Turbulent Fluid Flow Research Articles

Turbulent Fluid Flow and Heat Transfer Enhancement Using Novel Vortex Generator

One of several effective methods for improving heat transfer in heat exchangers is the use of vortex generators in channels. The flow of fluid through a channel with a vortex generator is disrupted by the development of recirculation zones on the step wall, which improves fluid mixing and heat transmission. The current work investigates the flow and heat transfer of turbulent fluid flow through channels with various vortex generator designs mathematically (triangular, half-circle, and quarter circle). The finite volume method (FVM) is used to discretize and solve the continuity, momentum, and energy equations. The SIMPLE algorithm approach is used to connect the pressure and velocity fields within the domain. The effect of geometrical factors such as step height (2, 3, 4, 5, and 6 mm) on the flow and heat fields is demonstrated and analyzed, as is the effect of Reynolds number. As the step height increases, so do the surface Nusselt number and skin friction coefficient, which peak at 4 mm. According to the findings, as Re increases, so does the average Nusselt number. The quarter circle vortex generator has the best thermal-hydraulic performance at a 4 mm amplitude height, followed by the triangle vortex generator. The simulation results are consistent with those found in the literature.

Thermo-hydraulic performance evaluation of turbulent flow and heat transfer in a twisted flat tube: A CFD approach

The numerical procedure for turbulence fluid flow and heat transfer in a twisted flat tube is presented in this work. For the cross-section of the flat tube, three different geometries are investigated. Water is a working fluid with constant thermophysical properties. The Reynolds number ranges from 5000 to 20000. Results show that by using twisted flat tubes, the efficiency has increased nearly 70% compared to the smooth tubes with the same surface area and hydraulic diameter. Also, the alternating helical direction has caused almost 60%, which seems to be a good choice. Due to the effect of helical pitch that affects the flow path, the alternating geometry was better in some Reynolds and not others. The results are explained using flow characteristics contours such as turbulent kinetic energy, tangential velocity, and temperature. Moreover, the results are presented in numbers and plots for a more accurate comparison.

On the Fuzzy Solution of Linear-Nonlinear Partial Differential Equations

In this article, we present the fuzzy Adomian decomposition method (ADM) and fuzzy modified Laplace decomposition method (MLDM) to obtain the solutions of fuzzy fractional Navier–Stokes equations in a tube under fuzzy fractional derivatives. We have looked at the turbulent flow of a viscous fluid in a tube, where the velocity field is a function of only one spatial coordinate, in addition to time being one of the dependent variables. Furthermore, we investigate the fuzzy Elzaki transform, and the fuzzy Elzaki decomposition method (EDM) applied to solving fuzzy linear-nonlinear Schrodinger differential equations. The proposed method worked perfectly without any need for linearization or discretization. Finally, we compared the fuzzy reduced differential transform method (RDTM) and fuzzy homotopy perturbation method (HPM) to solving fuzzy heat-like and wave-like equations with variable coefficients. The RDTM and HPM solutions are simpler than other already existing methods. Several examples are provided to illustrate the methods that have been offered. The results obtained using the scheme presented here agree well with the analytical solutions and the numerical results presented elsewhere. These studies are important in the context of the development of the theory of fuzzy partial differential equations.

Thermohydraulic performance and entropy generation in heat exchanger tube with louvered winglet tapes

The present article concerns with thermohydraulic performance, flow friction and entropy generation analysis in a heated tube contained with louvered winglet tape (LWT). The experiment was conducted in a uniform heat-fluxed test tube for turbulent fluid flow, Reynolds number (Re) ranging from 4760 to 29,260. The purpose of the LWT insert is to produce streamwise vortices assisting to induce impinging-jets onto the tube wall and to decline the pressure loss through the louver mounted on the winglet aside from providing rapid mixing of fluid flow. In the present experiment, the louvered winglets were mounted periodically on a double-sided straight tape with six different louver angles (θ = 0 ˗ 90°) and three winglet pitch ratios (PR = 1 ˗ 2) at a single relative winglet height (BR = 0.25) and a fixed attack angle (α) of 30°. There were two-types of LWT arrangements: inline and staggered louvered-winglet tapes (I-LWT and S-LWT). To examine the optimum thermohydraulic performance, an influence of θ at each PR on the rate of heat transfer and friction loss inside the tube was explored. The measured results disclosed that the Nusselt number (Nu) and friction factor (f) from using both types of LWTs rise considerably with the reduction of PR and θ. The entropy generation (S˙gen') was declined with the decrease in Re, PR and θ where the minimum S˙gen' was obtained for the I-LWT tube at PR = 1, θ = 0° and lowest Re. The peak thermal enhancement factors (TEF) of the S-LWT and the I-LWT were, respectively, around 2.22 and 2.18 at similar PR = 1, θ = 45°. The Nu, f and TEF correlations for using LWT insert were also reported.

