Flow Turbulence Intensity Research Articles

10801 管摩擦に及ぼすマイクロバブルの影響 : 流動特性と管摩擦の関係(GS-03【流体(1)】)

Ejection of microbubble within water has been investigated for several years as a method to achieve pipe friction loss reduction. However, the physical mechanism of this phenomenon is not yet understood. Therefore, the purpose of our study is to understand the reduction mechanism. We researched pipe friction loss reduction by the experiment, which was constructed by a circular pipe. Adherence of bubbles to the pipe wall had an effect on pipe friction loss. In the case of void fraction 0.1% ± 0.02%, microbubble influenced pipe friction loss reduction. Moreover, turbulence intensity calculated using Particle Image Velocimetry (PIV) measurement technic. In the result, turbulence intensity decreased in the case of the water injecting microbubble. As the result, we thought that microbubble controlled turbulence intensity of circular tube flow.

室内気流模型を対象としたシーディング手法の検討及び測定結果

This paper describes particle image velocimetry (PIV) air flow measurements in a room model of scale 1:10; the model has inlets and outlets across its width. The PIV measurements use three types of tracer particles: water mist, oil mist, and metal powder. The air flow distribution and turbulence intensity are measured by PIV with metal powder tracers in four combinations of inlet and outlet positions.The results are as follows;(1) Among the three tracer particles, metal powder yields the best air flow measurements in the 1:10-scale room model.(2) To obtain uniform luminance, an appropriate tracer particle concentration and increased laser output are required.(3) When the inlet and outlet are located midway across the scale model, the air flow velocity in the room in nearly symmetrical. In this case, air enters straight through the inlet, is sucked into the outlet, and is largely discharged. The air not discharged from the outlet collides with the side-wall of the outlet to forms a symmetrical vortex area.

An experimental study on the near flow field characteristics of non-circular jets

Subsonic turbulent free jet, issuing from a lobed contoured nozzle in to quiescent air is investigated experimentally. Results are compared with a cir ular jet from a nozzle of the same exit area and same contraction profile. Mean flow characteristics, turbulence intensities and Reynolds shear stresses in the near field region are investigated by using Hot-wire Anemometry. An overall decrease in turbulence intensities and enhanced flow entrainment in the near field of lobed nozzle are observed.

Numerical analysis of periodic flow unsteadiness in a single-blade centrifugal pump

In this paper, the CFD simulation and new flow unsteadiness analysis for a single-blade centrifugal pump with whole flow passage were carried out. The periodic flow unsteadiness has been quantitatively investigated in detail by defining unsteady intensity and turbulence intensity in both rotor and volute domains under design condition Q=33 L s−1. The results show that the distributions of flow unsteadiness are the functions of impeller rotating angle φ and have complex unsteady characteristics. The obvious Tu fluctuations can be also observed for different impeller positions. In addition, time-averaged unsteady intensity and time-averaged turbulence intensity were calculated by averaging the results of each mesh node for entire impeller revolution period to evaluate the strength distributions of flow unsteadiness directly and comprehensively. The accumulative results of an impeller revolution can directly show the positions and strength of the flow unsteadiness and turbulence intensity in both rotor and stator domains which can be an important aspect to be considered in the single-blade pump optimum design procedure for obtaining more stable inner flow of the pump and decreasing flow-induced vibration and noise. The flow unsteadiness in the side chamber cannot be neglected for an accurate prediction of the inner flow of the pump, and the optimizing design procedure for a single-blade pump impeller will not be accurate using CFD tool if the unsteady flow phenomenon in the side chamber is not considered.

Experimental and numerical investigations of shell-side thermo-hydraulic performances for shell-and-tube heat exchanger with trefoil-hole baffles

Orifice plates have been developed as the support of tubes for shell-and-tube heat exchangers, however, very few related academic literature are available. In the present work, we experimentally investigate the shell-side thermo-hydraulic performance of a shell-and-tube heat exchanger with trefoil-hole baffles (THB-STHXs) under turbulent flow regime. The test data demonstrate that the heat transfer rate is effectively enhanced on the shell side, meanwhile, the flow resistance increases substantially. Moreover, both the heat transfer performance (NuPr−0.333(μf/μw)−0.14) and pressure loss (Δp) rise with the increment of the Reynolds number. Based on the experimental results, correlations of the Nusselt number and pressure loss as a function of the Reynolds number are obtained respectively. To analyze the mechanisms of these thermo-hydraulic characteristics, numerical computation is carried out on the basis of a unit channel, which demonstrates that the trefoil-hole baffle could generate high-speed flush, intensive recirculation flow and high turbulence intensity level. As a result, the heat transfer rate is considerably enhanced and the flow resistance increases substantially as well.

