Subsonic choking in microchannel slip flow: Isothermal or adiabatic?

Some experimental evidence of subsonic choking in rarefied gas flow regime is available; however, the nature of the flow (isothermal/adiabatic) close to the choking point is not known. The theoretical limiting Mach number (Ma) for isothermal choked flows is 1/γ (where γ is the ratio of specific heat) and that for adiabatic choked flows is unity. In this work, we perform measurements for temperature, pressure, and mass flow rate at the inlet and outlet of a microchannel of aspect ratio 0.49 in the slip flow regime (4.04 × 10−3 < Kno < 7.04 × 10−3, where Kno is the outlet Knudsen number). We see some evidence of choking at Ma close to 1/γ with a shift in the choking point to Ma of unity. The measured static temperature is observed to be constant at microchannel inlet and outlet, indicating isothermal flow behavior for lower Ma values. The stagnation temperature is calculated to be nearly constant at the microchannel outlet for higher Ma values, indicating a shift in flow behavior to adiabatic. This study emphasizes the significance of temperature measurement for understanding the choking behavior. There is no active transfer of heat during the experiments, making the present work relevant to practical and real situations. This state-of-the-art study would be immensely useful while designing microchannels and microtubes for long-distance gas transportation, microelectromechanical systems, and space applications, where one needs to be careful about any occurrence of choking.