Flow Computational Fluid Research Articles

Some recent advances in computational heat transfer and fluid flow

Computational methods in heat transfer and fluid flow have gained significant progresses and stimulated strong interest and continued efforts in academic and industrial communities for a broad range of thermal engineering. The topics included in this special issue cover computational methods from microscopic to macroscopic modellings and related applied researches from single-phase to multi-phase flow phenomena, as well as an introduction of Prof. Wen-Quan Tao’s 50-year academic career in heat transfer. The research efforts documented in the papers clearly reflect the most recent advances in computational heat transfer and fluid flow. The vision of the paper shows some of the key achievements and main challenges which still remain in applied thermal engineering. Finally, some future perspectives in employing modern methods are given to highlight pathways of ongoing researches in understanding and predicting complex physical phenomena and processes in heat transfer and fluid flows.

Numerical Computations of Non-Newtonian Fluid Flow in Hexagonal Cavity With a Square Obstacle: A Hybrid Mesh–Based Study

Thermal flow phenomena in a double lid–driven enclosure have many potential applications in numerous engineering domains. The present article theoretically investigates the heat transfer analysis of power-law fluid in a hexagonal cavity embedded with a square obstacle. The upper and lower lid walls of the cavity are considered heated along with the walls of the centered embedded square, while the rest of the cavity walls are thermally insulated. In addition, the horizontal lid walls of the cavity are uniformly moving in opposite horizontal directions. The mathematical modeling of the nonlinear fluid has been developed considering the continuity, momentum, and energy equations subjected to the appropriate boundary conditions. Due to the nonlinearity of the governing partial differential equations and of the viscosity models, we use the numerical scheme based on the finite element method The stable finite element pair (ℙ2/ℙ1) has been selected for the discretization purpose. The discretized nonlinear system is solved with the Newton method in conjunction with a direct linear solver in the inner iterations. The thermal flow features are exposed via streamlines and temperature contours for a set of governing parameters such as Reynolds number (Re), Prandtl number (Pr), and Grashof number (Gr) along with the power-law index (n). The values of local and average Nusselt numbers are calculated for the involved parameters. It is noted that the square obstacle has a strong impact for the formulation of streamlines and isotherms. Moreover, the power-law index has a strong impact on the values of the average Nusselt number and kinetic energy. The kinetic energy of the system increases with an increasing value of Gr and n, while Reynolds number has opposite effects on kinetic energy.

Design and Analysis of a Self-Circulated Oil Cooling System Enclosed in Hollow Shafts for Axial-Flux PMSMs

With the every-increasing demands for miniaturization, the efficient cooling techniques for axial-flux permanent magnet synchronous machines (AF-PMSMs) have attracted great attentions. In this paper, the thermofluid behaviors of a 7-kW 4000-rpm AF-PMSM are numerically investigated based on a coupled model, and the computations are validated by thermal tests performed on the AF-PMSM prototype. To enhance the heat dissipation capacity of the rotor components, it is proposed a self-circulated oil hermetically cooling system, for which the cooling liquid circulates in a hollow shaft. During operation, the oil flow driven by the centrifugal blades forms a closed oil circuit, and can create an effective heat flow path from the hot areas to the cold regions. The oil cooling system is employed in the AF-PMSM to effectively maintain, and evenly distribute, the motor temperature rise. The cooling effectiveness is proved by fluid flow and thermal coupled analyses. To further improve the heat dissipation capacity, the self-circulated oil-cooling structure is modified based on the calculated fluid flow distribution. Finally, the structural parameters are optimized by Taguchi method to minimize both the temperature rise and the rotor weight increment, and the optimization results are numerically and experimentally validated.

