Laminar Region Research Articles

Predicting the Pressure Drop of Corrugated Sheet Structured Packings in Deep Vacuum Applications

Advanced corrugated sheet structured packings are considered a natural choice for<br /> deep vacuum distillation. In many of these applications that occur at absolute pressures<br /> below 0.01 bar at the top of the column, the low density gas/vapor driven by pressure<br /> ascends through an irrigated packed bed under laminar flow conditions. This implies that the packing geometry features aiming to reduce the form drag of advanced packing may not be as effective, if at all, as experienced in common applications where turbulent flow prevails. To consider this appropriately, a theoretically founded expression for laminar flow friction factor has been incorporated into Delft model (DM). With this extension, the predicted pressure drop within laminar flow region approaches closely that estimated using well-established empirical model available in software package SULCOL. In absence of adequate experimental evidence, extended DM was validated using newest data obtained at FRI with an advanced wire gauze structured packing in total reflux experiments carried out with paraxylene/orthoxylene system at 0.02 and 0.1 bar top pressure in a column with internal diameter of 1.22 m.

SENCR stabilizes vascular endothelial cell adherens junctions through interaction with CKAP4

SENCR is a human-specific, vascular cell-enriched long-noncoding RNA (lncRNA) that regulates vascular smooth muscle cell and endothelial cell (EC) phenotypes. The underlying mechanisms of action of SENCR in these and other cell types is unknown. Here, levels of SENCR RNA are shown to be elevated in several differentiated human EC lineages subjected to laminar shear stress. Increases in SENCR RNA are also observed in the laminar shear stress region of the adult aorta of humanized SENCR-expressing mice, but not in disturbed shear stress regions. SENCR loss-of-function studies disclose perturbations in EC membrane integrity resulting in increased EC permeability. Biotinylated RNA pull-down and mass spectrometry establish an abundant SENCR-binding protein, cytoskeletal-associated protein 4 (CKAP4); this ribonucleoprotein complex was further confirmed in an RNA immunoprecipitation experiment using an antibody to CKAP4. Structure-function studies demonstrate a noncanonical RNA-binding domain in CKAP4 that binds SENCR Upon SENCR knockdown, increasing levels of CKAP4 protein are detected in the EC surface fraction. Furthermore, an interaction between CKAP4 and CDH5 is enhanced in SENCR-depleted EC. This heightened association appears to destabilize the CDH5/CTNND1 complex and augment CDH5 internalization, resulting in impaired adherens junctions. These findings support SENCR as a flow-responsive lncRNA that promotes EC adherens junction integrity through physical association with CKAP4, thereby stabilizing cell membrane-bound CDH5.

Momentum and heat transfer from an asymmetrically confined rotating cylinder in a power-law fluid

In the present study, the momentum and heat transfer aspects from an isothermally rotating cylinder immersed in a power-law fluid, confined symmetrically and asymmetrically between two parallel walls are considered. The governing equations are solved numerically for the rotational Reynolds number (10−3≤Re≤40), Prandtl number (1≤Pr≤100), power-law index (0.02≤n≤1), blockage ratio (10−3≤β≤0.9999) and asymmetry ratio (10−4≤γ≤1) to elucidate the role of these parameters on the momentum and heat transfer characteristics. The results reported herein pertain to the two dimensional, steady, incompressible power-law fluid in the laminar flow region. The numerical strategy and method used here are validated by comparing the present results with some of the available experimental and numerical studies, showing good agreements. As far as can be ascertained, it is the first systematic numerical study of a rotating cylinder immersed in power-law fluids elucidating the complex interplay between the rheological and kinematic parameters. The flow field is visualized by plotting the streamlines and, hydrodynamic forces and torque acting on the rotating cylinder are computed. The numerical results are supplemented by a lubrication approximation analysis relevant to highly confined cases, i.e., large values of β. In this case, when confinement increases, the asymptotic scaling of the torque is given by ε−1/2 where ε denotes the non-dimensional gap between the cylinder surface and the nearest wall. The heat transfer characteristics are presented in terms of the isotherm contours, and the local and average Nusselt numbers as functions of the aforementioned governing parameters. The rate of heat transfer exhibits a positive dependence on the Reynolds number, Prandtl number, confinement and the degree of asymmetry while the shear-thinning behavior decreases the rate of heat transfer above the corresponding value in Newtonian fluids. Finally, the present results are consolidated by fitting them in the form of a predictive correlation for the average Nusselt number for different blockage ratios studied herein.

