Double-diaphragm shock tube - Comparison between theory and experiment

Abstract

HE double-diaphr agm shock-tube technique developed by Holbeche1>2 is an ideal way to cool shock-heated gases and vapors rapidly. It has been used to study vibrational relaxation1'4 and atomic recombination reactions,5'6 and is also suitable for the study of homogeneous nucleation. The usefulness of the technique, however, depends on knowing the flow properties throughout the region of interest. Treatments so far have assumed that shock reflection from the second diaphragm can be ignored. We describe a series of experiments performed to establish the presence of a reflected shock from the second diaphragm and give brief details of the theoretical analysis used to obtain flow properties. The shock tube of 76 mm diameter (see Fig. 1) has been described elsewhere.6 A scored aluminum disk and two sheets /of 0.025 mm aluminum foil were used for primary and secondary diaphragms, respectively. Pressure was measured by Kistler piezoelectric pressure transducers mounted 80 mm upstream and 100 and 455 mm downstream from the secondary diaphragm. Light scattered at 90 deg from an argon ion laser beam (476.5 nm) was also recorded 455 mm downstream from the secondary diaphragm. In each run highpurity argon and nitrogen were used as test and driver gas, respectively. Pressure measurements upstream from the secondary diaphragm indicate that shock reflection has occurred. A typical oscilloscope trace showing pressures upstream and downstream of the secondary diaphragm is given in Fig. 2. Previous analyses assumed no shock reflection. Inclusion of shock reflection in the flow analysis necessitates a full method of characteristics analysis, similar to that of Rudinger.7 In our analysis, the incident shock reflects from the secondary diaphragm (assumed planar) and travels back upstream into the test gas. At a later stage, the secondary diaphragm bursts instantaneously due to the increased pressure behind the reflected shock. A rarefaction wave is thus generated, the head of which travels upstream into the test gas; the tail travels downstream into the evacuated expansion section. The rarefaction wave head eventually catches up to the reflected shock front, and the resulting interaction causes the shock wave to decay. Particle paths are constructed through the characteristics mesh so that gas properties at any point in time and space are