Axisymmetric Sudden Expansion Flow Research Articles

Prediction of local loss coefficient for turbulent flow in axisymmetric sudden expansions with a chamfer: Effect of Reynolds number

This paper reports the pressure losses in turbulent flows through axisymmetric sudden expansions having a slight chamfer on the edge. A parametric study is performed for dimensionless chamfer lengths of 0–0.5, expansion ratios of 2–6, and chamfer angles of 0–45° in a Reynolds number range of 1×105–8×105. The chamfer effect on the expansion losses and its dependence on the Reynolds number are analyzed in detail along with a discussion of the relevant flow features. On the basis of numerical results, an existing correlation of the local loss coefficient is also extended to take into account the effect of the Reynolds number additionally.

Effects of Inlet Centre Line Turbulence on the Turbulent Flow through an Axi-symmetric Sudden Expansion

The turbulent fluid flow through sudden expansion passage has both fundamental scientific interest and numerous practical applications. The engineering applications such as the separated flow associated with pipelines, dump combustors, different ducting devices etc. are examples of the turbulent flow through the sudden expansion. In the present analysis the effect of centre line turbulence on reattachment length in an axi symmetric sudden expansion flow has been investigated. The change of size and strength of recirculation bubble has been studied, which is directly involved with reattachment length.

Prediction of local pressure drop for turbulent flow in axisymmetric sudden expansions with chamfered edge

The present study numerically investigates the turbulent flow in axisymmetric sudden expansions with a chamfered edge. With the aid of commercial CFD software, Fluent 12.0, an extensive set of numerical simulations is carried out for dimensionless chamfer lengths varying from 0.02 to 0.5, expansion ratios between 2 and 6, and chamfer angles of up to 45° at a Reynolds number of 3×105. First, we present and analyze the local pressure drop across the sudden expansions without a chamfer. The dependence of the pressure drop coefficient on the main geometric parameters is then thoroughly investigated. Finally, a new correlation is proposed for the local loss coefficient in sudden expansions with a chamfered edge, along with a discussion of the optimal chamfer angle.

303 円管急拡大流れにおける空間構造の遷移機構

303 円管急拡大流れにおける空間構造の遷移機構

Numerical Simulation of Unsteady Turbulent Flow in Axisymmetric Sudden Expansions

This paper is concerned with the numerical simulation of unsteady turbulent flows behind sudden expansions without inlet swirl. Time dependent simulations have been carried out using the VLES approach with the standard k-ε model. The expansion ratio investigated is in the range from 1.96–6.0. The simulations show that the flows in axisymmetric sudden expansions are inherently unstable when the expansion ratio is above a critical value. The precessing phenomenon, which features self-sustained precession of the global flowfield around the expansion centerline, is predicted successfully using CFD, with simulated oscillation frequencies that are in general agreement with reported data. For the case of expansion ratios from 3.5–6.0, a combination of a precession motion and a flapping motion in a rotating frame of reference is predicted in terms of the jet movement. Large-scale structures are identified in the downstream flowfield. Other important phenomena, such as the transition of the oscillation patterns, have also been predicted.

A PIV study of the laminar axisymmetric sudden expansion flow

The laminar flow through an axisymmetric sudden expansion was investigated experimentally using real-time digital particle image velocimetry. An expansion ratio (downstream-to-upstream pipe diameter ratio) of 2 was selected for the study. The measurements covered the regions of separation, reattachment and re-development. Two dimensional velocity maps were obtained on the vertical center plane for six Reynolds numbers between 20 and 211, based on the upstream pipe diameter and bulk velocity. The stream function distributions are calculated and presented from the streamwise and radial velocity maps. The dependence of reattachment length, redevelopment length and recirculating flow strength on the Reynolds number are determined. Results show that not only the reattachment length but also the redevelopment length downstream of reattachment is a linear function of the Reynolds number. The recirculation eddy strength, on the other hand, has a non-linear dependence on the Reynolds number which becomes weaker as the Reynolds number is increased. The results indicate no instability- or buoyancy-driven flow asymmetry in the range 20?Re? 211.

A general correlation for the local loss coefficient in Newtonian axisymmetric sudden expansions

Results from numerical simulations and guidance from an approximated corrected-theory, developed by Oliveira and Pinho (1997), (Oliveira, P.J. and Pinho, F.T. 1997. Pressure drop coeAcient of laminar Newtonian flow in axisymmetric sudden expansions. Int. J. Heat and Fluid flow 18, 518‐529) have been used to arrive at a correlation expressing the irreversible loss coeAcient for laminar Newtonian flow in axisymmetric sudden expansions. The correlation is valid for the ranges 1.5 < D2/D1 < 4 and 0.5 < Re < 200 with errors of less than 5%, except for 25 < Re < 100 where the error could be as much as 7%. The recirculation bubble length is also presented for the same range of conditions and the pressure recovery coeAcient was calculated for Reynolds numbers above 15. ” 1998 Elsevier Science Inc. All rights reserved.

