Channel Of Rectangular Cross-section Research Articles

Detonation suppression in hydrogen–air mixtures using porous coatings on the walls

We considered the problem of detonation suppression and weakening of blast wave effects occurring during the combustion of hydrogen–air mixtures in confined spaces. The gasdynamic processes during combustion of hydrogen, an alternative environmentally friendly fuel, were also considered. Detonation decay and flame propagation in hydrogen–air mixtures were experimentally investigated in rectangular cross-section channels with solid walls and two types of porous coatings: steel wool and polyurethane foam. Shock wave pressure dynamics inside the section with porous coating were studied using pressure sensors; flame front propagation was studied using photodiodes and high-speed camera visualization. For all mixtures, the detonation wave formed before entering the section with porous coating. For both porous materials, the steady detonation wave decoupled in the porous section of the channel into a shock wave and flame front propagating with a velocity around the Chapman–Jouguet acoustic velocity. By the end of the porous section, shock wave pressure reductions of 70 and 85% were achieved for the polyurethane foam and steel wool, respectively. The dependence of the flame velocity on the mixture composition (equivalence ratio) is presented.

Investigation on Convergence – Divergence Nozzle Shape for Microscale Channel in Harvesting Kinetic Energy

This paper presents performance evaluation of nozzle shapes on microscale channel by employing different types of NACA airfoils profile and conventional profile. The deploying nozzle used are NACA 0012, NACA 0021 and NACA 0024 airfoils while for conventional convergence-divergence nozzle diameter ratio (d2 / d1) in the range from 1/4 to 3/4 are applied. These nozzles are assembled on rectangular cross sectional microscale channel which has designated constant fluid flow velocity at the channel inlet. This study revealed reduction on diameter ratio increased dramatically fluid velocity but further reduction on diameter ratio exposed fluid flow to fluctuate which slightly slowing down the fluid velocity. Nevertheless, curved NACA profiles are favourable for convergence – divergence nozzle in microscale channel as it significantly improved flow characteristics by enhancing fluid velocity and resultant kinetic energy as compared to conventional profile.

Critical regime of gravity currents flowing in non-rectangular channels with density stratification

We present theoretical and experimental analyses of the critical condition where the inertial–buoyancy or viscous–buoyancy regime is preserved in a uniform-density gravity current (which propagates over a horizontal plane) of time-variable volume ${\mathcal{V}}=qt^{\unicode[STIX]{x1D6FF}}$ in a power-law cross-section (with width described by $f(z)=bz^{\unicode[STIX]{x1D6FC}}$, where $z$ is the vertical coordinate, $b$ and $q$ are positive real numbers, and $\unicode[STIX]{x1D6FC}$ and $\unicode[STIX]{x1D6FF}$ are non-negative real numbers) occupied by homogeneous or linearly stratified ambient fluid. The magnitude of the ambient stratification is represented by the parameter $S$, with $S=0$ and $S=1$ describing the homogeneous and maximum stratification cases respectively. Earlier theoretical and experimental results valid for a rectangular cross-section ($\unicode[STIX]{x1D6FC}=0$) and uniform ambient fluid are generalized here to a power-law cross-section and stratified ambient. Novel time scalings, obtained for inertial and viscous regimes, allow a derivation of the critical flow parameter $\unicode[STIX]{x1D6FF}_{c}$ and the corresponding propagation rate as $Kt^{\unicode[STIX]{x1D6FD}_{c}}$ as a function of the problem parameters. Estimates of the transition length between the inertial and viscous regimes are also derived. A series of experiments conducted in a semicircular cross-section ($\unicode[STIX]{x1D6FC}=1/2$) validate the critical values $\unicode[STIX]{x1D6FF}_{c}=2$ and $\unicode[STIX]{x1D6FF}_{c}=9/4$ for the two cases $S=0$ and $1$. The ratio between the inertial and viscous forces is determined by an effective Reynolds number proportional to $q$ at some power. The threshold value of this number, which enables a determination of the regime of the current (inertial–buoyancy or viscous–buoyancy) in critical conditions, is determined experimentally for both $S=0$ and $S=1$. We conclude that a very significant generalization of the insights and results from two-dimensional (rectangular cross-section channel) gravity currents to power-law cross-sections is available.

