Low Reynolds Number Range Research Articles

Experimental Study of Heat Transfer and Pressure Drop of Two-Pass Tube Banks.(Dept.M)

Tube bandles are widely used in heat exchangers for energy carriers and cooling product process, . as a cooler for compressor in gas turbine unit and refrigeration plants. Heat transfer and pressure drop of isolated two-pass tube banks bandles with baffles at different locations are experimentally studied. In the present study, five-heated test tubes are located at inlet of the first p ass. The test. tubes are heated at different rates-(Q). The measurements are taken for the five test tubes. The length of the baffles and its location are taken into account. Also the effect of the tube banks aspect ratio is studied. From the measurements, the local heat transfer coefficient and pressure drop are calculated. All the measurements are conducted at low Reynolds number range, (Re = 1000 to 3500) .

Aerodynamic characterization of bio-mimicked pleated dragonfly aerofoil

The work inspired by the dragonfly wing corrugation positioned at the front wing's radius section lying at 40% of the total wingspan of forewing from the root section. During gliding flight, dragonfly wings presumed to be an ultra-light aerofoil due to its well-defined cross-sectional corrugation. The aerodynamic simulation carried out to understand the aerodynamic performance of a bio-mimetic dragonfly corrugated airfoil at low Reynolds number range of 75000-150000 to explore the potential advantages of pleated airfoils at a varying angle of attack from 0° to 12°. CFD analysis accomplished by using ANSYS Fluent to understand the aerodynamic performance of the pleated aerofoil and its counterpart profiled aerofoil. The simulation result exhibits aerodynamic commission, i.e. the corrugated aerofoil's gliding ratio (CL/CD), is higher than that of a profiled aerofoil. The analysis demonstrates that the pleated corrugated aerofoil produces a comparable higher lift coefficient than that of profiled aerofoil NACA 0015 and reduces the separation bubble. The vortices trapped inside the valleys, which re-energize the flow and delay flow separation leads to an increment in lift coefficient (CL). These investigations amplify the demand of insect-inspired corrugated wing structure and make a possible application for improved design of pretended wings for micro air vehicles.

Effect of Reynolds number on turbulent channel flow over a superhydrophobic surface

The slip boundary and the near-wall statistics of a fully developed turbulent channel flow over a superhydrophobic surface (SHS) was investigated in a low Reynolds number (Re) range. The Re was varied from 6200 to 9400, based on the bulk velocity and the full-channel height. The root-mean-square of the surface roughness, normalized by the inner flow scaling, varied from 0.26 to 0.35 with increasing Re. Time-resolved, two-dimensional particle tracking velocimetry (PTV) was used to obtain the mean velocity profile in the linear viscous sublayer. Furthermore, time-resolved three-dimensional PTV was applied to obtain the Reynolds stresses. The estimated wall shear stress showed that the drag reduction of the SHS increased slightly from 37% to 42% when Re increased. With increasing Re, the slip velocity increased linearly from 0.25 m/s to 0.34 m/s, and the slip length reduced from 97.5 μm to 69.6 µm. When normalized using inner scaling, slip velocity and length remained constant with increasing Re. The mean velocity of the SHS demonstrated a log-law with the universal von Kármán constant but shifted upward by an amount equal to the normalized slip velocity. The SHS increased the dimensional Reynolds stresses in the near-wall region and attenuated them farther away from the wall. With increasing Re, the differences between the dimensional Reynolds stresses of the smooth surface and the SHS increased. However, when Reynolds stresses were normalized using friction velocity, the Reynolds stresses of the SHS overlapped for all the investigated Re and were larger than the normalized Reynolds stresses of the smooth surface.

