Numerical investigations on leakage performance and flow mechanism of spiral groove seal for shrouded blade in axial turbine

Flow field of shroud leakage flow for a single-stage axial turbine has been investigated in this article. The spiral groove seal (SGS) is adopted for shrouded rotor blade to reduce tip leakage and improve turbine aerodynamic performance. A series of three-dimensional (3D) computational fluid dynamics (CFD) simulations are performed to investigate leakage characteristics and flow mechanism of various configurations with different angle, depth, width, and grooves number of the SGS. The original staggered labyrinth seal (LS) is also calculated for comparison. The results illustrate that small spiral groove angle can create more axial flow resistance; meanwhile, it will increase grooves number existing in the axial direction. Groove depth and tooth width will influence the number, shape, and strength of vortex in the groove. The leakage mass flow can be reduced by 36% and isentropic efficiency of the turbine can be increased by 0.26% when spiral groove angle, depth, and width of the SGS are 1.5°, 1.8 mm, and 0.8 mm, respectively. Overall, the optimal SGS can influence vortex generation and enhance energy dissipation in shroud cavity to reduce the leakage and suppress mixing loss of leakage flow with the main flow to some extent. It can be attributed to the combination of throttling effect and pumping effect of the SGS that realize leakage reduction and efficiency improvement. As a result, the SGS can effectively improve tip leakage flow of shrouded blade in axial turbine.

Static and Rotordynamic Characteristics for Two Types of Novel Mixed Liquid Damper Seals With Hole-Pattern/Pocket-Textured Stator and Helically Grooved Rotor

Noncontacting liquid annular seals, such as helical groove seals, are widely used at the impeller interstage and shaft end in the liquid turbomachinery to reduce the fluid leakage and stabilize the rotor-bearing system. However, previous literatures have expounded that the helical groove seal possesses the poor sealing property at low rotational speed condition and suffers the rotor instability problem inducing by negative stiffness and damping, which is undesirable for the liquid turbomachinery. In this paper, to obtain the high sealing performance and the reliable rotordynamic capability throughout full operational conditions of machines, two novel mixed liquid damper seals, which possess a hole-pattern/pocket-textured stator matching with a helically grooved rotor, were designed and assessed for the balance piston location in a multiple-stage high-pressure centrifugal liquid pump. To assess the static and rotordynamic characteristics of these two types of mixed liquid damper seals, a three-dimensional (3D) steady computational fluid dynamics (CFD)-based method with the multiple reference frame theory was used to predict the seal leakage and drag power loss. Moreover, a novel 3D transient CFD-based perturbation method, based on the multifrequency one-dimensional stator whirling model, the multiple reference frame theory, and the mesh deformation technique, was proposed for the predictions of liquid seal rotordynamic characteristics. The reliability and accuracy of the present numerical methods were demonstrated based on the published experiment data of leakage and rotordynamic force coefficients of a helical groove liquid annular seal and a hole-pattern liquid annular seal. The leakage and rotordynamic force coefficients of these two mixed liquid damper seals were presented at five rotational speeds (0.5 krpm, 2.0 krpm, 4.0 krpm, 6.0 krpm, and 8.0 kpm) with large pressure drop of 25 MPa, and compared with three types of conventional helical groove seals (helical grooves on rotor, stator or both), two typical damper seals (hole-pattern seal, pocket damper seal with smooth rotor), and a mixed helical groove seal. Numerical results show that two novel mixed liquid damper seals both possess generally better sealing capacity than the conventional helical groove seals, especially at lower rotational speeds. The circumferentially isolated cavities (hole/pocket types) on the stator can enhance the “pumping effect” of the helical grooves for mixed helical groove seals, by weakening the swirl flow in seal clearance (which results in the increase of the fluid velocity gradient near the helically grooved rotor). What is more, the helical grooves on rotor also strengthen the dissipation of fluid kinetic energy in the isolated cavities, so the mixed liquid damper seals offer less leakage. Although the mixed liquid damper seals possess a slightly larger (less than 40%) drag power loss, it is acceptable in consideration of the reduced (∼60%) leakage for the high-power turbomachinery, such as the multiple-stage high-pressure centrifugal liquid pump. The present novel mixed liquid damper seals have pronounced rotordynamic stability advantages over the conventional helical groove seals, due to the obviously larger positive stiffness and damping. The mixed liquid damper seal with the hole-pattern stator and the helically grooved rotor (HPS/GR) possesses the lowest leakage and the largest effective damping, especially for higher rotational speeds. From the viewpoint of sealing capacity and rotor stability, the present two novel mixed liquid damper seals have the potential to become the attractive alternative seal designs for the future liquid turbomachinery.

