Turbulent Multiphase Flows Research Articles

Thermal performance of nanoparticle enhanced phase change material in a concentric cylindrical enclosure

This paper presents the flow and heat transfer characteristics of melting behavior of phase change material (PCM) in the presence of nanoparticles. The melting process occurs in a concentric cylindrical enclosure filled with PCM (ice) and aluminum oxide (Al2O3) nanoparticles. The inner and outer walls of the concentric cylinder are considered as hot and cold walls and maintained at a constant wall temperature. The problem is modeled as unsteady, two-dimensional turbulent multiphase flow using the Eulerian multiphase model and SST k-ω turbulence model. The numerical simulation is performed using Ansys Fluent 19.2. The primary and secondary phases are specified with thermo-physical properties of ice and aluminum oxide. The transient melting behavior of PCM is investigated for different nanoparticle volume concentrations by varying the temperature difference between the hot and cold walls of concentric enclosure. The results are analyzed by plotting the streamline, velocity and temperature contours. The results indicate that the dispersion of aluminum oxide nanoparticles into phase change material increases the heat transfer rate and decreases the melting temperature of PCM. The results also indicate that the melting rate and thermal performance are significantly enhanced with increase in volume concentrations and temperature difference.

Design Space Exploration of Turbulent Multiphase Flows Using Machine Learning-Based Surrogate Model

This study focuses on establishing a surrogate model based on machine learning techniques to predict the time-averaged spatially distributed behaviors of vaporizing liquid jets in turbulent air crossflow for momentum flux ratios between 5 and 120. This surrogate model extends a previously developed Gaussian-process-based framework applicable to laminar flows to accommodate turbulent flows and demonstrates that in addition to detailed fields of primitive variables, second-order turbulence statistics can also be predicted using machine learning techniques. The framework proceeds in 3 steps—(1) design of experiment studies to identify training points and conducting high-fidelity calculations to build the training dataset; (2) Gaussian process regression (supervised training) for the range of operating conditions under consideration for gaseous and dispersed phase quantities; and (3) error quantification of the surrogate model by comparing the machine learning predictions with the truth model for test conditions (i.e., conditions not used for training). The framework was trained using data generated by high-fidelity large eddy simulation (LES)-based calculations (also referred to as the truth model), which solves the complete set of conservation equations for mass, momentum, energy, and species in an Eulerian reference frame, coupled with a Lagrangian solver that tracks the dispersed phase. Simulations were conducted for the range of momentum flux ratios between 5 and 120 for liquid water injected into crossflowing air at a pressure of 1 atm and temperature of 600 K. Results from the machine-learned surrogate model, also called emulations, were compared with the truth model under testing conditions identified by momentum flux ratios of 7 and 40. L1 errors for time-averaged field quantities, including velocity magnitudes, pressure, temperature, vapor fraction of the evaporated liquid, and turbulent kinetic energy in the gas phase, and spray penetration and Sauter mean diameters in the dispersed phase are reported. Speedup of 65 was achieved with this emulator when compared against LES simulation of the same test conditions with errors for all quantities below 14%, thus demonstrating the potential benefits of using machine learning techniques for design space exploration of devices that are based on turbulent multiphase fluid flows. This is the first effort of its kind in the literature that demonstrates the application of machine learning techniques on turbulent multiphase flows.

Electromagnetic Effects on Solidification Defect Formation in Continuous Steel Casting

Understanding and reducing defects formed during continuous casting of steel are challenging because of the many inter-related, multiscale phenomena and process parameters involved in this complex process. Solidification occurs in the presence of turbulent multiphase flow, transport and capture of particles, superheat transport, and thermal–mechanical behavior. The application of electromagnetic fields provides an additional parameter to control these phenomena to reduce solidification defects. It is especially attractive because the field has the potential to be easily adjusted during casting to accommodate different casting conditions. This article briefly reviews how electromagnetic forces affect solidification defects, including subsurface hooks, particle capture, deep oscillation marks, depressions, cracks, breakouts, segregation, and shrinkage. This includes the related effects on superheat transport, initial solidification, surface quality, grain structure, internal quality, and steel composition distribution. Finally, some practical strategies regarding how to apply electromagnetics to improve steel quality are evaluated.