Understanding effect of fluid salinity on polymeric drag reduction in turbulent flows of slickwater fluids

Produced waters are brines drawn during the oil and gas production. These fluids are often re-injected into unconventional reservoirs along with certain additives and proppants in order to induce/sustain fractures and boost oil/gas production. The additives may transform these fluids into drag-reducing slickwater fluids that are pumped at high flowrates (turbulent conditions) to transport proppants at low viscosities. The drag-reducer polymeric additives in the slickwater fluids facilitate high flowrates by reducing drag during the flow. The present work investigated the influence of fluid salinity/hardness on polymer dynamics and its impact on the ability of the polymer to alleviate drag. The study focuses on two synthesized drag-reducing anionic acrylamide (APAM) polymers: first is a copolymer of acrylamide and acrylic acid (Base-APAM), and the second is a ter-polymer of acrylamide, acrylic acid and a functional group (Functionalized-APAM). The polymers were characterized using capillary viscometry, NMR and degree of ionicity.For evaluation of the drag reduction performance of the above polymers, a laboratory flow-loop was built to mimic the turbulent flow conditions in the wellbore. The experimental evaluation was conducted for the fluids with added polymers and range of fluid salinities to encompass plausible variations in the field. Additionally, the fluids’ viscosity performance was evaluated using conventional oil-field viscometers. The obtained experimental data was used to build a semi-empirical correlation that predict the polymer relaxation defined by Weissenberg number i.e., Wi, modelled as a function of fluid salinity and polymeric structure. The drag reduction performance may be derived based on the Re-Wi parameter space, and a correlation for the studied range of parameters has been constructed. The experimental data and models also evidently depicted that the synthesized ter-polymer provides a superior salt-tolerance compared to the co-polymer for the range of parameters studied.

Large-eddy simulation of turbulent pipe flow of Herschel-Bulkley fluids - Assessing subgrid-scale models

This paper investigates the turbulent pipe flow of viscoplastic fluids using large-eddy simulation (LES) and the OpenFOAM package. The Herschel-Bulkley model is employed to describe the viscosity function with shear-thinning and viscoplastic behavior. Many LES studies evaluating different subgrid-scale (SGS) models for Newtonian fluids are available in the literature. However, studies regarding LES and SGS models involving non-Newtonian fluids are scarce, despite the importance of the subject. The present paper focuses on evaluating the performance of the Dynamic Smagorinsky and Wall Adapting Local Eddy-viscosity (WALE) models, and the under-resolved direct numerical simulation (UDNS) with viscoplastic fluids. The first- and second-order turbulence statistics and flow features, such as mean wall shear stress, mean wall viscosity, friction factor, mean shear rate, and mean viscosity profiles are used to assess the different models. Simulations are carried out at two different generalized Reynolds numbers (ReG) of 5000 and 7500. The numerical results computed with LES are compared with direct numerical simulation (DNS) results and experimental data for turbulent pipe flow of a Newtonian fluid to validate the numerical method. LES results for the Herschel-Bulkley fluid show a good agreement for the first- and second-order turbulence statistics compared with DNS data. Moreover, mean flow quantities obtained with LES follow the trend of DNS data.

Long-Term Static and Dynamic Corrosion Stability of Nonwetting Surfaces.

