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.

The influence of circumferential inflow distorted on the performance and flow behavior of a high aspect ratio, low speed contra rotating fan is reported in this paper. The total pressure at the inlet is artificially distorted by means of 90 deg mesh sector with a porosity of 0.70. The performance of the contra rotating fan was studied under different speed combinations of the two rotors under clean and distorted inflow conditions. Detailed flow analyses were conducted under design and off-design conditions. In order to understand the effect of distortion and its extent, the distortion sector was rotated circumferentially at intervals of 15 deg to cover the entire annulus. Detailed measurements of the total pressure, velocity components, and flow angles were carried out at the inlet of the first rotor, between the two rotors, and at the exit of the second rotor. The study reveals a few interesting aspects on the effect of inflow distortion on the performance of a contra-rotating stage. For the design speed combination and lower rotational speed of rotor-2, a reduction in the overall operating range with a shift of the peak pressure point towards higher mass flow rate, was observed. It is observed that the effect of inflow distortion at the inlet of rotor-1 gets transferred in the direction of rotor-1 rotation and spreads across the entire annulus. The opposite sense of rotation of rotor-2 causes the distortion effect to get transferred in the direction of rotation of rotor-2 with an associated reduction in the total pressure near the hub. It is observed that a higher rotational speed of the second rotor has a beneficial effect on the overall performance due to the strong suction by generated higher rotational speed of rotor-2.

This paper explores the effect of speed ratio and axial spacing between high aspect ratio, low speed contra-rotating pair rotors on their aerodynamic performance. The blades were designed with a low hub-tip ratio of 0.35 and an aspect ratio of 3.0. Numerical and experimental studies are carried out on these contra-rotating rotors operating at a Reynolds number of 1.258 × 105 (based on blade chord). The first and second rotors were designed to develop a pressure rise of 1100 Pa and 900 Pa, respectively, for total mass flow rate of 6 kg/s when both operating at a design speed of 2400 rpm. The performance of the fan was evaluated based on variations of total pressure and flow angles at off-design operating conditions. The measurementsof total pressure rise, flow angles etc. are taken upstream of the first rotor and in between the two rotors and downstream of the second rotor. The performance of the contra rotating stage is mainly influenced by the axial spacing between the rotors and speed ratio of both the rotors. The study reveals that the aerodynamics of the contra-rotating stage and stall margin is significantly affected by both the speed ratio as well as the axial spacing between the rotors. It was found that with increasing the speed ratio, the strong suction generated by the second rotor, improves the stage pressure rise and stall margin. Lower axial spacing changes the flow incidence to the second rotor and thereby improves the overall performance of the stage. This however, is accompanied by an increased noise level. The performance is investigated at different speed ratios of the rotors at varying axial spacing. Detailed numerical simulations have been conducted using ANSYS CFX13© using mixing plane approach between rotors. Numerical simulations are compared with experimental results at off-design conditions. These results are validated using the experimental data. Numerical simulations are expected to provide deeper insight into the flow physics of contra-rotating rotors which may be difficult to capture experimentally.

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In this paper, results from an experimental study on the effect of circumferential inflow distorted on the performance and flow behavior of a high aspect ratio, low speed contra rotating fan, are reported. The total pressure at the inlet is artificially distorted by means of 90° mesh sector having porosity of 0.70. The performance of contra rotating fan was studied under variable speed combinations of rotors under clean and distorted inflow conditions. Detailed flow analyses were conducted under design and off-design conditions. In order to understand the extent of inlet distortion, the distortion sector was rotated circumferentially at intervals of 15° to cover the entire annulus. Detailed measurements of total pressure, velocity components and flow angles were carried out at the inlet of the first rotor, between the two rotors and at the exit of the second rotor. The study reveals a few interesting aspects on the effect of inflow distortion on the performance of a contra-rotating stage. It is observed that a higher rotational speed of the second rotor has a beneficial effect on the overall performance.

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The paper reports results of a detailed study of hub-and tip-strong radial inflow distortion on the performance of a low speed, high aspect ratio (3.0) contrarotating fan stage. The distortion screen (placed upstream of the first rotor) covers 1/3 of the span of the blade near the hub and the casing for hub-strong and tip-strong inflow distortions, respectively. The performance of the contrarotating fan was investigated under both the radially distorted inflows and compared with the clean inflow conditions under different speed combinations of rotor-1 and rotor-2. The hub-strong radial distortion reduces the overall performance (pressure rise and efficiency) as well as the operating range by about 2 to 4% under the different speed combinations. On the other hand, it is interesting to observe that the tip covered radial distortion not only improves the pressure rise, but also increases the overall operating range by about 6 to 8%. There was, however, a 6 to 8% decrease in efficiency in this case as compared to 12 to 15% for hubstrong radial inflow distortion. The results reveal the physics of the effects of radial inflow distortion on the overall performance of the contrarotating fan stage.

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