Reynolds Number Flow Research Articles

Experiment on bubble characteristics of turbulent bubbly jets in pipe crossflow

Air-water bubbly jets in crossflow are commonly encountered in various practical applications such as artificial aeration in oceans and submarine pipelines. This study reveals the bubble characteristics for bubbly jets in pipe crossflow by conducting a series of physical experiments. It was found that the horizontal distance from nozzle exit to where bubble centerline touches the top pipe wall can be expressed as a function of the Reynolds and Weber numbers of the ambient pipe flow, and the Reynolds numbers of both the air and water phases of bubbly jets. The centerline gas void fraction and bubble size increase with distances after bubbles touch the top wall, mainly because bubbles are prone to gathering to the centerline to form larger bubbles. Afterwards, the (horizontal) bubble velocity decreases mainly due to the resistance force from the top wall. Good agreement has been achieved between horizontal bubble velocity and the 1/7th power law for water velocity in the pipe, and a correlation was proposed for predicting bubble rise velocity. Furthermore, turbulence characteristics of bubbles (e.g., root-mean-square of bubble fluctuating velocity, bubble turbulence intensity) were investigated for the bubbly jets.

Experimental and Numerical Study of Droplet Impact on Radially Flowing Liquid Film

In this paper, a single droplet impact onto a radially flowing liquid film is investigated both experimentally and numerically. The interfacial evolution and dynamic features are studied in detail with regard to various droplet impact velocities and film flow rates. Also, the corresponding mechanisms are analyzed with the aid of numerical results using the coupled level set and volume of fluid method. Results show that the droplet impact on the radial film produces a larger crown radius in the direction perpendicular to the film flow compared with the unidirectional film. Increasing the impact velocity leads to increases in crown height and upstream radius, but it has a weak effect on the downstream radius. As for the effect of the film flow rate, its increase causes a rise in the secondary droplet number in the upstream but makes the finger jets associated with the crown splashing less developed. Besides, increasing the flow rate of the liquid film results in increases in the upstream crown height and downstream radius but decreases in the downstream crown height and upstream radius. Finally, a formula for predicting the threshold of splashing is built up based on the film flow Reynolds number and the droplet Weber number.

Experimental testing of solar-based air heater roughed with discrete V-down rib and staggered element.

A rough rectangular channel of solar-based air heater (SBAH) is made and tested. The work consists of an effort to perceive the proportion of heat discharge and frictional behavior of air passing over a roughened rectangular channel. The absorber surface of SBAH is roughed with discrete V-down rib and staggered element roughness having different values of relative rib pitch (P/e) that ranged from 6 to 14. Fixed parameters such as relative gap size (g/e), relative staggered element pitch (P'/P), numeral of gaps (Ng), relative staggered element size (r/g), and relative rib height (e/D) are considered 4, 0.4, 3,1, and 0.0433, respectively, all throughout the study. The flow Reynolds number (Re) changes from 4000 to 14,000; consequently the Nusselt number (Nu) and friction factor (f) reach up to 2.16 and 2.73 times, respectively, with respect to plane surface. The optimum rise in terms of thermal-hydraulic performance (THP) is gained analogous to a P/e of 10. The correlation for heat transfer function, R(e+), and roughness function, G(e+), is given to anticipate the performance of roughness.

Thermohydraulic experiments on a supercritical carbon dioxide–air microtube heat exchanger

Heat exchangers are critical components in supercritical CO2 Brayton cycles and provide necessary heat input, recovery, and dissipation. Tubular heat exchangers with unconventionally small tube sizes (tube diameters less than 5 mm) are promising components for supercritical CO2 cycles and potentially provide excellent structural stability with wide scope of application. This paper provides essential design and fabrication guidelines for a compact shell-and-tube heat exchanger with microtubes (with an inner diameter equal to 1.75 mm). A heat exchanger test rig is used to evaluate the thermohydraulic performance of this heat exchanger with supercritical CO2 and air as working fluids. Thermohydraulic data are reported for more than forty sets of experiments with varying Reynolds numbers for shell and tube flows. Critical performance metrics are calculated from the data and compared with predictions from a previously described numerical model. The average deviations between the experimental and model results fall within 10% for all critical metrics. This excellent agreement validates the numerical model for supercritical CO2 heat exchanger optimization and scale-up.

