Entrance Cross Section Research Articles

The motion of Brownian particles suspended in a non-uniform fluid flow

The effect of the type of fluid flow velocity profile on the motion of a Brownian particle in a circular tube has been studied using a Monte Carlo technique to simulate particle trajectories. Three types of fluid flow were studied: uniform (UF), parabolic (PF), and an initial uniform flow which evolves and may eventually attain the parabolic velocity profile, i.e. transient flow (TF). For UF and PF, radial and axial particle diffusion were simultaneously considered in some cases; for TF, where fluid dynamics equations had to be solved in addition to the Monte Carlo simulations, axial diffusion was neglected for fluid and particles, so that calculations had to be restricted to tube aspect ratios (radius/length) smaller than 0.1, a common scenario in most practical aerosol applications. A first series of simulations were performed to determine the overall particle penetration through the tube. For fixed tube geometry and particle diffusion coefficient, penetration in TF lied between those in PF (highest) and UF (lowest), in such a manner that, for a given value of the diffusion coefficient, the larger the degree of flow development (i.e., the smaller the values of the tube aspect ratio and the Reynolds number), the larger the penetration. These findings are coherent with those found in a former investigation dealing with the particle residence time in the tube. In a second series of simulation runs, calculations were done for selected initial locations of the particle within the tube entrance cross section, to examine the effect that the proximity of the particle starting position to the tube wall or to the tube axis has on variables such as penetration, mean first hitting time and length, mean first exit time, and mean axial velocity of lost and survived particles. When the fluid flow is not uniform but varies from place to place, the mean particle axial velocity is larger than that of the fluid. This counterintuitive fundamental result allows interpretation of the trends observed in penetration and residence time.

Mutation at the entrance of the quinone cavity severely disrupts quinone binding in respiratory complex I

In all resolved structures of complex I, there exists a tunnel-like Q-chamber for ubiquinone binding and reduction. The entrance to the Q-chamber in ND1 subunit forms a narrow bottleneck, which is rather tight and requires thermal conformational changes for ubiquinone to get in and out of the binding chamber. The substitution of alanine with threonine at the bottleneck (AlaThr MUT), associated with 3460/ND1 mtDNA mutation in human complex I, is implicated in Leber's Hereditary Optic Neuropathy (LHON). Here, we show the AlaThr MUT further narrows the Q-chamber entrance cross-section area by almost 30%, increasing the activation free energy barrier of quinone passage by approximately 5 kJ mol−1. This severely disrupts quinone binding and reduction as quinone passage through the bottleneck is slowed down almost tenfold. Our estimate of the increase in free energy barrier is entirely due to the bottleneck narrowing, leading to a reduction of the transition state entropy between WT and MUT, and thus more difficult quinone passage. Additionally, we investigate details of possible water exchange between the Q-chamber and membrane. We find water exchange is dynamic in WT but may be severely slowed in MUT. We propose that LHON symptoms caused by 3460/ND1 mtDNA mutation are due to slowed quinone binding. This leads to an increased production of reactive oxidative species due to upstream electron backup at the FMN site of complex I, thus resulting in a mt bioenergetic defect.

Causes and mitigation of gas temperature deviation in tangentially fired tower-type boilers

Gas temperature deviation is a common problem for tangentially fired boilers due to the residual swirling at the furnace exit. It can induce overheating or even explosion of superheaters and/or reheaters tubes. Tower design of tangentially fired boilers is expected to abate the deviation. However, large gas temperature deviation still exists in tower-type boilers. In this study, a numerical method that is validated by experimental data and designed value is used to investigate the gas temperature deviation in a 1000 MWe tangentially coal-fired tower-type boiler, from which the two main causes of the deviation are identified and a new method to mitigate the deviation is proposed. First, the traction of the induced draft fans skews the flue gas flow towards the rear wall, on which the furnace exit is located. Such gas flow skewness becomes aggravated along with the furnace height. Second, the dense panel heaters obstruct the gas flow, which further aggravates the gas flow skewness. Both in combination result in a highly non-uniform gas flow in the panel heaters zone and thus a large gas temperature deviation. In the 1000 MWe tower-type boiler under investigation, the gas temperature deviation between the left side and the right side on the entrance cross-section of the final superheater reaches 120 K. A new method, i.e., adding an arch nose under the furnace exit, is proposed to mitigate the deviation, in which the height of the arch nose is a decisive factor and needs to be carefully determined. A too low arch nose cannot eliminate the skewness of flue gas towards the rear wall while a too high arch nose skews the flue gas towards the front wall. The new mitigation method has been proven effective.

