid intra-cycle spatial variations that were consistent between consecutive cycles. During the compression stroke, chemical species tomography images of fuel vapour were reconstructed from data acquired using chemically selective spectral absorption by the hydrocarbon molecules, at an imaging rate of 2400 frames per second. Later in the compression stroke, the temporal evolution of the fuel vapour distribution in the plane of observation is relatively slow and displays inhomogeneities that are consistent between consecutive cycles. This is the first report of the use of tomography to image, within individual engine cycles, the in-cylinder evolution of both fuel spray droplet distribution and fuel vapour distribution.

Image optimization for chemical species tomography with an irregular and sparse beam array

High-speed tomographic imaging of hostile engineering processes using absorption-based measurements presents a number of difficulties. In some cases, these challenges include severe limitations on the number of available measurement paths through the subject and the process of designing the geometrical arrangement of these paths for best imaging performance. This paper considers the case of a chemical species tomography system based on near-IR spectroscopic absorption measurements, intended for application to one cylinder of a multi-cylinder production engine. Some of the results, however, are also applicable to other hard-field tomographic modalities in applications where similar constraints may be encountered. A hitherto unreported design criterion is presented for optimal beam geometry for imaging performance, resulting in an irregular array with only 27 measurement paths through the subject for the engine application. Image reconstruction for this severely limited geometry is considered at length, using both simulated and experimental phantom data. Novel methods are presented for the practical generation of gaseous phantoms for calibration and testing of the system. The propane absorption coefficient at 1700 nm is measured. Quantitative imaging of propane plumes in air is demonstrated, showing good localization of circular plumes with diameter as small as 1/5 of the subject diameter and excellent imaging of multiple plumes.