Pressure drop coefficient of laminar Newtonian flow in axisymmetric sudden expansions

Laminar flow of a Newtonian fluid in an axisymmetric pipe expansion has been studied numerically by means of the finite-volume approach. Predicted values of some of the overall flow characteristics, such as recirculation length, its strength, and centre location, were compared with available experimental data and correlations, and good agreement was found. The purpose of the work was to evaluate the pressure-loss coefficient C l for a range of Reynolds numbers and to compare the results with existing simplified theory, which is based on a one-dimensional (1-D) overall balance of energy and momentum. Considerable differences were found, which lead us to formulate corrected theoretical equations in the scope of the 1-D approximation. These corrections were evaluated from the numerical results and accounted for three effects: (1) differing actual and fully developed wall friction; (2) distortion of velocity profiles from the parabolic shape at the sudden expansion section; and (3) nonuniformity of pressure at the expansion plane. Predicted values of the loss coefficient agreed to within 4% with the corrected theory and were found to be proportional to the inverse of the Reynolds number for Re ≤ 17.5 [with effect (3) above predominant and accounting for up to 85% of C l ] and approximately constant for Re > 17.5 [with effect (1) above predominant and accounting for 20% of C l ]. Finally, a correlation for calculating the local loss coefficient as a function of the Reynolds number for the 1:2.6 sudden expansion and fully developed conditions is proposed.

Pressure drop coefficient of laminar Newtonian flow in axisymmetric sudden expansions

Laminar flow of a Newtonian fluid in an axisymmetric pipe expansion has been studied numerically by means of the finite-volume approach. Predicted values of some of the overall flow characteristics, such as recirculation length, its strength, and centre location, were compared with available experimental data and correlations, and good agreement was found. The purpose of the work was to evaluate the pressure-loss coefficient Cl for a range of Reynolds numbers and to compare the results with existing simplified theory, which is based on a one-dimensional (1-D) overall balance of energy and momentum. Considerable differences were found, which lead us to formulate corrected theoretical equations in the scope of the 1-D approximation. These corrections were evaluated from the numerical results and accounted for three effects: (1) differing actual and fully developed wall friction; (2) distortion of velocity profiles from the parabolic shape at the sudden expansion section; and (3) nonuniformity of pressure at the expansion plane. Predicted values of the loss coefficient agreed to within 4% with the corrected theory and were found to be proportional to the inverse of the Reynolds number for Re ≤ 17.5 [with effect (3) above predominant and accounting for up to 85% of Cl] and approximately constant for Re > 17.5 [with effect (1) above predominant and accounting for 20% of Cl]. Finally, a correlation for calculating the local loss coefficient as a function of the Reynolds number for the 1:2.6 sudden expansion and fully developed conditions is proposed.

The Axisymmetric Sudden Expansion Flow of a Non-Newtonian Viscoplastic Fluid

The flow of a non-Newtonian viscoplastic Bingham fluid over an axisymmetric sudden expansion is studied by numerically solving the governing fully-elliptic continuity and momentum equations. Solutions are obtained for a wide range of Reynolds and yield numbers in the laminar flow regime with constant fluid properties. The present work demonstrates that the finite-difference technique can successfully be employed to obtain solutions to separating/reattaching internal flows of Bingham plastics. The results demonstrate the strong effects of the yield and Reynolds numbers on both the integral and the local structure of the separating and reattaching flow. Higher yield numbers result in larger overall effective viscosities and thus faster flow recovery downstream of the sudden expansion. The reattachment length decreases with increasing yield numbers, eventually reaching an asymptotic nonzero value which, in turn, is dependent on the Reynolds number. The strength of the recirculating flow also decreases with increasing yield numbers.

Turbulent expansion flow of low molecular weight shear-thinning solutions

A Laser-Doppler anemometer and a pressure transducer were used to carry out detailed measurements of the mean and root mean square of the velocity and wall-pressure in an axisymmetric sudden expansion flow, with 0.4 and 0.5% by weight shear-thinning aqueous solutions of a low molecular weight polymer (6,000), after appropriate rheological characterisation. In spite of their very low molecular weight, these solutions still exhibited elongational elastic effects through drag reduction of up to 35% relative to Newtonian turbulent pipe flow, as shown by Pereira and Pinho (1994).

Premixed, turbulent combustion of axisymmetric sudden expansion flows

Velocity and low-frequency combustor pressure oscillations have been measured in a ramjet dump combustor model. The mean and root mean square (rms) values of the turbulent velocity field were obtained using a two-component laser Doppler velocimeter (LDV) system operating in the backscatter mode. Reacting flow data were obtained for premixed propane and air, while isothermal results were collected after replacing the propane with nitrogen. The velocity data indicated substantial differences between the two cases. Combustor pressure oscillation data were also obtained. The intensity and frequency of the oscillations were found to be dependent on the inlet velocity, combustor length, and equivalence ratio. Results showed that pressure oscillations were controlled by both vortex kinematics in the combustor and acoustic response of the inlet section.