Analysis of natural circulation frictional resistance characteristics in rod bundle channel

A considerable amount of research has gone into understanding the thermal-hydraulics characteristics of rod bundle channels which is of great importance for the nuclear industry. This paper will show that the development of the correlation of the frictional resistance coefficient in rod bundle channels under natural circulation conditions is well understood. This paper presents experimental studies of the single-phase resistance and obtains an improved correlation of the frictional resistance coefficient under natural circulation conditions based upon thermal-hydraulics characteristics which are well understood. It is also shown that this newly developed correlation closely predicts data.The experimental results show that the frictional resistance coefficient of a rod bundle channel under natural circulation is related to the physical properties of fluid and the size of the rod bundle channel. According to different flow regions, the relationship between the frictional resistance coefficient and Reynolds number is discussed separately, and the expressions of frictional resistance coefficient with Reynolds number in laminar flow region and transition region are obtained. After considering the influence of fluid properties on the frictional resistance coefficient, this paper introduces the kinematic viscosity correction factor to correct the frictional resistance coefficient. Finally, the complete expression of the frictional resistance coefficient was obtained. This newly developed predictive expression is in good agreement with experimental data in both the laminar and transition flow regions, with a relative error within ±5%. Thus we now have a predictor of the frictional resistance coefficient under natural circulation conditions based upon more complete thermal-hydraulic parameters in a static state.

Turbulence in intermittent transitional boundary layers and in turbulence spots

Direct numerical simulation data of bypass transition in flat-plate boundary layers are analysed to examine the characteristics of turbulence in the transitional regime. When intermittency is 50 % or less, the flow features a juxtaposition of turbulence spots surrounded by streaky laminar regions. Conditionally averaged turbulence statistics are evaluated within the spots, and are compared to standard time averaging in both the transition region and in fully turbulent boundary layers. The turbulent-conditioned root-mean-square levels of the streamwise velocity perturbations are notably elevated in the early transitional boundary layer, while the wall-normal and spanwise components are closer to the levels typical for fully turbulent flow. The analysis is also extended to include ensemble averaging of the spots. When the patches of turbulence are sufficiently large, they develop a core region with similar statistics to fully turbulent boundary layers. Within the tip and the wings of the spots, however, the Reynolds stresses and terms in the turbulence kinetic energy budget are elevated. The enhanced turbulence production in the transition zone, which exceeds the levels from fully turbulent boundary layers, contributes to the higher skin-friction coefficient in that region. Qualitatively, the same observations hold for different spot sizes and levels of free-stream turbulence, except for young spots which do not yet have a core region of developed turbulence.

Numerical investigation of laminar separation in flow across elliptic cylinders

This article presents the numerical studies on separate flow across elliptical cylinders of various axis ratios (AR) in the laminar region. The simulations calculated the laminar separation Reynolds number that marks separation of flow from surface and the critical Reynolds number that represents transition from steady to unsteady flow. The separate angles are expressed as the function of the axis ratio and Re in laminar region. The results are in good agreement with existing data.

Parametric Study of Heat Transfer and Pressure Drop Characteristics of a Rectangular Offset Strip Fin Compact Heat Exchanger

Compact heat exchangers are of great topics of interest while we are dealing with the enhancement of heat transfer rate. In this study, a rectangular offset strip fin compact heat exchanger is taken into consideration. The purpose of this study is to develop a numerical model to study the heat transfer characteristics as well as the pressure drop characteristics. The parametric study was done in FLUENT with a three dimensional computational domain in which air is selected as working fluid. The analysis was confined to the flow in laminar region and so the Reynolds number was limited to 1000. The variation of friction factor and Colburn factor with Reynolds number is also analysed. The results obtained were analysed and were also compared with an existing correlation. The findings of this study may serve as a helping tool to develop a correlation for fluid flow in offset strip fin geometry.