Generalized network modelling of two-phase flow in a water-wet and mixed-wet reservoir sandstone: Uncertainty and validation with experimental data

We use a generalized pore network model in combination with image-based experiments to understand the parameters that control upscaled flow properties. The study is focued on water-flooding through a reservoir sandstone under water-wet and mixed-wet conditions. A set of sensitivity studies is presented to quantify the role of wettability, pore geometry, initial and boundary conditions as well as a selection of model parameters used in the computation of fluid volumes, curvatures and flow and electrical conductivities.We quantify the uncertainty in the model predictions, which match the measured relative permeability and capillary pressure within the uncertainty of the experiments. Our results show that contact angle, initial saturation, image quality and image processing algorithm are amongst the parameters which introduce the largest variance in the predictions of upscaled flow properties for both mixed-wet and water-wet conditions.

Flow modulation and heat transport of radiatively heated particles settling in Rayleigh–Bénard convection

We investigate the effect of radiatively heated solid particles settling in Rayleigh–Bénard turbulent flow. Three-dimensional fluid flow computations were performed using direct numerical simulation, while the evolution of particle temperature, velocities, and positions are obtained by Lagrangian particle tracking. We consider particles whose diameters vary form 20 µm to 80 µm subject to thermal radiation and settle in Rayleigh–Bénard cell at Rayleigh number Ra=2×106. The results show that the sedimentation behaviors of the particles with different particle sizes are significantly different, and the trajectories of small particles are very chaotic compared to those of large particles. Consequently, small particles can absorb solar energy efficiently and result in gas temperature increase. As the particle size decreases, the thickness of the upper boundary layer becomes thinner, while the thickness of the lower boundary layer becomes thicker. The Nusselt number varies monotonically with a logarithmic function with the diameter of the particle on both the upper and lower walls.

Pore-scale modelling of fluid flow in porous media using the projection method for incompressible Navier-Stokes equations in irregular domains

This paper presents the results of numerical simulation of incompressible viscous flow in porous media, which comprise periodically arranged cylinders. This simulation is based on the numerical solution of the incompressible Navier-Stokes equations in irregular domains using the projection method on staggered grids, where the irregular boundary is represented by its level-set function at the pore-scale level. The main problem in numerical calculation of fluid flow through porous media occurs when the value of the porosity is close to 1 or is close to the threshold value since it is necessary to take a very fine numerical mesh, which requires additional computing power and increases the calculation time. There are exact analytical solutions for simple types of porous media which consist of periodically arranged cylinders. In this paper, the permeabilities of these porous media were numerically calculated and compared with the previous works based on the numerical solution of the Lattice-Boltzmann equation in irregular domains, when the fluid flow obeys Darcy’s law. The comparison of numerical and theoretical values of porosity shows that this method is sufficiently accurate for porosity values φ=0.2–0.8.

Improved prediction of heavy oil viscosity at various conditions utilizing various supervised machine learning regression

Fluid viscosity plays a major role in the petroleum industry. It’s required for fluid flow calculations through the reservoir and production systems. In this study, heavy oil viscosity measurements for seventy samples are reported at various temperatures with different API gravity. Three supervised machine learning regression (SMLR) models were developed to predict the viscosity as a function of API gravity, temperature, and density with a 5 K-fold validation technique for a better model representation. Results showed that the newly developed models have superiority over published models. The Gaussian process regression (GPR) and the regression ensembles tree (RET) showed the best performance with a mean absolute percent error of 0.7328%, a root mean square error of 0.6174 and a coefficient of determination of 0.9988 for the GPR where for the RET the mean absolute percent error is 0.5768%, the root mean square error is 3.514, and the coefficient of determination is 0.9971.