Influence of bundle porosity on shell-side hydrodynamics and mass transfer in regular fiber arrays: A computational study

CFD predictions of the effects of a fiber bundle porosity on shell-side hydrodynamics and mass transfer under conditions of steady laminar flow were obtained. Fluid was assumed to flow around regular hexagonal or square arrays of cylindrical fibers of different pitch to diameter ratios, yielding bundle porosities ranging from the theoretical minimum up to ∼1. A large number of axial, transverse and mixed flow combinations were simulated by letting the axial and transverse flow Reynolds numbers and the transverse flow attack angle vary. Both fully developed and developing conditions (entrance effects) were considered. The continuity and momentum equations, along with a transport equation for the concentration of a high-Schmidt number solute, were solved by a finite volume CFD code. Fully developed conditions were simulated by the well-established “unit cell” approach, in which the computational domain is two-dimensional and includes a single fiber with the associated fluid, periodic boundary conditions are imposed between all opposite sides and compensative terms are introduced to account for large-scale longitudinal or transversal gradients. Developing flow was studied by using a fully three-dimensional computational domain. Predictions were validated against experimental, computational and analytic literature results. The simulations showed that lattices with different porosities exhibit a qualitatively similar behavior, but differ significantly in important quantities such as the Darcy permeability, the Sherwood number and the hydrodynamic and mass transfer development length.

The reduction of pressure losses in thermally modulated vertical channels

The role played by patterned heating in reducing pressure losses within vertical conduits is investigated. The heating generates flow separation structures which reduce the direct contact between the stream and the sidewalls, thereby limiting the frictional resistance. This also modifies the temperature field thereby inducing a net buoyancy force which may either assist or oppose the pressure gradient required to maintain a fixed flow rate. If the flow Reynolds number is increased sufficiently, the separation structures may be washed away, which means that the pressure-gradient-reducing mechanism is eliminated. The details of the system response are a function of the form of spatial heating distribution, its intensity, the flow Reynolds number and the fluid Prandtl number. Carefully chosen heating of the two walls can induce a pattern interaction effect and a judicious choice of the two patterns can have as much as an order of magnitude effect on the system response.

Computational Fluid Dynamic Simulation of Leakage Acoustic Waves Propagation Model for Gas Pipelines

When leakage occurs for natural gas pipelines, acoustic waves generated at the leakage point will propagate to both ends of the pipe, which will be measured and processed to detect and locate the leakage. When acoustic waves propagate in the gas, the amplitude will attenuate and the waveform will spread, which decides the installation distance of acoustic sensors. Therefore, computational fluid dynamic (CFD) simulation research on the acoustic wave propagation model is accomplished and verified by experiments to provide the foundation for the acoustic leak location method. The propagation model includes two parts: amplitude attenuation model and waveform spreading model. Both can be obtained by the established CFD simulation model. Additionally, the amplitude attenuation model can be verified by the experiments. Then, the simulation method is applied to conclude the propagation model under variable conditions, including different flow directions, Reynolds numbers, and diameters. Finally, the experimental demonstration of the leak location based on the propagation model is given. The results indicate that not only the gas viscosity but also the gas flow can influence the propagation model, and the leak location method based on the propagation model is effective. Conclusions can be drawn that CFD simulation on the propagation model for natural gas pipelines is an efficient way to carry out research and provide the theoretical basis for acoustic leak location method application.

Similarity Analysis of Droplet Evaporation Trajectory in High-Temperature Gas Flow

In this study, a similarity analysis of droplet evaporation trajectory in high-temperature air crossflow based on numerical calculations was conducted. The procedure used for modeling the evaporation of a water droplet involved the use of a low boiling point liquid droplet. The trajectories of water and acetone droplets over an extensive temperature range of 695–1194 K were investigated, and the relationships for the corresponding parameters of the two different droplets, as well as the scale effect, were discussed. The results identify the existence of a comparative trajectory for different kinds of droplets, as well as the main dimensionless numbers governing the trajectory, by determining the Reynolds, Stokes, and Nusselt numbers. For each given gas flow Reynolds number, the droplet evaporation trajectory has a unique correspondence with the airflow temperature. Given this, a method of calculation to determine the modeling parameters of the droplet and crossflow was proposed. The applicability of the similarity modeling method of a single droplet to the spray droplets was examined, from which the spray structure in high-temperature crossflow was obtained.