Influence of nano-particles addition on hydrodynamics and heat transfer in laminar flow entrance region inside tube

The hydrodynamics and heat transfer through isothermal tube entrance region was investigated numerically in the presence of Nano-particles. The study was performed by a computer home-code. This code is based on finite volume method. The effects of Reynolds number and Nano-particle concentration on viscous boundary layer thickness, shear stress, coefficient of friction, thermal boundary layer thickness and Nusselt number was presented. Through the study Reynolds number varies from 500 to 2000 and Nano-particle concentration varied from 0 to 10%. Three different Nano-particles Al2O3, TiO2 and Cu were studied. While the Prandtl number for the base fluid was kept constant at 7 referring to water. In general, the results showed that an enhancement in Nusselt number by adding Nano-particles especially by using Cu Nano-particles. The maximum increase in Nusselt number reached 20.5% at Re = 1000, Nano-particle concentration = 10% for copper Nano-particle. Both average coefficient of friction and average Nusselt number were correlated in terms of Reynolds number, Nano-particle concentration, dimensionless entry length and Nano-particle type. Some of the results were compared with published data, a good agreement was found.

Numerical Research on the Vortex Center on the Forward-Swept 3-D Wind Turbine Blades at Low Rotational Speed

This paper studies the CFD simulation of forward three-dimensional (3-D) horizontal axis wind turbine (HAWT) blades. Using logarithmic grid and Q-criterion to learn the vortex dynamics around the blades at low rotational speed. The computational fluid dynamics (CFD) simulation uses Q-criterion to probe vortices and logarithmic grid to emphasize the micro-gridding effect of the turbulent boundary layer. The visualization & measurement of the simulation results give the coefficient of pressure (Cp). For forward 3-D wind turbine blade, at low rotational speed, the strongly accelerated laminar region surrounds the lower blade, and the decelerated tip blade region coalesce each other give rise to a reverse limiting streamline, eroding the laminar region further until a little is left on the tip of the blade. The "reverse limiting streamline" grows inward radially, the area is narrowing closing to the leading edge of the blade tip. The second side of the rolled-up vortex appears the velocity ratio (Uc/Ulocal) of the second vortices are higher than the main vortex cores. For radius R=1.547 m, U=12 m/s, at 210 RPM, CL and CD values reach a maximum with fully laminar tip conditions. While at 120 RPM, the CL and CD values reach a minimum in the absence of laminar tips. The results show the detailed vortex dynamic pattern surround the blades, give more understanding to design laminar 3-D blade toward a noiseless wind turbine system.

Instability wave-streak interactions in a hypersonic boundary layer at flight conditions.

The interaction of stationary streaks undergoing nonmodal growth with modally unstable instability waves in a hypersonic boundary-layer flow is studied using numerical computations. The geometry and flow conditions are selected to match a relevant trajectory location from the ascent phase of the HIFiRE-1 flight experiment; namely, a 7 degree half-angle, circular cone with 2.5 mm nose radius, freestream Mach number equal to 5.30, unit Reynolds number equal to 13.42 m-1, and wall-to-adiabatic temperature ratio of approximately 0.35 over most of the vehicle. This paper investigates the nonlinear evolution of initially linear optimal disturbances that evolve into finite-amplitude streaks, followed by an analysis of the modal instability characteristics of the perturbed, streaky boundary-layer flow. The investigation is performed with stationary direct numerical simulations (DNS) and plane-marching parabolized stability equations (PSE), in conjunction with partial-differential-equation-based planar eigenvalue analysis. The overall effect of streaks is to reduce the peak amplification factors of instability waves, indicating a possible downstream shift in the onset of laminar-turbulent transition. The present study confirms previous findings that the mean flow distorsion of the nonlinear streak perturbation reduces the amplification rates of the Mack-mode instability. More importantly, however, the present results demonstrate that the spanwise varying component of the streak can produce a larger effect on the Mack-mode amplification. The study with selected azimuthal wavenumbers for the stationary streaks reveals that a wavenumber of approximately 1.4 times larger than the optimal wavenumber is more effective in stabilizing the planar Mack-mode instabilities. In the absence of unstable first-mode waves for the present cold-wall condition, transition onset is expected to be delayed until the peak streak amplitude increases to nearly 35 percent of the freestream velocity, when intrinsic instabilities of the boundary-layer streaks begin to dominate the transition process. For streak amplitudes below that limit a significant net stabilization is achieved, yielding a potential transition delay that can exceed 100 percent of the length of the laminar region in the uncontrolled case.

