Prediction of Heat Transfer in a Jet Cooled Aircraft Engine Compressor Cone Based on Statistical Methods

The paper presents the setup and analysis of an experimental study on heat transfer of a jet cooled compressor rear cone with adjacent conical housing. The main goal of the paper is to describe the systematic derivation of empirical correlations for global Nusselt numbers to be used in the design process of a jet engine secondary air system. Based on the relevant similarity parameters obtained from literature, operating points are deduced leading to a full factorial design experiment to identify all effects and interactions. The varied similarity parameters are the circumferential Reynolds number, the non-dimensional mass flow, the non-dimensional spacing between rotor and stator, and the jet incidence angle. The range of the varied similarity parameters covers engine oriented operating conditions and is therefore suitable to predict Nusselt numbers in the actual engine component. In order to estimate measurement uncertainties, a simplified model of the test specimen, consisting of a convectively cooled flat plate, has been derived. Uncertainties of the measured quantities and derived properties are discussed by means of a linear propagation of uncertainties. A sensitivity study shows the effects of the input parameters and their interactions on the global Nusselt number. Subsequently, an empirical correlation for the global Nusselt numbers is derived using a multivariate non-linear regression. The quality of the empirical correlation is assessed by means of statistical hypotheses and by a comparison between measured and predicted data. The predicted values show excellent agreement with experimental data. In a wide range, accuracies of 15% can be reached when predicting global Nusselt numbers. Furthermore, the results of the sensitivity study show that pre-swirled cooling air does not have a positive effect on heat transfer

Analysis of Heat Flux Distribution during Brush Seal Rubbing Using CFD with Porous Media Approach

This paper discusses the question of heat flux distribution between bristle package and rotor during a rubbing event. A three-dimensional Computational Fluid Dynamics (3D CFD) model of the brush seal test rig installed at the Institute of Thermal Turbomachinery (ITS) was created. The bristle package is modelled as a porous medium with local non-thermal equilibrium. The model is used to numerically recalculate experimentally conducted rub tests on the ITS test rig. The experimentally determined total frictional power loss serves as an input parameter to the numerical calculation. By means of statistical evaluation methods, the ma in influences on the heat flux distribution and the maximum temperature in the frictional contact are determined. The heat conductivity of the rotor material, the heat transfer coefficients at the bristles and the rubbing surface were identified as the dominant factors

Similarity solutions of a Blasius flow with variable fluid properties and viscous dissipation

An analytical model of the Blasius flow is studied including temperature-dependent fluid properties and viscous dissipation. The friction coefficient and Nusselt number at the wall are calculated from the resulting dimensionless velocity and temperature fields. The variable properties model is compared to a constant properties model to verify if and under which conditions this simplification is valid. Air, water and oil are analyzed as fluids over a representative operating regime, respectively. For air, the variable properties do not influence the friction coefficient and the Nusselt number. For water, the influence of the variable properties is present for both parameters but limited since no large temperature difference can occur in water without a phase change. New correlations for the friction coefficient and Nusselt number were derived for water and oil over a large range of operating conditions. Viscous dissipation does not significantly affect these parameters for air and water because of their relatively low Prandtl numbers. The high Prandtl number of oil in combination with a viscosity that is strongly decreasing with increasing temperature, leads to a more complex behavior. The friction coefficient as well as the Nusselt number are strongly dependent on the fluid properties. Dissipation effects cannot be neglected above an Eckert number of around 0.01. The superposition principle to evaluate wall heat flux in experiments is based on the assumption of constant fluid properties. It can be used without restrictions for air but should be thoroughly checked for all other fluids, especially liquids, using the presented methodology

Extensive Experimental and Analytical Investigation of the Aerodynamic Flow Field of Labyrinth Seals with Innovative Liner Configurations

The state-of-the-art sealing systems in aircraft engines are labyrinth seals to regulate mass flow between rotating components. To prevent structural damage of vital parts during a rubbing process, abradable liner configurations with hollow body structures are applied on the stator. Compared to smooth stator surfaces, these structures increase the cross-sectional area of the sealing gap and, thus, lead to a higher leakage. The aim of this paper is to identify the main geometrical parameters influencing the aerodynamic behavior of various liner configurations. As a basis of an analytical study, an extensive experimental database is required. For this purpose, a test rig is set up at the Institute of Thermal Turbomachinery (ITS). Three liner structures (honeycomb, rhombus, and polyhedral structures), two seal fin configurations, and the effect of the adjusted nominal clearances (0.1-1.0mm) and different pressure conditions are investigated. The experimental results will be presented, discussed, and evaluated by means of established parameters. The focus is on the leakage of the seal, the discharge coeffcient, and the equivalent gap width. Additionally, the influence of the geometric shape of the liner structures is discussed. Flow separation and flow direction play the main role. With the results of this paper, optimization guidelines are derived for the design of innovative liner configurations

