Turbulent Flow Regime Research Articles

Numerical simulation of the thermal-hydraulic performance of solar collector equipped with vector generators filled with two-phase hybrid nanofluid Cu-TiO2/ H2O

In this research, the effect of vortex generators (VGs) with various geometric shapes and Cu-TiO2/H2O nanofluid (NF) on thermal-hydraulic (TH) performance, energy efficiency and exergy of a solar collector (SC) has been perused. In this research, the turbulent flow regime (TFR) was used to study the stated parameters. The QUICK algorithm is used for the discretization of the equations. According to the assumption of a TFR, the flow behavior of Cu-TiO2/ H2O NF in the Re range of 4000–16,000 was studied. In addition, the k-ω SST model was utilized to modelling the turbulent flow. Also, due to considering the NF as two-phase, the Eulerian two-phase method was used for modeling. Two-phase hybrid NFs with Cu and TiO2 nanoparticles in solid volume fractions of SVF=1, 3, and 5% were modeled. The findings of the research illustrate that convective heat transfer (CHT), Nuavg (average Nusselt number), and pressure drop (PD) increase with the increase of the inlet velocity of Cu-TiO2/ H2O NF flow. In addition, the results of PEC evaluation indicate that the adding VGs and changing their placement states is desirable in terms of PEC. In the SC along with the case D turbolator and SVF= 5 %, the exergy efficiency increases by 45.87 % with the increase of the Reynolds number (Re) from 4000 to 16,000.

Hydraulic Characterization of Ceramic Foam Filters Used in Aluminum Filtration.

Ceramic Foam Filters (CFF) are frequently used during the filtration of aluminum (Al) melts to produce high-quality products. In the present study, the physical and hydraulic characteristics of alumina (Al2O3)-based CFF from three different suppliers (A, B and C) have been thoroughly investigated. The filters' porosity and pore diameter, i.e., Window and Cell Feret diameters, were measured and the permeability of the different filters calculated based on pressure drop experiments. The comparison of the classification systems of CFF, i.e., Grade and PPI (Pore Per Inch) numbers, using statistical analysis of permeability and Window Feret diameter showed significant variations between the morphological and hydraulic properties of some CFFs of identical Grade and PPI numbers. Moreover, the Fanning friction factor was plotted as a function of interstitial Reynolds numbers (Rei), and laminar, transient, and turbulent flow regimes were identified. The relationship between the Fanning friction factor and the interstitial Reynolds numbers of all the filter samples investigated was processed using regression analysis, and a model equation developed to calculate the pressure drop over the CFF using the Window Feret diameter. The correlation between the experimental pressure drop values and the derived model equation indicates that empirical expressions for calculating the pressure drop over CFFs should be derived based on experimental measurements carried out at the velocity range of the application of the CFF, which is about 10 mm·s-1 for aluminum filtration.

Impact of vortex generator and two-phase MWCNT-MgO/water hybrid nanofluid on thermo-hydraulic performance and exergy and energy efficiency of a solar collector under Kuwait climatic conditions

The effect of curved vortex generators and two-phase MWCNT-MgO/water hybrid nanofluid (HNF) on thermo-hydraulic performance and exergy and energy efficiency of a solar collector in a turbulent flow regime is examined in this study. The simulations performed for the climatic conditions of Kuwait. The two-phase DWCNT-MgO/water HNF is modeled using the mixture model. The numerical study is performed for DWCNT-MgO/water HNF with a volume fraction (φ) range of 1–3%, Reynolds number (Re) range of 5000–20,000, and four Pitch Ratio (PR = 0 (Case A), PR = 0.1 (Case B), PR = 0.2 (Case C) and PR = 0.3 (Case D)). It is found that the average Nusselt number (Nuav) and pressure drop (Δp) are enhanced with Re and φ. At φ = 3% and Re = 20,000, placing the vortex generator with a PR = 0.3 (Case D) inside the solar collector causes the Nuav to improve by 83.09% as compared to the solar collectors without turbulators. In addition, the results demonstrated that the use of vortex generators is desirable in all PRs. For Case D and φ = 3%, the exergy efficiency is improved by 45.02% as Re is enhanced from 5000 to 20,000.

