Corneliu Balan

Influence of the Geometry on the Transient and Steady Flow of Lubricating Greases

The objective of this paper is to investigate the influences that geometry, roughness of the sensor system and gap size exert on the rheological measurements of a commercial lubricating grease. With this aim, creep and stress growth experiments, stepped strain rate ramps and dynamic measurements were performed using different geometries and roughness. A significant influence of gap size was found in transient and steady-state experiments in a certain range of strain rate. Roughening the surface of the geometry increases considerably the values of the stress in comparison to the measurements performed with smooth surfaces. The experimental results are discussed taking into account the wall slip phenomenon. In addition to this, different grooves made in the surfaces of the tools seem to eliminate or considerably decrease these wall slip effects. However, the influence of geometry or roughness was not detected in oscillatory tests performed inside the linear viscoelastic range.

Experimental procedure for the evaluation of the friction energy of lubricating greases

Abstract The paper is concerned with the study of a tribological system characterised by fluid friction. The main parameter of the system is the density of the friction energy, a physical quantity which is put in a direct relation with the viscous dissipation function of the lubricant. The lubricant under study is grease, a viscoelastic material which exhibits a yield stress at low strain rates. For this type of material, the shear stress function has a relevant plateau behaviour previous to the beginning of the flow. The rheological characteristics of the lubricant are determined in simple shear motions, both in creeping and stressing modes. The specific energy is graphically computed for these corresponding non-steady shear motions. Below the yield shear stress, an important time variation of the specific energy has been observed. This result is considered to be useful information on the non-steady tribological regime, implicit in the relative intensity of the associated wear process.

Rheological Behavior of Poly[(B‐alkylamino)borazine] in a Fiber Spinning Process

Polymer‐derived ceramics (PDCs) are innovative materials with a wide range of novel applications (micro fibers, protective coatings, porous materials, MEMS). Among high‐performance, non‐oxide ceramics, hexagonal boron nitride offers interesting potentialities as fibrous reinforcing agent for ceramic composites. Boron nitride can easily be produced from preceramic polymers and previous studies have shown the great potential of poly[(B‐methylamino)borazines], called polyMAB as melt‐spinnable precursors for the preparation of this type of ceramics. The goal of the present study is to provide a comprehensive structural and rheological characterization of polyMAB‐type polymers using a combination of thermal, structural, and chemical experiments, as well as rheological investigations and constitutive modeling, in order to predict and control polymer spinnability from the early stage of material formation. The results from dynamic shear rheology are consistent with the rheological behavior observed in spinning. The experimental measurements of the fiber diameter during steady spinning put in evidence the difference in extensional rheological behavior and spinnability between samples. Numerical simulations of 1D model of fiber spinning emphasis the importance of heat transfer at the exit from the die and relevant differences for the polymers under investigation in the evolution of spin extensional viscosity along the fiber. The present work evidences for the first time the rheological behavior of polyMAB samples and the link to their chemistry, especially in relation to the fiber spinning process.

A finite deformation formulation of the 3-parameter viscoelastic fluid

Abstract The paper addresses, with reference to the 3-parameter fluid, a method to extend constitutive laws for viscoelastic fluids from small to finite deformations. It relies upon the multiplicative decomposition of the deformation gradient tensor into elastic and inelastic parts. Also care is taken that the second law of thermodynamics is satisfied in every admissible process. To demonstrate the capabilities of the model obtained, simple shear and uniaxial extensional flow are discussed. It turns out that essential effects of material behaviour, which have been observed experimentally, can be predicted by the model.

A New Approach to Determine the Nonlinear Parameter of the Giesekus Constitutive Model

Considering the wealth of information that Fourier‐transform rheology provides, we apply it for the determination of the nonlinear parameter of the Giesekus equation. Instead of using the shear‐thinning as the representative nonlinear effect, we use the higher harmonics in the Fourier spectrum of the shear stress signal. The fluid is subjected to large amplitude oscillatory shear (LAOS) and its response is recorded. Deviations of this signal from the sinusoidal form are specific to each material. By fitting the material response, we can extract the values of the nonlinear parameter that best describe the fluid. It is also demonstrated that this method provides better results than the traditional viscosity‐fit method. The procedure is applied to semiconcentrated polymer (polyacrylamide) solutions.

