Investigation of Flow Structure in a Narrow Clearance of a Low Specific Speed Centrifugal Impeller

The disk friction loss is remarkably large in low specific speed centrifugal pumps, and an effective reduction method has not been established. Therefore, to develop such a method, the loss mechanism was investigated. To grasp the internal flow structure in the narrow clearance, both experimental and computational approaches were used. An experimental apparatus that imitates clearance between a rotating impeller disk and a stationary casing disk was used, and the static pressure distribution in the radial direction was measured. The internal flow where the disk friction loss occurs was investigated. In the case of outward flow, the static pressure decreased because the influence of the centrifugal force lessened toward the outer diameter side of the disk, as the flow rate surged. For this reason, the pressure gradient became steep. According to the CFD analysis, there was a vortex in the cross-section of the clearance. This vortex encouraged flow recirculation and promoted the increased of the circumferential velocity in the potential core. When the flow rate grew, the vortex diminished. The circumferential velocity gradient and the shear stress intensified. As a result, the disk friction escalated. In the case of inward flow, the pressure gradient became steep as the flow rate increase. There was a vortex in the clearance, the size of which lessened when the flow rate surged. The disk friction had a minimum value at the flow rate was 6e-4 m3/s. This research clarified that the vortex in the clearance has a remarkable effect on reducing the disk friction.

Numerical research on effect of dislocated turbine rims on hot gas ingestion

The flow characteristics in the rim clearances of turbines are sensitive to the turbine rim structures. From this standpoint, rotor–stator cavity models with dislocated clearances are proposed in this study to reveal the effect of dislocated turbine rim lips on hot gas ingestion and the clearance flow characteristics. The results show that the rim clearance flow is unstable, and Kelvin–Helmholtz vortices are induced by the relatively large difference in circumferential velocity of the flow fields inside and outside the rim seal. Moreover, the dislocated turbine rim lips decrease the effectiveness of the rim seal. Ultimately, the responses of the flow characteristics to the dislocated rims are determined to be primarily reflected in the effect of the forward or backward step flow on the Kelvin–Helmholtz vortices in turbine rims.

Numerical Investigation on the Flow Characteristics in a Cover-Plate Pre-Swirl System

This paper presents a numerical investigation on the flow characteristics in a cover-plate pre-swirl system. The Reynolds-averaged Navier-Stokes equations, coupled with the standard k-ε turbulent model, are adopted and solved. With the inlet total pressure and total temperature being constant, the influences of the temperature reduction and flow resistance by changing pressure ratios and rotational Reynolds numbers were conducted. Flow features in the pre-swirl nozzle, pre-swirl cavity, receiver hole and cover-plate cavity were summarized. The results obtained in this study indicate that the pressure ratio and rotational Reynolds number have a significant influence on the vortex structure of the pre-swirl system. As the air is accelerated by the pre-swirl nozzle, the difference of circumferential velocity between the air and the rotational domain would be reduced, and the static temperature of the air would be decreased. The pressure drop in the pre-swirl system mainly occurs in the pre-swirl nozzle and the pre-swirl cavity. In addition, with the increase of the pressure ratio, the air mass flow rate and the circumferential velocity of the air out of the nozzle increased, thereby leading to an increment in temperature reduction. Moreover, with the increasing of the rotational Reynolds number, the dimensionless mass flow rate and temperature reduction of the pre-swirl system, which are mainly determined by the flow incidence angle of cooling air at the receiver hole, will first increase to a maximum and then decrease.

Details of Shrouded Stator Hub Cavity Flow in a Multi-Stage Axial Compressor Part 2: Leakage Flow Characteristics in Stator Wells

The flow in shrouded stator cavities can be quite complex with axial, radial, and circumferential variations. As the leakage flow recirculates and is re-injected into the main flow path upstream of the stator, it deteriorates the near-hub flow field and, thus, degrades the overall aerodynamic performance of the compressor. In addition, the windage heating in the cavity can raise thermal-mechanical concerns. Fully understanding the details of the shrouded-hub cavity flow in a multi-stage environment can enable better hub cavity designs. In the first part of the paper, the influence of the hub leakage flow on compressor performance and its interactions with the primary flow were investigated. While the impact of hub leakage flow on the primary passage is readily available in the open literature, details inside the cavity geometry are scarce due to the difficulties in instrumenting that region for an experiment or modeling the full cavity geometry. To shed light on this topic, the flow physics in the stator cavity inlet and outlet wells are investigated in the present paper using a coupled CFD model with inclusion of the stator cavity wells for the Purdue 3-Stage (P3S) Axial Compressor, which is representative of the rear stages of a high-pressure-compressor in core engines. At the inlet cavity, the presence of at least one pair of vortices influences the trajectory of the cavity leakage flow. The amount of leakage flow also determines the size of the vortical structures, with larger clearances creating a smaller vortex and vice versa. After passing through the labyrinth seals, the leakage flow travels along the stator landing first and then transitions to the rotor drum. In general, a flow path closer to the rotor drum achieves higher circumferential velocity but also exhibits significant temperature rise. A rise in circumferential velocity directly corresponds to a rise in temperature. In addition, the windage heating increases with increasing seal clearance. Furthermore, the inlet well contributes the most to overall windage, nearly 50% of the total windage heating, while the labyrinth seals and outlet well account for very little.