One-sided GRP solver and numerical boundary conditions for compressible fluid flows

In the computation of compressible fluid flows, numerical boundary conditions are always necessary for all physical variables at computational boundaries while just partial physical variables are often prescribed as physical boundary conditions. Certain extrapolation technique or ghost cells are often employed traditionally for this issue but spurious wave reflections often arise to cause numerical instability. In this paper, we associate this issue with the one-sided generalized Riemann problem (GRP) solver motivated by the accelerated piston problem in gas dynamics so that the extrapolation technique can be actually avoided. In fact, the compatibility arguments naturally require to formulate the one-sided generalized Riemann problem and incorporate it into the numerical procedure of boundary conditions. Once the corresponding solver is available, the boundary control volume is treated in the same way as the usual interior control volume. As far as the interaction of nonlinear waves with physical boundaries, such a one-sided GRP solver shows significant benefits, as numerical experiments demonstrate, on avoiding spurious wave reflections at the computational boundaries.

A review on the application of lattice Boltzmann method for melting and solidification problems

Melting and solidification of pure metals and alloys are important research areas due to their practical applications, where the study of dynamic evolution of the interface offers a challenging task to the researchers. For the last two decades, lattice Boltzmann method (LBM) has been extensively used to model transport phenomena involving complex boundary at the phase interface for pure metals and alloys because of high computational efficiency and cost effectiveness of LBM. A state-of-art survey on application of LBM for melting and solidification phenomena is now presented in this work. This paper, first introduces the theory of thermal lattice Boltzmann method (TLBM) for heat and fluid flow, subsequently an elaborate coverage is presented on the methodologies used to study the melting-solidification phenomena of pure substances and alloy materials. The success of lattice Boltzmann (LB) method in investigating the morphological structure during solidification is specially emphasized. It is observed in this context that the phase field method (PFM) has evolved as the most popular choice for evaluating the microstructure. While the robustness of LBM can effectively handle the fluid flow calculation around the dendrites, phase field based lattice Boltzmann method (PFLBM) is very effective in simulating the growth of dendrites from the seed level to the very large scale. The realistic morphological structure can be accurately predicted due to the computational efficacy of PFLBM. A comprehensive coverage has been provided on application of PFLBM method in simulating the growth kinetics in alloys. Further, the application of the PFLBM in solidification has been showcased from open literature.

Improvements to the Fracture Pipe Network Model for Complex 3D Discrete Fracture Networks

AbstractFractures widely present in the subsurface and play a critical role in the fluid flow processes in porous media. The Fracture Pipe Network Model (FPNM) is an efficient method to represent and calculate fluid flow properties as a particular part of Discrete Fracture Networks (DFNs) method compared to direct numerical simulations. However, the current FPNM formulation can result in large deviations in computed fluid flow properties when applied to complex interconnected DFNs, although it can produce good results for simple DFNs. To enhance the performance and versatility of current FPNMs, four modifications to the FPNM formulation are introduced from different perspectives to improve the accuracy of pipe conductance assignment and ensure the correct topology of the fracture network. Two benchmarking examples with complex interconnected fractures and two real fractured samples are presented and the results show the modifications significantly improve the accuracy of computed fluid flow properties in complex DFNs.

Surrogate-Assisted Fine Particulate Matter Exposure Assessment in an Underground Subway Station.

With the increase in subway travelers, the air quality of underground enclosed spaces at subway stations has attracted much more attention. The study of pollutants exposure assessment, especially fine particulate matter, is important in both pollutant control and metro station design. In this paper, combining pedestrian flow analysis (PFA) and computational fluid dynamics (CFD) simulations, a novel surrogate-assisted particulate matter exposure assessment method is proposed, in which PFA is used to analyze the spatial-temporal movement characteristics of pedestrians to simultaneously consider the location and value of the pedestrian particulate generation source and their exposure streamline to particulate matter; the CFD model is used to analyze the airflow field and particulate matter concentration field in detail. To comprehensively consider the differences in the spatial concentration distribution of particulate matter caused by the time-varying characteristics of the airflow organization state in subway stations, surrogate models reflecting the nonlinear relationship between simulated and measured data are trained to perform accurate pedestrian exposure calculations. The actual measurement data proves the validity of the simulation and calculation methods, and the difference between the calculated and experimental values of the exposure is only about 5%.

