Numerical study on parametrical design of long shrouded contra-rotating propulsion system in hovering

The parametrical study of Shrouded Contra-rotating Rotor was done in this paper based on 2D axisymmetric simulations. The calculations were made with an actuator disk as double rotor model. It objects to explore and quantify the effects of different shroud geometry parameters mainly using the performance of power loading (PL), which could evaluate the whole propulsion system capability as 5 Newton total thrust generation for hover demand. The numerical results show that: The increase of nozzle radius is desired but limited by the flow separation, its optimal design is around 1.15 times rotor radius, the viscosity effects greatly constraint the influence of nozzle shape, the divergent angle around 10.5° performs best for any chosen nozzle length; The parameters of inlet such as leading edge curvature, radius and internal shape do not affect thrust greatly but play an important role in pressure distribution which could produce most part of shroud thrust, they should be chosen according to the reduction of adverse pressure gradients to avoid the risk of boundary separation

Experimental Analysis of the Aerodynamics of Long-Shrouded Contrarotating Rotor in Hover

This paper aims to quantify the benefits of a shrouded coaxial rotor configuration through experimental comparisons with free (not shrouded) rotors in hover. The experiment shows that both the figure of merit of contrarotating rotors and the system power loading are improved by the shroud inclusion. Improvements are induced by a suction effect at the inlet, which can be optimized by a regulation effect of the mass flow. Compared to free rotors, the strong suction peak formed on the shroud leading edge by a 65% increase in mass flow, allowing the shroud to contribute up to 56% of the total thrust. More uniform pressure distribution in the downstream rotor and less contraction of the slipstream decrease losses and increase the rotor efficiency. The shrouded system efficiency is further improved if the upstream rotor rotates slower than the rear one, for a given total shaft power, because a stronger pressure depression occurs upstream of the rotors to generate more mass flow. On the other hand, the system behavior is insensitive to the interrotor distance

Revisiting Froude’s Theory for Hovering Shrouded Rotor

This paper extends Froude’s momentum theory for free propellers to the analysis of shrouded rotors. A one-dimensional analytical approach is provided, and a homokinetic normal inlet surface model is proposed. Formulations of thrusts and power for each system component are derived, leading to the definition of optimum design criteria and providing insight into the global aerodynamics of shrouded rotors. In the context of micro-air vehicles applications, assessment of the model is conducted with respect to numerical data. Overall, comparison between numerical and analytical results shows good agreement and highlights the sensitivity of the model to viscous effects

Experimental Analysis of the Global Performance and the Flow Through a High-Bypass Turbofan in Windmilling Conditions

A detailed study of the air flow through the fan stage of a high-bypass, geared turbofan in windmilling conditions is proposed, to address the key performance issues of this severe case of off-design operation. Experiments are conducted in the turbofan test rig of ISAE, specifically suited to reproduce windmilling operation in an ambient ground setup. The engine is equipped with conventional measurements and radial profiles of flow quantities are measured using directional five-hole probes to characterize the flow across the fan stage and derive windmilling performance parameters. These results bring experimental evidence of the findings of the literature that both the fan rotor and stator operate under severe off-design angle-of-attack, leading to flow separation and stagnation pressure loss. The fan rotor operates in a mixed fashion: spanwise, the inner sections of the rotor blades add work to the flow while the outer sections extract work and generate a pressure loss. The overall work is negative, revealing the resistive loads on the fan, caused by the bearing friction and work exchange in the different components of the fan shaft. The parametric study shows that the fan rotational speed is proportional to the mass flow rate, but the fan rotor inlet and outlet relative flow angles, as well as the fan load profile, remain constant, for different values of mass flow rate. Estimations of engine bypass ratio have been done, yielding values higher than six times the design value. The comprehensive database that was built will allow the validation of 3D Reynolds-averaged Navier–Stokes (RANS) simulations to provide a better understanding of the internal losses in windmilling conditions

Effect study of design variables on hover performance of long shrouded contra-rotating rotors

This paper presents a experimental study on the design issues of long shrouded (L/D>1) coaxial propulsion system in hovering. It objects to explore the effects of different design variables, which include: shroud inlet radius and nozzle expansion ratio, rotor locations, rotational speed, rotor number and so on. Through applying the test bench with multiple possibilities of testing parameters and different combinations of shroud inlets and nozzles, the preliminary results show that the diverging nozzle limited by the flow separation and relative greater inlet radius are desired. Based on it, a shroud with a suitable combination of inlet and nozzle is chosen. The experiments on it reveal that: The system with double rotor at the upstream side of cylindrical section and with the second rotational speed higher than the first one performs best; compared to the shroud contribution, increasing rotor number doesn't improve the efficiency greatly

Experimental Analysis of the Flow through the Fan Stage of a High-Bypass Turbofan in Windmilling Conditions

A detailed study of the air flow through the fan stage of a high bypass turbofan in engine-out conditions is proposed, to address the key performance issues of this severe case of off-design operation. Experiments are conducted in the new turbofan test rig of ISAE, specially suited to reproduce windmilling operation in an ambient ground setup. The engine is equipped with conventional measurements to characterize the cycle performance and directional five-hole probes are used to characterize the flow across the fan stage. These first results confirm the findings of the literature that the fan outlet guide vane operates under severe off-design angle-of-attack. Massive flow separation is expected in the stator vanes, generating an important pressure loss that contributes to engine drag. This comprehensive database will allow the calibration and validation of 3D RANS simulations to provide a better understanding of the internal losses in windmilling conditions

Towards a fully coupled component zomming approach in engine performance simulation

This paper presents a fully-coupled zooming approach for the performance simulation of modern very high bypass ratio turbofan engines. This simulation is achieved by merging detailed 3D simulations and map component models into a unified representation of the whole engine. Today‟s state-of-the-art engine cycle analysis are commonly based on component mapping models which enable component interactions to be considered, while CFD simulations are carried out separately and therefore overlook those interactions. With the methodology discussed in this paper, the detailed analysis of an engine component is no longer considered apart, but directly within the whole engine performance model. Moreover, all links between the 3D simulation and overall engine models have been automated making this zooming simulation fully-integrated. The simulation uses the PROOSIS propulsion object-oriented simulation software developed by Empresarios Agrupados for whole engine cycle analysis and the computational fluid dynamics (CFD) code CEDRE developed by ONERA for the high fidelity 3-D component simulations. This methodology has been applied on an advanced very high bypass ratio engine developed by Price Induction. The proposed zooming approach has been performed on the fan stage when simulating Main Design Point as well as severe case of off-design conditions such as wind-milling. The results have been achieved within the same time frame of a typical CFD fully-converged calculation. A detailed comparison with upcoming test results will provide a first validation of the methodology and will be presented in a future paper