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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.

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This work presents the study of isothermal rarefied gas flows in converging microchannels. Experiments are carried out on microchannels of three different converging angles (4°, 8°, and 12°). Numerical investigation is carried out using commercial software to study the local behaviour of the flow parameters. The simulations show a sudden drop in the fluid temperature at the exit of the microchannel. Knudsen minimum, which was experimentally observed for the first time recently in diverging microchannels, is also noted here in the case of flow in converging cross section. It is interesting to note that, at the location of Knudsen minimum, the Knudsen number and the value of the minimum mass flow rate are same for both converging and diverging cross sections, for all the angles tested. This result implies the absence of any flow preference at high Knudsen numbers when the flow is subjected to converging and diverging orientations of the microchannel.

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