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TWO-PHASE FLOW HEAT TRANSFER AND PRESSURE DROP IN HORIZONTAL SCAVENGE PIPES IN AN AERO ENGINE

机译:航空发动机水平弯管中的两相流动传热和压降

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In modern aero engines the lubrication system plays a key role due to the demand for high reliability. Oil is used not only for the lubrication of bearings, gears or seals, but it also removes large amounts of the generated heat. Also, air from the compressor at elevated temperature is used for sealing the bearing chambers and additional heat is introduced into the oil through radiation, conduction and convection from the surroundings. The impact of excessive heat on the oil may lead to severe engine safety and reliability problems which can range from oil coking (carbon formation) to oil fires. Coking may lead to a gradual blockage of the oil tubes and subsequently increase the internal bearing chamber pressure. As a consequence, oil may migrate through the seals into the turbo machinery and cause contamination of the cabin air or ignite and cause failure of the engine. It is therefore very important for the oil system designer to be capable to predict the system's functionality. Coking or oil ignition may occur not only inside the bearing chamber but also in the oil pipes which carry away the air and oil mixture from the bearing chamber. Bearing chambers usually have one pipe (vent pipe) at the top of the chamber and also one pipe (scavenge pipe) at the bottom which is attached to a scavenge pump. The vent pipe enables most of the sealing air to escape thus avoid over-pressurization in the bearing compartment. In a bearing chamber sealing air is the dominant medium in terms of volume occupation and also the in terms of causing expansion phenomena. The scavenge pipe carries away most of the oil from the bearing chamber but some air is also carried away. The heat transfer in vent pipes was investigated by Busam, . Busam has experimentally developed a Nusselt number correlation for an annular flow in a vent pipe. For the heat transfer predictions in scavenge pipes no particular Nusselt number correlation exist. This paper intends to close the gap in this area. As part of the European Union funded research programme ELUBSYS (Engine LUBrication System Technologies), an attempt was done to simplify the oil system's architecture. In order to better understand the flow in scavenge pipes, high speed video was taken in two sections of the pipe (vertical and horizontal). In the vertical section the flow was a wavy annular falling film whereas the flow in the horizontal section was a an unsteady wavy stratified/slug flow. Heat transfer has been investigated in the horizontal section of the scavenge pipe, leaving the investigation on the vertical section for later. Thanks to the provided extensive instrumentation, the thermal field in, on and around the pipe was recorded, evaluated and also numerically modeled using ANSYS CFX version 14 Brand new correlations for two-phase flow heat transfer (Nusselt number) and for pressure drop (friction coefficient) in horizontal scavenge pipes are the result of this work. The Nusselt number correlation has been developed in such a way that smooth transition (i.e. no discontinuity) from two-phase into single phase flow is observed. This work was funded and conducted within the 7th EU Frame Programme for Aeronautics and Transport (AAT.2008.4.2.3).
机译:在现代航空发动机中,由于对高可靠性的需求,润滑系统起着关键作用。油不仅用于润滑轴承,齿轮或密封件,还可以消除大量产生的热量。而且,来自压缩机的高温空气被用于密封轴承腔,并且额外的热量通过来自周围环境的辐射,传导和对流而被引入油中。过多的热量对机油的影响可能导致严重的发动机安全性和可靠性问题,其范围可能从机油焦化(碳形成)到机油着火。结焦可能导致油管逐渐堵塞,从而增加轴承室内腔压力。结果,机油可能会通过密封圈迁移到涡轮机械中,并导致机舱空气受到污染或着火,并导致发动机故障。因此,对于油系统设计人员来说,能够预测系统的功能非常重要。结焦或起火不仅会在轴承腔内部发生,而且还会在从轴承腔带走空气和油混合物的油管中发生。轴承腔室通常在腔室的顶部具有一根管(排气管),在底部具有一根管(排气管),该管连接至排气泵。排气管可使大部分密封空气逸出,从而避免轴承室内的过压。在轴承腔中,就体积占用而言,密封空气是主要介质,从引起膨胀现象的角度来看,密封空气也是主要介质。扫气管带走了轴承腔中的大部分机油,但也带走了一些空气。排气管中的热传递由Busam,进行了研究。 Busam已针对排气管中的环形流实验性地开发了Nusselt数相关性。对于扫气管中的传热预测,不存在特定的努塞尔数相关性。本文旨在缩小这一领域的差距。作为欧盟资助的研究计划ELUBSYS(发动机润滑系统技术)的一部分,人们试图简化机油系统的结构。为了更好地了解清除管道中的流量,在管道的两个部分(垂直和水平)中拍摄了高速视频。在垂直部分中,流动是波浪形的环形降膜,而在水平部分中,流动是不稳定的波浪状分层/弹状流。已经在扫气管的水平部分研究了热传递,而在垂直部分进行了研究。由于提供了广泛的仪器,使用ANSYS CFX版本14记录,评估和评估了管道内,管道上和管道周围的热场。两相流动传热(努塞尔数)和压降(摩擦)具有全新的相关性水平扫气管中的系数是这项工作的结果。以这样的方式开发了努塞尔数相关性,使得观察到从两相流到单相流的平滑过渡(即,没有不连续性)。这项工作是在第七届欧盟航空和运输框架计划(AAT.2008.4.2.3)中资助和进行的。

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