가스터빈의 연소기 냉각을 위한 이중벽 냉각 구조에서 유출 구멍의 복합각도 변경에 따른 냉각 성능 변화에 대한 수치해석적 연구

For the cooling of the combustor liner, this investigation considers a double-wall cooling structure in which the compound angle and orientation angle of the effusion hole are 30° and ±30°. respectively. Also, to analyze the effect of orientation angle, data for comparison in a simple angle configuration with an orientation angle of 0° is provided. In this study, the heat transfer and flow characteristics of the double wall cooling structure by the change in orientation angle were numerically analyzed. The results of the numerical simulations was obtained using ANSYS FLUENT V 21.1 numerical codes with a shear stress transfer k-ω turbulence model. Data are provided for main flow Reynolds numbers from 142,000 to 149,000. and overall blowing ratios from 5.5 to 7.4. The use of compound and orientation angle arrangements inhibits the formation of horseshoe-shaped vortices,BRresulting in asymmetry of the counter rotating vortex. As a result, the lift-off of the coolant injected from the effusion holes is suppressed from the effusion cooling plate. When orientation angle is changed from 0° to ± 30°, the area-averaged film cooling effectiveness and area-averaged heat transfer coefficient increase by 7.7% and 12.4% overall, respectively. The skewed and non-symmetric shape of the horseshoe-shaped vortex is also increased by the use of orientation angle arrangement, which also provide localized improvements in surface thermal protection.

Large eddy simulations of turbulent heat transfer in packed bed energy storage systems

The present paper aims to study the effect of partial blocking and flow regime on the mutual turbulent interplay between porous and non-porous regions in packed bed energy storage systems (PBESSs). To this end, high-fidelity pore-scale large eddy simulations (LES) are conducted for two PBESS configurations, namely full blockage and partial blockage under the discharge process at three Re numbers 3600, 7200, and 14,400. The influences of the flow major features, including flow channelling and leakage, on the rate of heat transfer (Nusselt number) and pressure drop are investigated for various flow Reynolds (Re) numbers. Results demonstrate that the channelling effect inside the porous region strongly affects the temperature profiles and leads to local maximum peaks of Nusselt (Nu) number on the upper and lower sides of pore elements. For the partial blockage, it is observed that 79 % of the flow entering the porous block leaks from the porous region into the non-porous region through the porous-fluid interface at Re = 3600, which reduces by 26 % as the Re increases to 14,400. The flow leakage leads to the formation of counter-rotating vortex pair structures inside and over the porous block. It also causes local maximum peaks of Nu number at the lower sides of pore elements and changes the stagnation points' position at the leading edge of the porous block near the porous-fluid interface. Compared to the full blockage configuration, temperature profiles inside the porous block are less dependent on the Re number for the partial blockage case. Finally, the pressure drag force for the full blockage is about 21.4 and 30.9 times that of partial blockage at Re = 3600 and 14,400, respectively. Whereas at these Re numbers, the average Nu number for the full blockage is nearly 51.2 % and 57.3 % higher than that of the partial blockage. Consequently, blocking the entire fluid flow area may not necessarily be the best design, since it may result in excessive pressure drops without significant heat transfer enhancement.

Experimental and numerical investigation on heat transfer and pressure drop of SiO2 and Al2O3 oil-based nanofluid characteristics through the different helical tubes under constant heat fluxes

