Flow In Conical Diffuser Research Articles

Analysis of swirling and choking in diffuser using CFD analysis

Effect of swirl on static pressure evolution, total pressure, velocity component recirculation zones and wall shear stress such parameters in conical diffuser flow is studied. Higher pressure recovery is attained with swirl addition at the exit of the diffuser without addition tailpipe. Generally, swirling may not prevent boundary layer separation due to an intermediary recirculation zone appears hence tailpipe is suitable to allow a large-scale mixing for enhancing pressure recovery process. The present study involves the CFD analysis to predict swirl effect in the diffuser which is use in subsonic wind tunnel. 1/8th part of diffuser uses for CFD analysis. Also, it has 50swirl angle, area ratio 2 and length 1.5 m. Reynolds Average Navierstoke’s [RANS] and Shear Stress Transport [SST] use to solve closing problem and turbulence modeling respectively. For solving these numerical simulations, CFX5 Ansys based solver use. Governing equation may solved by finite volume method using Software. Turbulence effect is taken into account employing the k-ε model with an enhanced wall treatment. Mass flow (0.3445) i.e. 25 m/s velocity take as initial boundary condition for steady, incompressible as well as density based flow. Reynolds number kept constant as 30*105. Result show that swirl velocity component develops into a Rankine-vortex type or a Forced-vortex type. Input swirl intensity provides minimum energy loss and leading pressure recovery to an optimum level. Total pressure, static pressure and Mach number verses mass flow graphic result shows swirl uniformity at exit ofdiffuser.

Statistical and Spectral Analysis of the BulbT Conical Diffuser Flow

Statistical and Spectral Analysis of the BulbT Conical Diffuser Flow

Investigation of the BulbT conical diffuser flow dynamics with TR-PIV and pressure measurements

The three dimensional unsteady swirling flow inside the draft tube of a bulb turbine is investigated using time-resolved stereoscopic particle image velocimetry. The measurement plane located inside the conical diffuser, together with synchronous pressure measurements from thirteen sensors distributed across the draft tube, allow to link the draft tube flow dynamics to a known efficiency drop of the turbine. This paper presents the experimental setup and the first observations based on statistical analyses and instantaneous snapshots of the velocity field. After the efficiency drop, the intermittency and skewness of the draft tube pressure recovery are found to be reflected onto the velocity fields. Spectral analysis reveals the presence of a coherent and non-axisymmetric structure that rotates at the same frequency as the runner, but in the opposite direction.

Окружная неравномерность параметров потока в конических диффузорах и способ ее гашения

Окружная неравномерность параметров потока в конических диффузорах и способ ее гашения

Experimental investigations of the swirling flow in the conical diffuser using flow-feedback control technique with additional energy source

The previous experimental and numerical investigations of decelerated swirling flows in conical diffusers have demonstrated that water injection along to the axis mitigates the pressure fluctuations associated to the precessing vortex rope [1]. However, for swirling flows similar to Francis turbines operated at partial discharge, the water jet becomes effective when the jet discharge is larger than 10% from the turbine discharge, leading to large volumetric losses when the jet is supplied from upstream the runner. As a result, it was introduced a new approach for supplying the jet by using a fraction of the discharge collected downstream the conical diffuser [2]. This is called flow-feedback control technique (FFCT) and it was investigated experimentally in order to assess its capability [3]. The FFCT approach not requires additional energy to supply the jet. Consequently, the turbine efficiency is not diminished due to the volumetric losses injected even if around 10% of the main flow is used. However, the equivalent amplitude of the pressure pulsations associated to the vortex rope decreases with 30% if 10% jet discharge is applied [3]. Using 12% water jet discharge from upstream then the equivalent amplitude of the pressure pulsations is mitigated with 70% according to Bosioc et al. [4]. In our case, an extra 2% jet discharge is required in order to obtain similar results with FFCT. This extra discharge is provided using an additional energy source. Therefore, the paper presents experimental investigation performed with FFCT with additional energy source. The experimental results obtained with this technique are compared against FFCT and the swirling flow with vortex rope, respectively.

