Teemu Turunen-Saaresti

Lappeenranta University of Technology Laboratory of Fluid Dynamics Teemu. Turunen@lut. fi

Jaakko Larjola

Lappeenranta University of Technology Laboratory ofFluid Dynamics Jaakko. Larjola@lut. fi

Abstract An unsteady experimental and numerical investigation was conducted to a cen­trifugal compressor with a vaneless diffuser. The unsteady static pressure was measured at the diffuser inlet and outlet at three different circumferential po­sitions. Also a time-accurate numerical simulation was carried out. The whole compressor was modeled. Turbulence was modeled with the low Reynolds num­ber k — e turbulence model. The measurements were made in operation points near the surge, near the choke and at the design operation point of the compres­sor. The numerical simulations were conducted at the design operation point and at the operation point near the choke. The gathered numerical and experimental unsteady pressures were compared with each other.

Keywords: Unsteady pressure, centrifugal compressor, vaneless diffuser

1. Introduction

The ft>w field inside a radial compressor is highly three-dimensional and unsteady. The wakes from the blades of the impeller and the non-axisymmetry of the volute causes a circumferentially varying ft>w field in the diffuser. The ft>w field in the vaneless diffuser is examined by measuring unsteady pres­sure at the inlet and outlet of the diffuser and analyzing the whole compressor numerically.


K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 493-503. © 2006 Springer. Printed in the Netherlands.

The simultaneous solution of the three-dimensional unsteady Navier-Stokes equations in the impeller and volute requires a large amount of a computa­tional resources. Previously Fatsis et al. [1] have carried out three dimensional unsteady fbw calculation of the impeller using Euler solver. Hillewaert and Van den Braembussche [2] have carried out the same simulation with the in­clusion of a steady volute calculation. Koumoutsos et al. [3] have conducted viscous three-dimensional unsteady flow calculation to the centrifugal com­pressor stage with vaned diffuser but the volute was not modeled.

In this paper, the time-accurate simulation was conducted to the whole com­pressor and the unsteady static pressure was measured at the diffuser inlet and outlet at three different circumferential positions. In the time-accurate simula­tion, the computational mesh describing the rotational part of the compressor was rotated. The connection between the stationary and the rotating part of the mesh was handled by using the sliding mesh technique. The grid lines between the impeller blocks and the stator block were discontinuous, so a mass con­serving interpolation was made at every time step [4]. At first, a time-averaged solution was calculated for the starting point of the time-accurate simulation. Then, using dual time-stepping, in which a pseudo-time integration was per­formed inside every physical time step, the time-accurate calculation was per­formed. This way the time history could be taken into account. Turbulence was modeled with the Chien’s k — є model [5]. A parallel computation with six pro­cessors was used to achieve a tolerable computational time. The convergence was speeded up with the multigrid method.

The unsteady static pressure measurements were made with Kulite XTC – 190 pressure transmitter and the data was collected with a Tektronix oscillo­scope connected to the computer. The unsteady pressure was measured within the time of ten rotations of the impeller. The measuring frequency was 1 MHz. Three sets of data were collected at each measuring point. The exact rotation speed of the rotor was measured at the same time.