Superhydrophobic surfaces (SHSs) and lubricant-infused surfaces (LISs) are two classes of nonwetting surfaces that have drawn attention due to their advanced functional properties including corrosion inhibition. Yet there is a conspicuous lack of corrosion study of SHSs and LISs with respect to their fabrication and material parameters, especially at high temperatures and under dynamic flow conditions over long durations, which is sought to be addressed in this article. Considering copper SHSs and LISs, a full factorial combinatorial study of two facile texturing processes, electrodeposition and etching, two different functionalization agents, stearic acid and mercaptan, and two types of infused lubricants, Krytox 104 and DOWSIL 510, is presented, encompassing over 650 measurements on 90 tested surfaces. All fabricated surfaces demonstrated water repellency with a contact angle above 150° and a sliding angle below 7°. For the first time, the study examines high-temperature corrosion stability and long-term corrosion durability of the nonwetting surfaces in both static fluid and dynamic turbulent flow conditions over a period of 30 days. LISs and SHSs are shown to provide excellent corrosion inhibition over all tested corrosion conditions, with negligible presence of corrosion species on the surfaces and no deterioration of the texturing. The surfaces are also shown to rejuvenate easily to the initial wettability and corrosion resistance values. This study provides valuable insights into the selection of materials and processing parameters for the fabrication of nonwetting surfaces for the application of interest.

Thermal and Hydraulic Performances of Carbon and Metallic Oxides-Based Nanomaterials

For companies, notably in the realms of energy and power supply, the essential requirement for highly efficient thermal transport solutions has become a serious concern. Current research highlighted the use of metallic oxides and carbon-based nanofluids as heat transfer fluids. This work examined two carbon forms (PEG@GNPs & PEG@TGr) and two types of metallic oxides (Al2O3 & SiO2) in a square heated pipe in the mass fraction of 0.1 wt.%. Laboratory conditions were as follows: 6401 ≤ Re ≤ 11,907 and wall heat flux = 11,205 W/m2. The effective thermal–physical and heat transfer properties were assessed for fully developed turbulent fluid flow at 20–60 °C. The thermal and hydraulic performances of nanofluids were rated in terms of pumping power, performance index (PI), and performance evaluation criteria (PEC). The heat transfer coefficients of the nanofluids improved the most: PEG@GNPs = 44.4%, PEG@TGr = 41.2%, Al2O3 = 22.5%, and SiO2 = 24%. Meanwhile, the highest augmentation in the Nu of the nanofluids was as follows: PEG@GNPs = 35%, PEG@TGr = 30.1%, Al2O3 = 20.6%, and SiO2 = 21.9%. The pressure loss and friction factor increased the highest, by 20.8–23.7% and 3.57–3.85%, respectively. In the end, the general performance of nanofluids has shown that they would be a good alternative to the traditional working fluids in heat transfer requests.

Experimental study of single bubble breakage in turbulent flow field: Evaluation of breakage models

The breakage of bubbles in a turbulent fluid flow is studied to gain more insight into the gas bubbles breakage mechanisms and assist evaluating the breakage models used in Population Balance Modelling (PBM) for more accurate predictions. Experimental tests were performed for air bubbles in flows with different levels of turbulence intensity. A few of the existing models were evaluated against the experimental results and the measurements were used for determination of constant parameters of breakage functions. Moreover, new modifications to existing models were proposed to improve the prediction of breakage functions for the case of cascade breakage. The new functions proposed for the case of cascade breakage of bubble were also tested for liquid drop breakage. A discussion regarding the practicality of the conventional breakage models is presented.

Turbulent displacement flows of viscoplastic fluids in obstructed eccentric annuli: Experiments

We present an experimental study of turbulent displacement flows in eccentric annuli, where the displacing fluids are water, 0.1% and 0.2% xanthan gum solutions, and the in situ fluid is a viscoplastic, 0.15% Carbopol solution. We focus on the effect of a solid obstruction on the narrow side of the annular gap, analogous to a consolidated cuttings bed in well cementing operations. For comparison we include experiments with an unobstructed test section with eccentricity e set at ∼0.5. While the displacement flow is successful without the obstruction regardless of the displacing fluid, we find that the obstruction at e∼0.5 is mostly detrimental to removal of the yield stress fluid stuck downstream of it, and a decrease in Reynolds numbers via formulation of xanthan gum solutions contribute to a less effective displacement in all situations due to the decrease in fluctuating stresses. Upstream of the obstruction, we observe complete removal of the Carbopol, likely due to enhanced vorticity effects. However, at high eccentricity values of e∼0.7, the effect of the obstruction on the displacement of Carbopol appears to be negligible, especially in the less turbulent situations with the xanthan gum solutions where the stationary Carbopol layer covers the entirety of the solid blockage, both upstream and downstream. Thus, in a highly eccentric obstructed annulus, the eccentricity remains the dominant factor to hinder the displacement.

