Entrance Effects Research Articles

Mechanism and improvement for tail vehicle swaying of power-centralized EMUs

The swing phenomenon of the tail vehicle will reduce the stability of the train operation, and even seriously deteriorate the ride comfort of passengers. In response to the relevant railway department, this paper reported the different dynamics performances in tail vehicle swaying when power-concentrated EMUs passed through a tunnel under push/pull operation. To analyze the characteristics and mechanism of the EMU tail swaying, field tests and numerical simulations were conducted. Firstly, through the on-track test, it was found that under push operation, the EMU tail continued to sway at a frequency of 1.3 Hz inside the tunnel, while under pull operation, only a swing of 1.7 Hz appeared at the tunnel entrance. Subsequently, through a simulation analysis, it was found that under push operation, the vortex-induced vibration of the tail carbody occurred inside the tunnel, leading to the sustained swing with a carbody hunting frequency of 1.3 Hz; under pull operation, due to the aerodynamic effect of the tunnel entrance, the secondary lateral stop had an abnormal elastic collision with the bogie frame, and the lateral impact on the bottom of the tail carbody caused a swing of 1.7 Hz, which was verified by the field test. Finally, for the motor vehicle of the EMU tail, two improvement measures of yaw damper optimization were proposed to alleviate the EMU tail swaying inside the tunnel. Furthermore, the research results can provide a reference for the aerodynamic swing problem of other trains, which has certain engineering significance.

Entrance effect of curved endwall on heat transfer and flow structure in a confluent channel with pin fins

In advanced turbine blade internal cooling channels, the majority of the remaining coolant converges into the trailing edge and constitutes a confluent flow before the slot entrance. The entrance effects of curved endwalls (convex, concave) for confluent internal cooling channel are numerically investigated in this paper. The overall heat transfer performances and flow structures are compared in the slot channel with pin fin cooling at the five momentum ratios (M = 0.1–2.1). Among all the cases studied, the higher momentum inflow not only enhance heat transfer but also reduce pressure loss in the slot channel with concave endwalls. In particular, the substantial comprehensive heat transfer factor (η) increase reaches 29.4% higher than the flat endwalls at the given relative curvature height ratio of 1.83. For the models with convex endwalls, the jet-like flow improves the η at the most by 12.1% compared to the flat endwalls. As the curvature height ratio increases, the effects of curved endwalls diminish eventually. In addition, the momentum ratio determines the formation and specified impact of the above flow characteristics in the confluent flow. The curved endwalls exhibit more increase in the η at lower M.

A study on MHD Couette flow in a duct filled with porous materials at the thermal entrance and local thermal non-equilibrium effects

A study on MHD Couette flow in a duct filled with porous materials at the thermal entrance and local thermal non-equilibrium effects

Influence of Operation Parameters on Thermohydraulic Performance of Supercritical CO2 in a Printed Circuit Heat Exchanger

The performance of recuperative printed circuit heat exchangers is critical in supercritical CO2 (sCO2) power generation applications. This article presents a three-dimensional numerical model of sCO2 flowing in a printed circuit heat exchanger and investigates its thermohydraulic performance under different operation conditions. The simulations employ the standard k-ε turbulent model, and consider entrance effects, conjugate heat transfer, real gas thermophysical properties and buoyancy effects. The heat exchanger operation parameters cover mass flux from 254.6 to 1273.2 kg/m2s, inlet temperature 50–150 °C and outlet pressure 100–250 bar on the cold side, and 300–500 °C and 75–150 bar on the hot side. Results show that increasing CO2 mass flux leads to a significantly increased heat transfer coefficient, a slight increase in temperature difference between the hot and cold CO2, as well as larger pressure drop and lower friction factor on both sides. Increasing the cold CO2 pressure, decreasing the cold CO2 temperature, and increasing the hot CO2 temperature result in a higher heat transfer rate of the heat exchanger. Increasing the CO2 temperature on each side causes increased pressure drops on both sides. Increasing the CO2 pressure on each side reduces the pressure drop on each side.

