Flow-induced noise generation at the outlet of a capillary tube

Abstract

The aim of this work is the identification of noise generation mechanisms at the outlet of an adiabatic capillary tube used as throttling device in a cooling cycle. For this purpose, experiments are carried out on a R-600a test bench designed for systematic investigations on flow-induced capillary noise. The experiments are performed using a copper capillary tube with an inner diameter of 0.6 mm. The test bench enables process control with a continuous change of the capillary outlet pressure, while the inlet pressure and temperature remain constant. A similar process takes place in a refrigerator at the beginning of the on-cycle, when the compressor starts to pull down the evaporation pressure. Correlations between various operating conditions and the emitted airborne noise can thus be established. The evaluation of the sound pressure level profiles provides a method to indicate the change of flow regimes at the capillary tube outlet, using flow pattern maps from literature. We identify two different noise generation mechanisms: In case of intermittent flow regimes, pressure pulses evoked by the fluid lead to fluctuating unsteady noise excitations. During non-intermittent flow regimes, on the other hand, uniform noise excitations strongly correlate with the vapor velocity of the two-phase flow.