Mônica F. Naccache

Heat transfer to non-Newtonian fluids in laminar flow through rectangular ducts

Abstract Heat transfer to non-newtonian fluids flowing laminarly through rectangular ducts is examined. The conservation equations of mass, momentum, and energy are solved numerically with the aid of a finite volume technique. The viscoelastic behavior of the fluid is represented by the Criminale-Ericksen-Filbey (CEF) constitutive equation. Secondary flows occur due to the elastic behavior of the fluid, and, consequently, heat transfer is strongly enhanced. It is observed that shear thinning yields negligible heat transfer enhancement effect, when compared with the secondary flow effect. Maximum heat transfer is shown to occur for some combinations of parameters. Thus, there are optimal combinations of aspect ratio and Reynolds numbers, which depend on the fluids mechanical behavior. This result can be usefully explored in thermal designs of certain industrial processes.

Heat transfer to viscoplastic materials flowing laminarly in the entrance region of tubes

Abstract Heat transfer in the entrance-region flow of viscoplastic materials inside tubes is analyzed. The flow is laminar and the material viscosity is modeled by the Herschel–Bulkley equation. The conservation equations are solved numerically via a finite volume method. Two different thermal boundary conditions are considered, namely, uniform wall temperature and uniform wall heat flux. The effect of temperature-dependent properties is also investigated. The Nusselt number is obtained as a function of the axial coordinate, yield stress, and power-law exponent. Results show that the same trend is observed for the two thermal boundary conditions, but the Nusselt numbers are always higher for the isoflux-wall cases. The length of the entrance region decreases as the material behavior departs from the Newtonian one. Finally, it is observed that neglecting the temperature dependence of material properties may introduce important errors in the heat transfer coefficient.

Heat transfer to viscoplastic materials flowing axially through concentric annuli

Abstract Heat transfer in the entrance-region laminar axial flow of viscoplastic materials inside concentric annular spaces is analyzed. The material is assumed to behave as a generalized Newtonian liquid, with a modified Herschel–Bulkley viscosity function. The governing equations are solved numerically via a finite volume method. Two different thermal boundary conditions at the inner wall are considered, namely, uniform wall heat flux and uniform wall temperature. The outer wall is considered to be adiabatic. The effect of yield stress and power-law exponent on the Nusselt number is investigated. It is shown that the entrance length decreases as the material behavior departs from Newtonian. Also, it is observed that the effect of rheological parameters on the inner-wall Nusselt number is rather small.

Crossflow of viscoplastic materials through tube bundles

Abstract The flow of viscoplastic materials through staggered arrays of tubes is analyzed. The mechanical behavior of the materials is assumed to obey the generalized Newtonian liquid (GNL) model, with a viscosity function given by the biviscosity law. The governing equations of this flow are solved numerically using a finite-volume method with a non-orthogonal mesh. For a representative range of the relevant parameters, results are presented in the form of velocity, pressure and viscosity fields. The pressure drop is also given as a function of rheological and geometric parameters.

Flow of elasto-viscoplastic liquids through a planar expansion-contraction

The steady flow of incompressible elasto-viscoplastic liquids through a planar expansion–contraction is investigated. A novel constitutive model is employed to describe the mechanical behavior of the flowing liquids. Numerical solutions of the constitutive and conservation equations were obtained via a finite element method to investigate the role of elasticity, yield stress, and inertia. The fields of velocity, stress, elastic strain, and rate of strain were obtained for different combinations of the governing parameters. It was observed that these fields, as well as the shape and position of the yield surface, are all strong functions of elasticity, yield stress, and inertia. The trends observed agree well with previous numerical and visualization results available in the literature. The present work offers a detailed study on the effects of elasticity, presenting, in particular, the fields of elastic strain.

Identification of non-newtonian rheological parameter through an inverse formulation

In this work, we introduce an inverse formulation to be applied in the identification of a rheological parameter associated to non-Newtonian fluids. It is built upon a creeping flow through a 4 to 1 axisymmetric abrupt contraction. The fluid is modeled by the Generalized Newtonian Fluid constitutive equation. The viscosity function is based on the one proposed by Souza Mendes et al. (1995). It predicts an extensional elastic behavior, controlled by a rheological parameter q , which is the parameter determined via the proposed identification procedure. The numerical solution of the forward problem, needed in the iterative procedure introduced by the inverse formulation, is obtained through the finite volume method. A sensitivity analysis is also performed to evaluate the effect of the parameter q on the dimensionless pressure drop through the contraction. The optimization algorithm is based on an iterative method to find the minimum of the cost function, which is given by the least square difference between numerical and experimental values of the dimensionless pressure drop. The gradient method was used to update the parameter q , starting from the cost function gradient. The results obtained with the sensitivity analysis validated the adequacy of the proposed cost function, which is a key aspect on the identification formulation. Moreover, it shows that the method provides an attractive alternative for estimation of rheological properties.

