Reynolds Number Region Research Articles

Analysis of heat transfer and flow resistance of twisted oval tube in low Reynolds number flow

Heat transfer enhancement of twisted oval tube is mainly concentrated in the high Reynolds number region. In the present study, the low Reynolds k-ε model is employed to investigate heat transfer and flow characteristics of water flow inside twisted oval tube (TOT) for Reynolds number in the range of 50–2000. Three dimensional numerical study is conducted to study the effects of the geometric parameters on the performance of twisted oval tube for a uniform wall temperature case. The flattening of 1.2, 1.4, 1.63, 1.8 and 2.0, and the twisted pitch ratio of 0.17, 0.25, 0.33 and 0.5. Local distributions of Nusselt number and friction factor are presented. The filed synergy principle is applied to reveal heat transfer enhancement mechanism. The results show that the heat transfer performance of twisted oval tube has been enhanced while having an increasing of pressure drop. One of the key findings of this study is that laminar to turbulent flow transition point was identified and located at Re=500. The fluid is in laminar states with Reynolds number range of 50–250, while the fluid is in turbulent flow when the Reynolds comes to 500–2000. It is also found that the twisted oval tube performs well compared with the smooth tube due to the effect of secondary flow. The maximum enhancements factor PEC of 1.7 is obtained with flattening of 2.0, twisted pitch of 0.33 and Reynolds number of 350. Moreover, PEC of twisted oval tube increased with the increasing of flattening, but decreased with the increasing of twisted pitch ratio. These results would assist people in a comprehensive understanding of heat transfer performance of twisted oval tube in low Reynolds number flow, but also help in the design and development of compact heat exchanger.

Study in Optimum Shape of Wing with Waving Leading Edge In Low Reynolds Number Region

Study in Optimum Shape of Wing with Waving Leading Edge In Low Reynolds Number Region

Influence of flow shedding frequency on convection heat transfer from bank of circular tubes in heat exchangers under cross flow

Numerical study on confined flow over circular tube isolated from the bundle of circular tubes in a compact and widely spaced heat exchangers, to explore the influence of flow shedding frequency under forced convection heat transfer. The flow and heat transfer characteristics such as pressure, frictional and total drag coefficients, skin friction coefficient, Strouhal number, volume goodness factor, convective heat transfer coefficient, Nusselt number and effectiveness are estimated for different dimensionless transverse pitch ratios and Reynolds numbers. The highly confined flow past a circular tube makes the flow steady, highly attached and postponed the flow separation, flow shedding and also advances the laminar to turbulent transition. The increases in flow attachments and transitions have been confirmed by observing the frictional drag and skin friction coefficients and also through various contours respectively. Three different flow shedding nature has been observed when Reynolds number increases at different blockage ratios. It is confirmed that the heat transfer enhancement in compact and widely spaced heat exchanger at blockage ratios less than or equal to 3 does not depend on flow shedding frequency. Also, at low Reynolds number region, the heat transfer enhancement is due to highly attached flow. But the gradual increase in Reynolds number advances laminar to turbulent transition and responsible in heat dissipation.

Design and Analysis of Passive Y-Type Micromixers for Enhanced Mixing Performance for Biomedical and Microreactor Application

In this paper, endeavor has been made to design and analyze different Y-type micromixers by characterizing the mixing and flow behavior in ultra-low Reynolds number region. The effects of different geometric and flow parameters on the mixing and pressure drop are studied through computational fluid dynamics (CFD) simulations using COMSOL Multiphysics software. The parameters investigated are obstacle geometry, obstacle arrangement, obstacle depth, aspect ratio (AR), entrance angle ([Formula: see text], Reynolds number (Re) and obstacle packing factor (OPF). The simulation results reveal that rectangular shaped obstacles in staggered arrangement gives the best mixing. Increasing obstacle depth and OPF increases both mixing and pressure drop whereas increasing entrance angle enhances mixing but has negligible effect on pressure drop. Also both the mixing and pressure drop performance enhances with decreasing AR and lower Reynolds number gives better mixing in the lower laminar flow region.