Pressure and fuel effects on turbulent consumption speeds of H2/CO blends

This paper describes measurements and correlations of turbulent consumption speeds, ST,GC, of hydrogen/carbon monoxide (H2/CO) fuel mixtures over a range of conditions. This work is set within the broader context of understanding the sensitivity of the turbulent flame speed to chemical kinetic and diffusive properties of the reactive mixture. Turbulent consumption speed data were obtained at mean flow velocities, turbulence intensities (urms′/SL,0), equivalence ratios and pressures ranging from 4–50m/s, 5–45, 0.5–0.8, and 1–10atm, respectively. Experiments were conducted where the mixture equivalence ratio, ϕ, was adjusted at each fuel composition to have nominally the same calculated un-stretched laminar flame speed, SL,0. Consistent with prior studies, these data show both fuel composition and pressure effects on the turbulent flame speed. For example, measured ST,GC values of the 90/10 H2/CO are about 3 times larger than CH4 blends having the same SL,0, turbulence intensity, and operating conditions. Similarly, the 5atm data have ST,GC values that are consistently about 1.8 times larger than the 1atm data, at identical conditions and fuel compositions. These data are correlated with a scaling law derived from quasi-steady leading points concepts using detailed kinetics calculations of highly stretched flames. For a given pressure, these scalings do an excellent job in scaling data obtained across the H2/CO fuel composition and equivalence ratio range. However, the pressure sensitivities are not captured by this scaling, which may be more fundamentally a reflection of the non-quasi-steady nature of the flame leading points. In support of this argument, we show that the spread in the data can largely be correlated with the ratio of a leading point chemical time scale to a flow time scale.

Experimental and numerical study on heat transfer enhancement in a channel with Z-shaped baffles

The influence of baffle turbulators on heat transfer augmentation in a rectangular channel has been investigated experimentally and numerically. In the experiment, the baffles are placed in a zigzag shape (Z-shaped baffle) aligned in series on the isothermal-fluxed top wall, similar to the absorber plate of a solar air heater channel. The aim at using the Z-baffles is to create co-rotating vortex flows having a significant influence on the flow turbulence intensity leading to higher heat transfer enhancement in the tested channel. Effects of the Z-baffle height and pitch spacing length are examined to find the optimum thermal performance for the Reynolds number from 4400 to 20,400. The Z-baffles inclined to 45° relative to the main flow direction are characterized at three baffle- to channel-height ratios (e/H=0.1, 0.2 and 0.3) and baffle pitch ratios (P/H=1.5, 2 and 3). The experimental results show a significant effect of the presence of the Z-baffle on the heat transfer rate and friction loss over the smooth channel with no baffle. The Nusselt number, friction factor and thermal performance enhancement factor for the in-phase 45° Z-baffles are found to be considerably higher than those for the out-phase 45° Z-baffle at a similar operating condition. The in-phase 45° Z-baffle with larger e/H provides higher heat transfer and friction loss than the one with smaller e/H while the shorter pitch length yields the higher Nu, f and TEF than the larger one. The numerical work is also conducted to investigate the flow friction and heat transfer behaviors in the channel mounted with the 45° Z-baffles, and the numerical results are found in good agreement with experimental data.