Flow-Feedback for Pressure Fluctuation Mitigation and Pressure Recovery Improvement in a Conical Diffuser with Swirl

Our previous experimental and numerical investigations of decelerated swirling flows in conical diffusers have demonstrated that water jet injection along the symmetry axis mitigates the pressure fluctuations associated with the precessing vortex rope. However, for swirling flows similar to Francis turbines operated at partial discharge, the jet becomes effective when the jet discharge is larger than 10% from the turbine discharge, leading to large volumetric losses when the jet is supplied from upstream the runner. As a result, we introduce the flow-feedback approach for supplying the jet by using a fraction of the discharge collected downstream the conical diffuser. Experimental investigations on mitigating the pressure fluctuations generated by the precessing vortex rope and investigations of pressure recovery coefficient on the cone wall with and without flow-feedback method are presented.

Mitigation of pressure fluctuations in the discharge cone of hydraulic turbines using flow-feedback

Our previous experimental and numerical investigations of decelerated swirling flows in conical diffusers have demonstrated that water jet injection along the symmetry axis mitigates the pressure fluctuations associated with the precessing vortex rope. However, for swirling flows similar to Francis turbines operated at partial discharge, the jet becomes effective when the jet discharge is larger than 10% from the turbine discharge, leading to large volumetric losses when the jet is supplied from upstream the runner. As a result, we introduce in this paper a new approach for supplying the jet by using a fraction of the discharge collected downstream the conical diffuser. We present the technical implementation of this flow-feedback approach, and we investigated experimentally its capability in mitigating the pressure fluctuations generated by the precessing vortex rope. The main advantage of this flow-feedback approach is that is does not require additional energy to supply the jet and it does not decrease the turbine efficiency.

Numerical prediction of inner turbulent flow in conical diffuser by using a new five-point scheme and DLR k-ɛ turbulence model

The internal turbulent flow in conical diffuser is a very complicated adverse pressure gradient flow. DLR k-ɛ turbulence model was adopted to study it. The every terms of the Laplace operator in DLR k-ɛ turbulence model and pressure Poisson equation were discretized by upwind difference scheme. A new full implicit difference scheme of 5-point was constructed by using finite volume method and finite difference method. A large sparse matrix with five diagonals was formed and was stored by three arrays of one dimension in a compressed mode. General iterative methods do not work wel1 with large sparse matrix. With algebraic multigrid method(AMG), linear algebraic system of equations was solved and the precision was set at 10−6. The computation results were compared with the experimental results. The results show that the computation results have a good agreement with the experiment data. The precision of computational results and numerical simulation efficiency are greatly improved.

Numerical study of flow in conical diffuser with vortex generator jets

To develop vortex generator jet (VGJ) method for flow control, the turbulence flow in a 144°conical diffuser with and without vortex generator jets are simulated by solving Navier-Stokes equations with κ -e turbulence model. The diffuser performance, based on different velocity ratio (ratio of the jet speed to the mainstream velocity), is investigated and compared with the experimental study. On the basis of the flow characteristics using computation fluid dynamics (CFD) method observed in the conical diffuser and the downstream development of the longitudinal vortices, attempt is made to correlate the pressure recovery coefficient with the behavior of vortices produced by vortex generator jets.

348 管路型円すいディフューザの脈動流れ特性 : 広がり角度や振動数の影響(G05-6 流体機械(1),G05 流体工学)

Pulsating turbulent flow in conical diffusers having total divergence angles of 12, 16 and 24°were investigated experimentally. Distributions of the pressure and axial velocity and the power loss were obtained for the pulsating flows at the Womersley numbers α of 10 to 40, the mean Reynolds numbers of Reta =15000〜30000 and the oscillatory Reynolds number of Reos =10000. The power loss of the pulsating flow in the diffusers is larger than that of the steady flow and is highly dependent on the flow rate ratio. A discussion, furthermore, is given for the influences of the divergence angle and the Womersley number on the time-dependent profiles of the phase-averaged velocity and the turbulence intensity.

円すいディフューザの脈動流れ特性(O.S.2-2 内部流の実験と計算,OS2:乱流中の渦構造および流れの制御)

円すいディフューザの脈動流れ特性(O.S.2-2 内部流の実験と計算,OS2:乱流中の渦構造および流れの制御)

Characteristics of Pulsating Flow in Conical Diffusers

Characteristics of Pulsating Flow in Conical Diffusers

Suppression of Swirling Flow in a Conical Diffuser by use of J-groove.