In the present paper, a numerical and experimental study has been performed to investigate the heat transfer and pressure drop properties of Al2O3 and SiO2/Base oil nanofluid flow in a helical tube under constant heat fluxes. The nanofluids were prepared by using SN-300 base oil as a working fluid for 0.05%, 0.1%, 0.3%, and 0.5% mass concentrations. The experiments were carried out with two constant heat fluxes of 3950 W/m2 and 5280 W/m2 at 30, 40, 50, and 60 °C. The impact of various parameters, including flow Reynolds number, fluid temperature, two different nanoparticles, and various weight concentrations, on the heat transfer factor and pressure drop of the flow, are investigated. To examine the impact of different geometries on the heat transfer rate, simulations were conducted over four different helical tubes by changing the pitch circle diameter and steps. The results indicated that the heat transfer factor and pressure drop were increased by utilizing nanofluid instead of the base fluid. In addition, in the same mass concentration, the Al2O3 nanofluid increased the heat transfer coefficient more than the SiO2 nanofluid. The maximum heat transfer rate corresponds to 0.5% mass concentrations of Al2O3 and SiO2 nanofluids, which are 41.4% and 27.3% more than base oil, respectively. The helical tube compared to the straight tube increased heat transfer by 19.5%. Moreover, heat transfer was improved by 6% and 16.5%, respectively, by reducing helical pitch and pitch circle diameter.

Comparisons of cooling performance and flow characteristics of a combustor liner plate with compound angle and simple angle effusion holes

The present investigation considers a unique compound angle arrangement, never previously considered, with α = 30°, β = ±30°, such that the compound angle of the effusion holes changes from β = +30° to β = −30°, as adjacent rows of effusion holes are encountered. Also provided are data for comparison from a simple angle configuration with α = 30°, β = 0°. Coolant is supplied to both effusion hole arrays using arrays of impingement cooling jets. The results of the numerical simulations are obtained using the ANSYS FLUENT V 21.1 numerical code, with a shear stress transport (SST) k–ω turbulence model. Data are provided for main flow Reynolds numbers from 142,000 to 155,000, impingement flow Reynolds numbers from 7900 to 18,000, effusion flow Reynolds numbers from 10,400 to 23,600, and overall blowing ratios from 3.3 to 7.4. When β is changed from 0° to ±30°, area-averaged film cooling effectiveness values and area-averaged heat transfer coefficient magnitudes increase overall by approximately 2.89% and 11.7%, respectively. This is because the use of compound angle arrangements often inhibits the formation of the horseshoe-shaped vortex, and the resulting counter-rotating vortex legs, that often promote lift off of significant film concentrations from the test surface. The skewed and non-symmetric character of the horseshoe-shaped vortex is also increased by the use of compound angle arrangement, which also sometimes provides local improvements in surface thermal protection.

Numerical Analysis of the Pulsating Heat Transfer of Ferrofluid in Helically Fluted Tubes

The flow and heat transfer characteristics in the corrugated tube mainly depend on flow parameters and geometric configurations of the tube, such as corrugated cross-section, corrugated angle, corrugated arrangement, corrugated pitch and depth, pulsating flow frequency, coolant types, and Reynolds number. This paper presents the pulsating thermal characteristics of ferrofluid flowing in the fluted tube using the de-ionized water and 0.015% by volume as working fluid flowing in the test section. Eulerian two-phase turbulence model validation has been performed in both steady and pulsating flow. The finite volume approach discretizes the Eulerian two-phase model. Numerical results indicate that the longitudinal and transverse secondary flows are induced differently via the pulsating flow. The heat transfer characteristics of pulsating fluid flow are significantly larger than that of continuous fluid flow. A higher pulsating frequency induces a heat transfer enhancement. However, pulsating flow increases pressure due to more flow complexity and rough augmentation. Due to the disturbed fluid flow and higher swirling motion, the heat transfer augmentation increases and corresponds with the published results.

Flow and Performance Characteristics of S- Shaped Diffuser

This paper presents the effect of turning angle on the first and second curvature of S-shaped diffusers, one with a square inlet and rectangular outlet having a centre length of 800 mm and an area ratio of 2.5. A computer program based on the finite volume technique, using the Standard k–ε turbulence model, has been adopted and modified to predict the flow. The measurements of pressure distributions of the airflow in S-shaped diffusers are performed for twelve models of different turn angles such as 30º/30º, 60º/ 60º, 90º/ 90º, 120º/ 120º, 150º/ 150º, 180º/ 180º, 60º/90º, 90º/ 60º, 90º/ 120º, 120º/ 90º, 120º/ 150º, 150º/ 120º. In this paper, results are shown in the form of pressure recovery coefficient and pressure recovery efficiency. Furthermore, pressure and velocity contours have been developed for the entire study of all twelve designs of the diffuser. The results showed that the diffuser pressure recovery coefficient is strongly affected by the geometrical parameters of the S-shaped diffuser and inflow Reynolds number. The turning angle plays an important role in improving the S-shaped diffuser's performance. In this study it is concluded that diffuser 90º/ 120º performs best, it gives the highest pressure recovery coefficient and efficiency, whereas diffuser 30º/30º gives the lowest performance parameters.

