Search

This paper investigates turbulence characteristics and the parameters controlling the turbulent incompressible flow of a double-sided lid-driven cavity. The effects of varying Reynolds numbers (1-104 ≤ Re ≤ 2⋅105), speed ratios (0.05 ≤ S ≤ 1.0), and aspect ratios (0.5 ≤ K ≤ 2.0) on the turbulent quantities, such as kinetic energy, k, dissipation, s, turbulent viscosity, n,, are analyzed. The k-s turbulence model equations are solved using the FVM-based SIMPLE algorithm. Taguchi's approach uses an L16 orthogonal array to determine the optimal cavity parameters. The significance of the considered factors is estimated using the analysis of variance (ANOVA) method. The present study reveals that the turbulent quantities are significantly reduced by increasing the aspect ratio, speed ratio, and Reynolds number. Taguchi analysis suggests that the optimal fluid-flow rate is attained by combining S = 0.05, K = 0.5, and Re = 2⋅105. The ANOVA analysis shows the significant percentage contribution for parameters S and Reynolds number, which are approximately 62.29% and 30.21%, respectively. From the regression equation, vt avg has a positive relationship with both K and Reynolds number but a negative relationship with S. [ABSTRACT FROM AUTHOR]

In order to enhance the heat transfer effect and improve the energy utilization efficiency of the circular tube under turbulent flow, rhombic and triangular winglet vortex generators are proposed on the basis of rectangular winglet vortex generator. The effects of three vortex generators on flow and heat transfer in the tube are investigated by numerical simulations. The results indicate that, the rectangular and triangular winglets generate two pairs of longitudinal vortices and the triangular winglets generate four pairs of longitudinal vortices due to additional shrinkage regions. The multiple longitudinal vortices increase the mixing between the fluids, raise the temperature in the central region of the tube, and enhance the heat transfer effect. Furthermore, the triangular winglet enhanced tube has the largest direct flow area and the least effect on resistance. The triangular winglet enhanced tube had the best comprehensive performance, followed by the rhombic winglet enhanced tube, and finally, the rectangular winglet enhanced tube. The performance evaluation criterion reached 1.04-1.13, 1.05-1.15, and 1.08-1.21 for the three enhanced tubes in the given flow rate interval. In addition, the effect of inclination angle on the triangular winglet enhanced tube was further investigated, and the results show that the best comprehensive performance of enhanced heat transfer is achieved at an inclination angle of 45°, with a maximum performance evaluation criterion of 1.25. [ABSTRACT FROM AUTHOR]

The current study experimentally examines the impact of coil pitch on heat transfer behavior andfriction factor through helical coils for α-Al2O3-distilled water nanofluid turbulent flow. These tests were conducted on coils with coil pitches of 20 mm, 35 mm, and 50 mm. The nanoparticle volume fraction was 0.1%. The nanoparticles in a 0.1% volume concentration of nanofluid increased the heat transfer rate andfriction factor compared to those of distilled water. Increases in coil pitch also resulted in greater heat transfer efficiency. A correlation between the Reynolds number, the Prandtl number, and the curvature ratio of the coil was also shown to be connected to the Nusselt numbers for the flow of nanofluids within the coils. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive analysis of the dynamic and thermal behavior of air-flow within a heat exchanger equipped with two distinctive baffles: a perforated baffle and a partially inclined baffle. The influence of hole positioning in the perforated baffle on the overall performance of the heat exchanger is thoroughly investigated through a systematic examination of temperature curves at varying Reynolds number values. The results demonstrate significant enhancements in flow characteristics attributed to the presence of these baffles. The flow structure exhibits prominent main currents across the gaps and secondary currents through the holes. The inclusion of these barriers leads to significant deformations and the emergence of well-developed recycling cells in the form of vortices. Both the perforated and inclined baffles effectively reduce pressure values on their frontal regions, thereby mitigating friction losses. Furthermore, the introduction of a perforation in the lower part of the baffle induces a more turbulent flow compared to the other cases. This is attributed to the expansion of the recirculating cells, resulting in improved fluid mixing and subsequent enhancement of thermal energy gain. These findings offer valuable insights into the design and optimization of heat exchangers, enabling improved performance and efficiency in various engineering applications. [ABSTRACT FROM AUTHOR]

Static mixers are becoming increasingly popular because they are energyefficient, cost-effective, and easy to maintain. Mixing is an essential unit operation in many chemical industries. In this study, a modified high efficiency vortex static mixer was used to examine laminar flows in a rectangular duct. To encourage fluid rotation and improve mixing with heat transfer, the modified high efficiency vortex set was rotated by angles of 0°, 5°, 10°, 20°, 25°, and 30°. The Reynolds number varied from 3000 to 8000. The outcomes demonstrated that the performance of the mixing was significantly impacted by the modified high efficiency vortex set. The highest mixing efficiency was achieved with a rotation angle between 15° and 20°. Furthermore, the rotations reduced pressure loss in the system and enhanced heat transfer performance, by creating vortices. These results show how modified high efficiency vortex static mixers can improve mixing and heat transfer efficiency in turbulent flows, with prospective utilization across diverse chemical sectors. [ABSTRACT FROM AUTHOR]

