Browsing by Author "Chinyoka, Tiri"

Now showing 1 - 5 of 5
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    Complex fluid dynamical computations via the Finite Volume Method
    (2018) Rahantamialisoa, Faniry Nadia Zazaravaka; Chinyoka, Tiri
    Numerical simulations of the complex flows of viscoelastic fluids are investigated. The viscoelastic fluids are modelled, primarily, via the Johnson-Segalman constitutive model. Our Numerical approach is based on finite volume method, based on the Johnson-Segalman constitutive model and implemented on the OpenFOAM® platform. The Johnson-Segalman model also easily reduces to the Oldroyd-B model under certain conditions of the material parameters. Since computations using the Oldroyd-B model have been extensively documented in the literature, we take advantage of the mathematical modelling connection between the Johnson-Segalman and Oldroyd-B models to validate the accuracy of our Johnson-Segalman solver via reduction to the Oldroyd-B model. Numerical validation of our results is conducted via the most commonly used benchmark problems. The final aim of our work is to assess the viability and efficiency of our numerical solver via an investigation into the complex fluid dynamical processes associated with shear banding.
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    Computational Analysis of Shear Banding in Simple Shear Flow of Viscoelastic Fluid-Based Nanofluids Subject to Exothermic Reactions
    (Multidisciplinary Digital Publishing Institute, 2022-02-25) Khan, Idrees; Chinyoka, Tiri; Gill, Andrew
    We investigated the shear banding phenomena in the non-isothermal simple-shear flow of a viscoelastic-fluid-based nanofluid (VFBN) subject to exothermic reactions. The polymeric (viscoelastic) behavior of the VFBN was modeled via the Giesekus constitutive equation, with appropriate adjustments to incorporate both the non-isothermal and nanoparticle effects. Nahme-type laws were employed to describe the temperature dependence of the VFBN viscosities and relaxation times. The Arrhenius theory was used for the modeling and incorporation of exothermic reactions. The VFBN was modeled as a single-phase homogeneous-mixture and, hence, the effects of the nanoparticles were based on the volume fraction parameter. Efficient numerical schemes based on semi-implicit finite-difference-methods were employed in MATLAB for the computational solution of the governing systems of partial differential equations. The fundamental fluid-dynamical and thermodynamical phenomena, such as shear banding, thermal runaway, and heat transfer rate (HTR) enhancement, were explored under relevant conditions. Important novel results of industrial significance were observed and demonstrated. Firstly, under shear banding conditions of the Giesekus-type VFBN model, we observed remarkable HTR and Therm-C enhancement in the VFBN as compared to, say, NFBN. Specifically, the results demonstrate that the VFBN are less susceptible to thermal runaway than are NFBN. Additionally, the results illustrate that the reduced susceptibility of the Giesekus-type VFBN to the thermal runaway phenomena is further enhanced under shear banding conditions, in particular when the nanofluid becomes increasingly polymeric. Increased polymer viscosity is used as the most direct proxy for measuring the increase in the polymeric nature of the fluid.
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    Modelling and Analysis of Viscoelastic and Nanofluid Effects on the Heat Transfer Characteristics in a Double-Pipe Counter-Flow Heat Exchanger
    (2022-05-28) Mavi, Anele; Chinyoka, Tiri; Gill, Andrew
    This study computationally investigates the heat transfer characteristics in a double-pipe counter-flow heat-exchanger. A heated viscoelastic fluid occupies the inner core region, and the outer annulus is filled with a colder Newtonian-Fluid-Based Nanofluid (NFBN). A mathematical model is developed to study the conjugate heat transfer characteristics and heat exchange properties from the hot viscoelastic fluid to the colder NFBN. The mathematical modelling and formulation of the given problem comprises of a system of coupled nonlinear partial differential Equations (PDEs) governing the flow, heat transfer, and stress characteristics. The viscoelastic stress behaviour of the core fluid is modelled via the Giesekus constitutive equations. The mathematical complexity arising from the coupled system of transient and nonlinear PDEs makes them analytically intractable, and hence, a recourse to numerical and computational methodologies is unavoidable. A numerical methodology based on the finite volume methods (FVM) is employed. The FVM algorithms are computationally implemented on the OpenFOAM software platform. The dependence of the field variables, namely the velocity, temperature, pressure, and polymeric stresses on the embedded flow parameters, are explored in detail. In particular, the results illustrate that an increase in the nanoparticle volume-fraction, in the NFBN, leads to enhanced heat-exchange characteristics from the hot core fluid to the colder shell NFBN. Specifically, the results illustrate that the use of NFBN as the coolant fluid leads to enhanced cooling of the hot core-fluid as compared to using an ordinary (nanoparticle free) Newtonian coolant.
