Francois Cottier

Experimental and Numerical Study of the Thermal Performance of a Film Cooled Turbine Platform

Detailed surface measurements of the thermal performance of a film cooling system have been performed on the endwall of a nozzle guide vane (NGV) mounted in a linear cascade facility at EPFL. An external cooling scheme including several rows of fan-shaped and cylindrical cooling holes has been designed. By testing different cooling flow rates at a NGV exit Reynolds number of 1.7E+06 and Mach number of 0.88, detailed aerodynamic and heat transfer values were obtained destined to assess the design tools for film cooled platforms. The surface static pressure distribution and the film cooling effectiveness on the endwall surface have been experimentally determined. The measurements were obtained applying the pressure sensitive paint technique measuring the coolant gas concentration. An engine representative density ratio between the coolant and the external hot gas flow was achieved by the injection of CO2. The working conditions of the test case similar to realistic engine conditions allow for the validation of in-house CFD codes and the investigation of the reliability of modern commercial tools in such a complex cooling system. The numerical campaign has been performed on the same numerical grid, using the commercial codes FLUENT and CFX, used by EPFL and MTU respectively. A detailed analysis of the grid effects on the obtained results has been previously realised as well as the study of the influence of the modelling approximations. Three cooling mass flows have been simulated and the performance parameters of the film cooling system have been compared to the experimentally obtained data. Special emphasis has been put on the jet penetration effects and on the interaction of secondary flows with the coolant flow. The experimental and numerical efforts were part of the EU funded research project TATEF2 (Turbine Aero-Thermal External Flows 2).

Comparison of Numerical Investigations With Measured Heat Transfer Performance of a Film Cooled Turbine Vane

Detailed surface measurements of the heat transfer coefficient and the film cooling effectiveness by application of the transient liquid crystal method were carried out on a heavily film cooled nozzle guide vane (NGV) in a linear cascade wind tunnel at the EPFL as part of the European Research Project TATEF2 (Turbine Aero-Thermal External Flows 2). The external cooling setup included a showerhead cooling scheme and suction and pressure side of the airfoil several rows of fanshaped cooling holes. By testing two different cooling flow rates at a NGV exit Reynolds number of 1.46E+06, detailed aerodynamic and heat transfer measurement data were obtained that can be used for validation of numerical codes and design tools for cooled airfoils. The data include the NGV surface static pressure distribution and wall heat transfer and film cooling effectiveness obtained by application of the transient liquid crystal technique. An engine representative density ratio between the coolant and the external hot gas flow was achieved by using CO2 as coolant gas. For the coupled simulation of internal cooling and external flow the numerical model was composed of the cooling air feeding the internal plenum, the cooling holes, and the outer external flow domain. An unstructured mesh was generated for the simulations by applying two different commercial CFD codes (Fluent and CFX). Identical boundary conditions were chosen in order to allow for a direct comparison of both codes. The computations were carried in two ways, first using a builtin transition model and second by imposing fully turbulent flow starting at the leading edge. For both codes the same built-in turbulence models were applied. The computations were set up to solve for the aerodynamic flow quantities both within and around the test model and for the thermal quantities on the vane surface, i.e. heat transfer coefficient and film cooling effectiveness. The computational results from the two codes are compared and validated against the results from the experiments. The numerical results were able to confirm a suspicion that the cross flow in the feeding plenum causes an observed non-symmetry of the measured film cooling effectiveness at the outlet of some cooling holes.

Active Outer Ring Cooling of High-Loaded and High-Speed Ball Bearings

Bearings for aero engine applications are subjected to a high thermal impact because of the elevated speeds and loads. The high rate of heat generation in the bearing cannot be sustained by the materials used and, in the absence of lubrication, will fail within seconds. For this reason, aero engine bearings have to be lubricated and cooled by a continuous oil stream. The heat that is generated in the bearings through friction is transferred into the oil. Oil itself has limited capabilities and can only remove heat as long as its temperature does not reach critical limits. When the critical limits have been reached or even exceeded, the oil will suffer chemical decomposition (coking) with loss of its properties and subsequently cause a detrimental impact on the rotating machinery. Oil is normally transferred into the bearings through holes in the inner ring, thus taking advantage of the centrifugal forces due to the rotation. On its way through the bearing, the oil continuously removes heat from the inner ring, the rolling elements, and the bearing cage until it reaches the outer ring. Since the oil has already been heated up, its capability to remove heat from the outer ring is considerably reduced. The idea to provide the bearing with an “unlimited” quantity of oil to ensure proper cooling cannot be considered, since an increase in the oil quantity leads to higher parasitic losses (churning) in the bearing chamber and increased requirements on the engines lubrication system in terms of storage, scavenging, cooling, weight, etc., not mentioning the power needed to accomplish all these. In this sense, the authors have developed a method that would enable active cooling of the outer ring. Similar to fins, which are used for cooling electronic devices, a spiral groove engraved in the outer ring material would function as a fin body with oil instead of air as the cooling medium. The number of “threads” and the size of the groove design characteristics were optimized in a way that enhanced heat transfer is achieved without excessive pressure losses. An experimental setup was created for this reason, and a 167.5-mm pitch circle diameter (PCD) ball bearing was investigated. The bearing was tested with and without the outer ring cooling. A reduction of 50% of the lubricant flow through the inner ring associated with a 30% decrease in heat generation was achieved.