In order to understand the mechanism of cavity shedding and evolution, turbulent cavitating flows of the twisted hydrofoil were numerically investigated using the k-ε turbulence model and the ZGB cavitation model. The results of the numerical calculation and the experimental method are basically consistent, which confirms the feasibility of the numerical calculation model. This study has obtained the following conclusions. Firstly, the cavity shedding can be summarized into six stages, and the cavity shape, pressure and velocity field at different stages are displayed, analyzed and compared in detail. Secondly, the shedding of cavity and its evolution are mainly caused by the re-entrant jet and side-entrant jet, in which the former provides the kinetic energy and the latter plays the role of guiding the direction. Thirdly, under the convective shearing action of the re-entrant jet and the main flow, a strong vortex located in the mid-back edge of the hydrofoil is formed, which promotes the transformation of the cavity shape into a U-shaped structure. [ABSTRACT FROM AUTHOR]

In this work, the ability of a Reynolds stress model to compute turbulent homogeneous shear flow with significant compressibility effects is discussed. Several studies of compressible turbulent flows carried out in the past years have shown that the pressure strain correlation is mainly responsible for the strong changes in the magnitude of the Reynolds stress anisotropies. Two recent compressible models of this term are considered in conjunction with the standard model of the dissipation rate of the turbulent kinetic energy to predict compressible homogeneous flow highly sheared are tested. It is found that deficiencies appear in the calculations even if the pressure strain model is improved by compressibility corrections. Consistent with earlier studies, this deficiency is attributed to the use of the incompressible model for turbulent dissipation. However, a compressibility correction of this equation model uncovers the main focus of the present study. This correction makes the standard coefficients of this equation depend on the turbulent gradient Mach and Mach numbers. The proposed model is tested for low and strong compressibility cases from the DNS results of Sarkar. A comparison of the proposed model predictions with the DNS results shows good qualitative agreement. Therefore, compressibility correction for the incompressible model of the turbulent dissipation rate is found to be an important issue for the compressible homogeneous turbulent shear flow. [ABSTRACT FROM AUTHOR]

Non-Fickian transport of suspended sediment has been observed at field and laboratory scales. Such as turbulent bursting, resulting in complex dynamics for the sediment particles movement. The erosion and deposition of sediment have an impact on the hydraulic engineering and environment. This study makes an attempt to develop the variable-order fractional advection-diffusion equation (VOFADE) and variable-order Hausdorff fractal derivative advection-diffusion equation (VOHADE) models to describe the vertical distribution of suspended sediment in unsteady turbulent flows. From a classical viewpoint, the distribution of the concentration in sediment-laden flows is determined based on Fick's first law. However, the vertical diffusion of suspended particles exhibits the non-locality/space scale dependency and history memory/time scale dependency properties due to turbulent bursting. Moreover, previous literatures have indicated that turbulence structure changes with the water depth. Hereby, we employ the space-dependent VOFADE and VOHADE models to describe the vertical diffusion of suspended sediment in unsteady flows, and further test its applicability with the experimental data. Numerical simulation results confirm that the VOFADE and VOHADE models give a better agreement with the experimental data and can well characterise the space-dependent anomalous transport. Hence, the models proposed by this study may help to provide a powerful mathematical physical model in the quantification of suspended sediment transport. [ABSTRACT FROM AUTHOR]

The present paper presents the aeroacoustic calculation methodology for the H-Darrieus wind turbine. The CFD analysis, for different wind turbine blades’ angles of attack, coupled with the noise analysis, based on Lighthill and Prudmann models is performed. This type of turbine is of particular interest since it is insensitive to wind direction and can be used in urban areas. In this study commercial software, ANSYS is used for CFD and aeroacoustic analysis. The required turbulent flow field is calculated based on the standard k-ε model, and required model constants are obtained experimentally in a low-Mach number wind tunnel. The noise levels generated by operating turbine are calculated based on Lighthill and Proudman's aeroacoustic theories. It was found that the methodology presented can be efficiently used in noise analysis of vertical axes wind turbines and due to recent strict noise regulations has to be deployed at the early design stages. [ABSTRACT FROM AUTHOR]

The present paper highlights a computational analysis of air-flows around rectangular and triangular turbulators inside baffled heat exchanger channels in order to improve heat transfer between the fluid and their heated areas. The dynamic and thermal fields as well as fluid temperature curves at the outlet of the exchanger are studied. The computational study is conducted by utilizing SIMPLE algorithm with FLUENT system based on the finite volumes. The analysis clearly demonstrated the presence of highly turbulent flows and the appearance of many vortices in various regions of the exchanger. By comparing the different heat exchangers, it was found that the baffled channel fitted with rectangular turbulators produced high fluid temperature values at the channel outlet, indicating the significance of using this rectangular form of turbulators in order to enhance the interaction between the hot spaces and the used fluid. [ABSTRACT FROM AUTHOR]

Five wire-coil inserts with fixed wire diameter and different pitches, fitted inside a round tube have been experimentally studied in a transitional and low turbulent flow. Water was used as a working fluid at a wide range of flow conditions: 103 < Re < 104 and 3.9 < Pr < 10.0. The geometrical parameters of the inserts are: e/Di = 0.070, and p/e = 6.7, 9.0, 10.0, 12.5, and 15.0. The variation of the friction factor and heat transfer coefficients have been obtained and compared with those of the smooth pipe. Performance evaluation criteria for the cases FG-1a, FG-2a, and VG-2a have been used to evaluate the maximum and real benefit that can be obtained. The greatest benefit can be achieved with the pitch of the wire-coil insert p/e = 10.0. [ABSTRACT FROM AUTHOR]

