Evaluating viscoplastic properties with rheometry and PIV measurements in pipeline flows

Abstract

Non-Newtonian fluids, widely utilized in industries such as cosmetics, food processing, and petroleum, exhibit shear-dependent viscosity, necessitating precise rheological characterization for effective pipeline and equipment design. In the petroleum industry, for instance, oils can transition from Newtonian to non-Newtonian behavior under specific conditions, such as long-distance horizontal flow at high pressures and low temperatures (near crystallization). In these cases, oils often behave as viscoplastic fluids, requiring a minimum shear stress, known as yield stress, to initiate flow. The Herschel–Bulkley model is a well-established equation for describing the viscous behavior of such fluids through three rheological parameters: yield stress, power-law index, and consistency coefficient. The determination of these parameters is essential for computing flow characteristics, friction factors, and pressure drops—crucial for designing efficient transport systems. This study aims to characterize a viscoplastic fluid by determining its rheological properties from rheometric and in-situ measurements. To accomplish this, an experimental setup was developed using a model fluid prepared from an aqueous Carbopol and triethanolamine (neutralizing agent) solution. In addition to traditional rheometry, an in-situ approach was evaluated, integrating Particle Image Velocimetry (PIV) with differential pressure sensor data. The velocity profiles obtained enabled the reconstruction of shear rate profiles, while pressure drop data facilitated shear stress profile determination, allowing a flow curve reconstruction. Furthermore, the modified SoFA model (Suspension of Fractal Aggregates) was applied, utilizing Carbopol and Triethanolamine concentrations to estimate the rheological parameters and obtain the corresponding flow curve. A comparative analysis was conducted between serrated parallel-plate rheometry and the PIV–pressure drop method in a commercial 2 inch (0.053 m) pipeline under laminar flow of the aqueous Carbopol solution. The results confirmed that the Herschel–Bulkley model effectively fit the flow curves across all methodologies, with yield stress values deviating by less than 15%. However, consistency indices ($K$) obtained from PIV data were overestimated, likely due to the limited shear rate range at the low mean velocities tested. This study highlights the importance of integrating traditional rheometry with in-situ techniques for a comprehensive rheological characterization.