Accurate Prediction of Buffer Air Temperatures Using Lumped Heat Transfer Method

Abstract Buffer air system plays a vital role in gas turbine engines as it pressurizes bearing compartments, thermal conditioning of life limiting parts, purging the hot gas ingestion and antiicing. Buffer air is normally tapped from compressor section and routed to several locations of the engine through tubes, struts and ducts. Air supply is controlled though a buffer valve which sources air from different sources based on the flight condition. Accurate prediction of steady state buffer air temperature is critical in achieving gas turbine performance, durability and operability. It allows design community to determine optimum amount of buffer air which directly impacts gas turbine efficiency. As buffer air passes through different section of the engine by the time it reaches its intended destination, it losses or gains heat which eventually alter its temperature. Buffer air is currently modelled through a network of chambers and restrictors which is solved through an iterative solver for flow and energy convergence. Heat transfer is neglected in this solver and captures only enthalpy balance and pressure losses. An effort has been made to capture buffer air convective heat transfer as it passes through many channels and ducts on its routing. Lumped heat transfer method is employed and estimated the delta temperature b/w inlet and outlet of the corresponding passage. Outside and inside heat transfer coefficients are estimated using empirical correlations. Under steady state conditions, Final temperature of the buffer air would be the surrounding air temperature at that location and initial temperature of the air is its entry temperature. This delta temperate is incorporated in the buffer air network which then predicts corrected buffer air temperature for any flight conditions. With this methodology predicted buffer air Temperature is matched very close to thermo couple test data for various engine power conditions. Buffer air network model fidelity is greatly improved and gives greater confidence on air temperature for any flight conditions. Design studies are carried out and made a recommendation to incorporate thermal barrier coating on passages so that buffer temperature is retained which improves its thermal conditioning capabilities.