Contra-rotating Stage Research Articles

Numerical Analysis of Unsteady Phenomena in a Contra-Rotating Stage Based on the Reduction of Local Entropy Production

Abstract The primary purpose of this study is the reduction of local entropy production in a contra-rotating stage. As such, the unsteady flow phenomena and the impact of radial load distribution on these phenomena and local entropy production need to be clarified. In this study, the stress-blended eddy simulation (SBES) turbulence model is utilized to capture the vortices in the flow separation zone, and the γ-Reθ transition model is employed to predict the transition phenomenon within the boundary layer. Entropy production rate models suitable for different turbulence models are constructed separately to calculate local entropy production. Vortex visualization is achieved according to the λci criterion, and the relative vorticity change rate is used to analyze the components of the tip clearance vortices. The transition phenomenon is analyzed from the perspectives of both the Euler and the Lagrange descriptions. The primary findings can be summarized as follows: the transition begins earlier and progresses more rapidly in the rear rotor. Wake propagation, occurring at double the frequency, entropy production rate within the boundary layer changes in synchrony with the wall shear stress at the same frequency. Additionally, an investigation of the tip clearance vortices concludes that the main structure of the tip clearance vortices coincides with the flow pattern of the high entropy production rate region, and the flow structure related to the high divergence area is essential for considering subsequent optimization with the aim of reducing the entropy production rate.

Experimental and numerical studies on small contra-rotating electrical ducted fan engines

Electrical propulsion has been identified as one of the key fields of future research within the aerospace sector. The Institute of Aeronautical Engineering at the Universität der Bundeswehr München aims to contribute to the ongoing development of small-sized electrical ducted fan engines with a thrust in the range of 100 N. A special emphasis is placed on electrically powered contra-rotating fan stages. When compared to a conventional rotor–stator stage, contra-rotating fan stages allow for a more compact design, considering a given pressure ratio, or an increased pressure ratio at a constant fan diameter. Since numerous new aircraft concepts are presently being developed, a high demand for compact and powerful electrically driven engines arises. Electrically driven contra-rotating fan engines provide a high potential in terms of compactness, emissions and efficiency. Using electric motors offers the ability to overcome common issues, such as design and integration of a contra-rotating stage into a gas turbine. An innovative new engine design featuring such a contra-rotating stage is developed and tested at one of the Institute’s test benches for electrical propulsion. Key components are two brushless motors powering the fan stage, one for each rotor. Various operation points are investigated experimentally during an extensive test campaign. Experimental results are compared to results of numerical simulations computed by ANSYS CFX. Results indicate a good agreement between experiment and simulation. The engine is running very smooth throughout all tested operation points. Yet, intensive heating up of the electric motors and high-temperature zone are found to be an issue at higher rotation speeds.

Experimental investigation of a high aspect ratio, low speed contra-rotating fan stage with complex inflow distortion

This paper focuses on the response of a high aspect ratio, low speed contra-rotating fan with complex inflow distortion. The total pressure at the inlet is artificially distorted by means of two different sets of screens with different porosities to generate a hub-strong complex distortion and a tip-strong complex distortion. Detailed flow analyses were conducted for the design speed of rotor-1 in combination with off design speeds of rotor-2 both for the design and the peak pressure mass flow rates. 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, static 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 using three seven hole probes. The study reveals a few interesting aspects on the effect of complex inflow distortion on the flow behavior of a contra-rotating stage. The presence of a low porosity screen reduces the magnitude of axial velocity and generates higher spread of distortion near the localized region of placement of the screen. The hub-strong complex distortion has a greater effect of presence of this low porosity screen in both circumferential and radial directions. This leads to higher stagnation pressure variation at the inlet. On the other hand, for the tip-strong complex distortion case, the extent of distortion is observed to be higher in the circumferential direction towards the casing rather than radial. The localized improvement in the flow (in tip-strong inflow distortion) near the tip region improves the performance both in terms of pressure rise and efficiency of stage compared to hub–strong complex distortion.

Influence of Circumferential Inflow Distortion on the Performance of a Low Speed, High Aspect Ratio Contra Rotating Axial Fan

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.

A Study of Some Factors Affecting the Performance of a Contra-Rotating Axial Compressor Stage

An experimental investigation of the performance of a contra-rotating stage is reported. The influence of factors such as speed ratio of the two rotors, rotor stagger, pitch-chord ratio and axial spacing between the rotors is examined from tests on a 0.66 hub-tip ratio compressor. The study reveals that the performance of a contra-rotating stage is affected by all these factors. Axial spacing between the rotors and the speed ratio of the rotors both exhibit a strong influence on the stalling behaviour of the stage. It has been found that in a stage with close axial gap, rotating stall on the first rotor is suppressed if the second rotor is contra-rotated at a speed 50 per cent faster than the first rotor. This unique advantage of contra-rotation is not obtained if the axial gap is large. Measurements of sound pressure level are also reported to highlight the high noise problems associated with a contra-rotating stage.