Bejan’s numerical heat and mass flow visualization in turbulent boundary layer regime

Low Reynolds number k-ϵ model is employed to demonstrate the Bejan's numerical heat and mass flow visualization in turbulent boundary layer regime. The governing turbulence kinetic energy and its dissipation rate equations are included along with conservation equations. The turbulent flow equations are highly nonlinear and coupled in nature, hence, an unconditionally stable Crank–Nicolson finite difference approach is deployed. The computer-generated heatlines, masslines, streamlines, and isotherms for different control parameters are analyzed in the turbulent flow regime. Average momentum, thermal, and concentration diffusion rates are increased upon the increment in in laminar regime and are decreasing with rising in turbulent regime. The stream, heat and masslines deviate more in turbulent regime when compared to laminar regime. Further, the kinetic energy and dissipation rate contours are deviated more in turbulent heat and mass flow regime for different values of . Mainly, in this research paper, the authors made an attempt to demonstrate the turbulent heat and mass flow visualization about a vertical plate using boundary layer approximations through LRN k-ϵ turbulence model.

Numerical study on forced convection heat transfer of TiO2/water nanofluid flow inside a double-pipe heat exchanger with spindle-shaped turbulators

Heat exchangers have a substantial role in industrial processes. Efficient designs must be introduced to balance heat exchanger effectiveness and pressure drop in order to attain the anticipated tradeoff between the system's size and efficiency. Herein, a numerical study has been implemented on the impact of using nanofluid inside a double-pipe heat exchanger equipped with spindle-shaped turbulators on the heat transfer rate and pressure drop in a turbulent flow regime. The two-phase mixture method has been employed and the characteristic equations, namely, continuity, momentum, energy, turbulent kinetic energy, turbulent losses, and the particle volume fraction equation have been solved for studying the flow. The numerical finite volume method has been utilized to discretize and solve the equations. The results show that by adding 0.01 volumetric fraction of nanoparticles to the base fluid, the transfer heat transfer coefficient increases by 11.45% at Re 4000, and 11.91% at Re 7000, in comparison with the base fluid. Also, although increasing the volume fraction of nanoparticles reduces the heat exchanger performance index, however, this reduction is slight when associated with the base fluid rate in the volume fraction of 0.01.

Numerical investigation of the effect of the turbulator geometry (disturber) on heat transfer in a channel with a square section

The principal moot point in this investigation is heat removal from surfaces with high heat flux, which is the use of tabulators and parts with a particular geometry, it has been chosen as a solution to this moot point. The primary hypothesis in this investigation is to increase fluid heat transfer by increasing turbulence and heat transfer by increasing the plane of heat transfer and establishing a vortex flow. The fundamental idea and novelty of this investigation is the simultaneous use of a turbulator (to improve turbulence and provide more effective heat transmission) and increasing the contact surface (through the installation of parts with unique geometry), which can be obtained from different turbulator used in other geometries. In this research, the limited volume method to solve governing equations in three-dimensional space and Cartesian coordinates has been used on the network using Ansys Fluent software. In order of comparison, turbulators SLT and then TRT is compared to other turbulators (TRT, SHT, RET) It has the highest Nusselt number, and in the Reynolds numbers in the turbulent flow regime, they have the most significant reduction in the friction coefficient.

