Suppression Of Secondary Flows Research Articles

Suppression of secondary flows in a centrifugal impeller by optimisation design

PurposeThe purpose of this study is to suppress secondary flows and improve aerodynamic performance of a centrifugal impeller.Design/methodology/approachA multi-objective optimisation design system was described. The optimization design system was composed of a three-dimensional (3D) inverse design, multi-objective optimisation and computational fluid dynamics (CFD) analysis. First, the control parameter ΔCp for the secondary flows was derived and selected as the optimisation objective. Then, aimed at minimising ΔCp, a 3D inverse design for impellers with different blade loading distributions and blade lean angles was completed and multi-objective optimisation was conducted. Lastly, the improvement in the distribution of secondary flows and aerodynamic performance of the optimal impeller was demonstrated by CFD analysis.FindingsThe study derived the control parameter ΔCp for the secondary flows. ΔCp can indicate the distribution of secondary flows both near the blade pressure and suction surfaces. As ΔCp decreased, secondary flows decreased. The blade loading distribution with fore maximum blade loading at the shroud and aft maximum blade loading at the hub, coupled with a small negative blade lean angle, could help suppress secondary flows and improve aerodynamic efficiency.Originality/valueA direct control method on internal flow field characteristic-secondary flows by optimisation design was proposed for a centrifugal impeller. The impeller optimisation design process saves time by avoiding substantial CFD sample calculations.

Three-Dimensional Inverse Design Method for Hydraulic Machinery

Hydraulic machinery with high performance is of great significance for energy saving. Its design is a very challenging job for designers, and the inverse design method is a competitive way to do the job. The three-dimensional inverse design method and its applications to hydraulic machinery are herein reviewed. The flow is calculated based on potential flow theory, and the blade shape is calculated based on flow-tangency condition according to the calculated flow velocity. We also explain flow control theory by suppression of secondary flow and cavitation based on careful tailoring of the blade loading distribution and stacking condition in the inverse design of hydraulic machinery. Suggestions about the main challenge and future prospective of the inverse design method are given.

Influence of the Presence of the Lorentz Force and its Direction on the Suppression of Secondary Flow in Two Different Orifices: A Numerical Study using OpenFOAM

Influence of the Presence of the Lorentz Force and its Direction on the Suppression of Secondary Flow in Two Different Orifices: A Numerical Study using OpenFOAM

Experimental and Computational Study of the Effect of Momentum-Flux Ratio on High-Pressure Nozzle Guide Vane Endwall Cooling Systems

High-pressure (HP) nozzle guide vane (NGV) endwalls are often characterized by highly three-dimensional (3D) flows. The flow structure depends on the incoming boundary layer state (inlet total pressure profile) and the (static) pressure gradients within the vane passage. In many engine applications, this can lead to strong secondary flows. The prediction and design of optimized endwall film cooling systems is therefore challenging and is a topic of current research interest. A detailed experimental investigation of the film effectiveness distribution on an engine-realistic endwall geometry is presented in this paper. The film cooling system was a fairly conventional axisymmetric double-row configuration. The study was performed on a large-scale, low-speed wind tunnel using infrared (IR) thermography. Adiabatic film effectiveness distributions were measured using IR cameras, and tests were performed across a wide range of coolant-to-mainstream momentum-flux and mass flow ratios (MFRs). Complex interactions between coolant film and vane secondary flows are presented and discussed. A particular feature of interest is the suppression of secondary flows (and associated improved adiabatic film effectiveness) beyond a critical momentum flux ratio. Jet liftoff effects are also observed and discussed in the context of sensitivity to local momentum flux ratio. Full coverage experimental results are also compared to 3D, steady-state computational fluid dynamics (CFD) simulations. This paper provides insights into the effects of momentum flux ratio in establishing similarity between cascade conditions and engine conditions and gives design guidelines for engine designers in relation to minimum endwall cooling momentum flux requirements to suppress endwall secondary flows.

Mechanisms of Lean on the Performance of Transonic Centrifugal Compressor Impellers

Transonic centrifugal compressors with high performance are required in modern gas turbine engines and turbochargers. Lean of the blades is one of the crucial features that have a significant influence on their performance. This paper numerically investigates mechanisms by which lean affects the performance of a transonic impeller with twin splitters. Lean is defined relative to the tangential direction, and the variation range of lean angle has been chosen from to . Results show that the variation of efficiency value is 4.5% and the optimal lean angle occurs near 0 deg. Lean influences the flowfields through the effects on the shock structure, the spanwise pressure gradient, the axial-to-radial flow separation, and the secondary flow structure. The induced second blade surface vortex with positive lean tends to migrate the low-momentum flow from hub to shroud, whereas the induced inward spanwise pressure gradient with negative lean enhances the axial-to-radial flow separation, both of which increase the tip leakage loss and lead to the efficiency drop of leaned designs. The optimal lean angle is deduced to be the result of a trade-off between the suppression of axial-to-radial flow separation and the suppression of secondary flow.