The effect of the design on the heat transfer is numerically investigated by using the wavy inner tube in a double-pipe heat exchanger. A wavy inner tube was used in the design to give a turbulent effect to the fluid along the inner tube of a double tube heat exchanger. In numerical study, ANSYS 12.0 Fluent code program was used, and the basic protection equations were solved for steady-state, 3-D and turbulent flow conditions. The study was examined at Reynolds numbers ranging from 2700-5300. The obtained results were compared with the experimental data performed under the same conditions. As a result of this comparison, after it was seen that the results obtained from the numerical analysis and the experimental results were compatible with each other, the wave number of the inner tube was increased and analyzed with the ANSYS fluent code program. When the data obtained as a result of the analyzes were evaluated, it was seen that the highest heat transfer was obtained from the 16 wave tube heat exchanger, which has the highest number of waves and under counter flow conditions. The increase in heat transfer increased by 270% compared to the straight tube. [ABSTRACT FROM AUTHOR]

The paper presents the numerical results of modelling pollutant transport from a low source behind a bluff-body imitating a building within a non-isothermal boundary-layer. The main goal of the study is to estimate the tracer gas dispersion in a complex turbulent separated flow behind a building in the presence of interference of the atmospheric boundary-layer and local flows. In the fist part of the study we compare numerical approaches URANS and IDDES for turbulent flow prediction on a configuration for which experimental data are available. It is shown that detached eddy simulation approach predicts correctly the main separated flow features and demonstrates a reliable correlation with the experimental data on mean velocity, pollutant concentration and temperature fields. In the second part of the study, the influence of unstable thermal stratified flow on the tracer gas transport around a building is analyzed using IDDES method. The unstable thermal flow regime considered in the study affects the distribution of the pollutant concentration in the re-circulation zone behind the building. The presence of additional buoyancy effects leads to an increase in the gas concentration on the leeward wall of the body and gas transport from a ground region to a height greater than in the case with the neutral boundary-layer. [ABSTRACT FROM AUTHOR]

The spatial structure of a swirling turbulent flow has been investigated based on the three-component laser Doppler anemometry method in an isothermal laboratory model of a four-vortex furnace. The structure of the vortex cores of the flow with the shape of a deformed vertical elliptical cylinder is visualized using the "minimum total pressure" criterion. The spectrum of velocity pulsations indicates the absence of unsteady periodic vortex structures, which means the occurrence of a stable vortex flow in the volume of the combustion chamber. [ABSTRACT FROM AUTHOR]

This experimental study deals with the heat transfer and friction effects of sinusoidal part turbulators for single-phase flows occurring in a circular shaped pipe. Turbulators with three different radius values are placed in the pipe to make the flow turbulent. In this way, changes in Nusselt number and friction coefficient are examined. As a result of the experiments made with Reynolds numbers in the range of 6614-20710, the increase rates of the Nusselt numbers of turbulators with 20 mm, 110 mm, and 220 mm radius compared to the empty pipe were obtained as 153.49%, 85.36%, and 52.09%, respectively. As a result of the decrease in the radius, there was an increase in the Nusselt number and the friction factor. Parallel to the Nusselt number, the highest friction factor was obtained in the smallest radius turbulator. It was found that the thermal enhancement factors of 110 mm and 220 mm radius turbulators increased by 179.54% and 132.95%, respectively, compared to the 20 mm radius turbulator. Similarly, it was determined that the thermal enhancement factor of the 110 mm radius turbulator increased by 20% compared to the 220 mm radius turbulator. [ABSTRACT FROM AUTHOR]

The article presents the research results related to the influence of turbulence on the efficiency of the combustion chamber of gas turbine. An artificial increase in the intensity of turbulence is considered as a way to improve the formation of a fuel-air mixture. Turbulent flow is formed due to the installation of guide swirlers at the entrance to the device for creating a fuel-air mixture – a micro module. The angle of rotation of the swirler blades is selected. Theoretical research, mathematical software modelling, as well as an aerodynamic experiment have been carried out. As a result, design solutions are provided that significantly increase the efficiency and reliability of the gas turbine combustion chamber. In the course of the study, guide vanes were selected, and their design was established. The recommended swing angle of the swirler guide vanes is 40°. The recommended depth of the fuel injector inside the chamber is 1.0 gauge. [ABSTRACT FROM AUTHOR]

Fluent simulates the water-sand flow around a cylinder. Monitoring lines are set up at different positions in the cylindrical surface and the very near wake area behind the cylinder, in order to explore the speed difference of fluid and sand in the water-sand two-phase flow in the boundary-layer and the very near wake area. The results show that the sand particles stay for the longest time on the back of the cylindrical surface and in the very near wake area, and a small part of the sand particles are sticky on the back of the cylindrical pier. When the height of the cylinder is z/D ∈ (1.57, 3.14), the turbulent flow on the cylindrical surface is fully developed. The dynamic pressure of the flow field in the very near wake area behind the cylinder fluctuates greatly, and the water-sand flow is extremely unstable. At the monitoring position of the cylinder, there is a sudden decrease in the velocity of the fluid, while the velocity of the sand particles changes little and remains finally at about –0.02 m/s. The water-sand flow field near the wall changes drastically, but the velocity change of sand particles has obvious hysteresis compared with fluid. When leaving the near-wall position but still in the cylindrical wake area (x/D ≤ 3), the changes in the water-sand flow field are more intense and and the velocity of the sand particles is still slightly larger than the fluid velocity. [ABSTRACT FROM AUTHOR]