Effect of radial clearance on the dynamic behavior of adjustable journal bearings in turbulent and laminar flow regimes

Radial clearance critically affects journal bearing performance, specifically the lubrication performance. This study investigates the lubrication characteristics of a journal bearing with adjustable radial clearance and compares the dynamic behavior of the bearing under laminar and turbulent flow regimes. The adjustable bearing exhibits two states of oil film pressure at different radial clearances. When the radial clearance is reduced from 100%cr to 80%cr, the oil film pressure is present only on the lower bearing bush; after being reduced to 70%cr, the upper bearing bush achieves effective lubrication; and after reduction from 70%cr to 30%cr, the oil film pressure increases more than six-fold. The turbulent flow model of the clearance adjustable bearing is then established considering the lubricant flow state, and the turbulence correction factors for different flow states are calculated. The journal bearing exhibits better lubrication characteristics under laminar than under turbulent flow conditions, and in some operating conditions, the stable velocity range calculated according to laminar flow theory may become unstable in turbulent flow conditions. Therefore, applying turbulence assumptions to design the operating speed range of rotating machinery under such conditions yields a greater safety threshold, which can reduce the risk of instability. The stability of the rotor bearing system is closely related to the radial clearance, and reducing the radial clearance will improve the stability of the system. In the turbulent flow state, the oil film pressure changes drastically, thus reducing the system stability. Moreover, the larger the radial clearance of the bearing, the more prominent the turbulence effect. Thus, reducing the bearing clearance prevents drastic changes in oil film pressure and improves the stability of the system.

Assessment of The Acoustic Scattering Matrix of a Heat Exchanger Using ssCFD-LNSE Simulation

The risk of thermoacoustic instability is present in any combustion appliance. The instability results from a closed loop feedback between unsteady combustion, heat-transfer and acoustic modes of the system. To predict the system acoustics all constituting elements of the appliance need to be modelled. A heat-exchanger is the element where the gas faces complex fluid dynamics and heat transfer processes. Therefore, modelling of (thermo)-acoustic properties of a heat exchanger is challenging. In this paper, a computational approach is proposed to characterize the acoustic properties of a generic heat exchanger in both laminar and turbulent flow regimes. A hybrid Computational Fluid Dynamics - Computational Aero-Acoustics (CFD-CAA) method is used based on full linearized Navier-Stokes equation, called ssCFD-LNSE. The fundamental idea in this approach is to efficiently model acoustic wave propagation with inclusion of mean flow and temperature fields. ssCFD-LNSE is performed by splitting the quantities of the total field into a mean part (obtained from CFD) and a (acoustic) perturbation part modelled within the LNSE solver. The goal of this research is to assess the two-port acoustic scattering matrix of an array of tubes, as a generic model of heat-exchanger, with a hot cross flow.

Mixing performance of a non-Newtonian fluid in a coaxial agitated impeller reactor

BackgroundThe power demand and mixing performance of three different coaxial mixers, fitted with either a Cowles turbine, 4-pitched blade turbine or 3-blade propeller impeller as the inner element, and anchor impeller as the outer element, were evaluated in a counter rotating mode in the laminar, transition, and turbulent flow regimes. MethodsCMC solution (sodiumcarboxymethyl cellulose) was used as the non-Newtonian fluid. The quantitative correlations for CMC solutions were established to calculate the power draw and the mixing time of the three coaxial mixers. The inner agitators which produce axial flow were identified to be more efficient than those which produce radial flow. Significant findingsResults show that the 4-pitched blade turbine/anchor impeller is the optimized combination among the three coaxial mixers tested, providing the shortest mixing time given the same power consumption. Based on mixing efficiency criteria, this study confirmed that the 4-pitched blade turbine/anchor impeller counter-rotating mode consistently yields the best results in the full flow regimes.

The Effect of Threaded Rod Inserts and Aluminum Oxide–Water Based Nanofluids on Performance of a U-Bend Double Pipe Heat Exchanger

