An efficient method of measuring impedances of fluid machinery manifolds

Abstract

Knowledge of the acoustical impedances is required to understand the fluid oscillations in the manifolds of positive displacement machinery. This paper presents an impedance measurement method which puts emphasis on the lower frequencies. The technique is based on providing a known volume velocity excitation to the manifold through an electrodynamic, shaker driven, oscillating piston. Harmonic, random and transient excitations have been utilized. Digital data acquisition and processing techniques are used to acquire piston displacement and acoustic pressure signals for input and transfer impedances. The proposed method is direct and efficient. It is applied to the measurement of the acoustical impedances of a two-cylinder refrigeration compressor discharge system. Various aspects of the measurement procedures are discussed along with the recommendations.