چكيده به لاتين
Abstract:
Compressors do work in a certain range of capacities, delivering convinced pressures, known as the compressor operating ranges. Although, the compressor design point is a particular point on the compressor compression operating curve, but all other operation points within the curves could be acknowledged as compressor performance with less efficiencies in comparison to the maximum one which could be achieved in the design point. Performing outside the compressor's functional range may causes aerodynamic instabilities. At a critical condition, aerodynamic instabilities initially would result in the occurrence of the stall phenomenon as a local instability. Stall maybe followed by the surge phenomenon as a global instability. The occurrence of instabilities significantly reduces the compressor pressure, which results in the overall efficiency loss. With the furtherance of the stall phenomenon, despite a significant reduction in the efficiency, the compressor is driven to the surge conditions, which is accompanied by stopping the compressor operation.
Generally, two major methods are used to delay the stall phenomenon; the active and the passive control techniques. Using a casing treatment is one of the most important and effective methods of stall passive control in order to delay the stall phenomenon and prevent the surge.
In the present dissertation, the effect of a vane-recessed casing treatment on the performance of an axial flow compressor has been investigated. Initially, a geometry model of a row of guide vanes with single stage of a low-speed axial compressor, containing inlet area, a row of rotor blades and a row of stator blades has been considered. The aforementioned configuration has been analyzed numerically; numerical and experimental results have been compared. The single stage compressor with guide vanes has been studied using three different geometries of the vane-recessed casing treatment: semi-tubular, semi-circular and semi-elliptical with seven rotor blade axial chord exposures of 23.2%, 33.3%, 43.4% 53.5%, 63.6%, 73.7% and 83.8%, all with 120 number of vanes inside the casing treatment. The obtained results of the simulations of employing casing treatment are compared against the experimental results. The pressure ratio, pressure rise coefficient and efficiency at the overall conditions have been calculated based on the (total to total) and (total to static) pressures. The stall margin improvement in addition to the sacrificed efficiency have been evaluated for all the three geometries. It is concluded from the numerical studies, that the semi-tubular casing treatment with 43.4% rotor blade axial chord exposure has provided the highest efficiency and the ratio of the pressure rise. This casing treatment provides exclusively 21.55% for the stall margin improvement. The highest stall margin improvement corresponds to the semi-tubular casing treatment with a rotor blade axial chord exposure of 33.3%, which has been calculated to be 29.68%. It is worth noting, although this rotor blade axial chord exposure has provided the best stall margin improvement, this enhancement has been gained at a cost of sacrificing 0.95% of the maximum overall efficiency based on the total to total pressures. On the other hand, the semi-circular casing treatment has shown a different behavior. The highest overall efficiency based on the total to total pressures, has been obtained in the rotor blade axial chord exposure of 63.6% which has increased the maximum efficiency by 0.99%. The stall margin has also been improved by about 24.9% in 63.6% exposure. The highest stall margin improvement in the semi-circular casing treatment has been obtained for the exposure of 43.4%, which calculated as 29.16%. Moreover, the behavior of the semi-elliptical recirculated cavity casing treatment is similar to that of a semi-circular treated casing, where the highest overall efficiency based on the total to total pressures, has been obtained at the rotor blade axial chord exposure of 63.6%. This exposure has raised the maximum efficiency by 0.91% as well as improving the stall margin by an amount of 22.9%. Furthermore, the highest stall margin improvement in the semi-elliptical casing treatment with 43.4% exposure has been obtained to be 28.34%.
Keywords: Stall, Surge, Casing Treatment, Vane-Recessed Casing Treatment, Semi-Tubular, Semi-Circular, Semi- Elliptical, Stall Margin Improvement, Sacrificed Efficiency.