Double pipe heat exchangers, owing to their simplicity in construction and ease of maintenance, are being widely used in refineries, food processing units and pharma industries. However, they have a limitation that they can only operate at low heat loads. In order to make them suitable for higher loads, it is essentially required to enhance the rate of heat transfer. In this work, an attempt is made to this end using a combination of threaded rods and water-based aluminum oxide nanofluids. Experimental investigations were carried on a counter-flow double pipe return bend heat exchanger (DPHE) using water-aluminum oxide (Al2O3) nanofluids with threaded rod inserts. The volume concentration of Al2O3 nanofluid added in water was 0.03% and 0.05%. The entire analysis was carried out under turbulent flow regime by varying the mass flow rates of cold water from 3 to 15 LPM in steps of 2 LPM, with the hot water flow rate being fixed at 6 LPM. The Reynolds number range considered was from 3000 to 30000. From the obtained results, it was found that the maximum enhancement in the Nusselt number and friction factor for 0.05% concentration of the nanofluid and threaded rod inserts respectively is 89.95% and 32.46% more than the plain tube. The maximum thermal performance factor was found to be 1.75 for the combination of 0.05% nanofluid and inserts.

Numerical Study on the Heat Transfer Characteristics of Cu-Water and TiO2-Water Nanofluid in a Circular Horizontal Tube

A numerical simulation of convective heat transfer coefficient (hconv) was studied with Cu-Water and TiO2-Water nanofluids flowing through a circular tube subjected to uniform wall heat flux boundary conditions under laminar and turbulent regimes. Four different concentrations of nanofluids (ɸ = 0.5, 1, 1.5 and 2%) were used for the analysis and the Reynolds number (Re) was varied from laminar (500 to 2000) to turbulent flow regime (5000 to 20,000). The dependence of hconv on Re and ɸ was investigated using a single-phase Newtonian approach. In comparison to base fluid, average hconv enhancements of 10.4% and 7.3% were noted, respectively, for the maximum concentration (ɸ = 2%) and Re = 2000 for Cu-Water and TiO2—water nanofluids in the laminar regime. For the same ɸ under the turbulent regime (Re = 20,000), the enhancements were noted to be 14.6% and 13.2% for both the nanofluids, respectively. The random motion (Brownian motion) and heat diffusion (thermophoresis) by nanosized particles are the two major slip mechanisms that have more influence on the enhancement of hconv. In addition, the Nusselt number (Nu) of the present work was validated for water with the Shah and Dittus Boelter equation and found to have good agreement for both the regimes.

Regime transitions in stratified shear flows: the link between horizontal and inclined ducts

We present the analytical solution for the two-dimensional velocity and density fields within an approximation for laminar stratified inclined duct (SID) flows where diffusion dominates over inertia in the along-channel momentum equation but is negligible in the density transport equation. We refer to this approximation as the hydrostatic/gravitational/viscous in momentum and advective in density (HGV-A) approximation due to the leading balances in the governing equations. The analytical solution is valid for laminar flows in a two-layer configuration in the limit of long ducts. The non-dimensional volume flux within the HGV-A approximation is given by $Fr^* ={{Re}}_g/(AK)$ , which is a control parameter with ${{Re}}_g$ the gravitational Reynolds number, $A$ the aspect ratio of the duct and $K$ a geometrical parameter that depends on the tilt of the duct and is obtained from the analytical solution. This analytical solution was validated against results from laboratory experiments, and allows us to gain new insight into the dynamics and properties of SID flows. Most importantly, constant values of $Fr^*$ describe, in both horizontal and inclined ducts, the transitions between increasingly turbulent flow regimes: from laminar flow, to interfacial waves, to intermittent turbulence and sustained turbulence.

The effect of polymeric drag reducing agent on pressure drop reduction in circular pipes: Experimental and statistical investigation

The present work deals with investigating the effect of adding Polyacrylonitrile (PAN) as a drag-reducing agent on the water flow’s hydraulic performance within horizontal smooth circular pipes experimentally and statistically. Several experiments were performed under various operating circumstances such as fluid flow rates at the 6,8,10 l/min for preparing turbulent flow regimes, concentrations of 25,100, 300 ppm of PAN, and inner pipe diameters of 1/2,3/4 in, and temperatures of 30, 40, and 50 °C. The maximum percentage of drag reduction (DR%) was 56 at a concentration of 300 ppm, the flow rate of 6 l/min, temperature of 30 °C and the diameter of 3/4 in. Using the Taguchi method, the results were statistically analyzed. The maximum estimated DR% value was 40.187%, and the error between the predicted and experimental values was about 10%. Moreover, the PAN concentration had a contribution of 96.5% in the polymeric solution DR. Finally, there was a new correlation to predict friction coefficient in terms of the Prandtle-Karman relation with the newly set slope as a linear function of polymer concentration. There was an excellent consistency between this correlation and the experimental data.

