Technologies and studies of gas exchange in two-stroke aircraft piston engine:A review

The in-cylinder gas exchange process is crucial to the power performance of two-stroke aircraft piston engines,which is easily influenced by complex factors such as high-altitude performance variation and in-cylinder flow characteristics.This paper reviews the development history and characteristics of gas exchange types,as well as the current state of theory and the validation methods of gas exchange technology,while also discusses the trends of cutting-edge technologies in the field.This paper provides a theoretical foundation for the optimization and engineering design of gas exchange systems and,more importantly,points out that the innovation of gas exchange types,the modification of theoretical models,and the technology of variable airflow organization are the key future research directions in this field.

Effect of scavenge port angles on flow distribution and performance of swirl-loop scavenging in 2-stroke aircraft diesel engine

Swirl-Loop Scavenging(SLS) improves the performance of 2-stroke aircraft diesel engine because the involved swirl may not only benefit the scavenging process, but also facilitate the fuel atomization and combustion. The arrangement of scavenge port angles greatly influences incylinder flow distribution and swirl intensity, as well as the performance of the SLS engine. However, the mechanism of the effect and visualization experiment are rarely mentioned in the literature. To further investigate the SLS, Particle Image Velocimetry(PIV) experiment and Computational Fluid Dynamics(CFD) simulation are adopted to obtain its swirl distribution characteristics, and the effect of port angles on scavenging performance is discussed based on engine fired cycle simulation. The results illustrate that Reynolds Stress Turbulence model is accurate enough for in-cylinder flow simulation. Tangential and axial velocity distribution of the flow, as well as the scavenging performance, are mainly determined by geometric scavenge port angles ageom and βgeom . For reinforcement of scavenging on cross-sections and meridian planes, ageomvalue of27° and βgeom value of 60° are preferred, under which the scavenging efficiency reaches as high as 73.7%. Excessive swirl intensity has a negative effect on SLS performance, which should be controlled to a proper extent.

An experimental method to obtain the hard alpha anomaly distribution for titanium alloy aeroengine disks

A probability-based damage tolerance methodology has been proposed to improve the recognition of material anomalies, especially hard alpha(TiN) anomalies for aeroengine rotor disks. A key input to this method is hard alpha anomaly distribution, which reflects the occurrence rate and size of anomalies present in the finished part material of titanium rotors. Since anomalies rarely occur naturally, an experimental method is proposed to obtain the anomaly distribution for titanium alloy aeroengine disks to reflect and equivalently replace the manufacturing development in titanium industry. In general, the anomaly distribution information can be divided into two parts: the Probability of Detection(POD) curve and the detected anomaly distribution, which contains the size and frequency data of the detected anomalies. The distribution can be established based on several appropriate assumptions and derivation steps with different confidence levels of POD curves and detected anomaly distributions. In this case, the distribution can be obtained in a relatively short time as a key input to the probability-based damage tolerance methodology. Then the Probability of Failure(POF) can be calculated, and the value is found to vary with different confidence levels. On this basis, the conservative estimated POF can be obtained in conjunction with confidence levels.

Review of the development of the probabilistic damage tolerance assessment of life-limited parts in compliance with the airworthiness regulations

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 aeroengines.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.

Numerical and experimental investigation on dynamic performance of bump foil journal bearing based on journal orbit

Widespread usage of bump-type foil journal bearing(BFJB) in oil-free microturbomachinery requires accurate predictions of dynamic performance characteristics, although it remains a challenging issue because BFJB reflects nonlinear both structurally and aerodynamically.This paper presented a simple experimental method to semi-directly obtain the minimum film thickness and dynamic stiffness of BFJB using the journal orbit. Numerical calculations and simulations are conducted to validate the experimental method. The micro-deformation and interaction of various foils are taken into consideration to improve the model precision. The results from the numerical model regarding the BFJB dynamic characteristics are compared with the experimental results coming from a dedicated test rig, which shows that the experimental results fluctuate obviously and agree not well with the numerical results at the start stage due to the presence of dry friction at that time, nevertheless, they show fantastic agreement as soon as a gas film is gradually generated to separate the shaft from the top foil. Therefore, the proposed experimental method is effective to predict film thickness and dynamic characteristics during the period from the lift-off time to the land-off time. The dynamic characteristics, along with the journal orbits also can be used to rapidly predict the dynamics behavior of rotor-bearing systems.

Investigation into gas lubrication performance of porous gas bearing considering velocity slip boundary condition

Porous gas bearings(PGBs)have a proactive application in aerospace and turbomachinery.This study investigates the gas lubrication performance of a PGB with the condition of velocity slip boundary(VSB)owing to the high Knudsen number in the gas film.The Darcy-Forchheimer laws and modified Navier-Stokes equations were adopted to describe the gas flow in the porous layer and gas film region,respectively.An improved bearing experimental platform was established to verify the accuracy of the derived theory and the reliability of the numerical analysis.The effects of various parameters on the pressure distribution,flow cycle,load capacity,mass flow rate,and velocity profile are demonstrated and discussed.The results show that the gas can flow in both directions,from the porous layer to the gas film region,or in reverse.The load capacity of the PGB increases with an increase in speed and inlet pressure and decreases with an increase in permeability.The mass flow rate increases as the inlet pressure and permeability increase.Furthermore,the simulation results using VSB are in agreement with the experimental results,with an average error of 3.4%,which indicates that the model using VSB achieves a high accuracy.The simulation results ignoring the VSB overrate the load capacity by 16.42%and undervalue the mass flow rate by 11.29%.This study may aid in understanding the gas lubrication mechanism in PGBs and the development of novel gas lubricants.