Study On The Cavitation Mechanism And Control Methods Of The Throttling Section In Valves

Valves are important flow control components in the nuclear power industry,the petrochemistry industry,and the pharmaceutical industry,but there is often cavitation appears around the throttling section under high-pressure difference.The occurrence of cavitation can not only cause great damage to the structure of valves but also may induce valve vibration and the valve noise.To ensure the normal operation,the investigation of the cavitation mechanism and the methods to control cavitation intensity at the throttling section of valves has great importance.Supported by National Natural Science Foundation of China through the No.51805470 and the No.51875514,and the Key Research and Development Program of Zhejiang Province through the No.2019C01025,and the Natural Science Fund Key Project of Zhejiang Province through the No.LZ17E050002,this dissertation focused on the driving valves with linear motion form(take the globe valve with typical structure as an example)and the valves with rotary motion(take the mechanical heart valve as an example,which is regarded as check valve in this dissertation),and analyzed the cavitation characteristics of the throttling section in valves with and without motion,and proposed the methods to reduce the cavitation intensity.The main contents and achievements are as follows:(1)The test rig is built,and experiments are conducted to obtain the relationship between the flux and the pressure drop of throttle orifice.Numerical models to analyze the flow characteristics and cavitation characteristics are established.The cavitation characteristics of a micro scale throttle orifice are investigated,and the effects of the structural scale on cavitation are discussed by comparing with the macro scale throttle orifice.It shows that the errors between the numerical results and the experimental results are within 10%,so the numerical methods have enough accuracy.The velocity,pressure and cavity distributions inside micro scale throttle orifice are similar to the macro scale throttle orifice,but the inception pressure ratio is lower than the macro scale throttle orifice,especially when the ratio between the length and the diameter of the throttle orifice is less than 1.(2)Based on numerical methods,the flow characteristics and cavitation characteristics of the throttling section in globe valves with no motion are studied.The cavitation mechanism and the effects of cavitation on the transverse force acting on the valve stem are analyzed,and the effects of the structural parameters of the valve body and boundary conditions on cavitation intensity are discussed.It shows that when fluids flow through globe valves,the pressure between the valve core and the valve seat has obvious variation and there is a low-pressure region at the location where the valve throat and the outlet connects,which leads to the formation of cavitation.If the upstream pressure remains unchanged,there is a higher cavitation intensity under lager valve opening,and the transverse force acting on the valve stem decreases due to the occurrence of cavitation,while the upstream velocity remains unchanged,there is a higher cavitation intensity under smaller valve opening,and the transverse force acting on the valve stem increases due to the occurrence of cavitation.Besides,the increase of the deviation distance,the bending radius,and the curvature radius leads to the decrease of the cavitation intensity,and the decrease of upstream pressure or the increase of the downstream pressure can also reduce the cavitation intensity inside globe valves.(3)Based on the active dynamic mesh method,the cavitation characteristics during the opening and closing processes of the throttling section in globe valves are analyzed.The curve of the flow characteristic,the transverse force acting on the valve stem,and the cavitation characteristics during the opening and closing phases are compared,and effects of the opening and closing velocity,and the shape of the valve core on cavitation intensity are investigated.It shows that the globe valve has a quick open flow characteristic.The maximum transverse force appears when globe valves are at the middle opening,but the transverse force acting on the valve stem with a flat valve core is higher than the valve stem with the trapezoid valve core.During the closing phase,the cavitation intensity is higher if the closing velocity is higher for the globe valves with flat valve core.For the glove valves with the trapezoid valve core,cavitation intensity decreases firstly then increases if the closing velocity is small.Therefore,for globe valves with flat valve core,the closing velocity should be slow,but for globe valves with the trapezoid valve core,the closing velocity should be slow at first then the closing velocity should be increased.(4)Based on the passive dynamic mesh method,the flow characteristics and cavitation characteristics during the rotational process of the throttling section in check valves are analyzed.The feasibility of the application of numerical methods in the study of cavitation at the throttling section of check valves is verified,and the cavitation mechanism and methods to control the cavitation intensity are discussed.It shows that the water hammer effect formed at the instant of valve closure and the high-speed jet formed due to the squeeze effect of the valve disc and valve housing are the main reasons that lead to the occurrence of cavitation phenomenon.The possibility of cavitation occurrence and cavitation intensity can be expressed in terms of the valve closing velocity,and cavitation intensity increases with the increase of valve closing velocity.The valve closing velocity and cavitation intensity are higher if the cross-section of the downstream channel is epicycloid,while the cross-section of the downstream channel is straight,the closing velocity and cavitation intensity are lower but the check valves cannot be fully opened.When the crosssection of the downstream channel is a circle with varying diameters,an appropriate length needs to be chosen to reduce the cavitation intensity,and the length should be 10 mm if the valve diameter is 25 mm.The geometry of the valve disc also affects the cavitation intensity,and the increased deviation distance of the rotational axis and the increased thickness of the valve disc can reduce the cavitation intensity,but larger deviation distance of the rotational axis can also lead to a decrease of the opening angle.