A combined numerical and experimental study is conducted to test the heat transfer enhancement and friction factor characteristics for a rectangular duct fitted with three cases of ribs turbulators: continuous ribs, intermittent-continuous-intermittent ribs, and intermittent ribs. Experiments are conducted within a turbulent flow for Reynolds numbers values varied from 10000 to 35000, pitch ratio equal to 5 and height ratio of 0.33. The numerical study carried out using ANSYS FlUENT17.2. The turbulence is modeled by using k-ɛ model. The results showed that the case of intermittent ribs provide the highest over performance factor while the continuous ribs indicate less overall performance factor among the considered cases. In addition, the results show that the highest values of the friction factor are marked from the case of intermittent ribs and then the case of intermittent-continuous- intermittent ribs followed by continuous rib case. The continuous rib case showed the lowest friction factor. The experimental results showed a good agreement with the computational results. [ABSTRACT FROM AUTHOR]

Heat transfer is considerably influenced by flow stagnation, separation and reattachment regions due to the ribbed plates. Placing the ribs such as fins, turbulators that trigger the flow separation, enhances the heat transfer inside the channel by increasing the turbulence intensity. The flow separation is caused by disturbing the thermal and hydrodynamic development lengths. Moreover, these ribs also make an impact that increases the heat transfer by enlarging the heat transfer area. However, the ribs lead to the increment of the required pumping power in the meantime due to the increasing pressure loss in such systems. This aforementioned method is used for the heat exchangers, the solar collectors, the cooling of electronic devices. The investigation of the flow characteristics is very crucial to understand the heat transfer mechanism in the ducts for this reason. In the present paper, the flow characteristics between the plates have been experimentally researched. Particle image velocimetry system in the open water channel of Selcuk University Advanced Technology Research and Application Center has been used. The smooth plates have been taken as the reference model and used for the comparison with the plates having the rectangular cross-sectional ribs. The ribs with various heights of 0.1 ≤ h′ = h/H ≤ 0.3 have been symmetrically placed on the internal surfaces of the plates via several spacing values of 0.5 ≤ S′ = S/H ≤ 1 for varying Reynolds numbers as 10000 ≤ Re ≤ 20000. As a result, the flow characteristics have been given in terms of the contour graphics for velocity vector field, velocity components and vorticity. [ABSTRACT FROM AUTHOR]

In the paper, the Navier-Stokes-type equations of the complex fluid-flows, the equations of the complex turbulent flows, and Euler-type equations of the complex fluid-flows based on the theory of the new vector calculus with respect to monotone functions are investigated for the first time. [ABSTRACT FROM AUTHOR]

In this study, the effects of cylindrical aluminum and Cr-Ni alloy pins placed in different arrangements on the inner wall of the pipe in the turbulent flow, the effects of heat transfer and flow characteristics on different Reynolds numbers have been experimentally investigated. The experiments were carried out under forced flow and constant heat flow conditions. Air is preferred as the fluid and the fluid velocity is adjusted between Reynolds number of 10000 and 50000. It has been observed that the Nusselt values obtained over the number of Reynolds number for the 5 different test tubes are arranged in a line from large to small, sequential row aluminum pin, sequential row Cr-Ni alloy pin, diagonal row aluminum pin, diagonal row Cr-Ni alloy pin, plain tube. There are also CFD analysis for each material, arrangements and pins geometry sets. On the other hand, it was determined that friction coefficient is directly proportional to the increase of heat transfer coefficient. As a result, it is observed that experimental results are compatible with both literature and numerical study. [ABSTRACT FROM AUTHOR]

Thermal-hydraulic characteristics of water based ZrO2 nanofluids has been investigated in a segmental baffled shell and tube heat exchanger in turbulent flow regime. The effect of Reynolds number, nanoparticle loading, mass-flow rate, and tube lay-out has been analysed on overall heat transfer coefficient. The effect of Reynolds number on the tube side pressure drop and convective heat coefficient have also been discussed. The effect of shell side mass-flow rate was also investigated on shell side heat transfer coefficient determined using Bell-Delaware method. The nanoparticle volume concentration is taken very low i. e. 0.2%, 0.4%, and 0.8%, respectively. The improvement in both tube side convective heat transfer coefficient and overall heat transfer coefficient has been observed. The maximum improvement in the convective heat transfer coefficient is found to be 14.1% for 0.8% ZrO2 nanofluids. However, the percentage enhancement in tube side pressure drop was higher than the percentage increment in the tube side heat transfer coefficient. [ABSTRACT FROM AUTHOR]

The baffling technique is well-known for its efficiency in terms of enhancement of heat transfer rates throught channels. However, the baffles insert is accompanied by an increase in the friction factor. This issue remains a great challenge for the designers of heat exchangers. To overcome this issue, we suggest in the present paper a new design of baffles which is here called graded baffle-design. The baffles have an up- or down-graded height along the channel length. This geometry is characterized by two ratios: up-graded baffle ratio and down-graded baffle ratio which are varied from 0-0.08. For a range of Reynolds number varying from 104 to 2 x 104, the turbulent flow and heat transfer characteristics of a heat exchanger channel are numerically studied by the computer code FLUENT. The obtained results revealed an enhancement in the thermohydraulic performance offered by the new suggested design. For the channel with a down-graded baffle ratio equal to 0.08, the friction factors decreased by 4-8%. [ABSTRACT FROM AUTHOR]

