The geared turbofan is a promising concept for civil aircraft jet engines. With the introduction of a gearbox between the low-pressure turbine and the fan, both components can rotate at their respective optimum speed. The geared turbofan enables a lower specific fuel consumption as well as jet engine noise reductions. A planetary gear train is usually chosen for the transmission with the sun gear connected to the low-pressure turbine. This high-speed reduction gear train needs to transmit high loads with a high efficiency in limited installation space. To ensure a safe operation of the gear train, the thermal behavior of the gears needs to be understood. The heat generated by the meshing processes is dissipated by oil impingement cooling. While the field of Elastohydrodynamic lubrication yields good results for the heat generation, no validated model for the impingement cooling process is available in literature. In this study, an analytical model is developed and validated against experimental data. First, the surface area of the oil film on the gear tooth flank formed by the impinging oil jet is calculated. Second, the heat transfer from the gear tooth flank to the oil film is determined. The fluid motion is modeled as an oil film that is flung off the gear tooth flank by centrifugal forces. In addition to the film flow, the presented model takes into account the temperature dependence of the viscosity of the oil and the initial oil film height. The effect of a lubrication oil film on the gear tooth flank before the oil jet impinges is included and its effect on the heat transfer is assessed. The analytical model agrees well with experimental results over the entire range of investigated operating conditions. Finally, a discussion on the effect of several assumptions in the derivation of the analytical model is presented. The validated analytical model can be used as an efficient tool for the design of gear trains with impingement cooled spur gears.
To design gearboxes with very high power densities, an effective means of cooling the gears and knowledge of the achievable heat transfer coefficients are necessary. In this paper, a method to measure heat transfer coefficients for oil injection cooled gears is presented. Contrary to other experimental investigations, a single hollow spur gear is used. To measure heat transfer coefficients, a temperature gradient between the gear and the hot oil needs to be induced. This is achieved by injecting hot oil at realistic temperatures and cooling the inside diameter of the gear. This enables the measurement of heat transfer coefficients in absence of any dissipative or frictional losses, decreasing the measurement uncertainty. In addition, the novel method yields spatially resolved HTC data. The uncertainty of the method is assessed using Monte Carlo simulations. Experimental results for various operating conditions are presented. For all investigated oil flowrates, the same characteristic behavior of the average heat transfer coefficient versus rotational speed was observed. This observation can be explained by using a kinematic model of the oil jet. The geometry of the gear and the cooling arrangement and the spatially resolved HTC data presented in this paper provide a complete basis for the validation of numerical simulations.
This paper presents experimental and numerical investigations of oil leakage across a conventional labyrinth seal commonly found in aero engine bearing chambers. Measurements and simulations were carried out in order to investigate the influence of chamber geometry and operating conditions on the reliability of the oil seal against leakage. The main goal of the experiments was to determine a minimum required pressure difference Δpleak to prevent oil from leaving the bearing chamber for any given operating point. To determine this variable, the pressure inside the test rig was continuously lowered from a high pressure difference until oil was found to leave the bearing chamber. Using two pressure supplies, this pressure could be negative or positive. The results show that the minimum pressure depends on component design and rotational speed. While certain component designs may increase this pressure at low rotational speeds, thereby creating a safety margin for oil leakage, the opposite effect can manifest itself at higher rotational speeds. Selected operating points were simulated using computational fluid dynamics employing the Volume-of-Fluid (VoF) approach. A comparison of the experimental and numerical results shows good qualitative agreement of the two phase flow phenomena inside the bearing chamber.
Understanding the heat transfer characteristics of impingement cooling of high-speed high-power gears is essential to design a reliable gearbox for a new generation of jet engines. However, experimental data on the impingement cooling of gears is limited in the literature. The experimental setup at the Institute of Thermal Turbomachinery aims at closing this gap. It includes a rotating gear instrumented with thermocouples. The measured temperatures are used to determine a spatially resolved heat transfer coefficient distribution on the gear tooth. The iterative evaluation approach applied in the post-processing of the experimental data is validated with two reference cases. First, it is shown that the interpolation of temperature data between thermocouple locations leads to inaccurate results and would not be valid for the evaluation of the experiments, even if the number of thermocouples were increased. The iterative evaluation approach can reproduce the reference heat transfer coefficient distributions very accurately even with a low spatial resolution of temperature data. A new iterative method based on the Levenberg-Marquardt algorithm is implemented within this study. The new method generally converges faster than the existing method. The difference in required computational time is negligible in the easy to evaluate high heat transfer case, whereas a speed-up of up to three times is observed in the relatively cumbersome evaluation of the low heat transfer case.
Understanding the heat transfer characteristics of impingement cooling of high-speed high-power gears is essential to design a reliable gearbox for a new generation of jet engines. However, experimental data on the impingement cooling of gears is limited in the literature. The experimental setup at the Institute of Thermal Turbomachinery aims at closing this gap. It includes a rotating gear instrumented with thermocouples. The measured temperatures are used to determine a spatially resolved heat transfer coefficient distribution on the gear tooth. The iterative evaluation approach applied in the post-processing of the experimental data is validated with two reference cases. First, it is shown that the interpolation of temperature data between thermocouple locations leads to inaccurate results and would not be valid for the evaluation of the experiments, even if the number of thermocouples were increased. The iterative evaluation approach can reproduce the reference heat transfer coefficient distributions very accurately even with a low spatial resolution of temperature data. A new iterative method based on the Levenberg-Marquardt algorithm is implemented within this study. The new method generally converges faster than the existing method. The difference in required computational time is negligible in the easy to evaluate high heat transfer case, whereas a speed-up of up to three times is observed in the relatively cumbersome evaluation of the low heat transfer case.
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