Investigation of macro-turbulent flow structures interaction with a vertical hydrokinetic river turbine

Reported are a series of river velocity measurements taken one diameter upstream of a 25 kW vertical axis river kinetic turbine deployed in the Winnipeg River at Pointe du Bois, located 150 km northeast of Winnipeg. A load-bank located on the river shore provides a resistive load for the turbine generator. An acoustic Doppler velocimeter measures velocity fluctuations at a sample rate of 200 Hz. Analysis of acoustics Doppler velocimeter data measured upstream of the turbine quantifies the variation in the mean and fluctuating components of river flow velocity. Results show that in this particular test site the river contains eddies with the same order of magnitude as the turbine diameter or greater. Measurements upstream of the operating turbine indicate reduction in the mean velocity with eddies breaking down into smaller sizes. These smaller eddies increase the turbulence intensity of the incoming flow and increase the high frequencies energy level in the power spectra. ► Velocity measurements upstream of a 25-kW vertical turbine in a river are obtained. ► Solutions for ADV signal quality and signal despiking provided. ► Inflow is non-uniform and contains large eddies in the river. ► Operating turbine increases turbulence intensity and breaks large eddies in upstream. ► Higher turbulence intensity induces higher levels of fatigue damage.

Experimental Study of Flow Fields around a Perforated Breakwater

This study investigates flow fields and energy dissipation due to regular wave interaction with a perforated vertical breakwater, through velocity data measurement in a two-dimensional wave tank. As the waves propagate through the perforated breakwater, the incoming wave energy is reflected back to the ocean, dissipated due to very turbulent flows near the perforations and inside the chamber, and transmitted through the perforations of the breakwater. This transmitted energy is further reduced due to the presence of the perforated back wall. Hence most of the energy is either reflected or dissipated in the vicinity of the structure, and only a small amount of the incoming wave energy is transmitted through the structure. In this study, particle image velocimetry (PIV) technique was employed to measure two-dimensional instantaneous velocity fields in the vicinity of the structure. Measured velocity data was treated statistically, and used to calculate mean flow fields, turbulence intensity and turbulent kinetic energy. For investigation of the flow pattern, time-averaged mean velocity fields were examined, and discussed using the cross-sections through slot and wall for comparison. Flow fields were obtained and compared for various cases with different regular wave conditions. In addition, turbulent kinetic energy was estimated as an approach to understand energy dissipation near the perforated breakwater. The turbulent kinetic energy was distributed against wave height and wave period to see the dependence on wave conditions.

An experimental study on laminar-turbulent transition at high free-stream turbulence in boundary layers with pressure gradients

We report here the results of a study on measurements and prediction of laminar-turbulent transition at high free-stream turbulence in boundary layers of the airfoil-like geometries with presence of the external pressure gradient changeover. The experiments are performed for a number of flow cases with different flow Reynolds number, turbulence intensity and pressure gradient distributions. The results were then compared to numerical calculations for same geometries and flow conditions. The experiments and computations are performed for the flow parameters which are typical for turbomachinery applications and the major idea of the current study is the validation of the turbulence model which can be used for such engineering applications.

Heat Transfer Characteristics of an Array of Impinging Gaseous Slot Jets

A comprehensive experiment was conducted for heat transfer characteristics for an array of impinging gaseous slot jets to a flat plate with strong turbulence (nozzle exit Reynolds number Re=22500~64700).Find that turbulence intensity of flow has an important influence on local forced convective heat transfer coefficient. Meanwhile, the nozzle-to-plate spacing and nozzle exit Reynolds number Re would affect the mean forced convective heat transfer coefficient of the slot jets. And heat transfer efficiency of slot jets has been set to show the relation between ability of the jets and energy consumption of gas supply.

Double-average mean flow and local turbulence intensity profiles from PIV measurements for an open channel flow with rigid vegetation

This contribution presents particle image velocimetry measurements for an open channel stationary uniform and fully developed flow of water over a horizontal flat bed of uniform glass beads in presence of a staggered array of vertical cylindrical stems. The main objective was to explore and quantify the influence of the stems-to-flow relative submergence, h v /h, over the mean flow and local turbulence intensities. A comparison with measurements for the non-vegetated flow over the same granular bed is presented. Results indicate a remarkable influence of h v /h over the whole flow field. The time-average mean flow presents a strong spatial variation in the layer of the flow occupied by the stems. The local velocity fluctuations are strongly affected by the presence of the stems, with regions in between the stems where they reach peaks that are several times larger than those encountered in the flow in absence of vegetation. The turbulence intensity profiles are noticeably different when compared to those measured in the non-vegetated flow conditions. From previous works it was possible to derive an equation for the mean velocity, U v , of the flow through the vegetated layer of height h v . The prediction of this equation is in good agreement with the uniform value for the double-average longitudinal velocity profile in this layer. A final brief discussion about the possible impact of these vegetated-flow features on the sediment transport is presented.