The experimental results shown that the mode of droplet disintegration dominates in the laminar flow, and the intensive fragmentation is prevalent in the turbulent flow during almost the entire time of heating. Typical dependences of the time of droplet heatup before disintegration or fragmentation on the temperature, flow rate, structure and regime (laminar and turbulent) are established. The studies are conducted with heated air and flue gases to ensure the application of the research results in the technology of thermal and flame cleaning of liquids from irregular impurities. It is shown that in the flow of combustion products the droplet disintegration occurs 15-20% faster than in the air-flow. In this case, the explosive puffing is more often realized. At high-temperatures (more than 400 °C) the characteristics of the explosive droplet disintegration in the studied flows are almost identical (differences in disintegration times do not exceed 5% at different flow turbulization). At lower temperatures, the disintegration times differ 3-4 times for the range Re = 2200-3400. In this case, the more Reynolds number is, the more intense is the fragmentation of two-component droplets throughout the heating time. Due to explosive disintegration of intensely evaporating two-component droplets the growth of the relative area of evaporation was 10-25 times. [ABSTRACT FROM AUTHOR]

The results of the simulation of car radiator operation with a large range of changes in the volume flow rate of liquid inside the tubes were presented. The change of the flow regime from laminar through transitional to turbulent flow was taken into account. Semi-empirical and empirical relationships for the Nusselt number on the liquid-side in the laminar, transitional, and turbulent range were used. The Nusselt number on the air side was determined using empirical power-type correlation. The friction factor in the transition flow range was calculated by linear interpolation between the values of the friction factor for the Reynolds number equal to 2100 and 3000. The water and air temperature at the outlet of the heat exchanger were calculated using effectiveness-number of transfer units method. The heat-flow rate from water to air was calculated as a function of the water-flow rate to compare it with the experimental results. The calculation results agreed very well with the results of the measurements. [ABSTRACT FROM AUTHOR]

Swirl is a tangential velocity component of the fluid flow and is often present in the conical diffuser as a result of rotating machinery in the upstream section. The present experimental work is dedicated to study the effect of moderate swirl on wide angle conical diffuser performance and flow development. The experiments were performed in a low-speed open circuit wind tunnel. There are two different diffusers having a cone angle of 14° (with an area ratio 3.0) and 20° (with an area ratio 4.2) were selected for this investigation. The flow parameters have been measured using DANTEC DYNAMICS make constant temperature hot-wire anemometer (CTA). The results showed that the moderate swirl can significantly improve the stalled diffuser (20° cone angle) performance; however, it has a little effect on the diffuser (14° cone angle) having incipient turbulent boundary layer separation. It was confirmed that the introduction of moderate swirl reduces the chances of flow separation in wide angle conical diffusers. [ABSTRACT FROM AUTHOR]

The paper presents a numerical simulation of two-phase turbulent flow in straight horizontal channels of pneumatic transport with non-circular cross-section. For the granular flow simulation, we have chosen the flow of solid particles of quartz, flour, and ash in the flow of air, which is transporting fluid. During the modeling of the flow, the transported solid particles are reduced to spherical shapes. A correction of the stress model of turbulence is performed by taking into account the influences of the induction of secondary flows of the second order in the gas phase. The full Reynolds stress model was used for modeling the turbulence, and the complete model is used for the turbulent stresses and turbulent temperature fluxes. All numerical experiments were conducted for the same initial flow conditions and a single uniform grid was adopted for all numerical experiments. The flow is observed in a straight channel of a square cross-section and dimensions of sides of 200 mm and the length of 80 Dh. During the simulation, the fineness of the numerical grid was also tested, and the paper shows results of the numerical grid of the highest resolution beyond which the fineness does not influence the obtained results. The paper offers graphics of velocities of the solid particles transported by the transporting fluid (air) along the channel. [ABSTRACT FROM AUTHOR]

This work presents the results of an experimental study of thermohydraulic performance of rectangular channel having discrete V-pattern baffle attached on the broad wall. Measurements have been carried out for the aspect channel ratio of 10, Reynolds number from 3000 to 21000, relative baffle height value of 0.50, relative baffle pitch value of 1.5, relative gap width value of 1.0, flow attack angle value of 60°, and relative discrete distance values of 0.26 to 0.83. The heat transfer and friction factor data obtained were compared with the data obtained from a smooth wall channel under similar operating conditions. In comparison to the smooth wall channel the discrete V-pattern baffle channel enhanced the Nusselt number and friction factor by 3.89 and 6.08 times, respectively. The overall thermal performance parameter is found superior for the relative discrete distance of 0.67. Discrete V-pattern baffle roughness shape has also been shown to be overall thermal performance higher in comparison to other continuous (without discrete) V-pattern baffle shape rectangular channel. [ABSTRACT FROM AUTHOR]