Development of the loss coefficient correlation for cross flow between graphite fuel blocks in the core of prismatic very high temperature reactor-PMR200

The core of the very high temperature reactor (VHTR) PMR200 (a prismatic modular reactor rated at 200MW of thermal power) consists of hexagonal prismatic fuel blocks and reflector blocks made of graphite. If the core bypass flow ratio increases, the coolant channel flow is decreased and can then lower the heat removal efficiency, resulting in a locally increased fuel block temperature. The coolant channels in the fuel blocks are connected to bypass gaps by the cross gap, complicating flow distribution in the VHTR core. Therefore, reliable estimation of the bypass flow is highly important for the design and safety analysis of the VHTR core. Because of the complexity of the core geometry and gap configuration, it is challenging to predict the flow distribution in the VHTR core. To analyze this flow distribution accurately, it is necessary to determine the cross flow phenomena, and the loss coefficient across the cross gap has to be evaluated to determine the flow distribution in the VHTR core when a lumped parameter code or a flow network analysis code that uses the correlation of the loss coefficient is employed. The purpose of this paper is to develop a loss coefficient correlation applicable to the cross gap in the PMR200 core. The cross flow was evaluated experimentally using the difference between the measured inlet and outlet mass flow rates. Next, the applicability of a commercial computational fluid dynamics (CFD) code, CFX 15, was confirmed by comparing the experimental data and CFD analysis results. To understand the cross flow phenomena, the loss coefficient was evaluated; in the high Reynolds number region, the cross flow loss coefficient on Reynolds number is nearly constant regardless of the Reynolds number, whereas it varies with the gap size in the low Reynolds number region. Finally, the loss coefficient correlation was proposed on the basis of experiments and the CFD analysis. The developed correlation was compared with existing correlations, and the developed one shows better agreement with the experimental results than the existing ones. Hence, the developed correlation will be applied to the flow network code to analyze flow distribution in the PMR200 core.

Characteristics of flow field and pressure fluctuation in complex turbulent flow in the third elbow of a triple elbow piping with small curvature radius in three-dimensional layout

In this study, a flow visualization and pressure measurement were conducted by using an experimental setup including test sections of 1/7-scale models of the cold-leg piping of Japan sodium-cooled reactor with high Reynolds number region up to about one million. Regarding the flow field, flow separation appeared in the intrados of the third elbow. However, the separation region was smaller than that observed in the first elbow in the direction normal to the mean flow and was larger in the streamwise direction. This can be considered because of the swirling flow vgenerated downstream of the second elbow which flowed into the third elbow with a little reduction. From the pressure fluctuation test, it was found that prominent frequencies of the pressure fluctuation appeared at about 0.4 in Strouhal number, which corresponds to a nondimensional frequency, in the region from 0 D to 0.4 D downstream of the elbow outlet, where D is the diameter of the piping. And weak peaks of about 0.7 in Strouhal number were observed in the region far 0.75 D downstream of the outlet. The peak values were about 1/4 of those in the first elbow. This means that the excitation source in the third elbow becomes smaller than that in the first elbow for the fully developed inflow case.

循環制御型高揚力翼の開発

Small unmanned aerial vehicles (SUAV) have recently attracted considerable research and development interest owing to their small size, portability, and limited operational risk. In SUAV, the Reynolds number is typically very low because they are small and fly at low speeds. The Re strongly influences the airfoil performance, and at Re = 105, the lift-drag (L/D) ratio decreases significantly and the airfoil performance deteriorates. Since SUAV must be able to carry payloads such as communication devices and observation equipment, the challenge is to maintain sufficient payload capacity. Therefore we proposed the Circulation-controlled high-lift wing that can maintain a high L/D ratio and increase more payload in the low Reynolds number region (Re = ～105). In this paper, we conducted with the flow visualization test to verify the prevent effect of the peeling of airflow by the smoke-wire method.

Feedback flow control of a low-Re airfoil by flap actuators

The present paper describes the applicability of the active flow control device, mini electromagnetic flap actuators attached on the leading edge of an airfoil, for the flow separation under both the steady and the unsteady flow conditions in the low Reynolds number region. At first, lift and drag have been measured for a wide variety of the wind speed Reynolds numbers and the angles of attack for the steady flow condition. Then, effects of some simple feedback flow controls, where the time-dependent signal of the lift-drag ratio have been used to detect the stall and served as a trigger to start the actuation, have been explored under the unsteady flow condition for evading the stall. In every low Reynolds number ranging from 30 000 to 80 000, the present actuators worked quite well to delay the stall, increasing in the lift and delaying the stall angle of attack. These aerodynamic modifications by the flap actuators obtained from the steady flow were found to be available even if the manipulation of the actuators started after the stall. Activation threshold of the lift-drag ratio as the input for the feedback control was determined from a stall classification map obtained under the steady flow experiment. Effectiveness of this feedback control was then demonstrated under the condition of the wind speed decrease (Reynolds number from 80 000 to 40 000) keeping the angle of attack constant at 11°, at which the stall occurs without the active control. Immediately after the sudden velocity decrease, the decrease in the lift-drag ratio were detected and the dynamic actuations were successfully started, resulting in evading the stall and keeping high and stable lift. An additional operation of the feedback, in which the running actuation is turned off when the lift-drag ratio shows lower than the second threshold value after operation, was revealed to be effective to keep the high lift force under the condition combined with the wind speed increase and decrease within the low Reynolds number range treated in this study.

