Smaïne Kouidri

Numerical Modelization of the Flow in Centrifugal Pump: VoluteInfluence in Velocity and Pressure Fields

A 3D-CFD simulation of the impeller and volute of a centrifugal pump has been performed using CFX codes. The pump has a specific speed of 32 (metric units) and an outside impeller diameter of 400 mm. First, a 3D flow simulation for the impeller with a structured grid is presented. A sensitivity analysis regarding grid quality and turbulence models were also performed. The final impeller model obtained was used for a 3D quasi-unsteady flow simulation of the impeller-volute stage. A procedure for designing the volute, the nonstructured grid generation in the volute, and the interface flow passage between the impeller and volute are discussed. This flow simulation was carried out for several impeller blades and volute tongue relative positions. As a result, velocity and pressure field were calculated for different flow rates, allowing to obtain the radial thrust on the pump shaft.

Inverse Design Method for Centrifugal Impellers and Comparison with Numerical Simulation Tools

A process that enables us to improve the design of 2D centrifugal and helico-centrifugal pumps is presented. First of all, the definition of the impeller geometry as well as the analysis of its global performances are carried out starting from the mean streamline method (1D), based at the same time on ideal models and experimental correlations. A second stage of optimisation is achieved from a quasi three-dimensional (Q3D) method, by studying the meridional flow and blade-to-blade flow. Finally, 3D flow solution is performed by CFD tools. Nowadays, we have a group of tools which help the designers improve the performance of new machines. These digital tools are built around two computer programs, HELIOX developed for design and performance analysis in any centrifugal and mixed flow pumps equipped with volute or deswirl vanes, and also the module REMIX that gathers the meridional flow analysis and the simplified blade-to-blade one. To validate this procedure, a centrifugal machine with a volute (NS32) was modified and studied with it, and the results were simultaneously compared with the previous trial runs and with the software CFX-BladeGEN+ and CFX-TASCflow. The results for a machine equipped with a deswirl (VM51) are also presented.

Influence of Impeller Geometry on the Unsteady Flow in a Centrifugal Fan: Numerical and Experimental Analyses

The aim of this study is to evaluate the influence of design parameters on the unsteady flow in a forward-curved centrifugal fan and their impact on the aeroacoustic behavior. To do so, numerical and experimental studies have been carried out on four centrifugal impellers designed with various geometrical parameters. The same volute casing has been used to study these impellers. The effects on the unsteady flow behavior related to irregular blade spacing, blade count and radial distance between the impeller periphery and the volute tongue have been studied. The numerical simulations of the unsteady flow have been carried out using computational fluid dynamics (CFD) tools based on the unsteady Reynolds averaged Navier Stokes (URANS) approach. The study is focused on the unsteadiness induced by the aerodynamic interaction between the volute and the rotating impeller blades. In order to predict the acoustic pressure at far field, the unsteady flow variables provided by the CFD calculations have been used as inputs in the Ffowcs Williams-Hawkings equations (FW-H). The experimental part of this work concerns measurement of aerodynamic performance of the fans using a test bench built according to ISO 5801 (1997) standard. In addition to this, pressure microphones have been flush mounted on the volute tongue surface in order to measure the wall pressure fluctuations. The sound pressure level (SPL) measurements have been carried out in an anechoic room in order to remove undesired noise reflections. Finally, the numerical results have been compared with the experimental measurements and a correlation between the wall pressure fluctuations and the far field noise signals has been found.

Incident Turbulence Interaction Noise from an Axial Fan

The development of analytical methods allowing the prediction of the noise radiated by an airfoil in a turbulent flow is an active research topic. Its extension to the blades row is still in progress. However, their validation by well measurements is missing. In this paper, a prediction method of broadband noise, due to the incident turbulence and radiated in free field by a subsonic low-pressure axial fan, is reviewed then validated. This approach is an extension of the model initially established for an isolated airfoil placed in a turbulent flow to rotating blades. The formulation providing the noise produced by an airfoil in rectilinear motion is used for the calculation of the instantaneous acoustic spectrum generated by a blade segment considering the rotation effects. The acoustic calculation is based on the Amiet formulation allowing the determination of the far field acoustic power spectral density using a statistical description of the upstream turbulence. A low-pressure axial fan without guide vanes is exposed to a range of turbulent flow fields generated by five different inflow arrangements. Turbulence inflow properties and far field sound were both measured allowing comparison with the predicted results. The mean flow velocity distribution in the plane of the rotor and the turbulent intensities are measured using hot wire anemometry. The local spatial correlation lengths in circumferential direction are derived via a crosscorrelation method employing two hot wire probes at various angular distances. The results thus obtained were used to calculate the power spectral density of the inflow velocity fluctuations, which are the input data for the aeroacoustic model. As expected, the inflow arrangements have a significant influence on both the statistical parameters of the flow field in the plane of the fan rotor and the broadband noise radiated by the fan. The directivity feature of the broadband turbulence interaction noise shows that the main radiation lobe is located along the fan axis. The predicted acoustic power spectra show a good agreement with the experimental results and especially for inflow arrangements generating low turbulence intensity.