Thermal control of crystalline silicon photovoltaic (c-Si PV) module using Docosane phase change material (PCM) for improved performance

As crystalline silicon photovoltaic (c-Si PV) module demonstrates circa 0.45% drop in conversion efficiency for every 1 °C cell temperature rise above 25 °C standard test condition (STC), thermal control of the system is critical to improving its efficiency especially for elevated temperature climate deployment. This research demonstrates the utilisation of Docosane C22H46 paraffin wax phase change material (PCM) to lower the operating temperature of c-Si PV module operating in hot climatic conditions. Employing computational fluid flow fluent analysis in Ansys software package, the thermal responses of finite element (FE) models of the module to environmentally induced loads are simulated for 24-hour period. The results are validated analytically. Findings reveal that temperature difference between ambient and solar cell is maximum at noon at 23.7 °C and monotonically decreases afterwards to zero at 6.00 am and 8.00 pm. The implementation of PCM delivers a PV-PCM module that is 10.88% more conversion efficient than the conventional c-Si PV module. Further findings show the improved efficiency results from PV-PCM module achieving cell temperature reduction of 78.4% at peak heat flux and 31.1% overall. The PV-PCM module produces 6.1% more voltage, 2.1% more electric power and 8.1% more maximum power daily than the conventional c-Si PV module. In addition, it is found to be 1.05% more efficient and has 34.0% longer fatigue life. The PCM usage demonstrates effectiveness in control of thermal characteristics of c-Si PV module by improving its performance – crucial for its reliable operations in hot climates.

Re-examining the partially saturated-cells method for incompressible flows with stationary and moving bodies

We revisit the Partially-Saturated-Cells method (PSC) in the context of incompressible single-phase flows past stationary and moving bodies. This methodology adds a solid collision operator to the Lattice Boltzmann Equation, resulting in a unified evolution equation everywhere in the domain that suitably accounts for the boundary conditions. The unified evolution equation, which also allows for a smooth transition from solid cells to fluid cells, is dictated by the solid volume fraction, obtained using a simple geometric approach. We prove that the PSC approach exhibits nominal second-order accuracy in presence of straight and curved boundaries for pressure, although spatial order of accuracy in velocity degrades to first-order. Our analysis also reveals that PSC is approximately discretely conservative in the incompressible limit with the mass conservation errors being dependent on the lattice size and relaxation time. For simulations involving moving body problems, the method is found to introduce acceptable level of spurious force oscillations, which rapidly decrease with increasing grid resolution. To the best of the authors' knowledge, the present investigations pertaining to discrete conservation and spurious force oscillations in the PSC approach have never been discussed in literature. Extensive numerical experiments are performed involving both stationary and moving bodies, with imposed and induced motion, to assess the applicability of the PSC approach. Our findings illustrate that the inclusion of added mass effects are important for accurate force and torque computations in case of accelerating solids. Numerical investigations, that include unsteady flows featuring three degrees-of-freedom fluid-particle interactions clearly highlight the ability of the PSC approach in resolving complex hydrodynamical phenomena. This in-depth evaluation of the PSC approach conclusively demonstrates its efficacy as a promising computational framework for efficient and robust computations of incompressible fluid flows over a range of flow scenarios.

Specific Aspects in Numerical Simulation of Complex Processes in Gas Turbine Engine Bearing Chamber

The complex interrelation of thermal and hydraulic processes in a gas turbine engine bearing chamber requires modelling methods based on the multiphase flow mechanics and Computational Fluid Dynamics (CFD) to predict the fluid distribution and heat transfer phenomena. This paper presents a study of different approaches to CFD modelling of multiphase oil-air flow in the bearing chamber. The Volume of Fluid and Eulerian multiphase models, Steady and Transient solvers, “Realizable k-ε” and “k-ω SST” turbulence models were analysed. The Eulerian Wall Film Model implemented in ANSYS Fluent was applied to model an oil film formation on the bearing chamber walls. The CFD results were compared with available experimental data to formulate practical recommendations for precise modelling of processes in the bearing chamber.

