Positive Preswirl Research Articles

Inlet preswirl dependence research on three different labyrinth seals

In this paper, an angular velocity is proposed to calculate the air inlet preswirl condition. The potential influences on rotordynamic and leakage performance of inlet preswirl ratios ranging from −1–1 are considered in the traditional labyrinth seal (LS), diaphragm labyrinth seal (DLS) and hole diaphragm labyrinth seal (HDLS). The simulation results show that the inlet preswirl of the LS has little influence under low rotation speeds, while the negative influence on the higher rotation speed for the positive preswirl is clear. The maximum improvement of effective damping under the −1 preswirl ratio compared to zero preswirl conditions for LS, DLS and HDLS can reach 132.1%, 240.6% and 385.1%, respectively, when that of effective stiffness for DLS and HDLS can improve 14.8% and 90.1%, respectively, and that of leakage reduction for DLS and HDLS can reach 4.12% and 4.5%, respectively. Meanwhile, the average leakage under different conditions of HDLS can obtain 5.70% reduction than that of LS and 1.78% than DLS, which is better than another new seal, which was proposed by other scholars. Further discussions on detailed flow areas show that the existence of holes brings more energy dissipation for HDLS, which brings lower leakage among the three different seals. Comparing DLS and HDLS also shows a potential combination to obtain a new type of seal with higher stability for rotor-seal systems and lower leakage.

Effect of hole arrangement patterns on the leakage and rotordynamic characteristics of the honeycomb seal

The honeycomb seal is a vital component to reduce the leakage flow and improve the system stability for the turbomachines. In this work, a three-dimensional model is established for the interlaced hole honeycomb seal (IHHCS) and the non-interlaced hole honeycomb seal (NIHHCS) to investigate its leakage and rotordynamic characteristics by adopting computational fluid dynamics (CFD). Results show that the hole arrangement patterns have little impact on the pressure drop and turbulence kinetic energy distribution for the seals, and the IHHCS possesses a slightly lower leakage flow rate than the NIHHCS. Moreover, the numerical results also show that the NIHHCS possesses a better rotordynamic performance than the IHHCS at all investigated conditions. Both seals show a larger k and a lower Ceff with the increase of the positive preswirl ratios and rotational speeds, while the negative preswirl ratios would reduce the k and improve the Ceff. The NIHHCS possesses a higher absolute value of Ft for all operating conditions, this could explain the distinction of Ceff for both seals at different working conditions.

Numerical study on the influence of pre-swirl angle on internal flow characteristics of centrifugal pumps

The effect of inlet pre-swirl on the performance of a centrifugal pump is studied by numerical simulation. The governing equations are Navier–Stokes equations and the shear stress transport k–ω turbulence model. The numerical results show that the optimal operating point moves from the low flow region to the high flow region as the pre-swirl angle shifts from positive to negative. It is found by contours of Omega–Liutex that the positive pre-swirl angle is able to weaken the vortex on the blade suction and reduce the energy dissipation. On both the 0.5Q0 and 1.2Q0 operating conditions, the proportion of entropy production loss in the impeller and volute is about 60% and 30%, respectively. As the pre-swirl angle changes from negative to positive, the entropy production loss in the inlet and outlet pipes increases slowly, and the entropy production loss in the volute and impeller shows a decreasing trend and the peak area of entropy loss moves toward the outlet. Under the four pre-swirl angles, the main frequency is always the passing frequency of the blade. The pre-swirl angle affects the pressure fluctuation at the main frequency but has little effect at the secondary frequency. The change in velocity pulsation amplitude in the impeller in the positive pre-swirl angle is smaller than that in the negative pre-swirl angle. As a result, for the positive pre-swirl angle, the turbulent kinetic energy density in the impeller is low, and the energy loss is low, compared with negative pre-swirl. Under the low flow condition (0.5Q0), the change in velocity pulsation amplitude in the inertial range of the energy spectrum under negative pre-swirl is greater than that under positive pre-swirl.

Numerical Investigation of Transient Flow Characteristics in a Centrifugal Compressor Stage with Variable Inlet Guide Vanes at Low Mass Flow Rates

This study numerically investigates the beneficial effects of positive pre-swirl on the aerodynamic performance and internal flow field in a centrifugal compressor stage with variable inlet guide vanes (VIGVs) at low mass flow rates. Four positions of VIGV are considered, including 0°, 30°, 45°, and 60° angle. The latter three positions of VIGV induce positive pre-swirl. Numerical results show that as positive pre-swirl increases, the aerodynamic performance curve of the stage moves in the low mass flow rate direction. In the three cases of positive pre-swirl, there was an improvement of approximately 9.95% of stall/surge margin greater than in conditions with no pre-swirl. The regulation of IGV can effectively improve the unstable flow of the compressor stage at low mass flow rates. A low frequency that has a great influence on the internal flow of the compressor stage is found, and the unstable flow caused by low frequency is analyzed by the combination of streamline distribution, spectrum analysis, vector, entropy increase, and modal decomposition method. Meanwhile, the modal decomposition method and flow field reconstruction techniques are used to investigate the coherent flow structures caused by low frequency under different guide vane openings.