VarRhoTurbVOF 2: Modified OpenFOAM volume of fluid solvers with advanced turbulence modeling capability

varRhoTurbVOF contains a set of OpenFOAM volume of fluid (VOF) solvers for turbulent isothermal multiphase flows, which are variable-density incompressible. Unlike their official counterparts, where Favre-averaged and Reynolds-averaged velocities coexist in different equations, new solvers use Favre-averaged velocities consistently in all equations. This major update introduces three main improvements to the previous version of varRhoTurbVOF. First, the implementation is extended to VOF solvers for isothermal and non-isothermal phase change two-phase flows, where the flow is no longer incompressible. Second, in order to introduce backward compatibility and to avoid code duplication, the turbulence model construction procedure is redesigned such that solvers can determine whether the variable-density effect is considered or not in the turbulence modeling part based on the input file at run time. Third, the Egorov turbulence damping model for ω-based turbulence models is implemented with its most recent developments. Plus, an extension to ϵ-based turbulence models is developed and implemented. Program summaryProgram Title: varRhoTurbVOFCPC Library link to program files:http://dx.doi.org/10.17632/4t8z8vzyvs.2Developer’s repository link:https://github.com/wenyuan-fan/varRhoTurbVOF_2Licensing provisions: GPLv3Programming language: C++Journal reference of previous version: Wenyuan Fan, Henryk Anglart, 2020. varRhoTurbVOF: A new set of volume of fluid solvers for turbulent isothermal multiphase flows in OpenFOAM, Computer Physics Communications, 247, 106876.Does the new version supersede the previous version?: YesReasons for the new version: Implementation of new solvers and models; code redesign to maintain backward compatibility.Summary of revisions: The updated version mainly has three new features in comparison with the previous version: •The issue addressed in [1] exists in all OpenFOAM VOF solvers where the variable-density effect is neglected in the turbulence modeling. In order to solve this issue for phase change VOF solvers, two additional solvers, namely, varRhoInterPhaseChangeFoam and varRhoInterCondensatingEvaporatingFoam, are implemented based on their official counterparts.•The procedure of constructing the turbulence model inside individual solvers is redesigned. Using the new design, solvers will construct turbulence models with or without the variable-density effect being considered based on the user input. When the latter mode is activated, solvers will behave just like their official counterparts. In such a way, the backward compatibility of the code is achieved without introducing code duplications.•The Egorov turbulence damping model [2], which is a popular approach to phenomenologically modify turbulence behaviors near a two-phase interface, is implemented. In addition to the original model, recent modifications, e.g., a new length scale calculation and asymmetric damping treatments [3], are implemented as well. The Egorov model only works with ω-based turbulence models. In the new release, the model is extended to ϵ-based turbulence models following the idea proposed in [4] such that the turbulence damping model is applicable to commonly used turbulence models.Nature of problem: Within the VOF framework, the mixture has a density changing with phases. However, in quite a few OpenFOAM VOF solvers, the variable-density effect is considered in the momentum equation but neglected in the turbulence modeling. As a result, Favre-averaged velocities and Reynolds-averaged velocities are used in the momentum equation and turbulence quantity equations, respectively and simultaneously, introducing a severe self-inconsistency. Solutions to this problem have been proposed in [1] with no backward compatibility support. Also, no phase change solvers are supported in the previous version. Another issue with turbulent VOF simulations is that, even with the correct implementation, certain modifications are still needed to correct the turbulence behavior around the interface.Solution method: The extension to phase change VOF solvers is straightforward according to [1]. Regarding the construction of turbulence models, two additional fields, namely, rhoTurb and rhoPhiTurb, are created and used in turbulence models. If the user chooses to use the variable-density turbulence models, real two-phase density rho and mass flux rhoPhi will be assigned to rhoTurb and rhoPhiTurb, respectively. Otherwise, rhoTurb and rhoPhiTurb will use unity and volume flux phi, respectively. This run time selection design enables solvers to operate in two modes, which guarantees the backward compatibility and avoids code duplications. The implementation of the turbulence damping model also takes the backward compatibility into account. At run time, the code will automatically determine whether the mixture density should be included in the turbulence damping model by checking the dimension of the corresponding equation.Additional comments: A manual is provided to introduce how the turbulence damping modeled is formulated and how to use the model. It should be emphasized that the Egorov damping model is a phenomenological model. Model parameters should be carefully selected for any given mesh and flow condition.

