Swirl flow in conical diffusers

Dr. Dejan Ilić, Assist. Prof., University of Belgrade, Faculty of Mechanical Engineering

This paper comprises complex experimental investigations of the turbulent swirl flow in three conical diffusers (various diffuser angles). The incompressible swirl flow field is induced by the axial fan runner, which is set in the initial part of the straight pipe section followed by a conical diffuser. The average values of pressure and fluid velocity are determined using classical probes measuring the profiles of total and static pressure, as well as flow angle, i.e. profiles of the average circumferential and axial velocity components at different sections along the conical diffuser, for different regimes. Ratio of swirl flow loss coefficients and loss coefficients in pure axial flow, for tree conical diffusers is determined. With LDA system specific velocity components in certain sections of a diffuser were measured and statistical properties of generated turbulence were calculated. Comparison of the results of measurements of the average velocity of classical probes and LDA system is also shown.

Keywords: diffuser, swirl flow, turbulence, LDA measurements.

Experimental investigations and statistical analysis of turbulent swirl flow in a straight pipe

Prof. Dr. Milan Lečić, Dr. Aleksandar Ćoćić, Assist. Prof., University of Belgrade, Faculty of Mechanical Engineering, Jela Burazer, PhD student, Res. Assist., Institute Goša, Belgrade

This paper presents the results of our own velocity field measurements in a straight pipe swirl flow. The studies were conducted using the original hot wire probe. Because of the special shape of the probe, it was possible to get four measurement points in the viscous sublayer. Based on the measured voltage signal from the probe, time-averaged velocity field and statistical moments of the second and third order are calculated. Mathematical and physical interpretation of statistical characteristics and structure of turbulent swirl flow in time domain are presented. On the basis of these results a closer insight into turbulent transport processes can be obtained, as well as usefull conclusions necessary for turbulent swirl flows modeling.

Keywords: turbulent swirl flow, hot wire probe, velocity field.

Hydraulic performance of impellers with semi-parabolic blade profiles and modeling of the impeller's mixing losses with CFD

Dr.-Ing.Vladimir Škara, Research Engineer, Hydraulic Design & Fluid Dynamics Technology Development, Group Research, Innovation & Technology WILO SE, Dortmund

State of the art design of centrifugal pumps is unimaginable without the use of the CFD simulations. Presented research focuses on the design of the radial and mixed flow impellers based on the Ansys CFX turbo system. The workflow was modified so that the geometry input was provided usingthe WILO in-house impeller tool. Ansys Turbogrid was used for mesh generation and Ansys CFX was used for CFD simulation. Impeller design is a process of gradual limitation of the impeller’s degrees of freedom, by the choice of impeller design parameters. Initially, main impeller dimensions are defined in respect with impeller’s hydraulic targets, for ex.: zLA, b2, d1a, d1i, β2b. The second step determines the impeller’s meridional contour. The third step determines the impeller’s blade itselfwith the distributions of blade angles along the blade length. The fourth and the final step defines the blade’s profile. Each of the design steps contains specific design features and one of the important design features which may influence the overall hydraulic performance is the so called:“semi-parabolic” blade profile. Hydraulic performance of three impellers for the pump with specific speed of nq=80 was observed. One of the impellers has standard asymmetric profile used at WILO, the other two are having parabolic and semi-parabolic blade profiles. Hydraulic performances of all impellers were evaluated within the single channel CFD simulation. It was showed that the impeller with semi-parabolic profile shows best hydraulic performance in respect to the highest delivery head, lowest NPSH3%at design point and in overload, with the most the uniform outlet flow. Gülich1 states that the non-uniform velocity distributions are one of the main source of energy losses, especially in pumps with high specific speeds and that is hardy possible to predict such losses theoretically. Therefore he suggested an approach for the evaluation of the mixing losses within CFD simulations. This research shows also practical considerations for modelling of the mixing losses within the Ansys CFX. The observation of flow non-uniformities may give the designer useful informationregarding the quality of the impeller outlet flow and thus enable him to optimize the impeller performance design even further. The semi-parabolic blade profile feature enables the designer to further load already high loaded impeller blades. This provides the possibility for achieving more compact and in this respect, more cost effectively optimized pump design.

Keywords: centrifugal pump design, impeller blade profiles, semi-parabolic blade profile, CFD simulations, Ansys CFX, mixing losses, velocity non-uniformities.

