Effect Of Wall Confinement Research Articles

Effects of wall confinement on flame topologies and lean blowout characteristics of partially premixed DME/air flames in a gas turbine model combustor

Dimethyl ether (DME) is a promising alternative fuel for gas turbines, considering its renewable nature and potential for large-scale synthesis from CO2 feedstock. However, there are limited studies on the swirl combustion characteristics of DME, particularly regarding the stability performance. This work investigates the effects of wall confinement on flow, mixing, and lean blowout (LBO) characteristics of partially premixed DME/air swirl flames in a gas turbine model combustor. Flow and flame macrostructures are captured using particle image velocimetry and planar laser-induced fluorescence (PLIF) measurements, respectively. The results show remarkable differences in flame topologies between unconfined and confined flames, especially the formation of inner recirculation zone and outer recirculation zone. The LBO limits are measured over various Reynolds numbers and flame dynamics approaching LBO are captured using 10 kHz OH* chemiluminescence imaging. Unconfined flames have unexpectively lower LBO limits (∼0.4) than the LBO limits of confined flames (over 0.5). Approaching LBO, unconfined flames display a quiet blowout mode featuring swirling spiral blow-off behaviors. In contrast, confined flames exhibit a violent blowout mode featuring large-scale quasi-instability behaviors, including periodic events of local extinction, flame lift-off, and re-ignition. Based on the numerical simulations and simultaneous OH- and CH2O-PLIF measurements, the large discrepancy in the measured LBO limits between the two configurations is found to result from different re-ignition processes related to the low-temperature ignition before the final blowout. These findings highlight the crucial roles of wall confinement in determining the stable flame topologies, LBO characteristics, and mechanisms driving the LBO physics for partially premixed DME/air swirl flames.

Wall confinement effects on the dynamics of cavitation bubbles in thin tubes

Cavitation is a common phenomenon in nature and has numerous applications. In contrast to a cavitation bubble in a free domain, a cavitation bubble in a thin tube is restricted by the tube wall, which is expected to significantly affect bubble evolution, but its mechanism is still unclear. In this study, the dynamics of a cavitation bubble in a thin circular tube is studied by numerical simulation, focusing on the confinement effects of the tube. The results show that besides affecting the size and lifetime of the bubble, the confinement effects of the tube lead to the generation of counter jets and a ring jet during the contraction process of the bubble, and the curvature of the two counter jets determines the ring jet's peak velocity. When the bubble deviates from the midpoint of the tube in the axial direction, the two sides of the bubble along the axial direction show asymmetric behaviors, which results in the bubble migrating toward the midpoint. The tube diameter, tube length, liquid viscosity, and initial bubble position can significantly influence the degree of confinement effects, which can be characterized by the variations of several key indicators, such as bubble size, lifetime, degree of deformation, counter jet velocity, ring jet velocity, and axial migration of the bubble.

Confinement effects on energy harvesting by a heaving and pitching hydrofoil

Wall confinement effects on the energy harvesting performance by a flapping hydrofoil (aspect ratio = 4.5) have been investigated in a circulating water flume at a Reynolds number of 50,000. Measurements of hydrodynamic forces are taken for three different confinement configurations (unconfined, one-wall and two-wall confinement) and a series of confinement levels for each configuration. Compared with the unconfined situation, a significant improvement of efficiency performance is obtained for strong two-wall confinement due to the enhancement of the hydrodynamic forces, while only a modest increase is observed in the one-wall confinement configuration. Results show that the heave component of efficiency is the primary contributor to the performance improvement. A parametric study, varying reduced frequency and pitching amplitude, shows that as the confinement increases, the optimal energy harvesting occurs at larger values of the reduced frequency and pitching amplitude, and can reach a Lanchester–Betz efficiency as high as 50%.

PENINJAUAN EFEK PENGEKANGAN DINDING PADA HUBUNGAN KOLOM , BALOK DAN DINDING AKIBAT BEBAN BERULANG LATERAL SERTA IMPLEMENTASINYA PADA PEMBANGUNAN RUMAH TAHAN GEMPA DI KABUPATEN ACEH BESAR

The function of wall which arranged from brick even from concrete in function as partition could increase strength and durability of frame structure, but actually the contribution of wall in increasing strength and ductility is often ignored. It is happened because of lack of acknowledge of behavior of frame and wall caused of axial loading that consist of cyclic lateral loading as the effect of wind load and earthquake.. In this case study it was aimed to observe the effect of wall confinement at column, beam and wall joint caused of lateral cyclic loading and it implementation on earthquake resistant housing building in Aceh Besar sub district. The strength and ductility increasing on the frame with wall and without wall (Husna, 2008) proofed that there were contribution of the increasing of strength and ductility of frame. Drift ratio of frame with wall without bed joint was 0.74. % and drift ratio of frame with wall and bed joint was 0.84 %.. In the other hand drift ratio of portal without wall was 44.85 %.Key words : strength, ductility, frame, full wall, partial wall, axial cyclic loading

