Investigation On Hydrodynamic Characteristics And Pressure Fluctuation For Pump As Turbine

Centrifugal pump running in inverse mode(pump as turbine)is a kind of high pressure fluid energy recovery device.With the characteristic of simple structure,low cost and convenient maintenance,pump as turbine is widely used in petrochemical,coal chemical industry,seawater desalination industry and so on,but the pump as turbine generally has the problem of conversion efficiency is not high and working condition is not stable.In general there are two reasons: one is the guide vane is not added to pump as turbine,the other is that to the high pressure fluid in chemical industry,in addition to the liquid,a certain amount of gases are also contained.The gas has a great influence on the recovery performance of hydraulic turbine.In order to solve the above problems,based on the centrifugal pump as turbine,the external characteristics of the pump as turbine was studied by numerical calculation and experiment,and the accuracy of the numerical calculation method is verified by the experimental results.The guide vane is added to the pump as turbine,under the pure liquid working conditions,the effect of guide vane on the hydraulic performance in hydraulic turbine is studied,then under the gas-liquid two phase working conditions,the effect of gas volume fraction on the hydraulic performance and the distribution rules of pressure fluctuation in hydraulic turbine are studied,two main results in this paper are shown as follows:1.The efficiency and the running stability of pump as turbine can be improved by guide vane.When the guide vane numbers equal(Z+1),(Z+3)and(Z+5)(Z represents the impeller blade number),the efficiency of pump as turbine with guide vane respectively is improved 0.58%,1.45% and 2.00%.To the pump as turbine,an optimal guide vane number is existent,under which the effieciency of turbine is improved,the inlet circulation of impeller and the distribution of blade loading are the most uniform,the useful work areas of fluid are increased,the energy conversion characteristic in high flow rate is increased,the fluctuation amplitude and the dominant frequency amplitude are also the minimum,all of which is good to the performance of pump as tturbine.In this paper,the optimal guide vane number is(Z+3).2.Under the gas-liquid two phase conditions,the effieciency of pump as turine is higher than that of pure liquid conditions,to a same flow rate,with the gas volume fraction increases the efficiency of pump as turbine reduced.At the optimal flow rate,the efficiency of pump as turbine atk? =0.05 is respectively 0.8%,2.2% and 2.8% higher than that of k? =0.1,0.15 and 0.2.Under gas-liquid two phase conditions,the flow losses of pump as turine is higher than that of pure liquid conditions,to a same flow rate,with the gas volume fraction increases the flow losses in pump as turbine are also increased.At the optimal flow rate,the flow losses in pump as turbine at k? =0.05 is respectively 5.1%,11.9% and 16.3% lower than that of pure liquid conditions.To a same flow rate,under different gas volume fractions,the distribution trends are consistent,the higher the gas volume fraction,the smaller the blade loads.At high flow rate,the obvious curvature change of blade loads appeared,which is cased by the change of static pressure in pressure surface,the main working area is the impeller center.Under the low gas volume fraction and high flow rate,the radial force fluctuation amplitude and dominant frequency amplitude are the maximum.Under gas-liquid two phase conditions,the distribution of gas is not uniform,the wake effect appeared at the dynamic and static coupling interface,in the impeller outlet the gas gathered obviously.The time domain of pressure fluctuation in pump as turbine changes periodically,and near the volute tongue,the two times pressure fluctuation appeared.In one rotating circle the pressure fluctuation numbers equal the impeller blade numbers,and the dominant frequency equals the impeller blade frequency,and no subordinate appeared.The pressure fluctuation in impeller is the most intense,which in volute and guide vane is the second intense,and in draft tube is the lightest.When the gas volume fractionk? is greater than 0.05,the gas rate has obvious effect on the pressure fluctuation.The more the gas rate,the smaller the maximum fluctuation amplitude and the dominant frequency amplitude.