Advancements on the propagation mechanism of a detonation wave in an obstructed channel

Utilising a recently developed technique, involving high-speed schlieren photography shot through a soot-coated glass sheet, new details of the propagation of combustion waves in obstructed channels have been revealed. In this study, a channel equipped with 50% blockage ratio obstacles was used to examine the repeated detonation initiation and failure processes responsible for the large CJ detonation velocity deficits observed in the quasi-detonation regime. Using a combination of simultaneous schlieren images, soot foil records, and average velocity measurements, experiments were carried out in mixtures of stoichiometric hydrogen–oxygen at initial pressures between 9 kPa and 30 kPa in a 3.66 m long, 7.62 cm by 2.54 cm rectangular cross-section channel. Results indicate continuous detonation propagation through the core of the channel for sufficiently reactive mixtures, while fast-flame propagation occurred for weaker mixtures which do not exhibit detonation initiation at the obstacle face. Two unique propagation modes, one symmetrical and one asymmetrical about the channel centreline, were found to occur at the DDT limit that resulted in average combustion wave velocities between that of the fast-flame and product speed of sound. Local detonation initiation at the obstacle face, following shock reflection, was found to be governed by both the incident shock strength and the distance between the lead shock and trailing flame. For sufficient shock strength and shock-flame spacing, the resulting detonation waves produce a shock interaction at the channel centreline that results in the formation of an axially propagating overdriven detonation that decays in strength with distance. For these quasi-detonations, the average wave velocity over many obstacles is governed by the frequency of these detonation initiation events. These centreline detonation initiation events were typically symmetrical across the channel width, producing a bell-shaped cellular region on the soot foil. However, asymmetrical detonation initiation events, originating at one sidewall, were also observed that produced a narrow vertical band of fine cells resulting from the head-on collision of the detonation wave propagating transversely through the compressed gas region between the lead shock and flame.

Characterization of Flow Structures Induced by Highly Rough Surface Using Particle Image Velocimetry, Proper Orthogonal Decomposition and Velocity Correlations

High Reynolds number flow inside a channel of rectangular cross section is examined using Particle Image Velocimetry. One wall of the channel has been replaced with a surface of a roughness representative to that of real hydropower tunnels, i.e. a random terrain with roughness dimensions typically in the range of ≈10% - 20% of the channels hydraulic radius. The rest of the channel walls can be considered smooth. The rough surface was captured from an existing blasted rock tunnel using high resolution laser scanning and scaled to 1:10. For quantification of the size of the largest flow structures, integral length scales are derived from the auto-correlation functions of the temporally averaged velocity. Additionally, Proper Orthogonal Decomposition (POD) and higher-order statistics are applied to the instantaneous snapshots of the velocity fluctuations. The results show a high spatial heterogeneity of the velocity and other flow characteristics in vicinity of the rough surface, putting outer similarity treatment into jeopardy. Roughness effects are not confined to the vicinity of the rough surface but can be seen in the outer flow throughout the channel, indicating a different behavior than postulated by Townsend’s similarity hypothesis. The effects on the flow structures vary depending on the shape and size of the roughness elements leading to a high spatial dependence of the flow above the rough surface. Hence, any spatial averaging, e.g. assuming a characteristic sand grain roughness factor, for determining local flow parameters becomes less applicable in this case.