Investigation of Natural Helium Circulation Inside Dual Channels Prismatic Modular Reactor

Natural helium circulation inside a dual-channel prismatic modular reactor was numerically investigated to comprehend the passive safety features using the new RHYS/ASYST VER4 package. The natural circulation was studied at Depressurized Cooldown (DC) at heat fluxes of 860–2293 W/m2 with a pressure of 413.7 kPa and Pressurized Cooldown (PC) at pressures of 413.47–689.12 kPa with a flux of 1720 W/m2. A reversal of the heat transfer near the riser exit was seen, combined with a decrease in helium and riser wall temperatures due to the end effect. The pressure has an inverse effect on the heat transfer, where the temperature decreases by 19 % as the pressure increases from 413.47 to 689.12 kPa. The heat transfer was studied under the DC and PC. Both entrance and end effects are introduced with the heat transfer coefficient. Also, the nondimensional groups (Nu, Gr, Pr, and Ra) were discussed.

Mixed Convection Heat Transfer Characteristics of Al2O3 – MWCNT Hybrid Nanofluid under Thermally Developing Flow; Effects of Particles Percentage Weight Composition

In this research, an experimental investigation of hybrid nanofluid mixed convection heat transfer characteristics is conducted. The research specifically investigates the effects of percentage weight composition (PWC) of nanoparticles in the hybrid nanofluids on mixed convection heat transfer characteristics along the lamina, transition and turbulent regions. Transition boundaries, thermal entrance effects, and the influence of tube axial position were also critically investigated and analysed experimentally. Three hybrid nanofluids of Al2O3 – MWCNT (i.e., Al2O3 (60%) – MWCNT (40%), Al2O3 (50%) – MWCNT (50%) and Al2O3 (40%) – MWCNT (60%)) were prepared using two-step method and then subjected to a constant heat flux through a horizontal circular copper tube with an internal diameter of 8mm. Results show a significant change in heat transfer characteristics with different PWCs. Al2O3 (60%) – MWCNT (40%) have shown a better heat transfer enhancement among the three fluids investigated. Its Nusselt number has an enhancement of more than 5 % better than the other two fluids. Along the transition regime, critical Reynold numbers (Recr) of the three nanofluids were found to have differed slightly, with Recr = 2020, 2000 and 2100 for Al2O3 (60%) – MWCNT (40%), Al2O3 (50%) – MWCNT (50%) and Al2O3 (40%) – MWCNT (60%) respectively. Mixed convection effects were found to be more significant with Al2O3 (60%) – MWCNT (40%) than with the other two fluids. At the axial position of 63.75, its mixed convection strength (Ὠ) was about 41%, which is the highest among the three fluids. Its strength of mixed convection was also found to deteriorate to about 14.75% with an increase in axial distance from the tube inlet. A similar observation was also noticed with other fluids. Thermal entrance effects were only found to be significant at x/d = 15 and 31.25, as their influence diminishes with an increased x/d distance from the tube inlet. It was concluded that both mixed convection and thermal entrance effects resulted in heat transfer enhancement, especially in the lamina region. Their influences decrease with an increase in axial distance from the tube inlet. Mixed convection influences were only present in the lamina and transition region, and their strength was reduced with an increase in Reynold number and axial position.

Discharge Formula and Hydraulics of Rectangular Side Weirs in the Small Channel and Field Inlet

In this study, experimental investigations were conducted on rectangular side weirs with different widths and heights. Corresponding simulations were also performed to analyze hydraulic characteristics including the water surface profile, flow velocity, and pressure. The relationship between the discharge coefficient and the Froude number, as well as the ratios of the side weir height and width to upstream water depth, was determined. A discharge formula was derived based on a dimensional analysis. The results demonstrated good agreement between simulated and experimental data, indicating the reliability of numerical simulations using FLOW-3D software (version 11.1). Notably, significant fluctuations in water surface profiles near the side weir were observed compared to those along the center line or away from the side weir in the main channel, suggesting that the entrance effect of the side weir did not propagate towards the center line of the main channel. The proposed discharge formula exhibited relative errors within 10%, thereby satisfying the flow measurement requirements for small channels and field inlets.