Large eddy simulation of the destruction of convecting hot fluid pockets through a cold channel flow

Hot fluid pockets or hot spots can be found in many engineering systems, such as chemical reactors, internal heat engines and gas turbines. They are inherently fluid parcels with rapid temperature rising in comparison with the base medium and usually convect with the flow inertia. Due to the higher energy content, hot fluid pockets can change thermal characteristics of the system. Ignoring the destruction of them, which has been mainly missed in the literature, can therefore change the related predictions. The destruction of the hot fluid pockets is so investigated in this paper using a large eddy simulation and some statistical indices are used to reveal the coherence of the pockets. The results show that the convecting hot pocket can be significantly affected by hydrodynamic and thermal conditions, such that it may loss the initial tempo-spacial distribution completely. It is found that the hot spot behavior in lower Reynolds number range is not as regular as that at the higher Reynolds number range. Furthermore, the real thermal boundary conditions of convective heat transfer on the walls can completely change the destruction pattern in comparison to that in the adiabatic combustor. The extent of the destruction is various, depending on the flow conditions. The current results will help better prediction for systems involving hot fluid pockets.

Study of the flow distribution in parallel micro-channels with a triangular manifold

Flow distribution and pressure drop in parallel micro-channels are two effective parameters on the performance of different devices. These two parameters are affected by different factors, such as the manifold geometry, channels geometry, flow rate and fluid direction of inlet flow. In the present work, the structure of the inlet manifold (the triangular geometry, with straight and curved walls) has been studied as the main subject. However, the effect of the flow rate (as the Reynolds number) and fluid direction of inlet flow has been studied on the flow distribution and pressure drop with these manifold geometries. The results show that in the low-Reynolds number range, with increasing the Reynolds number, the flow distribution does not change, but the pressure drop increases. Also, the vertical direction of the inlet flow in comparison with the horizontal direction is preferable. Flow distribution with triangular manifolds with curved walls is more uniform than with straight walls, while, in straight walls, equilateral triangle is a better choice. At the end of this consideration, the effect of geometry parameters (such as the channel number, channel width and depth of curvature) on the non-uniformity parameter was studied with concave manifolds.

Transition to chaos in a cross-corrugated channel at low Reynolds numbers

The pressure-gradient driven flow in a three-dimensional cross-corrugated channel is investigated based on large eddy simulations. The channel geometry is highly tortuous so that the flow unsteadiness can be triggered at a moderate Reynolds number. The objective of this paper is to provide a better understanding of the laminar-to-chaotic transition in this type of channel. The transition from steady to chaos is found to occur at a low Reynolds number range between 77.4 and 113.1. The nonlinear dynamics is analyzed based on the power spectra of the velocity and reconstructed phase space. The route to chaos is identified, which favors the Ruelle-Takens-Newhouse scenario. Moreover, the transition from quasiperiodic mode to chaotic mode is accompanied with temporal intermittencies. The fluid dynamics is analyzed. It is found that the cross-corrugated geometry prompts a recirculating-and-rotating wake behind each contact corner. Each wake is enfolded by a pair of curved free shear layers. They are destabilized by the Kelvin-Helmholtz instability, leading to the periodic flow oscillation. Subsequently, the centrifugal instability sets in and promotes a type of primary vortex structures, forming streamwise “zig-zag” vortex streets. The competition between the adjacent vortex streets leads to a quasiperiodic flow. Temporal intermittencies emerge as the Reynolds number is increased. Finally, the periodicities in both the streamwise and spanwise directions are broken, and the flow becomes chaotic. When further increasing the Reynolds number to 343.1, Taylor-Görtler-like vortexes and necklace-like vortexes are formed in the channel.