Heat transfer coefficient measurements using infrared thermography technique

The measurement and modeling of surfaces temperature at the impact with fluid jet is of great importance for many engineering applications. The goal of the present study is to develop an experimental procedure to determine the convection heat transfer coefficients in natural and forced flows in the vicinity of a vertical surface. An infrared camera FLIR Therma-Cam SC 3000 is used for the surface temperature evaluation, in parallel with the temperature measurements using a set of thermocouples. A vertical electrically heated disk will be the configuration for this study. A cold air jet is directed perpendicular to the surface, the corresponding Reynolds number and the distance from the wall being changed in order to investigate their influence on the measured heat transfer coefficient.

The role of human saliva on the elongational properties of a starch-based food product

In the medical context of swallowing disorders, human saliva may play an essential role, affecting the rheological properties of starch-based thickened products, due to the effect of the alpha-amylase, a protein enzyme naturally present in saliva. The aim of this work was to compare the effect of three different salivary solutions on the uniaxial properties of a starch-based food product (SBFP). Extensional rheology properties of SBFP after addition of solution containing a commercial alpha-amylase, mechanically stimulated saliva, or in-vivo stimulated whole human saliva, were measured by using a CaBER (Capillary Break-up Extensional Rheometer) device. Extensional and oscillatory shear results for the different types of salivary solutions were then analysed. These results showed that in-vivo stimulated whole human saliva provides an increased SBFP elasticity, while the alpha-amylase dramatically decreases its viscoelastic functions in oscillatory shear. The results obtained also suggest that mechanically stimulated saliva and alpha-amylase aqueous solutions do not offer adequate information concerning the real extensional properties of the bolus when blended with whole human saliva.

Coalescence of liquid drops with free surfaces

The present study investigates the coalescence phenomenon of pendant drops in the presence of liquid free surfaces. The experimental setup is composed of a glass cell placed on a lab-jack and a needle connected to a micro-syringe which controls the volume of the droplet. We acquired videos over the dynamics of the phenomenon with a fast CMOS camera attached to an objective with a microscopic magnification. Depending on the fluids properties (in our case water, respectively solutions of glycerin in water) and the distance between the needle and the fluid surface, the coalescence phenomenon evolves in different manners. The study emphasizes the effect of viscosity over the dynamics of the droplet and on the evolution of the capillary bridge between the needle and the free surface.

Free surface flow over the broad — Crested weir

The free surface flow around immersed bodies is a current CFD problem in applied fluid mechanics. The aim of this paper is to present a comparative study between experiments and numerical simulations of the flow around immersed bodies in vicinity of the free surface. The free surface flows in a planar channel were experimentally investigated in subcritical regim at Reynolds number Re = 3500, respectively Froude number Fr = 0.06. Numerical simulations are performed with the FLUENT code, RNG k-ε turbulent model and the volume of fluid (VOF) model for calculating the free surface geometry being used. The computational results showed a close agreement with experimental data obtained in the laboratory for the flow over the broad - crested weir with smooth and patterned surfaces.

Capillary breakup extensional properties of whole human saliva

Saliva studies are of great interest in the area of biorheology. This human body fluid plays an essential role in dysphagia since it is directly related to the swallowing process and to the rheology of food bolus. The aim of this work was to discuss the capillary breakup extensional properties of acidic and mechanically stimulated human saliva, and to point out the effect of saliva centrifugation on the elasticity of xanthan based solutions. Extensional rheology of alpha-amylase substitute solution was also studied and compared to whole human saliva through CaBER experiments (Capillary Break-up Extensional Rheometer). Results provided a general view about the complexity of this biofluid and highlighted the usefulness of CaBER device in achieving fast results concerning the elongational properties of human saliva.