Multiphase flow behaviour and hazard prediction of pyroclastic density currents

Pyroclastic density currents (PDCs) are dangerous multiphase flows originating from volcanic eruptions. PDCs cause more than a third of volcanic fatalities globally and, therefore, development of robust PDC hazard models is a priority in volcanology and natural hazard science. However, the complexity of gas–particle interactions inside PDCs, as well as their hostile nature, makes quantitative measurements of internal flow properties, and the validation of hazard models, challenging. Within the last decade, major advances from large-scale experiments, field observations and computational and theoretical models have provided new insights into the enigmatic internal structure of PDCs and identified key processes behind their fluid-like motion. Recent developments have also revealed important links between newly recognized processes of mesoscale turbulence and PDC behaviour. In this Review, we consider how recent advances in PDC research close the gaps towards more robust hazard modelling, outline the need to measure the internal properties of natural flows using geophysical methods and identify critical future research challenges. Greater understanding of PDCs will also provide insights into the dynamics of other natural gravity currents and high-energy turbulent multiphase flows, such as debris avalanches and turbidity currents. Pyroclastic density currents are complex multiphase flows originating from volcanic eruptions and account for almost a third of volcanic fatalities globally. This Review discusses recent advances in understanding of the complex internal processes within pyroclastic density currents and how these influence the flow dynamics and hazard footprints.

Modelling polydisperse nanoparticle size distributions as produced via flame spray pyrolysis

The correct description of the particulates produced via the flame spray pyrolysis (FSP) process is essential for scale-up studies and new reactor design. With the combination of computational fluid dynamics (CFD) and population balance models (PBM) it is possible to predict the formation and evolution of nanoparticles in such reactors, and estimate final product characteristics. The solution of such PBM, however, can be very costly, given the fact that the phenomena involved in the process require for a bivariate approach. In this work, the direct quadrature method of moments (DQMoM) is employed in order to obtain a polydisperse solution of the PBM, due to its relatively straight forward adaptation from uni- to bivariate cases. Furthermore, the reacting turbulent multiphase flow of the burning spray is described by an Eulerian-Lagrangian framework and solved with the use of CFD. The production of zirconium dioxide nanoparticles from a solution of zirconium n-propoxide in ethanol and propanol is investigated, and experimental data obtained through transmission electron microscopy (TEM) is used for model validation. Accuracy of about 90% is obtained for the first two moments of the distribution and the number of primary particles per agglomerate show good agreement with experiments.

A hybrid particle volume-of-fluid method for curvature estimation in multiphase flows

We present a particle method for estimating the curvature of interfaces in volume-of-fluid simulations of multiphase flows. The method is well suited for under-resolved interfaces, and it is shown to be more accurate than the parabolic fitting that is employed in such cases. The curvature is computed from the equilibrium positions of particles constrained to circular arcs and attracted to the interface. The proposed particle method is combined with the method of height functions at higher resolutions, and it is shown to outperform the current combinations of height functions and parabolic fitting. The algorithm is conceptually simple and straightforward to implement on new and existing software frameworks for multiphase flow simulations thus enhancing their capabilities in challenging flow problems. We evaluate the proposed hybrid method on a number of two- and three-dimensional benchmark flow problems and illustrate its capabilities on simulations of flows involving bubble coalescence and turbulent multiphase flows.

High speed visualization of self-oscillation of axisymmetric cavitating cavity in a square vortex chamber

An experimental investigation of the multiphase turbulent flow in a square vortex chamber was carried out to examine the self-oscillation of a cavitating cavity in a wide range of flow rates and pressure conditions. Such phenomenon is observed behind turbine runner when the hydraulic units operate at full load regime at high flowrate, and leads to significant flowrate and pressure pulsations in the entire hydraulic system. The dynamics of the cavity over time, and the freaquency of the volumetric oscillations frequency was studied via high speed visualization. The grey-scale images of the vortex cavity were binarized. By assuming an axially symmetric flow, the volume of cavity for each frame was integrated, and further investigated using FFT. Quantitative information on the structure of a single-phase flow was also obtained using the PIV technique.