Effects of shear-thinning nature and opposing buoyancy from a square geometry in a confined framework

ABSTRACTNumerical computations were brought up to study the effect of opposing buoyancy mixed convection (Ri = 0 to − 1) flow of power law shear-thinning fluids past a confined cooled square bluff body at Prandtl numbers (Pr) = 1, 50 and Reynolds numbers (Re) = 1–40. Irrespective of the n, Ri, Pr, and Re, the flow separation is delayed with increasing confinement (β). The vortex shedding and flow separation start earlier for shear-thinning fluids than Newtonian fluid. For opposing buoyancy (Ri < 0), the vortex shedding starts earlier on increasing Pr (except for n = 0.2, Ri = −1). Also, the periodic unsteady transition appears at some higher value of Re on increasing Ri for fixed Pr. The drag coefficient (CD) value reduces with the decrease in n, whereas the maximum CD is noted for Newtonian fluids. The maximum augmentation in the heat transfer was reached about 9 and 36% on comparing with Newtonian fluids and forced convection case, respectively, and also the corresponding maximum compression in heat transfer was found about 15 and 5%, respectively. The numerical results have also been correlated for CD and the Colburn jh factor values for various Re, Pr, Ri, and n. In surplus, the effects of wall confinement ranging from β = 25 to 50% on flow separation and engineering output parameters were studied in a steady regime.

Effects of wall confinement and rheology of non‐Newtonian nanofluids on mixed convection phenomenon of a square cylinder in a vertical channel

AbstractThe mixed convective momentum and heat transfer phenomena of confined square cylinders in non‐Newtonian nanofluids are numerically investigated. The experimental thermophysical properties of alumina‐water‐based nanofluids are adopted from literature and these nanofluids obey shear‐thinning power‐law type non‐Newtonian behavior. The square cylinder is confined in a vertical channel with a confinement ratio of 0.1333. The flow is assumed to be two‐dimensional and the fluid is allowed to flow in upward direction across the confined square cylinder in the vertical channel. The aiding/opposing buoyancy in the flow is incorporated in terms of Richardson number (Ri) in the range of –2 to 2. The ranges of other dimensionless parameters considered are: Reynolds number, Re: 1 to 40; and volume fraction of nanoparticles, ϕ: 0.005 to 0.045. This range of volume fraction of nanoparticles (i.e., ϕ = 0.005 to 0.045) corresponds to the power‐law index (n) of a non‐Newtonian nanofluid in the range of n = 0.88 to 0.5, respectively. Prior to obtaining new results, the solution methodology is validated with existing literature counterparts. Finally, effects of the Reynolds number, Richardson number, and the rheology of non‐Newtonian nanofluids on streamline patterns, surface pressure, surface vorticity, drag coefficients, isotherm contours, local and average Nusselt numbers are delineated.

Flame size measurements of premixed propane-air mixtures ignited by microwave-enhanced plasma

The effects of microwave-enhanced plasma ignition on unsteady, expanding, premixed propane-air flames were studied using Schlieren technique in a constant volume chamber at various equivalence ratios and different microwave exposure durations. The plasma was generated by a spark discharge in engine-like conditions. Under these conditions, the dominant factors affecting the flame were stretching, heat transfer, and wall confinement effects. The spark was enhanced using microwaves, which accelerated the electrons in the spark, generating a non-thermal plasma. The flame size, flame speeds were computed from the visualized flame fronts. The experimental results indicated that an unsteady ignition kernel precedes the flame propagation and that there are multiple regimes prior to the final steady regime, in which the flame propagation is sustained. There is a minimum flame size which spark kernel must achieve, below which the flame could not sustain for successful combustion. The measured minimum flame size is a function of mixture equivalence ratio, and it decreases with increasing equivalence ratio. Increasing the microwave energy resulted in higher initial flame velocity and a reduction in the time taken to reach the critical flame size. Measurement of microwave-enhanced plasma size at non-reactive conditions indicated that plasma size is larger than spark-only case. At lean mixture conditions, enhancing the spark by microwave addition proven to enlarge the size of the ignition kernel, and achieve the required minimum flame size, that successfully helps in transitioning the initial spark kernel to a propagating flame, whereas spark-only operation failed to sustain the flame.

