## 2 Pulp Suspensions Imaging Overview
[PERSON] and colleagues [45] used PIV combined with LDA to further understand the mechanisms of plug flow and near-wall behaviour in fibre flow through a pipe at concentrations of up to 4.7% (w/w) with a flow rate of 0.003 m\({}^{3}\)/s; the obtained results showed that the pulp suspension flow in the plug regime can be very inhomogeneous in the region near the pipe wall, particularly at low flow velocities, becoming more structured with an increase in flow velocity. Among the difficult field techniques, X-ray computed tomography is the safest one; it has excellent spatial resolution and image reconstruction is easy, but due to the source and construction of detection components, its operation is slow [9]. X-ray usage for fibre flow characterisation has rarely been reported in the literature; however, flash X-ray radiography has been used to characterise pulp flows similar to those that can be found in the paper industry. Flash X-ray is an X-ray radiography method that uses an intense burst of radiation that is produced for a short time period, making it possible to capture images of high-speed events that are hidden by, for instance, dust, smoke or light [9]. [PERSON] used Flash X-ray [46] in order to image tungsten fibres with a median diameter of 25 \(\upmu\)m (acting as tracers) in a wood fibre suspension with a concentration of 2% (w/w). He was able to asses fibre mass distribution, confirming that this technique could be used for pulp flow characterisation. Later, [PERSON] and co-workers [47; 48] used Flash X-ray to characterise and quantify gas behaviour in several suspensions containing fibres; they used it to visualise air flow in diverse systems, particularly in one composed of air/water/wood fibres, with fibre consistencies of 0.5%, 1% and 1.5% (w/w), while varying the average injection velocity of the air between 0.1 and 12.5 cm/s and maintaining the remaining mixture injection velocity at a constant rate in the pipe. Gamma ray has also been tried for inspection of pulp suspensions, particularly for acquiring density/void fraction information [49]. Gamma ray densitometry [10] uses Gamma radiation, which is an electromagnetic radiation similar to X-rays, but which is produced by the natural decay of radionuclides. It is reliable for the determination of densities, it is inexpensive, and it is portable; however, it does not allow local information to be obtained (the collimated beam of the Gamma ray produces a line averaged value and local information in points of the chord cannot be obtained) [50]. Also, Gamma rays' intensities in the domain are quite dependent on the thickness of the pipe wall, and their accuracy depends on the radiation reaching the detector, which is affected by the material and thickness of the pipe wall [50]. [PERSON] et al. [51] combined Flash X-ray tomography with Gamma ray densitometry to study flow structures, gas holdup, and the geometry of the gas bubbles in a gas-pulp-liquid system; in their work, low-fibre suspension consistencies were used--up to 1.5% (w/w)--and superficial flow velocities of gas and liquid/pulp suspensions in the ranges 0-26 and 21-51 cm/s, respectively. The study of larger-scale systems can be addressed by Electrical Impedance Tomography (EIT), which makes it possible to obtain good images of the distribution of gas-liquid-solid mixtures [52]. Low cost, portability and ease of upscaling are some of the advantages of EIT. However, its spatial resolution is lower than that of other tomographic techniques. This is mainly due to the number of matchless conductivity measurements that can be obtained, which is strongly tied to the current injection-voltage acquisition system protocol and to the number of measuring electrodes [53]. The reconstruction procedure is poorly conditioned and non-linear [53]. Consequently, the amount of fibres or particles present in the suspension cannot be precisely calculated, with the obtained tomographic images representing the distribution of variation in conductance/impedance, rather than absolute values.