Two-phase stratified flow in horizontal pipes: A CFD study to improve prediction of pressure gradient and void fraction

Improved understanding of flow regime effects on design-influencing engineering quantities is of primary importance. This work is focused on the numerical prediction of pressure gradient and void fraction in a horizontal pipe where gas-liquid stratified flow is present in different operating conditions. The problem was modeled with unsteady, multiphase CFD (computational fluid dynamics) simulations. Volume Of Fluid (VOF) method was used as multiphase model. To define the numerical methodology, this study provides details on the influence of discretization grid and turbulence model on the simulation accuracy. It shows that mesh density on pipe cross-section is the most important grid parameter to focus on. Different turbulence models are required depending on the gas velocity and on its turbulence flow regime. Transition SST is able to model all the operating conditions but Realizable k-ε is adopted to further increase the accuracy of the results. A general underestimation of the pressure gradient is reported with an average error of − 6.43 % and − 16.21 % for a liquid superficial velocity of 0.04 m/s and 0.06 m/s respectively. Comparison with two-fluid 1D models shows that CFD simulations are the most accurate tools for predicting the pressure gradient at gas superficial velocities lower than 1.3 m/s. The implementation of a drift flux model shows a good agreement between experimental results and CFD simulations concerning void fraction estimation. CFD results are also used to underline the physical phenomena limiting the performance of 1D models.

Exergetic, Energetic, and entropy production evaluations of parabolic trough collector retrofitted with elliptical dimpled receiver tube filled with hybrid nanofluid

In this paper, a numerical study was performed on a parabolic trough collector receiver tube containing elliptical dimpled fins with different elliptical ratios (ER = 0.66–1.66) under turbulent flow regime. The C++ code was developed to perform a non-uniform heat flux on outside of the receiver tube to get approximation real working conditions. Different mono and hybrid nanofluids and their combinations with different nanoparticle volume fractions (2.0 %TiO2/Syltherm800, 2.0 %Al2O3/Syltherm800, 0.5 %TiO2-1.5 %Al2O3/Syltherm800, 1.0 %TiO2-1.0 %Al2O3/Syltherm800, and 1.5 %TiO2-0.5 %Al2O3/Syltherm800) were used as a working fluid. The study provides a detailed analysis of the hydro-thermal characteristics, exergetic, and entropy production of the parabolic trough collector. 1.5 %Al2O3-0.5 %TiO2/Syltherm800 hybrid nanofluid shows the best increase in results obtained in this study using elliptical ratio of a/b = 5/3. The use of the elliptical ratio of a/b = 5/3, the average Nusselt number increases with 38 % compared to the receiver tube without elliptical dimpled fin. Total entropy generation was reduced up to 16.82 % for the elliptical ratio of a/b = 5/3. With the inserting elliptical dimpled fins of a/b = 5/3, the Performance Evaluation Criterion was detected as 1.81 compared to while the thermal efficiency increases from 74 % to 80 % at the lowest Reynolds numbers.

РАСЧЕТ ТЕПЛООТДАЧИ В ПЛОСКОМ КАНАЛЕ ПРИ ТУРБУЛЕНТНОМ РЕЖИМЕ ТЕЧЕНИЯ

In the article formulas for calculating the heat transfer coefficient in a flat channel under a turbulent flow regime are obtained. When constructing the formulas, data from direct numerical simulation of the heat transfer process obtained by various research groups were used.

