LDA Measurements on a Turbulent Gas/Liquid/Fibre Suspension

Abstract

T he three-phase system gas/liquid/fi bre suspension is of great importance in many industrial systems, particularly in the manufacture of pulp. Both separating a gas phase from a fi bre suspension and mixing them are important processes. Pulp fi bre processing is commonly carried out in the medium-consistency (MC) range where the mass concentration of the suspension is between 0.03 and 0.15. MC technology has the potential to decrease not only water and energy consumption but also equipment dimensions. This is of particular importance in connection with the closing of mills. The disadvantages are associated with the more diffi cult processing of the fi bre suspension in operations such as screening, pumping, washing and mixing.The accumulation of gas in centrifugal pumps was one of the fi rst problems encountered when MC technology was introduced (Gullichsen and Härkönen, 1981). Centrifugal forces in the pumps caused an effi cient phase separation and accumulated gas around the pumping wheel prevented further operation. The problem was solved early with vacuum pumps removing excess air from the centre of the pump. Flotation de-inking of wastepaper involves the use of air bubbles to remove hydrophobic particles from wastepaper slurries. Ideally, a homogeneous system would provide more effi cient fl otation but bubble sizes and rise velocities are also very important. Radiographic images reveal that gas fl ow characteristics are substantially altered in the presence of fi bres (Heindel and Monefeldt, 1998).In pulp bleaching operations, gaseous reagents are often mixed into pulp suspensions. The introduction of ozone bleaching in particular has, however, caused complications. The reaction is extremely fast, which makes good gas dispersion essential. Poor dispersion leads to low selectivity, producing unevenly bleached fi bres and degraded fi bres. Byrd et al. (1992) review the literature of ozone bleaching and delignifi cation. When gases are mixed into pulp suspensions, it is important to achieve good mixing over the macro-scale, fi bre-scale and microscale. This ensures the uniform distribution of reagents throughout the suspension and leads to uniform bleaching. Bennington (1993) was the fi rst to make a thorough investigation of the mixing of gases into MC pulp suspensions. Using a high-speed video system, he identifi ed six different fl ow regimes, which were correlated to the power input, pulp consistency and volume fraction of the gas phase. Most of these regimes are less turbulent than is the case in this work. As far as is known, however, no detailed fl ow characterization has been performed on this three-phase fl ow system. Detailed fl ow measurements on a turbulent fi bre suspension (without gas) by Laser Doppler Anemometry (LDA) were reported in earlier work by Andersson and Rasmuson (2000). The measurements were performed on a refractive-index matched glass-fi bre suspension in the 3-20% wt. range. It was found that the liquid fl ow of such suspensions underwent two transitions: the Detailed continuous...

A theoretical model of turbulent fiber suspension and its application to the channel flow

“…is the apparent viscosity of the suspension, μ = f ( ) 3 6 ln 2 n l r μ π [18] with μ being the viscosity of the suspending fluid, n the number of fibers per unit volume, l the fiber half-length and r the fiber aspect-ratio. kl a and ijkl a are the second-and fourth-orientation tensor of fiber [19,20] : …”

“…The presence of the fibers also flattened the profiles, indicating an increased momentum transfer. Pettersson and Rasmuson [3] performed detailed measurements of mean and RMS velocities by LDA of a turbulent gas/fiber/liquid suspension in a rotary shear tester. Plotting RMS and mean velocities versus impeller speed and power input indicates that both decrease with increasing gas and fiber contents.…”

The Yield Stress of Gas/Liquid/Fibre Suspensions

“…The velocities that were used for mixing were clearly above the second point of transition introduced by Andersson and Rasmuson (2000) and discussed by Pettersson and Rasmuson (2004). Another rheometer was, therefore, used for comparison.…”

“…The shear tester has been used as a mixing vessel in earlier studies Rasmuson, 2000, Pettersson andRasmuson, 2004) and is described in detail in Andersson and Rasmuson (2000). It is a hexagonal vessel with six baffl es and a six-bladed impeller (see Figure 1).…”

“…Glass fi bres were mainly used, which provided the possibility to visually observe most parts of the suspension. The same glass fi bres were used in earlier studies of the fl ow dynamics of fi bre suspensions (Pettersson and Rasmuson, 2004). In the present study, obtaining a well mixed stable pulp suspension with gas was found to be diffi cult.…”