scholarly journals Experimental Study on Reynolds Number Evolution of Gas-Filled Coal

Geofluids ◽  
2019 ◽  
Vol 2019 ◽  
pp. 1-10
Author(s):  
Chao Liu ◽  
Minghui Li ◽  
Honggang Zhao
Keyword(s):  
Reynolds Number ◽  
Axial Stress ◽  
Mass Density ◽  
Confining Pressure ◽  
Reynolds Numbers ◽  
Nonlinear Flow ◽  
Equivalent Diameter ◽  
Fluid Viscosity ◽  
Flow State ◽  
Actual Situation

The flow state of gas in coals is very complicated. We should pay attention to whether the permeability calculated by Darcy’s law is in accordance with the actual situation. We conducted an experiment on coal permeability and deformation under fixing confining pressure and increasing axial stress conditions. The objective is to investigate the variation of Reynolds number Re. In this study, the dynamic evolution of the Reynolds number is calculated under the relevant assumptions. The Reynolds number increases with an increase in the axial stress. In addition, the larger the value of initial Reynolds numbers, the greater the value of Re in the postpeak, and the possibility of nonlinear flow state is higher. Further, if the mass density (ρ) and fluid viscosity (μ) are constant, the decrease in the amplitudes of the flow rate is less than the increase in the equivalent diameter of the seepage path. Moreover, the tensile stress generated around the pores and fractures parallel or nearly parallel to the axial stress direction with increase in the axial stress results in an increase in the Reynolds numbers and equivalent diameter of the seepage path increase due to the development, expansion, and penetration of the cracks.

Mixing of Matter by a Falling Spherical Particle in a Still Liquid

2015 ◽  
Author(s):  
Toru Koso
Keyword(s):  
Reynolds Number ◽  
Spherical Particle ◽  
Mixing Time ◽  
Reynolds Numbers ◽  
Fluid Viscosity ◽  
Liquid Viscosity ◽  
Turbulent Diffusion Coefficient ◽  
Number Range ◽  
Particle Reynolds Number ◽  
Photochromic Dye

The mixing of liquid mass caused by a spherical solid particle falling in a still liquid in a pipe was investigated by visualization and noninvasive concentration measurement using a photochromic dye. A spherical particle with diameter of 4.76 mm was dropped in a kerosene-paraffin mixture liquid with a photochromic dye. The photochromic dye was activated by an ultraviolet light and was subjected to the mixing by the particle wake. The falling velocity of particle was changed by using 8 different densities of particle. The effect of the particle Reynolds number on the mixing was investigated for the Reynolds number range from 10 to 2490. The effect of liquid viscosity on the mixing time was also investigated using two liquids having different viscosity. The visualized dye patterns indicated the mixing process depended strongly on the particle Reynolds number. For the Reynolds numbers higher than 300, the particle shed the vortices behind the particle and the dye was mixed isotropically by large-scale vortices. For the Reynolds numbers lower than 300, the dye was drawn straightly by a laminar wake of the particle. The concentration of the mixed dye was measured using the photochromic concentration measuring (PCM) technique to discuss the mass mixing quantitatively. The turbulent diffusion coefficient (TDC) was evaluated for the cases the dye was mixed by the vortices. It was found that the evaluated TDCs showed strong time-dependency, which was attributed to the change in scale and whirling velocity of wake vortices. The maximum TDC depended on the falling velocity regardless of the fluid viscosity. The mixing time depended strongly on the liquid viscosity. The mixing time of the TDC was suggested to be governed by the viscous decay time and expanding time of vortices in the pipe. The amount of dye drift was evaluated for the cases the particle wake was laminar. It was found that the dye drift increased sharply just after the particle passing and then saturated. The final dye drift increased gradually with increasing Reynolds number.

