turbulent flow

In the present study, the heat transfer and flow friction characteristics of a circular tube with coiled spring inserts are experimentally reported for a fully developed turbulent flow regime. Experimental investigations were performed in a circular concentric tube in a tube heat exchanger in the Reynolds number (Re) range of 8000–32,000 with water as a working fluid. The average Nusselt number ratio (Nua/Nup) and average friction factor ratio (fa/fp) with and without inserts are reported to be in the range of 1.79–2.79 and 2.44–4.17, respectively, for the tested six geometries of the inserts. The Nusselt number ratio (Nua/Nuc) based on equal pumping power criteria is also reported and found to be in the range of 0.94–1.24. The effects of varying pitch to length of insert ratio (p/l) and diameter of insert to the inner diameter of tube ratio (dc/Di) on heat transfer and pressure drop are reported, and empirical correlation is given for Nusselt number in terms of Reynolds number (Re), pitch to length of insert ratio (p/l), insert diameter to inner diameter of tube ratio (dc/Di), and Prandtl number (Pr).

In the present study, we numerically investigated the twin turbulent jets characteristics and turbulent quantities when a solid object is placed between the two nozzles. The two jets are assumed to be isothermal, incompressible and fully developed. Turbulence is modeled by the k-e Realizable model. The set of Reynolds averaged Navier Stokes equations are solved iteratively by the Fluent software. The numerical model is validated with the experimental results available in the literature. For many Reynolds numbers, it was found that velocities and its gradients decay along the longitudinal direction. The placement of a solid object between the twin jet affects the flow structure behind nozzles due to the curvature effect of the solid object. The converging region is disappeared and the combined points axial position increases with Reynolds number. The evolution is almost linearly with an increase in Reynolds number. The effect of  turbulence intensity at the exit of the nozzle is also examined. For a fixed Reynolds number, the axial combined position increases almost linearly with turbulence intensity.

From a regression analysis perspective, this paper focused on literature about TiO2 nano particles. The particles on focus entailed those that had been suspended in ethylene glycol and water – at a ratio of 60:40. Indeed, regression analysis has gained application in contexts such as the turbulent Reynolds number, especially with the aim of establishing the impact of the ratio of the base fluid on heat transfer coefficients, as well as the target materials’ thermal properties. From the findings, this study infers that when the water-ethylene glycol mixture is used at a ratio of 60:40, the rate of heat transfer is higher than that which is obtained when water is used solely. Additional findings established from the examination of the impact of material concentration and temperature on the rate of nanofluids’ heat transfer suggested that as temperature increases, the rate of heat transfer decreases. However, it was noted that an increase in concentration exhibits a positive correlation with the nanofluids’ rate of heat transfer whereby an increase in the former parameter (concentration) leads to an increase in the latter (rate of nanofluids heat transfer).

In this research, turbulent MHD convection of liquid metal with constant and variable properties is investigated numerically. The finite volume method is applied to model the fluid flow and natural convection heat transfer in a square cavity. The fluid flow and heat transfer were simulated and compared for two cases constant and variable properties. It is observed that for the case variable properties in high Hartmann numbers (Ha) the temperature slope near the hot wall is more than the cold wall. For both cases, the temperature gradient near the hot and cold walls is high. By applying magnetic field and increasing the Ha the temperature slope reduces so at Ha=800 the profile is linear. In the case constant properties, the slope of temperature profile near the vertical walls is the same and the temperature profiles pass from one point at the center of the cavity. However,in the case variable properties as it was expected the temperature profile doesn’t pass one point and the slope of temperature profile at high Hartmann numbers near the hot and cold walls is partly different. Furthermore, it is indicated that for the case constant properties the Nusselt number is less than the case variable properties.

One of the main concerns of researchers is the separation of suspended particles in a fluid. Accordingly, the current study numerically investigated the effects of a conical section on the flow pattern of a Stairmand cyclone by simulating single-cone and dual-cone cyclones. A turbulence model was used to analyze incompressible gas-particle flow in the cyclone models, and the Eulerian–Lagrangian approach was employed to examine particle movement. Despite the simplicity of cyclone geometry, internal two-phase flow in such devices is very complicated and anisotropic. This flow was therefore analyzed using a Reynolds stress model. The numerical results were then compared with those of experimental studies. To track calcium carbonate particles, drag and gravity forces were considered in the Lagrangian model. The findings indicated that adding a second conical section at the bottom of the cyclones increases tangential velocity and expands the Rankine vortex region. Moreover, an increasing trend of descending flow occurs. Increasing the number of conical sections elevates pressure drop at all velocities. Finally, the dual-cone cyclone has higher efficiency than the typical cyclone because the smaller end of the former limits particle motion and increases collection performance.