Friction factor and mean velocity profile for pipe flow at high Reynolds numbers

The friction factor for a fully developed pipe flow is examined at high Reynolds numbers up to ReD = 1.8 × 107 with high accuracy using the high Reynolds number actual flow facility “Hi-Reff” at AIST, NMIJ. The precise measurement of the friction factor is achieved by the highly accurate measurement of the flow rate, and the measurement uncertainty is estimated to be approximately 0.9% with a coverage factor of k = 2. The result examined here is obviously different from the Prandtl equation and the experimental results from the superpipe at Princeton University. The deviation of the present result from the Prandtl equation in the lower Reynolds number region is approximately 2.5% and −3% at the higher Reynolds number. For ReD < 2.0 × 105, the present friction factor obtained here agrees very well with the results at the superpipe, but a deviation is observed for ReD > 2.0 × 105, and it increases with the Reynolds number and reaches −6% at ReD = 1.0 × 107. The Kármán constant estimated by the measured friction factor is 0.385. Using inner scale variables estimated by the present friction factor, the velocity profile measured by laser Doppler velocimetry in the same measurement configuration for the friction factor is normalized in order to observe the consistency of the Kármán constants between both the measurements. The Kármán constant estimated by the measured velocity profiles for ReD > 3.0 × 105 is 0.382.

A burst wave-induced plasma actuator for controlling separated flow over a backward-facing step at low Reynolds numbers

Recently, dielectric barrier discharge plasma actuators have been extensively studied as flow control devices for controlling separated flow. It is known that a periodic disturbance is an effective way to excite a separated shear layer and control the separation in the low Reynolds number region around Re=500. In this study, the fundamental transient behavior of a burst wave-induced plasma actuator (BWPA) was investigated to control separated flow over a backward-facing step at low Reynolds numbers. The time-resolved induced velocity in quiescent air was measured using particle image velocimetry, and we found that the induced velocity exponentially grew and decayed independently of the burst frequency and duty ratio. For the flow over a backward-facing step, the reattachment process was drastically improved by BWPA operation when the oscillation frequency of the induced flow matched the acceptance frequency of the separated shear layer. We show the relations among BWPA operating parameters and the reattaching behavior of the separated flow.

Circulation-controlled high-lift wing for small unmanned aerial vehicle

Small unmanned aerial vehicles (SUAVs) have recently attracted considerable research and development interest owing to their small size, portability, and limited operational risk. In SUAVs, the Reynolds number Re is typically very low because they are small and fly at low speeds. The Reynolds number strongly influences the airfoil performance, and at Re = 105, the lift-drag (L/D) ratio decreases significantly and the airfoil performance deteriorates. Since SUAVs must be able to carry payloads such as communication devices and observation equipment, the challenge is to maintain sufficient payload capacity. We propose a high-lift wing that can maintain a high L/D ratio and increase more payload in the low Reynolds number region (Re = ~105) by small size airfoil (SUAV), thereby expanding the application range of SUAVs. A high L/D ratio is achieved by converting the top and bottom face of the wing into a driven belt, thereby actively adding momentum to the flow around the wing. A test was conducted at a Reynolds number of 1.1 × 105 chord. The maximum L/D ratio increased by a factor of 1.67. A flow visualization test was also conducted at a Reynolds number of 105. Flow attachment was observed up to an angle of attack of 28° with the device; in comparison, the flow separated at an angle of attack of 11° in a non-driven wing.

M413 ダクト内バックステップ下流の局所熱伝達特性 : 低レイノルズ数域における検討(GS5,6 熱工学(5),修士研究発表セッション)

M413 ダクト内バックステップ下流の局所熱伝達特性 : 低レイノルズ数域における検討(GS5,6 熱工学(5),修士研究発表セッション)

Modeling how shark and dolphin skin patterns control transitional wall-turbulence vorticity patterns using spatiotemporal phase reset mechanisms.

