High-efficiency design of a mixed-flow pump has been carried out based on numerical analysis of a three-dimensional viscous flow. For analysis, the Reynolds-averaged Navier-Stokes equations with a shear stress transport turbulence model were discretized by finite-volume approximations. Structured grid system was constructed in the computational domain, which has O-type grids near the blade surfaces and H/J-type grids in other regions. The numerical results were validated with experimental data for the heads and hydraulic efficiencies at different flow coefficients. The hydraulic efficiency at the design flow coefficient was evaluated with variation of the geometric variables, i.e., the area of the discharge and length of the vane in the diffuser. The result has shown that the hydraulic efficiency of a mixed-flow pump at the design condition is improved by the modification of the geometry.

High-efficiency design of a mixed-flow pump

This paper presents an optimization design method for centrifugal compressors based on one-dimensional calculations and analyses. It consists of two parts: (1) centrifugal compressor geometry optimization based on one-dimensional calculations and (2) matching optimization of the vaned diffuser with an impeller based on the required throat area. A low pressure stage centrifugal compressor in a MW level gas turbine is optimized by this method. One-dimensional calculation results show that D3/D2 is too large in the original design, resulting in the low efficiency of the entire stage. Based on the one-dimensional optimization results, the geometry of the diffuser has been redesigned. The outlet diameter of the vaneless diffuser has been reduced, and the original single stage diffuser has been replaced by a tandem vaned diffuser. After optimization, the entire stage pressure ratio is increased by approximately 4%, and the efficiency is increased by approximately 2%.

/pdf/a-new-optimization-method-for-centrifugal-compressors-based-xzmqkniidv.pdf

A New Optimization Method for Centrifugal Compressors Based on 1D Calculations and Analyses

The matching of a vaned diffuser with a centrifugal impeller is examined with a one-dimensional (1D) analysis combined with extensive experimental data A matching equation is derived to define the required throat area of the diffuser relative to the throat area of the impeller at different design speeds and validated by comparison with a wide range of compressor designs The matching equation is then used to give design guidelines for the throat area of vaned diffusers operating with impellers at different tip-speed Mach numbersAn analysis of test data for a range of high pressure ratio turbocharger compressor stages is presented in which different matching between the diffuser and the impeller has been experimentally examined The test data includes different impellers with different diffuser throat areas over a wide range of speeds It is shown that the changes in performance with speed and diffuser throat area can be explained on the basis of the tip-speed Mach number which causes both the diffuser and impeller to choke at the same mass flow Based on this understanding, a radial compressor map prediction method is extended to include this parameter, so that more accurate maps for matched and mismatched vaned diffusers can be predictedCopyright © 2014 by ASME

https://library.e.abb.com/public/032ab8398281326b85257da5007742ba/The%20Matchin%20of%20a%20Vaned%20Diffuser.pdf

The Matching of a Vaned Diffuser With a Radial Compressor Impeller and Its Effect on the Stage Performance

\n Multi-objective design optimization was applied to the impeller and volute of a centrifugal pump using surrogate-based optimization techniques and three-dimensional Reynolds-averaged Navier–Stokes (RANS) analysis. The objective functions used to improve the hydraulic performance and operating stability of the pump were the hydraulic efficiency at the design condition and the flow rate at which the maximum volute pressure recovery coefficient occurs. Three design variables were selected based on the results of a sensitivity analysis: the blade outlet angle, the constants in determining the impeller outlet width, and the cross-sectional area of the volute. Using response surface approximation (RSA), surrogate models were constructed for the objective functions based on numerical results at experimental points obtained by Latin hypercube sampling (LHS). The representative Pareto-optimal solutions obtained by the multi-objective genetic algorithm (MOGA) show enhanced objective function values compared to the baseline design. The results of unsteady calculation show that the flow instability of the centrifugal pump was successfully suppressed by the optimization.

