Heat Transfer by Impingement Cooling of Spur Gears

Abstract

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.