Experimental Investigation of Flow Drag and Turbulence Intensity of a Channel Flow with Rough Walls

Turbulent flow over rough walls is investigated through acoustic doppler velocimeter measurements. Smooth rods with a diameter of 6 mm are used as roughness elements. The rods are arranged at the channel bottom wall in three ways: longitudinally (along the flow direction); transversely (orthogonally to flow direction); and mesh-shaped (in a staggered mesh). The transverse roughness elements produce higher disturbance and flow drag than longitudinal roughness. Both turbulence intensity and flow drag for mesh-shaped roughness are not significantly different from those of transverse roughness, indicating that the transverse roughness elements mainly affect turbulence characteristics. Both turbulence intensity and flow drag are greatest for transverse rough walls at w/k = 7; likewise, both increase with decreasing w/k for longitudinal rough walls. Compared with channel flow over a smooth wall, the turbulence intensity increases considerably, while the flow drag only increases slightly when w/k is small for the three arrangements. This is beneficial for enhancing heat transfer and mixing in channel flows with relatively small flow resistance.

The Investigation of Back-Mixing Particles near Dust Outlet in Cyclone Separator with Tangential Inlet

Based on experimental and computational fluid dynamics analysis, the phenomenon of particle back-mixing near the dust outlet in cyclone separator with tangential inlet was studied. The results show that particle back-mixing appears near the dust outlet geometry. Particle back-mixing can be divided into dust hopper back-mixing and discharge cone back-mixing for different generation mechanism. The upward flow coming from dust hopper, which occupies 17.7% of the inlet gas, can induce dust hopper back-mixing. The particle mass flow rate that caused by dust hopper back-mixing occupies 46.6% of total inlet particle mass flow rate. Precessing vortex core, bias flow and high turbulent intensity near the dust outlet can induce discharge cone back-mixing. For both dust hopper back-mixing and discharge cone back-mixing, particle back-mixing is serious near the dust outlet geometry, which occupies 56.8% of total inlet particle mass flow rate. Particle which is smaller than 18μm can mix backward. The axial distribution of particle concentration decreases sharply in a range of 1.5 D (cyclone diameter) height above the dust discharge port. At last, only 2.6% of back-mixing particles with diameter no bigger than 13μm escape from vortex finder. This effect on separator efficiency increases with the particle diameter decreases.

Numerical Simulation of Gas Flow Pattern in Cyclone Spray Scrubber

The numerical simulation of the fluid flow is presented by CFD technique to characterize the flow pattern of cyclone spray scrubber. In this process, the Reynolds-averaged Navier-Stokes equations with the Reynolds stress turbulence model (RSM) for fluid flow are solved by use of the finite volume method based on the SIMPLE pressure correction algorithm in the fluid computational domain. According to the computational results, the tangential velocity, axial velocity and turbulence intensity of the gas flow are addressed in the different flowrate. The results indicate that the CFD method can effectively reveal the mechanism of gas flow in the cyclone spray scrubber.

Wind tunnel study on wind and turbulence intensity profiles in wind turbine wake

In recent years, there has been a rapid development of the wind farms in Japan. It becomes very important to investigate the wind turbine arrangement in wind farm, in order that the wake of one wind turbine does not to interfere with the flow in other wind turbines. In such a case, in order to achieve the highest possible efficiency from the wind, and to install as many as possible wind turbines within a limited area, it becomes a necessity to study the mutual interference of the wake developed by wind turbines. However, there is no report related to the effect of the turbulence intensity of the external flow on the wake behind a wind turbine generated in the wind tunnel. In this paper, the measurement results of the averaged wind profile and turbulence intensity profile in the wake in the wind tunnel are shown when the turbulence intensity of the external wind was changed. The wind tunnel experiment is performed with 500mm-diameter two-bladed horizontal axis wind turbine and the wind velocity in wake is measured by an I-type hot wire probe. As a result, it is clarified that high turbulence intensities enable to the entrainment of the main flow and the wake and to recover quickly the velocity in the wake.

