Propulsion and Power Research最新文献

This paper addresses the necessity to make a physical interpretation of a highly complex three-dimensional tip clearance flow field study for high-speed mixed-flow compressor having stage exit static pressure to inlet total pressure ratio of 3.8 with 39,836 rpm rotor speed. The four different tip configurations namely the constant (λ = 0.016 and 0.019) and variable (λ = 0.011 (inlet)-0.019 (exit) and 0.019 (inlet)-0.022 (exit)) tip clearances were numerically analysed using available experimental data-set. The numerical investigation reveals that in contrast to the classic jet-wake pattern, two anomalous velocity profiles formed at the impeller exit which results in pressure losses in the vaneless diffuser. Near the impeller inlet, the tip leakage flow rolls up to discrete tip leakage vortex structure for each tip clearance configuration. This results in the formation of a region of momentum deficit, recirculation zone, which gets weakened as it moves downstream. The tip clearance configuration is observed to profoundly influence the extent and vorticity of the tip leakage vortex. In the splitter blade passage, the tip leakage flow and Coriolis flow interact with passage flow, resulting in the formation of two secondary passage vortices that move downstream along the pressure and suction surface of the splitter blade. The tip clearance configuration directly influences the impeller exit jet-wake pattern by modulating the secondary passage vortices trajectory and vorticity. Moreover, off-design analysis for tip clearances λ = 0.016 and λ = 0.019, depict distinctive tip leakage vortex characteristics. When operating near the stall conditions (80% of design mass flow rate), λ = 0.019 exhibits bubble shape tip leakage vortex breakdown occurring near the impeller inlet. This result in a substantial change in the tip leakage vortex nature; expansion of the recirculation zone and early weakening of the vorticity in the tip leakage vortex. It is observed that vortex breakdown plays a vital role in characteristics of the passage flow field structure and compressor performance near the stall conditions.

“Ionic wind” generators are used as the main propulsion system in ion propulsion unmanned aerial vehicles (UAVs). Owing to the large size and poor stiffness of the electrode array in the propulsion system, the electrode array is prone to deformation under the flight load. In this work, the thrust characteristics and static aeroelastic properties of “ionic wind” propulsion systems were analyzed in detail. The simulation model for an “ionic wind” propulsion system was established by coupling a two-dimensional gas discharge model with a gas dynamics model. The influences of electrode voltage, spacing, size, and shape on the performance of the propulsion system were investigated. The fluid-solid interaction method was used to solve static aeroelastic characteristics under deformation. The aerodynamic and thrust performances of the elastic state and the rigid state were compared. It was found that the operating voltage, the distance between two electrodes, and the emitter radius had greater impacts on the thrust of the propulsion system. The propulsion system had a small contribution to the lift but a large contribution to the drag. In the elastic state, the lift coefficient accounted for 12.2%, and the drag coefficient accounted for 25.8%. Under the action of the downwash airflow from the wing, the propulsion system formed an upward moment around the center of mass, which contributed greatly to the pitching moment derivative of the whole aircraft. In the elastic state, the pitching moment derivative accounted for 29.7%. After elastic deformation, the thrust action point moved upward by 28.7 mm. Hence, the no lift pitching moment is reduced by 0.104 N·m, and the pitching moment coefficient is reduced by 0.014, causing a great impact on the longitudinal trimming of the whole aircraft.

One of the crucial factors affecting the carrying capacity of the cryogenic liquid launch vehicle is the effective volume of the tank. Theoretical and experimental investigations on vortex breaker mechanisms have proposed promising schemes applied in the oxygen tank of the liquid-propellant launch vehicle to ensure the normal operation of the engine. In this paper, the liquid surface profile functions of the laminar core when the vortex generates were derived based on the Rankine vortex model. The dimensionless residual volume V/d³ and the Froude number were applied to compare the theoretical prediction of critical height with the actual simulation data of liquid oxygen. This comparison method can improve the model's accuracy. The efficiency of different basic shapes of vortex breakers was tested by conducting CFD modelling on a non-vertical outflow tank under a specific operating condition. Simulation results suggest negligible effects of heat transfer and surface tension. A circular plate is considered the optimal vortex breaker shape in traditional vertical outflow tanks, while a higher optimize efficiency was discovered in the half baffle basic shape in a non-vertical outflow tank by comparing the dimensionless residual volume and flow coefficient. A 34.26% reduction in flow resistance of half baffle breaker can be reached when applying a twenty-degree outlet pipe chamfering setting compared to a zero-degree chamfer. Considering practical operating limitations, it is concluded that a vortex breaker mechanism in a half baffle basic shape with a radius of 2.5d and a height of 4/d is the optimal scheme, which is suitable for all types of tanks. Its optimization efficiency of the residual volume reduction is about 56.68% compared to a no-breaker installation case. Lastly, a general equation based on CFD simulation for predicting the residual volume under a certain outflow velocity was proposed: $V/d^3\cong \alpha F{r}^{0.3}$, which trend is consistent with that of mathematical prediction $V/d^3\cong \alpha F{r}^{1/3}$. This consistency proves the accuracy and applicability of optimization strategy in this paper.

Probabilistic damage tolerance is a critical method to understand and communicate risk and safety. This paper reviews recent research on the probabilistic damage tolerance design for life-limited parts. The vision of the probabilistic damage tolerance assessment is provided. Five core parts of the probabilistic damage tolerance method are introduced separately, including the anomaly distribution, stress processing and zone definition, fatigue and fracture calculation method, probability of failure (POF) calculation method, and the combination with residual stress induced by the manufacturing process. The above currently-available risk assessment methods provide practical tools for failure risk predictions and are applied by the airworthiness regulations. However, new problems are exposed with the development of the aero-engines. The time-consuming anomaly distribution derivation process restricts the development of the anomaly distribution, especially for the developing aviation industries with little empirical data. Additionally, the strong transient characteristic is prominent because of the significant temperature differences during the take-off and climbing periods. The complex loads then challenge the fatigue and fracture calculation model. Besides, high computational efficiency is required because various variables are considered to calculate the POF. Therefore, new technologies for the probabilistic damage tolerance assessment are provided, including the efficient anomaly distribution acquisition method based on small samples, the zone definition method considering transient process, and stress intensity factor (SIF) solutions under arbitrary stress distributions combined with the machine learning method. Then, an efficient numerical integration method for calculating failure risk based on the probability density evolution theory is proposed. Meanwhile, the influence of the manufacturing process on residual stress and the failure risk of the rotors is explored. The development of the probabilistic damage tolerance method can meet the requirement of the published airworthiness regulation Federal Aviation Regulation (FAR) 33.70 and guide the modification or amendment of new regulations to ensure the safety of the high-energy rotors.