Fully compressible multiphase model for computation of compressible fluid flows with large density ratio and the presence of shock waves

We propose a fully compressible multiphase model for simulating compressible interfacial flows. The mathematical model is formulated on the basis of the typical conservation laws for mixtures, including mass, momentum, and energy balance. The mass balance equation in each phase is considered to discretely conserve the mass of each phase; therefore, it is always mass conservative for each phase. The numerical solution procedure for this model is developed using a high-resolution shock-capturing finite-volume method based on the Harten–Lax–van Leer-contact (HLLC) Riemann solver and total variation diminishing (TVD) time-integration schemes for simulating unsteady multiphase flows and capturing strong shocks. The finite-volume Riemann solver is combined with a monotonic upstream-centered scheme for conservation laws (MUSCL) and compressive limiters to maintain a sharp interface. The numerical framework yields a conceptually simple but robust and versatile numerical procedure. Numerical results of benchmark Riemann problems demonstrate the ability of the proposed method to obtain sharp interfaces, free of spurious oscillations, high accuracy, conservation properties, and thermodynamic consistency over various Mach numbers.

Optimization of Thermo-Physical Properties For The Binary System Of Acetone–Water At 303.15-318.15 K By Response Surface Quadratic Model

Thermo bodily residences of binary liquid combos are very beneficial in biotechnology, pharmaceutical, chemical and petroleum refining industries for improved favoured products from diverse raw materials. The physical property data on mixed solvents are important for the theoretical and applied areas of research and are frequently used in many chemical and industrial processes such as design of new process and process equipment (fluid flow, mass transfer or heat transfer calculation) and designing of bioreactor/fermenter. The objective of the current paper is to determine the density, viscosity, ultrasonic velocity, refractive index and surface tension of acetone (1) + water (2) at temperatures of 303.15K to 318.15K for the complete composition degrees and atmospheric pressure. These experimental values were used to calculate the respective excess homes in conjunction with a few acoustic properties. The experimental facts and excess residences have been used to calculate the interacting coefficients and well known deviations from different existing models. Also the new model equations have been evolved with the aid of the use of Design Expert application for density, viscosity, ultrasonic velocity, refractive indices and surface tension. Experimental consequences have been analysed on the premise of molecular interactions among element molecules with the assistance of FT-IR spectrum. Acetone-water binary structures of the prevailing observer have positive values of excess molar volumes and deviations in isentropic compressibility over whole composition range and at all temperatures which suggests sturdy interactions among the components of binary mixtures. Thermodynamic observations of acetone and water mixtures were made. Negative viscosity deviation of acetone-water combinations suggests robust dipole-dipole interactions within the device. Fourier remodel infrared spectroscopy also indicates the sturdy interaction made at 3589 cm-1 and based on the response surface method outcomes, more interplay takes area at zero. Five and zero six moles of acetone and water combination and the consequences are conformed with reference to the R2 cost (0.89). Excess molar extent and isentropic compressibility were decided and kinds of interactions have been discussed primarily based on the derived properties.

A fluid dynamic model for Single Well Chemical Tracer tests with variable petrophysical and pre-flushing parameters