In this study, the stirring mechanism of shear-thinning fluids benefiting from four blades in turbulent flow is considered. The fluid is studied inside a stirred cylindrical tank with a flat bottom. The height of fluid is equal to the cylinder’s diameter and the impeller is positioned centrally. A CFD simulation has been carried out and three-dimensional turbulent flow is numerically analyzed using the Shear Stress Transport k-ω (k- ω SST) model. The parameters related to power consumption including attack angle and flow index were studied. The power consumed during the mixing of the shear thinning liquids under a specific Reynolds number and attack angle is less than that consumed when the fluid used is water, which is a Newtonian fluid. As the power law index decreases, the corresponding power consumption also declines. At a certain attack angle and power law index, an increase in the Reynolds number first significantly decreases power consumption; beyond a given range, the consumption plateaus. To validate the numerical simulation results, the findings derived on the basis of the power number used in this work were compared with the test results of other studies, and good agreement was observed.

In this study, the numerical analysis of turbulent flow and heat transfer of double pulsating impinging jets on a flat surface has been investigated. The unsteady two-dimensional numerical solution for two similar and dissimilar jets was performed using the RNG k-ε model. The results showed that the RNG k-ε model has more satisfactory predictions of the Nusselt number distribution. Comparisons show that for two identical jets with constant frequency and amplitude, increasing Reynolds number leads to the considerable increase of time-averaged Nusselt number. Also, with increasing oscillation amplitude, the averaged Nusselt number of surface increased. The results show that increasing the phase difference angle of pulsating jets leads to the increase of mixing between jets, which consequences the increase of Nusselt number in this zone. It should be mentioned that for two jets by equal frequency and phase angle, increasing oscillating amplitude of one jet leads to an asymmetric distribution of the Nusselt number. In this case, the averaged Nusselt number between two jets increased. Furthermore, the array of double jets with different oscillating type (intermittent and sinusoidal) leads to the increase of averaged Nusselt number considerably in the stagnation region between the jets.

The objective of this paper is the numerical study of the flow through an axial fan and examining the effects of blade design parameters on the performance of the fan. The axial fan is extensively used for cooling of the electronic devices and servers. Simulation of the three-dimensional incompressible turbulent flow was conducted by numerical solution of the (RANS) equations for a model. The SST- k-ω and k-ε turbulence models are applied in the simulations which are done using CFX software. The comparison between available experimental data and simulation results indicates that the SST k-ω model gives more accurate results than the k-ε model. The results also show that in separation regions and vortices, the pressure will decrease. Hub area and blade root contain large vortices. The effects of changes in the blade geometry and the number of blades on the fan performance are studied in detail. For the primary fan model with the different number of blades (4, 5, and 6), the maximum mass flow rate of 800 CFM is obtained. Hence, the number of blades had negligible effects on the maximum flow rate. By 3o% decreasing in the chord of the blades, the maximum mass flow rate of the fan with the different number of blades (5, 6 and 8) will be reduced to 500 CFM. Therefore, in order to increase the maximum mass flow rate, the chord and the width of blades should be increased. On the other hand, by increasing blades from 4 to 6 in the primary model, the maximum outlet pressure has been increased by 32%. Furthermore, it was found that in high flow rates, an increment in the number of blades had no effect on the produced static pressure.

Present study examines turbulent structures of a rough bed open-channel flow in the context of deterministic approach. Instantaneous velocity field is measured in different hydraulic conditions using two dimensional Particle Image Velocimetry (PIV) in vertical plane and Stereoscopic PIV in horizontal plane. Different techniques and quantities such as swirl strength, two-point and cross-correlations of the swirl strength and fluctuating velocity are estimated to control formation of hairpin vortex. Our observations show in the area far above roughness elements, there is fairly a good agreement with the observations of previous studies for smooth-bed boundary flow. This consistency demonstrates that there is a similarity between outer region of the smooth and rough-bed layer flow. Moreover, some of the observations in the present study such as contourmap of instantaneous vorticity and swirling strength or observed inclination angle in contourmap of two-point correlations of the fluctuating velocity and swirl contourmap can be considered as signatures of the hairpin vortex. Given these observations, due to the lack of direct observation of the hairpin vortex and similarity of the observed features with signatures of other types of coherent structures, it is still too hard to assurly opine that in this condition, hairpin vortex is present.

