The present investigation is undertaken to study the performance of an existing stage of an industrial centrifugal compressor with a vaneless diffuser and is aimed at improving the efficiency of the compressor stage through the use of a low-solidity (LSD) vaned diffuser. The experiments were conducted at a tip Mach number of 0.35. The LSD vane is formed from a standard aerofoil profile with marginal modification in the trailing edge region. The study was conducted at a solidity ratio of 0.81, which is considered as the optimum for the chosen stage. The overall stage performance for different diffuser vane setting angles was evaluated in terms of flow coefficient, head coefficient, polytropic efficiency and static pressure recovery coefficient. The performance parameters are normalized with respect to those of the vaneless diffuser at design flow. Improvement in performance as well as static pressure recovery was observed with LSD vanes as compared with a vaneless diffuser configuration. Variation in blade loading was studied from measurements of static pressure on the pressure and suction surfaces of LSD vanes at different vane setting angles. It was observed that the vane setting angle has a significant effect on stage performance and also on the blade loading.
In an effort to improve the performance of centrifugal compressors, detailed studies were carried out on three different impeller-diffuser configurations covering a wide range of design flow coefficients. These were two-dimensional impellers chosen from a multi-stage compressor series with identical blade geometry. The impeller exit widths covered were 24.5, 18.5, and 11.5 mm, respectively. Extensive experimental studies have been carried out on the chosen stage configurations with parallel walled vaneless diffusers (VLDs) as well as low solidity vaned diffuser (LSD). For all the tested configurations, the aerofoil shape of the diffuser vanes remained same while the diffuser width, setting angle, and solidity were chosen as variables. The performance for each of the stages with LSDs was compared with that of the corresponding VLD. Significant improvement in performance in terms of polytropic efficiency, head coefficient, and static pressure recovery coefficient was observed with LSD vanes as compared with the VLD. Significant improvement in performance was noticed from low flow till design point for all the tested cases. The study revealed that among the tested configurations, there is an optimum width at impeller exit and diffuser for which the performance of the stage was the best. Performance in terms of static pressure recovery coefficient was also evaluated for all the tested configurations and compared. All the studies were conducted for solidity ratio of 0.81. The effect of setting angle on wall static pressure coefficient was also studied at 24° and 28° for the flow coefficients corresponding to 80, 100, and 120 per cent of design flow. Flow surveys at impeller exit for the three cases covering impellers/parallel walled diffusers of different widths have also been carried out to know the flow behaviour in the passage between the impeller and the diffuser.
This paper presents the results of numerical investigations of a turbulent, swirling and recirculating flow without combustion inside a reverse flow gas turbine combustor. In order to establish the characteristics of fuel distribution patterns of the fuel spray injected into swirling flows, flow fields are analyzed inside the swirl combustor for varying amount of swirl strength using a commercial CFD code fluent 6.1.22. Three Dimensional computations are performed to study the influence of the various parameters like injection pressure, flow Reynolds number and Swirl Strength on the fuel distribution patterns. The model predictions are compared against the experimental results, and its applicability over a wide range of flow conditions was investigated. It was observed from the CFD analysis, that the fuel decay along the axis is faster with low injection pressures compared to higher injection pressures. With higher Reynolds numbers the fuel patterns are spreading longer in the axial direction. The higher momentum of the air impedes the radial mixing and increases the constraint on the jet spread. The results reveal that an increase in swirl enhances the mixing rate of the fuel and air and causes recirculation to be more pronounced and to occur away form the fuel injector. The CFD predictions are compared with the experimental data from the phototransistor probe measurements, and good agreement has been achieved.
Numerical simulation of impeller and low solidity vaned diffuser (LSD) of a centrifugal compressor stage is performed individually using CFX- BladeGen and BladeGenPlus codes. The tip mach number for the chosen study was 0.35. The same configuration was used for experimental investigation for a comparative study. The LSD vane is formed using standard NACA profile with marginal modification at trailing edge. The performance parameters obtained form numerical studies at the exit of impeller and the diffuser have been compared with the corresponding experimental data. These parameters are pressure ratio, polytropic efficiency and flow angle at the impeller exit where as the parameters those have been compared at the exit of diffuser are the static pressure recovery coefficient and the exit flow angle. In addition, the numerical prediction of the blade loading in terms of blade surface pressure distribution on LSD vane has been compared with the corresponding experimental results. Static pressure recovery coefficient and flow angle at diffuser exit is seen to match closely at higher flows. The difference at lower flows could be due to the effect of interaction between impeller and diffuser combinations, as the numerical analysis was done separately for impeller and diffuser and the effect of impeller diffuser interaction was not considered.
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