Numerical tracking and turbulent dispersion of sorbent particles during gas desulfurization in pulverized coal combustion furnace

Ivan Tomanović, PhD student, University of Belgrade, „Vinča“ Institute of Nuclear Sciences, Laboratory for Thermal Engineering and Energy

A two phase, gas-particle flow is considered. Pulverized coal and calcium-based sorbent particlesmotion is simulated duringdesulfurization of flue gases inside of the boiler furnace. It is important to determine trajectories of particles in the furnace, in order to monitor the particles heat and concentration history. A two-way coupling of the phases is considered –influence of the gas phase on the particles, as well as the influence of particles on the gas phase. Particle–to–particle interactionsare neglected.Mutual influence of gas and dispersed phase is modelled by corresponding terms in the transport equations for gas phase and the equations describingthe particles motion. Gas phase is modelled in Eulerian field, while the particles are tracked in Lagrangian field. Turbulence is modelled by the standard k-ε model, with additional terms for turbulence modulation. Dispersion and residence time of sorbent particles in the furnace have a considerable influence on the desulfurization process.

Keywords: two phase flow, turbulence, reactive flow, model

Mathematical modelling of turbulent flow in rectangular ducts

Branislav Stanković, PhD student, University of Belgrade, „Vinča“ Institute of Nuclear Sciences, Laboratory for Thermal Engineering and Energy

The essential ideas of investigations of turbulent flow in a straight rectangular duct are chronologically presented. Fundamentally significant experimental and theoretical studies for mathematical modeling and numerical computations of this flow configuration are analyzed. An important physical aspect of this flow is the presence of secondary motion in the plane perpendicular to the streamwise direction, which is of interest from both the engineering and the scientific viewpoints. The key facts for the task of turbulence modelling and optimal choice of turbulence model for this flow case are obtained through careful examination of the physical mechanisms that generate the secondary flow.

Keywords: turbulent flow, rectangular duct, secondary flows, symmetry, opposite pairs, driving mechanisms, turbulence models.

Analysis of wake effect in wind power plants

M.Sc. Saša Rakić, Dr. Željko Đurišić, Assist. Prof., University of Belgrade, Faculty of Electrical Engineering

Wind turbines are grouped in a wind farm in order to yield as much energy as possible from the wind in a specific location. This should be done efficiently, with а minimal number of wind turbines, and also with a minimal space between them, due to the economy of land. However, minimization of the distances between wind turbines within a wind farm causes an increase of the so called wake effect. When the turbine extracts energy from the wind, a wake evolves downstream of the turbine. If another nearby turbine is operating within this wake it will be exposed to a degraded quality of air flow, which can lead to substantial decrease in power production and increase of dynamic loading due to increased turbulence. This paper gives the elementary characteristics of wake effect, causes of wake effect and the aerodynamics of disturbed air flow behind the turbine will be described, as well as the effects that it has on wind farm operation. Calculations will be illustrated using real measurement data of wind speed from a location near the village Bavanište in Vojvodina.

Keywords: wind farm, wake effect, wake modelling.

Numerical research of a vortex tube performance using OpenFOAM software

Jela Burazer, PhD student, Institute Goša, Belgrade, Dr. Aleksandar Ćoćić, Assist. Prof., Prof. Dr. Milan Lečić, University of Belgrade, Faculty of Mechanical Engineering

Energy separation is a spontaneous process of total temperature redistribution inside the fluid flow. A device known by this phenomena is called vortex tube. Inside this device with no moving parts, a pressurised gas being injected tangentially in respect to the tube's axis, is separated into cold and hot fluid flows. The flow inside this device is swirling, turbulent and compressible. A modification of an OpenFOAM solver is presented, where modified solver is able to capture the temperature differencing. The computational domain is 2D, with axysimmetric flow assumed. For turbulence modelling a standard k-e and k-w SST models are used. After showing the abilities of the modified solver regarding energy separation phenomena, a parametric analysis of the performance of the counterflow vortex tube is presented. The influences of L/D ratio and Prt number on the temperature on hot and cold outlets is examined.

Keywords: vortex tube, swirl, turbulence, compressibility, OpenFOAM, energy separation.