Parametric study and application of a data-mining model in 2D and 3D micro-fin tubes

• The application of least squares-support vector regression method in micro-fin study. • The best 2D and 3D micro-fins had similar design parameters. • The best 2D micro-fin tube enhanced the efficiency index up to %18. • Importance of micro-fin’s axial pitch over height ratio on the efficiency index. Micro-fins (<0.5 mm tall) are an engineered roughness with the advantage of reducing thermal resistance and the disadvantage of increased pressure drop when applied inside a tube in heat-exchange applications. The competing effects highlight the need for careful optimization that identifies micro-fin surfaces with the potential to match heat exchanger design needs. Hence, the objectives of this study are chosen to enable efficient optimization in future studies. The main goals are: (1) study the effects of micro-fin design variables on heat transfer and friction factors; and (2) evaluate the potential of a data-mining model as a surrogate of computational fluid dynamic ( CFD ) models in 2 dimensional ( D ) and 3 D micro-fin tubes. This study applied conductive and convective heat transfer and turbulent fluid flow simulation to evaluate the performance of different 2 D and 3 D micro-fin tubes. Different configurations were generated by varying micro-fin height ( e ), helix angle ( α ), number of starts ( N f ), and discontinuity features. Coupled solid and periodic fluid domains were applied in ANSYS Fluent 19.1 . Performance was mapped for 210 different simulations (including a smooth tube) using a realizable k - ε turbulence model at Reynolds number ( Re ) of 48,928. Two different least squares-support vector regression ( LS - SVR ) models were employed to estimate the Colburn j factor as a function of geometric variables and the Fanning friction factor ( f ) as a function of geometric variable and j factor. Results of the parametric study showed that the best 2 D micro-fin tube can enhance efficiency index ( η ) up to 1.18. Results of the LS - SVR model showed that the percentage of average absolute error ( AAE ) between simulated and estimated j and f factors are 2.05% and 2.93% for 3 D micro-fin tubes, respectively.

Paul Andrews Libby Contribution to Zone-Conditioned Average Transport Equations in Intermittent Turbulent Flows

ABSTRACT Professor Paul A. Libby has made lasting contributions to the theory and modeling of turbulent premixed flames. A less known subject of his research relates to the conditioning of the local and instantaneous Navier-Stokes equations for either the turbulent or the irrotational flow regions separated by a thin interface. He postulated an evolution equation for the intermittency function, a generalized function introduced by experimentalists in the decades of 1960 and 1970, which is unity in the vorticity-containing turbulent flow and zero in the surrounding irrotational flow. This intermittency function equation allowed the derivation of transport equations for average variables conditioned either to the turbulent or the vorticity-free regions. Paul A. Libby formalism is explained in this paper. Further research triggered by Libby’s seminal work is then reported. An exact expression is obtained for his “intermittency creation” term, which allows a simple physical interpretation of the entrainment and zone interactions. This methodology is applied to an intermittent scalar in a turbulent flow of a variable density fluid. Various types of intermittent flows with zonal discrimination, based on vorticity, scalar and scalar-gradient criteria, are presented. The analogy of turbulent/nonturbulent interfaces with propagating turbulent premixed flames is remarked.

Friction factor estimation for turbulent flow of Herschel-Bulkley and power law fluids in pipes