Numerical investigation on leakage and rotordynamic performance of the honeycomb seal with swirl-reverse rings

Honeycomb seals are a crucial component to restrict the leakage flow and improve system stability for the turbomachines and aero-engines. In this work, the leakage and rotordynamic performance of honeycomb seals with the Swirl-Reverse Ring (SRR) is predicted by employing the approach of Computational Fluid Dynamics (CFD) and the multifrequency whirling model theory. Numerical results show that the positive preswirl flow and circumferential velocity can be effectively weakened for the honeycomb seal as SRR is introduced. The obtained results also suggest that the direct stiffness, direct damping, and effective stiffness will not reduce, for the honeycomb seal is introduced to SRR. However, the honeycomb seal with SRR can significantly reduce the cross-coupled stiffness and remarkably enhance the effective damping. Meanwhile, the absolute value of negative tangential force acting on the rotor surface significantly increased for the honeycomb seal introduced SRR. Therefore, the whirling velocity of the rotor would be weakened. This phenomenon would be conducive to improve the stability of the rotor. Besides, the performance of SRR can be further enhanced when SRR possesses a smaller bending angle, and a higher arrangement density and height.

Prestall Disturbances and Stall Inception for an Eccentric Low-Speed Axial Compressor With Inlet Swirl

Abstract Owing to the manufacturing and assembly error or the fatigue of long-time operation, nonaxisymmetric tip clearance is actually a common phenomenon in compressors. It is also well accepted that tip leakage flow, associated with tip clearance size and loading, has a strong influence on the performance and stall inception of compressors. The work of this paper, based on an eccentric compressor with/without inlet swirl distortion, is aimed at strengthening the understanding of stall inception for a real geometry compressor experimentally. Results indicate that rotor tip blockage and flow unsteadiness vary evidently around the circumference. For this compressor, the maximum tip flow unsteadiness and blockage happens at the location near the minimum clearance under the condition of clean inlet flow. Before the occurrence of stall inception, disturbances arise and vanish intermittently within the region of high unsteadiness. However, it fails to rotate due to the inhibition of the low unsteadiness region. Once the most robust region is no longer able to suppress disturbances, stability finally breaks down and stall inception generates. After exerting inlet paired swirl, unsteadiness within the region of positive preswirl decreases significantly and the maximum unsteadiness location shifts, while the increase for the region with negative preswirl is nearly negligible. As a result, stall margin of the compressor is improved.

Effect of bristle pack position on the rotordynamic characteristics of brush-labyrinth seals at various operating conditions

Over the last few decades, the research on the effect of bristle pack position on the rotordynamic characteristics of the brush-labyrinth seals is not sufficient. To this end, two kinds of brush-labyrinth seals for the bristle pack element installed upstream of the labyrinth teeth named BSU and installed downstream of the labyrinth teeth called BSD were used to investigate the effect of bristle pack position on the rotordynamic characteristics of the brush-labyrinth seals. Using the numerical model combining the porous medium model and the whirling rotor method, the rotordynamic characteristics of the BSU and BSD at various operating conditions including four kinds of pressure ratios, five kinds of inlet preswirl speeds and four kinds of rotor spinning speeds were conducted. The obtained results show that the effects of operating conditions on rotordynamic coefficients for the different seal configurations are different. The direct stiffness, cross-coupled stiffness and direct damping of the BSU are lower than those of the BSD. The rotordynamic coefficients of the BSU are more insensitive to the operating conditions variation. From the perspective of the seal stability, the BSU is a better brush-labyrinth seal configuration at high pressure ratio, high positive preswirl or high rotor spinning speed conditions. While in the case of low pressure ratio, low positive preswirl or low rotor spinning speed conditions, the BSD is a better choice.

On the Thermodynamic Process in the Bulk-Flow Model for the Estimation of the Dynamic Coefficients of Labyrinth Seals

The impact of sealing equipment on the stability of turbomachineries is a crucial topic because the power generation market is continuously requiring high rotational speed and high performance, leading to the clearance reduction in the seals. The accurate characterization of the rotordynamic coefficients generated by the seals is pivotal to mitigate instability issues. In the paper, the authors propose an improvement of the state-of-the-art one-control volume (1CV) bulk-flow model (Childs and Scharrer, 1986, “An Iwatsubo-Based Solution for Labyrinth Seals: Comparison to Experimental Results,” ASME J. Eng. Gas Turbines Power, 108(2), pp. 325–331) by considering the energy equation in the steady-state problem. Thus, real gas properties can be evaluated in a more accurate way because the enthalpy variation, expected through the seal cavities, is evaluated in the model. The authors assume that the enthalpy is not a function of the clearance perturbation; therefore, the energy equation is considered only in the steady-state problem. The results of experimental tests of a 14 teeth-on-stator (TOS) labyrinth seal, performed in the high-pressure seal test rig owned by GE Oil&Gas, are presented in the paper. Positive and negative preswirl ratios are used in the experimental tests to investigate the effect of the preswirl on the rotordynamic coefficients. Overall, by considering the energy equation, a better numerical estimation of the rotordynamic coefficients for the tests with the negative preswirl ratio has been obtained (as it results from the comparison with the experiments). Finally, the numerical results are compared with a reference bulk-flow model proposed by Thorat and Childs (2010, “Predicted Rotordynamic Behavior of a Labyrinth Seal as Rotor Surface Speed Approaches Mach 1,” ASME J. Eng. Gas Turbines Power, 132(11), p. 112504), highlighting the improvement obtained.

