Turbulent Flow Reynolds Number Research Articles

Experimental study on single-phase heat transfer performance of internal helically finned tubes covering low Reynolds number in turbulent flow

ABSTRACT This paper experimentally investigated the heat transfer performance of two internal helically finned tubes covering low Reynolds number in turbulent flow. A water-ethylene glycol mixture flowed inside and R134a boiled outside. The numbers of fin, the helix angles, and the ratios of fin height to diameter for the two test tubes were 38 and 60, 60° and 45°, and 0.0534 and 0.0222, respectively. The experimental Reynolds number was lower to 5400, covering the critical Reynolds number Re cr for turbulent flow. The data was divided into good and poor linearity regions by the modified Wilson plot method, which was much suitable for this case than that the data was treated as one region. The j factors showed ascending trend for Re below Re cr and descending trend for Re above Re cr. Enhancement factors of friction factor or j factor of the test tubes increased with increasing Reynolds number and eventually approached at the maximum for the Reynolds number above Re cr. The test tubes exhibited maximum efficiency indexes of 2.1 and 1.88, indicating superior overall performance compared to the majority of existing tubes. By utilizing the experimental data, a correlation was established basing on the critical Reynolds number, resulting in over 96% of the predicted errors falling within the range of ± 8%. This study has the potential to provide valuable insights for industrial implementation and the advancement of general correlation development.

DESIGNATION OF THE HEAD LOSS COEFFICIENT IN FITTINGS CFD

The optimization of pumping systems is important because it leads to an increment of improved energy efficiency. This study analyzes the hydraulic behavior of clean water through pressurized fittings which is analyzed numerically. The numerical experiment is carried out using Ansys Fluent software. The flow characterization through the fittings was performed using the coefficient of minor losses (KL). Two short radius elbows of 45° and 90°, and a PVC tee (simple) were used. The procedure used for the development of the numerical experimentation in this study is as follows: geometry construction, discretization, boundary conditions, configuration of the numerical simulation parameters, validation, CFD parametric study, and the analysis of results. The results of the flow simulation on the meshes showed a high sensitivity to the mesh size. To determine the optimal element size, a mesh independence study was performed, which consisted of refining the mesh to stable values. The numerical results were compared with values calculated using conventional equations for minor losses in the fittings. The KL values were characterized by two methods: in the first one the behavior of KL with respect to Reynolds number in turbulent flow was studied, in the second one the relative percentage of loss error was determined in comparison with previous investigations on this subject. The numerical results showed an acceptable relative error for the three simulated fittings.

Turning Guide Vane Effect on Internal Cooling of Two-Passage Channel With Parallel Ribs

Abstract The present work investigates the effects of various guide vane designs on the heat transfer enhancement of rotating U-duct configuration with parallel 45-deg ribs. The ribs were installed on the bottom wall of the channel, which has a constant heat flux boundary condition. The channel has a square cross section with a 5.08 cm hydraulic diameter. The first and second passes are 514 mm and 460 mm, respectively. The range of Reynolds number for turbulent flow is up to 35,000. The channel rotates at various speeds up to 600 rpm, which brings the maximum rotation number of 0.75. Several computational fluid dynamics simulations are carried out for this study to understand the effect of guide vanes on flow and heat transfer in serpentine channels under various operating conditions.

Experimental study on single-phase flow in horizontal internal helically-finned tubes: The critical Reynolds number for turbulent flow

A correlation of the critical Reynolds number for turbulent flow in horizontal helically finned tubes is proposed in this study based on the analysis of experimental data from the current and six previous studies. In the experiment, the main parameters of the two tubes (Tube-1 and Tube-2), include the number of fin (Ns), helix angle (α), and the ratio of fin height to diameter (e/Di), which are 38 and 60, 60° and 45°, and 0.0534 and 0.0222, respectively. Aqueous ethylene glycol was used as the test fluid. Pressure drop data were obtained under isothermal condition with Reynolds number spanning from 3100 to 39,500 and Prandtl number spanning from 13.8 to 49.2. Results showed that the critical Reynolds numbers for turbulent flow in Tube-1 and Tube-2 were 11,000 and 17,000, respectively. The proposed correlation, which correlated the critical Reynolds number with four parameters (i.e., e, Di, α, and Ns) and a constant, predicted all the 14 groups of critical Reynolds number within 10% and could be applicable to internal helically finned tubes with 0.01 < e/Di < 0.0534, 18° < α < 60°, and 25 < Ns < 82. This work offers a reference for the general correlation for heat transfer coefficient and friction factor of the internal helically finned tubes.