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    Modelling the flow behaviour of gas bubbles in a bubble column
    (2009) McMahon, Andrew Martin; Rawatlal, Randhir; Harrison, STL; Chinyoka, Tiri
    The bubble column reactor is commonly used in industry, although the fluid dynamics inside are not well understood. The challenges associated with solving multi phase flow problems arise from the complexity of the governing equations which have to be solved, which are typically mass, momentum and energy balances. These time-dependent problems need to include effects of turbulence and are computationally expensive when simulating the hydrodynamics of large bubble columns. In an attempt to reduce the computational expense in solving bubble column reactor models, a "cell" model is proposed which predicts the velocity flow field in the vicinity of a single spherical bubble. It is intended that this model would form the fundamental building block in a macroscale model framework that does predict the flow of multiple bubbles in the whole column. The non-linear Navier-Stokes (NVS) equations are used to model fluid flow around the bubble. This study focusses on the Reynolds number range where the linear Stokes equations can be used to accurately predict the flow around the bubble. The Stokes equations are mathematically easier to solve than the NVS equations and are thus less computationally expensive. The validity of the NVS model was tested against experimental data for the flow of water around a solid sphere and was found to be in close agreement for the Reynolds number range 25 to 80. The simulation results from the Stokes flow model were compared with those from the NVS flow model and were similar at Reynolds numbers below 1. The application is then in the partitioning of the bubble column into regions governed by either Stokes or NVS equations.
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    Non-isothermal dynamics of thin-film free-surface and channel flows of non-Newtonian nanofluids
    (2022) Khan, Idrees; Chinyoka, Tiri
    Numerical modelling of the dynamic behaviour of generalized-viscoelastic-fluidbased nanofluids (GVFBNs) and viscoelastic-fluid-based nanofluids (VFBNs) has a number of industrial applications such as in new battery technologies and phasechange heat transfer devices. The computational results have shown that for certain flow parameters values, some of the non-Newtonian fluids also known as complex fluids (e.g. worm-like micellar solutions, granular flows, polymer solutions and some polymer melts) reveal flow instabilities within the flow field, such as the emergence of regions of different shear bands due to the flow induced material non-homogeneities. It has also been observed that it is becoming increasingly clear that the thermal runway phenomenon should not be ignored in polymers or other complex fluids since it may, in some instances, be as important as the complex rheology in differentiating susceptibility order for different types of nanofluids, for instance Newtonian fluid Based Nanofluids (NFBN), Generalized Newtonian Fluid-Based Nanofluids (GNFBN), Viscoelastic-fluid based nanofluids (VFBN) and Generalized viscoelastic fluid based nanofluids (GVFBN). These computational observations laid the foundation of this thesis. We have investigated the improvement of heat transfer for GVFBN and VFBN by homogenously mixed spherical shape nanoparticles. To incorporate the nanoparticles in the governing equations we use a single phase nanofluid modelling approach. Our mathematical models are governed by a system of non-linear, highly coupled, time-dependent Partial Differential Equations (PDEs). We developed computational solutions in Matlab software for the resulting system of equations by using an efficient semi-implicit finite-difference method, combined with a Crank-Nicolson scheme. In addition, the effects of nanoparticles on fluid velocity, extra stresses, temperature, and thermal conductivity are explored. Comparisons of the numerical results for the nanofluids with those from the literature without nanoparticles show excellent agreement.