Development and Research of Crosshead-Free Piston Hybrid Power Machine

This article considers the development and research of a new design of crosshead-free piston hybrid power machine. After verification of a system of simplifying assumptions based on the fundamental laws of energy, mass, and motion conservation, as well as using the equation of state, mathematical models of the work processes of the compressor section, pump section, and liquid flow in a groove seal have been developed. In accordance with the patent for the invention, a prototype of a crosshead-free piston hybrid power machine (PHPM) was developed; it was equipped with the necessary measuring equipment and a stand for studying the prototype. Using the developed mathematical model, the physical picture of the ongoing work processes in the compressor and pump sections is considered, taking into account their interaction through a groove seal. Using the developed plan, a set of experimental studies was carried out with the main operational parameters of the crosshead-free PHPM: operating processes, temperature of the cylinder–piston group and integral parameters (supply coefficient of the compressor section, volumetric efficiency of the pump section, etc.). As a result of numerical and experimental studies, it was determined that this PHPM design has better cooling of the compressor section (decrease in temperature of the valve plate is from 10 to 15 K; decrease in temperature of intake air is from 6 to 8 K, as well as there is increase in compressor and pump section efficiency up to 5%).

Numerical Investigation on the Static and Rotordynamic Characteristics for Two Types of Novel Mixed Helical Groove Seals

Non-contracting annular seals, such as helical groove seals, are widely used between the impeller stages in the liquid turbomachinery to reduce the fluid leakage and stabilize the rotor-bearing system. However, previous literature has expounded that the helical groove seals possess the poor sealing property at low rotational speed condition and face the rotor instability problem inducing by negative stiffness and damping, which is undesirable for liquid turbomachinery. In this paper, to obtain the high sealing performance and the reliable rotordynamic capability for full operational conditions of the machine, two novel mixed helical groove seals, which possess a hole-pattern/pocket-damper stator matching with a helically-grooved rotor, were designed and assessed for a multiple-stage high-pressure centrifugal liquid pump. In order to assess the static and rotordynamic characteristics of these two types of mixed helical groove seals, a three-dimensional (3D) steady CFD-based method with the multiple reference frame theory was used to predict the seal leakage and drag power loss. Moreover, a proposed 3D transient CFD-based perturbation method, based on the multi-frequency one-dimensional stator whirling model, the multiple reference frame theory and a mesh deformation technique, was utilized for the predictions of seal rotordynamic characteristics. The accuracy of the numerical methods was demonstrated based on the experiment data of leakage and rotordynamic forces coefficients of published helical groove seals and hole-pattern seal. The leakage and rotordynamic forces coefficients of these two mixed helical groove seals were presented at five rotational speeds (0.5 krpm, 2.0 krpm, 4.0 krpm, 6.0 krpm, 8.0 kpm) with large pressure drop of 25MPa, and compared with three types of conventional helical groove seal (helical grooves on rotor, stator or both), and two types of damper seals (hole-pattern seal, pocket damper seal with smooth rotor). Numerical results show that the mixed groove seals possess generally better sealing capacity than the conventional helical groove seals, especially at low rotational speed conditions. The circumferentially-isolated cavities (hole or pocket) on the stator enhance the “pumping effect” of the helical grooves for mixed helical groove seals, what is more, the helical grooves also strengthen the dissipation of kinetic energy in the isolated cavities, thus the mixed helical groove seal offers less leakage. Although the mixed helical groove seals possess a slightly larger drag power loss, it is acceptable in consideration of reduced leakage for the high-power turbomachinery. The present novel mixed helical groove seals have pronounced stability advantages over the conventional helical groove seal, due to the obvious large positive stiffness and increased damping. The mixed helical groove seal with the hole-pattern stator and the helically-grooved rotor (HPS/GR) possesses the lowest leakage and the largest effective damping, especially for the high rotational speeds. From the viewpoint of sealing capacity and rotor stability, the novel mixed groove seals are better seal concepts for liquid turbomachinery.

Pump Grooved Seals: A Computational Fluid Dynamics Approach to Improve Bulk-Flow Model Predictions

In multiple stage centrifugal pumps, balance pistons, often comprising a grooved annular seal, equilibrate the full pressure rise across the pump. Grooves in the stator break the evolution of fluid swirl and increase mechanical energy dissipation; hence, a grooved seal offers a lesser leakage and lower cross-coupled stiffness than a similar size uniform clearance seal. To date, bulk-flow modelbulk-flow models (BFMs) expediently predict leakage and rotor dynamic force coefficients of grooved seals; however, they lack accuracy for any other geometry besides rectangular. Note that scalloped and triangular (serrated) groove seals are not uncommon. In these cases, computational fluid dynamics (CFD) models seals of complex shape to produce leakage and force coefficients. Alas, CFD is not yet ready for routine engineer practice. Hence, an intermediate procedure presently takes an accurate two-dimensional (2D) CFD model of a smaller flow region, namely a single groove and adjacent land, to produce stator and rotor surface wall friction factors, expressed as functions of the Reynolds numbers, for integration into an existing BFM and ready prediction of seal leakage and force coefficients. The selected groove-land section is well within the seal length and far away from the effects of the inlet condition. The analysis takes three water lubricated seals with distinct groove shapes: rectangular, scalloped, and triangular. Each seal, with length/diameter L/D = 0.4, has 44 grooves of shallow depth dg ∼ clearance Cr and operates at a rotor speed equal to 5,588 rpm (78 m/s surface speed) and with a pressure drop of 14.9 MPa. The method validity is asserted when 2D (single groove-land) and three-dimensional (3D) (whole seal) predictions for pressure and velocity fields are compared against each other. The CFD predictions, 2D and 3D, show that the triangular groove seal has the largest leakage, 41% greater than the rectangular groove seal does, albeit producing the smallest cross-coupled stiffnesses and whirl frequency ratio (WFR). On the other hand, the triangular groove seal has the largest direct stiffness and damping coefficients. The scalloped groove seal shows similar rotordynamic force coefficients as the rectangular groove seal but leaks 13% more. For the three seal groove types, the modified BFM predicts leakage that is less than 6% away from that delivered by CFD, whereas the seal stiffnesses (both direct and cross-coupled) differ by 13%, the direct damping coefficients by 18%, and the added mass coefficients are within 30%. The procedure introduced extends the applicability of a BFM to predict the dynamic performance of grooved seals with distinctive shapes.

