A study of the flow field of an axial flow hydraulic turbine with a collection device in an open channel

Significance 

Increasing strict regulations on carbon emissions have significantly favored the development of alternative renewable energy sources. In particular, hydraulic energy has attracted significant attention of researchers owing to its great potential large-scale harvesting that will be of high value in solving the current global energy crisis.

Among the available hydraulic turbines, those with open channels are widely preferred over those with closed channels due to their ability to operate under low-pressure heads thus minimizing the overall costs. However, low rotational speeds in open channel based hydraulic turbines have limited their electricity generating capacities. As such, the development of more efficient open channel based hydraulic turbines is highly desirable. In fact, open channels like canals and rivers do not require weir intake as they can use the water flow.

In order to maximize hydraulic energy generation, understanding the flow field and performance characteristics of hydraulic turbines is of great importance. To realize this, various methods including single-phase flow analysis have been developed. Additionally, considering that open channel turbines can be operated under different configurations i.e. series and parallel, it’s equally important to understand the effects of both external and internal flow on the performance of the turbines. Alternatively, studies have shown that flow field in open channel turbines are affected by free, side and bottom surfaces. However, understanding the flow field with the free surface has been identified as a promising solution in enhancing the performance of hydraulic turbines in open channels.

Recently, Ibaraki University researchers: Dr. Yasuyuki Nishi, Mr. Genki Sato, Mr. Daishi Shiohara, Prof. Terumi Inagaki and Dr. Norio Kikuchi investigated the multiphase flow in axial flow hydraulic turbines in open circulating water tanks. Particularly, the study was based on the particle image velocimetry measurement techniques and the multiphase flow analysis taking into consideration the free surface. Furthermore, they performed another single-phase flow analysis without taking into consideration the free surface so as to validate the flow field characteristics. Their main objective was to enhance hydraulic power generation using the open channel axial flow hydraulic turbines. Their work is currently published in the journal, Renewable Energy.

The authors observed consistent results in both free surface shape and flow field hydraulic turbines inlet and outlets. In addition, different vortexes occurred at the downstream of the hydraulic turbines due to the influence of the channel bottom and free surface. On the other hand, depending on the nature of the vortex, the process and velocity recovery distance equally varied. For instance, a shorter velocity recovery distance in the overall slipstream was exhibited in the multiphase flow analysis as compared to the single-phase flow analysis.

In summary, the Japanese researchers placed the hydraulic turbines in series against the flow to confirm the influence of channel bottom and free surface on the types of vortexes occurring downstream of the hydraulic turbines and in turn on the process and velocity recovery distance. Altogether, the study provides vital information that will pave way for further analysis of the flow conditions that will, in turn, lead to high-performance of the hydraulic turbines.

A study of the flow field of an axial flow hydraulic turbine with a collection device in an open channel - Advances Engineering
Fig.1 Streamlines around the hydraulic turbine obtained from the multiphase flow analysis
A study of the flow field of an axial flow hydraulic turbine with a collection device in an open channel - Advances Engineering
Fig.2 Absolute velocity vectors and vorticity at each downstream position obtained from the multiphase flow analysis

Reference

Nishi, Y., Sato, G., Shiohara, D., Inagaki, T., & Kikuchi, N. (2019). A study of the flow field of an axial flow hydraulic turbine with a collection device in an open channelRenewable Energy130, 1036-1048.

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