A 3D Moment of Fluid method for simulating complex turbulent multiphase flows

This paper presents the moment of fluid method as a liquid/gas interface reconstruction method coupled with a mass momentum conservative approach within the context of numerical simulations of incompressible two-phase flows. This method tracks both liquid volume fraction and phase centroid for reconstructing the interface. The interface reconstruction is performed in a volume (and mass) conservative manner and accuracy of orientation of interface is ensured by minimizing the centroid distance between original and reconstructed interface. With two-phase flows, moment of fluid method is able to reconstruct interface without needing phase volume data from neighboring cells. The performance of this method is analyzed through various transport and deformation tests, and through simple two-phase flows tests that encounter changes in the interface topologies. Exhaustive mesh convergence study for the reconstruction error has been performed through various transport and deformation tests involving simple two-phase flows. It is then applied to simulate atomization of turbulent liquid diesel jet injected into a quiescent environment. The volume conservation error for the moment of fluid method remains small for this complex turbulent case.

A population balance model for large eddy simulation of polydisperse droplet evolution

In the context of many applications of turbulent multi-phase flows, knowledge of the dispersed phase size distribution and its evolution is critical to predicting important macroscopic features. We develop a large eddy simulation (LES) model that can predict the turbulent transport and evolution of size distributions, for a specific subset of applications in which the dispersed phase can be assumed to consist of spherical droplets, and occurring at low volume fraction. We use a population dynamics model for polydisperse droplet distributions specifically adapted to a LES framework including a model for droplet breakup due to turbulence, neglecting coalescence consistent with the assumed small dispersed phase volume fractions. We model the number density fields using an Eulerian approach for each bin of the discretized droplet size distribution. Following earlier methods used in the Reynolds-averaged Navier–Stokes framework, the droplet breakup due to turbulent fluctuations is modelled by treating droplet–eddy collisions as in kinetic theory of gases. Existing models assume the scale of droplet–eddy collision to be in the inertial range of turbulence. In order to also model smaller droplets comparable to or smaller than the Kolmogorov scale we extend the breakup kernels using a structure function model that smoothly transitions from the inertial to the viscous range. The model includes a dimensionless coefficient that is fitted by comparing predictions in a one-dimensional version of the model with a laboratory experiment of oil droplet breakup below breaking waves. After initial comparisons of the one-dimensional model to measurements of oil droplets in an axisymmetric jet, it is then applied in a three-dimensional LES of a jet in cross-flow with large oil droplets of a single size being released at the source of the jet. We model the concentration fields using $N_{d}=15$ bins of discrete droplet sizes and solve scalar transport equations for each bin. The resulting droplet size distributions are compared with published experimental data, and good agreement for the relative size distribution is obtained. The LES results also enable us to quantify size distribution variability. We find that the probability distribution functions of key quantities such as the total surface area and the Sauter mean diameter of oil droplets are highly variable, some displaying strong non-Gaussian intermittent behaviour.

VarRhoTurbVOF: A new set of volume of fluid solvers for turbulent isothermal multiphase flows in OpenFOAM

The volume of fluid (VOF) method is a popular approach for multiphase flow modeling. The open-source computational fluid dynamics (CFD) software, OpenFOAM, implements a variety of VOF-based solvers and provides users a wide range of turbulence models. Since isothermal multiphase flows under the VOF framework belong to the variable-density incompressible flow category, the isothermal VOF-based solvers in OpenFOAM fail to use the correct turbulence models. varRhoTurbVOF is designed to solve this issue and with the hope to replace all the corresponding existing solvers in the future. With the object-oriented paradigm, varRhoTurbVOF guarantees the usability, reusability and maintainability of the codes. Aside from turbulence modeling, all other features in the original solvers are preserved in varRhoTurbVOF. Program summaryProgram Title: varRhoTurbVOFProgram Files doi:http://dx.doi.org/10.17632/4t8z8vzyvs.1Licensing provisions: GPLv3Programming language: C++Supplementary material:http://dx.doi.org/10.17632/7mp25kyb4p.4Nature of problem: Under the VOF framework, the flow of the isothermal mixture belongs to the variable-density incompressible flow category. For such flows, VOF-based solvers of OpenFOAM fail to construct the correct governing equations for turbulence modeling. varRhoTurbVOF contains a set of newly designed VOF-based solvers which could use the desired governing equations for turbulence quantities.Solution method: varRhoTurbVOF creates a new class for variable-density incompressible turbulence models, which allows reusing the existing turbulence model template classes. A set of VOF-based solvers are then created to be able to construct variable-density incompressible turbulence models.