Making Better Swirl Brakes Using Computational Fluid Dynamics: Performance Enhancement From Geometry Variation

A fluid with a large swirl (circumferential) velocity entering an annular pressure seal influences the seal cross-coupled dynamic stiffness coefficients and hence it affects system stability. Typically comprising a large number of angled vanes around the seal circumference, a swirl brake (SB) is a mechanical element installed to reduce (even reverse) the swirl velocity entering an annular seal. SB design guidelines are not readily available and existing configurations appear to reproduce a single source. By using a computational fluid dynamics (CFD) model, the paper details a process to engineer a SB upstream of a sixteen-tooth labyrinth seal (LS) with tip clearance Cr = 0.203 mm. The process begins with a known nominal SB* geometry and considers variations in vane length (LV* = 3.25 mm) and width (WV* = 1.02 mm), and stagger angle (θ* = 0°). The vane number NV* = 72 and vane height HV* = 2.01 mm remain unchanged. The SB-LS operates with air supplied at pressure PS = 70 bar, a pressure ratio PR = exit pressure Pa / PS = 0.5, and rotor speed Ω = 10.2 krpm (surface speed ΩR = 61 m/s). Just before the SB the pre-swirl velocity ratio = average circumferential velocity U / shaft surface speed (ΩR) equals α = 0.5. For the given conditions, an increase in LV allows more space for the development of vortexes between two adjacent vanes. These are significant to the dissipation of fluid kinetic energy and thus control the reduction of α. A 42% increase in vane length (LV = 4.6 mm) produces a ∼ 43% drop in swirl ratio at the entrance of the LS (exit of the SB), from αE = 0.23 to 0.13. Based on the SB with LV = 4.6 mm, the stagger angle θ varies from 0° to 50°. The growth in angle amplifies a vortex at ∼ 70% of the vane height while it weakens a vortex at 30% of HV. For θ = 40°, the influence of the two vortexes on the flow produces the smallest swirl ratio at the LS entrance, αE = −0.03. For a SB with LV = 4.6 mm and θ = 40°, the vane width WV varies from 0.51 mm to 1.52 mm (± 50% of WV*). A reduction in WV provides more space for the strengthening of the vortex between adjacent vanes. Therefore, a SB with greater spacing of vanes also reduces the inlet circumferential velocity. For WV = 0.51 mm, αE further decreases to −0.07. Besides the design condition (α = 0.5), the engineered SB having LV = 4.6 mm, θ = 40° and WV = 0.51 mm effectively reduces the circumferential velocity at the LS entrance for other inlet pre-swirl ratios equaling α = 0 and 1.3. Rather than relying on extensive experiments, the CFD analysis proves effective to quickly engineer a best SB configuration from the quantification of performance while varying the SB geometry and inlet swirl condition.

Experimental investigation on nozzle diameter of vortex gripper

Purpose Vortex grippers use tangential nozzles to form vortex flow and are able to grip a workpiece without any physical contact, thus avoiding any unintentional workpiece damage. This study aims to use experimental and theoretical methods to investigate the effects of nozzle diameter on the performance. Design/methodology/approach First, various suction force-distance curves were developed to analyze the effects of nozzle diameter on the maximum suction force. This study determines the tangential velocity distribution on the workpiece surface by substituting the experimental pressure distribution data into simplified Navier-Stokes equations and then used these equations to analyze the effects on the flow field. Subsequent theoretical analysis of the distribution of pressure and circumferential velocity further validated the experimental results. Next, by rearranging these relationships, the study considered the effects of nozzle diameter on the inherent vortex gripper characteristics. In addition, this study developed various suction force-energy consumption curves to analyze the effects of nozzle diameter. Findings The results of this study indicated that the vortex gripper’s circumferential velocity and maximum suction force decrease with increasing nozzle diameter. Nozzle diameter did not significantly affect the inherent frequency of the vortex gripper-workpiece inertial system or the corresponding suspension stability of the workpiece. However, an increase in nozzle diameter did effectively increase the vortex gripper’s suspension region. Finally, as the nozzle diameter increased, the energy required to achieve the same maximum suction force decreased. Originality/value This study’s findings can enable optimization of nozzle design in emerging vortex gripper designs and facilitate informed selection among existing vortex grippers.

