Study On Cavitation Characteristics And Flow Field Of Tubular Turbine Under Different Clearances

The tubular turbine unit has the advantages of small investment,short construction period,large flow rate,high efficiency,excellent cavitation performance,and fewer blades,which is widely used in the development of low-head hydraulic resources.In the actual operation of the unit,there is a gap between the runner blade and the runner chamber.The gap leakage will affect the internal flow field of the cavitation of the tubular turbine.For example,the turbulence intensity in runner region increases,the head and efficiency decrease,the energy transport and conversion in runner region decrease,which seriously affects the stable operation of tubular turbine units.Therefore,studying the cavitation characteristics of tubular turbine with different clearances and mastering the flow law of cavitation internal flow field are of great significance to ensure the safe and stable operation of tubular hydraulic machinery.In this paper,the real turbine model of a hydropower station is taken as the research object.Based on SST k-(?) model and ZGB cavitation model,the steady cavitation of tubular turbines with clearances of 3.5 mm,4 mm and 5.5 mm under three flow conditions is calculated.It is concluded that cavitation first appeared in the blade outlet side,with the decrease of cavitation coefficient,it gradually developed into the inlet side.At the same flow rate,the efficiency decreases with the decrease of cavitation number,and the efficiency decreases most obviously when the critical cavitation occurs.Cavitation area increases with the increase of gap value.Under the same gap value,with the increase of flow rate,the cavitation coefficient increases,and the cavitation area is larger.Then,the unsteady cavitation calculation of tubular turbine is carried out to study the influence of different clearances on the cavitation flow characteristics of tubular turbine.It is concluded that under the critical cavitation,the pressure distribution of the blade front and the middle section of the runner is uniform,from the inlet to the outlet,pressure of blade working face decreases gradually,from the blade working face to the back,the pressure of the middle section of the runner gradually decreases,from hub to shroud,the pressure pulsation coefficient increases gradually,the main frequency of pressure pulsation is the rotational frequency,and the secondary frequency is 0.5 times the rotational frequency,the wave of pressure fluctuation increases gradually from cavitation region to non-cavitation region,from the inlet to the outlet,the pressure fluctuation amplitude decreases along the flow direction.When the flow rate is constant,the larger the clearance value is,the smaller the high pressure area of the blade working face and the middle section of the runner is,and the larger the pressure fluctuation amplitude is.When the gap value is constant,the larger the flow rate is,the larger the high-pressure area of the blade working face and the middle section of the runner is,and the smaller the pressure pulsation amplitude is.Finally,studying on cavitation internal flow field of different blade tip clearance.It is concluded that under critical cavitation,with the increase of radial coefficient,the turbulent kinetic energy increases first and then decreases and then increases,the absolute value of axial velocity increases first and then decreases,and the turbulent kinetic energy reaches the maximum in the middle of the gap..From the inlet to the outlet,along the flow direction,the turbulence intensity decreases,and the absolute value of axial velocity decreases.Under the same flow condition,with the increase of gap value,the turbulence intensity increases,while the axial velocity changes little.This is because the generation of gap leakage vortex makes the fluid flow more disordered.When the gap value is constant,the turbulent kinetic energy increases with the increase of flow rate,and the maximum absolute value of axial velocity increases significantly.