제어핀이 달린 수중 물체의 공동 수치해석

The evolution of the cavity and the variation of the drag for an underwater body with control fins are investigated through a numerical analysis of the steady cavitating turbulent flow. The continuity and the steady-state RANS equations are numerically solved using a mixture fluid model for calculating the multiphase turbulent flow of air, water and vapor together with the SST k-ω turbulence model. The method of volume of fluid is applied by the use of the Sauer’s cavitation model. Numerical solutions have been obtained for the cavity flow about an underwater body shaped like the Russian high-speed torpedo, Shkval. Results are presented for the cavity shape and the drag of the body under the influence of the gravity and the free surface. The evolution of the cavity with the body speed is discussed and the calculated cavity shapes are compared with the photographs of the cavity taken from an underwater launch experiment. Also the variation of the drag for a wide range of the body speed is investigated and analyzed in details.

V-ONSET (Vertical Octagonal Noncorrosive Stirred Energetic Turbulence): A vertical water tunnel with a large energy dissipation rate to study bubble/droplet deformation and breakup in strong turbulence.

A vertical water tunnel facility has been constructed to study the dynamics of turbulent multiphase flow. The new system features several unique designs that allow us to study bubble deformation and breakup in strong turbulence: (i) The mean flow can be adjusted to balance the rising velocity of buoyant bubbles/droplets so that they can stay in the view area for an extended period of time. (ii) Turbulence is generated and controlled using a 3D-printed jet array that can fire 88 random high-speed momentum jets with the individual jet velocity of up to 12 m/s. This component allows us to attain turbulence with a high energy dissipation rate (≥0.1 m2/s3), which is orders-of-magnitude higher than most of the existing turbulent multiphase flow facilities. (iii) Turbulence generated in the test section is nearly homogeneous and isotropic, and the turbulent fluctuations are also decoupled from the mean flow. The resulting turbulence intensity can be varied between 20% and 80% with the speed of the mean flow at around 0.2 m/s. (iv) This system has an octagonal test section that allows six cameras to image and reconstruct the 3D shape of deforming bubbles/droplets in turbulence. The same set of cameras was also used for tracking tracers in the surrounding turbulent flow. Both the reconstruction and particle tracking were completed using our in-house codes that were parallelized to run on high-performance computing clusters efficiently.

Wavelet-spectral analysis of droplet-laden isotropic turbulence

The spectrum of turbulence kinetic energy for homogeneous turbulence is generally computed using the Fourier transform of the velocity field from physical three-dimensional space to wavenumber $k$. This analysis works well for single-phase homogeneous turbulent flows. In the case of multiphase turbulent flows, instead, the velocity field is non-smooth at the interface between the carrier fluid and the dispersed phase; thus, the energy spectra computed via Fourier transform exhibit spurious oscillations at high wavenumbers. An alternative definition of the spectrum uses the wavelet transform, which can handle discontinuities locally without affecting the entire spectrum while additionally preserving spatial information about the field. In this work, we propose using the wavelet energy spectrum to study multiphase turbulent flows. Also, we propose a new decomposition of the wavelet energy spectrum into three contributions corresponding to the carrier phase, droplets and interaction between the two. Lastly, we apply the new wavelet-decomposition tools in analysing the direct numerical simulation data of droplet-laden decaying isotropic turbulence (in absence of gravity) of Dodd & Ferrante (J. Fluid Mech., vol. 806, 2016, pp. 356–412). Our results show that, in comparison to the spectrum of the single-phase case, the droplets (i) do not affect the carrier-phase energy spectrum at high wavenumbers ($k_{m}/k_{min}\geqslant 128$), (ii) increase the energy spectrum at high wavenumbers ($k_{m}/k_{min}\geqslant 256$) by increasing the interaction energy spectrum at these wavenumbers and (iii) decrease the energy at low wavenumbers ($k_{m}/k_{min}\leqslant 16$) by increasing the dissipation rate at these wavenumbers.