Assessment of the Loss Map of a Centrifugal Compressor’s External Volute

A volute’s loss coefficient is highly sensitive to Mach number, circumferential velocity and flow rate at volute inlet. In case of a backswept impeller, these parameters are coupled to each other. An increased flowrate leads to a steeper absolute flow angle at impeller exit and hence to a decrease of circumferential velocity. The absolute Mach number is also altered. Therefore, in order to investigate the effects of flowrate and flow angle separately, one would have to vary the diffuser width together with the flowrate, keeping the flow angle constant. This corresponds to coupling the volute with aerodynamically similar impellers, designed for higher and lower flowrates. Since this is elaborate, there is no adequate study available in open literature, assessing a volute’s global loss map. In this work, a new numerical approach for the prediction of a volute’s representative loss map is presented: The volute is calculated by means of steady CFD as a standalone component. The inlet boundary conditions are carefully selected by means of 1D and applied together with different diffuser widths. This allows for separate investigation of the impacts of flow angle, flow rate and Mach number. Validation against full stage CFD confirms the applicability of the standalone model. The results exhibit that minimum losses do not necessarily occur at the theoretical matching point but either when the volute is smaller or bigger, depending on the inlet flow angle. Investigations of the loss mechanisms at different operating conditions provide useful guidelines for volute design. Finally, the validity of these study’s findings for volutes with different geometrical features is examined by comparison with experimental data as well as with fullstage CFD.

Numerical Investigation of Fluid Flow between Rotating Permeable Cylindrical Surfaces

The paper presents numerical simulation results concerning fluid flow in the annular channel of a hydrodynamic filter comprising a perforated protective screen located between another perforated protective screen and a filtering screen, both cylindrical. We investigated the effects of the following two parameters on the flow structure: the perforated area of the protective screen and the width of the annular channel between the protective and filtering cylindrical screens. We established that increasing the annular channel width and the perforation area of the protective screen leads to secondary vortex structures forming in the channel. We obtained circumferential velocity distribution in the channel formed by the protective and filtering screens of the hydrodynamic filter. We show that, in the bracket of modal and design parameters under consideration, a power curve with an exponent in the 2.4--3.3 range may be used to approximate the circumferential velocity profile. We discovered that the structural and modal parameters of the channel between the rotating permeable cylindrical surfaces control the intensity of the deterministic separation process components. Channel width and perforation area are structural parameters; angular velocity is a modal parameter. Arranging the flow in a hydrodynamic filter in the way proposed makes it possible to decrease the intensity of random separation process components in multi-phase media.

Design and Justification of a Combined Tillage Machine

The tillage of soil with minimum energy consumption can be achieved by breaking the bonds between soil aggregates with tensile deformations. The design of a combined tillage machine is proposed and its technological parameters are justified. The proposed machine includes a frame with a flat working element with a leg, top and bottom rotary agitators with a drive mechanism placed behind a flat working element above each other. The design feature is that the circumferential velocity of a soil-destroying element of the lower rotary agitator is higher than the translational velocity of a machine, and the circumferential velocity of the upper rotary agitator is higher than the circumferential velocity of the lower rotary agitator. Besides, the proposed shape of a rotary agitator made along the Archimedes’ spiral does not allow soil to be collapsed by the back of a cutting edge. This reduces specific energy consumption at high quality of soil loosening. The purpose of the machine is to reduce specific energy consumption of soil treatment. This is achieved by the fact that the axis of rotation of the upper rotary agitator is shifted backwards in the direction of machine movement relative to the axis of rotation of the lower rotary agitator by h = R · sin · (arccos (– 1/λ)), where R – radius of the upper rotary agitator, λ – kinematic parameter characterizing the operation mode of the upper rotary agitator. Thus, the destruction of a pre-stressed soil formation due to tensile stresses caused by the mutual arrangement of working elements and the interconnection of machine operation modes contributes to the reduction of energy consumption.