Пространственные эффекты при взаимодействии ударной волны с продольным каналом газа пониженной плотности

AbstractThree-dimensional interaction of a shock with lateral low-density gas channel of round, elliptic or rectangular cross-section is numerically studied using Euler’s equations. The structure of formed shock wave precursor is described in detail. Internal shear layer instabilities in three-dimensional flow are shown to develop faster than in axisymmetric case. Moderate amplification of high-pressure jet cumulation effect is noted for elliptic and rectangular channel cases. Dependence of precursor growth rate on cross-section shape is studied. It is found that stretching of cross-section shape significantly increases the duration of linear precursor growth phase.

Flow Electrification of Liquids in Rectangular Channels—Comparison of Different Theoretical Models

This paper deals with flow electrification phenomenon of liquids in channels of rectangular cross section. Different theoretical models are described and compared. For all the models, it is assumed that the flow and the diffuse layer are fully developed. The space charge density conveyed by the flow is computed. First, two cases are examined in the case of weak space charge density, the exact rectangular channel solution is compared with the approximate solution of two parallel planes. This comparison shows a rather small difference between the two models. Then, in the case of two parallel planes assumption, the charge conveyed is computed without any hypothesis on the magnitude of the space charge density and compared to the solution obtained for a weak space charge density commonly assumed. This comparison shows a big difference between the two models concerning the determination of the space charge density on the wall, and, therefore, the zeta potential [1] .

Analytical solution of hydrodynamics and heat exchange problem in a porous rectangular channel for thermal boundary conditions of the second kind

In a three-dimensional formulation, an analytical solution of the Darcy-Brinkman equation is obtained together with the two-temperature Schumann model equations for the unidirectional flow of a Newtonian medium in a laminar regime through a horizontal porous channel of a constant rectangular cross-section with known thermal flows at the boundary for small Darcy numbers. The analysis of the specific structure of a compact porous heat exchanger is presented, where the possibilities of the obtained solution for justification its geometrical dimensions during the cooling process of the surface with a heat release of 100 W/cm2 are demonstrated.

Numerical Investigation of Shock-Train Response to Inflow Boundary-Layer Variations

A dataset of normal shock trains in a rectangular cross-section channel has been created from direct numerical simulations in an effort to quantify the impact of inflow confinement ratio on the shock-train structure. To this end, the inlet boundary-layer momentum thickness was varied while the bulk inflow and outflow conditions remained constant. The simulations show that the shock train is displaced upstream as the inflow confinement ratio increases. Also, an increase in boundary-layer momentum thickness results in a reduction of the normal-like portion of the lambda-shock structures in the channel core. This leads to more numerous but weaker bifurcating shocks as well as an increase of the shock-train length. When the inflow boundary-layer thickness is varied temporally, the complex shock-train response depends on the excitation frequency. A resonant frequency is observed when different components of the shock train exhibit the highest amplification in terms of pressure jumps. Further, the domain upstream of the leading shock acts as a low-pass filter, which has the end result of limiting the axial shock-train motion. Nevertheless, even a small oscillation in boundary-layer momentum thickness of 0.45 mm (0.6% of the channel height) is seen to increase the shock-train length by two orders of magnitude (4 cm).

Numerical study of thermohydraulic performance of solar air heater duct equipped with novel continuous rectangular baffles with high aspect ratio

Turbulent flow and convective heat transfer of air inside channel of rectangular cross-section, containing rectangular baffles with inclined upper part planted on the opposite surface of absorber plate is investigated numerically under solar air heater boundary conditions. For a fixed value of heat flux (1000 W/m2) and the range of Reynolds number from 4000 to 18,000, the effect of four baffle blockage ratios, (BR = 0.7, 0.82, 0.92, 0.98) and four baffle-pitch spacing ratios, (PR = 2, 4, 6, 8) in sixteen configurations on thermohydraulic behavior were confirmed.By means of commercial CFD code Fluent 6.3, the Reynolds average Navier Stokes formulation was computed with RNG k-ε model to simulate the fully turbulent air flow through a baffled rectangular duct. However, the configuration of BR = 0.7 and PR = 2 at a Re = 5000, yields the highest thermohydraulic performance factor THPF of about 0.857, with both increment in heat transfer and friction factor, which noted to be 2.16 and 15.95 times of those of the smooth duct, respectively.Attempts were carried out to explain the mechanisms of fluid behavior in the presence of this type of obstacles and their impact on both fields, thermal and dynamic.

