Pressure Losses Generated by a Thixotropic Fluid When Subject to an Oscillating Flowrate

Rapid variations in the fluid velocity field influence pressure loss calculations. In this paper, we propose numerical methods for estimating pressure losses in a circular pipe when the flow rate oscillates. The method is described for Newtonian and two non-Newtonian rheological behaviors: the power law and the Quemada models. Also, as drilling fluids are usually thixotropic, i.e., their rheological behavior depends on the shear history, an expansion of the Quemada model is proposed to account for shear history effects. A laboratory flow-loop has been assembled and measurements conducted with a non-thixotropic aqueous solution of Carbopol and a thixotropic potassium chloride solution of xanthan gum. The measurements were analyzed and compared with estimates made with the proposed models. It is found that when applying a square wave oscillating flowrate to a non-thixotropic fluid, large surge and swab pressure spikes are generated. The same square wave signal does not produce pressure spikes when circulating a thixotropic fluid; on the contrary the acceleration and deceleration fronts are largely attenuated. When applying a triangular or sinusoidal wave form to the flowrate while circulating a non-thixotropic fluid, the peak-to-peak pressure gradient gets progressively larger when the oscillation amplitude increases or the signal period reduces, compared to the expected value when estimating the pressure losses with the steady state approximation. However, under the same conditions, when circulating a thixotropic fluid, the peak-to-peak pressure gradients are lower than those estimated with the steady state approximation.

Yielding and structure under flow of gels and glasses of rod-like colloids

Σε αυτή τη διατριβή, ερευνούμε τη δυναμική, τη φασική συμπεριφορά και την γραμμικήκαι μη γραμμική ρεολογική απόκριση κολλοειδών διασπορών ραβδωτών σωματιδίων μεαπωστικές και ελκτικές αλληλεπιδράσεις με έμφαση τις μετασταθείς καταστάσεις υάλων(glasses) και πηκτωμάτων (gels). Χρησιμοποιούμε τη ρεολογία σε συνδυασμό μεδιάφορες οπτικές τεχνικές, συμπεριλαμβανομένης της συνεστιακής μικροσκοπίας για ναδιερευνήσουμε το ρόλο της μικροδομής στις μηχανικές ιδιότητες καθώς και στοφαινόμενο της διαρροής (yielding) των αιωρημάτων ραβδωτών κολλοειδών. Κολλοειδήσωματίδια πυριτίου, μικρομετρικού μεγέθους, σε διάφορες συγκεντρώσεις και διαλύτεςμε διαφορετικές, ελκτικές και απωστικές αλληλεπιδράσεις, χρησιμοποιήθηκαν ως τοβασικό πειραματικό σύστημα-μοντέλο. Η παρούσα διατριβή χωρίζεται σε τέσσερα κυρίωςμέρη. Το πρώτο μέρος συζητά το ρόλο του περιορισμού στη μη γραμμική ρεολογικήαπόκριση υάλων ραβδωτών κολλοειδών με απωστικες αλληλεπιδράσεις. Το δεύτερο μέροςπαρουσιάζει το ρόλο της διατμητικής ιστορίας (pre-shear history) στη γραμμικήρεολογική απόκριση κολλοειδών πηκτωμάτων σφαίρικών ή ραβδωτών κολλοειδών πυριτίουμε ελκτικές αλληλεπιδράσεις. Το τρίτο μέρος της διατριβής εξετάζει το φαινόμενοτης διαρροης πυκνών υάλων απωστικών ράβδωτών κολλοειδών ενώ στο τέταρτο καιτελευταίο μέρος εξετάζεται η σχέση μεταξύ της διάτμησης και του προσανατολισμούτων σωματιδίων σε πυκνά αιωρήματα και υάλους ραβδωτών κολλοειδών που παρουσιάζουνφαινόμενα διατμητικής πάχυνσης (shear thickening)

Flocculation of Clay Suspensions by Anionic and Cationic Polyelectrolytes: A Systematic Analysis

The characteristics of clayey suspensions, majorly composed of quartz microparticles, in the presence of anionic and cationic polyelectrolytes were investigated using different techniques. A wide range of clay concentrations was used, i.e., from 0.07 to 1000 g/L for different experimental techniques, based on the fact that the clay concentration possible to analyze with selected experimental methods was significantly different. The optimum flocculant to clay ratio was defined as the ratio that gives the fastest initial floc growth by static light scattering or fastest initial settling velocity by settling column experiments. In case of anionic polyelectrolyte, it was observed that the optimum flocculant dose depends on the amount of cations present in the system. For suspensions made with demi-water, a lower optimum flocculant dose (<1 mg/g) than for suspensions prepared in tap water (2.28 mg/g) was observed. At these lower salinities, the supernatant remained turbid in all the experiments and was, therefore, not a good measure for optimal anionic based flocculation. The equilibrium floc size at a given shear rate was found to be independent on the shear history of the floc and only dependent on the current applied shear. This was confirmed by both light scattering and rheological analysis. In case of cationic polyelectrolyte, the optimum flocculant ratio (5–6 mg/g) corresponded to the ratio that gives the lowest electrophoretic mobility for each clay concentration and to the ratio that gives the fastest settling velocity for the highest clay concentrations (12–15 g/L), where static light scattering measurements were not possible. All investigation techniques, therefore, proved to be good indicators for predicting the optimum flocculant to clay ratio. For the lowest concentrations (1.75–8.7 g/L) studied by settling column measurements, the optimum flocculant ratio was observed to increase with decreasing clay concentration, for fixed mixing conditions. The optimum flocculant to clay ratio was not always corresponding to the clearest supernatant and the size of flocs at optimum dosage was dependent on the mixing efficiency. The equilibrium floc size at a given shear rate was found to be dependent on the shear history of the floc and the current applied shear. This was confirmed by both light scattering and rheological analysis.