Effect of Vibration on Forced Convection Heat Transfer for SiO2–Water Nanofluids

In the process of heat transfer, the fluid type and external parameters have a significant impact on heat transfer performance. For this reason, the physical properties, pressure differences, and heat transfer rates of SiO2–water nanofluids have been experimentally investigated in a straight circular pipe. Experimental results revealed a great difference in physical properties between SiO2–water nanofluids and purified water. The friction factor of low-volume-concentration nanofluids was slightly increased for laminar flow and tended to be almost independent of the Reynolds number for turbulent flow. The heat transfer coefficient can be enhanced either by adding nanoparticles to purified water or by imposing a transverse vibration on the heat transfer surface. Using these two methods at the same time (compound heat transfer enhancement), heat transfer performance is much better than that with either method alone. The largest increase of about 182% was observed under conditions of compound heat transfer enhancement.

Experimental study on the convective heat transfer enhancement in single-phase steam flow by a support grid

Single-phase flow occurs in the fuel rod bundle of a pressurized water reactor, during the normal operation period or at the early stage of the reflood phase in a loss-of-coolant accident scenario. In the former period, the flow is single-phase water flow, but in the latter case, the flow is single-phase steam flow. Support grids are required to maintain a proper geometry configuration of fuel rods within nuclear fuel assemblies. This study was conducted to elucidate the effects of support grids on the convective heat transfer in single-phase steam flow. Experiments were made in a square array 2×2 rod bundle. The four electrically-heating rods were maintained by support grids with mixing vanes creating a swirl flow. Two types of support grids were considered in this study. The two types are geometrically similar except the blockage ratio by different mixing vane angles. For all test runs, 2kW power was supplied to each rod. The working fluid was superheated steam with Re = 2,301–39,594. The axial profile of the rod surface temperatures was measured, and the convective heat transfer enhancement by the presence of the support grids was examined. The peak heat transfer enhancement was a function of not only the blockage ratio but also the Reynolds number. Given the same blockage ratio, the heat transfer enhancement was sensitive to the Reynolds number in laminar flow, whereas it was nearly independent of the Reynolds number in turbulent flow.

Friction Coefficient (F)-Reynolds Number (Re) Relationship in Non-cohesive Suspended Sediment Laden Flow thorough Pervious Rockfill Dams

Recently pervious rockfill detention dam is used as a flood mitigating structure. Analysis of hydraulics of turbulent flow through this kind of dam is mostly done using Darcy-Weisbach equation. So far, many attempts have been made to study the friction coefficient as a function of Reynolds Number in turbulent flow of clean water through pervious rockfill detention dams, while this subject has remained intact for sediment laden flow of water. In this paper relationship between friction coefficient and Reynolds Number for sediment laden flow through highly pervious rockfill dams was investigated. Required data for a regression analysis obtained by conducting a series of laboratory tests to calibrate and validate a proposed power law friction coefficient-Reynolds Number relationship. A changeable bed slop Plexiglas flume, an adjustable rate sediment feeder and a recirculating flow electro pump system were used in present study. The tests were carried out on four different rectangular laboratory rockfill dams and three different non-cohesive suspended sediments. A power law relationship was obtained with a correlation coefficient of 0.74 using two thirds of laboratory measured friction coefficient and Reynolds Number. The obtained relationship was validated employing the remaining unused data with a Mean Square Error of 0.29 which is an acceptable agreement. A new power law relationship was found between friction coefficient and Reynolds Number in sediment laden flow through pervious rockfill dams. This new relationship is the only one thus has been proposed for the sediment laden flow of water through pervious rockfill dams.

Aerosol Deposition Measurements as a Function of Reynolds Number for Turbulent Flow in a Ninety-Degree Pipe Bend

An experimental and numerical investigation of the effect of the Reynolds number (Re) on the deposition of aerosol particles in a 90° pipe bend for turbulent flow was performed. Deposition fraction data were measured for a range of Stokes numbers (Stk) at different flow Re (10,250, 20,500, and 30,750) higher than those of most previous studies where Re was ⩽10,000. The data show good agreement with previous studies for Stk > 0.4, demonstrating that increased Re does not significantly alter the trend of deposition fraction with Stokes number (Stk) in this range. However, a noticeable increase in deposition was detected for 0.1 ⩽ Stk ⩽ 0.4. At Stk = 0.15, an increase in Re from 10,250 to 30,750 caused a factor of 2.6 increase in deposition fraction from 0.14 to 0.36. Numerical simulations were completed, using the Reynolds Averaged Navier-Stokes (RANS) equations with the Shear Stress Transport turbulence model. Modeling with inertial impaction only (i.e., neglecting turbulent dispersion), the results accura...