Thermal Decomposition of a Single AdBlue® Droplet Including Wall–Film Formation in Turbulent Cross-Flow in an SCR System

The selective catalytic reduction (SCR) methodology is notably recognized as the widely applied strategy for NOX control in exhaust after-treatment technologies. In real SCR systems, complex unsteady turbulent multi-phase flow phenomena including poly-dispersed AdBlue® spray evolve with a wide ranging relative velocity between the droplet phase and carrier gas phase. This results from an AdBlue® spray that is injected into a mixing pipe which is cross-flowing by a hot exhaust gas. To reduce the complexity while gaining early information on the injected droplet size and velocity needed for a minimum deposition and optimal conversion, a single droplet with a specified diameter is addressed to mimic a spray featuring the same Sauter Mean Diameter. For that purpose, effects of turbulent hot cross-flow on thermal decomposition processes of a single AdBlue® droplet are numerically investigated. Thereby, a single AdBlue® droplet is injected into a hot cross-flowing stream within a mixing pipe in which it may experience phase change processes including interaction with the pipe wall along with liquid wall–film and possible solid deposit formation. First of all, the prediction capability of the multi-component evaporation model and thermal decomposition is evaluated against the detailed simulation results for standing droplet case for which experimental data is not available. Next, exploiting Large Eddy Simulation features the effect of hot turbulent co- and cross-flowing streams on the dynamic droplet characteristics and on the droplet/wall interaction is analyzed for various droplet diameters and operating conditions. This impact is highlighted in terms of droplet evaporation time, decomposition efficiency, droplet trajectories and wall–film formation. It turns out that smaller AdBlue® droplet diameter, higher gas temperature and relative velocity lead to shorter droplet life time as the droplet evaporates faster. Under such conditions, possible droplet/wall interaction processes on the pipe wall or at the entrance front of the monolith may be avoided. Since the ammonia (NH3) gas generated by urea decomposition is intended to reduce NOX emission in the SCR system, it is apparent for the prediction of high NOX removal performance that UWS injector system which allows to realize such operating conditions is favorable to support high conversion efficiency of urea into NH3.

Twente mass and heat transfer water tunnel: Temperature controlled turbulent multiphase channel flow with heat and mass transfer.

A new vertical water tunnel with global temperature control and the possibility for bubble and local heat and mass injection has been designed and constructed. The new facility offers the possibility to accurately study heat and mass transfer in turbulent multiphase flow (gas volume fraction up to 8%) with a Reynolds-number range from 1.5 × 104 to 3 × 105 in the case of water at room temperature. The tunnel is made of high-grade stainless steel permitting the use of salt solutions in excess of 15% mass fraction. The tunnel has a volume of 300 l. The tunnel has three interchangeable measurement sections of 1 m height but with different cross sections (0.3 × 0.04 m2, 0.3 × 0.06 m2, and 0.3 × 0.08 m2). The glass vertical measurement sections allow for optical access to the flow, enabling techniques such as laser Doppler anemometry, particle image velocimetry, particle tracking velocimetry, and laser-induced fluorescent imaging. Local sensors can be introduced from the top and can be traversed using a built-in traverse system, allowing, for example, local temperature, hot-wire, or local phase measurements. Combined with simultaneous velocity measurements, the local heat flux in single phase and two phase turbulent flows can thus be studied quantitatively and precisely.