Pump Grooved Seals: A CFD Approach to Improve Bulk-Flow Model Predictions

In multiple stage centrifugal pumps, balance pistons, often comprising a grooved annular seal, equilibrate the full pressure rise across the pump. Grooves in the stator break the evolution of fluid swirl and increase mechanical energy dissipation; hence, a grooved seal offers a lesser leakage and lower cross-coupled stiffness than a similar size uniform clearance seal. To date bulk-flow models (BFMs) expediently predict leakage and rotor dynamic force coefficients of grooved seals; however, they lack accuracy for any other geometry besides rectangular. Note scalloped and triangular (serrated) groove seals are not uncommon. In these cases, computational fluid dynamics (CFD) models seals of complex shape to produce leakage and force coefficients. Alas CFD is not yet ready for routine engineer practice. Hence, an intermediate procedure presently takes an accurate two-dimensional (2D) CFD model of a smaller flow region, namely a single groove and adjacent land, to produce stator and rotor surface wall friction factors, expressed as functions of the Reynolds numbers, for integration into an existing BFM and ready prediction of seal leakage and force coefficients. The selected groove-land section is well within the seal length and far away from the effects of the inlet condition. The analysis takes three water lubricated seals with distinct groove shapes: rectangular, scalloped and triangular. Each seal, with length/diameter L/D = 0.4, has 44 grooves of shallow depth dg ∼ clearance Cr, and operates at a rotor speed equal to 5,588 rpm (78 m/s surface speed) and with a pressure drop of 14.9 MPa. The method validity is asserted when 2D (single groove-land) and 3D (whole seal) predictions for pressure and velocity fields are compared against each other. The CFD predictions, 2D and 3D, show the triangular groove seal has the largest leakage, 41% greater than the rectangular groove seal does, albeit producing the smallest cross-coupled stiffnesses and whirl frequency ratio. On the other hand, the triangular groove seal has the largest direct stiffness and damping coefficients. The scalloped groove seal shows similar rotordynamic force coefficients as the rectangular groove seal but leaks 13% more. For the three seal groove types, the modified BFM predicts leakage that is less than 6% away from that delivered by CFD, whereas the seal stiffnesses (both direct and cross-coupled) differ by 13%, the direct damping coefficients by 18%, and the added mass coefficients are within 30%. The procedure introduced extends the applicability of a BFM to predict the dynamic performance of grooved seals with distinctive shapes.

Second Stage Optimization of Helical Groove Seals Using Computational Fluid Dynamics to Evaluate the Dependency of Optimized Design on Preswirl

Helical groove seals are non-contacting annular seals used in pumping machinery to increase the efficiency and, in the case of the balance drum, to manage the axial force on the thrust bearing. Prior work has shown that optimization of helical groove seals can reduce the leakage by two thirds given a desired pressure differential or, conversely, can significantly increase the pressure differential across the helical groove seal given a flow rate. This study evaluates the dependency of the optimal helical groove seal design on the inlet preswirl, which is the ratio of the inlet circumferential velocity to the rotor surface speed. To accomplish this goal, second stage optimization from the previously optimized helical groove seal with grooves on the stator and water as the working fluid were conducted at a series of preswirls ranging from −1 to 1. Optimization is performed using ANSYS CFX, a commercial computational fluid dynamics software and mesh independence is confirmed for the baseline case. For each preswirl case, design of experiments for the design parameters of groove width, groove depth, groove spacing, and number of grooves was performed using a Kennard-Stone Algorithm. The optimized solution is interpolated from the simulations run by using multi-factor quadratic regression from the 30 simulations in each optimization and the interpolated solution is simulated for comparison. In addition to evaluating the optimized solution’s dependency on preswirl, the viability of using swirl breaks or swirl promoting inlet passages to improve the overall efficiency of the seal is discussed. Finally, the power loss performance is evaluated for each of the seal designs simulated so that potential trade-offs can be evaluated. Overall, the results show that increasing preswirl can increase the efficiency of the helical groove seal both by improving power loss and by improving leakage.