Study On Flow And Heat Transfer And Optimization Of Scavenge Structure In Bearing Chamber Based On Oil/Air Two-phase Flow

The bearing chamber is an important part of aero-engine lubricating system.With the rising of the aero-engine rotor speed and the gas temperature at the turbine inlet,the heat load of the hot zone bearing chamber in aero-engine is also increasing.Tough and bad working environment will increase the chance of degeneration,coking,and even the phenomenon such as fire for lubricating oil in bearing chamber.Therefore,the lubricating oil flow,the heat transfer and oil scavenge structure of the bearing cavity have been more and more important to the lubricating system and even the working reliability and the service life of the whole aircraft engine.There is complex oil-gas two-phase flow in the bearing cavity of the aircraft engine.Understanding and grasping this two-phase flow field’s characteristics is necessary for the further research and analysis on heat character,which will provide the direction and guidance for the improvement and optimization of the bearing cavity scavenge structure.First of all,this paper used DPM method and CLSVOF method to simulate the oil flow on the cavity wall of a typical bearing chamber.For the air phase,DPM and CLSVOF are both described in the Euler systems.For the oil phase,DPM is described in the Lagrange systems.On the inner wall,the film assumption is used in DPM.However,CLSVOF uses Euler systems to describe the oil phase.The simulate results were compared to the experiment results in a published paper to verify the reliability of these methods.Secondly,basing on the authentic working condition of aero-engine,the bearing chamber oil film thickness and the heat transfer experiment was carried out.The ultrasonic measuring distance system was developed to measure the wall film thickness.And then,the experiment result was compared with CFD result to verify the accuracy of two-phase flow calculation model.At the same time,the temperature of outer wall,inner wall and the oil near inner wall were collected to calculate the heat exchange between oil film and the cavity.Thirdly,the simulation of test piece was carried out,using fluid-structure coupling with heat transfer.The simulate results were in good agreement with experimental data and it could accurately reflect the characteristics of heat transfer of bearing chamber.On this basis,for one conventional bearing chamber oil scavenge structure,two kinds of optimized structure were proposed:Slope-scavenge structure and Flank-scavenge structure.The oil scavenge characters were compared through calculation and analysis between the conventional structure and optimized structures.For the quantitative evaluation of bearing chamber oil scavenging effect,the concept of oil-scavenge efficiency was presented.The flow field characteristics,heat transfer characteristics and oil scavenge characteristics of three kinds structure were also compared.Two kinds of optimized structure have a positive effect to the oil scavenge efficiency.At last,a further design of the optimized bearing chamber oil scavenge structure and vent structure experiment parts were presented,which can expectedly provide data support for the optimization design of the bearing chamber in the next phase of work.Based on the job description,the main conclusions were as follows:（1）.In the chamber inner wall,the oil film thickness calculated from DPM matches well with the experiment result.The site where the oil film calculated from DPM is thick matches well with the site where the oil volume fraction calculated from CLSVOF is large.At a higher oil flow rate,oil accumulation becomes more evident in the bearing chamber,so that the film assumption for DPM is no longer valid.The results acquired from CLSVOF can effectively respond to the real oil-gas two-phase flow in the bearing chamber.（2）.In the different cases,with the increasing of oil flow rate,the film thickness in the inner wall becomes thicker,but it isn’t in the direct proportion to the flow rate.As the shaft speed is increased,the film thickness becomes thinner.Subsequently,the changing law of film thickness around the circumferential direction,starting from 0°to 360°is acquired.（3）.Heat transfer between oil film flow and bearing chamber inner wall will increase with the rise of rotary speed.The nature logarithmic of average Nusselt number Nu around circumference along the bearing chamber wall is directly proportional to 0.3 power of the nature logarithmic of oil supply Reynolds number Re_oil and 0.345 power of the nature logarithmic of rotary shaft rotational Reynolds number Re_s;The nature logarithmic of local Nusselt number Nu_l near wall is directly proportional to 0.7 power of the nature logarithmic of local Reynolds number Re_l.（4）.The flank-scavenge structure works best with the working condition of low rotary speed and low oil flow rate,and that,conversely,the slope-scavenge structure needs higher rotary speed to exhibit its excellent performance.These two improved structures will alleviate the accumulation of oil on the bearing chamber wall and distribute heat transfer evenly.Deep-trench design on these two structures obtains higher oil returning efficiency than a shallow one;more importantly,it will reduce the problem of the rapid decline of oil returning efficiency with the increase of rotation,especially under the condition of high speed.（5）.Generally,the flank-scavenge structure has better oil scavenge effect and heat transfer effect at low rotary speed.But,when the shaft is working on high speed,the slope-scavenge structure will perform better than the former.