Experimental study and large eddy simulation for the turbulent flow around four cylinders in an in-line square configuration

The turbulent flows around four cylinders in an in-line square configuration with different spacing ratios of 1.5, 2.5, 3.5 and 5.0 have been investigated experimentally at subcritical Reynolds numbers from 11,000 to 20,000. The mean and fluctuating velocity distributions were obtained using the laser Doppler anemometry (LDA) measurement. The digital particle image velocimetry (DPIV) was employed to characterize the full field vorticity and velocity distributions as well as other turbulent quantities. The experimental study indicated that several distinct flow patterns exist depending on the spacing ratio and subcritical Reynolds number for turbulent flow. The three-dimensional numerical simulations were also carried out using the large eddy simulation (LES) at Reynolds number of 15,000 with the spacing ratio of 1.5 and 3.5. The results show that the LES numerical predictions are in good agreement with the experimental measurements. Therefore, the three-dimensional vortex structures and the full field instantaneous and mean quantities of the flow field such as velocity field, vorticity field, etc., which are very difficult to obtain experimentally, can be extracted from the simulation results for the deepening of our understanding on the complex flow phenomena around four cylinders in in-line configuration.

Navier–Stokes Analysis of a Regular Two-Dimensional Roughness Pattern Under Turbulent Flow Regime

The present work deals with the flow characteristics induced by a two-dimensional textured surface. The texture consists of identical and equally spaced rectangles with characteristic lengths at least one order of magnitude larger than the clearance of the thin film. Periodic boundary conditions enable the analysis of a single groove and the complete Navier–Stokes analysis is carried on for turbulent flow Reynolds numbers. The analysis is performed for shear driven flows (Couette), pressure driven flows (Poiseuille), and combined Couette–Poiseuille flows. First, the presence of inertial forces generated by the groove is emphasized by the momentum balance performed for the computational cell. The peculiar effect of the groove is also shown by the rotor and the stator shear stresses variations. Finally, it is shown that despite the presence of fluid inertia forces, cell-averaged rotor, and stator shear stresses obtained for pure Couette or Poiseuille flows can be added or subtracted to obtain with good accuracy the characteristics of combined shear and pressure driven flows.

Factors affecting the accuracy of pressure measurements in vascular stenoses from phase-contrast MRI.

In this work the effects of noise, resolution, and velocity (flow) on the measurement of intravascular pressure from phase-contrast (PC) MRI are discussed. To elucidate these effects, we employed an axisymmetric geometry that enabled us to calculate pressures in <2 min on a Sun Ultra SPARC 10 workstation. To determine the effects of vascular stenoses, we fabricated several stenotic phantom geometries (with 50%, 75%, and 90% area stenoses), and performed both MRI and computational fluid dynamics (CFD) simulations for various flow rates for these phantom geometries. Noise with Gaussian statistics was added to the velocity field obtained from the CFD simulations. The pressure maps obtained directly from CFD simulations for our phantom geometries were compared with pressure maps derived by our algorithm when 1) the input was noise-corrupted velocity data from CFD, and 2) the input was PC-MRI data collected from the phantoms. The quantitative effects of noise, resolution, and flow rate on the accuracy of pressure measurements were determined. We found that for flow rates below the Reynolds number for turbulent flow, resolution is a more significant determinant of accuracy than SNR. Furthermore, if other parameters remain constant, increased flow rates may result in decreased accuracy.

밀폐된 원통내부에서 회전체의 형상이 회전유동에 미치는 영향에 관한 실험적 연구

Two-dimensional, angle-resolved LDV(Laser Doppler Velocimetry) measurements of the turbulent rotating flow field in a confined cylinder have been performed. The configurations of interest are flows between a rotating upper disk with a rod attached by a disk or impeller(??= 45°, 90°) and a stationary lower disk in a confined cylinder. The mean now velocity as well as the turbulent intensity of the now field have been measured. The results show that the now is strongly dependent on the position of the impellers or the disk, negligibly affected by the Reynolds number in turbulent flow. It is observed that the mixing effect of the axial now impeller(??= 45°) is better than that of the radial flow impeller(??= 90°) or a disk.

Experimental Study for the Mixing Effect of the Driven Bar on Rotating Flow in a Closed Cylinder

The experiment is conducted on the rapidly rotating incompressible flow within a confined cylinder using LDV(Laser Doppler Velocimetry). The configurations of interest are the flows between a rotating upper disk with a bar and a stationary lower disk enclosed within a cylinder. The flow is considered to be an axisymmetric undisturbed basic flow. The results show that the flow is strongly dependent on the radius and the shape of bar but is negligibly affected by the Reynolds number in turbulent flow. It is observed that in the lid-driven case the main forms near the wall as the Reynolds number increases. The thin bar causes the second axial flow due to the suction effect and the thick bar causes the main flow to be pulled toward the surface of the bar. The step bar shows the dual effect of the two. 1:2 tilt bar shows that the main flow distributes wider than the other cases in which interference occurs due step bar.