Porosity and permeability are two fundamental reservoir parameters. We study how important large variations in their values are for residual oil saturation estimates from Single Well Chemical Tracer tests. Although porosity and permeability do not enter the classical chromatography formulae, or variations thereof, that does not necessarily imply that they are irrelevant in all real scenarios. This is because porosity and permeability govern how fluids are distributed within the oil-bearing formation, and thus influence dispersion, temperature, rate of hydrolysis of the primary tracer, pH, partitioning etc., all of which may affect the residual oil saturation estimates. We focus on coarsening and fining upwards sedimentary sequences, but we also consider constant porosity scenarios. In addition, we examine how spatial variations in residual oil saturation influence the single value ‘average’ obtained by the tracer test. The impact of pre-flushing on the estimated residual oil saturation estimate is investigated as well. An axially symmetric finite element model was developed that calculates fluid flow in the wellbore as well as in the oil-bearing target formation; reservoir cooling caused by the injection of cold brine; transport of solutes in the brine; and pH driven changes in the rate of hydrolysis of ethyl acetate. A Reynolds Averaged Navier-Stokes equation with an algebraic turbulence model was applied in the wellbore to calculate fluid flow there, whereas the Brinkman equation was used in the porous target formation. The temperature of the brine pumped into the target as a function of time was calculated analytically for a down casing model. pH changes induced by the acetic acid produced by the hydrolysis of ethyl acetate are buffered by solutes in the injected brine as well as by calcite in the oil-bearing formation were accounted for. The transport of solutes calculations account for fluid advection, diffusion, dispersion as well as temperature dependent partitioning of ethyl acetate between the residual oil and the injected brine. We use test data based on published values and a brine composition that is realistic for a sandstone reservoir. The synthetic tracer production curves generated by the model vary only modestly between the various porosity, permeability, residual oil saturation and pre-flushing models. A simple and widely used chromatography formula was applied to estimate the residual oil saturation from the synthetic tracer curves. This yields 18–19% for all porosity-permeability scenarios when the true constant value is 22%. We also studied six cases with variable Sor. In these cases, the chromatographic formula underestimates the average residual oil saturation by 1–3% except in two models where residual oil saturation increases with increasing porosity; then, the estimate is 8% too low. More work is needed to understand why. In summary, we find that variable porosity and permeability do not significantly increase the estimation error relative to constant models, except when the residual oil saturation varies spatially – then, the error may be much larger. Finally, four pre-flushing models all yield 18% residual oil saturation for a true constant value of 22%, i.e., the error is like the tests without pre-flushing.

A method for generating moving, orthogonal, area preserving polygonal meshes

A new method for generating locally orthogonal polygonal meshes from a set of generator points is presented in which polygon areas are a constraint. The area constraint property is particularly useful for particle methods where moving polygons track a discrete portion of material. Because Voronoi polygon meshes have some very attractive mathematical and numerical properties for numerical computation, a generalization of Voronoi polygon meshes was formulated that enforces a polygon area constraint. Area constrained moving polygonal meshes allow one to develop hybrid particle-mesh numerical methods that display some of the most attractive features of each approach. It is shown that this mesh construction method can continuously reconnect a moving, unstructured polygonal mesh in a pseudo-Lagrangian fashion without change in cell area/volume, and the method's ability to simulate various physical scenarios is shown. The advantages are identified for incompressible fluid flow calculations, with demonstration cases that include material discontinuities of all three phases of matter and large density jumps.

On air entrapment onset and surface velocity in high-speed turbulent prototype flows

In a free-surface spillway, the upstream flow is non-aerated and the flow becomes a strong air-water mix downstream of the onset location of air entrapment. Field observations were conducted over a steep spillway chute, and detailed quantitative measurements were undertaken in real-world high-speed flows with strong turbulence and very high Reynolds numbers within the range 107 to 108. The data showed that the onset of air entrapment is a complicated transient three-dimensional process in high-speed strongly-turbulent flows. A robust optical flow (OF) technique was applied and provided physically-meaningful surface velocities in the non-aerated flow region. The streamwise velocities were reasonably close to ideal fluid flow calculations, with large streamwise surface velocity fluctuations, in the non-aerated flow region. Overall, the study demonstrated the application of optical techniques to prototype spillway flows, provided that some careful validation was undertaken.

Computation of Electric Charged Micro-particles and Fluid Flows in Filtration Process

Computation of Electric Charged Micro-particles and Fluid Flows in Filtration Process