Flow field evolution and entrainment in a free surface plunging jet

We investigate ambient fluid entrainment and near-field flow characteristics of a free surface plunging jet for five Reynolds numbers ranging from 3000 to 10000 using time-resolved stereo particle image velocimetry (SPIV). We present time-averaged velocities, RMS velocity fluctuations, mean entrainment and unsteady flow features and compare them with previous studies on free jets. We find that plunging jets have a smaller potential core length, and earlier decay of the mean centerline velocity. The peak RMS velocity fluctuations occur at a location significantly upstream compared to the free jets reported in the literature. Near-field ambient fluid entrainment of plunging jets is measured for the first time and is found to be considerably higher than free jets in the low Reynolds number range. For the plunging jet case at Re = 3000, faster jet decay, higher levels of turbulent intensity in the near-field, and augmented mass entrainment result from strong primary vortices that give the turbulent/non-turbulent interface (TNTI) its convoluted shape which facilitates both bulk entrapment of ambient fluid and small scale nibbling because of larger surface area. These primary vortices occur right below the free surface and disintegrate into secondary structures at axial locations that are upstream compared to those of free jets. At higher Reynolds numbers, primary vortices are smaller in size, weak in swirling strength, and disintegrate prematurely, resulting in suppressed mixing and reduced entrainment efficiency.

Vortex flowmeter with enhanced turndown ratio based on high-frequency pressure sensors and improved convection velocity estimation

A novel vortex flowmeter by using pressure sensors and improved convection velocity estimation was proposed to extend the lower operation range of flowrate. The two sensors were mounted on the positions of 0.2D45° and 1.0D45° according to the signal strength and vortex wavelength criterion. The high-frequency pressure sensors were designed to acquire the undistorted signals of vortex-induced pressure fluctuation and further compute the vortex convection velocity. Aiming at the problem of multiple peaks in cross-correlation calculation, a modified transit time estimation technique combined with a moving-average-filter-based cross correlation function were introduced and verified by the tests. Then, the mean convection coefficient was obtained and the performance of the transit-time-based method was analyzed and compared with that of the frequency-based method. It indicated that the novel vortex flowmeter is robust at low Reynolds number range, which achieves a turndown ratio of 8:1 with an accuracy of ±2% in the Reynolds number range from 1.53×104 to 1.23×105. A remarkable improvement of turndown ratio is achieved compared to the original frequency-based method of 3:1. The proposed system of pressure sensors shows good prospect for the gas metering in small pipes due to the feature of non-invasion and sufficient high frequency response.

Study on mass transfer of ethyl acetate in polymer adsorbent by experimental and theoretical breakthrough curves

Abstract A new polymeric adsorbent with highly hypercrosslinked structure was developed for the removal of VOCs from polluted air. The purpose of this work is to obtain the intraparticle mass transfer coefficient of the adsorbent particles. Adsorption experiments for obtaining breakthrough curves were carried out with a fixed bed system. A dynamic mathematical model for the fixed bed adsorption system was developed. By model fitting, the overall mass transfer coefficient was determined when the deviation error was minimum. Then, the intraparticle mass transfer coefficient of the adsorbent was determined when the external mass transfer resistance was eliminated at higher velocities. Furthermore, a linear relationship of the intraparticle mass transfer coefficient and equilibrium coefficient at lower inlet gas concentrations range was correlated. Moreover, an equation for predicting external mass transfer coefficient at low Reynolds number range at room temperature was obtained.

Effect of variation of cylindrical pin fins height on the overall performance of microchannel heat sink

Numerous researches are performed to take account of the heat transfer in the microchannel heat sink, due to its high efficiency and application in various fields of microelectronics. An attempt has been made to numerically investigate the effects of variation of cylindrical pin fin height, entrenched on bottom wall of microchannel. Three cases of microchannel heat sink are prepared: case 1: plain rectangular channel; case 2 and case 3 with decreasing pin fin height and with increasing pin fin height in the direction of flow, respectively. Also, diameter of pin fin is varied to obtain effects of any underlying flow feature on heat transfer augmentation. The analysis is performed for single-phase fluid deionized ultra-filtered water with temperature-dependent properties, for low Reynolds number range of 150–300. Higher Nusselt number is obtained for case 2, whereas lower pressure drop is obtained in case 3. The overall thermal performance of case 3 with increasing pin fin height outperforms the corresponding case 2 with decreasing pin fin height for the same pin fin diameter, due to the velocity distribution and reduced pressure drop in the downstream region in microchannel, which shows that the downstream region of microchannel heat sink has a significant impact in terms of the overall efficiency while establishing pressure drop as essential characteristics.