Experimental investigation of heat transfer and pressure drop in a mini-channel shell and tube heat exchanger

This study experimentally investigated heat transfer and pressure drop on tube side of a mini-channel shell and tube heat exchanger (MC-STHE) prototype designed and manufactured using Kern method. Mini-channel copper tubes with an inner diameter of 2 mm and a length of 240 mm were used on the tube side of the MC-STHE. Baffles with a 25% baffle cut and a shell with an inner diameter of 30 mm were used on the shell side. The Reynolds number on the tube side was changed from 1000 to 10,000 while that on the shell side was kept constant at 680. The experimental convective heat transfer coefficients and friction factors for the tube side were compared with correlations for macro-pipes commonly used in the literature. The experimental convective heat transfer coefficients were in good agreement with the Shah and Sieder-Tate correlations in the laminar region and in good agreement with the Petukhov, Hausen and Gnielinski correlations in the transition region while the experimental friction factors showed a similar tendency to the correlation modified by Shah. In addition, the Nusselt number and friction factor correlations have been proposed for use in designing similar MC-STHEs. The optimal operating range of Reynolds number for MC-STHEs is 4000 < Re <5000 based on the Colburn factor while it is 4000 < Re <6000 based on the flow area goodness factor.

Influences of Flow Attack Angles and Flow Directions on Heat Transfer Rate, Pressure Loss, and Thermal Performance in Heat Exchanger Tube with V-Wavy Surface

Numerical investigations on flow and heat transfer characteristics in the heat exchanger tube with the V-wavy surface are presented. The finite volume method with the SIMPLE algorithm is selected to solve the present problem. The effects of flow attack angles (α = 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, and 60°) and flow directions (V-tip pointing downstream known as “V-Downstream” and V-tip pointing upstream known as “V-Upstream”) for the V-wavy surface on flow and heat transfer patterns are considered for both laminar and turbulent regions. The laminar regime is studied in the range Re = 100–1200, while the turbulent region is investigated in the range Re = 3000–10,000. The mechanisms on flow and heat transfer in the test section are reported. The numerical results reveal that the V-wavy surface changes the flow structure in the test section. The vortex flow is produced by the V-wavy surface. The vortex flow disturbs the thermal boundary layer on the heat transfer surface that is the reason for heat transfer and thermal performance enhancements. The optimum flow attack angles of the V-wavy surface for laminar and turbulent regimes are concluded.

Neural-network-based error reduction in calibrating utility ultrasonic flow meters

In this paper, a Multi-Layer Perceptron Neural Network (MLPNN) based calibration is proposed for a utility ultrasonic flow meter. The measured flow range is from 0.2 to 4 m3 per hour. Due to the presence of transient flow in a significant portion of the flow range, as well as very low flow velocity in the laminar flow region, nonlinear effects appear that cause a variable bias error with fluid flow changes. Therefore, in order to reduce the bias error and increase the accuracy of the flow meter measurement, the use of neural network is considered in the flow meter calibration due to its nonlinear mapping ability. Neural network training is done based on input and target data. The sing-around method is used to measure signal time of flight. In this study, the goal is to achieve an error of less than 1.5%. To evaluate the suggested method, two-point calibration and bracketing method were also used and the results were compared. Since calibration equations is a mapping between the flow meter and prover or master meter reading, the objective is to reach a method that is able to predict or map the flow rate with a smaller error. Although the two-point calibration method resulted in good statistical agreement between the flow computer output and base values at data collection points, it failed in attaining the above objective, but the bracketing technique worked with a prediction error slightly better than 1.5%. On the other hand, using a MLPNN produced a maximum prediction error of 1.21%, and thus, the best output. This shows that the use of Artificial Intelligence as an efficient calibration tool is worth further exploration.