Suppression of secondary flows in a double suction centrifugal pump with different loading distributions

Secondary flow is one of the main reasons for low efficiency in double suction centrifugal pump. In a 3-D inverse design method, the pump blade could be designed by a specified loading distribution to control the flow field in pump. In order to study the influence of loading distribution on secondary flow of a double suction centrifugal pump, the external characteristics and the internal flow field of the pump with three kinds of loading distributions are analysed by using CFD approach. According to the simulation results, it is found that the form of fore-loading distribution at shroud and aft-loading distribution at hub could improve the optimal efficiency and broaden the high efficiency area of the pump. Furthermore, the secondary flow in impeller exit region and volute could be significantly suppressed if the slope of loading distribution curve of shroud is set to be −0.7.

Suppression of Secondary Flow through Sidewall Converging in Model Horizontal Longitudinal MOCVD Processes

Suppression of Secondary Flow through Sidewall Converging in Model Horizontal Longitudinal MOCVD Processes

Suppression of Secondary Flows in a Mixed-Flow Pump Impeller by Application of Three-Dimensional Inverse Design Method: Part 1—Design and Numerical Validation

This paper describes the design of the blade geometry of a medium specific speed mixed flow pump impeller by using a three-dimensional inverse design method in which the blade circulation (or rVθ) is specified. The design objective is the reduction of impeller exit flow nonuniformity by reducing the secondary flows on the blade suction surface. The paper describes in detail the aerodynamic criteria used for the suppression of secondary flows with reference to the loading distribution and blade stacking condition used in the design. The flow through the designed impeller is computed by Dawes’ viscous code, which indicates that the secondary flows are well suppressed on the suction surface. Comparison between the predicted exit flow field of the inverse designed impeller and a corresponding conventional impeller indicates that the suppression of secondary flows has resulted in substantial improvement in the exit flow field. Experimental comparison of the flow fields inside and at exit from the conventional and the inverse designed impeller is made in Part 2 of the paper.

Suppression of Secondary Flows in a Mixed-Flow Pump Impeller by Application of Three-Dimensional Inverse Design Method: Part 2—Experimental Validation

In Part 1 of this paper, a mixed-flow pump impeller was designed by a fully three-dimensional inverse design method, aimed at suppressing the secondary flows on the blade suction surface. In this part, the internal flow fields of the impeller are investigated experimentally, using flow visualization and phase-locked measurements of the impeller exit flow, in order to validate the effects of secondary flow suppression. The flow fields are compared with those of a conventional impeller, and it is confirmed that the secondary flows on the blade suction surface are well suppressed and the uniformity of the exit flow fields is improved substantially, in both circumferential and spanwise directions. The effects of tip clearance and the number of blades for the inverse designed impeller are also investigated experimentally and numerically.

Small channels: Their experimental formation, nature, and significance

AbstractThe manner in which small channels are generated, from plane beds beneath sheet flows, has been experimentally elucidated. On plane, erodible, sand beds, the transition from thin, supercritical sheet flows to the channelled condition was studied over ranges of discharge, slope, and temperature. Secondary flow of the second kind, its action facilitated by steep vertical velocity gradients in the primary flows, caused sheet‐flow instability. Along junctions between neighbouring secondary cells, both either raised or lowered elements of the primary flow. In the latter case, fast surface water was lowered to the bed, causing relatively intense, local, longitudinal scour. Dislodged grains were moved divergently to either side, leaving straight, central trenches. Development of positive feedback between cells and trenches led to rapid enlargement of the latter and concomitant growth of paired levees. The resulting structures, ‘protochannels’, were themselves ephemeral, developing two types of instability associated with secondary flow of the first kind. Firstly, small deviations from bilateral symmetry were enhanced, causing evolution into meandering channels. Secondly, headcutting led to multiple tributary development and, at resulting confluences, the action of strong pairs of secondary cells led to the development of braiding channels. Because they are shortlived, protochannels are but rarely seen in nature. Their seeding is markedly temperature‐sensitive, reflecting their frictional origin.The erosive power of shallow overland flow largely depends on flow‐energy concentration by secondary flow, firstly into channels, then within the channels themselves. Suppression of secondary flow, as by intense raindrop bombardment, can stabilize sheet flows.In deeper water, the effects of secondary flow appear relatively less dramatic. However, even if such motion is weak, bedload divergence from attachment lines can favour entrainment locally and thus affect bed geometry. Analogy between our results and river behaviour appears close and. on continental shelves where water must often flow as sheets, structures resembling giant protochannels evidently persist.