Space-filling single square and square fractal grids induced turbulence: Reynolds stress model parameters-optimization

The employment of grid-induced turbulent flow structures as a passive means of augmenting solid-fluid heat transfer is receiving considerable attention, especially in HVAC applications. However, there presently exists a gap in simulation approaches, where industry-accessible CFD packages are unable to accurately express crucial second-order turbulent flow statistics generated by space-filling single square grids (SSGs) and square fractal grids (SFGs). The present study reports a successful application of a revised Reynolds Stress Model (RSM), which accurately replicates the streamwise distributions of centerline mean flow velocity and turbulence intensity in the lee of one SSG (operating under three different flow Reynolds number ReDh) and five geometrically different SFG test cases after undergoing Nelder-Mead downhill simplex optimization of key RSM kernel parameters. The optimized RSM presents a disagreement of, at worst, 4.30% and 9.98%, respectively against experimental hot-wire anemometry measurements of first-order and second-order statistics, and is the first known instance of the RSM being validated for turbulence intensity predictions of SSG- and SFG-induced turbulence. Examination of RSM parameters reveals that the pre-factors for the rates of turbulence dissipation production and destruction (C1,ε and C2,ε) hold greatest effect on simulation accuracy, with additional optimization of the turbulent viscosity pre-factor (Cμ) required for SFG cases. This is attributed to the effect of enhanced turbulent transport due to the cascading and multiscale nature of SFG turbulence, which insofar could not be replicated by Reynolds-Averaged Navier-Stokes (RANS) models. The values of optimized C1,ε, C2,ε, and Cμ range between 1.057 to 1.697, 2.226 to 2.556, and 0.17 to 2.27, respectively. This leads to the largest deviation of −26.6%, 33.1%, and 200% for C1,ε, C2,ε, and Cμ, respectively, when compared to their corresponding default values. With regards to the sensitivity of this parameter set on grid design, it is shown that the grid's fractal iteration number N and thickness ratio tr have greatest influence on the variation of C1,ε, C2,ε, and Cμ, while the effect of ReDh is insignificant. Overall, this study presents an alternative approach to capture the anisotropic and inhomogeneous nature of SSG- and SFG-induced turbulence for industrial heat-transfer applications via an accessible RANS package, which was previously constrained to expensive DNS and LES studies.

Investigation of an electrolysis system with boron-doped diamond anode and gas diffusion cathode to remove water micropollutants.

Using electrolysis systems to degrade organics in wastewater encourages this technique to remove micropollutants (MPs) in different types of water. In this work, a cell consisting of an anode as a boron-doped diamond (BDD) electrode combined with a gas diffusion (GDE) cathode without a separator showed that MPs degradation can be effectively achieved. Investigating different operating parameters, it was stated that applying a low current density (2 mA/cm2) and setting the Reynolds number of the electrolyte flow through the cell at the laminar range raised the treatment time by 3-fold at the same energy demand. This arrangement increased the MPs removal. Some substances like diclofenac were removed up to 84% at a longer treatment time of 180 min coupled with an increase in energy demand. The results at the mentioned parameters indicated an adequate generation rate of radicals needed to remove MPs and the oxidation reactions were promoted. The results show high potential to the investigated electrolysis system in removing MPs in wastewater under considering the need for further reduction of the energy demand.