Variable density effects on axisymmetric sudden-expansion flows

The flow through an axisymmetric sudden expansion is investigated numerically. A low-Reynolds-number full Reynolds stress closure that can account for viscous effects near a wall and thus avoids the use of wall functions is used to close the time-averaged Navier-Stokes equations. The calculated results are compared with recent laser Doppler measurements and show that the recirculating flow and reattachment behavior are correctly predicted by the low-Reynolds-number full Reynolds stress model. Since density variation complicates mixing, coaxial flow of two different gases into an axisymmetric sudden expansion is also investigated using the full Reynolds stress model, but with the model modified to account for variable density effects. The scalar field away from the wall is calculated by solving the modelled scalar flux transport equations, but in the near-wall region, the scalar field is approximated by a constant Schmidt number. Good agreement is obtained between predictions and measurements. However, the comparison indicates that the additional terms due to density variation in the momentum and scalar flux transport equations are of importance near a wall and the governing equations for the scalar field should be modified to account for near-wall viscous dissipation effects because turbulence in this region is not isotropic.

Rotation effects on axisymmetric sudden-expansion flows

The dump combustor of a ramjet propulsion system was simulated by a Plexiglas tube of 63.5 mm I.D. and 76.2 cm long. A convergent nozzle with an exit diameter of 43.2 mm was fitted to the inlet of the tube. Therefore, this arrangement provided a sudden expansion and gave a step height of 10.15 mm. Tube rotation effects on mixing in the dump combustor were examined by rotating the Plexiglas tube about its own axis at a constant speed. The flow downstream of the sudden expansion was studied using a one-color, one-component, laser-Doppler anemometer. Two conditions were investigated in detail, one with the tube rotating at 840 rpm and another with the tube stationary. In addition, three other experiments with different tube rotational speeds were carried out to determine the dependence of reattachment length on rotation. The average inlet velocity was set at ~ 10.6 m/s for all experiments. The results show that the primary reattachment length decreased as rotation increased because of the destabilizing effect due to the large shear created by the rotating tube wall. A detailed comparison of the flow characteristics with and without rotation present is made, and the effects of rotation on the flow are analyzed.

Turbulent mass flux measurements using a laser/hot-wire technique

A laser-Doppler anemometer and a hot-wire type concentration probe are used to measure the velocity and species concentration fields simultaneously in an axisymmetric sudden-expansion flow. The presence of the probe has a negligible effect on the Doppler signals. Furthermore, the presence of small liquid droplets in the flow does not affect the hot-wire measurements. Resulting measurements of axial velocity and species concentration are compared with corresponding data obtained by applying the LDA and hot-wire probe separately to measure the flow properties. Excellent agreement is obtained. Therefore, the measured mass flux is valid and reliable. The technique is used to examine the behavior of the turbulent mass flux in an axisymmetric sudden-expansion flow.

Inlet centerline turbulence effects on reattachment length in axisymmetric sudden-expansion flows

The important parameters that affect the reattachment length in an axisymmetric sudden-expansion flow are examined. It is found that inlet centerline turbulence stands out as the most important; namely increased inlet centerline turbulence causes the reattachment length to decrease.

Rotation effects on inhomogeneous mixing in axisymmetric sudden-expansion flows

Rotation effects on variable-density co-axial jet mixing in an axisymmetric sudden-expansion geometry are investigated experimentally. A central air jet surrounded by an annular helium/air jet is injected into a circular tube with an enlarged cross-sectional area. Two cases are examined in detail; one stationary and another with a constant speed of rotation of 840 r.p.m. about the tube axis. Results show that mixing between helium and air is greatly enhanced by rotation. Consequently, the recirculation region downstream of the sudden expansion is reduced and the reattachment length is decreased by about a factor of three. However, the flow at 30 step heights downstream of the sudden expansion is essentially identical for both the stationary and rotation cases.

A contribution on the effects of inlet conditions when modelling stenoses using sudden expansions

Using three diameter ratios, and up to four inlet velocity profiles, the laminar flow in axisymmetric sudden expansions is investigated numerically. For inlet Reynolds numbers of 250 or lower, the vortex reattachment length is found to be a linear function of the Reynolds number regardless of either the diameter ratio or inlet velocity profile; however, the inlet velocity profile plays an important role in contributing to a non-linear relationship between the vortex length as a function of the expansion step height. The wall shear-stress distribution in the larger pipe is found to depend strongly upon the diameter ratio, inlet velocity profile and Reynolds number.