In this paper, using vortex blob method (VBM), turbulent flow in a channel is studied and physical concepts of turbulence are obtained and discussed. At first, time-averaged velocities, and , and then their fluctuations are calculated. To clarify turbulence structures, velocity fluctuations and are plotted. It is observed that turbulence structures occupy different positions and move with convection velocity. To verify the second law of thermodynamics, averaged vorticity and its fluctuations as well as averaged entropy and its fluctuations are calculated. Contours of these fluctuations show that their positions coincide with the positions of turbulence structures and both positions move with the same velocity. Correlation coefficient of velocity fluctuations between two points, and temporal correlation coefficient at a point, which have significant role in understanding physics of turbulence, are calculated and plotted. Having obtained these coefficients, time and space micro-scales and then turbulence energy dissipation rate () are obtained. Also, spatial-temporal correlation coefficients is calculated and then for turbulence structures microscale of time (memory), microscale of space (size) and convection velocity of structures are found. These scales estimate their life and size. Having obtained dual correlation coefficients, spectral studies of the velocity fluctuations, and , are performed, which include both frequency amplitude (related to temporal correlation coefficient) and wave number (related to special correlation coefficient). In fact, spectral study of fluctuations is Fourier transform (Cosine) of these fluctuations. Finally, dropping rate of this transform is compared with available data in turbulence literature.

The forced convection heat transfer of turbulent Al2O3-water nanofluid flow inside the grooved tubes with the different aspect ratio of the rectangular grooves is numerically investigated. The governing equations have been solved using finite volume method (FVM) coupled with SIMPLE algorithm. It is assumed the heat flux is constant on the grooved walls. The Single-phase approach is applied for the computation of the nanofluid flow. The Nanoparticles volume fraction is in the range of 0-5% and flow Reynolds number is in the range of 10,000-35,000. Comparisons between the numerical results and
available experimental data show that among different turbulence models, k-ε model with enhanced wall treatment gives the better results. The results show that the heat transfer coefficient increases with nanoparticles volume fraction and Reynolds number but it is accompanied by pressure drop augmentation. From the results, it is concluded that the grooved tubes with Al2O3-water nanofluid flow are thermodynamically advantageous. The Correlations for heat transfer coefficients have been presented for grooved tubes in different aspect ratios using the numerical results. The optimum geometric ratios in which the entropy generation is minimized are also determined.

The thermo-hydraulic behavior of the air flow over a two dimensional ribbed channel was
numerically investigated in various rib-width ratio configurations (B/H=0.5-1.75) at different Reynolds numbers, ranging from 6000 to 18000. The capability of different
turbulence models, including standard k-ε, RNG k-ε, standard k-ω, and SST k-ω, in predicting the heat transfer rate was compared with the experimental results and it was
showed that the k-ε turbulent models best adapt with the measured data. Four main
parameters, namely, the Nusselt number, friction factor, skin friction factor, and the thermal enhancement factor were examined through the simulations. Results indicate that an increase in the Reynolds number caused the Nusselt number to increase and the friction factor to drop. It was found that the thermal enhancement factor augmented by an increase in the Reynolds number, and also, for a wider rib, i.e. at the higher the B/H ratio, a lower thermal enhancement factor was obtained.

Changes in rib-height to channel-height ratio (e/H) has a significant effect on the heat transfer and pressure drop characteristics inside corrugated channels. In current paper, the variation of (e/H) was investigated numerically as well as a deep concern for finding the adequate turbulent model. In this regards, the governing equations were solved by a finite volume approach in a wide range of rib height to channel-height ratio (0.06 < e/H < 0.26) and the Reynolds numbers (5400 < Re < 23000). The predicted results reveal that the RNG k - ε turbulence model provides better agreement with available experimental data than other turbulence models. The computed results not only confirm the noticeable effects of (e/H) on the heat transfer performance and pressure drop but also, demonstrate the optimum corrugation design limits