Identification of dynamic phenomena in pipes supplied with a pulsating flow of gas

Abstract

Pulsating flow is typical of displacement machines such as piston engines, compressors, or other displacement devices. The cyclic movement of the working element (piston, valve, membrane, etc.) generates pulsations, which, from a dynamic point of view, can be considered as excitation in the system composed of a flowing gas and pipes in which the flow takes place. This study shows that under certain conditions, a significant increase of pulse amplitudes occurs and transient values of main flow parameters are up to a few times higher than that for the equivalent steady flow conditions. This unsteadiness can be harmful or even damaging; however, in some cases it can be profitable (e.g. induction ramming of combustion engines). In order to investigate which conditions are favourable to obtain such effects in pipes, four different systems, distinguished by a different level of choking at the pipe outlet section, were tested. The experimental tests performed in the range of excitation frequencies of 0—200 Hz have shown that systems with low choking at the pipe outlet are more prone to strong dynamic effects, characterized by considerable growth of pulse amplitudes and an intensive reverse flow occurring in the proximity of resonance frequencies. In systems with high choking at the pipe outlet section, those effects were very weak. Characteristic frequencies, for which the resonance takes place, depend on the pipe length and can be determined theoretically with a simple acoustic model or a one-dimensional model ‘ x— t’ describing a pulsating flow in the pipe. The experimental verification of those resonance frequencies can be conducted with the use of transient flow parameters recorded by appropriate fast response sensors.