Entropy Production Analysis for Hump Characteristics of a Pump Turbine Model

The hump characteristic is one of the main problems for the stable operation of pump turbines in pump mode.However,traditional methods cannot reflect directly the energy dissipation in the hump region.In this paper,3D simulations are carried out using the SST k-ω turbulence model in pump mode under different guide vane openings.The numerical results agree with the experimental data.The entropy production theory is introduced to determine the flow losses in the whole passage,based on the numerical simulation.The variation of entropy production under different guide vane openings is presented.The results show that entropy production appears to be a wave,with peaks under different guide vane openings,which correspond to wave troughs in the external characteristic curves.Entropy production mainly happens in the runner,guide vanes and stay vanes for a pump turbine in pump mode.Finally,entropy production rate distribution in the runner,guide vanes and stay vanes is analyzed for four points under the 18 mm guide vane opening in the hump region.The analysis indicates that the losses of the runner and guide vanes lead to hump characteristics.In addition,the losses mainly occur in the runner inlet near the band and on the suction surface of the blades.In the guide vanes and stay vanes,the losses come from pressure surface of the guide vanes and the wake effects of the vanes.A new insight-entropy production analysis is carried out in this paper in order to find the causes of hump characteristics in a pump turbine,and it could provide some basic theoretical guidance for the loss analysis of hydraulic machinery.

Design of Synchronous Drive Mechanism of Opposed-Piston Hydraulic-Output Engine

In order to improve the thermal power conversion capacity of the internal combustion engine,combined with existing opposed-piston two-stroke engine（ OP2S） and hydraulic free piston engine（HFPE）,the integral structure for a newtype of opposed-piston hydraulic-output（ OPHO） engine has been designed,an operating principle has been introduced,the composition of its synchronous drive mechanism has been carefully analyzed,and a mathematical model has been built. In addition,the kinematics models of both the mechanism and the conventional crank-link mechanism have been established by utilizing MATLAB,and the movement rules of the pivotal moving components have been obtained. According to the simulation results,the piston movement of this newtype of opposed-piston hydraulic-output engine reveals a prominent asymmetry compared to the conventional crank-link engine. Under a fixed engine revolving speed,the compression time of the opposedpiston hydraulic-output engine is shortened while the expanding time is lengthened,thus the gas turbulence intensity is strengthened around the top dead center（ TDC） position. Meanwhile,the piston obtains a longer isometric process compared to conventional engines,which could be benefitial to enhance the combustion efficiency.

On the hydrodynamics of hydraulic machinery and flow control

Hydraulic machinery mainly includes turbine and pump, which is closely related to national economy and people＇s livelihood involving aerospace industry, marine engineering, hydropower engineering, petroleum industry, chemical industry, mining industry, biomedical engineering, environmental engineering, agricultural water-soil engineering, etc.. The internal flow of hydraulic machinery is extremely complex, and its characteristics can be summarized as high Reynolds number, multi-scales, inhomogeneous and vortex-dominant unsteady turbulence which interact with the rotating dynamic boundary（rotor blade）. Based on the analysis of the internal flow characteristics of hydraulic machinery, the author and his research team successively proposed a rotation correction model, a curvature corrected filter-based model, a scalable detached eddy simulation method, and a non-linear hybrid RANS/LES turbulence model to capture unsteady flow structures and then predict hydraulic performance and dynamic characteristics more accurately. According to the analysis on the internal flow, the corresponding flow control measures were put forward. It was verified by experiments that these methods could significantly improve the hydraulic performance, anti-cavitation performance and dynamic characteristics（pressure pulsation and vibration） of hydraulic machinery in a certain range of operating conditions. In addition, the mechanism how flow control measures influence internal flow was analyzed in depth, aiming at finding a feasible and effective way to improve hydraulic performance, anti-cavitation performance and dynamic characteristics of hydraulic machinery.