Experimental investigation of the effects of airflow nonuniformity on performance of a fin-and-tube heat exchanger

The effects of airflow nonuniformity on the thermal–hydraulic performance of a fin-and-tube heat exchanger were investigated experimentally. The test subject was a four-depth-row fin-and-tube heat exchanger with face split tube circuitry. Nonuniform inlet air distribution was generated by partially blocking the heat exchanger entrance cross-section. It was found that airflow nonuniformity causes thermal effectiveness deterioration and pressure drop increase. The size of effectiveness deterioration and pressure drop increase depend on the degree of airflow nonuniformity as well as on the orientation between airflow nonuniformity and tube-side fluid circuitry. For most of the observed nonuniform airflow profiles, which had low or moderate degrees of airflow nonuniformity, the effectiveness deterioration was between 5% and 10%, while the pressure drop increase was between 10% and 20%. For severely nonuniform airflow profiles the effectiveness deterioration was up to 30% and the pressure drop increase was up to 90%.

Effects of Magnetic Particles Entrance Arrangements on Mixing Efficiency of a Magnetic Bead Micromixer

In this study, a computer code is developed to numerically investigate a magnetic bead micromixer under different conditions. The micromixer consists of a microchannel and numerous micro magnetic particles which enter the micromixer by fluid flows and are actuated by an alternating magnetic field normal to the main flow. An important feature of micromixer which is not considered before by researchers is the particle entrance arrangement into the micromixer. This parameter could effectively affect the micromixer efficiency. There are two general micro magnetic particle entrance arrangements in magnetic bead micromixers: determined position entrance and random position entrance. In the case of determined position entrances, micro magnetic particles enter the micromixer at specific positions of entrance cross section. However, in a random position entrance, particles enter the microchannel with no order. In this study mixing efficiencies of identical magnetic bead micromixers which only differ in particle entrance arrangement are numerically investigated and compared. The results reported in this paper illustrate that the prepared computer code can be one of the most powerful and beneficial tools for the magnetic bead micromixer performance analysis. In addition, the results show that some features of the magnetic bead micromixer are strongly affected by the entrance arrangement of the particles.

Effects of Magnetic Particles Entrance Arrangements on Mixing Efficiency of a Magnetic Bead Micromixer

Abstract In this study, a computer code is developed to numerically investigate a magnetic bead micromixer under different conditions. The micromixer consists of a microchannel and numerous micro magnetic particles which enter the micromixer by fluid flows and are actuated by an alternating magnetic field normal to the main flow. An important feature of micromixer which is not considered before by researchers is the particle entrance arrangement into the micromixer. This parameter could effectively affect the micromixer efficiency. There are two general micro magnetic particle entrance arrangements in magnetic bead micromixers: determined position entrance and random position entrance. In the case of determined position entrances, micro magnetic particles enter the micromixer at specific positions of entrance cross section. However, in a random position entrance, particles enter the microchannel with no order. In this study mixing efficiencies of identical magnetic bead micromixers which only differ in particle entrance arrangement are numerically investigated and compared. The results reported in this paper illustrate that the prepared computer code can be one of the most powerful and beneficial tools for the magnetic bead micromixer performance analysis. In addition, the results show that some features of the magnetic bead micromixer are strongly affected by the entrance arrangement of the particles.