Measurements and analysis of turbulent consumption speeds of H 2/CO mixtures

This paper describes measurements of global turbulent consumption speeds, S T,GC , of hydrogen/carbon monoxide (H 2/CO) mixtures. The turbulent flame properties of such mixtures are of fundamental interest because of their strong stretch sensitivity, and of practical interest since they are the primary constituents of syngas fuels. Data are reported at mean flow velocities and turbulence intensities of 4 < U 0 < 50 m/s and 1 < u rms ′ / S L , 0 < 100 , respectively, for H 2/CO blends ranging from 30% to 90% H 2 by volume. Two sets of experiments are reported. In the first, fuel blends ranging from 30% to 90% H 2 and mixture equivalence ratio, ϕ, were adjusted at each fuel composition to have nominally the same un-stretched laminar flame speed, S L, 0. In the second set, equivalence ratios were varied at constant H 2 levels. The data clearly corroborate results from other studies that show significant sensitivity of S T,GC to fuel composition. In particular, at a fixed u rms ′ and S L, 0, values of S T,GC increase by a factor of almost 2 when H 2 levels are increased from 30% (at ϕ = 0.61) to 90% (at ϕ = 0.48). Moreover, S T,GC in the 90% H 2 case is three times larger than the ϕ = 0.9 CH 4/air mixture with the same S L, 0 value. An important conclusion from this work is these fuel effects are not simply a low turbulence intensity phenomenon – they clearly persist over the entire range of turbulence intensities used in the measurements. We also describe physics-based correlations of these data, using leading points concepts and detailed kinetic calculations of the stretch sensitivity of these mixtures. These results are used to develop an inequality for negative Markstein length flames that bounds the turbulent flame speed data and show that the data can be collapsed using the maximum stretched laminar flame speed, S L,max , rather than S L, 0.

Study on the Mechanism of the Position Adjustable Rotors for Sediment Deposition Reduction

For to reduce and protect the sediment deposition at downstream river course under the tide gates, the position adjustable rotors for sediment deposition reduction is designed. This paper through the physical experiment study on the flow characteristics by the rotor disturb at the condition of the steady uniform flow in the rectangle open channel. The test shows through fluent actuation the rotor revolving, which will strengthen the flow turbulence intensity. The maximum value of the flow turbulence intensity increased 2∼4 times within 5 times blade length and 0.1<y/H<0.6 depth of the water scope. The influence length scope of the rotor is away from until 16 times blade length. The change rule of the Longitudinal and vertical relative turbulence intensity along the channel is about -1/2 power relationship with the distance away from the rotor.

Influence of primary air ratio on flow and combustion characteristics and NOx emissions of a new swirl coal burner

Abstract Cold airflow experiments on a small-scale burner model, as well as in situ experiments on a centrally fuel-rich swirl coal combustion burner were conducted. Measurements were taken from within a 300 MW e wall-fired pulverized-coal utility boiler installed with eight of centrally fuel-rich swirl coal combustion burners in the bottom row of the furnace during experiments. Various primary air ratios, flow characteristics, gas temperature and gas species concentrations in the burner region were measured. The results of these analyses show that with decreasing primary air ratio, the swirl intensity of air, divergence angles and maximum length and diameter of the central recirculation zone all increased, and the turbulence intensity of the jet flow peaked but decayed quickly. In the burner nozzle region, gas temperature, temperature gradient and CO concentration increased with decreasing primary air ratio, while O 2 and NO x concentration decreased. Different primary air ratios, the gas temperatures and gas species concentrations in the side-wall region varied slightly.

Hydrodynamic instability of hyperconcentrated flows of the Yellow River

Sediment concentrations of Chinese rivers are among the highest worldwide. If the sediment concentration exceeds a certain value, the flow dramatically changes exhibiting properties different from low-concentration sediment-laden flows, both in the movement and the pattern of sediment transportation. The hyperconcentrated flow exhibits distinct phenomena, such as “instability flow”, “clogging river”, or “blast flow”. Such flow phenomena are related to the formation, development and break-up of a hyperconcentrated laminar layer, and also reflect the transition from laminar to turbulent and return to laminar flow. The turbulence intensity of the hyperconcentrated flow is normally weak with turbulence largely damped as the sediment concentration becomes sufficiently high. It was found that the instability of hyperconcentrated flows is not only controlled by the critical Reynolds number, but also related to sediment concentration, distribution of the sediment in water, and the size and density of the sediment particles.