Constant Property Newtonian Fluid Flow and Heat Transfer overCascaded Fins

Turbulent flow and heat transfer characteristics were studied and analyzed numerically for a constant property Newtonian fluid flowing through a two-dimensional horizontal rectangular cross section channel with staggered, transverse cascaded rectangular-triangular fins (CRTFs) and a constant temperature along both walls. The governing equations based on model used to describe the turbulence phenomenon, are solved by the finite volume method using the SIMPLEC-algorithm. Computations were carried out in the fully-developed regime for different Reynolds numbers, and geometric locations. In particular, velocity and temperature fields, skin friction coefficient, and local and average Nusselt numbers were obtained. This study can be a real application in the field of heat exchangers and air plane solar collectors.

Ice Jams in Straight and Sinuous Channels: Insights from Small Flumes

AbstractThis paper presents insights from two small flumes used to compare ice conveyance and jam initiation between straight and sinuous channels of rectangular cross section, and to investigate i...

An Analytical Solution to Neumann-Type Mixed Boundary Poiseuille Microfluidic Flow in Rectangular Channel Cross-Sections (Slip/No-Slip) including a Numerical Technique to Derive It

In most microfluidic applications, pressure-driven Poiseuille flow in a contained cross-section with no-slip boundary conditions is the underlying fluid-mechanical model. Solutions for this problem exist for many known cross-sections. We have recently demonstrated a simple method to solve the relevant Poisson equation using a finite difference scheme in a spreadsheet analysis tool such as Microsoft Excel. The numerical solutions obtained from such a spreadsheet are close-to-exact to the analytical solutions with errors on the order of only a few percent. However, there are numerous applications in microfluidics for which the no-slip boundary condition is not valid. Examples include drag-reducing air-retaining surfaces as well as open-channel flow. For these scenarios few to no analytical models exist. In this paper, we derive an analytical model for mixed boundary conditions (slip/no-slip) in two dimensions in a rectangular channel cross-section. We also demonstrate that the equivalent numerical solution can be derived conveniently by adaption of the spreadsheet. In general, mixed boundary-type flow scenarios are especially difficult to solve analytically whereas numerical solutions can be derived using Microsoft Excel within seconds.

Thin-film flow in helically wound shallow channels of arbitrary cross-sectional shape

We consider the steady, gravity-driven flow of a thin film of viscous fluid down a helically wound shallow channel of arbitrary cross-sectional shape with arbitrary torsion and curvature. This extends our previous work [D. J. Arnold et al., “Thin-film flow in helically-wound rectangular channels of arbitrary torsion and curvature,” J. Fluid Mech. 764, 76–94 (2015)] on channels of rectangular cross section. The Navier-Stokes equations are expressed in a novel, non-orthogonal coordinate system fitted to the channel bottom. By assuming that the channel depth is small compared to its width and that the fluid depth in the vertical direction is also small compared to its typical horizontal extent, we are able to solve for the velocity components and pressure analytically. Using these results, a differential equation for the free surface shape is obtained, which must in general be solved numerically. Motivated by the aim of understanding flows in static spiral particle separators used in mineral processing, we investigate the effect of cross-sectional shape on the secondary flow in the channel cross section. We show that the competition between gravity and inertia in non-rectangular channels is qualitatively similar to that in rectangular channels, but that the cross-sectional shape has a strong influence on the breakup of the secondary flow into multiple clockwise-rotating cells. This may be triggered by small changes to the channel geometry, such as one or more bumps in the channel bottom that are small relative to the fluid depth. In contrast to the secondary flow which is quite sensitive to small bumps in the channel bottom, the free-surface profile is relatively insensitive to these. The sensitivity of the flow to the channel geometry may have important implications for the design of efficient spiral particle separators.