The purpose of the study is to examine the validity of J-groove in controlling and suppressing swirl flow in conical diffuser, for draft surge suppression in a Francis turbine which is caused by the swirling flow from runner outlet into a draft tube. "J-groove" composed of shallow grooves mounted parallel to the pressure gradient on the diffuser wall is a very simple passive device to suppress several anomalous phenomena in turbomachinery. The experimental study has been performed using conical diffuser of divergent angle 20 degree. The measured results of velocity distribution in the diffuser show that considerable reduction of swirl intensity is attained by use of J-grooves. Besides, the amplitude of pressure fluctuation caused by rotation of vortex core around the dead water region near the diffuser inlet is reduced by J-grooves.

Behavior of mean and fluctuating skin friction in a conical diffuser with instantaneous flow reversals

Measurements were made of the mean and fluctuating skin friction in an 8° total divergence angle conical diffuser using modern pulsed-wire anemometry. The development of streamwise and spanwise fluctuating skin friction, skewness and flatness factors of streamwise skin friction, and probability density distribution of the wall shear stress show the rapidly changing nature and complexity of the wall layer turbulence structure in an axisymmetric severe adverse pressure gradient flow. The relative intensities of skin friction increase consistently and achieve very high values in the final stages of diffuser flow. The quantitative instantaneous backflow and the mean and fluctuating skin friction distributions were examined to determine their interrelationship. The initiation of instantaneous reversals was found to strongly influence the near-wall flow field and to coincide with the precipitous drop in skewness factor of the streamwise fluctuating wall shear stress, to approximately 10% of its initial value near the diffuser inlet. The flatness factor of the streamwise fluctuating wall shear stress also decreases in the latter stages, indicating a lesser inrush of high-velocity fluid from the central region. Comparison with conventional Preston tube data indicated the appropriateness of pulsed-wire skin friction measurements in severe adverse pressure gradient flows. In this diffuser the minimum streamwise wall shear stress near the diffuser exit is close to 3% of its inlet value, indicating that the flow approaches mean-flow separation. The present results of mean and fluctuating skin friction also differ substantially from those reported by other investigators using hot-film anemometry, both in level and shape of profiles. The differences are attributed to the rectification errors in hot-film anemometry at high turbulence intensities (near-wall turbulence level over 80%) and the appreciable instantaneous backflow (instantaneous reversals up to 30% of the time) present in the final stages of conical diffuser flow.

Measurement of instantaneous flow reversals and velocity field in a conical diffuser

An 8° total divergence angle conical diffuser fed with a fully developed turbulent pipe flow at its inlet was experimentally investigated. Through effective use of pulsed-wire anemometry, some quantitative information about instantaneous reversals was obtained in conical diffuser flow. Profiles of streamwise mean and fluctuating velocities, skewness factor, and flatness factor are presented for the stations in the final stages of the diffuser flow, where appreciable instantaneous reversals were detected in the wall region. Results are also presented for the stations in the earlier stages of the diffuser, where smaller instantaneous backflow was present. Comparative results of conventional pressure measurements and hot-wire anemometer measurements were obtained to determine quantitatively the effects of instantaneous flow reversals on these techniques. In the regions of instantaneous backflow, the pulsed-wire technique was found most accurate for measuring the mean and fluctuating velocities. The hot-wire and pulsed-wire anemometer results were also used to obtain guidelines for hot-wire use in highly turbulent instantaneously reversing flows. The integral constraint of continuity was used at the diffuser stations to evaluate measurement techniques for obtaining mean velocity in a conical diffuser flow. The distributions of streamwise mean and fluctuating wall shear stress were measured with a pulsed-wire skin friction probe. Pulsed-wire wall shear stress probe results indicated serious errors in the values of the mean wall shear stress obtained by the conventional Preston tube in the region of appreciable instantaneous backflow. Some of the separation prediction criteria reported in the literature were evaluated for their validity using the present results of quantitative instantaneous backflow. These measurements illustrate important characteristics of instantaneous flow reversals and the velocity field in the highly turbulent axisymmetric flow developing under a severe adverse pressure gradient.