Numerical investigation into power extraction by a fully passive oscillating foil with double generators

Abstract A new concept of energy conversion system based on fully passive oscillating foil with double generators to extract energy from pitching and swing motions respectively is proposed and numerically tested. The shedding vortices in the wake drive a C-shape foil to pitch around a pivoting axis, and the resulting periodic lift force drives the foil to swing around an arm. Two-dimensional incompressible Navier-Stokes simulations under Re from 1100 to 11000 are carried out to study the fluid-foil interaction. The effects of pitching damping factor, swing damping factor and incoming flow speed are systematically studied. Numerical results demonstrate that the use of pitching generator not only provides extra pitching power, but also improves the swing power. It is also found that the maximum power coefficient and efficiency increase with flow speed slightly in the laminar flow region, but the influence of Re is very limited. The analysis of the correlation between pitching and swing motions shows that the energy conversion system possesses the possibility of connecting pitching motion with swing motion and further simplifying the system.

An analytical technique for the optimal designs of tube-in-tank thermal energy storage systems using PCM

Latent heat thermal energy storage (LHTES) using phase change material (PCM) has many application potentials, in view of the fact that it has higher thermal energy storage capacity than the sensible heat thermal energy storage such as stratified water storage (SWS). However, the stored thermal energy in an LHTES system is not all effective in discharging due to the additional heat transfer resistance between PCM and heat transfer fluid (HTF). An LHTES system could have a higher effective storage capacity than an SWS system only if it is well designed. The optimal design of an LHTES system could be realized only after the effective energy storage ratio, Est, is analyzed in the design stage. In comparison to the laborious computational analysis method, in this work, we developed an analytical technique from the effectiveness-NTU theory to calculate the required heat transfer length and predict Est of the basic unit in any tube-in-tank LHTES system. The technique was first compared with the numerical simulation method we reported earlier, and then used to find the optimal design under various constraints. It was shown that heat transfer enhancement in PCM could effectively improve the effective storage capacity of an optimal design. However, the maximum Est was limited by the thermal resistance in HTF when the enhancement in PCM was over a certain threshold. It was demonstrated that the use of HTF in a low-Re turbulent region could enhance heat transfer in HTF and achieve an Est as high as that in a laminar region while obtaining a higher discharging rate if heat transfer in PCM was well enhanced. This analysis provided quantitative guidelines on designing optimal tube-in-tank LHTES systems.

The Effect of Rheology Model of an Activated Sludge on to the Predicted Losses by an Elbow

This study presents a numerical investigation of flow and rheological behaviour of activated sludge. These materials are usually driven by pumps in wastewater treatment plants. Because of the correct sizing of the pipeline systems, which is of great importance from the point of view of efficiency, the friction losses and loss coefficients of the components have to be known. These are well-known in the case of Newtonian fluids but few data are available if the rheological properties are non-Newtonian. Three non-Newtonian models (Ostwald, Bingham, Herschel-Bulkley) are investigated related to the friction factor of a straight pipe, the loss coefficients of an elbow and to the pressure drop on this element. For our study the rheological data were used from the literature, where the same sample origin was diluted or concentrated to achieve three different TSS (total suspended solids) contents for the same sludge (7.4 g/l; 6.2 g/l; 3.6 g/l). Moreover, modified Reynolds-number definitions are tested related to the non-Newtonian models in the case of the laminar, transition and turbulent regions.

Safety profile characterization of a cosmetic silanol compound in vivo

The heat transfer process to a non-Newtonian pseudoplastic fluid inside a scraped surface heat exchanger has been experimentally analysed. The scraping device consists of a rod with semicircular pieces mounted on it, which are in contact with the inner surface of the pipe. The whole moves axially and thus, the pieces scrape the inner surface of the pipe, in order to avoid fouling formation and enhance heat transfer.Pressure drop, heat transfer and power consumption measurements, using non-Newtonian pseudoplastic fluids, have been carried out in static and dynamic conditions of the scraper. Four flow regions have been identified: attached laminar, detached laminar, transitional and turbulent flow regions. A generalization method for the fluid viscosity that includes the effects of the non-Newtonian behaviour has been used. Friction factor and Nusselt number have been successfully correlated by employing the generalized viscosity on pressure drop and heat transfer results, both in static and dynamic conditions, for three of the four identified flow regions (it was not possible to get correlations in the transition to turbulent flow region). The study shows that, despite the high power consumption of the scraping movement, the device is suitable for an industrial process using non-Newtonian fluids, since it prevents fouling, increases heat transfer, provides flexibility and enhances the final product quality. Furthermore, the obtained correlations are a valuable tool in the design of heat exchangers.