Heat Transfer Effectiveness and Coefficient of Pressure Drop on the Shell Side of a Staggered Elliptical Tubes Bank

2014 ◽  
Vol 493 ◽  
pp. 134-139 ◽  
Author(s):  
Utomo Kukuh W. Budi ◽  
Kamal Samsul ◽  
Suhanan ◽  
I. Made Suardjaja
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Pressure Drop ◽  
Reynolds Numbers ◽  
Equivalent Diameter ◽  
Shell Side ◽  
Low Reynolds Numbers ◽  
Wind Velocities ◽  
Low Reynolds ◽  
Tube Banks

The effectiveness of heat transfer and the pressure drop coefficient of staggered elliptical tube banks are studied experimentally. The bank consists of 11 elliptical tubes of 0.75 equivalent diameter in an arrangement of 4-3-4. The major and the minor sub-axis of each tube are 24.70 mm and 12.35 mm respectively, and therefore the aspect ratio (AR) of the tube is 2.0. The geometric parameters of the bank are ST = 24.70 mm, SL = 37.00 mm and minimum frontal area B = 12.35 mm. Seven mid-tubes are internally heated by electrical heater of 69.6 Watt each. Experiment is conducted in a sub sonic wind tunnel and run with the wind velocities of 1 m/s 12.6 m/s which correspond with Reynolds number of = 346-6904. The results show that the effectiveness (ε) varied from 2144.44 to 15.26. It decreases exponentially at low Reynolds numbers and tended asymptotically at higher Reynolds number. The coefficient of pressure drop (CΔp) ranges from 7.21 to 4.41 decreases continuously at low Reynolds number and asymptotic at higher one.

Reynolds number effects on lipid vesicles

2012 ◽  
Vol 711 ◽  
pp. 122-146 ◽  
Author(s):  
David Salac ◽  
Michael J. Miksis
Keyword(s):  
Reynolds Number ◽  
Stokes Flow ◽  
Scaling Laws ◽  
Circulatory System ◽  
Reynolds Numbers ◽  
Two Dimensions ◽  
Fluid Viscosity ◽  
Maximum Tension ◽  
Wide Range ◽  
Human Circulatory System

AbstractVesicles exposed to the human circulatory system experience a wide range of flows and Reynolds numbers. Previous investigations of vesicles in fluid flow have focused on the Stokes flow regime. In this work the influence of inertia on the dynamics of a vesicle in a shearing flow is investigated using a novel level-set computational method in two dimensions. A detailed analysis of the behaviour of a single vesicle at finite Reynolds number is presented. At low Reynolds numbers the results recover vesicle behaviour previously observed for Stokes flow. At moderate Reynolds numbers the classical tumbling behaviour of highly viscous vesicles is no longer observed. Instead, the vesicle is observed to tank-tread, with an equilibrium angle dependent on the Reynolds number and the reduced area of the vesicle. It is shown that a vesicle with an inner/outer fluid viscosity ratio as high as 200 will not tumble if the Reynolds number is as low as 10. A new damped tank-treading behaviour, where the vesicle will briefly oscillate about the equilibrium inclination angle, is also observed. This behaviour is explained by an investigation on the torque acting on a vesicle in shear flow. Scaling laws for vesicles in inertial flows have also been determined. It is observed that quantities such as vesicle tumbling period follow square-root scaling with respect to the Reynolds number. Finally, the maximum tension as a function of the Reynolds number is also determined. It is observed that, as the Reynolds number increases, the maximum tension on the vesicle membrane also increases. This could play a role in the creation of stable pores in vesicle membranes or for the premature destruction of vesicles exposed to the human circulatory system.

The Effect of Reynolds Number on the Ventilation Efficiency in a Wind Tunnel

2004 ◽  
Author(s):  
Jun Li ◽  
Ibrahim Yavuz ◽  
Ismail B. Celik ◽  
Steven E. Guffey
Keyword(s):  
Reynolds Number ◽  
Turbulence Model ◽  
Three Dimensional ◽  
Concentration Field ◽  
Reynolds Numbers ◽  
Ethanol Vapor ◽  
Vapor Concentration ◽  
Equivalent Diameter ◽  
Three Dimensional Model ◽  
Rng Turbulence Model

The present work is concerned with the effect of the ventilation intensity on the worker exposure in a tunnel when the worker is facing the downstream direction and a gaseous contaminant is released in an arm length of his reach. A three-dimensional model of a manikin which was used in the experiments was created in order to study the effect of the mean inlet velocity which can be characterized by the Reynolds number based on the equivalent diameter of the head of the manikin. For this study, turbulent flow was assumed to enter the ventilation tunnel and exit at the other end from an exhaust duct. The scalar transport method was employed to determine the ethanol vapor concentration field. The results with the low_Re RNG turbulence model are compared to the ones with the RNG turbulence model. The results with the RNG k-ε turbulence model seem to agree better with the experimental data at higher Reynolds numbers. At lower Reynolds numbers there are significant differences between experiments and predictions.

