| Cavitation is a common phenomenon in the centrifugal pumps.It usually reduce the performance of hydraulic machinery and produce vibration and noise.It will also cause damage to materials in severe cases.One of the most effective ways to improve cavitation performance is to install an inducer before high speed centrifugal pumps.Inducer can improve the cavitation allowance of centrifugal pumps,because it has better cavitation performance and it can increase the inlet pressure of impeller.Based on the Reynolds N-S equation,RNG k-?turbulence model and Rayleigh-Plesset cavitation model,a three-dimensional numerical calculation is employed to study the flow characteristics and the development law of cavitation of one certain LNG pump.The main contents and conclusions are as follows:1.Cavitation evolution process and steady-state characteristics of flow field in centrifugal pumpsBubbles,static pressure and flow lines in centrifugal pumps was studied under different NPSH_a.According to bubbles distribution and the change of head,the cavitation evolution is divided into four stages:initial cavitation,critical cavitation,developing cavitation and completed cavitation.The development of cavitation under different cavitation stages is got by the distribution of the cavitation bubble on the surface of the inducer and the impeller cascades and the blades.It shows that inducer has great cavitation performance.It can work even on cavitation condition.The velocity of the fluid is fast at mass flow,so that the dynamic pressure of the fluid is increased and the static pressure is reduced.Therefore,cavitation is more likely to occur at a mass flow rate.On the other hand,the dynamic pressure is lowered and the static pressure is increased at small flow,and cavitation is not likely to occur.2.Influence of the inlet sweepback of the inducer on the cavitation performance of the centrifugal pumpThe cavitation flow characteristics of the centrifugal pump under the regime of eight inducer inlet sweepback are studied.The sweepback of the inducer has a great influence on the cavitation performance of the pump.The cavitation performance increases significantly with the sweepback within a certain range.The blades begin to work on liquid and the pressure increase gradually when fluid enters inducer and continues to move towards the rim.So the cavitation performance is improved at the outer edge of the inlet which is most susceptible to cavitate.The greater the sweepback angle,the longer the pressurization process is,the better the cavitation performance is.The efficiency and the head are no longer increased and the cavitation performance is reduced when the sweepback reaches a certain value.So the sweepback exists the optimal value.The optimal sweepback of this pump is 270°.3.The influence of matching of number of inducer and impeller blades on cavitation performanceThe nine different orthogonal combinations between the number of inducer blades and impeller blades(2,3,4&6,7,8 respectively)are carried out to get the cavitation flow characteristics of the centrifugal pump.The number of impeller blades has a great influence on the pump head and efficiency.According to the infinite number of blade theory,the pump head and efficiency both increase with the number of impeller blades in a certain range.The number of inducer blades has a greater influence on the cavitation performance.The larger the number of blades,the greater the displacement of the liquid by the blades is,the higher the flow velocity of the fluid and the lower the pressure is,so cavitation is more likely to occur.Therefore,the number of inducer blades has a greater influence on the cavitation performance,and the number of impeller blades has less influence. |
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