Numeriacal Simulation And Experimental Study On The Transient Process Of The Air System In Aeroengine

The secondary air system is an important functional system in aeroengine,which performs a variety of functions such as cooling hot ports,sealing cavities and controling of bearing axial loads.In the transient process or the emergency situation of engine,because of the unstability of the main gas flow and cooling air flow the secondary air system is susceptible to experience complex transient flow.Due to the cavities capacity,fluid flow inertia,propagation and reflection of the pressure wave,the airflow parameter and external disturbance cannot change consistently,which has a delay,lag,and fluctuation.The appearance of the transient response of the air system will bring a series of hazards for the normal work of the engine,which include unpredictable expansion of engine parts,hot gas ingestion and bearing oil leakage.Therefore,the transient response of air system has gradually became a frontier topics in the field of modern engine.This paper made a profound study on the component transient performance,transient network calculation and characteristic test,which is of great significance to correct understanding of the transient flow,and provides the theoretical support and guidance for the optimization and precise design and failure analysis of the air system.First of all,the typical transient response phenomena in secondary air system were investigated based on the basic concepts of dynamic process.According to the gas dynamics basic theory,the causes of transient phenomena were analyzed in two aspects,external disturbance and system physical properties.Four parameters were introduced to quantitatively analyze the transient response characteristics of air system.Secondly,using the method of lumped parameters and method of characteristic line respectively,the one-dimensional mathematical models of transient response components(cavity and pipe)were built which can be used to simulate the transient flow in cavity and pipe.In addition,URANS has been applied to simulate the transient flow in straight-through and stepped labyrinth seals,and the evolution of flow field,the variation of the characteristic parameters and influencing factors were analyzed.Thirdly,on the basis of the network method of fluids,the node pressure residual equation of transient network was deduced.The variation curves of pressure and mass flow rate at different time were abtained by computing for a air flow pipeline.Compared with the numerical results,the transient network algorithm of air system was verified.Finally,experiment system of transient characteristics of air system was set up congsidering the structural features in actual cavities and experimental feasibility.The test pieces of static and rotor-stator cavity were designed and machined.At the same time,the design and equipments selection of the air supply system,power system,measurement system and valve control system were carried out.A series of experiments for single component and multiple components were conducted.Then,the inlet pressure and temperature,as well as cavities dynamic pressure were tested,which were used to verify the accuracy of the one-dimensional cavity model and URANS.Through the above research work,several important conclusions were obtained in this paper.(1)The transient response phenomena in secondary air system were caused by two reasons,one was the effect of external disturbances which is the necessary conditions of the creation of the phenomena(e.g.rapid changes in main airflow parameters),the other was the influence of physical properties in system or components(e.g.cavities capacity,fluid flow inertia,fluid compressibility and rotation effect).It is possible to quantitatively analyze the characteristic parameters with the method of analysis and evaluation for transient characteristics of air system.(2)Through the calculation of the stransient flow in disk cavity,pipe and labyrinth seals,it is found that the cavity pressure and outlet flow rate were gradually increased,and the inlet flow rate first increased rapidly to the maximum and then decreased to reach a constant when the inlet pressure suddenly increases.The pipe inlet flow rate increased dramatically at first and then increased continuously to be stable.The mass flow rate and pressure of the labyrinth seals first experienced the oscollation process and then gradually stabilized.The results showed that disk cavity,pipe and labyrinth seals had the delaying effects on disturbance at different degree.The response time of disk cavity was longest,about 76 ms,that were similar between pipe and labyrinth seals.Therefore,it is necessary to consider the influence of the transient characteristics of labyrinth seals.(3)The transient network algorithm of air system was proposed.In the modelling process of the transient air system,internal nodes represent the virtual nodes introduced by man-made,and the cavities were treated as response components.By analyzing the flow characteristics of the resistance components and the transient characteristics of the response components,the calculation formulas for different components were derived.After establishing the node pressure residual equation of transient network,the mass flow rate,pressure and temperature at different time were finally calculated through iterative solution.(4)The experimental results of the static cavity indicate that the response time increase with the increase of the inlet pressure;the inlet and outlet position in static have little effect on the response process;the greater the outlet area,the shorter the response process.The experimental results of the rotor-stator cavity show that the response time decrease with the increase of the inlet pressure;the low rotational speed on the response process has little effect,when the speed continues to increase the response time gradually increased.The test results of mutiple components suggest that when the valve opening the inlet pressure of the static cavity increased quickly and then reached a stable value;the pressure of the static cavity,plenum chamber and rotor-stator cavity rose gradually,and the response time of them were increased.The chloser the measuring points is to the disturbance source,the shorter the alg time.