Self-propelled swimming of a flexible plunging foil near a solid wall

Numerical simulations are conducted to investigate the influences of a solid wall on the self-propelled swimming of a flexible plunging foil. It is found that the presence of a solid wall enhances the cruising speed, with the cost of increasing input power. Rigid foil can achieve high percentage increase in cruising speed when swimming near a solid wall, but the propulsive efficiency may be reduced. Foils with some flexibility can enjoy the enhancements in both cruising speed and propulsive efficiency. Another advantage of the flexible foils in near-wall swimming is that smaller averaged lateral forces are produced. The effects of wall confinement on the wake structure and the vortex dynamics are also studied in this paper. The results obtained in this study shed some light on the unsteady wall effect experienced by aquatic animals and also inform the design of bio-mimetic underwater vehicles which are capable of exploiting the wall effect.

Flow and Heat Transfer Phenomena around an Equilateral Triangular Bluff Body: Effect of Wall Confinement

This paper focuses on the effects of wall confinements (or blockage ratios) on the flow and heat transfer characteristics around a long equilateral triangular bluff body placed in a horizontal channel for Reynolds number (Re) range 1 to 80 and blockage ratio range 0.1 to 0.5 for air as the working fluid. The governing continuity, Navier–Stokes and energy equations along with appropriate boundary conditions are solved by using a finite volume method-based commercial solver Ansys Fluent. The total drag coefficient decreases with an increasing value of Re for a fixed value of the blockage ratio; however, it increases with an increasing value of the blockage ratio for a fixed value of Re due to the fact that the channel walls exert an extra retardation force on the obstacle. The onset of flow separation is delayed as the value of the blockage ratio increases. The critical Re (i.e., the transition to a time-periodic regime) exists between 45 and 46, 46 and 47, 58 and 59, and 79 and 80 for the blockage ratios of 0.1, 0.125, 0.25, and 0.5, respectively. The simple correlations for wake length, total drag coefficient, and average obstacle Nusselt number are obtained for the range of conditions covered.

Dynamics of a compound droplet in shear flow

The deformation dynamics of a compound liquid droplet in shear flow is numerically investigated in two- and three-dimensional space. The computational model is based on the immersed boundary method. This accurately and efficiently tracks the interfaces of immiscible multi-phase fluids. We extend a recently developed volume-conserving immersed boundary method for two-phase fluid flow to ternary compound droplet flows. For long time simulations, we also apply a surface remeshing algorithm. Chorin’s projection method is employed, and the resulting system of discrete equations is solved by a multigrid technique. We study the effects of radius, interfacial tension ratios, and inner droplet location on the deformation of a compound droplet, and compute the inclination angles of inner and outer droplets. Simulation results indicate that the angle of the inner droplet is always greater than or equal to that of the outer one. The effect of wall confinement on compound droplet deformation is compared with that of a simple droplet. The result shows that the more confined the wall is, the more different the compound and simple droplets’ behavior.

Momentum and Heat Transfer Phenomena of Confined Spheroid Particles in Power-Law Liquids

Effects of wall confinement and the power-law fluid behavior index on momentum and heat transfer phenomena of confined spheroid particles are numerically investigated over wide ranges of conditions such as the Reynolds number, Re, 1–100; Prandtl number, Pr, 1–1000; particle aspect ratio, e, 0.5–2.5; wall confinement factor, λ, 2–5; and power-law fluid behavior index, n, 0.4–1.8. Some of the key findings include that the size of the recirculation wake at the rear end of the particle increases with the increasing particle aspect ratio and/or Reynolds number and/or wall factor. The total drag coefficient (Cd) decreases with the increasing wall factor and/or Reynolds number and/or the decreasing power-law index. Regardless of values of the Reynolds and Prandtl numbers and regardless of the shape of the spheroid particles, the average Nusselt number increases with the decreasing power-law index and/or with the decreasing wall factor.

Effect of confinement on droplet deformation in shear flow

The dynamics of a single droplet under shear flow between two parallel plates is investigated by using the immersed boundary method. The immersed boundary method is appropriate for simulating the drop-ambient fluid interface. We apply a volume-conserving method using the normal vector of the surface to prevent mass loss of the droplet. In addition, we present a surface remeshing algorithm to cope with the distortion of droplet interface points caused by the shear flow. This mesh quality improvement in conjunction with the volume-conserving algorithm is particularly essential and critical for long time evolutions. We study the effect of wall confinement on the droplet dynamics. Numerical simulations show good agreement with previous experimental results and theoretical models.