A magnetohydrodynamic turbulent channel flow under the influence of a wallnormal magnetic field is investigated using the Large-Eddy-Simulation technique and k-equation subgrid-scale-model. Therefore, the new solver MHDpisoFoam is implemented in the OpenFOAM CFD-Code. The temporal decay of an initial turbulent field for different magnetic parameters is investigated. The rms values of the averaged velocity fluctuations show a similar, trend for each coordinate direction. 80% of the fluctuations are damped out in the range between 0 < Ha < < 75 at Re = 6675. The trend can be approximated via an exponential of the form exp(-a·Ha), where a is a scaling parameter. At higher Hartmann numbers the fluctuations decrease in an almost linear way. Therefore, the results of this study show that it may be possible to construct a general law for the turbulence damping due to action of magnetic fields. [ABSTRACT FROM AUTHOR]

This review paper provides data about research activities at the School of the turbulent swirling flow at the Faculty of Mechanical Engineering, University of Belgrade, conducted in the period 1941 up to date. An overview is provided of the main directions in this research area. First papers dealt with the turbulent swirling flow in hydraulic turbines to be continued by the experimental and analytical approaches on the axial fans pressure side. The complexity of 3-D, non-homogeneous, anisotropic turbulent velocity fields required complex experimental and theoretical approach, associated with the complex numerical procedures. Analytical approaches, complex statistical analyses and experimental methods and afterwards CFD employed in the research are presented in this paper. The 150 scientific papers, numerous diploma works, several master of science (magister) theses, six Ph. D. theses and two in progress, 40 researchers, national and international projects are the facts about the School. Scientific references are chronologically presented. Numerous abstracts from scientific conferences, presentations, projects with industry and lectures are not given here. [ABSTRACT FROM AUTHOR]

A cost-effective reduction of NOx emission from utility boilers firing pulverized coal can be achieved by means of combustion modifications in the furnace. It is also essential to provide the pulverized coal diffusion flame control. Mathematical modeling is regularly used for analysis and optimization of complex turbulent reactive flows and mutually dependent processes in coal combustion furnaces. In the numerical study, predictions were performed by an in-house developed comprehensive three-dimensional differential model of flow, combustion and heat/mass transfer with submodel of the fuel- and thermal-NO formation/ destruction reactions. Influence of various operating conditions in the case-study utility boiler tangentially fired furnace, such as distribution of both the fuel and the combustion air over the burners and tiers, fuel-bound nitrogen content and grinding fineness of coal were investigated individually and in combination. Mechanisms of NO formation and depletion were found to be strongly affected by flow, temperature and gas mixture components concentration fields. Proper modifications of combustion process can provide more than 30% of the NOx emission abatement, approaching the corresponding emission limits, with simultaneous control of the flame geometry and position within the furnace. This kind of complex numerical experiments provides conditions for improvements of the power plant furnaces exploitation, with respect to high efficiency, operation flexibility and low emission. [ABSTRACT FROM AUTHOR]

Experimental investigation of the free turbulent swirl jet behind the axial fan, by use of the three-component laser Doppler anemometry is presented in this paper. Axial fan generates Rankine swirl. Measurements have been performed in five vertical measuring sections downstream for one rotation number 1500 rpm. Time averaged velocity components and their profile transformation downstream were investigated. Statistical moments of the second, third and fourth order have been calculated and presented. The highest turbulence levels are reached in the central zone, more precisely - shear layer zone and in the injection zone. Skewness and flatness factors differ from normal Gaussian distributions. Turbulence swirl flow anisotropy is investigated by applying invariant maps. Most of the obtained results are positioned in the region of the three-component isotropic turbulence and axisymmetric expansion. [ABSTRACT FROM AUTHOR]

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 type of flow is 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 a task of turbulence modeling and optimal choice of the turbulence model are obtained through careful examination of physical mechanisms that generate secondary flows. [ABSTRACT FROM AUTHOR]

Paper gives a review of the most important results of turbulence research achieved by the Laboratory for Thermal Engineering and Energy at the Vinca Insitute of Nuclear Sciences. Paper presents detailed overview of the history of the scientific research provided in the laboratory, from the beginning in the mid-60s to today, pointing out the main reasons initiating the investigations in this field. After the first period, which was mainly devoted to the research of the structure of the turbulence, since the beginning of the 80s, research is mainly oriented to the flows at high temperatures including chemical reactions and to the development and improvement of differential mathematical models as a modern and very efficient tool in the technological development. This research significantly contributed to the development of pulverized coal burners, plasma-chemical reactors, and optimization of pulverized coal fired boilers operating parameters and prediction of the greenhouse gases emissions. Most recent period includes experimental and numerical studies of the coherent structures in turbulent fluid jets, mathematical modeling of various turbulent thermal flow processes including two-phase turbulent flow in the multiphase heat exchangers and mathematical modeling of the atmospheric boundary layer. [ABSTRACT FROM AUTHOR]

A very high resolution minimal channel flow direct numerical simulation was used to examine, virtually, the ability of various multi-sensor hot-wire probe configurations to measure the statistics of velocity gradient components. Various array and sensor configurations and the spatial resolution of probes with these configurations were studied, building on designs and investigations of various authors. In contrast to our previous studies, which focused on turbulent vorticity, vorticity-velocity correlations, dissipation and production rate, here the measurement accuracy of each component of the velocity vector gradient tensor is analyzed separately. The results of the study show that the virtual experiments compare well with a physical experiment, and that such virtual experiments are a powerful tool to examine the accuracy of velocity gradient measurements. The cross-stream gradients needed to determine the vorticity components can be measured with sufficient accuracy with most of the array and sensor configurations of vorticity probes used so far. A systematic error of some of the gradient measurements can appear due to the array or sensor configurations. None of the examined probe designs can measure, with sufficient accuracy, the streamwise velocity gradients, directly or indirectly, using the continuity equation. [ABSTRACT FROM AUTHOR]

