Contents I. Introduction
Two-phase flow imaging is a new technique developed rapidly in recent years, which has great developmental potential and wide industrial application prospect [1]. One kind of attractive techniques to identify flow regime is the tomographic technique, which is to obtain an image of a predetermined plane of the investigated object, i.e., identification of a distribution within the physical process. Practically, this is done by a number of sensors being mounted around the object to be imaged, which ideally should be nonintrusive and noninvasive. However, the tomography technique, particularly in process engineering, is of limitation in the number of sensors arranged and consequently in the relative lower spatial resolution due to the limited integral measurement data obtained. There are many kinds of process tomography techniques, such as X-ray, -ray, electrical, optical, ultrasonic, and magnetic resonance imaging [2]. Among them, computerized tomography (CT) and electrical capacitance tomography (ECT) are regarded as the two types of the most attractive techniques. The basic principle of ECT is to measure the capacitance between electrode pairs. The sensitivity field for ECT is nonlinear; the sensitivity distribution inside the field depends on the measured media, i.e., it has the property of “soft field.” As a result, it has a low spatial resolution, although it can be very fast for a flow measurement. The basic principle of CT is to measure the attenuation of the intensity of the radiation described by the Beer–Lambert law. The sensitivity field is not influenced by the distribution of the components in the process being imaged, i.e., it has the property of “hard field.” Compared with the ECT technique, CT provides higher spatial resolution up to 1% of a column diameter and even more details at the scale of 200–400 [2]. Generally, CT imaging refers to X-ray and gamma-ray sources. The distinctions between them are mainly based on the origins of the radiation rays. Gamma rays are originated from nuclear processes propagating in free space and have wavelengths shorter than . The discrete energy spectrum of gamma rays provides a stable monochromatic photon beam. Thus, gamma-ray CTs enable better phase resolution than X-ray CTs, and the reconstruction process is simplified. X-rays have typical wavelengths in the range from to . They are produced either as discrete wavelengths during individual transitions among the inner most tightly bound electrons of the atom or in the process of retardation of charged particles. Although X-rays have greater intensity than gamma rays, they have a continuous energy spectrum with lower penetration capability and are always needed for the expensive and complicated high-voltage power supplies. In recent years, the CT technique has been acknowledged as a useful and important technique in the study of multiphase flows [2], [6]–[8], including gamma densitometries and gamma-ray/X-ray CTs. Standard gamma densitometry measures along only one path for obtaining the density of the material inside the volume; the phase distribution in the multiphase flow cannot be obtained. However, spatially resolved measurements can be made by applying tomographic reconstruction algorithms to the results of measurements along many different paths.