Use of one-dimensional modelling in estuary management: entrance depth –– model calibration

The Wairoa River, a barrier enclosed estuary situated in Hawke’s Bay, New Zealand was modelled using a one-dimensional hydrodynamic model. Water level data obtained during a flood and a bathymetric survey were available but the entrance cross-section had not been surveyed. This paper describes the calibration of the model to jointly optimise the selection of the Manning coefficient and the depth of the estuary entrance.

Tidal asymmetry of a shallow, well-mixed estuary and the implications on net sediment transport: A numerical modelling study

Tidal asymmetry plays a pivotal role in the transport of sediment and morphological change in shallow inlet/estuarine systems, particularly those with extensive tidal flats and channels. This study examined tidal circulation patterns of a shallow and well-mixed estuary on the central coast of New South Wales, Australia. A numerical modelling study was carried out by applying a depth-averaged flow model (MIKE 21 HD). The model was calibrated and successfully validated against recently acquired hydrodynamic data. Model results reveal the presence of a number of eddies within the estuary, which may have substantial influence on net sediment transport when combined with wave effects. Tidal phase duration and magnitude asymmetries indicate that, in contrast to typical flood/ebb dominance, double ebb-dominance is in deep channels and the entrance cross-section, whereas the flood ramp area shows double flood-dominance. The implications of tidal asymmetry on net bed-load sediment transport were inferred from the duration and magnitude asymmetries of the above threshold velocity. The ebb-directed net transport occurs at main tidal channels and may result in further deepening channels, whereas a flood-directed net transport in the flood ramp area may cause an accumulation of sediment on the delta or a landward progradation of the tidal flat. The present-day estuary is characterised as ebb-dominated, tends to flush bed-load sediment seaward more effectively and may represent more stable geometries.

PNPI wide-aperture fan neutron supermirror analyzer of polarization

A wide-aperture fan neutron supermirror analyzer of polarization is presented in the paper. The analyzer has been made in PNPI (Gatchina, Russia) for the neutron reflectometer NeRo in GKSS (Geesthacht, Germany). The analyzer is installed in front of the two coordinate position-sensitive detector with sensitive area 250×250mm2. The neutron wavelength is 4.35Å. The analyzer consists of 90 channels, and its entrance cross-section is 90×160mm2. In the paper, the results of tests of the analyzer are presented.

Accumulation of frazil ice along a river bend

This paper discusses the role of transverse currents on frazil ice accumulation along a river bend. To this end, field observations of a frazil ice jam along the ShiYaoBu bend reach of the Yellow River, in China, and a model study in the laboratory were carried out. Field measurements of flow velocities verified that there are two transverse circulations under an ice cover. The characteristics of frazil ice accumulation along a river bend have been discussed. The laboratory model study shows that the transverse accumulation of frazil ice depends not only on the hydraulic variables, such as Froude number, but also on the central angle of the river bend (inclined to the entrance cross section). Equations describing the transverse accumulation of frazil ice along a bend have been established. The dependency of the accumulation thickness in the longitudinal direction on the Froude number and ice discharge has been derived.

Forced convection with viscous dissipation in the thermal entrance region of a circular duct with prescribed wall heat flux

The thermal entrance forced convection in a circular duct with a prescribed wall heat flux distribution is studied under the assumptions of a fully developed laminar flow and of a negligible axial heat conduction in the fluid, by taking into account the effect of viscous dissipation. The solution of the local energy balance equation is obtained analytically by employing the Laplace transform method. The effect of viscous dissipation is taken into account also in the region upstream of the entrance cross-section, by assuming an adiabatic preparation of the fluid. The latter hypothesis implies that the initial condition in the entrance cross-section is a non-uniform radial temperature distribution. Two special cases are investigated in detail: an axially uniform wall heat flux, a wall heat flux varying linearly in the axial direction.