Effects of a High Porous Material on Heat Transfer and Flow in a Circular Tube

2008 ◽  
Vol 131 (2) ◽  
Author(s):  
Koichi Ichimiya ◽  
Tetsuaki Takeda ◽  
Takuya Uemura ◽  
Tetsuya Norikuni
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Porous Material ◽  
Entropy Generation ◽  
Circular Tube ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Working Fluid ◽  
Equivalent Diameter

This paper describes the heat transfer and flow characteristics of a heat exchanger tube filled with a high porous material. Fine copper wires (diameter: 0.5 mm) were inserted in a circular tube dominated by thermal conduction and forced convection. The porosity was from 0.98 to 1.0. The working fluid was air. The hydraulic equivalent diameter was cited as the characteristic length in the Nusselt number and the Reynolds number. The Nusselt number and the friction factor were expressed as functions of the Reynolds number and porosity. The thermal performance was evaluated by the ratio of the Nusselt number with and without a high porous material and the entropy generation. It was recognized that the high porous material was effective in low Reynolds numbers and the Reynolds number, which minimized the entropy generation existed.

Numerical Simulation of Cell Motility at Low Reynolds Number

2012 ◽  
Author(s):  
Thomas F. Scherr ◽  
Chunliang Wu ◽  
W. Todd Monroe ◽  
Krishnaswamy Nandakumar
Keyword(s):  
Numerical Simulation ◽  
Reynolds Number ◽  
Cell Motility ◽  
Reynolds Numbers ◽  
Fluid Viscosity ◽  
Forward Movement ◽  
Fluid System ◽  
Length Scales ◽  
Velocity Fluctuations ◽  
Fluid Movement

As length scales decrease to microns, the mechanism for swimming becomes unfortunately counter-intuitive. In the macro-world, where human intuition has developed, we swim by accelerating the liquid around us. For microorganisms, which swim at Reynolds numbers much less than unity, Stokes law does not permit accelerations. As such, the fluid movement is governed entirely by the local boundaries of the microorganism and the fluid viscosity dampens velocity fluctuations rapidly as distance away from the swimmer increases. A well known byproduct of this, Purcell’s “Scallop Theorem”, forbids reciprocal motions to generate net forward movement [1]. To overcome this, flagella propagate waves down their length and cilia have asymmetric beats. This type of motility has been described as zero-thrust swimming since the net force on the organism-fluid system must be zero [2].

scholarly journals Marginally turbulent flow in a square duct

2007 ◽  
Vol 588 ◽  
pp. 153-162 ◽  
Author(s):  
MARKUS UHLMANN ◽  
ALFREDO PINELLI ◽  
GENTA KAWAHARA ◽  
ATSUSHI SEKIMOTO
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Secondary Flow ◽  
Reynolds Numbers ◽  
Computational Domain ◽  
Flow State ◽  
Square Duct ◽  
Vortex Pattern ◽  
Marginal State ◽  
Secondary Flow Structure

A direct numerical simulation of turbulent flow in a straight square duct was performed in order to determine the minimal requirements for self-sustaining turbulence. It was found that turbulence can be maintained for values of the bulk Reynolds number above approximately 1100, corresponding to a friction-velocity-based Reynolds number of 80. The minimum value for the streamwise period of the computational domain is around 190 wall units, roughly independently of the Reynolds number. We present a characterization of the flow state at marginal Reynolds numbers which substantially differs from the fully turbulent one: the marginal state exhibits a four-vortex secondary flow structure alternating in time whereas the fully turbulent one presents the usual eight-vortex pattern. It is shown that in the regime of marginal Reynolds numbers buffer-layer coherent structures play a crucial role in the appearance of secondary flow of Prandtl's second kind.

Low Reynolds number flow around and heat transfer from a heated circular cylinder

2010 ◽  
Vol 1 (1-2) ◽  
pp. 15-20 ◽  
Author(s):  
B. Bolló
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Lift Coefficient ◽  
Reynolds Numbers ◽  
Low Reynolds Numbers ◽  
Reynolds Number Flow ◽  
Two Dimensional Flow ◽  
Number Flow ◽  
Low Reynolds ◽  
Increasing Temperature

Abstract The two-dimensional flow around a stationary heated circular cylinder at low Reynolds numbers of 50 < Re < 210 is investigated numerically using the FLUENT commercial software package. The dimensionless vortex shedding frequency (St) reduces with increasing temperature at a given Reynolds number. The effective temperature concept was used and St-Re data were successfully transformed to the St-Reeff curve. Comparisons include root-mean-square values of the lift coefficient and Nusselt number. The results agree well with available data in the literature.