Many slow-moving biological systems like seashells and zebrafish that do not contend with wall turbulence have somewhat organized pigmentation patterns flush with their outer surfaces that are formed by underlying autonomous reaction-diffusion (RD) mechanisms. In contrast, sharks and dolphins contend with wall turbulence, are fast swimmers, and have more organized skin patterns that are proud and sometimes vibrate. A nonlinear spatiotemporal analytical model is not available that explains the mechanism underlying control of flow with such proud patterns, despite the fact that shark and dolphin skins are major targets of reverse engineering mechanisms of drag and noise reduction. Comparable to RD, a minimal self-regulation model is given for wall turbulence regeneration in the transitional regime—laterally coupled, diffusively—which, although restricted to pre-breakdown durations and to a plane close and parallel to the wall, correctly reproduces many experimentally observed spatiotemporal organizations of vorticity in both laminar-to-turbulence transitioning and very low Reynolds number but turbulent regions. We further show that the onset of vorticity disorganization is delayed if the skin organization is treated as a spatiotemporal template of olivo-cerebellar phase reset mechanism. The model shows that the adaptation mechanisms of sharks and dolphins to their fluid environment have much in common.

Investigation of the semi-dimple vortex generator applicable to fin-and-tube heat exchangers

The present study examines the air side performance of the fin-and-tube heat exchangers having semi-dimple vortex generator or plain fin geometry. A total of eight samples are made and tested with the corresponding fin pitch (Fp) being 1.6 mm and 2.0 mm and the number of tube row (N) are 1 and 2. The inlet air flow direction is also being tested upon the proposed semi-dimple VG. Test results indicate that the heat transfer performance of the proposed semi-dimple VG with N = 1 at a smaller fin pitch of 1.6 mm is slightly higher than that of plain fin geometry. For N = 1 with a larger fin pitch of 2.0 mm, the semi-dimple VG is about 10% higher than that of plain fin geometry. The difference in heat transfer performance amid VG and plain fin geometry becomes more pronounced with N = 2 and is especially evident when Fp = 2.0 mm due to mixing contribution. In general, the difference between plain and semi-dimple geometry becomes more conspicuous at a larger fin pitch because of the comparatively effectively swirled motion. Both geometries show a dependence on fin pitch at N = 1 but the effect is almost negligible when N is increased to 2. The inlet air flow direction casts negligible influence on the heat transfer performance of semi-dimple VG. However, the friction factors for the opposite air flow operation is lower than that of normal operation, especially in low Reynolds number region.

A Multi-Domain Coupled Numerical Method for a Flat Tube Bank Fin Heat Exchanger with Delta-Winglet Vortex Generators

Along with the improvement of numerical methods and the development of computer power, using a numerical method to design a tube bank fin heat exchanger becomes increasingly possible. In real working conditions, the conjugate heat transfer process arises in this type of exchanger. To provide a feasible numerical model and its relevant method to properly assess the thermal performance of exchanger, in this article, a multi-domain coupled numerical method is presented, in which the conduction in solid regions and the convection in flow regions are calculated together with a fully coupled manner. The reliability of a numerical method was tested through comparing average numerical results with experimental results of a real heat exchanger. The results reveal that the multi-domain coupled numerical method can achieve more accurate positions of the interfaces, arrange the orthogonal grids close to the interfaces, and use more reasonable criteria of convergence. Thus, we have achieved reasonable results that agree with the experimental data of the real heat exchanger under the working condition within 10% deviation in the Reynolds number region studied, and got overall heat transfer coefficient of a flat tube bank fin heat exchanger.

S0520202 マイクロチャネルにおける高分子溶液の不安定流動を用いた混合に対する2液の組み合わせの影響([S052-02]複雑流体の流動現象(2))

In this study, we have investigated the mixing properties in microchannels utilizing unstable flows generated with polymer solutions. Microchannels with abrupt contraction-expansion parts consisted of asymmetric rectangular block gates were tested and mixing procedure in it was visualized with combination of dyed and undyed solutions. Dilute aqueous solutions of polyethylene oxide (PEO) were employed to generate unstable vortexes at the contraction flow parts. The degree of mixing (DM) was estimated from image processing of pictures taken. The DM obtained from combination of PEO-PEO solutions with a same polymer concentration was compared with that obtained from water-PEO solution mixing, where the PEO solution had twice the concentration as PEO-PEO system. Although the maximum DM was obtained at relatively low Reynolds number region in the cases of water-PEO systems, the mixings with PEO-PEO solutions have showed gradually increase in DM with Reynolds number. It is considered that difference in generation and instability of vortexes before the block gates depending on the combinations of the solutions affected to the mixing properties.