The structure of instantaneous reversals in highly turbulent flows

Structure of instantaneous flow reversals has been measured in a highly turbulent axisymmetric diffuser flow using pulsed-wire anemometry. In this 8° nominal included angle conical diffuser, the adverse pressure gradient (APG) is strong enough to cause appreciable instantaneous flow reversals (instantaneous backflow up to 30% of the time), but the time-averaged flow is non-separated. The results are compared with the other severe APG separating flows reported in literature. An increase in entry Reynolds number indicated a decrease in the size of near-wall instantaneous reversals region as well as a decrease in the magnitude of instantaneous backflow. Also, the region of instantaneous reversals moves slightly downstream at appreciably higher Reynolds numbers. The initiation and growth of instantaneous reversals in a conical diffuser was found to strongly influence the wall-layer and the central region. Present results also suggest that the instantaneous backflow should be considered for modelling of instantaneously-separating diffuser flows. In the final stages of a conical diffuser, the magnitudes of cross-stream pressure gradient were found to be appreciably larger than that of the longitudinal pressure gradient, indicating that accurate representation of a conical diffuser flow can not be achieved without considering V-momentum equation. A comparison of various separating flows revealed remarkable similarity of instantaneous reversals regions and distributions even in different flow configurations.

Asymptotic velocity defect profile in an incipient-separating axisymmetric flow

Presence of a new asymptotic mean velocity profile (velocity defect law) was found concomitant with appreciable incipient separation (instantaneous detachment > 20 % of the time) and extremely high-turbulence intensities in the final stages of an 8-deg total divergence angle conical diffuser fed with a fully developed turbulent pipe flow. Pulsed-wire anemometry was used for the measurement of incipient flow separation, mean velocities and skin-friction distributions. The conical diffuser flow shows some attributes .of quasiequilibrium in the final onethird length. In the second-half of the diffuser the mean velocity profiles indicate a distinct point of inflection, the wall regions have less sensitivity to the initial conditions, and the flow exhibits some insensitivity toward the local boundary conditions. A comparison with the other asymptotic velocity defect profiles in Perry-Schofie ld coordinates revealed the distinctly different shape of the present asymptotic profile found in the conical diffuser. The validity of this asymptotic mean velocity profile was confirmed at two Reynolds numbers. Application of rapid distortion theory indicated coexistence of the regions of slowly and rapidly changing turbulence in the conical diffuser. Results of the present study also support the theory that turbulence is not universal and depends on the initial and boundary conditions.

Effects of incipient separation on the mean and turbulent fluctuating velocities in a conical diffuser flow

Effects of incipient separation on the mean and turbulent fluctuating velocities in a conical diffuser flow

Prediction of turbulence quantities for swirling flow in conical diffusers

Turbulent swirling flow through 12 and 20 deg included angle diffusers, with moderate inlet swirl numbers sufficient to avoid wall flow separation, are predicted by a k-e and an algebraic Reynolds stress turbulence model with a two-layer wall function. Good agreement with experimental data for the mean velocities and turbulence quantities is obtained by using an algebraic Reynolds stress turbulence model and a k-e turbulence model with a suitable choice of the wall treatment. Both the amplification of the peak in turbulence quantitites and its increased distance from the diff user wall, when compared to those of fully attached flow, are predicted very well for solid-body rotation swirling flow. The effect of different inlet swirl profiles on the flow behavior is also discussed.

Comparison of k–ϵ and algebraic Reynolds stress models for swirling diffuser flow

AbstractA brief overview of classes of turbulent swirling flow in conical diffusers is given, together with a description of appropriate numerical schemes for each class. Numerical results obtained for the class of moderate swirl in a 20° diffuser and for the class of no swirl in an 8° diffuser are compared with experimental results. The results are obtained using a multi‐sweep scheme solving the full steady state time‐averaged Navier–Stokes equations. Turbulence quantities are approximated using two types of algebraic Reynolds stress model and two types of k–ϵ model. One of the algebraic Reynolds stress models includes extra production terms associated with the Christoffel symbols in cylindrical co‐ordinates, and one of the k–ϵ models includes a swirl‐related modification to the ϵ equation. It is demonstrated that the standard k–ϵmodel gives poor prediction of the mean flow, and it is necessary to at least use the modified form or one of the two algebraic Reynolds stress models.