Effects of Wall Confinement and Power‐Law Fluid Viscosity on Nusselt Number of Confined Spheres

AbstractEffects of wall confinement on the heat transfer phenomena of confined spherical particles in power‐law fluids are numerically investigated. The spherical particle is held at constant temperature and a power‐law fluid is flowing past the particle in a confining tube. Heat transfer takes place from the heated sphere to the surrounding fluid. The governing conservation equations of mass, momentum, and energy are solved using computational fluid dynamics‐based software. The numerical solver is thoroughly validated by comparing the present average Nusselt numbers with existing literature values. On the basis of numerical results, an empirical correlation for the average Nusselt number of a confined sphere in Newtonian and power‐law fluids is proposed which can be used in new applications.

Microchannel flows with superhydrophobic surfaces: Effects of Reynolds number and pattern width to channel height ratio

Superhydrophobic surfaces are widely adopted for reducing the flow resistance in microfluidic channels. The structures on the superhydrophobic surfaces may consist of longitudinal grooves, transverse grooves, posts, holes, etc. In this paper their effective slip performances are systematically studied and compared in detail through numerical simulations. The numerical results show that channel wall confinement effects have a positive influence on the effective slip length for square posts and longitudinal grooves, and a negative influence for square holes and transverse grooves. Square posts, holes, and transverse grooves all exhibit deteriorating effective slip performances at higher Reynolds numbers, while the effective slip performance of longitudinal grooves remains independent of the Reynolds number. For small pattern width to channel height ratios and at low Reynolds numbers, for low shear-free fractions, the effective slip length of square posts is equivalent of that of transverse grooves, and both geometries yield effective slip lengths which are in turn lower than those of square holes and longitudinal grooves. With increasing shear-free fractions, the effective slip length of square posts surpasses that of square holes and longitudinal grooves, but it becomes lower than that of longitudinal grooves at high Reynolds numbers or large pattern width to channel height ratios. Scaling laws for the effective slip length of superhydrophobic surfaces with square posts, square holes, and transverse grooves have previously been reported for shear-driven flows. This study extends the validity of these scaling laws to pressure-driven channel flows, even at high Reynolds numbers. The findings in this study serve as a useful guide for applications involving the reduction in flow resistance in microchannels containing superhydrophobic surfaces.

Wall boundary model for primitive chain network simulations

In condensed polymeric liquids confined in slit channels, the movement of chains is constrained by two factors: entanglement among the chains and the excluded volume between the chains and the wall. In this study, we propose a wall boundary (WB) model for the primitive chain network (PCN) model, which describes the dynamics of polymer chains in bulk based on coarse graining upon the characteristic molecular weight of the entanglement. The proposed WB model is based on the assumptions that (i) polymers are not stuck but simply reflected randomly by the wall, and (ii) subchains below the entanglement length scale behave like those in bulk even near the wall. Using the WB model, we simulate the dynamics of entangled polymer chains confined in slit channels. The results show that as the slit narrows, the chains are compressed in the direction normal to the wall, while they are expanded in the parallel direction. In addition, the relaxation time of the end-to-end vector increases, and the diffusivity of the center of mass decreases. The compression in the normal direction is a natural effect of confinement, while the expansion is introduced by a hooking process near the wall. The trends revealed that the relaxation time and diffusivity depend on the increase in friction due to an increased number of entanglements near the wall, which is also associated with the hooking process in the PCN model. These results are expected within the assumptions of the PCN model. Thus, the proposed WB model can successfully reproduce the effects of wall confinement on chains.

Effect of blockage ratio on wake transition for flow past square cylinder

Two-dimensional numerical investigations are carried out to study the behavior of wake transition for flow past square cylinder confined between two parallel walls. It is known from past literature that nature of wall confinement affects the behavior of vortex shedding behind the obstacle. The present work reports dependence of the critical Reynolds number ( Re cr ) for onset of planar vortex shedding on blockage ratio. The effect of wall confinement on characteristic features of the wake, such as wake bubble size and shear layer behavior has been studied. The characteristic relationship between Strouhal number and flow Reynolds number has been presented for different values of blockage ratios. The temporal characteristics of the wake are presented.

The Wall and Multivalent Counterion Effects on the Electrostatic Force between Like-Charged Spherical Particles Confined in a Charged Pore

The effect of wall confinement (wall charge and wall–sphere separation distance) on the electrostatic force between two charged spheres confined in a long charged pore in symmetric and asymmetric electrolytes have been quantified by solving the nonlinear Poisson–Boltzmann equation (PBE), using adaptive finite elements combined with error minimization techniques. The computed force indicated the strong effect of the wall potential on the reduction of the repulsive force for all type of electrolytes. The influence of the wall effect was reduced when the valence of the electrolyte was increased. A significant reduction in the repulsive force between the two spheres was also observed when the distance between the pore wall and the sphere surface was reduced. A smaller long-range repulsive interaction was observed between spheres when the solutions contained multivalent counterions as compared with a monovalent solution. However, at short ranges of separation distances multivalent counterions increase the electrostatic repulsive force between the spheres. The effect of the dimensionless radius of the spheres on the electrostatic force between them has been determined and a significant reduction observed as the dimensionless radius was reduced.

