GT2017-63815

EXPERIMENTAL INVESTIGATION OF OIL SHEDDING FROM AN AERO ENGINE BALL BEARING AT MODERATE SPEEDS R Santhosh Santhosh Rudrasetty nottingham ac uk G2TRC The University of Nottingham UK Jee Loong Hee eaxjlh exmail nottingham ac uk G2TRC The University of Nottingham UK Kathy Simmons Kathy Simmons nottingham ac uk G2TRC The University of Nottingham UK Graham Johnson Graham johnson nottingham ac uk G2TRC The University of Nottingham UK David Hann David Hann nottingham ac uk G2TRC The University of Nottingham UK Michael Walsh Michael walsh rolls Rolls Royce plc Derby UK ABSTRACT In civil aero engine transmission system bearings are used for shaft location and load support An experimental test facility in the University of Nottingham s Gas Turbine Transmissions Research Centre G2TRC was designed and commissioned to investigate oil behaviour as it exits an engine representative ball bearing In the rig oil is delivered to the bearing inner race and cage via under race feed at three delivery locations i e front mid and rear of the bearing assembly An electromagnetic load system is designed and implemented to allow engine representative axial loads up to 35 kN to be applied to the bearing This paper details the rig design including the load and under race lubrication systems and gives information about bearing oil shedding mechanisms observed In this phase of testing high speed images are acquired at shaft speeds between 1000 and 7000 rpm at an oil flowrate of 5 2 litres per minute and bearing axial load of 10 kN The work presented here focusses on oil shedding from the bearing cage Oil shedding behaviour from aeroengine ball bearing is identified to share many similarities to that observed in the past for shedding from rotating disks and cups However it is shown that it not possible to predict the conditions at which transition in flow regimes will occur for the aeroengine bearing on the basis of correlations for simpler geometries spinning disks and cups The work presented here is the first observation of flow regimes in an aeroengine ball bearing involving high resolution high speed imaging INTRODUCTION Meeting the targets of ACARE and Flightpath 2050 requires aeroengines that are more efficient and lighter in weight thereby reducing specific fuel consumption and with lower emissions and noise levels One of the ways to increase engine efficiency is to improve gas turbine efficiency Glahn et al 1 This can be accomplished by increasing either one or both of the following a turbine inlet temperature and b overall pressure ratio Smaller engine cores are also desirable as this reduces weight and also engine volume reducing drag However smaller hotter engines pose more challenges for the design of bearing chambers Computational approaches to design are attractive but required experimental data for validation which is one of the valuable aspects of an experimental study such as this Aeroengine bearing chambers house the bearings that locate and support the shafts They contain rotating and static components and are typically sealed by air blown labyrinth seals fed by the compressor bleed Oil is supplied to the bearings for lubrication and cooling typically via under race feed or targeted jet exiting into the chamber from which it is scavenged Heat is removed from the hot walls of the chambers by the oil films that form there The current bearing chamber design process includes assumptions about the way oil leaves the bearings and travels in the chamber and on the walls Improved bearing chamber design is therefore strongly dependent on improved data as well as improved design methods Heat and oil management are very important with coking and even in extreme cases oil firing resulting if a chamber operates outside its intended conditions Suter et al 2 Therefore it is imperative to understand the heat transfer phenomenon in these bearing chambers It is also important at the same time to characterize the oil film flow inside the bearing chamber for improvements in efficiency of secondary air system This is a challenging task because the two phase flow inside the bearing chamber is highly complex In practice it is likely that safe oil system design is assured by supplying more oil than is actually required for cooling and Proceedings of ASME Turbo Expo 2017 Turbomachinery Technical Conference and Exposition GT2017 June 26 30 2017 Charlotte NC USA GT2017 63815 1Copyright 2017 ASME lubrication Peduto 3 attributes this to lack of understanding of flow phenomenon and associated heat transfer inside oil systems The heat transfer process in the bearing chamber is primarily convective in nature Glahn and Wittig 4 Oil film on the bearing chamber walls with significant tangential momentum is responsible for most percentage of heat transfer occurring in the bearing chamber Rear bearing chambers in addition to convection heat transfer are expected to transfer heat flux imposed by conduction from the combustion chambers and turbine Convective heat transfer in bearing chambers depends on many factors including oil film thickness and oil film velocity Glahn and Wittig 4 and on the rate of droplet impingement on the wall film Peduto 3 The local heat transfer coefficient is significantly affected by droplet impingement In addition the smaller secondary droplets produced by droplet impingement may be more difficult to predict control and separate from the airflow Therefore it is imperative to understand the droplet production processes mechanisms and empirically relate them to key non dimensional numbers pertinent to rig engine running conditions This will in turn enable prediction of heat transfer correlations The first step towards understanding the droplet production mechanism is to characterize the flow from the bearing cage as this is a significant source of droplets into the core flow Initial visualization of oil shedding from the bearing cage reported in the present work discussed in detail in the Results section