Effect of Different Shear Rates on Particle Microstructure of Cementitious Materials in a Wide Gap Vane-in-cup Rheometer

Rheological properties of cementitious suspensions are affected not only by their mixture composition but also by process-related factors such as shear history. To enable a model-based description, investigations were carried out on the effect of shear history (shear rate variation over time) on the cement paste agglomeration state. Therefore, a Focused Beam Reflectance Measurement (FBRM) system and a wide gap rheometer were coupled to study the relation between shear history and in-situ chord length distribution simultaneously, indicating particle agglomeration. Hence, the effect of average shear rates (resulting from the applied shear profile), as well as shear rate distribution within the gap (local shear rates) on the particle agglomeration state have been investigated. The rheological properties of cement paste were evaluated with the Reiner–Riwlin approach. Furthermore, the agglomeration state of the particles was compared for different average shear rates and local shear rates at various positions of the FBRM probe. The results show that the median chord length increases in all positions when the average shear rate is decreased, indicating increasing particle agglomeration. Moreover, due to variable local shear rates at different FBRM probe positions, different agglomeration states are observed, resulting from two factors, shear rate dependent particle agglomeration and shear-induced particle migration.

The Influences of Cement Hydration and Temperature on the Thixotropy of Cement Paste

The rheological properties of fresh cement paste are highly influenced by a large number of parameters, among which the most important factors are the applied shear stress, and the shear history, the age of the sample and the temperature. The effects of these parameters on the yield stress (designated as structural limit stress in this work), the viscosity and the structural recovery rate (i.e., the change in dynamic viscosity with time at rest) were studied. In parallel, the changes in ion composition of the carrier liquid, mineral phase content and granulometry were investigated. The results reveal that all investigated rheological parameters exhibit an approximated bi-linear trend with respect to the degree of hydration, with a period of quasi-constant properties until a degree of hydration of approximately 0.07, followed by a non-linear increase. This increase could be attributed to the formation of calcium hydroxide (CH) and calcium-silicate-hydrate (C-S-H) via calorimetry results. With regard to the effect of the shear history of the sample on the rheological properties, the structural limit stress showed a minor dependency on the shear history immediately after the end of shearing, which, however, vanished within the first minute at rest. The same is true for the structural recovery rate. The presented results give detailed insights into the influences of hydration and shear on the rheological properties—especially the thixotropy—of fresh cement pastes.

Shear thinning and thickening of cemented paste backfill

Cemented paste backfill (CPB) is considered to be a concentrated suspension in which tailings are bonded together by the hydraulic binder and water, and it has a high solid volume concentration (≥50 vol.%). Although the shear thinning and thickening of CPB has been extensively reported in literature, the shear history effects have been ignored in previous studies. In this paper, by using rheometer and Focused Beam Reflectance Measurement, the relationship between the rheological properties and microstructure of the paste under different shear histories was studied. The results have shown that at a low shear rate, CPB revealed shear thinning, low yield stress and low index parameters; while exhibited shear thickening, high yield stress and high consistency index when at high shear rates of shear history. This agreed with the general trends shown in the FBRM analysis. It was proposed that the action of shear is beneficial to particle dispersion, whereas a high shear rate history tends to promote the aggregation of particles. It was revealed that both shear thinning and thickening of paste are related to the situation of particles (flocculation, dispersion and aggregation), and shear history effects play an important role in rheological properties of CPB.

Differential Scanning Calorimetry contribution to a better understanding of the aging of gelled waxy crude oils

Below Wax Appearance Temperature (WAT), waxy crystals appear within the crude oil and make it viscous with yield stress and shear thinning properties. Particular attention has been paid during the past works on different parameters such as temperature, pressure, shear history, etc. Another important parameter is the holding time of the sample once it has gelled and left at rest under isothermal conditions. Actually, the network of waxy crystals is known to change with time. This phenomenon has been particularly observed in the case of deposit that is expected to harden. The set of rheological tests and calorimetric analysis that were performed on a real waxy crude oil confirm that a gel formed with waxy crystals may evolve with time and that the extent of this phenomenon depends on the thermo-mechanical past of the sample. Actually, a strengthening of the gel during holding time is observed in the case of fast cooling because the sample gets supersaturated. Aging takes place through isothermal crystallization that lasts a few minutes. The results have also shown that slowly cooled samples do not lead to any aging.

Experimental difficulties often encountered with sludge rheological properties determination and advices to perform reliable measurements

Rheological parameters being of great importance for sludge process management, they are increasingly studied. However, experimental procedures may strongly impact their determination. Sample volume, measuring device depth and roughness but also mechanical history, have to be well-controlled to ensure reproducible results. Indeed, even if shear history can be erased with a sufficient preshear for diluted sludge, this paper clearly established that no steady state can be achieved for concentrated sludge. The longer the shear history, the lower the rheological characteristics: Reproducible results are hardly obtained. More importantly, slippage appeared to occur even with surfaces of moderate roughness and the phenomenon is all the more important that the dry matter is high. From all these observations, an experimental procedure based on the control of preshear and rest periods has been defined. Advices and precautions to observe are given in order to ensure reproducible and obtain unaltered results.