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This article reports a parametric study on the performance of a high aspect ratio, low speed contra-rotating fan stage. Parameters namely speed ratios of the two rotors and axial spacing between the rotors that play a significant role on the overall performance of the contra-rotating fan stage were evaluated. The rotors have a low hub-tip ratio of 0.35 and chord of 45 mm. The two rotors were designed to develop a pressure rise of 1100 Pa and 900 Pa, respectively, when operating at 2400 r/min and developing a mass flow rate of 6 kg/s. In order to evaluate the performance of the designed rotors, measurements of total pressure at the entry of rotor-1, between the rotors and the exit of rotor-2 were taken using total pressure probe rake, 4-hole probe and a Kiel probe rake. The experiments were conducted for different speed combinations of rotor-1 and rotor-2. All these speed combinations were studied separately for different axial spacing. The performance plots revealed the existence of two stall limits namely, partial stall and full stall. For lower throttle positions, rotor-2 was observed to stall. Further lowering the mass flow rate; reduces the pressure rise capacity of the stage due to stalling of rotor-1 as well as rotor-2. For the design speed operation of rotor-1 in combination with an off-design speed of rotor-2, the flow parameters change significantly. A higher rotational speed of rotor-2 generates a stronger suction effect leading to an overall improvement in the performance of the whole stage. The effect of variation in the axial spacing between the rotors was also studied. The strongest suction effect between the rotors was observed at an axial spacing of 0.9 chord.

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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.