Analytical and Numerical Simulations of the Two-Phase Flow Heat Transfer in the Vent and Scavenge Pipes of the CLEAN Engine Demonstrator

摘要

Advanced aircraft engine development dictates high standards of reliability for the lubrication systems, not only in terms of the proper lubrication of the bearings and the gears, but also in terms of the removal of the large amounts of the generated heat. Heat is introduced both internally through the rotating hardware and externally through radiation, conduction, and convection. In case where the bearing chamber is in close proximity to the engine's hot section, the external heat flux may be significant. This is, for example, the case when oil pipes pass through the turbine struts and vanes on their way to the bearing chamber. There, the thermal impact is extremely high, not only because of the hot turbine gases flowing around the vanes, but also because of the hot cooling air, which is ingested into the vanes. The impact of this excessive heat on the oil may lead to severe engine safety and reliability problems, which can range from oil coking with blockage of the oil tubes to oil fires with loss of part integrity, damage, or even failure of the engine. It is therefore of great importance that the oil system designer is capable of predicting the system's functionality. As part of the European Research program efficient and environmentally friendly aero-engine, the project component validator for environmentally friendly aero-engine (Wilfert, et al., 2005, " CLEAN-Validation of a GTF High Speed Turbine and Integration of Heat Exchanger Technology in an Environmental Friendly Engine Concept, " International Symposium on Air Breathing Engines, Paper No. ISABE-2005-1156; Gerlach et al., 2005, "CLEAN-Bench Adaptation and Test for a Complex Demo Engine Concept at ILA Stuttgart, " International Symposium on Air Breathing Engines, Paper No. ISABE-2005-1134) was initiated with the goal to develop future engine technologies. Within the scope of this program, MTU Aero Engines has designed the lubrication system and has initiated an investigation of the heat transfer in the scavenge and vent tubes passing through the high thermally loaded turbine center frame (TCF). The objective was to evaluate analytical and numerical models for the heat transfer into the air and oil mixtures and benchmark them. Three analytical models were investigated. A model that was based on the assumption that the flow of air and oil is a homogeneous mixture, which was applied on the scavenge flow. The other two models assumed annular two-phase flows and were applied on the vent flows. Additionally, the two-phase flow in the scavenge and vent pipes was simulated numerically using the ansys CFX package. The evaluation of the models was accomplished with test data from the heavily instrumented test engine with special emphasis on the TCF. Both the analytical and the numerical models have demonstrated strengths and weaknesses. The homogeneous flow model correlation and the most recent correlation by Busam for vent flows have demonstrated very good agreement between test and computed results. On the other hand the numerical analysis produced remarkable results, however, at the expense of significant modeling and computing efforts. This particular work is unique compared with published investigations since it was conducted in a real engine environment and not in a simulating rig. Nevertheless, research in two-phase flow heat transfer will continue in order to mitigate any deficiencies and to further improve the correlations and the CFD tools.