Thermal Entrance Heat Transfer of an Adiabatically Prepared Fluid With Viscous Dissipation in a Tube With Isothermal Wall

Abstract Forced convection in the thermal entrance region of a circular duct is analyzed. Viscous dissipation effects are taken into account under conditions of laminar hydrodynamically developed flow. The duct wall is assumed to be isothermal in the region downstream of the entrance cross section. The prescription of the initial condition at the entrance cross section is coherent with the assumption of a non-negligible viscous heating in the whole duct. The special case of an adiabatic-wall preparation of the fluid in the upstream region is considered. This adiabatic preparation results in a non-uniform entrance temperature distribution. The governing equations are solved analytically by separation of variables. Important differences are pointed out in the comparison of the solution with those available in the literature, which are based on the assumption of a uniform temperature distribution in the entrance cross section.

Finite Element Analysis of Tilting of Metal in Bar Rolled with Three-roll Mill

Rolled metal bars and rods often exhibit tilting, which is rotation of the material around the pass line under rolling. Tilting may result in a defective shape. High dimensional accuracy is demanded in 3-roll rolling because 3-roll rolling is used for finishing passes. Therefore, the design of calibers and a rolling sequence that can reduce tilting is important from industrial aspects. In this study, three-dimensional FE analysis is applied to the prediction of metal tilting under 3-roll rolling in an oval-round pass and a hexagon-oval pass. The difference in the characteristics of tilting according to the contact contour between metal and the roll is investigated by classifying the characteristics of tilting into two modes, with the aim of clarifying the threshold for the occurrence of overturning and restoration resulting from tilting. Predicted results agree well with the experimental measurements of tilting, and the validity of method for analyzing tilting is confirmed. Moreover, it is found that the mechanism of tilting differs according to the contact contour. In an oval-round pass that is of a convex contact type, the bar tilt angle is difficult to decrease once tilting has occurred. In a hexagon-oval pass that is of a concave contact type, although the bar tilt angle tends to increase when the entrance cross section is constrained strongly, the bar tilt angle tends to decrease after constraint becomes relatively small.

Diversion tunnel of the Irganaiskaya waterworks facility and prospects of its use in the period of erection of the dam to intermediate levels

General information on the diversion tunnel and statement of the problem of its use at normal maximum operating level NMOL = 521 m. The Irganaiskaya waterworks facility is being built in Dagestan on the Avarskoe Koisu River. The design engineering solutions for this waterworks facility and the stage of the construction have been considered in [1]. Excess water volumes of this waterworks facility are passed through the right-bank combined diversion and operational tunnel-kind spillway designed for freeflow operation in all stages of flow passage. The diversion tunnel is the first story of the spillway, which serves for passing flows in the initial period of the erection of the waterworks facility and in temporary operation. This structure consists of an intake channel, intake portal, slightly inclined part of the tunnel, and a tail device. The gretaer part of the slightly inclined tunnel and the tail device are to be used later in the period of continuous operation after the erection of the sluiceway entrance and inclined part of the sluice of the upper (second) story. Some data on the structure of the diversion tunnel and of laboratory hydraulic studies performed for its design are presented in [2]. The full length of the diversion tunnel is 965 m, and a two-span gates chamber is constructed in its intake portal (Fig. 1). The sizes of the entrance cross-section of the gates chamber (in front of the slots of the guard gates), which determine the diversion capacity of the tunnel, are h b = 6 4.5 m; the sizes of the main gates are 6.3 3.5 m. The operating gates were actuated by hydraulic actuators mounted at a level of 522.0 m of the upper part of the intake portal of the tunnel. Metallic facing exists in the gates chamber only on the zone adjoining the gate slots. Behind the gates chamber and the bottleneck transition part, the tunnel has a trough shape with a height of 11 m and a width of 8 m; the interface of the trough and of the vertical side walls of the tunnel is rounded. On the upstream region (in front of the connection between the tail reach of the tunnel and the inclined conduit of the operational spillway) the tunnel has a sloped bottom (the slope is 0.006) and on the downstream discharge region the slope is 0.0235. In the greater part of the length the diversion tunnel is made of concrete of class B30, but in the upstream region the Power Technology and Engineering Vol. 39, No. 6, 2005

