Mathematical model of volutes and annular chambers of centrifugal compressor stages

Abstract

The outlet nozzle is one of the elements of the centrifugal compressor flow path. Outlet nozzles of centrifugal compressors are divided into two types: volute and annular chamber. The experience of using the mathematical model of outlet nozzles has shown the need of improvement. The new model for calculating outlet nozzle was developed. To simplify the calculation process, a constant density along the outlet nozzle length is adopted in the mathematical model. The circumferential component of the velocity is also determined approximately, by the constancy circulation equation without taking into account viscosity. After determining dimensions of the outlet nozzle, the pressure loss in it is calculated. Friction and expansion losses in the outlet diffuser are calculated by analogy with a straight circular diffuser. The friction coefficient is calculated for cases of a hydraulically smooth and rough surfaces. In volutes and annular chambers losses are calculated in the radial and meridional planes. In the radial plane, the main ones are friction loss. In the meridional plane, the main ones are expansion loss. For a trapezoid volute, they are considered depending on the wall divergence angle. If there is a crossover before the outlet nozzle, losses are also considered in it. In off-design operating modes, incidence losses are added. The presented model was implemented in the new version of the Universal modeling method. The mathematical model was identified by the results of turboexpander units and turbochargers tests.