Volume 21, Issue 2

The composition of high concentration ash slurry requires careful selection of particle size distribution (PSD) to achieve the required rheological properties for efficient disposal through pipelines. In the present study, the maximum static settled concentration, C W-max , tests and rheological measurements were carried out for a total eighteen nos. of ash samples (nine nos. of fly ash samples and nine nos. of fly ash-bottom ash mixture samples) in the concentration range of 60 - 70 wt%. It was observed that the C W-max value reached maximum for the composition of mixture slurry consisting of fly ash and bottom ash with d50 as 6.4 mm and 144 mm respectively at a fixed blend ratio (weight ratio of fly ash to bottom ash) of 4:1. This was attributed to the packing effect and was correlated to the ratio of surface to surface separation for the coarse bottom ash particles, b, to the average fly ash particle size, d 50-f , to achieve higher solids concentration. The rheological behaviour of the ash slurry samples were described by non-Newtonian power law model in the range of solids concentration studied. Also a substantial reduction in viscosity was observed for the same composition of mixture slurry sample which was attributed to the poly-dispersive characteristics of the ash particles. It was indicated that the slurry viscosity was very much influenced by particle size ratio (l = d large /d small ) and volume fraction of solids. The study revels that the blending of fly ash and bottom ash at a controlled PSD may be employed for preparation of high concentration ash mixture slurry for pipeline transport.

In this paper, the role of extensional viscosity in different paper coating processes was evaluated. Numerical models for the various coating application processes (blade coating, film coating, and curtain coating) were constructed to calculate the extension rate. Different rheological methods were employed to measure extensional viscosity versus extension rate for model coating colour systems. Pilot coater trials were carried out to study the performance of each model coating colour in curtain, blade and film coating. It was demonstrated that extensional viscosity of model coating colours depends on the rate of extension, and colours can be extension thickening or extension thinning. With the numerical calculation results of extension rate in coating application processes, the extensional viscosity test results of model coating colours were matched to their performance in pilot coater experiments. It was shown that increasing the extensional viscosity measured at the appropriate extension rate that exists in blade or rod metering increased blade or rod load. Misting in film coating was reduced by low extensional viscosity measured at the appropriate extension rate. Cratering in curtain coating was reduced by increasing extensional viscosity measured at the appropriate extension rate. It was further concluded that the extensional viscosity can be used to predict the coating performance only if it is measured at the prevailing extensional rate of the specific coating process speed.

This paper presents an experimental facility that allows simultaneous viscosimetric and Particle Image Velocimetry measurements on concentrated suspensions in a wide-gap Couette rheometer. The experimental procedure is detailed: the optical characteristics of the index-matched suspension are carefully studied, the bottom end effect on both the viscosimetric measurements and the recorded velocity profiles are analysed. First the experimental procedure is tested on a Newtonian fluid whose viscosity is known. The spatial and time resolutions of our device are shown to be 200 mm and 100 ms. The precision of the local viscosity measurement is evaluated to better than 4 %. Then we show that the device can be used to characterize the rheological behaviour of a 47 %- concentrated suspension of 30 mm spheres. According to the particles large size, the Brownian motion can be neglected. However, colloidal interaction are still noticeable.

The rheological parameters (elastic modules and the yield stress) of binary mixtures of highly concentrated emulsions with different droplet sizes can be several times lower than additive values in a certain range of concentration. This is related to the proper packing of small droplets between larger ones without compression of droplets. While the yield stress is practically absent for these uncompressed droplets, the rather high storage modulus demonstrates the significance of interdroplet interaction in this system.

This work takes a phenomenological approach to modeling the rheology of polymer/clay nanocomposites in ẏ ≤ 1 s-1 based on experimental observations [10]. The total stress was divided to three contributions: Matrix stress, ỡ M , inter-particle (matrix/particle) stress, ỡ P , and hydrodynamic stress ỡ p' . Based on the superposition of complex viscosities, h*, plotted against strain rate amplitude, g0w, at different nonlinear strain amplitudes, a modified Bingham-type constitutive equation proposed by Doiraswamy et. al [16] was used to model ỡ M + ỡ P while ỡ H was modeled by using constitutive equation proposed by Lipscomb et. al [25] for ellipsoidal particles. The comparison between experimental and modeling results showed that steady hydrodynamic stress in simple shear flows scales with complex viscosities in oscillatory experiments when compared at ẏ= y o w. On the basis of this observation, the network-like behavior of the polymer nanocomposite was attributed to retarded chain dynamics as a result of polymer/clay interactions. In order to take into account the thixotropic behavior of network structure, the constitutive equation proposed by Coussot [18] was employed for modeling ỡ M + ỡ P . Both Coussot and Doraiswamy equations gave a reasonable quantitative prediction of transient stress in simple shear flow up to shear rates as high as ẏ = 0.1 s -1 .