Two-Phase Flow Pressure Drop in Corrugated Tubes Used in an Aero-engine Oil System

In modern aero-engines, the lubrication system holds a key role due to the demand for high reliability standards. An aero-engine bearing chamber contains components like bearings and gears. Oil is used for lubrication and for heat removal. In order to retain the oil in a bearing chamber, pressurized seals are used. These are pressurized using air from the compressor. In order to avoid overpressurization of the bearing chamber, air/oil passages are provided in the bearing chamber. At the top, a vent pipe discharges most of the sealing air and at the bottom, a scavenge pipe is used for discharging the oil by means of a pump (scavenge pump). The scavenge pipe is setup in most cases by tubes of circular or noncircular cross sections. When the scavenge pipe has to be routed in a way that sharp bends or elbows are unavoidable, flexible (corrugated) pipes can be used. Because of the corrugation, considerable flow resistance with high-pressure drop can result. This may cause overpressurization of the bearing compartment with oil loss into the turbomachinery with possibility of ignition, coking (carbon formation), or contamination of the aircraft’s air conditioning system. It is therefore important for the designer to be capable to predict the system’s pressure balance behavior. A real engine bearing chamber sealed by brush seals was used for generating different air/oil mixtures thus corresponding to different engine operating conditions. The mixtures were discharged through a scavenge pipe which was partly setup by corrugated tubes. Instead of a mechanical pump, an ejector was used for evacuating the bearing chamber. An extensive survey covering the existing technical literature on corrugated tube pressure drop was performed and is presented in this paper. The survey has covered both single-phase and multiphase flows. Existing methods were checked against the test results. The method which was most accurately predicting lean air test results from the rig was benchmarked and was used as the basis for extending into a two-phase flow pressure drop correlation by applying two-phase flow multiplier techniques similar to Lockhart and Martinelli. Comparisons of the new two-phase flow pressure drop correlation with an existing correlation by Shannak are presented for mixtures like air/oil, air/water, air/diesel, and air/kerosene. Finally, numerical analysis results using ansys cfx version 15 are presented.

Ejector Scavenging of Bearing Chambers: A Numerical and Experimental Investigation

Oil system architecture in aero engines has remained almost the same for the last 30 years. At least one oil feed pump is responsible for distributing pressurized oil into the bearing chambers, and several scavenge pumps are responsible for evacuating the bearing chambers from the oil and the air mixture. Air is used as the sealing medium in bearing chambers and is the dominant medium in terms of volume occupation and expansion phenomena. In order to simplify the oil system architecture and thus improve the systems reliability with less mechanical parts and also decrease weight, an ejector system has been designed for scavenging bearing chambers. The idea behind the ejector is to use high-pressure oil from the feed pump and use it for feeding the ejectors primary jet. Through the momentum transfer between the pressurized oil at the jets tip and the two-phase mixture of air and oil from the bearing chamber, the mixture will be discharged into the oil tank. In order to design the ejector for aero engine applications, engine-relevant performance conditions had to be considered. The design was performed using a one-dimensional analysis tool and then considerably refined by using the numerical tool ansys cfx. In a further step, the ejector was manufactured out of pure quartz glass and was tested in a lube rig with a bearing chamber, which has evolved from a real engine application. In the bearing chamber, engine-relevant performance conditions were simulated. Through the provided instrumentation for pressures, temperatures, and air/oil flows, the performance characteristics of the ejector were assessed and were compared to the analytic and numerical results. A high-speed camera was used to record the two-phase flow downstream of the bearing chamber in the scavenge pipe. This work is part of the European Union-funded research program Engine LUBrication System TechnologieS (ELUBSYS) within the 7th EU Frame Programme for Aeronautics and Transport (AAT.2008.4.2.3).