PIV and LDA investigation of the turbulent swirl flow behind the axial fan in pipe and jet

Novica Janković, PhD student, Research Assist., Dr. Đorđe Čantrak, Assist. Prof., University of Belgrade, Faculty of Mechanical Engineering, Philipp Mattern, PhD student, Karlsruhe Institute of Technology, Institute of Fluid Machinery, Assist. Mag. Sc. Slobodan Tašin, University of Novi Sad, Faculty of Technical Sciences

Experimental investigation of the turbulent swirl flow behind the axial fan in built in the test rig with free inlet and ducted or free outlet is reported. Axial fan, designed to generate Rankine vortex, has nine blades. The fan has been used without the outlet guide vanes, though it was designed for this case. The impeller diameter is Da=0.399m, while the dimensionless hub ratio Di/Da =0.5, where Di is the average hub diameter. The blade angle at outer diameter was positioned at the angle of 30°. Two test rigs have been built for studying the turbulent swirl flow in pipe. The first case is studied in the test rig consisted only of the straight pipe section 27.74D long, where D=0.4m is the inner pipe diameter. One-component laser Doppler anemometry (LDA) and stereo particle image velocimetry (SPIV) have been used in the first test rig in the measuring section 3.35D, measured from the test rig inlet. Region of interest size was app. 180mm x 90mm in the pipe cross-section, while 130mm x 80mm in the vertical meridian section. The second test rig is designed with the same inlet conditions, which are followed by the 20D long aluminium pipe followed by the exhaust hose, volume flow meter, valve and booster fan. Novel measuring technique, high speed SPIV (HSS PIV), has been used for the measurements in the second test rig in the section 2.1D downstream the fan's trailing edge. The HSS PIV sampling rate was 2kHz and it was performed in a cross-section, as well in a horizontal meridian plane perpendicular to that cross-section. Fan rotation number in the first test rig had values n=1000 and 1500 rpm, while in the second test rig 1200 rpm. Turbulent velocity field non-homogeneity and anisotropy is shown by use of the LDA system. Moments of the second and higher orders reveal complex mechanisms in the turbulent swirl flow. Vortex core dynamics was studied on the basis of the SPIV experimental results - instant and mean velocity fields. HSS PIV experimental results showed three-dimensionality and non-homogeneity of the generated turbulent swirl flow. Experimentally determined and calculated invariant maps revealed three-component isotropic turbulence in the vortex core region, especially in the pipe axis zone. In addition, results of the first experiments in the turbulent swirl flow jet behind the axial fan have been reported. Axial fan was again adjusted at the rotation numbers n=1000 and 1500 rpm. Velocity field non-homogeneity is studied by use of the three-component LDA system. Delicate LDA technique and flow seeding conditions are discussed.

Keywords: turbulence, axial fan, swirl, jet, stereo PIV, high speed stereo PIV, LDA, invariant maps.

Fluid-structure interaction with a hybrid RANS-LES turbulence model for applications in transonic flow domain

Bojan Šekutkovski, PhD student, Dr. Ivan Kostić, Assoc. Prof., Dr. Aleksandar Simonović, Assoc. Prof., University of Belgrade, Faculty of Mechanical Engineering

Current industrial practice for prediction of fluid-structure interaction (FSI) phenomena, such as flutter, is heavily based on linear methods. These methods involve many of design limitations and envelope restrictions for aircraft. In this work novel hybrid RANS-LES turbulence model (k-Omega Shear Stress Transport Scale-Adaptive Improved Delayed Detached Eddy Simulation) is tested and implemented in the FSI procedure and is applied in transonic flow. The turbulence model combines the advanced capabilities of the existing SST, SAS and IDDES turbulence models. Strongly coupled three-dimensional (3D) FSI solver is combined with the density based solver, turbulence model and large deformation updated Lagrangian finite volume structural solver in order to resolve standard computational fluid dynamics (CFD) and FSI benchmark cases of transonic flow. The numerical results of Onera M6 and AGARD 445.6 validation cases are presented and compared with the existing experimental results. Discretization of the governing equations is performed by cell-centered finite volume method (FVM) on unstructured meshes. The emphasis is made on turbulence modeling which appears to have a major impact to the prediction of FSI behavior in transonic flow domain. Described FSI solver is custom written and implemented in OpenFOAM.

Keywords: fluid-structure interaction, finite volume method, RANS-LES, transonic turbulent flow, OpenFOAM.