The strange flight behaviour of slowly spinning soccer balls

The strange three-dimensional flight behaviour of slowly spinning soccer balls is one of the most interesting and unknown phenomenon associated with the trajectories of sports balls. Many spectators have experienced numerous exciting and emotional instances while observing the curious flight behaviour of these balls. We examine the aerodynamic mechanisms of erratic ball behaviours through real flight observations, unsteady force measurements and flow pattern visualisations. The strange behaviour is elucidated by the relationship between the unsteady forces on the ball and the wake flow. The irregular changes in position for twin longitudinal vortices have already been discovered in the supercritical Reynolds number region of a sphere with a smooth surface. This finding is applicable to the strange behaviour of the flight of soccer balls with this supercritical flow. The players, spectators, and television viewers will gain greater insight into the effects of soccer ball flights.

Skin friction reduction in tubes with hydrophobically structured surfaces

Skin friction drag is dominant in the total drag in a tube flow. In this study pore-type poly (dimethylsiloxane) microstructures are used to reduce the contact area between water and the tube wall. The water is lifted up by the microstructures with a hydrophobic nature so that the water flows on a solid/air composite surface. The solid/air composite surface results in skin friction reduction. The hydrophobicity of the tube wall surface is adjusted by the solid fraction of the surface. Three tube models with different hydrophobicities (contact angle: No Pore = 106°, Pore30 = 131°, Pore110 = 148°) were fabricated and their flow velocities according to the water height were measured. The water flow velocities in a tube with a higher contact angle were higher than those in a tube with a lower contact angle. The relative velocities of a Pore110 tube over a No Pore tube rapidly decrease as the water height increases, and they are saturated over Reynolds number(Re) = 1,400. On the other hand, the relative velocities of a Pore30 tube slowly decrease and they are saturated over Re = 900. A Pore110 tube with a superhydrophobic surface exhibited a high skin friction reduction in the low Reynolds number region (∼ 1,400). In the saturated relative velocity region, the velocities of the Pore110 and Pore30 tubes were on average 3.3% and 2.1% higher than that of the No Pore tube. These results indicate that a tube with a higher contact angle exhibits relatively lower skin friction and delays the velocity saturation point better than a tube with a lower contact angle in the laminar flow region.

Feedback Control of a Flow around a Symmetric Airfoil (Applicability of the Electromagnetic Actuators for the Wind Speed Decrease)

Lift and drag of a symmetric airfoil with miniature electromagnetic actuators have been measured in the low Reynolds number region from 3 × 104 to 1× 105. It was found that the coefficient of the maximum lift increased as the Reynolds number was increased. The actuation made the maximum lift larger than that in the no-actuation case for each Reynolds number, although its effectiveness was the largest at the lowest Reynolds number of 3× 104 and became smaller as the Reynolds number was increased. Even in the case of the wind speed decrease, it was demonstrated that the actuation was effective to avoid the stall. Feedback control using increase/decrease of angle of attack was also effective to maintain the high lift in the case of the wind speed decrease and the additional feedback operation of actuators worked well to suppress the abrupt decrease of the lift.

S052032 プラズマアクチュエータによる複数の分岐・合流部を有する流れの制御([S052-03]プラズマアクチュエータ(3))

An experimental study is performed for multi-passage channel flow with the dielectric barrier discharge plasma actuator(DBD-PA). The multi-passage channel is a reverse flow type, and consists of the five branch passages. The position in which the actuator is installed can be changed in four positions of the outside wall in main channel. The Reynolds number based on the bulk velocity and channel height at the main channel is varied from 600 to 2000. The wave form with voltage of 5 kV and frequency of 2.5 kHz is applied to the electrode of actuator. The wall static pressure is measured, and the pressure loss and flow rate are evaluated. The results show that the pressure loss decreases and the flow rate in each branch passage improves by the effect of induced flow with the plasma actuator. The effect is remarkable in the low Reynolds number region.