In situ flow state characterization of molten resin at the inner mold in injection molding

AbstractOnline viscosity information on processing lines can reflect the material flow resistance and offer valuable guidance for manufacturing across various industries. Considering the accuracy, devices, and processes involved in injection molding, characterizing the melt's flow state during material processing poses a significant challenge. To reduce investment in viscometers, avoid influencing the components' surface aesthetics due to the installation of sensors, and make the flow state detect online in mold, this study designs a rheometric mold with cylindrical runners for identifying the in situ viscosity of molten resin during injection molding. The detection conditions of injection speed and cavity pressure variations, the entrance effect, and the viscous dissipation for Polycarbonate are analyzed under various conditions. The in situ viscosity is identified and compared with the standard cross‐WLF model. The result shows that the melt velocity and cavity pressure variations during the filling process create a stable environment for in situ rheological characterization and the detected viscosity is related to the shear rate, melt temperature, and channel dimension in injection molding. The designed mold with cylindrical runners for determining the in situ thermal‐rheological behavior of polymer is distinguished successfully and exhibits prospects for the development of low‐cost, nondestructive, and inner‐mold measurement in manufacturing applications.

Unified non-equilibrium simulation methodology for flow through nanoporous carbon membrane.

The emergence of new nanoporous materials, based, e.g., on 2D materials, offers new avenues for water filtration and energy. There is, accordingly, a need to investigate the molecular mechanisms at the root of the advanced performances of these systems in terms of nanofluidic and ionic transport. In this work, we introduce a novel unified methodology for Non-Equilibrium classical Molecular Dynamic simulations (NEMD), allowing to apply likewise pressure, chemical potential, and voltage drops across nanoporous membranes and quantifying the resulting observables characterizing confined liquid transport under such external stimuli. We apply the NEMD methodology to study a new type of synthetic Carbon NanoMembranes (CNM), which have recently shown outstanding performances for desalination, keeping high water permeability while maintaining full salt rejection. The high water permeance of CNM, as measured experimentally, is shown to originate in prominent entrance effects associated with negligible friction inside the nanopore. Beyond, our methodology allows us to fully calculate the symmetric transport matrix and the cross-phenomena, such as electro-osmosis, diffusio-osmosis, and streaming currents. In particular, we predict a large diffusio-osmotic current across the CNM pore under a concentration gradient, despite the absence of surface charges. This suggests that CNMs are outstanding candidates as alternative, scalable membranes for osmotic energy harvesting.

Developed and quasi-developed macro-scale flow in micro- and mini-channels with arrays of offset strip fins

We investigate to what degree the steady laminar flow in typical micro- and mini-channels with offset strip fin arrays can be described as developed on a macro-scale level, in the presence of channel entrance and sidewall effects. Hereto, the extent of the developed and quasi-developed flow regions in such channels is determined through large-scale numerical flow simulations. It is observed that the onset point of developed flow increases linearly with the Reynolds number and channel width but remains small relative to the total channel length. Furthermore, we find that the local macro-scale pressure gradient and closure force for the (double) volume-averaged Navier–Stokes equations are adequately modeled by a developed friction factor correlation, as typical discrepancies are below 15% in both the developed and developing flow region. We show that these findings can be attributed to the eigenvalues and mode amplitudes, which characterize the quasi-developed flow in the entrance region of the channel. Finally, we discuss the influence of the channel side walls on the flow periodicity, the mass flow rate, as well as the macro-scale velocity profile, which we capture by a displacement factor and slip length coefficient. Our findings are supported by extensive numerical data for fin height-to-length ratios up to 1, fin pitch-to-length ratios up to 0.5, and channel aspect ratios between 1/5 and 1/17, covering Reynolds numbers from 28 to 1224.

