Centrifugal compressors are widely applied in aircraft engines and their efficiency is of paramount importance. This is why developing a high-accuracy one-dimensional performance prediction method is highly desired. Precise performance prediction or analysis enables designers to make quick assessments and necessary refinements of the geometry during the early design stages of centrifugal compressors.
The current one-dimensional performance prediction methods are divided into three categories: zero-zone, single-zone, and two-zone. The zero-zone technique is a non-dimensional map analysis method for which efficiency is predicted from empirical correlations according to non-dimensional parameters such as tip-speed, Mach Number and flow coefficient. These techniques are used in the early design stage, and for the limited inputs requirements, they only give fairly rough estimations.
The single-zone technique, which is more-geometry-based, divides the loss in the centrifugal compressors into numerous individual components and a loss model formulated for each component. Researchers have proposed several collections of loss models, but purely empirical and semi-empirical models in these collections significantly limit their application.
The two-zone technique, which is a more advanced method, has been proposed to model jet-wake flow pattern in flow analyses. However, effectiveness parameters may vary significantly with flow parameters such as Rynolds number, blockage, blade-loading, incidence, Mach number, etc. Even when the effectiveness parameters are set as constants, the lack of a detailed guideline for parameter selection forces designers to rely on their own experience. The fact that there is still no specific definition of the two zones to differentiate between the primary and secondary zone is another shortfall of the two-zone method.
In all, the two-zone method falls short of a basic guideline for parameter selection despite its upside of trying to model real flow structure with introduced parameters. The single zone method predicts the loss from basic physics, but some loss mechanisms aren’t sufficiently understood yet. This is why PhD candidates Pengcheng Xu, Liming Xuan and led by Professor Zhengping Zou from Beihang University in China developed a new hybrid performance prediction method combining the loss models of the single-zone technique and the division idea of the two-zone technique. The researchers introduced two additional parameters for wake zone analysis to specifically analyze the loss mechanisms not clearly understood in the single-zone method. Their research work is currently published in journal, Aerospace Science and Technology.
Going by the two-zone definition, the proposed hybrid system splits the flow into a jet zone and a wake zone. The authors adopted the well-understood models in the jet zone analysis as an alternative to the empirical parameters of the two-zone method. In the wake zone analysis, the researchers introduced two parameters as an alternative to the endwall loss model in the single-zone model. For the first time, the authors were able to completely define the wake zone and proposed a sensitivity analysis. They then presented the loss models for the jet zone and parameter selection guideline for the wake zone. The authors finally validated their proposed hybrid method on four centrifugal compressors and compared the results with the single and two-zone methods.
The novel hybrid method implicitly took into account the endwall loss with the two introduced wake zone parameters on the assumption that all the endwall loss was contained in the wake zone. Models for the wake zone parameter predictions were proposed by evaluating the dominant factors from one-dimensional level. The authors then extracted the coefficients from sufficient datasets using statistical methods. They found it to be convenient to update them from corresponding database when a new design feature needs to be introduced. The researchers then validated the hybrid method on four centrifugal compressors, and they were able to accurately predict both the shape and performance curves values. They reported an efficiency error of less than 1% at design points and no more than 4% at off-design points.
The findings of Pengcheng Xu and colleagues show that compared with the single-zone and two-zone methods, the proposed hybrid method gives more reliable and accurate predictions in the whole operating range.
Pengcheng Xu, Zhengping Zou, and Liming Xuan. A hybrid performance prediction method for centrifugal compressors based on single-zone and two-zone models. Aerospace Science and Technology, issue 108 (2021), 106358.