Investigation of heat transfer in liquid-metal flows under fusion-reactor conditions

The effect discovered in studying a downward liquid-metal flow in vertical pipe and in a channel of rectangular cross section in, respectively, a transverse and a coplanar magnetic field is analyzed. In test blanket modules (TBM), which are prototypes of a blanket for a demonstration fusion reactor (DEMO) and which are intended for experimental investigations at the International Thermonuclear Experimental Reactor (ITER), liquid metals are assumed to fulfil simultaneously the functions of (i) a tritium breeder, (ii) a coolant, and (iii) neutron moderator and multiplier. This approach to testing experimentally design solutions is motivated by plans to employ, in the majority of the currently developed DEMO blanket projects, liquid metals pumped through pipes and/or rectangular channels in a transvers magnetic field. At the present time, experiments that would directly simulate liquid-metal flows under conditions of ITER TBM and/or DEMO blanket operation (irradiation with thermonuclear neutrons, a cyclic temperature regime, and a magnetic-field strength of about 4 to 10 T) are not implementable for want of equipment that could reproduce simultaneously the aforementioned effects exerted by thermonuclear plasmas. This is the reason why use is made of an iterative approach to experimentally estimating the performance of design solutions for liquid-metal channels via simulating one or simultaneously two of the aforementioned factors. Therefore, the investigations reported in the present article are of considerable topical interest. The respective experiments were performed on the basis of the mercury magneto hydrodynamic (MHD) loop that is included in the structure of the MPEI—JIHT MHD experimental facility. Temperature fields were measured under conditions of two- and one-sided heating, and data on averaged-temperature fields, distributions of the wall temperature, and statistical fluctuation features were obtained. A substantial effect of counter thermo gravitational convection (TGC) on averaged and fluctuating quantities were found. The development of TGC in the presence of a magnetic field leads to the appearance of low-frequency fluctuations whose anomalously high intensity exceeds severalfold the level of turbulence fluctuations. This effect manifest itself over a broad region of regime parameters. It was confirmed that low-energy fluctuations penetrate readily through the wall; therefore, it is necessary to study this effect further—in particular, from the point of view of the fatigue strength of the walls of liquid-metal channels.

Flame front propagation in a channel with porous walls

Propagation of the detonation front in hydrogen-air mixture was investigated in rectangular cross-section channels with sound-absorbing boundaries. The front of luminescence was detected in a channel with acoustically absorbing walls as opposed to a channel with solid walls. Flame dynamics was recorded using a high-speed camera. The flame was observed to have a V-shaped profile in the acoustically absorbing section. The possible reason for the formation of the V-shaped flame front is friction under the surface due to open pores. In these shear flows, the kinetic energy of the flow on the surface can be easily converted into heat. A relatively small disturbance may eventually lead to significant local stretching of the flame front surface. Trajectories of the flame front along the axis and the boundary are presented for solid and porous surfaces.

Numerical Simulation of Multidimensional Modes of Gaseous Detonation

ABSTRACTProcesses observed in experiments with the initiation and propagation of divergent detonation in a narrow gap between plates was simulated. The detonation wave is formed in a round tube adjoined to one of the plates at a right angle. Complicated cellular structures of the 2D detonation have been obtained. The process of formation of 3D cellular detonation in channels of square, rectangular, circular, and elliptical cross sections was studied. The process of spontaneous formation of spin detonation in channels of circular cross section was considered. Calculations of the transition of spin detonation to the channel of larger or smaller diameter were conducted and conditions of self-sustained spin detonation were formulated. Detonation is initiated in a quiescent combustible mixture by an instantaneous homogeneous electric discharge in a finite-width zone near the flat closed end of the channel. All computations of 3D detonation were performed in a correct formulation for the entire channel of large length (1 m) rather than only in a certain zone moving together with the wave.