GROUNDWATER CONTAMINATION DOWNSTREAM OF A CONTAMINANT PENETRATION SITE. II. HORIZONTAL PENETRATION OF THE CONTAMINANT PLUME

ABSTRACT Part I of this study (Rubin, H.; Buddemeier, R.W. Groundwater Contamination Downstream of a Contaminant Penetration Site Part 1: Extension–Expansion of the Contaminant Plume. J. of Environmental Science and Health Part A (in press).) addressed cases, in which a comparatively thin contaminated region represented by boundary layers (BLs) developed within the freshwater aquifer close to contaminant penetration site. However, at some distance downstream from the penetration site, the top of the contaminant plume reaches the top or bottom of the aquifer. This is the location of the “attachment point,” which comprises the entrance cross section of the domain evaluated by the present part of the study. It is shown that downstream from the entrance cross section, a set of two BLs develop in the aquifer, termed inner and outer BLs. It is assumed that the evaluated domain, in which the contaminant distribution gradually becomes uniform, can be divided into two sections, designated: (a) the restructuring section, and (b) the establishment section. In the restructuring section, the vertical concentration gradient leads to expansion of the inner BL at the expense of the outer BL, and there is almost no transfer of contaminant mass between the two layers. In the establishment section, each of the BLs occupies half of the aquifer thickness, and the vertical concentration gradient leads to transfer of contaminant mass from the inner to the outer BL. By use of BL approximations, changes of salinity distribution in the aquifer are calculated and evaluated. The establishment section ends at the uniformity point, downstream from which the contaminant concentration profile is practically uniform. The length of the restructuring section, as well as that of the establishment section, is approximately proportional to the aquifer thickness squared, and is inversely proportional to the transverse dispersivity. The study provides a convenient set of definitions and terminology that are helpful in visualizing the gradual development of uniform contaminant concentration distribution in an aquifer subject to contaminant plume penetration. The method developed in this study can be applied to a variety of problems associated with groundwater quality, such as initial evaluation of field data, design of field data collection, the identification of appropriate boundary conditions for numerical models, selection of appropriate numerical modeling approaches, interpretation and evaluation of field monitoring results, etc.

Modeling of nonstationary vacuum arc plasma jet interaction with a neutral background gas

A plasma beam formed by a stationary vacuum arc source with a copper cathode is considered as a supersonic hydrodynamic jet interacting by collisions with a neutral atmosphere of background gas (argon). The plasma jet propagation in the neutral atmosphere is calculated numerically using a one-dimensional approximation. The values assumed at the entrance cross section of the plasma jet were ion temperature 104 K, electron temperature 3×104 K, ion concentration ni=1019 m−3 plasma jet mass velocity 1.5×104 m/s, and neutral concentration of (0.1,1,2,5)×ni. Two cases were analyzed:(1) isothermal gas motion and (2) gas motion with energy transfer from the ions to the neutrals. It is found for the isothermal case that the plasma front expands with decreasing velocity, but it never decreases down to the speed of sound. A snowplow effect is found with respect to the neutral gas. The neutral gas is accelerated by the plasma jet up to supersonic velocity. With ion–neutral energy transfer the plasma jet deceleration leads to shock discontinuities for all studied neutral densities.