showed that the oil disintegration mechanism is strikingly similar to flow patterns witnessed in rotating disks and cups Therefore it is essential to gain a fundamental knowledge of flow patterns in these configurations Rotary cup and disc atomisers find application in chemical agricultural food and metallurgical industries Hinze these values are used in calculating Reynolds and Weber numbers The oil lubricates the bearing and flows radially outwards due to centrifugal action The entire oil feed circuit is controlled via a LabVIEW interface Figure 2 BEARING OIL SHEDDING TEST RIG 3Copyright 2017 ASME The study of oil behaviour in the vicinity of the bearing is the focus of the present work In particular the aim is to visualize and characterize the oil flow exiting from the bearing as this will enhance understanding of the complex two phase flow inside the chamber This is a challenging task because the timescales associated with the oil behaviour are very small as the rotational speeds are high High speed imaging by an IDT Vision high speed camera Model OSV3 is employed in the present study The images are acquired at a high frequency of 16100 fps with 1024 372 pixel resolution The imaged area corresponds to 111 7mm 40 5 mm A Nikon AF Micro Nikkor 60mm f 2 8D zoom lens is coupled to the camera for the imaging Test cases The present work aims to characterize oil flow from the bearing cage edge as the shaft speed is increased in steps Thirteen shaft speeds are considered between 0 7000 rpm The input oil flow rate is maintained constant at 5 2 lpm total combined feed to front mid and rear of the bearing The bearing axial load is maintained at 10 kN throughout the experiments RESULTS In the present section different oil break up mechanisms from the bearing cage edge are identified as the shaft rotational speed is increased in steps from 0 rpm to 7000 rpm at constant input oil flow rate and bearing axial load For the Aeroshell 390 oil employed in the present study the corresponding Weber number based on cage speed is in the range is 1 5 106 6 5 107 is obtained during testing from image analysis and is calculated to be 45 50 of shaft rotational speed over the experimental parametric space The oil disintegration mechanism from the cage edge of the ball bearing rotating at moderate speeds is strikingly similar to flow patterns witnessed in rotating discs and cups The geometrical similarity is illustrated in Figure 3 A zoomed in section of the bearing chamber assembly is shown in Figure 3b Here the resemblance of flow past the cage edge of the bearing assembly to the flow past the edge of a rotating cup with horizontal axis of rotation is illustrated Figure 4 identifies key elements of bearing cage geometry of relevance to the present experiments Locations 1 and 2 are the sharp edges of the bearing cage The stationary surface of the bearing chamber is marked as surface 3 Oil supplied to the bearing is delivered via three sets of holes at axial locations under the cage and inner race Some of the oil delivered to the location under the cage at the front of the rig exits through the gap between the cage and the inner race A subsidiary test done with the front oil feed off shows that a negligible amount of oil from the other two feed locations exits at the front Figure 5 shows a series of still images obtained with the high speed camera illustrating the oil shedding and break up behaviour as shaft speed is increased The different oil break up mechanisms are classified as a direct drop formation b primary ligaments laminar sheet and c ruffled sheet secondary ligaments Figure 3 RESEMBLANCE OF FLOW PAST CAGE EDGE TO THE FLOW PAST EDGE OF A ROTARY CUP Figure 4 CUT SECTION OF BALL BEARING DEPICTING BEARING CAGE EDGES MARKED AS LOCATIONS 1 AND 2 AND STATIONARY SURFACES MARKED AS 3 The transition from one mode to another is brought about by increasing shaft rotational speed Each of these modes is explained below describing the instability mechanism responsible for its formation a Direct drop formation At a low shaft speed of 1200 rpm Figure 5a the oil exiting between cage and inner race remains attached to the cage surface between locations 1 and 2 shown by the dotted line in Figure 4 due to surface tension effects An oil torus is formed around location 2 and due to centrifugal action drops are formed at the large diameter bulges of this torus which are dispersed 4Copyright 2017 ASME circumferentially outwards as oil droplets The term bulges is used to represent the pearl like blob of fluid breaking up from the edge of oil sheet This can be seen in Figure 5a This mode of oil disintegration is direct drop formation similar to drop formation mode in rotary cup oil shedding Figure 1a Although understanding droplet generation from the bearing cage at low shaft rotational speed may not be as important for aeroengine bearing chambers as high speed behaviour it is useful from a CFD validation perspective It is also important from an academic point of view to understand the fundamental fluid mechanics over the complete parametric ranges of bearing chamber operation b Primary ligaments laminar sheet As the shaft speed is increased to 1800 rpm Figure 5b centrifugal force overcomes the surface tension and viscous forces at location 1 to form an oil torus at this location instead of flowing along the surface to location 2 Two break up shedding mechanisms are intermittently observed at this shaft speed In the first bulges from the torus become thin to form primary ligaments By ligaments we mean thread like structures whose aspect ratio length to thickness ratio is 1 This mode is similar to ligament break up mode depicted in Figure 1b In the present study these are referred to as primary ligaments because we identified another class of ligaments discussed later in this subsection that we are terming secondary ligaments The difference between primary and secondary ligaments is described later It is important to note that the