Characterization of ducted contra-rotating propeller propulsions

This paper presents the experimental studies of the efficiency of open and ducted contra-rotating propeller systems operating in the low Reynolds number range. Eight off-the-shelf propellers were selected with a diameter in the range from 139 mm to 377 mm and seven ducts were built with the duct length of 0.28–0.53 the propeller diameter. Static and wind tunnel experiments were conducted. The maximum increase in the static thrust coefficient and power loading for the ducted contra-rotating propeller systems over the open systems was found to be 25% and 50%, respectively. This performance improvement for the medium size ducted systems is smaller than that observed in previous studies for ducts longer than the 0.8 propeller diameter but greater than for ducts shorter than the 0.15 propeller diameter. The thrust coefficient decreases with an advance ratio increase. The power loading of both open and ducted systems drops dramatically after reaching maxima.

High-performance airfoil with low reynolds-number dependence on aerodynamic characteristics

In the low Reynolds number range below Re SD and Ishii airfoils are known as high performance low Reynolds number airfoils with a relatively high lift to drag ratios Although the aerodynamic characteristics at particular Reynolds numbers have been thoroughly studied research is lacking on how Reynolds numbers affect the aerodynamic characteristics that become pronounced for general low Reynolds number airfoils This study investigates the Reynolds number dependence on the aerodynamic performance of these two airfoils The results demonstrate that the Ishii airfoil is particularly less dependent on the Reynolds number of the lift curve compared to the SD airfoil Although there is a slight difference in the high angles of attack above the maximum lift coefficient the lift slope hardly changes even when the Reynolds number varies

Dissimilarity between heat and momentum transfer in transitional channel flow

There is a strong similarity between heat and momentum transfer, and thus simultaneous achievement of enhancement of heat transfer and reduction of momentum transfer is a challenging goal for turbulence control. In this study, we perform direct numerical simulation in a low Reynolds number range for transitional channel flow. In the simulations, stripe-like localized structures and spot-like localized turbulence structures are observed, and dissimilarity in which heat transfer dominates momentum transfer is significant for spot-like structures. We explain the mechanism of emergence of this dissimilarity.

The effect of cubic stiffness nonlinearity on the vortex-induced vibration of a circular cylinder at low Reynolds numbers

The vortex-induced vibration (VIV) of a circular cylinder elastically supported by linear and cubic springs is investigated numerically at low Reynolds numbers. The cylinder has a low mass ratio and zero structural damping. Nine dimensionless cubic stiffness nonlinearity strength values are considered. It is found that within the parameter space examined, the VIV response for the linear and softening springs can be divided into four regimes, namely the initial, upper, lower and desynchronised regimes. When the softening spring nonlinearity gets stronger, there exist a reduction in the peak amplitude and shifts in the initial-upper branch and upper-lower branch transitions to lower Reynolds number ranges. In contrast, as the hardening spring nonlinearity increases, the response envelope moves to a higher Reynolds number range and the profile of the initial and upper branches becomes smoother with the lower branch gradually disappearing. In the hardening spring case, the beating response is observed near the low end of the initial branch up to the high end of the initial branch. The modulations in the vibration amplitude gradually diminish with increasing Reynolds number. The cubic spring results coincide with those of the linear spring when they are presented with the equivalent reduced velocity. Due to the low Reynolds number range considered, the majority of the vortex shedding is in the 2S mode. The wake in the cases with smaller vibration amplitudes exhibits a single-row configuration. Whereas, a double-row vortex street is mainly observed in the upper branch. Disorders in the wake are found to be associated with beating responses which have larger vibration amplitudes.