Effect of saltating sediment load on the determination of the mean velocity of overland flow

Where overland flow velocity is measured using dye or salt tracing, the mean velocity is often determined by multiplying the velocity of the leading edge of the tracer plume by a correction factor α. Flume experiments show that a varies with both Reynolds number and sediment load. For sediment‐free flow over a sand‐covered bed, α is less than the theoretical value of 0.67 in laminar flow and increases rapidly with Reynolds number in transitional flow and more slowly with Reynolds number in turbulent flow. For sediment‐laden flow, α decreases as sediment load increases in transitional and turbulent flows. Saltating sediment extracts momentum from the flow, causing velocities near the bed to decrease and α to decrease. A multiple‐regression equation is developed which can be used to predict the value of α from Reynolds number and sediment load in transitional and turbulent overland flows.

MASS TRANSFER IN AC ELECTROLYSIS: EXTENSION OF A FILM MODEL TO TURBULENT FLOW ON A ROTATING HEMISPHERE

Turbulent mass transfer to a rotating hemisphere during AC electrolysis was examined with a film model. The analytical results for the limiting AC current density were compared to the experimental data obtained with sinusoidal, square and triangular wave AC. The limiting AC current density in turbulent flow was found to depend on both the Reynolds number as a dimensionless AC frequency K = (ω/Ω)Sc1/3. The prediction by the film model agreed with the experimental data to within ±15%. For a given root-mean-square of applied AC, the periodic concentration overpotential decreased in the order of quare, sinusoidal, and triangular AC. The phase shift between the applied AC and the concentration overpotential wave decreased with increasing Reynolds number in turbulent flow.

Turbulence effects on some aerodynamic parameters of a circular cylinder at supercritical numbers

Aerodynamic parameters such as lift coefficient, Strouhal number and drag coefficient are required for estimating the response of structures due to wind action. These parameters for a circular cylinder are sensitive to a number of influences. Among these, effects of Reynolds number and turbulence are particularly prominent. Experimental data have been obtained from pressure and force measurements in a wind tunnel for different levels of turbulence intensity at critical and supercritical Reynolds numbers up to 10 6. Blockage corrected results together with some full scale measurements have illustrated that some of these aerodynamics parameters become relatively insensitive to Reynolds number effects earlier in turbulent flow than in smooth flow. It is shown that modelling of full scale configurations with circular features can be performed satisfactorily provided the test Reynolds number in turbulent flow with turbulence intensity greater than 4%, is in excess of 2×10 5; tests at Reynolds numbers below this value are shown to have little relevance to such full scale structures.

Application de la méthode du fil chaud à la mesure de la turbulence dans l'eau deuxième partie exécution des mesures et résultats

Once the general measurement procedure had been finalised during the tests described in Part I, measurements were carried out to determine the characteristics of the turbulence produced in the tank. This took place in the following stages : A) Choice of final parameters, especially rotating arm speed. Different turbulence values are obtained by varying the speed of the rotating arm as the turbulent system is determined by the mechanical generator characteristics. B) Actual measurements. 1. Fluctuating velocity. As might be expected a substantial decrease was observed in the turbulent velocities from the bottom part of the tank near the generator to the free surface. 2. Spectra. These measurements too are more difficult than with conventional wind tunnel screen layouts as the characteristic apparent frequencies of the motion resulting from the spatial frequencies of the turbulent motion and average (rotating arm) speed are low. The spectrum shows a very typical Kolmogoroff zone, but the dissipation zone does not conform to the (-7) exponent law predicted by the Heisenberg theory. 3. Correlations. The correlation between longitudinal velocity fluctuations and distance between the two measurement points was measured and the resulting curve used to define the integral transverse correlation length. 4. Micro-scales. These were measured directly with the aid of a differentiation system and calculated from the spectra. A certain number of measurement difficulties were experienced with probe dimensions as compared to measurement lengths and differentiation system pass bands. C) Interpretation of measurement results. 1. Effective turbulent velocities. These velocities were found to satisfy an equation of the 'diffusion' type surprisingly accurately. The energy diffusion 'coefficient' was determined by matching this equation to the experimental curve. 2 and 3. Relationship between characteristic turbulence quantities. The study of the (approximately isotropic) turbulence behind the screen yielded certain relationships between the characteristic quantities. Comparison with the tank measurements showed that these relationships are at least approximately preserved in spite of differences between the basic parameters of the movements obtained, especially Reynolds number. 4. The Kolmogoroff constant was determined by spectrum measurement and by calculating dissipated power from the micro-scale measurement data. Despite fairly appreciable scatter the values found agree well with other researchers' data. As the Reynolds number for turbulent flow in the tank is about 300, it is seen that the critical Reynolds number for the existence of a Kolmogoroff zone is invariably less than 500, which is the figure some researchers have suggested. 5. The measurements also especially showed up the need for very different special instrumentation from that used in air flows, even though the definitions of the measured quantities may be the same in both cases.