primary ligaments emanating from the cage edge location 1 have a controlling solid surface The primary ligaments are formed due to the combined effect of Kelvin Helmholtz K H and Rayleigh Taylor R T instabilities Bizjan et al 9 In particular the initial disturbance required for wave formation on the oil surface is induced by K H instability caused due to velocity difference at the air oil interface These initial disturbance waves are strengthened by R T instability interfacial instability caused due to density difference which occurs when a lighter fluid pushes a heavier fluid These waves are damped by both viscosity and surface tension However at high enough rotational speed when the centrifugal effect overcomes surface tension and viscosity effects the unstable waves grow exponentially Depending on their respective wavelength the RT instability generated unstable waves grow at different rates Subsequently the wave with the fastest growth rate becomes dominant and causes oil film to break up in the form of ligaments The reader is referred to Kamiya 10 for a detailed analytical analysis involving expressions for growth rate of disturbance and the wave number which determines the number of bulges on the cage edge and thus number of ligaments in the analysis of ligaments produced by a spinning disk The fate of the primary ligaments is further governed by Plateau Rayleigh instability In particular each primary ligament granulates to an array of large main droplets from which secondary satellite droplets are subsequently formed The thinning of the primary ligaments to eventually form a conglomerate of droplets is driven by the aim to reduce system surface energy by reducing the surface area A system always prefers to be in the minimum energy state in order to attain equilibrium At the same shaft speed of 1800 rpm another oil break up mechanism is observed where the torus forms into a laminar sheet for some part of shaft rotational cycle Figure 5c The term laminar sheet is selected because the sheet is quite smooth and relatively undisturbed These sheets extend radially from the cage edge till the equilibrium radius At this radius the centrifugal force equals the contracting action of surface tension At this circular periphery a thick rim of oil is produced which further breaks down to threads droplets This type of disintegration mechanism has been termed rim disintegration by Frazer 7 A prominent characteristic of such sheets is that the sheet is not ruffled anywhere throughout its extent This laminar sheet mode is similar to the so called sheet break up observed in previous studies Figure 1c The transition from the ligament break up mode to the sheet type of disintegration is reported in Liu et al 6 as occurring when there was increase in either a shaft rotational speed or b oil flow rate In the present study the two modes i e primary ligaments and laminar sheet are observed at a constant rotational speed of 1800 rpm We postulate that the reason for simultaneous occurrence of both these modes at same shaft speed may be caused by marginal intermittent fluctuation in input oil flow rate through the gap between the cage and the inner race As a result during one part of the rotational cycle the disintegration mechanism is ligaments whereas for the remainder of the cycle the oil flow rate just increases such that the preferred break up mode is laminar sheet The slight variation in oil flow rate affects the break up mechanism in the sense that the variation of flow rate is directly linked to the extent of surface wetness Laminar sheet break up is witnessed when the cage surface is fully wet whereas primary ligaments are observed when the cage surface is partially wet In a recent study by Peng et al 11 of liquid on a spinning disk two break up modes were also intermittently observed namely direct drop and ligament break up modes and the authors concluded that the abovementioned wettability issue was responsible As shaft speed is further increased to 2600 rpm the observed behavior is similar to 1800 rpm but the primary ligaments are observed to grow in number while simultaneously thinning to form fine threads compare the ligaments in Figure 5b to those in Figure 5d The laminar sheet is still intermittently observed 5Copyright 2017 ASME Figure 5 HIGH SPEED VISUALIZATION IMAGES DEPICTING TRANSITION FROM DIRECT DROP FORMATION TO PRIMARY LIGAMENTS LAMINAR SHEET AND FINALLY TO RUFFLED SHEET SECONDARY LIGAMENTS c Ruffled sheet secondary ligaments With a further increase in shaft speed the proportion of time for which there is a sheet present increases until at 4500 rpm there is complete disappearance of primary ligaments and the sheet appears as illustrated by Figure 5e The sheet break up mechanism observed at this critical speed is fundamentally different from the laminar sheet formation of Figure 5c In particular at higher speed the shearing effect of the gas surrounding the radially propagating oil sheet superimposes exponentially growing waves extending from cage edge and which are perpendicular to the liquid flowlines These waves are the same as those observed by Frazer et al 7 in the spinning cup study and can be either sinuous or dilatational or a combination of both Frazer et al 7 termed this type of sheet break up mechanism wave disintegration The oil sheet disintegrates through half wavelengths of oil being torn off A prominent feature of a wave disintegrated sheet is that the sheet is ruffled throughout its extent The ligaments subsequently breaking from these ruffled sheets are termed secondary ligaments in the present study The key difference between the primary ligaments and the secondary ligaments is that in the latter mode of break up the rim location 1 in Figure 4 is no longer the controlling solid surface These secondary ligaments are torn off from the edge of the sheet they are formed due to wave disintegration followed by the subsequent formation of unstable threads In contrast primary liga