Effects of superhydrophobic surfaces on the flow around an NACA0012 hydrofoil at low Reynolds numbers

In the present study, the effects of superhydrophobic surface on the flow around an NACA0012 hydrofoil are experimentally investigated at low Reynolds number range of 0.2–1.0 $$\times 10^{4}$$ . The velocity fields were measured using two-dimensional digital particle image velocimetry in a water tunnel while varying the angle of attack from $$0^\circ$$ to $$20^\circ$$ . The spray-coating of hydrophobic nanoparticles was used to create superhydrophobic surfaces. Depending on the Reynolds number and angle of attack, we found that the effects of superhydrophobic surface show up differently, which is determined by the relative strength of turbulence caused by both the surface slip (influence of trapped air pockets) and roughness, compared to that of background (i.e., uncontrolled) flow. In general, the superhydrophobic surface imposes a little influence on the wake behind a hydrofoil when the angle of attack is very low (attached flow) or high (fully separated flow). At intermediate angles of attack (flow separates between the leading and trailing edges), however, it is measured that the flow over superhydrophobic surface has a stronger turbulence, and thus, the enhanced shear-layer instability forces the early vortex rollup in the wake and reduction of vortex formation length. Interestingly, there is a transitional range of angle of attack, in which this effect is reversed, and thus, the vortex rollup is slightly delayed. This trend can be explained based on the changes in the uncontrolled flow structures, and finally, we classify the effects of superhydrophobic surface in terms of Reynolds number and angle of attack, in the considered ranges of both.

Direct Simulation of Interstitial Heat Transfer Coefficient Between Paraffin and High Porosity Open-Cell Metal Foam

The interstitial heat transfer coefficient (IHTC) is a key parameter in the two-energy equation model usually employed to investigate the thermal performance of high porosity open-cell metal foam/paraffin composite phase change material. Due to the existence of weak convection of liquid paraffin through metal foam during phase change process, the IHTC should be carefully determined for a low Reynolds number range (Re = 0–1), which however has been rarely addressed in the literature. In this paper, a direct simulation at foam pore scale is carried out to determine the IHTC between paraffin and metal foam at Re = 0–1. For this purpose, the cell structures reflecting realistic metal foams are first constructed based on the three-dimensional (3D) Weaire–Phelan foam cell to serve as the representative elementary volume (REV) of metal foam for direct simulation. Then, by solving the Navier–Stokes equations and energy equation for the REV, the influences of Reynolds number (Re), Prandtl number (Pr), foam porosity (ε), and pore density (PPI) on the dimensionless IHTC, i.e., the Nusselt number Nuv, are investigated. According to the numerical results, a correlation of Nuv at Re = 0–1 is proposed for metal foam/paraffin composite material, which covers both diffusion-dominated interstitial heat transfer region (Re ≤ 0.1) and convection-dominated interstitial heat transfer region (0.1 < Re ≤ 1). Finally, the applicability of this correlation in the two-energy equation model for solid–liquid phase change of paraffin in metal foam is validated by comparing the model predicted melting front with that of experimental observations made in this study. It is found that the IHTC correlation proposed in this study can be used in the two-energy equation model for well predicting the phase change process of paraffin in metal foam.

Numerical analysis of the laminar forced convective heat transfer in coiled tubes with periodic ring-type corrugation

Wall curvature and wall corrugation represent two of the most used passive techniques to enhance convective heat transfer. The effectiveness of wall curvature is due to the fact that it gives origin to a secondary fluid motion orthogonal to the main flow, while wall corrugation is used to disrupt the development of the boundary layers, by enhancing the convective heat transfer mechanism. The compound use of the two techniques has been investigated in literature, mainly experimentally, but further investigation is still needed. In particular, it has been experimentally observed that this compound enhancement technique brings an additional heat transfer augmentation in the majority of applications whereas in the very low Reynolds number range the surface average performances of corrugated coils are lower than the one shown by smooth wall coils.This paper deepened the knowledge on this phenomenon presenting a numerical investigation of the effect induced by a periodic ring-type corrugation on the laminar convective heat transfer in coiled tubes. The study considered the laminar flow in the Reynolds and Dean number range 25-100 and 6-24 respectively. The investigation was particularly focused on the Dean's vortices destruction mechanism, induced by the wall corrugation and on the consequent breakdown of the average Nusselt number.