Numerical simulation of heat transfer characteristics of supercritical liquid hydrogen through triangular U-tube in moderator

In this study, the heat transfer characteristics of supercritical hydrogen in triangular U-tubes with different radius were investigated. In order to compare the heat transfer capacity of triangular U-tubes, and to explore the heat transfer characteristics of supercritical liquid hydrogen inside them, a three-dimensional numerical simulation was carried out by Fluent. The unstructured grid and RNG k-ε turbulence model were used to encrypt the grid. Meanwhile, the grid independence and the effectiveness of the turbulence model are verified. The Tw, Tf and Nu along the triangular U-tube at different q and Re were compared. The result shows that Tw, Tf and Nu increase with the increase of q and Re. The change of Re was found to be largest for Nu. The effect of secondary flow on Nu was similar to the entrance effect of upstream straight pipe, and the effect of secondary flow on Nu was slightly larger than the entrance effect. This work provides a useful technical guidance for thermal design and thermal management of neutron source and target station, as well as novel ideas for flow and heat transfer of supercritical liquid hydrogen in U-tube.

Pressure drop of turbulent flow across pipe bends

Pressure drop of turbulent flow across 90[Formula: see text] round bends with 10mm inside diameter investigated by employing the shear stress transport [Formula: see text] turbulent model for numerous curvature ratios and flow conditions. A total of 10 flow velocities varied from 1 to 10 m/s and 17 curvature radii between 5 and 300 mm were examined. Velocity profiles were evaluated at eight locations to examine the entrance effect. The simulation results were verified by comparing the evaluated Darcy friction factor with a well-known published correlation. The obtained results of bend pressure drop demonstrate the domination of the curvature ratio effect on the pressure drop over the influence of other factors. The computed pressure loss across the bend in terms of pressure loss coefficient [Formula: see text] and equivalent length to diameter ratio [Formula: see text] shows a good agreement with various published data in the literature with [Formula: see text] maximum average error. The coincidence between the results and the published data verifies the accuracy of the employed model to evaluate the pressure drop of turbulent flow throughout 90[Formula: see text] bends for the applied boundary conditions.

Understanding flow dynamics in membrane distillation: Effects of reactor design on polarization

Optimization and design of full-scale membrane distillation (MD) systems usually require Sherwood and Nusselt correlations that are developed from lab-scale systems. However, entrance effects in lab-scale systems can significantly impact heat, mass and momentum transfer in the reactor, therefore affect the accuracy of the developed experimental Sherwood and Nusselt correlations. Here, Computational Fluid Dynamics (CFD) simulations using OpenFOAM are performed to understand the effects of right-angled bends and inlet design on flow dynamics, temperature and concentration polarization in MD systems. Simulation results show that the presence of right-angled bends and inlets with sudden expansions lead to the formation of Dean vortices. Dean vortices enhance perpendicular mixing in MD systems and reduce both temperature and concentration polarization. Temperature and concentration polarization coefficients in MD systems with right-angled bends and inlets with sudden expansions vary significantly for the same volumetric flow rate. Our studies show that lab-scale systems with the same volumetric flow rate but different designs lead to significantly different Nusselt and Sherwood correlations. This study demonstrates the importance of CFD-informed design of lab-scale systems to minimize entrance effects and suppress Dean vortices for consistent model development and calibration across multiple scales.

The Elasticity of Polymer Melts and Solutions in Shear and Extension Flows.

This review is devoted to understanding the role of elasticity in the main flow modes of polymeric viscoelastic liquids-shearing and extension. The flow through short capillaries is the central topic for discussing the input of elasticity to the effects, which are especially interesting for shear. An analysis of the experimental data made it possible to show that the energy losses in such flows are determined by the Deborah and Weissenberg numbers. These criteria are responsible for abnormally high entrance effects, as well as for mechanical losses in short capillaries. In addition, the Weissenberg number determines the threshold of the flow instability due to the liquid-to-solid transition. In extension, this criterion shows whether deformation takes place as flow or as elastic strain. However, the stability of a free jet in extension depends not only on the viscoelastic properties of a polymeric substance but also on the driving forces: gravity, surface tension, etc. An analysis of the influence of different force combinations on the shape of the stretched jet is presented. The concept of the role of elasticity in the deformation of polymeric liquids is crucial for any kind of polymer processing.