A computational investigation of the effect of surface roughness on heat transfer on the stator endwall of an axial turbine

A numerical investigation of the effect of stochastic surface roughness on vane endwall heat transfer was conducted. The effect of equivalent sand grain roughness height was explored and compared with available experimental data. Steady-state computations using ANSYS CFX 14.0 in conjunction with the shear stress transport turbulence model were performed. Computations were conducted for fully turbulent flow conditions, since this best reproduces the conditions for the corresponding measurements. Roughness measurements were conducted at different locations along the vane passage. Exploration of these measurements indicated roughness Reynolds number values from the transitional and fully rough regime. The roughness model supplied in CFX was applied to explore the impact of surface roughness on heat transfer. Numerical heat transfer results in the vane passage were determined from a set of computations at the same operating point consisting of an adiabatic and a heat flux calculation. Calculations were conducted with a systematic variation of equivalent sand grain roughness heights and compared with experimental data. Results are presented for smooth and rough wall calculations at two different flow conditions.

Two-Phase Flow Heat Transfer and Pressure Drop in Horizontal Scavenge Pipes in an Aero-engine

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 turbomachinery 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 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 (2004, “Druckverlust und Warmeuebergang im Entlueftungssystem von Triebwerkslagerkammern (Pressure Drop and Heat Transfer in the Vent System in an Aero Engine’s Bearing Chamber),” Ph.D. thesis, Logos Verlag, Berlin, Germany) and Flouros (2009, “Analytical and Numerical Simulation of the Two Phase Flow Heat Transfer in the Vent and Scavenge Pipes of the CLEAN Engine Demonstrator,” ASME J. Turbomach., 132(1), p. 011008). 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 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.

Two-Phase Flow Heat Transfer and Pressure Drop in Horizontal Scavenge Pipes in an Aero Engine

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 [1], [2]. 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 [23].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).Copyright

Active Outer Ring Cooling of High Loaded and High Speed Ball Bearings

Bearings for aero engine applications are subjected to a high thermal impact because of the elevated speeds and loads. The high rate of heat generation in the bearing cannot be sustained by the materials used and in the absence of lubrication will fail within seconds. For this reason aero engine bearings have to be lubricated and cooled by a continuous oil stream. The heat which is generated in the bearings through friction is transferred into the oil. Oil itself has not unlimited capabilities and can only remove heat as long as its temperature does not reach critical limits. When the critical limits have been reached or even exceeded the oil will suffer chemical decomposition (coking) with loss of its properties and subsequently causing a detrimental impact on the rotating machinery. Oil is normally transferred into the bearings through holes in the inner ring thus taking advantage of the centrifugal forces due to the rotation.In its way through the bearing the oil continuously removes heat from the inner ring, the rolling elements and the bearing cage until it reaches the outer ring. Since the oil has already been heated up its capability to remove heat from the outer ring is considerably reduced. The idea to provide the bearing with an “unlimited” quantity of oil to ensure proper cooling cannot be considered since an increase in the oil quantity leads to higher parasitic losses (churning) in the bearing chamber and increased requirements on the engine’s lubrication system in terms of storage, scavenging, cooling, weight, etc, not mentioning the power needed to accomplish all these.In this sense, the authors have developed a method which would enable active cooling of the outer ring. Similar to fins which are used for cooling electronic devices, a spiral groove engraved in the outer ring material would function as a fin body with oil instead of air as the cooling medium. The number of “threads” and the size of the groove design characteristics were optimized in a way that enhanced heat transfer is achieved without excessive pressure losses.An experimental set up was created for this reason and a 167.5mm PCD (Pitch Circle Diameter) ball bearing was investigated. The bearing was tested with and without the outer ring cooling. A reduction of 50% of the lubricant flow through the inner ring associated with a 30% decrease in heat generation was achieved.Copyright

Impact of Screens Around Bearings on the Flow and Heat Transfer in the Vent and Scavenge Oil Pipes in Bearing Chambers

The aim of this paper is to investigate, first, the effects of screens introduced around bearings and, second, the use of protruded instead of flush installed vent pipes. The investigation focuses on the air and oil flow distributions and on the heat transfer in the scavenge and the vent pipes in an aeroengine bearing chamber. The flow distribution has an impact on the pipes wall temperature distribution with the likelihood of generating hot spots. High temperatures may cause substantial effects on the health of the lubrication system. Problems may range from oil quality degradation to oil self ignition. A steady state CFD analysis of the heat transfer involving the two-phase air and oil flow in these pipes is performed using the ANSYS CFX package. It was demonstrated that whereas screens around bearings reduce the parasitic losses and vent protrusion reduces the oil flow to the air/oil separator, however, due to the oil flow distribution the thermal effects may lead to high material temperatures and to malfunctions in the engine lube system.