Influence of viscosity temperature dependence on the spectral characteristics of the thermoviscous liquids flow stability equation

2019 ◽  
Vol 14 (1) ◽  
pp. 52-58 ◽  
Author(s):  
A.D. Nizamova ◽  
V.N. Kireev ◽  
S.F. Urmancheev
Keyword(s):  
Reynolds Number ◽  
Spectral Characteristics ◽  
Wave Number ◽  
Critical Parameters ◽  
Fluid Viscosity ◽  
Flat Channel ◽  
Stability Equation ◽  
The Stability ◽  
Thermoviscous Fluid ◽  
Sommerfeld Equation

The flow of a viscous model fluid in a flat channel with a non-uniform temperature field is considered. The problem of the stability of a thermoviscous fluid is solved on the basis of the derived generalized Orr-Sommerfeld equation by the spectral decomposition method in Chebyshev polynomials. The effect of taking into account the linear and exponential dependences of the fluid viscosity on temperature on the spectral characteristics of the hydrodynamic stability equation for an incompressible fluid in a flat channel with given different wall temperatures is investigated. Analytically obtained profiles of the flow rate of a thermovisible fluid. The spectral pictures of the eigenvalues of the generalized Orr-Sommerfeld equation are constructed. It is shown that the structure of the spectra largely depends on the properties of the liquid, which are determined by the viscosity functional dependence index. It has been established that for small values of the thermoviscosity parameter the spectrum compares the spectrum for isothermal fluid flow, however, as it increases, the number of eigenvalues and their density increase, that is, there are more points at which the problem has a nontrivial solution. The stability of the flow of a thermoviscous fluid depends on the presence of an eigenvalue with a positive imaginary part among the entire set of eigenvalues found with fixed Reynolds number and wavenumber parameters. It is shown that with a fixed Reynolds number and a wave number with an increase in the thermoviscosity parameter, the flow becomes unstable. The spectral characteristics determine the structure of the eigenfunctions and the critical parameters of the flow of a thermally viscous fluid. The eigenfunctions constructed in the subsequent works show the behavior of transverse-velocity perturbations, their possible growth or decay over time.

scholarly journals Turbulence model performance for ventilation components pressure losses

2021 ◽  
Author(s):  
Karsten Tawackolian ◽  
Martin Kriegel
Keyword(s):  
Reynolds Number ◽  
Turbulence Model ◽  
Pressure Loss ◽  
Low Reynolds Number ◽  
Turbulence Models ◽  
Reynolds Numbers ◽  
Test Case ◽  
Pressure Losses ◽  
Loss Coefficients ◽  
Near Wall

AbstractThis study looks to find a suitable turbulence model for calculating pressure losses of ventilation components. In building ventilation, the most relevant Reynolds number range is between 3×104 and 6×105, depending on the duct dimensions and airflow rates. Pressure loss coefficients can increase considerably for some components at Reynolds numbers below 2×105. An initial survey of popular turbulence models was conducted for a selected test case of a bend with such a strong Reynolds number dependence. Most of the turbulence models failed in reproducing this dependence and predicted curve progressions that were too flat and only applicable for higher Reynolds numbers. Viscous effects near walls played an important role in the present simulations. In turbulence modelling, near-wall damping functions are used to account for this influence. A model that implements near-wall modelling is the lag elliptic blending k-ε model. This model gave reasonable predictions for pressure loss coefficients at lower Reynolds numbers. Another example is the low Reynolds number k-ε turbulence model of Wilcox (LRN). The modification uses damping functions and was initially developed for simulating profiles such as aircraft wings. It has not been widely used for internal flows such as air duct flows. Based on selected reference cases, the three closure coefficients of the LRN model were adapted in this work to simulate ventilation components. Improved predictions were obtained with new coefficients (LRNM model). This underlined that low Reynolds number effects are relevant in ventilation ductworks and give first insights for suitable turbulence models for this application. Both the lag elliptic blending model and the modified LRNM model predicted the pressure losses relatively well for the test case where the other tested models failed.

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