Turbulent entrainment in finite-length wind farms

In this article, we present an entrainment-based model for predicting the flow and power output of finite-length wind farms. The model is an extension of the three-layer approach of Luzzatto-Fegiz & Caulfield (Phys. Rev. Fluids, vol. 3, 2018, 093802) for wind farms of infinite length, and assumes dependence of key flow quantities, such as the wind farm bulk velocity, on the streamwise distance from the farm entrance. To assist our analysis and validate the proposed model, we undertake a series of large-eddy simulations with different turbine spacing arrangements and layouts. Comparisons are also made with the top-down model with entrance effects of Meneveau (J. Turbul., vol. 13, 2012, N7) and data from the literature. The finite-length entrainment model is shown to be capable of capturing the power drop between contiguous rows of turbines as well as describing the advection and turbulent transport of kinetic energy in both the entrance and fully developed regions. The fully developed regime is approximated only deep in the wind farm, after approximately 15 rows of turbines. Our data suggest that for the cases considered in this study, the empirical coefficients that can be used to describe turbulent entrainment and transfers above the wind farm exhibit little dependence on the farm layout and may be considered constant for modelling purposes. However, the flow field within the wind farm layer can be strongly modulated by the turbine density (spacing) as well as the array layout, and to that extent it can be argued that they are both primary factors determining the wind farm power output.

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

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

Geometrical effect of natural convection in an air channel with a cavity at the turning section on thermal behavior and heat removal performance

The geometrical effect of natural convection in an air channel with a cavity at the turning section was investigated experimentally by controlling the vertical upper gap (G, 100–75 mm) and horizontal lower channel gap (B, 100 - 300 mm) and varying the heat load conditions (P, 1.35–6.20 kW). The thermal behavior in the channel was considered in terms of the temperature distributions of the walls, and the air temperature field was estimated from the measured air temperature. Additionally, the heat removal performance was analyzed for changes in each parameter to find the critical design parameter on cooling performance.The heat load condition positively affected the heat removal performance, but it did not affect the trend of the wall temperature distributions or the shape of the temperature field in the channel. A lower channel gap directly affected the cooling in the lower channel (cavity) between the lower curved wall and bottom but did not appreciably affect the cooling in the upper vertical channel between both vertical walls and overall heat removal performance (RSDMax = 3.2 %). As a result, the most critical parameter was the upper vertical channel gap because it affected both lower and upper channels and played a vital role in determining the overall heat transfer performance (RSDMax = 11.0 %). Additionally, it was confirmed that the location where the peak wall temperature appeared changed as the vertical gap size was altered.Interestingly, we found that the transition phenomenon eliminating the entrance effect occurred above the mid-height of the vertical channel in all experimental cases, and it was directly related to the heat removal performance. The temperature profiles developed smoothly along the height in only the G175 cases, which were the maximum heat removal performance cases, owing to the minimized entrance effect in the channel. In contrast, the temperature profiles below the mid-height of the vertical channel changed rapidly due to the entrance effect in other cases.

S–CO2 heat transfer characteristics analysis in PCHE and vertical channel

Numerical simulations are applied to investigate supercritical carbon dioxide (S–CO2) and heat transfer characteristics in the case of uniform/nonuniform heating conditions in the straight vertical channel and printed circuit heat exchanger (PCHE). The proposed S–CO2 heat transfer correlation agrees well with the experimental values. Results show that high heat flux let S–CO2 heat transfer deterioration likely to occur. Heat transfer deterioration effect will be enhanced as S–CO2 is in area of the gas-like region. Heat transfer enhancement effect will be more significant when S–CO2 is in the area of the liquid-like region. The concept of total length is proposed which is the channel length affected by heat transfer deterioration and entrance effect together. The affected length is 55–70 times of the equivalent diameter in the case of 140 kW/m2 heat flux; the affected length is 85–125 times of the equivalent diameter in the case of 300 kW/m2 heat flux. Changes in dominance of h and λ of PCHE suggest that the convective heat transfer and heat conduction resistance strength of the fluid layer is changing. This study provides guidance for PCHE design and heat transfer enhancement.