Vacuum arc plasma jet interaction with neutral ambient gas

Vacuum arc plasma jet (VAPJ) propagation in a neutral nitrogenatmosphere has been calculated numerically on the basis of a hydrodynamicdescription. It was assumed that the VAPJ was emitted isotropically from apoint source located z0 = 20 mm in front of the entrance of a straight duct100 mm in diameter in which an axial magnetic field of 0.02-0.05 T wasimposed. Plasma densities of n0 = 1018 and 1019 m-3 and neutralgas densities nn0 = 0.001-0.1×n0 were used as initial conditionsat the duct entrance. A three-fluid model was used, with the ion temperaturehaving an initial value at the duct entrance of Ti = 104 K, while constanttemperatures of Te = 3×104 K and Tn = 103 K were assumed for theelectrons and neutrals, respectively.It was found that the plasma jet decelerates down to the critical point, i.e. where the jet velocity is equal to the local speed of sound. The distance ofthe critical point from the entrance cross section decreased from zc = 4×R (R = radius of duct) for nn0 = 0.001×n0 down to zc = Rfor nn0 = 0.1×n0. It was found that increasing the magnetic field alsodecreased zc. It was also found that the electrons and ions have rotationalmotion components, in opposite directions, and that friction with the neutralgas decreases the rotational velocities.

ON THE COMPRESSION WAVE GENERATED WHEN A HIGH-SPEED TRAIN ENTERS A TUNNEL WITH A FLARED PORTAL

An analysis is made of the compression wave generated when a high-speed train enters a tunnel with a flared portal. Nonlinear steepening of the wavefront in a very long tunnel is responsible for an intense, environmentally harmful, micro-pressure wave , which propagates as a pulse from the distant tunnel exit when the compression wave arrives, with amplitude proportional to the maximum gradient in the compression wavefront. The compression wave profile can be determined analytically for train Mach numbers M satisfying M2⪡1, by regarding the local flow near the tunnel mouth during train entry as incompressible . In this paper, the influence of tunnel portal flaring on the initial thickness of the compression wave is examined first in this limit. The shape of the flared portal is “optimal” when the pressure gradient across the front is constant and an overall minimum, so that the pressure in the wavefront increases linearly . This linear behaviour is shown to occur for a flared portal extending a distance ℓ into the tunnel from the entrance plane (x=0) only when the tunnel cross-sectional area S (x) satisfiesS (x)A=1[A/AE−(x/ℓ)(1−A/AE)], −ℓ<x<0,where x increases negatively with distance into the tunnel, A is the cross-sectional area in the uniform section of the tunnel (x<−ℓ), and AEis the tunnel entrance cross-section. The optimum portal is achieved by adjusting the value of A/AEto make the pressure gradient continuous, and a formula is derived for this ratio for tunnels of semi-circular cross-section. For optimal flaring, the pressure rises linearly as the front of the train traverses the flared section of length ℓ, and the thickness of the compression wavefront ∼ℓ/M .A formula is proposed for extrapolating these predictions to train Mach numbers as large as 0·4, which is expected to be typical of future high-speed rail operations. It is validated for the special case of a circular cylindrical tunnel, for which an exact solution is known for arbitrary subsonic Mach numbers, and by comparison with scale model experiments using trains of various nose profiles.

Laser-induced spatial structures in a chemically active gas-phase medium illuminated by annular diffracting beams

The laws governing the propagation of diffracting beams in a two-component gaseous mixture are investigated on the basis of a computer simulation. It is discovered that several regions with high concentrations of the reaction product form. It is also found that the beam focus can move both in the direction of beam propagation and in the counter direction, and the change in its position can occur abruptly. It is shown that as time goes on, a high concentration of the reaction product is achieved in the central unilluminated region of the medium near the entrance cross section on account of diffusion (i.e., a diffusional concentration growth occurs). This leads to the formation of a dynamic concentration maximum in this region of the medium.

Spatial distribution of neutrons guided through a monolithic tapered lens

The cold neutron transmission through a monolithic polycapillary lens is studied by scanning a pinhole beam across the entrance cross section of the lens. The relative transmission as a function of bending radius of a tapered channel is obtained. The results are explained in terms of a reduced critical angle for total reflection for the tapered channels. A technique for qualitative examination of monolithic lenses is proposed.