Numerical Flow Simulations in the Agnew micro hydro turbine

Đorđe Novković, PhD student, Teaching Assist., University of Priština, Faculty of Technical Sciences, Serbia, Prof. Dr. Milan Lečić, University of Belgrade, Faculty of Mechanical Engineering, Jela Burazer, PhD student, Res. Assist., Institute Goša, Darko Radenković, PhD student, Teaching Assist., University of Belgrade, Faculty of Mechanical Engineering

Numerical flow simulations in the Agnew micro hydro turbine using Ansys CFX software have been performed in this paper. Simulations were performed under assumption of steady state flow in the turbine. Different two-equation turbulence models were used during this process. For each of selected turbulence model, several different operating regimes were simulated. During it, good numerical stability and convergence of solution were obtained. Based on the numerically obtained results, performance curves were formed for each of used turbulence model. Such formed performance curves were mutually compared. In turbine’s optimal operating regime, velocity profiles and static pressure distribution in a radnom selected cross section of the draft tube were calculated for each of used turbulence model. After that, obtained velocity profiles and obtained distributions of static pressure were mutually compared.

Keywords: CFD, small turbine, Agnew turbine, turbulence modeling.

School of the turbulent swirl flow at the Faculty of Mechanical Engineering University of Belgrade

Prof. Dr. Milan Lečić, Dr. Đorđe Čantrak, Assist. Prof., Dr. Aleksandar Ćoćić, Assist. Prof., University of Belgrade, Faculty of Mechanical Engineering

Almost half a century occurs and strengthens School of the turbulent swirl flow at the Faculty of Mechanical Engineering University of Belgrade. It was born with the first papers of Prof. Dr.-Ing. Ivo Vušković (1912-2005) in 1966 and Prof. Dr.-Ing. Zoran Protić (1922-2010) in 1970. The founder of this school in the former Yugoslavia was Prof. Dr.-Ing. Zoran Protić (1922-2010). Researchers and followers of this school are Prof. Dr. Miroslav Benišek, Prof. Dr.-Ing. Svetislav Čantrak, Dr. Bojan Vukašinović, Prof. Dr. Milan Lečić, Assist. Prof. Aleksandar Ćoćić, Assist. Prof. Dr. Đorđe Čantrak, Assist. Prof. Dejan Ilić, Research Assist. Novica Janković and Research Assist. Jela Burazer. Prof. Dr. Miloš Nedeljković, Princ. Res. Fellow Dr. Slavica Ristić and Prof. Dr. Jelena Ilić are coauthors in some papers published by this school members, as well some foreign researchers. In this paper are presented some of the most important results of these authors in the field of turbulent swirl flow. These results have been fulfilled in Mag. Sc. and PhD thesis, as well in papers published in journals or conference proceedings. The major part of the research is related to the experimental investigation of the turbulent swirl flow in pipe, diffuser and jet behind the axial fan. At the Faculty of Mechanical Engineering in Belgrade are designed and built experimental test rigs for this purpose. Various experimental techniques have been employed. First original classical probes, constructed and manufactured by Prof. Dr. Miroslav Benišek, were used. Prof. Dr. Petar Vukoslavčević, Member of the Montenegrin Academy of Sciences and Arts designed and produced original hot-wire anemometry probes. Particle Image Velocimetry (PIV) has been introduced in this school in 2007. by Dr. Đorđe Čantrak, and later laser Doppler anemometry. PhD student Novica Janković has recently emplemented 3D LDA system in the research of the turbulent swirl flow in jet. Computational fluid dynamics (CFD) has been also widely used, starting from simplified equations to resolving the turbulent swirl flow with the open source code OpenFOAM. This software was used for calculations in pipe and founder of the work in this software is Dr. Aleksandar Ćoćić. Research Assist. Jela Burazer has joined this research with the idea to resolve velocity and temperature fields in the Ranque-Hilsch vortex tube.

Keywords: turbulent flow, classical probes, HWA probes, LDA, PIV, OpenFOAM.


Recent advances and open challenges in high-speed combustion physics

Dr. Javier Urzay, Center for Turbulence Research, Stanford University

Great efforts have been dedicated over the last 50 years to the research and development of high-speed aerospace vehicles for terrestrial transportation, space exploration, and long-range global-strike weapons. However, superlative challenges related to mixing and combustion were identified very early in the history of development of high-speed propulsion. This lecture will review recent advances in the understanding of the fundamental physics and formulation of high-speed combustion, including autoigniting fuel sprays and supersonic flames. The presentation will emphasize the importance of interpreting dimensionless parameters in canonical problems for enabling understanding of more complex physical scenarios.

Keywords: spray combustion, supersonic combustion.