Two-phase simulation of entropy generation and thermo-hydraulic behavior of a therminol/CuO-diamond nanofluid in a heat exchanger

Many researchers have optimized the thermo-hydraulic performance of heat exchangers (HE) used in industries. Using passive methods, such as nanofluid, turbulator, and fins can be beneficial in the field. In this paper, two-phase entropy generation and thermo-hydraulic behavior of hybrid nanofluid CuO-diamond/Therminol hybrid nanofluid in a HE equipped with high fins, micro fins, and turbulator are numerically examined. The volume fraction of nanoparticles changes from 0 to 4% and the flow Reynolds numbers (Re) are 14,000, 18,000, 22,000, and 26,000. Two-phase Eulerian-mixture method, k-ω turbulence model, finite volume method, and SIMPLEC algorithm are employed for the simulations. It is concluded that in terms of pressure drop, the use of tubes with micro fins is more favorable. The maximum values of the average Nusselt number (Nu) correspond to the heat exchanger equipped with high fins. The largest change in the average Nusselt number at all Reynolds numbers and volume fractions investigated is 32.67%, which occurs at a Reynolds number of 22,000 and a volume fraction of 4%. From the point of view of the total entropy generation in the presence of a hybrid nanofluid, it is more desirable to use a heat exchanger with micro fins.

Impact of Wall Temperature on Aerothermal Characteristics of an Array of Surface Microstructures

Abstract The aero-thermal behavior of surface microstructures is of wide relevance, especially given the development of additive manufacturing (AM). Of particular interest is the interaction between fluid flow and heat transfer. In this work, two contrasting configurations, a flat plate boundary layer and an array of hemispheric microstructures are examined at three wall-inflow temperature ratios (TR): cooled (TR = 0.5), adiabatic (TR = 1) and heated wall (TR = 1.5). Due to compensation between fluid viscosity and velocity gradient in the boundary layer, the heat transfer effects may appear deceptively small if judged using the common aerothermal parameters (Cf, Nu). The authors find instead the local Reynolds number to be more usefully indicative of such aerothermal interaction. The scale-resolving large eddy simulations (LES) simulations at a range of Reynolds numbers show that the cooled wall case is characterized by a markedly earlier transition which takes place at a much lower (by 50%) bulk flow Reynolds number compared to a near-adiabatic case. Furthermore, it is shown that the incompressible flow LES solutions fail to capture the early transition under the same cooling condition. Finally, a regrouping of the nondimensional parameters (CD, Nu) with TR is proposed leading to a more unified characterization for easier scaling of wall heat transfer effects in practical applications.

A scale sensitive filtered sub-grid drag model for fluidized gas-particle flows

A new sub-grid drag model is proposed for fluidized gas-particle flows which stands among those derived from filtering over results of meso-scale highly resolved simulations with microscopic two-fluid modeling. Filtered sub-grid models frequently correlate to meso-scale effects alone, taking no account of any micro-scale or macro-scale effects, a clear drawback in view of the lack of scale separation characteristic of gas-particle fluidized flows. The currently proposed sub-grid drag model accounts for the micro-scale effect through a range of particle Froude numbers, while the macro-scale effect is accounted for in average, by considering ranges of average solid volume fractions and gas flow Reynolds numbers. For the sake of comparing the current model to similar models, we use the expression “accounting” to mean the background of consideration of micro and macro-scale aspects that are accounted for in the models. In this particular sense, the proposed model represents a gain over previous versions.

Heat Transfer and Fluid Dynamics Study in Solar Air Heater Employing Impinging Circular Air Jet Array for Effective Jet Stability

Abstract This experimental and numerical investigation is intended to increase heat transfer, reduce pumping power, and present important guidelines on the use of confined submerged air jet impingement in solar air heater designs that employ an array of circular air jets for high thermohydraulic performance. In order to increase thermohydraulic performance, a novel approach is adopted, i.e., by improving jet stability and consequently jet deflection that gives origination to four solar air heater designs for this investigation. The obtained numerical results revealed that thermohydraulic performance improves significantly by stabilizing the air jet array, which means lowering the jet deflection. The thermohydraulic performance of design-I is high compared to design-II, design-III, and design-IV, both at equal mass flow rate and Reynolds number. Moreover, design-I presents 20%, 33%, and 52% relative improvement in deviation angle of the jet core compared to design-II, design-III, and design-IV, respectively, at a minimum mass flow rate of 0.01 kg/s and Reynolds number of Re=2000, respectively. In addition, correlations to define heat transfer and fluid dynamics characteristics are established. An important guideline upon the use of solar air heater with jet impingement is that a solar air heater of short length and height (jet and plate spacing), and large width should be used for high thermohydraulic performance.