A heat transfer and friction factor correlation for low air-side Reynolds number applications of compact heat exchangers (1535-RP)

In this article, an experimental investigation of the heat transfer and pressure drop of compact heat exchangers with louvered fins and flat tubes was conducted within a low air-side Reynolds number range of 20< ReLp < 200. Using an existing low-speed wind tunnel, 26 sample heat exchangers of corrugated louver fin type, were tested. New correlations for Colburn factor j and Fanning friction factor f have been developed using eight nondimensional parameters based on the experimental data. Within the investigated parameter ranges, it seems that both the j and f factors are better represented by two correlations in two flow regimes (one for ReLp = 20 – 80 and one for ReLp = 80 – 200) than single regime correlation in the format of power-law. The results support the conclusion that airflow and heat transfer at very low Reynolds numbers behaves differently from that at higher Reynolds numbers.

Turbine vane film cooling: Heat transfer evaluation using high-order discontinuous Galerkin RANS computations

A high-order accurate CFD solver, based on the Discontinuous Galerkin (DG) finite element method, is here employed to compute the heat transfer, with and without film coolant injection, around a turbine vane extensively tested in a wind tunnel. The numerical solution makes also use of a high-order polynomial representation of the airfoil curved boundary in order to minimize the numerical sources of error, leaving possibly only those related to the physical model adopted. The objective of the work is therefore twofold: on the one hand to provide a detailed investigation, often beyond the reach of the experiments, of the complex flow field arising in a film-cooled gas turbine cascade, on the other hand to ascertain the limits of the Reynolds-averaged Navier-Stokes (RANS) approach and its associated turbulence model when using high-order accurate methods. The DG formulation is briefly reviewed, as well as the experimental apparatus and the measuring technique, and then the code is applied to the computation of various test cases characterized by different reference Reynolds and Mach numbers. Two-dimensional results (up to seventh-order accurate) obtained both with the high- and low-Reynolds version of the k-ω model employed are presented. Reasonably good agreement between experimental and numerical results is obtained, even though the outcomes are far from being completely satisfactory especially for flow regimes in the low Reynolds number range. This is due to the lack of suitable modeling of the laminar-turbulent transition process taking place around the blade leading edge. Such a complex phenomenon is out of reach of the modeling capabilities of the high-Re k-ω model, while can be roughly mimicked by the low-Re version of the model, which is able to provide a delayed onset of the turbulence quantities along the blade surface.Third-order accurate computation of the three-dimensional turbine vane are also presented in this work and compared with available measurements to investigate the relevant fluid flow phenomena occurring and to discuss significant issues related to an accurate prediction of the turbine wall heat transfer.

Thrust generation and wake structure for flow across a pitching airfoil at low Reynolds number

In this work, we present detailed particle image velocimetry (PIV) based investigation of wake structure of a pitching airfoil. PIV measurements have been carried out for NACA0015 airfoil at Re = 2900 with reduced frequency range of 1.82–10.92 and pitching angle of 5∘. Two different wake structures (reverse Kármán shedding and deflected vortex shedding) are observed over this parameter range. The vorticity decreases substantially over a distance of two chord-lengths. The velocity profile indicates a jet-like flow downstream of the airfoil. It is shown that the jet-like flow downstream of the airfoil is however not a sufficient condition for the generation of thrust. The vortex strength is found to be invariant of the pitching frequency. Certain differences from the reported results are noted, which may be because of difference in the airfoil shape. These results can help improve understanding of the flow behavior as the low Reynolds number range is not well studied.