Experimental Investigation of the Oil Jet Heat Transfer for an Aero Engine Gearbox

Geared turbofan engines have the potential to propel future civil aircraft engines more efficiently. A planetary gearbox between the low-pressure turbine and the fan enables the operation of both components at their respective optimum rotational speeds. This makes it possible to achieve higher bypass ratios and thus a better propulsion efficiency. A crucial part of the planetary gearbox design is the cooling and lubrication of the gears. Sufficient heat removal from the gear tooth flanks is necessary to ensure reliable operation without the risk of gear failure through pitting or scoring. Fast rotating and highly loaded gears are cooled with impinging oil jets according to current design guidelines. This impingement cooling process comprises a complex, multi-phase flow with heat transfer. Previous experimental, numerical and analytical investigations have shown that the cooling process depends both on the highly unsteady liquid flow dynamics and on the heat conduction in the oil film formed on the gear tooth flank. In this study, the gear is replaced by a cylinder in order to be able to study the impingement cooling on a rotating surface without the influence of unsteady flow phenomena. A hollow cylinder is instrumented with 42 thermocouples across the surface, which are all connected to a telemetry system. A single oil jet is directed radially onto the outer cylinder surface. The measured temperatures are subsequently corrected using a new algorithm to reduce systematic measurement errors without distorting the data. The corrected temperatures are used to calculate the Nusselt number distribution across the cylinder surface by means of a finite element analysis. A parameter study is performed to identify the influence of the parameters oil flow rate, oil viscosity and rotational speed of the cylinder on the heat transfer. The fundamental results of the present study enable a better understanding of the heat transfer on impingement cooled cylinders and spur gears

Experimental investigation of the oil jet heat transfer on meshing spur gears

Designing an adequate cooling system for a high-speed high-power gearbox of a geared turbofan requires a thorough understanding of the cooling capabilities of the utilized oil jet impingement. An experimental setup is employed to determine the heat transfer coefficient on gear teeth in various non-meshing and meshing configurations, which incorporate inclined jets with varying distances between the impingement and the meshing zones. The direction of heat transfer is inverted in the experiments to allow for a feasible setup with the rotating gears, where impinging oil jets heat the hollow instrumented gear as its inner surface is cooled via air jet impingement. Measurements with varying oil volume flow rates and rotational speeds are carried out. The losses are analyzed to enable an isolated investigation of the heat transfer between the oil and the gear via measured temperatures on the gear teeth. Heat transfer coefficients are compared at the lower rotational speed with relatively small meshing losses. The meshing in the experimental setup does not have a significant influence on the mean heat transfer coefficient. The spatial distribution of the heat transfer coefficient is slightly affected by the meshing teeth as the distribution gets more uniform with decreasing distance between the impingement and meshing zones

CFD study of oil-jet gear interaction flow phenomena in spur gears

Oil-jet lubrication and cooling of high-speed gears is frequently employed in aeronautical systems, such as novel high-bypass civil aero engines based on the geared turbofan technology. Using such oil-jet system, practitioners aim to achieve high cooling rates on the flanks of the highly thermally loaded gears with minimum oil usage. Thus, for an optimal design, detailed knowledge about the flow processes is desired. These involve the oil exiting the nozzle, the oil impacting on the gear teeth, the oil spreading on the flanks, the subsequent oil fling-off, as well as the effect of the design parameters on the oil flow. Better understanding of these processes will improve the nozzle design phase, e.g. regarding the nozzle positioning and orientation, as well as the nozzle sizing and operation. Most related studies focus on the impingement depth to characterize the two-phase flow. However, the level of information of this scalar value is rather low for a complete description of the highly dynamic three-dimensional flow. Motivated by the advancements in numerical methods and the computational resources available nowadays, the investigation of the oil-jet gear interaction by means of computational fluid dynamics (CFD) has come into focus lately. In this work, a numerical setup based on the volume-of-fluid method is presented and employed to investigate the two-phase flow phenomena occurring in the vicinity of the gear teeth. The setup consists of a single oil-jet impinging on a single rotating spur gear. By introducing new metrics for characterizing the flow phenomena, extensive use of the possibilities of modern CFD is made, allowing a detailed transient and spatially resolved flow analysis. Thus, not only the impingement depth, but also the temporal and spatial evolution of wetted areas on the gear flanks, as well as the evolution of the oil volume in contact with the gear flanks are extracted from the simulation data and compared in a CFD study. The study consists of 21 different simulation cases, whereby the effect of varying the jet velocity, the jet inclination angle, the jet diameter, and the gear speed are examined. Consistent results compared to a simplified analytical approach for the impinging depth are obtained and the results for the newly introduced metrics are presented