Bio-inspired self-agitator for convective heat transfer enhancement

Convective heat transfer plays an important role in both the fundamental research and the development of high-performance heat exchangers. Inspired by blades of grass vibrating in the wind, we developed a self-agitator for convective heat transfer enhancement. Because of fluid-structure interactions, the agitator, with self-sustained vibration, can generate strong vortices to significantly break the thermal boundary layer and improve fluid mixing for enhanced convective heat transfer. In particular, we establish a methodology to link the vorticity field at a preferred frequency to the optimal improvement in the convective heat transfer. To identify the self-agitator preferred frequency, mode analysis is performed with simulation results using dynamic mode decomposition. Experimental results are also obtained to further validate the proposed approach. These results show that the proposed self-agitator design can improve the convective heat transfer by 120% in a conventional heat exchanger without additional pumping power requirements and can achieve a Nusselt number of up to 30 within the laminar flow region, which is improved by 200% with the same Reynolds number compared to the clean channel.

Large-eddy simulation of airfoil flow near stall condition at Reynolds number 2.1 × 106

This paper investigates an airfoil flow involving a turbulent transition and separations near the stall condition at a high Reynolds number Rec = 2.1 × 106 (based on the freestream velocity and the airfoil chord) and provides the wall-resolved large-eddy simulation (LES) database for near-wall models in LES. The present results are compared with the existing experimental and computational data. The wall-resolved LES with the finest mesh (Δξ+,Δη+,Δζ+: chordwise, wall normal, spanwise ≲25, 0.8, 13) and the widest spanwise extent (approximately 5% of the chord length) resolves the key phenomena of the flow (i.e., laminar separation, transition to turbulence, turbulent reattachment, turbulent boundary layer development, and turbulent separation) and well predicts turbulence statistics. The present LES also clarifies unsteady flow features associated with shear-layer instability: the high frequency unsteadiness of St ≃ 130 (based on the freestream velocity and the airfoil chord) at the laminar separation bubble near the leading edge and low frequency unsteadiness of St ≃ 2 at the turbulent separation near the trailing edge. The characteristic frequencies can be scaled to 0.035 and 0.033 by the local momentum thickness and the shear layer velocity which are similar to the natural frequency of the laminar and turbulent shear layer, respectively. With regard to the near-wall modeling in LES, the obtained database indicates that the pressure-gradient term in the mean streamwise-momentum equation is not negligible at the laminar and turbulent separated regions. This fact suggests that the widely used equilibrium wall model is not sufficient, and the inclusion of the pressure-gradient term is necessary for wall modeling in LES of such an airfoil flow. Additionally, influences of computational mesh resolution and spanwise extent on the computational results in wall-resolved LES are investigated.

Improving car radiator performance by using TiO2-water nanofluid

The most recent developments in nanotechnology have lead to improvements in original uses of nanofluids in car motor cooling. In the present study, enhancement of car engine radiator by TiO2-water nanofluid as a coolant of car engine radiator was investigated experimentally. In order to determine the effect of TiO2-water nanofluid on radiator’s performance, experiments were performed with pure water and TiO2-water nanofluid separately and results were compared with other studies on vehicle engine system FIAT DOBLO 1.3 MJTD ENG. The main objective was to check the aspects of heat transfer of the TiO2-water nanofluid as a substitution to the customary coolant system. For this purpose, experiments were carried out using a TiO2 nanofluid with 0.1, 0.2 and 0.3% volume concentrations with flow rates of 0.097 and 0.68 m3/h in laminar floe region, where Reynolds number ranged from 560 to 1650. Our results show that the friction factor decreases when Reynolds number and the volume concentration are increased. Moreover, TiO2-water nanofluid with 0.2% concentration can enhance the effectiveness of car radiator by 47% as compared to 0.1 and 0.3% concentrations and pure water as a coolant. Finally, the average heat transfer coefficient was directly affected by the increase in Reynolds number and volume concentration fraction of the nanofluid.