Control of extrudate swell and instabilities using a rotating roller die

• The moving walls (rotating rollers) provide a flexible mean to control die swell and instabilities • The smooth convergent geometry of the rotating die prevents volume instabilities normally related to entrance effect. However very large stresses on the melt are generated in the stationary mode so much so that surface roughness instabilities, not usually reported with polystyrene, appear on the extrudate surface. • This surface roughness resembles sharkskin but the mechanism leading to it is different to that leading to sharkskin. • This observation is a first as PS is reported to never exhibit surface roughness. • The rotating die is a versatile tool to produce sheet of various thicknesses as the gap can be changed by moving the rollers up and down. • The experimental data presented are underpinned by simple theoretical considerations, showing the importance of the wall shear stress and the changes in extension at the die exit. Thermoplastics extruded from dies will always swell and above a critical flow rate display instabilities (sharkskin, stick-spurt or gross melt fracture). Prior research has shown that the best way to suppress these instabilities is to reduce the entry converging angle using a smooth convergence and induce permanent wall slip. In this research we go a step further by allowing the walls to move using a rotating roller die. Thus both extrudate swell and flow instabilities become controllable. This paper presents data to support this claim. The practical benefits are important as stable operation at higher flow rates become permissible. Also, by providing extra control variables, this device becomes a useful tool to help unravel the causes of the various instabilities that arise in polymer melt die extrusion. A first from this research, using this roller die geometry we were able to tease out surface roughness instability with polystyrene hitherto not reported.

Fluid flow and heat transfer in a rectangular ribbed channel with a hierarchical design for turbine blade internal cooling

• New design concept to develop high-performance internal cooling of turbine blades. • Greatly reduced pressure loss is achieved with negligible heat transfer reduction. • The underlying physics of pressure drop and heat transfer performance is clarified. • Various designs are examined to verify the feasibility of the new design concept. For internal cooling of a turbine blade, various advanced rib turbulators can markedly contribute to the heat transfer enhancement while suffering a great increase in pressure loss. In those designs, ribs with the same configuration are periodically and evenly mounted on the channel wall. In this context, this work proposes a hierarchical design concept to optimize the rib arrangement with the desired reduction in pressure loss. In terms of the rib height, this new design concept is implemented to construct three new rib configurations. Based on an established turbulence model, three-dimensional (3D) numerical simulations are entirely adopted to verify the feasibility of the new configuration in a wide Reynolds number range. The numerical results demonstrate that the optimal configuration with a linearly decreasing rib height can greatly reduce the pressure loss with a slight heat transfer deterioration. The negligible reduction in the heat transfer performance results from the enhanced fluid impingement on the reattachment region because of the lowering effect of the mainstream, although small ribs weaken the fluid impingement. The marked pressure drop reduction comes from the combination of the lowering effect and small ribs which constrains the separation vortex behind ribs. Furthermore, the comparison of the overall thermal performance is carried out considering a wide range of the Reynolds number, pitch ratios, and aspect ratios. The optimal configuration can greatly enhance the overall thermal performance up to 138.3% for the factor ( Nu / Nu 0 )/( f / f 0 ) and up to 32.5% for the factor ( Nu / Nu 0 )/( f / f 0 ) 1/3 . Eliminating the entrance effect of developing flow, the increment in the overall thermal performance is considerably reduced but still keeps at a high level. Finally, it is significantly highlighted that as a simple but effective improvement, the hierarchical design concept presents great potential in developing high-performance internal cooling of turbine blades.