Determining the appropriate friction factor value for calculating the frictional pressure loss of non-Newtonian fluids in a pipe is a critical task. This study aims to obtain an explicit friction factor correlation to accurately calculate the pressure losses for turbulent flow of yield pseudoplastic fluids in both smooth and rough pipes. A series of experiments are carried out with carboxymethyl cellulose (CMC) solutions using pipes of three different relative roughness. The Herschel-Bulkley model is found to be the most appropriate model to describe the yield pseudoplastic behavior of the CMC solutions used in this study. Based on the experimental friction pressure data obtained from the Izmir Katip Çelebi University flow loop, an explicit correlation is developed here. The new friction factor equation is compared with not only experiments in this study, but also experimental data presented by Okafor and Evers (1992), Subramanian (1995) and Vjargah et al. (2017). Results show that the proposed friction factor correlation is more consistent and better in agreement with experimental data than existing correlations in the literature. • This is the first study to experimentally investigate the turbulent flow of yield-pseudoplastic fluids in rough pipes. • A new explicit friction factor correlation for determining the pressure losses of non-Newtonian fluids accurately in rough and smooth pipes is proposed. • Experiments are carried out with various CMC solutions using pipes of three different relative roughness. • The Herschel-Bulkley model are found to be the most appropriate model to describe the yield pseudoplastic behavior of the CMC solutions. • The proposed friction factor correlation is more consistent and better in agreement with experimental data than existing correlations in the literature.

Model-assisted deep learning of rare extreme events from partial observations.

To predict rare extreme events using deep neural networks, one encounters the so-called small data problem because even long-term observations often contain few extreme events. Here, we investigate a model-assisted framework where the training data are obtained from numerical simulations, as opposed to observations, with adequate samples from extreme events. However, to ensure the trained networks are applicable in practice, the training is not performed on the full simulation data; instead, we only use a small subset of observable quantities, which can be measured in practice. We investigate the feasibility of this model-assisted framework on three different dynamical systems (Rössler attractor, FitzHugh-Nagumo model, and a turbulent fluid flow) and three different deep neural network architectures (feedforward, long short-term memory, and reservoir computing). In each case, we study the prediction accuracy, robustness to noise, reproducibility under repeated training, and sensitivity to the type of input data. In particular, we find long short-term memory networks to be most robust to noise and to yield relatively accurate predictions, while requiring minimal fine-tuning of the hyperparameters.

Simulation of a Perforated Vertical Wellbore with Near Wall Porous Media Effect

This paper aims to predict the effect of porous media permeability and perforations parameters on the pressure drop and productivity index, for the perforated vertical wellbore with six perforations and for two phase angles. The first 60ᵒ phase angle with helical distribution and the second 180ᵒ phase angle with normal distribution. In this study, a Computational Fluid Dynamics (CFD) software has been used to simulate a model of 3-D turbulent fluid flow with stander k−∈, steady-state, and single phase. The effect of the permeability of porous media, inlet mass flow rate from porous media, perforations length, and diameter of perforations are studied, for two cases of the phase angles. The results of this study show that, the pressure drop decreases with increasing permeability pf porous media, so the productivity index increasing. Also, increase of inlet mass flow rate from porous media causing an increase in the pressure drop. The perforations length has a few effects on the pressure drop and productivity index, while the diameter of perforations has a greater effect on the pressure drop.

3D numerical and experimental modelling of multiphase flow through an annular geometry applied for cuttings transport

Accurate estimation of volume fraction and pressure gradient is considered indicating parameters of efficient cuttings transportation. It is vital, in this regard, to consider all parameters that can affect cuttings volume fraction and pressure drop during enrollment of the drilling process. The analysis was conducted based on the turbulent flow of a power-law fluid through an annular domain by employing the Finite Volume Method. In addition, dimensional relationships were developed with the Buckingham-π theorem. Before carrying out simulations, the numerical schemes were validated using actual measurements made with the flow loop system available in Texas A&M University at Qatar. The simulation results demonstrated the followings: (i) using a power-law type drilling fluid with a shear-thinning character would reduce energy consumption for an inclination greater than 45°; (ii) inclination angles from 45° to 60° would be least desirable for an effective cuttings transportation with a turbulent Ostwald-de Waele fluid.

Meshless approach to the large-eddy simulation of the continuous casting process

The paper discusses the numerical solution of the large-eddy formulation for modelling the turbulent fluid flow with solidification in the continuous casting of steel. This industrially relevant problem is solved by a meshless method for the first time. The solid-liquid system is formulated within the continuum mixture formulation of the governing, mass momentum and energy equations. The Darcy porous media model describes the mushy region. The large-eddy formulation is based on the Smagorinsky model for sub-grid scale viscosity and the van Driest damping function. The synthetic turbulence is prescribed at the molten steel inlet. The generated turbulent fluctuations are isotopic and correlated in time through an asymmetric time filter. Explicit time discretisation, combined with meshless local radial basis function collocation method, is employed. Five-noded subdomains, non-uniform node arrangement and multiquadrics shape functions are used for solving a two-dimensional model. The sensitivity to different node arrangements, time steps, inlet temperature, and velocity boundary conditions is elaborated and successfully verified by comparison with our previously published meshless k − ε turbulence model of the same process.

