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This paper presents an experimental study of isothermal rarefied gas flow through a tube with sudden expansion in the slip flow regime. The measurements reported here are for nitrogen flowing at low pressures in conventional tubes with sudden expansion area ratios of 1.48, 3.74, 12.43 and 64. The flow is dynamically similar to gas flow in a microchannel as the Knudsen number $(0. 0001\lt \mathit{Kn}\lt 0. 075)$ falls in the slip flow regime; the Reynolds number in the smaller section (${\mathit{Re}}_{s} $) ranges between 0.2 and 837. The static pressure along the wall is measured for different mass flow rates controlled by a mass flow controller and analysed to understand the flow behaviour. The velocity profiles are obtained through a momentum balance and using the pressure measurements. A discontinuity in the slope of pressure at the sudden expansion junction is noted and given special attention. The absence of flow separation is another key feature observed from the measurements. The streamlines are found to be concave near the junction. It is demonstrated that the flow ‘senses’ the oncoming sudden expansion junction and starts adjusting itself much before reaching the junction; this interesting behaviour is attributed to an increased axial momentum diffusion and wall slip. The additional acceleration of the central core of the gas flow causes an increase in the wall shear stress and a larger pressure drop as compared with a straight tube. These results are not previously available and should help in improving understanding of gaseous slip flows.

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An experimental study of nearly isothermal rarefied gas flow near the sudden contraction junction of a tube is presented in this paper. The measurements are performed with nitrogen gas flowing at low pressures in conventional tubes with sudden contraction area ratios of 1.48, 3.74, 12.43, and 64. The flow is dynamically similar to gas flow in a microchannel as the Knudsen number (0.0001 < Kn < 0.032) falls in the slip flow regime. The Reynolds number in the smaller section (Res) ranges between 0.2 and 837. The static pressure measurements are analyzed to understand the flow behavior. The static pressure variation along the wall and uniform radial pressure profile near the junction indicates absence of flow separation and vena contracta. The static pressure variation in both the tubes approaches the pressure variation as that of an isolated straight tube at a certain critical Knudsen number for a given area ratio. The velocity field is obtained through a momentum balance and using the flow measurements. The effect of larger momentum diffusivity and slip at the wall, restricts any deviation in velocity profile from its parabolic nature at the junction and suppresses flow separation and vena contracta. The larger inertia force at the sudden contraction junction causes larger acceleration of the flow near the junction in the smaller tube as compared to that of the straight tube. The larger pressure drop in the sudden contraction is a result of the extent of flow compression and additional acceleration near the junction in the smaller tube as compared to the straight tube. This paper reports a set of new results that are expected to help in improving understanding of gaseous slip flows.

Study of rarefied gas flow in converging and diverging cross sections is crucial to the development of micro-nozzles and micro-thrusters. In other practical cases too, a microchannel may not always be straight and may include diverging and converging sections in the flow path. In this context, isothermal rarefied gas flow in microchannels of longitudinally varying cross section is studied experimentally in this work. The primary objective is to investigate the existence of Knudsen minimum in microchannels of varying cross sections. The effect of geometrical cross section and fluid properties on the Knudsen minimum are also investigated by performing experiments on three divergence angles (4°, 8°, and 12°) and three different gases (argon, nitrogen, and oxygen) to prove the robustness of the result. The Knudsen minimum, which is one of the characteristic features of rarefied flows, is experimentally observed for the first time in a microchannel of varying cross section. The position of the Knudsen minimum (at Kn ≈ 1) is seen to depend only weakly on the divergence angle and fluid properties.