Gas Pulsations: A Shock Tube Mechanism

Gas pulsations are defined presently as a macro flow rate and/or pressure fluctuation with relatively low frequency and high amplitude. They commonly exist in HVACR, energy and other processing industries, and are widely accepted to be mainly caused by PD type gas machinery such as reciprocating or rotary compressors, expanders and Roots type blowers. Moreover, they are believed to be responsible for system vibrations, noises and fatigue failures. Naturally, as important a matter as gas pulsations, there have been tremendous R/D efforts from both academia and industry focused in this area, especially since the late 1980s. The most well known works are acoustic models based on small perturbation and CFD methods aimed at solving nonlinear unsteady differential equations for pulsating flows. Both approaches have been successful in calculating gas pulsations at off-design conditions of either an under-compression, UC (over-expansion, OE) or over-compression, OC (under-expansion, UE). However, due to the transient nature of pulsation phenomena, some fundamental questions still remain to be answered, such as: What is the physical nature of gas pulsations? What exactly causes them to happen? Where and when are they generated? How are they different from acoustical waves and how to predict their behaviors such as amplitude, travelling direction and speed at source? This paper attempts to answer these questions by taking a different approach: applying the classical Shock Tube Theory to gas pulsation phenomena. The results not only confirm the findings of the previous workers, but also reveal the nature of gas pulsations as a composition of strong bi-directional waves and an accompanying unidirectional through-flow. Moreover, the pressure pulsations consist of pressure waves (coalescing into a quasishockwave) and a fan of finite expansion waves travelling in opposite directions, while the flow pulsation is simply the induced unidirectional flow as these strong waves sweep across the gas. It will be further demonstrated that the most dominant gas pulsations are the direct results from either an OC (or UE) or an UC (or OE) suddenly discharging at the compressor or expander outlet. Therefore its location of generation, magnitude, travelling directions and speed can be predicted based on design parameters and operating conditions of those machines. Based on this new insight, an effective pulsation control method called Pulsation Trap can be devised that tackles the non-linear waves and strong induced flow simultaneously and right at the predicted sources.