Flow about a circular cylinder between parallel walls

The flow about a body placed inside a channel differs from its unbounded counterpart because of the effects of wall confinement, shear in the incoming velocity profile, and separation of vorticity from the channel walls. The case of a circular cylinder placed between two parallel walls is here studied numerically with a finite element method based on the vorticity–streamfunction formulation for values of the Reynolds number consistent with a two-dimensional assumption.The transition from steady flow to a periodic vortex shedding regime has been analysed: transition is delayed as the body approaches one wall because the interaction between the cylinder wake and the wall boundary layer vorticity constrains the separating shear layer, reducing its oscillations. The results confirm previous observations of the inhibition of vortex shedding for a body placed near one wall. The unsteady vortex shedding regime changes, from a pattern similar to the von Kármán street (with some differences) when the body is in about the centre of the channel, to a single row of same-sign vortices as the body approaches one wall. The separated vortex dynamics leading to this topological modification is presented.The mean drag coefficients, once they have been normalized properly, are comparable when the cylinder is placed at different distances from one wall, down to gaps less than one cylinder diameter. At smaller gaps the body behaves similarly to a surface-mounted obstacle. The lift force is given by a repulsive component plus an attractive one. The former, well known from literature, is given by the deviation of the wake behind the body. Evidence of the latter, which is a consequence of the shear in front of the body, is given.

Numerical study of blockage effects on aerodynamic characteristics of an oscillating rectangular cylinder

Abstract The flow around a stationary and oscillating square cylinder was numerically simulated at various blockage ratios in order to study the effects of wall confinement on the aerodynamic characteristics of the cylinder. Numerical methods include Direct Simulation (DS) with upwind scheme for laminar flow at Re = 200, 400, 103, and k−e model for turbulence flow at Re = 4 × 103. The simulations were conducted at various blockage ratios of H L = 0.04, 0.067, 0.085, 0.112, 0.166, 0.244, 0.303 and 0.4 . For stationary cases, simulations were carried out at Reynolds numbers of Re = 200−103. For oscillation cases, simulations were done at Reynolds number of Re = (1, 4) × 103 for a square cylinder oscillating at forced oscillatory frequencies of Stc = 0.1, 0.25 and 0.3. Using the SIMPLE algorithm, simulation results show that drag and lift forces and Strouhal numbers all increase with the increase of blockage ratios (i.e. the decrease of the interval of the two parallel confining walls), and the aerodynamic characteristics agree reasonably well with those from experiments. For stationary cases, the lift and drag forces, and vortex-shedding Strouhal numbers all increase with the increase of blockage ratios. The flow streamlines and vorticity contours show conspicuous features of the wake under high blockage ratios. The first-sinking phenomenon due to blockage effect can be successfully captured at Reynolds numbers of only Re = 103 and can be confirmed to be caused by the change of non-reattachment of separated shear layers to reattachment flow by the blockage effects. For oscillation cases, the lock-in features are successfully captured. The behavior of phase difference, i.e. the stable/unstable features, against the forced oscillatory frequency cannot be almost influenced by the blockage effects.

Effects of wall confinement and temperature on laminar-flame propagation

A premixed laminar flame, freely propagating between parallel plates, was modelled two-dimensionally in terms of the conservation equations for energy and mass species. The flame system was taken to represent the global combustion of a kerosene-air mixture, with chemical kinetics of Arrhenius temperature-dependence and an over-all reaction order of 1·5. The modelling equations were solved numerically to yield the flame speed and two-dimensional flame structure at a certain distance separating the parallel plates. Solutions were obtained at various plate separations and wall temperatures in order to study the effect of wall confinement and temperature on laminar-flame propagation. The results showed that the wall started to affect the flame speed and maximum flame temperature slightly when the plate separation assumed values less than about 1 cm, while the heat-release rate exhibited an approximately linear decrease with the decrease in plate separation throughout the range studied. On the other hand, whereas the axial-flame structure showed only a slight variation with the distance separating the parallel plates, the transverse structure was much affected by the wall confinement and temperature where interesting results were obtained regarding the interaction between the flame and its containing walls.