Analytical models of induction heating of casing in a producing well

The article is devoted to the study of heat exchange processes in a well during induction heating of metal casing section. Heating of the column leads to an increase in the temperature of the fluid flow in the column. Due to short-term exposure, heat marks appear in the flow. Observation of their evolution forms the basis of active thermometry, a new method in borehole geophysics. Analytical solutions have been obtained for calculating the temperature changes of the upstream and casing during induction heating. Analytical solutions are compared with numerical simulation in Ansys Fluent*. It is shown that the discrepancy between the results of calculations based on analytical and numerical models decreases with an increase in the Reynolds number and the transition to a turbulent flow regime, with active mixing of the flow in the well in it is cross section. The developed models will be used in the planning of geophysical studies in wells by the method of active thermometry. Keywords: active thermometry; borehole; induction heating; column temperature; Nusselt number; Laplace transformations.

Analysis of diffusion and dispersion processes in water distribution networks through the use of the Péclet number threshold

To model the water quality within the distribution networks, simplified models are used (such as EPANET), which use an advective-reactive approach and neglect relevant phenomena, such as diffusion and dispersion, for low values of the Reynolds number. Although valid in the presence of a purely turbulent flow regime, as they do not produce substantial modelling errors, these simplifications are not applicable equally to all distribution networks. Therefore, the present study defines a threshold of the Péclet number, which relates the effectiveness of mass transport by advection with the effectiveness of mass transport by dispersion or diffusion, in order to establish a priori the behaviour of the water distribution network and find the most suitable model to use. The numerical analysis was applied to a real case study (network of Oreto-Stazione) using EPANET advective model and EPANET-DDR (Dynamic-Dispersion-Reaction) model, developed by the authors.

Laminar and turbulent flow development study in a rectangular duct with 180° sharp bend by using stereo particle image velocimetry and liquid crystal thermography measurements

This work presents a detailed insight into the flow progression and surface heat transfer distribution across the sharp 180° bend of a two-pass rectangular duct for laminar (Re = 800) and turbulent (Re = 6500) in-flow conditions. Stereoscopic particle image velocimetry (stereo PIV) as well as two-dimensional and two-component PIV measurements and liquid crystal thermography techniques are appropriately used for flow and heat transfer characterization across the complete sharp 180° bend. The centrifugal instabilities arise due to the sharp bend, which induces the secondary flows in the form of counter-rotating vortex pairs commonly known as Dean vortices. These secondary vortices play a significant role in the localized laminar–turbulent transition and turbulence augmentations for laminar and turbulent inflow conditions. Subsequently, quantitative analysis shows that complete 180° turning of flow resulted in intense augmentation of spatially averaged turbulence quantities. Root mean square (RMS) fluctuations in the transverse direction V¯T|rms increase by 298% and 186% for respective flow conditions. Augmentation of ∼ 287% (laminar) and 260% (turbulent) in the wall-normal RMS fluctuations (V¯N|rms) are observed. These augments in transverse and wall-normal velocity fluctuations result in a very sharp amplification of spatially averaged turbulent kinetic energy (k¯), that is, 1825% for inlet laminar and 928% for inlet turbulent flow regimes.

Spectral analysis of the transition to turbulence downstream a delta winglet pair vortex generator in an airflow channel

The purpose of this experimental study is to provide a detailed analysis of the transition to turbulence in an airflow channel perturbed by a delta winglet pair vortex generator. Measurements of instantaneous velocity are carried out by a laser Doppler anemometry technique. Experiments are conducted for Reynolds numbers (based on the hydraulic diameter) ranging from Re = 400 to Re=12 000 and at several axial position downstream of the vortex generator. The flow behavior is characterized in each regime by means of a frequency analysis. We identified the inlet conditions corresponding to laminar weakly unsteady, transitional, and fully turbulent flow regimes. Furthermore, we have constructed a correlation between the Strouhal number and the Reynolds number that characterizes the vortex instability mechanism. Finally, under turbulent flow conditions, the dissipation rate was estimated, which showed an increase followed by an exponential decrease with the distance downstream of the vortex generator.