Four quadrant axial flow compressor performance

[摘要] ENGLISH ABSTRACT:The aims of this thesis are to identify all possible modes of operaton for a multi-stage axial flowcompressor; then to characterise the performance, attempt to numerically model operation, anddetermine the main flow field features for each mode.Four quadrant axial flow compressor operation occurs when the direction of flow throughthe compressor or the sign of the pressure difference across the compressor reverses, or anycombination of these. Depending on the direction of rotation of the compressor, six modesof operation are possible in the four quadrants of the performance map. The rotor rotates inthe design direction for three modes, and in the opposite direction for the other three. Thestationary-rotor pressure characteristic is S-shaped and passes through the second and fourthquadrants.A three-stage axial flow compressor operating in the incompressible flow regime was usedfor the experimental investigation. Flow through the compressor was reversed or augmented bymeans of an auxiliary axial flow fan. Compressor performance was measured by means of staticpressure tappings, a turbine anemometer calibrated to measure forward and reversed volumetricflow and a load cell for torque measurement. The inter-blade row flow fields were measured withpneumatic probes and 50 μm cylindrical hot film probes.Three dimensional single blade-passage Navier-Stokes simulations were performed using theNumeca FineTurbo package. Steady state simulations used a mixing plane approach. A nonlinearharmonic approximation was used for time-unsteady simulations.Unstalled first quadrant operation was unremarkable, and good agreement was obtained betweenexperimental and numerical data. A single stall cell was detected experimentally duringstalled operation, which was not modelled numerically.In the fourth quadrant for positive rotation, (windmilling), the compressor acts as an inefficientturbine. Flow separates from the pressure surface of the blade, rendering the steady-statemixing plane approach unsuitable.The performance characteristic curves for second quadrant for positive rotation, are discontinuouswith those of first quadrant operation. The temperature rise in the working fluid issignificantly higher than at design point. Periodic flow structures occurring across two bladepassages were detected at all flow coefficients investigated, invalidating numerical modelling assumptions.Better agreement was obtained between experimental and numerical data from a casefound in literature.If the compressor operates as a compressor in reverse (third quadrant operation), significantseparation occurs on the pressure surface of all blades, and flow conditions resemble severe firstquadrant stall. Separation becomes less severe at larger flow rates, allowing numerical simulation,though this is sensitive to the initial flow field.In the the part of the second quadrant, where the compressor rotates in reverse, it operatesas a turbine. The blade angles and the direction of curvature match the flow angles and turningwell, leading to high turbine efficiencies. Numerical simulations yielded good agreement withmeasured results, but were again sensitive to the initial flow field.Fourth quadrant operation with negative rotation occurs when flow is forced through thecompressor in the design direction. Large separation bubbles are attached to the pressure surfaces of rotor and stator blades, so virtually all throughflow occurs near the hub and casing

[发布机构] Stellenbosch University