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Abstract
Centrifugal compressors are utilized in various fields and are used in vast applications. Their operational performance maps are significant to be studied, modified and enhanced Unfortunately, such maps that described by experimental results do not cover each condition. Furthermore, map enhancement utilized by different proposed active control methods to extend the safe margin may not be revealed by such experiments. This is due to expenses as well as the uncovering operational zones. Therefore, map prediction is important, however, it Is very complex becausee of its nonlinearity as well as unstable region that are not easily to be assigned. Consequently, the present work introduc a methodology that predicts different map operation conditions specified by stable operation, choking and unstable conditions. Such methodology was also used to enhance the compressor characteristics at different speeds by extending the safety operational methods. covered by experiments, the present
Margin using control In order to assign different stable operation not methodology Introduced two-dimensional model based on the determination of actual Euler head. This was achieved using thermodynamic relation, velocity triangles while considering different losses exerted in the compressor. Six different losses were considered and represented by empirical formulas published by previous researches. Pre- matching of the simulated model results with the experimental results was achieved by the consideration of these losses. Therefore, compressor stable operation at different speeds other than covered by the experimental results could be predicted. Furthermore, this led to the assignment of different losses and their effects were revealed. The efficiency at different conditions was estimated. Furthermore, the predicted data could be further used for enhancement of performance map.
The present methodology was extended to estimate the choke line. This was achieved by using the formula of Dixon (1967) for chocking. The predicted data of the present model was substituted in such formula to predict choking condition at different speed. However, the specific heat ratio was herein replaced with polytropic Index in such to achieve good matching with different experimental results. Accordingly, the locus of choke line at different conditions was determined.
Furthermore, the present methodology was used to specify the stall and surge lines. This was accomplished by substituting the predicted data of the present model into the model of Moore-Greitzer (1976) dedicated for instability prediction. Thereafter, stability analysis method was exploited to assess the system whether the system stalls or surge according to the roots of the eigenvalue of the state equations of Moore-Greitzer model. The effect of compressibility in compression system was demonstrated and how it shifted the surge line to the left side. Furthermore, comparison with equation of pampreen (1993) for surge prediction was demonstrated to show the adverse effect of non-considering the compressibility in surge line determination.
The present methodology was used to enhance the compressor performance map. This was achieved by extending the safe margin by using two different controllers that were the closed coupled valve, and variable drive speed. The two controller methods were used to achieve compressor stabilization and to extend the operating margin on the plenty ofpressure drop. Consequently, the compressor in this case could operate in stable condition at lower mass flow rates. However, compared with close coupled valve, variable speed drive method shifted the compressor map towards lower flow rate with less pressure drop. Inlet cooling was herein introduced to enhance the compressor pressure ratio that was dropped when utilizing drive speed or CCV methods. combination of inlet cooling with one of these two methods enhanced each of the two methods and improved the compressor performance map.