Numerical determination of bubble size distribution in Newtonian and non-Newtonian fluid flows based on the complete turbulence spectrum

Gas-liquid mass transfer in non-Newtonian fluids is a crucial aspect of the bioprocess industry. Mass transfer is analyzed using the coefficient kLa, which is limited by the rheology since it exerts a barrier to the fluid deformation, significantly affecting the oxygen diffusivity and the bubble breakup and coalescence. However, the traditional mathematical expressions to model the bubble size distribution from bubble breakup and coalescence in turbulent flows of Newtonian fluids are restricted to the inertial sub-range of turbulence where the kinetic energy is dominated only by the microscales. Application of the Newtonian models to non-Newtonian fluids could result in inaccurate predictions by not considering the continuous phase rheology. The main goal of this research is the numerical determination and experimental comparison of bubble sizes in different axial positions of a bioreactor stirred by a Rushton turbine. Emphasis was placed on the viscosity effects on simulating bubble dispersion in a Newtonian fluid (water) and its comparison with a non-Newtonian fluid (0.4 % CMC). The mathematical framework is constructed by coupling the hydrodynamics (through computational fluid dynamics CFD) and bubble breakup and coalescence from a turbulence perspective using the complete energy spectrum that considers the contributions from the energy containing, inertial, and dissipation sub-ranges. This is achieved by including the second-order structure–function. The results of bubble sizes and kLa were compared with experimental data, and acceptable agreement was achieved. Therefore, it is shown that the viscous effects were captured numerically by the entire energy spectrum and improved the predictions of the kLa and bubble sizes compared to the traditional structure function turbulence models.

Numerical simulation of the calcium hydroxide/calcium oxide system dehydration reaction in a shell-tube reactor

The Ca(OH)2/CaO system is a promising candidate for thermochemical energy storage because of its high energy density, low cost and negligible heat loss. Coupling mechanisms of multiple physiochemical processes still need further investigation and the system performance should be further improved. In this work, for the first time, a three-dimensional numerical model is developed to study the thermochemical energy storage process by Ca(OH)2 dehydration reaction in a shell-tube reactor. The turbulent heat transfer fluid flow in the shell side, and the steam flow, heat transfer and dehydration reaction in the tube side are comprehensively taken into account. Effects of operation conditions and reactor geometry parameters on the dehydration process are also investigated in detail. Five indicators are proposed to comprehensively evaluate the reactor performance, including reaction time, energy storage density, heat exchange power, heat exchange efficiency and energy storage efficiency. The results reveal that higher inlet temperature and inlet velocity of heat transfer fluid can enhance the heat transfer and accelerate the reaction. Lower porosity, although increases the energy storage density, leads to higher flow resistance and impedes the reaction. With deep understanding of the physiochemical processes, 16 more tubes are properly added in the by-pass flow region, leading to shorter reaction time and higher energy storage amount. The low values of heat exchange efficiency and energy storage efficiency reveal that the input heat cannot be efficiently transferred to the tube side. To improve the energy conversion performance of reactor, additional schemes are discussed including regenerative HTF, optimization of reactor geometry, increase of reactant thermal conductivity, reducing tube-side flow resistance and extending HTF residence time. The numerical model and simulation results in the present work could be helpful for optimizing the shell-tube Ca(OH)2/CaO reactor and improving the thermochemical energy storage performance.

Machine-learning energy-preserving nonlocal closures for turbulent fluid flows and inertial tracers

Machine learning turbulence closures for non-homogeneous and non-isotropic flows is a challenging task. The novelty of the presented approach is the adoption of a universal constraint associated with the energy preservation of the nonlinear terms, which is valid for any turbulent system. This constraint is embedded in the training process, and in combination with nonlocal representations in space and time, results in significant improvement for the resulted coarse scale models.