Sediment-wear morphology prediction method of hydraulic machine based on differential-quadrature method

Damage caused by abrasive wear is a significant problem for hydraulic machinery in rivers with sediments. Traditional prediction methods of abrasive wear morphology with the fixed geometrical boundary are difficult to reflect the physical reality of wear because of the interaction of sediment wear, flow pattern and flow boundary. In this study, a gradual wear method based on the idea of differential-quadrature is established to investigate the asymptotic wear behavior of hydraulic machinery. The average wear rate and depth of voxels on the geometrical boundary can be obtained by the unsteady numerical simulation with RNG [Formula: see text]-[Formula: see text] turbulent model and DPM multiphase flow model. Segmented voxels of geometrical boundary of flow are reconstructed after a calculation period until the end of the total running time. So, the progressive wear morphologies of flow-passage walls can be revealed step by step. The prediction method is used to predict the wear of the clearance flow in the guide vane. The numerical results are in good agreement with the actual wear depth on the wear surfaces at the actual running time, which indicates that the specified prediction method in this paper is applicable and effective.

Novel trends in modelling techniques of Pelton Turbine bucket for increased renewable energy production

Pelton turbines have been used for harvesting clean energy from water jet for over 100 years. The wide range of their applicability and the robustness with minimal onsite monitoring and carbon free energy production makes them one of the most desired hydro turbines for renewable energy production. Pelton turbine buckets are subject to complex turbulent multiphase flows with free surfaces, thus received lot of attention from Computational Fluid Dynamics (CFD) researchers to visualise the flow patterns. In addition, the bucket geometry optimization has been a prevalent research stream using analytical and graphical methods. Both of these investigations fields have resulted in significant improvement in the performance of the Pelton Turbine system. However, the design investigations for each feature are carried out independently due to the complexity in incorporating large number of design parameters. Thus, analysing the complex fluid flow on each pre-optimized design was rather challenging due to expensive computational needs of CFD and limited manufacturing possibilities. Today, development of accurate and inexpensive CFD models and innovative manufacturing technology such as rapid prototyping has made complex freeform shape possible to simulate and manufacture. Hence, any bucket designs disregarded in optimization on the basis of manufacturing feasibility can now be possible to manufacture and is worth investigating. This will require the combination of CFD and design optimization field of investigation with novel analysis approach. This paper examines these fields of studies with the view to establish the background for such novel approach. This approach could be a foundation for design optimization of turbomachinery or any reaction surface leading to increased production of renewable and sustainable energy from existing resources.

Thermal-hydraulic performance evaluation of gas-liquid multiphase flows in a vertical sinusoidal wavy channel in the presence/absence of phase change

Turbulent gas-liquid multiphase flows with and without phase change in a vertical wavy channel are addressed. The multiphase flow field is resolved using the volume of fluid method (VOF), and the flow equations are discretized and numerically solved by the well-known finite volume method. As a multiphase system without mass transfer, air/water flow is considered. It is shown that numerical simulation is well capable of predicting the various multiphase flow regimes ranging from slug to bubbly flows inside wavy channels. Moreover, accurate predictions of overall pressure drop are provided by numerical solutions for various air and water flow rates and the phase shift angle between wavy channel walls. Additionally, condensing flows of refrigerant R134a are simulated inside wavy channels. It is found that for almost all the cases considered in the present study, the convective heat transfer coefficient is higher in wavy channels in respect to straight channels. However, a significant pressure drop penalty is observed especially for high mass fluxes across wavy channels. Therefore, the use of the wavy channels for the enhancement of condensing heat transfer is only advisable for low mass fluxes with the phase shift angle of 180°.

Development of a multiphase cavitation solver and its application for ventilated cavitating flows with natural cavitation

The aim of this article is to develop a cavitation solver capable of dealing with the cavitating flow where more than three phases coexist based on the OpenFOAM platform. A new phase change class coupled with newly reported cavitation models and turbulence models has been implemented in the solver. Firstly, validations have been carried out using cases of a two-dimensional cavitating hydrofoil and a three-dimensional projectile near the free surface. The pressure distribution and the gas-liquid interface profile agree well with the experimental data. The solver is then applied for a base-ventilated cavitating hydrofoil with different angle of attack where natural cavitation could occur simultaneously on the hydrofoil surface. The cavity shape as well as the transient cavity shedding process are correctly predicted using the PANS turbulence model. The resulting velocity and pressure distributions are clearly analyzed, giving great insights into the formation and deformation mechanisms of the ventilated cavity. The developed solver is proved to be accurate and efficient in solving the details of turbulent multiphase flows with phase change occurring and has great potential for future applications.