Comparative studies on the propagation of rotating stall in a liquefied natural gas cryogenic submerged pump-turbine in both pump and turbine mode

This paper employs a dynamic and sliding mesh in the simulation of both uncoupled and coupled surge and roll motions of a tidal stream turbine, utilizing a modified actuator-line method. The modification involves the relocation of blade elements in relation to the grid. Detailed analyses are conducted on the Cp and Cz variations in surge, roll, and coupled motions at various frequencies and amplitudes. It is observed that changing the amplitude and frequency of surge and roll motions both impacts the amplitude of Cp and Cz. Interestingly, the Cp and Cz variations in surge motion are inversely proportional to velocity variations, while they are directly proportional in roll motion. The influence of the surge motion on Cp Cz plays a major role, while the addition of the roll motion increases the mean values of Cp and Cz. Due to the combination of the wake characteristics of both surge and roll, the coupled motion wake exhibits a contraction–expansion oscillation pattern. In a coupled motion with equal periods, the ring and strip tail vortex characteristics of both motions are apparent. A surge period increment diminishes the surge's tail vortex characteristic, whereas an increase in the roll period gradually erodes the roll's tail vortex characteristic. The coefficient variation of the tangential and normal forces (cn, ct) in combined motion mirror that of surge motion, presenting a convex table per surge cycle with depressions at the 1/2T and 1T points. The peak of cn and ct in surge motion are approximately 0.28 and 0.03, respectively, while in roll motion, they are around 0.261 and 0.025. The exploration of cyclic stress impacts on the turbine, and the potential instability on the platform could be valuable directions for future research.

Effects of surge and roll motion on a floating tidal turbine using the actuator-line method

Fernández-Rodríguez

2023

A precisive calculation method of volumetric and hydraulic efficiency of centrifugal pumps

Zhang

2023

This paper proposes a new method to calculate the volumetric efficiency and hydraulic efficiency of centrifugal pumps based on the principle of energy balance. Two efficiencies are calculated by means of a low specific speed centrifugal pump handling media with different viscosities at best efficiency points and are compared with those of two existing methods. The results manifest that the definition of two efficiencies in the present paper is more precisive and sensitive to the change of liquid viscosity.

Study on the dynamics and wake characteristics of a floating tidal stream turbine with pitch motion under free surface

Xu,

Zhang,

Peng

et al. 2023

To improve knowledge of the unsteady hydrodynamic characteristics of a floating horizontal-axis tidal turbine (HATT) in complex marine environments, a computational fluid dynamics (CFD) numerical method is established to analyze the functioning of a HATT under free surface and pitch conditions: an amplitude of 2.5°–12.5° and a period of 3–15 s. The appropriateness of the CFD method is validated against piled turbine tests in a circulating flume, which turned out to be only 1.43% off at the rated tip speed ratio. The power and thrust coefficients exhibit periodic variation with a frequency twice the pitch's motion. From the ecological perspective, the wake velocity field presents radial warps accompanied by several ellipsoidal low-velocity core regions, which corresponds to the fused vortex rings in the vortex zone. Notably, the wake is more influenced with large rather than small amplitudes and periods; meanwhile, the wake is more sensitive to frequency changes than to amplitude changes. The results could lay a foundation for future research into more complex motion response of floating tidal turbines.