Results of experimental investigations of the turbulent swirl flow in three straight conical diffusers with various diffuser total angles are presented in this paper. All three diffusers have the inlet diameter 0.4 m and total divergence angles 8.6°, 10.5°, and 12.6°. The incompressible swirl flow field is generated by the axial fan impeller, and for each diffuser several regimes were achieved by changing rotation number. Original classical probes were used for measurements. The distributions of the average main swirl flow characteristics along the diffuser are shown. Distributions of the inlet Boussinesq number, outlet Coriolis coefficient, ratio of the swirl and completely axial flow loss coefficients at conical diffuser on the inlet swirl flow parameter are also presented. [ABSTRACT FROM AUTHOR]

This paper presents results of our own velocity field measurements in a straight pipe swirl flow. These studies were conducted using an originally designed hot wire probe. Due to the specially tailored shape of the probe, it was possible to get four measurement points in the viscous sublayer. The time-averaged velocity field and the statistical moments of the second and third order are calculated based on the measured velocity components. Mathematical and physical interpretations of statistical characteristics and structures of turbulent swirl flow in the time domain are presented. On the basis of these results, deeper insight into turbulent transport processes can be obtained, as well as useful conclusions necessary for turbulent swirl flows modeling. [ABSTRACT FROM AUTHOR]

The reactants are generally injected into the industrial furnaces by jets. An effective method to act on combustion in such systems is to control the way injection jets. The present study concerns the control of an air-flow generated through a coaxial burner. The effects of passive control on a turbulent flow were investigated experimentally. The principal idea consists in adding small obstacles on the outlet side of the burner’s annular jet with an aim of increasing the turbulence intensity in the vicinity close to the edge of injection and of having an act on the central flow. The objective of this study consists to study of a swirling flow in a circular pipe and conceiving of a control system significantly improving the mixture on a non-reactive flow in order to apply it to a reactive situation. The various profiles speed for various values of the air-flow injected into the annular tube will be presented in order to propose the effectiveness of our inspecting device in terms of improvement of the mixture between the two jets as well as the mixture with the ambient air. The particle image velocimetry technique has been used to characterize the dynamic field. Results show that the control by adding small obstacles has a considerable effect on the dynamic behavior. [ABSTRACT FROM AUTHOR]

Partially open cavities are encountered in various engineering systems such as electronic cooling devices and cooling for gas turbine blades, instead of conventional film cooling slots. Flow is to be imparted over the partially open cavity where it induces a shear layer and a shear driven vortex within the cavity, which is subjected to cooling effect at its wall. Depending on the opening ratio, heat and mass transfer occur between the main flow and the trapped vortex through the shear layer. In the present study, RANS simulations of such flow have been conducted for circular and square cavities to investigate the effect of opening ratio on the heat and mass transfer characteristics. The simulations were established on a rigorous numerical approach and proper validation with laser Doppler velocimetry measurements of turbulent flow in circular cavity. Based on the hydraulic diameter of the cavity, opening ratios ranging from 0.2 to 1.0 were investigated for a Reynolds number of 3∙105. Generally, the maximum Nusselt number was achieved at higher opening ratios for both circular and rectangle cavities. On the other hand, the maximum dimensionless temperature gradient, θ, inside the cavity was achieved at L/D = 0.2 for both cavity configurations. [ABSTRACT FROM AUTHOR]

Heat transfer and pressure drop characteristics of three different passive inserts are experimentally investigated for individual and compound insertion. Insert cross-section is altered along the length of test section for compound insertion. Test runs were conducted in a concentric circular tube in tube heat exchanger in the Reynolds number range of 8000 to 32000 with water as a working fluid. Enhancements in Nusselt number and friction factors are reported to be in the range of 38-234% and 55-524%, respectively, over plain tube. The average performance ratios based on equal pumping power are also reported and found in the range of 0.63-1.53. Based on experimental results, optimum combination for compound insertion is proposed. [ABSTRACT FROM AUTHOR]

In the present paper a numerical study of natural turbulent convection in a tall cavity filled with nanofluids. The cavity has a heat source embedded on its bottom wall, while the left, right, and top walls of the cavity are maintained at a relatively low temperature. The working fluid is a water based nanofluid having three nanoparticle types: aluminum, Cu, and CuO. The influence of pertinent parameters such as Rayleigh number, the type of nanofluid, and solid volume fraction of nanoparticles on the cooling performance is studied. Steady forms of 2-D Reynolds- аveraged-Navier-Stokes equations and conservation equations of mass and energy, coupled with the Boussinesq approximation, are solved by the control volume based discretisation method employing the SIMPLE algorithm for pressurevelocity coupling. Turbulence is modeled using the standard k-ε model. The Rayleigh number is varied from 2.491009 to 2.491011. The volume fractions of nanoparticles were varied in the interval 0 ≤ φ ≤ 6%. Stream lines, isotherms, velocity profiles, and temperature profiles are presented for various combinations of Rayleigh number, the type of nanofluid, and solid volume fraction of nanoparticles. The results are reported in the form of average Nusselt number on the heated wall. It is shown that for all values of Rayleigh number, the average heat transfer rate from the heat source increases almost linearly, and monotonically as the solid volume fraction increases. Finally, the average heat transfer rate takes on values that decrease according to the ordering Cu, CuO, and Al2O3. [ABSTRACT FROM AUTHOR]