Numerical Comparison of Rotordynamic Characteristics for a Fully Partitioned Pocket Damper Seal and a Labyrinth Seal With High Positive and Negative Inlet Preswirl

Pocket damper seals (PDSs) are used as replacements for labyrinth seals in high-pressure centrifugal compressors at the balance-piston location or center seal location to enhance rotordynamic stability. A concern exists that this enhanced stability will be lost at high positive inlet preswirl. Numerical results of frequency-dependent rotordynamic force coefficients and leakage flow rates were presented and compared for a fully partitioned PDS (FPDS) and a labyrinth seal at high positive and negative inlet preswirl, using a proposed transient computational fluid dynamics (CFD) method based on the multifrequency elliptical orbit whirling model. The negative preswirl indicates a fluid swirl in a direction opposite to rotor rotation at seal inlet. Both seals have identical diameter and sealing clearance. The full 3D concentric CFD model and mesh were built for the labyrinth seal and FPDS, respectively. The accuracy and availability of the present transient CFD numerical method were demonstrated with the experiment data of frequency-dependent rotordynamic coefficients of the labyrinth seal and FPDS at zero and high positive preswirl conditions. The numerical boundary conditions include two high positive preswirl, two high negative preswirl, and a zero preswirl. Numerical results show that the effect of inlet preswirl on the direct force coefficients is weak, but the effect on the cross-coupling stiffness and effective damping is dramatic. Both seals possess negative effective damping at lower excitation frequencies due to positive preswirl, and the crossover frequency of effective damping term increases with increasing positive preswirl. Negative preswirl produces negative cross-coupling stiffness and positive effective damping over the whole excitation frequency range. Increasing negative preswirl is a stabilizing factor for annular gas seals, which results in a significant increase in the effective damping and a decrease in the crossover frequency. It is desirable to reduce the inlet preswirl to zero or even negative through applications of negative-swirl brakes and negative injection devices.

Analysis of Aerodynamic Design Characteristics of Flade Fan

Numerical simulation was conducted to explore how the load distribution and pre-swirl distribution influence the aerodynamic performance for the Flade fan. The Flade fan faces the design challenge of full span supersonic flow because of the position above the tip of main fan, so it is necessary to introduce full span positive pre-swirl. The full span positive pre-swirl introduced by IGV helped to reduce the rotor inlet relative Mach number and could effectively control the shock loss and the shock-boundary layer loss on the full span. Load distribution could effectively control the conflict between high tangential velocity and low pressure ratio, and change the degree of reaction to make the absolute Mach number at the rotor outlet satisfy the design request of stator. Stall characteristics analysis on Flade show that the tip clearance flow is still far away from the leading edge of blade, and a wide range of low-speed and low-energy region appeared at the root lead to the onset of stall.

Novel IGV Designs for Centrifugal Compressors and Their Interaction With the Impeller

In centrifugal compressors, variable inlet guide vanes (IGVs) are used to control the mass flow rate with negligible change in pressure ratio and shaft speed. The efficient operation of IGVs is limited to the range of aerodynamic stability of their vane profiles. Thin symmetric profiles, which are usually used, tend to exhibit flow separation already at low setting angles, resulting in a rapid decrease in the global efficiency. Three different guide vane profiles, including a symmetric reference profile, a two-piece tandem profile with a variable angle, and an s-cambered profile, together with two auxiliary profiles, were considered. These variants were numerically and experimentally investigated at different setting angles, covering negative and positive inlet swirls, and at different operating points. The results show that tandem and s-cambered vanes have superior aerodynamic performance in comparison with the symmetric reference profile and extend the operating range of the compressor over which the efficiency remains high. The tandem profile shows better aerodynamic performance than the s-cambered profile at negative inlet swirl, while the asymmetric s-cambered profile offers an extended range of high efficiency at positive preswirl. Considering manufacturing requirements and cost, the s-cambered profile is preferred for applications with positive preswirl.

An experimental study on the static and dynamic characteristics of pump annular seals. 2nd report. The dynamic characteristics for small concentric whirling motion.

The dynamic characteristics of annular seals for turbopumps have been experimentally investigated. In the experiment, fluid forces acting on the seal were measured for various parameters such as the preswirl velocity, the pressure difference between inlet and outlet of the seal, the whirling amplitude, and the ratio of whirling speed to spinning speed of the rotor. Influence of these parameters on the dynamic characteristics was investigated from the experimental results. As a result, the dynamic characteristics are largely affected by the preswirl velocity. The positive preswirl velocity plays an instabilizing role, and the negative preswirl velocity plays a stabilizing role in the rotor system. The axial flow through the seal also gives a stabilizing effect to the rotor system.