Rotating stall is an unsteady flow phenomenon that appears in both axial and centrifugal compressors. It is detrimental to the performance of the compressor, significantly narrowing its operating range. Numerical modeling of this phenomenon has been a major area of investigation for axial compressors with some success. While stall occurs less often in centrifugal compressors than axial ones, it can be much harder to predict. Some preventive measures are known but are mostly rules of thumb developed through experimental experience. This work focuses on the detection of impeller rotating stall using computational fluid dynamics. A compressor was chosen that has demonstrated rotating stall instabilities with different diffuser lengths and return vanes. Unsteady numerical simulations were performed on full 360° models of this geometry. The transient simulations were conducted using distributed processing on high-performance servers. The objective was to determine the accuracy in which rotating stall can be captured in simulations. For this purpose, simulation results were compared to experimental results for the same compressor and show a good correlation between the experimental and numerical tests.

This study conducted a detailed technical analysis of small-scale solar–bio-hybrid power generation systems using Rankine (steam turbine) and Brayton (gas turbine) cycles. Thermodynamic models were developed to characterize the state of working fluid and select the most suitable solar collection technology for individual power generation systems. Net capacity factor of power generation and utilization efficiencies of solar and biogas energy were used as parameters to evaluate energy generation and conclude the preferred system configuration. The analysis concluded that the steam turbine system has better global efficiency (67.7%) than the gas turbine system (55.7%), while the gas turbine system has better electricity generation efficiency (27.0%) than that (5.6%) of the steam turbine system. The effects of different climates on the selection of suitable hybrid systems were also investigated to delineate suitability and feasibility of different hybrid systems. In addition, the method used in this study can also be applied to investigate and optimize other small-scale hybrid renewable energy generation systems.

Rotating stall is an unsteady flow phenomenon that appears in both axial and centrifugal compressors. It is detrimental to the performance of the compressor, significantly narrowing its operating range. Numerical modeling of this phenomenon has been a major area of investigation for axial compressors with some success. While stall occurs less often in centrifugal compressors than axial ones, it can be much harder to predict. Some preventive measures are known but are mostly rules of thumb developed through experimental experience. This work focuses on the prediction of the onset flow rate of impeller rotating stall using computational fluid dynamics. Two different types of unsteady numerical simulations were performed on full 360° models of a compressor that exhibited rotating stall. The transient simulations were conducted using distributed processing on high-performance servers. Fast Fourier transform analysis was used to determine the mass-flow fluctuations in the impeller that can be associated to the phenomenon. The frequency analysis results were then used to determine the onset flow rate for impeller rotating stall. The simulation results and fast Fourier transform analysis were compared to experimental results for the same compressor and show a good correlation between the experimental and numerical tests.

A step-by-step technique has been implemented in the analytical study of heat transfer and pressure gradient characteristics of refrigerants R-410A, R-407C, R-22, and R-134a used as cooling media in the condenser of a steam power plant. Refrigerants are optimized to replace water/air as coolant in the condenser of a steam power plant. Refrigerants have much lower temperatures and much higher heat transfer rates than water or air. The thermal resistances that affect heat transfer characteristics and surface condenser performance are included. The effect of inlet refrigerant temperature and mass flow rate are reported for the four refrigerants. Calculations are performed at two inlet refrigerant temperatures −21 °C and −30 °C and mass flow rate ranging from 92.905 to 132.905 kg/s. The results revealed that the overall heat transfer coefficient, heat transfer rate, and condensation rate increased with refrigerant mass flow rate, with higher values at lower inlet refrigerant temperatures. For a given refrigerant mass flow rate and inlet temperature, the analytical study indicated that R-410A has higher values of overall heat transfer coefficient, heat transfer rate and condensation rate than R-407C, R-22, and R-314a, respectively. Moreover, it is found that R-410A, at −30 °C and 132.905 kg/s, is superior in condensing all steam entering the condenser than the other refrigerants; this corresponds to higher exergy efficiency. The condenser pressure was observed to be slightly higher for R-410A than the other refrigerants.