Influence of the Type of Oxidant in the Combustion of Natural Gas Inside an Aluminum Melting Furnace

The fuel used as energy source for aluminum melting is of extreme importance for a better performance of the process. However, the type of oxidant can also lead to better performance, leading to a greater preservation of the equipments. Air is more abundant and cheaper, however due to the presence of nitrogen, there is undesirable NOx formation. An alternative is to employ pure oxygen. Although it is more expensive, it can lead to a cleaner and much more efficient combustion process, by significantly altering the combustion aspects inside the furnace, such as the shape of the flame and the distribution of temperature and heat flux. In the present work, numerical simulations were carried out using the commercial package FLUENT, analyzing different cases with pure oxygen and air as the oxidant for the combustion of natural gas. The results showed the possible damages caused by the process if long or too intense and concentrated flames are present.© 2006 ASME

Rising bubbles in yield stress materials

We present an experimental investigation of bubbles rising in elasto-viscoplastic media. The effects of medium rheology and bubble size on the bubble velocity and shape are investigated. In addition, a discussion regarding the influence of the stress history on the bubble path is presented. Experimental tests are performed using yield stress materials which also present elastic behavior. Despite the difficulty in determining the relative importance of the different forces that are present in this flow, the results obtained illustrate the effect of yield stress, inertia, elasticity, and buoyancy on the dynamics and shape of the bubble.

Combustion performance of an aluminum melting furnace operating with liquid fuel

The characteristics associated with the delivery of the fuel to be used as the energy source in any industrial combustion equipment are of extreme importance, as for example, in improving the performance of the combustion process and in the preservation of the equipment. A clean and efficient combustion may be achieved by carefully selecting the fuel and oxidant, as well as the operational conditions of the delivery system for both. In the present work, numerical simulations were carried out using the commercial code FLUENT for analyzing some of the relevant operational conditions inside an aluminum reverb furnace employing liquid fuel and air as the oxidant. Different fuel droplets sizes as well as inlet droplet stream configurations were examined. These characteristics, associated with the burner geometry and the fuel dispersion and delivery system may affect the flame shape, and consequently the temperature and the heat flux distribution within the furnace. Among the results obtained in the simulations, it was shown the possible damages to the equipment, which may occur as a result of the combustion process, if the flame is too long or too intense and concentrated.

Enhanced Fluid Rheology Characterization for Managed Pressure Drilling Applications

Silva, Thiago Pinheiro da; Naccache, Mônica Feijó (Advisor). Enhanced fluid rheology characterization for Managed Pressure Drilling. Rio de Janeiro, 2016. 114p. MSc. Dissertation – Departamento de Engenharia Mecânica, Pontifícia Universidade Católica do Rio de Janeiro. Enhanced fluid rheology characterization for Manage Pressure Drilling. Hydraulics play an important role in many oil field operations including drilling, completion, fracturing, acidizing, workover and production. In Managed Pressure Drilling (MPD) applications, where pressure losses become critical to accurately estimate and control the well within the operational window, it is necessary to use the correct rheology for a precise mathematical modelling of fluid behavior. The standard API methods for drilling fluid hydraulics employ Herschel-Bulkley (HB), Power Law (PL) or Bingham plastic as rheological models. This work summarizes the results of an extensive study on issues and relevant aspects related to the equipment and methods used to characterize the drilling fluids for MPD applications, as well as the operational implications that diverge from conventional practices. A comparison of fluid rheology characterization is made using high precision rheometers versus conventional FANN35 methods. Subsequently, a comparison of rheology model selection proposed by API 13B and by Non Linear Regression (NLR) is presented. Further investigation of shear rate ranges is presented in a MPD “typical” annular geometry. Results obtained via Computational Fluid Dynamics (CFD), and with the formulas suggested in API RP 13D are compared. To conclude, the effects of measurements, data treatment (Curve Fit), and environment (laboratory observations versus field experiences) in the accuracy of fluid rheology characterization and annulus pressure loss estimation are presented and discussed.