Numerische Simulation der Ölstrahl-Zahnrad-Interaktion bei Flugtriebwerken mit hohem Nebenstromverhältnis: Stand der Forschung

Anforderungen aus der Flugtriebwerksentwicklung erfordern eine noch effizientere Gestaltung des Ölsystems zur Kühlung und Schmierung von schnelllaufenden Untersetzungsgetrieben. Deshalb unternehmen Wissenschaftler weltweit Anstrengungen, um die bei hohen Drehzahlen angewandte Ölstrahlschmierung und -kühlung zu erforschen. Fortschritte bei den CFD Methoden und gestiegene Rechenkapazitäten führen zu einem gestiegenen Einsatz numerischer Methoden zur Untersuchung der Ölstrahl-Zahnrad-Interaktion, wobei hauptsächlich die Volume-of-Fluid und die Smoothed Particle Hydrodynamics Methode eingesetzt werden. In diesem Übersichtsartikel wird der Stand der Forschung in diesem Bereich dargestellt und die Vor- und Nachteile der eingesetzten Methoden skizziert

Contamination of Brush Seals by Oil and Salt and Its Impact on Rubbing and Hysteresis Behaviour

The literature already contains some experimental, analytical and numerical investigations on the rubbing and hysteresis behaviour of brush seals. What the investigations have in common is that they were carried out with new and uncontaminated seals, or that such a condition was assumed. The influence of contamination has not been explicitly investigated yet. Particularly in stationary gas and steam turbines, foreign substances can accumulate on and in the bristle package during steady-state operation. In the case of a rubbing event with a contaminated brush seal, e.g., during shutdown of the machine, the process is not expected to be comparable to that assumed in the presence of a new, uncontaminated seal. The present paper is dedicated to the question of the influence of contamination on the total frictional power loss generated during rubbing and the distribution of heat fluxes in friction contact. For this purpose, rub tests with two seals were carried out on the brush seal test rig of the Institute of Thermal Turbomachinery (ITS) in new conditions. Subsequently, the sealing packages were contaminated with oil or a salt mixture. After the treatment, the rub tests were repeated and compared with the previous tests. In addition, stiffness measurements were used to assess the degree of contamination. A strong influence on the rubbing behaviour by the contamination was detected. Contamination causes the flexibility of the bristle package to be greatly reduced. As a result, especially at the beginning of the first measurements, the total power losses and rotor heat inputs are strongly increased. This flexibility is partly regained in the course of the measurements. As expected, contamination also influences the hysteresis behaviour of the seal. A highly increased leakage rate after rubbing could be observed, because the bristles remained close to their deflected positions. In the case of the salted seal, however, an improvement in the leakage performance could be observed after several repeat tests

Heat Transfer by Impingement Cooling of Spur Gears

Lower specific fuel consumption as well as noise reduction are the main goals in the sector of civilaeronautics engineering nowadays. One prominent concept of achieving these goals is the gearedturbofan engine, in which a planetary gear box is installed between the low pressure spool and thefan. This allows the low pressure turbine as well as the fan to rotate at optimum speeds. This way, thesame power can be generated by fewer stages in the faster rotating turbine, which in turn compensatesthe additional weight of the gear box. The main advantage of the geared turbofan is the possibilityto further increase the fan diameter and therefore improve the propulsion efficiency by means of ahigher bypass ratio. One crucial feature of the gear box is the cooling system needed to ensure safeoperating conditions during all phases of the flight envelope. For an efficient cooling system, optimizedwith respect to weight and cost, the heat transfer between the cooling fluid and the gears needs tobe understood thoroughly. In this study, the impingement cooling of spur gears by oil jets is forthe first time examined analytically and compared to experimental results. This provides knowledgeabout the evolution of the heat transfer coefficient distribution resulting from the cooling fluid flowrate and the gear speed, as well as a deep understanding of the underlying phenomena causing thisbehavior. The analytical solution process comprises of two calculation steps. First, the size of the oilfilm is calculated and secondly, the heat transfer across this surface is evaluated while the oil film isflung off the tooth flank by centrifugal forces. The parameters varied in this study were the oil flowrate, the rotational speed of the spur gear and the oil jet angle. The theoretical results are in goodagreement with the experimental data. The theoretical approach can therefore be applied as a newand efficient tool to estimate the global heat transfer coefficient of impingement cooled spur gears.Furthermore, the validated tool can be used as boundary condition for thermal models of spur gearsand help optimize the impingement cooling oil systems