In this study, thermo-physical and geometrical parameters affecting entropy generation of nanofluid turbulent flow such as the volume fraction, Reynolds number, and diameter of the channel and microchannel with circular cross-section under constant flux are examined analytically. Water is used as a base fluid of nanofluid with nanoparticles of Ag, Cu, CuO, and TiO2. The study is conducted for Reynolds numbers of 20000, 40000, and 100000, volume fractions of 0, 0.01, 0.02, 0.03, and 0.04, channel diameters of 2, 4, 6, and 8 cm and microchannel diameters of 20, 40, 60, and 80 micrometers. Based on the results, the most of the generated entropy in channel is due to heat transfer, and also, with increasing the diameter of the channel, Bejan number increases. The contribution of entropy generation due to heat transfer in the microchannel is very poor and the major contribution of entropy generation is due to friction. The maximum amount of entropy generation in channel belongs to nanofluids with Ag, Cu, CuO, and TiO2 nanoparticles, respectively, while in the microchannel this behavior is reversed, and the minimum entropy generation happens in nanofluids with Ag, Cu, CuO, and TiO2 nanoparticles, respectively. In channel and microchannel, for all nanofluids except for the water-TiO2, with increasing volume fraction of nanoparticles, the entropy generation decreases. In channel and microchannel, the total entropy generation increases as Reynolds number augments. [ABSTRACT FROM AUTHOR]

The paper considers two-phase gas-solid turbulent flow of pneumatic transport in straight horizontal channels with a non-circular cross-section. During turbulent flow, a specific flow phenomenon, known as secondary flow, occurs in these channels in the cross-sectional plane. The existence of strong temperature gradients in the cross-sectional plane of the channel or the cases of curved channels result in the appearance of the secondary flow of the first kind. However, in straight channels with a non-circular cross-section, in the developed turbulent flow mode, a secondary flow, known as Prandtl's secondary flow of the second kind, is induced. The paper presents a numerical simulation of a developed twophase turbulent flow by using the PHOENICS 3.3.1 software package. Reynolds stress model was used to model the turbulence. The paper provides the data on the changes in turbulent stresses in the channel cross-section as well as the velocities of solid particles transported along the channel. [ABSTRACT FROM AUTHOR]

This paper reports experimental studies on friction factor, Nusselt number, and thermal hydraulic performance of a tube equipped with the classic three modified internally grooved tubes. Heat transfer andfriction factor characteristics and pressure drop results have been obtained experimentally for a fully developed water flow in a grooved tube is also reported. Tests were performedfor Reynolds number ranges from 5000-13500 for different geometric grooved tubes (circular, square, and rapezium). The ratio of length-to-diameter is 38.69 D. Among the grooved tubes, heat transfer enhancement obtained up to 47% for circular grooved tube, 31% for square grooved tube, and 52% for trapezoidal grooved tube in comparison with the smooth tube. It has been observed that thefriction factor high in the case of square grooved tube than those of other tubes. [ABSTRACT FROM AUTHOR]

This study deals numerically with a heat transfer in a turbulent flow over a shallow cavity. Two different configurations of the incoming flow are considered: a boundary layer flow and a plane wall jet flow, in order to examine the wall jet outer layer effect on the heat transfer. This layer is an important additional turbulence source in the wall jet flow. Reynolds number and turbulence intensity effects were investigated in the boundary layer incoming flow case. The cavity depth to nozzle height ratio effect was examined in the wall jet incoming flow case. The numerical approach is based on k-ω standard turbulence model. This study reveals that the heat transfer is very sensitive to the incoming flow characteristics. The turbulence intensity increase accelerates the reattachment of the shear layer at the cavity floor and enhances the heat transfer. The reattachment phenomenon seems to be less affected by the Reynolds number. However, an increase in this parameter ameliorates the heat transfer. It was also observed a heat transfer enhancement in the wall jet incoming flow case as compared to that of a boundary layer. Likewise, it was found that the augmentation of the cavity depth to the jet nozzle height ratio improves even more the heat transfer. The maximum heat transfer occurs upstream of the reattachment. [ABSTRACT FROM AUTHOR]

Flow characteristics over dimpled surface are studied numerically for a fully developed turbulent channel. The results show that dimples can effectively activate the near wall turbulence and cause an increase of total drag. The dimple depth also plays an important role in enhancement of hear transfer. [ABSTRACT FROM AUTHOR]

Heat transfer and friction characteristics were numerically investigated, employing elliptical tube to increase the heat transfer rate with a minimum increase of pressure drop. The flow rate of the tube was in a range of Reynolds number between 10000 and 100000. The FLUENT software is used to solve the governing equation of CFD (continuity, momentum, and energy) by means of a finite volume method. The electrical heater is connected around the elliptical tube to apply uniform heat flux (3000 W/m²) as a boundary condition. Four different volume concentrations in the range of 0.25% to 1% and different TiO2 nanoparticle diameters in the range of 27 nm to 50 nm, dispersed in water are utilized. The CFD numerical results indicate that the elliptical tube can enhance heat transfer and friction factor by approximately 9% and 6% than the circular tube, respectively. The results show that the Nusselt number and friction factor increase with decreasing diameters but increasing volume concentrations of nanoparticles. [ABSTRACT FROM AUTHOR]