Computational analysis of the wave motions in micro-shock tube flow

In recent times, shock tube flows have been widely employed in many micro-scale devices in the fields of propulsion technology, micro-heat engines, particle delivery systems, and so on. The very small length scales in such micro-shock tubes make the flow physics more complicated compared to the ordinary macro-shock tubes. The major differences in the flow features are the profound influences of wall effects and rarefaction effects. The rarefaction effect alters the boundary layer structure by imparting additional velocity and thermal gradients to the wall-bounded fluid. These phenomena can strongly affect the micro-shock tube flow characteristics such as shock-contact wave speeds, wave propagations, hot and cold zone properties. The main objective of the present work is to produce a detailed understanding on the wave propagation characteristics in a micro-shock tube under rarefied conditions using computational fluid dynamics methods. The shock-contact interface movement under different operating conditions such as Knudsen number and pressure ratio are investigated in detail and compared with the macro-scale shock tube flows. The difference between the numerical and analytical works and their cause is identified and discussed. The results obtained show that the shock strength attenuates rapidly for micro-shock tubes compared to macro-shock tubes. The shock-contact propagation and the distance between them in a micro-shock tube have a strong dependence on rarefaction effects. The more the rarefaction effects are, lesser will be the shock-contact distance. The shock-contact distance decreases as the pressure ratio increases. A strong attenuation in shock strength can also be observed as the rarefaction increases.