Field-flow fractionation in microfluidic channels of low aspect ratio

ABSTRACTThe shape of the steady-state three-dimensional flow velocity profile established in carrier liquid flowing inside the rectangular cross-sectional channel for field-flow fractionation should be taken into account to optimize the separation. The central parts of this profile in the planes parallel to the main channel walls are flat with almost identical flow velocities which drop down to zero at the side walls. The separated species transported by the flow in the close-to-side walls regions move with lower average velocities compared to the species transported in the central part of the channel and are undesirably broadened. The hydrodynamic splitting of the carrier liquid at the entry of the channel where the sample is injected only into the central part of the channel eliminates the excessive zone broadening. The aspect ratio of the breadth to the thickness of the channel ratio can thus be reduced. The effect of various aspect ratios on the shape of the flow velocity profile is calculated and the results are used to optimize the aspect ratio of microfluidic channels. The experiments carried out by microthermal field-flow fractionation confirmed that the aspect ratio cannot be reduced to a value of 1, proposed by other authors.

Gravity currents in a linearly stratified ambient fluid created by lock release and influx in semi-circular and rectangular channels

We present an experimental investigation, supported by a theoretical model, of the motion of lock-release, constant inflow, and time varying inflow gravity currents (GCs) into a linearly stratified ambient fluid at large Reynolds number. The aim is the experimental validation of a simple model able to predict the slumping phase front speed and the asymptotic self-similar front speed for rectangular and circular cross section channels. The first investigated system is of Boussinesq type with the dense current (salt water dyed with aniline) released in a circular channel of 19 cm diameter and 400 cm long (605 cm in the inflow experiments), half-filled of linearly stratified ambient fluid (salt water with varying salt concentration). The second system has the same components but with a channel of rectangular cross section of 14 cm width, 11 cm ambient fluid depth, and 504 cm length. The density stratification of the ambient fluid was obtained with a computer controlled set of pumps and of mixing tanks. For the experiments with inflow, a multi-pipes drainage system was set at the opposite end with respect to the inflow section, computer controlled to avoid the selective withdrawal. The numerous experiments (28 for circular cross section, lock release; 26 for circular and 14 for rectangular cross section, constant inflow (fluid volume ∝tα, with α = 1); 6 for circular cross section, linearly increasing inflow (α = 2)), with several combination of the stratification parameter (0 < S < 1) confirm the theory within ≈30% (≈40% for a single series of experiments), which is considered a good result in view of the various underlying simplifications and approximations. The results on the front speed of the GCs are discussed in the presence of the internal waves, which have a celerity given by a theoretical and experimentally tested model for the rectangular but not for the circular cross section. The theoretical analysis of internal waves in circular cross sections has been extended and experimentally validated.

Acoustic wave effect on a stability of convective flow in a horizontal channel subjected to the horizontal temperature gradient

The effect of acoustic wave propagating in longitudinal direction on stationary convective flow in a horizontal channel of rectangular cross-section in the presence of horizontal temperature gradient and its stability is studied. It is shown that in the presence of sufficiently intensive acoustic wave stationary flow is of coaxial character, the fluid moves in the direction of temperature gradient near sidewalls and in the opposite direction in the interior of the channel. At Pr< ∼0.2 hydrodynamic shear instability mode is most dangerous. This mode is strongly stabilized by acoustic wave at low Prandtl numbers. The range of Prandtl number values where acoustic wave can stabilize the basic flow becomes wider with the channel width growth. At high Prandtl number values the spiral instability modes are most dangerous. The oscillatory spiral mode is slightly stabilized by acoustics. The monotonic spiral mode is destabilized by acoustic wave, especially in the wide channels. In general, for all instability modes the sidewalls suppress the development of perturbations and stationary flow in the channel is more stable than that in plane horizontal layer. In the channel of square cross-section the oscillatory spiral mode is completely suppressed.