Characterization of Inertance Tubes Using Resonance Effects
Michael A. Lewis, Peter E. Bradley, Ray Radebaugh, Zhihua Gan
Inertance tubes can be characterized by their inertance, compliance, and resistance. All three of these impedance components are present during normal measurements of inertance tube impedance. As a result, in comparing experimental results with models it is often difficult to find the fundamental cause of disagreement. In previous inertance tube measurements we have observed resonance conditions when the reservoir volume is properly sized. The resonance is analogous to an LC resonance of electrical systems with L analogous to the inertance and C analogous to the compliance of the reservoir volume. This paper discusses how we make use of this resonance to separate the various impedance components of the inertance tube and compare them with models. Frequency is varied about the resonance frequency for fixed pressure amplitude to find the resonance frequency, the minimum impedance, and the half-width of the resonance peak. Other measurements are made with zero reservoir volume and with large reservoir volume. With this set of measurements we show how to separate the impedance components to compare them with models. Various methods for mass flow measurements at the inlet to the inertance tube are compared. These include hot-wire anemometry, pressure drop across stacked screen, and pressure drop across a laminar flow element. The inertance tubes investigated here are 5.74 mm and 8.71 mm in diameter with lengths of 2.36 and 3.59 meters respectively. Average pressures range from 1.5 to 2.5 MPa and pressure ratios extend up to 1.3 to give acoustic powers ranging from about 200 to 1500 W. The reservoir volumes are sized to produce resonance frequencies near 60 Hz.