
Aeronautical and Space-Rocket Engineering
Аuthors
Central Institute of Aviation Motors named after P.I. Baranov, Moscow, Russia
e-mail: bmklinskiy@ciam.ru
Abstract
The article adduces the method for the high-pressure compressor stable operation margin experimental determining by the compressed air injection from the test bench compressor into the engine combustion chamber up to achieving the compressor operation instability boundary at the constant value of the reduced compressor rotating frequency of nHPC.cor = const as applied to the bypass turbofan with the flows mixing.
The high-pressure compressor tests as part of a turbofan engine are being conducted to determine:
- compressor characteristics in the absence of the flow disturbances at the engine inlet, including under conditions corresponding to the maximum flight altitude at the minimum instrument flight speed;
- compressor operation stability margin and its ampleness while the flow heterogeneity simulating at the engine inlet, corresponding to the heterogeneity level at the air intake channel of the power plant.
The purpose of the work consisted in experimentally determining the effect of the total pressure values changes in a wide range, as well as the amount of flow inhomogeneity prior to the turbofan engine entering on the changes in the available stable operation margin of the high-pressure compressor.
The article presents the results of studying the high-pressure compressor operation stability margin as a part of a bypass turbojet engine with both grid flow heterogeneity simulator and a “smooth” inlet in a wide range of the total pressure changes at the engine inlet in the layout of a common standard jet nozzle and an option with separate nozzles of a gas generator and an external the engine circuit.
The obtained data revealed that the grid placed in front of the engine simulates adequately the circumferential unevenness of the pressure field at the air intake outlet of the aircraft power plant. The average cross-sectional intensity herewith of the flow pulsations while the tests on the test bench is 1.35 times higher at high air flow rates, and is 1,7 times less at low the flow rate than in the air intake duct.
In the present experiment, the grid placed at the turbofan inlet affected mainly the high-pressure compressor operation stability boundary shifting.
The article demonstrated that reduction of the total pressure value and flow non-uniformity at the inlet of the tested bypass turbofan lead to the compressor stable operation boundary and operation modes line on the compressor characteristic shifting towards each other. Thus, with a in the total pressure P*in decrease from 78.5 to 24.5 kPa, the available reserves of compressor stable operation margin in the range of the reduced HPC rotor speed nHPC.cor = 0,826–0,860 decreased by the ΔKsustainability = –(6.3–7.7)%. This fact limited as well the range of the bypass turbofan control due to the high-pressure compresor rotational speed increase in the engine layout with two separate nozzles, corresponding to the “lower” stall while the grid placing (Win = 1.3%) by 1.86%, and with the total pressure P*in decrease from 78.5 to 24.5 kPa by 6.7%. The combined effect of the total pressure reducing and installing a special grid at the engine inlet can reduce the available of stable compressor operation margin to ΔKsustainability ≈ –(9.5–12.7)%.
The method of compressed air injecting into the combustion chamber of the tested turbofan engine from a test bench source ensured the of the operating point movement on the pressure branch of the HPC characteristic to the boundary of the stable operating modes in the entire range of checked operating modes, including the modes of total pressure reducing prior to the turbofan engine and flow inhomogeneity simulating at the inlet.
Keywords:
low-pressure compressor add stages, high-pressure compressor, attached inlet pipeline, stalling and surging, compressor operation stability margine, flow non-uniformityReferences
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