DC Flow Investigations in Thermoacoustic Prime Mover

In thermoacoustic device, the nonlinear phenomena are the source of secondary flows superimposed on the oscillating flow. To date, the acoustic streaming have been only characterized for progressive and standing waves in current systems. A method to characterize theoretically the secondary flows in thermoacoustic systems with more complex geometries is developed in this paper. This method is applied in the case of an annular thermoacoustic prime mover. The streaming fields are then provided. As the determination of the streaming fields requires the knowledge of the first order acoustic parameters, a linear model is used. The theoretical results of the acoustic pressure are compared to the experimental data. A relative error less than 10% is obtained, allowing the validation of the linear model.Copyright

Investigation on streaming sources in thermoacoustic prime mover

Thermoacoustic devices either prime mover, heat engines or refrigerators are not known for their high efficiency. Even though these systems have many advantages regarding environmental constraints, they are not yet used in the industrial applications. Energy conversion efficiency improvement of thermoacoustic systems is now in the priority of the thermoacoustic community. One of the reasons of the relative low efficiencies is in the physical understanding which is not well achieved. The appearance of steady mass flow of second order usually called streaming and superimposed to the oscillating flow in these systems is shown as an important dissipating energy phenomenon. From energy consideration and despite their low level, this DC flow involves heat transfer to the wall which is undesirable loss mechanism. This phenomenon which is a quite old topic is still widely investigated experimentally and theoretically. The design, construction and performance measurements of the traveling wave thermoacoustic engin...

Measurement of wall pressure fluctuations for noise prediction in axial flow fans

The unsteady pressure field on fan blades is an important investigation topic. Both numerical simulation and experimental techniques are used in order to achieve this purpose. However neither has given yet entire satisfaction. The CFD tools using the resolution of the averaged Navier Stokes equations do not really give the unsteady aerodynamic characteristics of the flow needed for an accurate noise prediction. In addition, tools using large eddy simulation are still expensive for industrial users in the case of a complex geometry such as turbomachinery. When a lifting surfaces goes through turbulence, pressure fluctuations occur on their surfaces that can radiate noise. To calculate these fluctuations and thus the noise requires a theoretical model of the unsteady aerodynamics. The validation and development of these models require data and understanding from experiments. Unsteady surface pressure measurements were carried out on one fan blade with an array of pressure transducers with high sensitivity. ...

Numerical strategies for investigation of gust‐airfoil interaction

The noise generated by the interaction between a gust and an airfoil in a uniform flow is investigated. This problematic is of major industrial interest, regarding fans, turbomachinery, or wind turbine applications. A two‐dimensional symmetric Joukowski‐type airfoil is immersed without incidence in a flow at Mach number 0.5, disturbed by a harmonic gust at 45° of incidence (4th CAA Workshop on Benchmark Problems, 2004). Our methodology is first to perform a high‐order direct resolution of Eulers equations of the disturbed flow over the airfoil and the associated acoustic emission, which is taken as a reference simulation. Second, the near aerodynamic field is simulated with Fluent 6.3 solver based on finite volume method with second‐order schemes. The aerodynamic data thus obtained are used for far field acoustic prediction, based on Ffowcs Williams and Hawkings analogy. Finally, following another hybrid approach, the noise is predicted by using integral formulations with source field from the DNS. The a...

Numerical Study of Time Domain Approach Applied to the Prediction of Noise Radiation From Rotating Blades

Aeroacoustic formulations in time domain are frequently used to model the aerodynamic sound of airfoils, the time data being more accessible. The formulation 1A developed by Farassat, integral solution of the Ffowcs Williams and Hawkings equation, holds great interest because of its adequacy for surfaces in arbitrary motion. The aim of this work is to study the numerical sensitivity of this model to specified parameters and the geometry used in the calculation. The numerical algorithms, spatial and time discretizations, and approximations used for far-field acoustic simulation are presented. A parametrical study of the relevant criteria is carried out based on the Isom’s and Tam’s test cases. A helicopter blade airfoil as defined by Farassat to investigate the Isom’s case is used in this work. According to Isom, the acoustic response of a dipole source with a constant aerodynamic load ρ0 c0 2 is equal to the thickness noise contribution. In practice, this observation is subject to numerical errors that are not systematically well controlled. Variations of these errors depending on the time step, Mach number and the source-observer distance are studied. The analysis is then extended to the Tam’s test case. Tam test case has the advantage of providing an analytical solution for the first harmonic.© 2007 ASME