Aerothermal performance of fan outlet guide vanes in modern geared turbofan engines

Examensarbete för masterexamen
Björkman, Albin
Mahaveer, Paudan Hassan
In the process of making aero engines more energy efficient, hybrid electrical concepts with electrical generators at the core of the engine are being explored. This brings additional challenges to the engine architecture. This thesis will focus on exploring the possibility of cooling these electrical generators using already existing surfaces in the engine. In particular, the possibility of using the fan outlet guide vanes (OGV) as heat sinks. Two different sets of OGV are tested, in additional to the original set for the engine, a low aspect ratio OGV of larger size with lower blade count is investigated. The proposed idea is to have pipes transport a cooling fluid from the generator to the OGV in order to cool down the generator. The require ment on this cooling system is the ability to reject 20 kW of heat at a minimum pressure drop in the pipes. This problem is tackled by doing CFD RANS simula tions on the low pressure section of the aero engine, consisting of the outer and inner end walls of the engine, together with the turbofan and OGV. As aircraft operate at different conditions, these have to be considered in running the simulations. The operating conditions tested in this thesis is cruise, when the aircraft is traveling at a high velocity at a high altitude, and idle, when the airplane is stationary on the ground with the engine running at a low speed. These simulations are run using the k - ω SST turbulence model along with the γ - θ turbulence transition model, and the object of the simulations is to capture the factors that contribute to heat rejection, namely heat transfer coefficient on the OGV and mass flow though the engine. This data is then used together with theoretical and empirical data to design a piping system going through the OGV. The design of the piping system is not based on simulation data, and standard fluid mechanics and heat transfer equations are used to estimate the possible heat rejection for a certain pipe configuration. With these two connected, a optimization code based on the genetic algorithm is used to create multiple optimal piping configurations by maximising heat rejection and minimising pressure drop. The piping parameters used is number of pipe passes, distance between pipes, and first pipe location in the OGV. An additional parameter is used containing a number of different coolants, to determine the most suitable candidate. The result of this is a Pareto front, containing data to design a piping system based required heat rejection to give minimal pressure drop. In the idle case, where it is most difficult to meet the requirement, the heat rejection possibilities range from 20 - 120 kW, with a required pumping power of 10−1 - 102 W. For the low count OGV, the heat rejection is in range 20 - 100 kW and pressure drop ranges from 10−2 - 102 W.
CFD, RANS, SST, OGV Heat transfer, HTC, Cooling system, Turbofan engine, Fan OGV, Low aspect ratio OGV
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