Design Optimization of the Impeller and Volute of a Centrifugal Pump to Improve the Hydraulic Performance and Flow Stability

Variable inlet prewhirl is an effective way to suppress compressor instability. Compressors usually employ a high degree of positive inlet prewhirl to shift the surge line in the performance map to a lower mass flow region. However, the efficiency of a compressor at high inlet prewhirl is far lower than that at zero or low prewhirl. This paper investigates the performances of a centrifugal compressor with different prewhirl, discusses the mechanisms thought to be responsible for the production of extra loss induced by high inlet prewhirl and develops flow control methods to improve efficiency at high inlet prewhirl. The approach combines steady three-dimensional Reynolds average Navier-Stockes (RANS) simulations with theoretical analysis and modeling. In order to make the study universal to various applications with inlet prewhirl, the inlet prewhirl was modeled by modifying the velocity condition at the inlet boundary. Simulation results show that the peak efficiency at high inlet prewhirl is reduced compared to that at zero prewhirl by over 7.6 percentage points. The extra loss is produced upstream and downstream of the impeller. Severe flow separation was found near the inlet hub which reduces efficiency by 2.3 percentage points. High inlet prewhirl works like a centrifuge gathering low-kinetic-energy fluid to hub, inducing the separation. A dimensionless parameter C is defined to measure the centrifugal component of flow. As for the extra loss produced downstream of the impeller, the flow mismatch of impeller and diffuser at high prewhirl causes a violent backflow near the diffuser vanes’ leading edges. An analytical model is built to predict diffuser choking mass flow which proves that the diffuser flow operates outside of stable conditions. Based on the two loss mechanisms, hub curve and diffuser stager angle were modified and adjusted for seeking higher efficiency at high prewhirl. The efficiency improvement of a modification of the hub is 1.1 percentage points and that of the combined optimization is 2.4 percentage points. During optimizing, constant distribution of inlet prewhirl was found to induce reverse flow at the leading edge of the blade root, which turned out being uncorrelated with blade angle. By revealing loss mechanisms and proposing flow control ideas, this paper lays a theoretical basis for overcoming the efficiency drop induced by high inlet prewhirl and for developing compressors with high inlet prewhirl.Copyright © 2015 by ASME

Loss Mechanisms and Flow Control for Improved Efficiency of a Centrifugal Compressor at High Inlet Prewhirl

The centrifugal compressor for a marine use turbocharger with its design pressure ratio of 3.2 was tested with a vaneless diffuser and various vaned diffusers. Vaned diffusers were chosen to cover impeller operating range as broad as possible. The analysis of the static pressure ratio in the impeller and the diffusing system, consisting of the diffuser and scroll, showed that there were four possible combinations of characteristics of impeller pressure ratio and diffusing system pressure ratio. The flow rate, Q P , where the impeller achieved maximum static pressure ratio, was surge flow rate of the centrifugal compressor determined by the critical flow rate. In order to operate the compressor at a rate lower than Qp, the diffusing system, whose pressure recovery factor was steep negative slope near Q P , was needed. When the diffuser throat area was less than a certain value, the compressor efficiency deteriorated ; however, the compressor stage pressure ratio was almost constant. In this study, by reducing the diffuser throat area, the compressor could be operated at a flow rate less than 40 percent of its design flow rate. Analysis of the pressure ratio in the impeller and diffusing systems at design and off-design speeds showed that the irregularities in surge line occurred when the component that controlled the negative slope on the compressor stage pressure ratio changed.

The Experimental Study of Matching Between Centrifugal Compressor Impeller and Diffuser

The centrifugal compressor for a marine use turbocharger with its design pressure ratio of 3.2 was tested with a vaneless diffuser and various vaned diffusers. Vaned diffusers were chosen to cover impeller operating range as broad as possible. The analysis of the static pressure ratio in the impeller and the diffusing system — consists of the diffuser and scroll — showed that there were four possible combinations of characteristics of impeller pressure ratio and that of diffusing system pressure ratio.The flow rate, QP, where the impeller achieved maximum static pressure ratio was the critical flow rate determined surge flow rate of the centrifugal compressor. In order to operate the compressor less than QP, the diffusing system whose pressure recovery factor was steep negative slope near QP was needed. Using the diffuser throat area less than a certain value, the compressor efficiency deteriorated, however the compressor stage pressure ratio was almost constant. In this study, the compressor by reducing the diffuser throat area could be operated at the flow rate less than 40% of its design flow rate. Analysis of the pressure ratio in the impeller and diffusing systems at design and off design speeds showed that the irregularities in surge line occurred when the component which controlled the negative slope on the compressor stage pressure ratio changed.Copyright © 1997 by ASME

/pdf/the-experimental-study-of-matching-between-centrifugal-441qsrv7r5.pdf

Hidefumi Nakao

Papers

The Experimental Study of Matching Between Centrifugal Compressor Impeller and Diffuser

The Experimental Study of Matching Between Centrifugal Compressor Impeller and Diffuser