Research On Characteristics Of Super-low Specific Speed Centrifugal Pumps During Transient Period

The super-low specific speed centrifugal pump is generally refers to centrifugal pump that the specific speed ns is less than or equal to 30, it was widely used in aerospace, agricultural irrigation, urban water supply, petrochemical, and other areas of the national economy because of the characteristics of small flow rate and high head. Generally, the operation condition of super-low specific speed centrifugal pump is basically stable. With the application development and system increasing complexity of super-low specific speed centrifugal pump, in addition to steady-state condition, the super-low specific speed centrifugal pump still need to run in a variety of transient condition in the short term, such as process of aircraft short takeoff and landing, large angle to climb, rocket launch and boost separation. These processes are often accompanied by radical changes of speed, flow rate and pressure. At this point, it will show the transient effect, which is different from the steady-state process. It will produce seriously influence on the pump itself and even the safe and reliable operation of the whole system. Therefore, it has important engineering significance and academic value to carry out the research on hydraulic design methods and the performance of the super-low specific speed centrifugal pump during transient periods.In this paper, a super-low specific speed centrifugal pump was kept as the research object. Emphatically researches on numerical calculation and experimental test of super-low specific speed pump during startup period under shut-off condition were conducted. The differences of internal flow mechanism and the external characteristic during steady-state and transient process were analyzed. In order to further reveal the transient characteristics of super-low specific speed pump, experimental research on characteristics during rapid decompression process at inlet was conducted in the end. In this paper, the main work and research results are as follows:1、The research status of super-low specific centrifugal pump and development history of research characteristics of centrifugal pump during transient progress were summarized systematically. The impeller, volute and suction chamber of a specific speed ns=25 super-low specific speed centrifugal pump was designed based on the method of increasing discharge coefficient.2 、 Based on the CFD numerical simulation technology, three-dimensional unsteady numerical calculation under steady-state condition was conducted of the super-low specific speed centrifugal pump. The energy characteristics of super-low specific speed pump were predicted based on the results of numerical simulation. Analysis of inner flow field, radial force on the impeller and pressure fluctuation in each section of the volute were analyzed. The results show that:(1)The radial force on the impeller reduce gradually with the increase of flow rate. The main fluctuation frequencies of radial force are all blade frequency or integer times of blade frequency. The rotor-stator interaction of impeller and volute is the main factors influencing radial force ripple.(2)The pressure fluctuation peak at each section of the volute is decreasing along the flow direction. The main fluctuation frequencies of radial force are all blade frequency or integer times of blade frequency. The rotor-stator interaction of impeller and volute is the main factors influencing pressure fluctuation.(3)Under small flow rate condition, there are massive whirlpools in the impeller passage. With the increase of flow rate, flow field distribution in impeller passage is uniform gradually.3、A test system which is suitable for test of characteristics of super-low specific speed centrifugal pump during the transient period was set up in this paper. Tests of energy characteristics, head at shut-off condition, cavitation characteristics, transient characteristics during startup period under shut-off condition and rapid decompression process at inlet of super-low specific speed pump were conducted. The results show that:(1)Motor speed linearly increase trend over time during the closed-valve startup process under three different accelerated speed. At the end of the startup process, motor speed achieves maximum value.(2)There was a significant higher impact head than the steady working condition at the last of the three different start acceleration startup processes, and also, the impact head is decreasing with the decrease of start acceleration.(3)The main fluctuation frequencies of vibration acceleration in the direction of X, Y and Z at pump outlet are all axle frequency or integer times of axle frequency during startup period under shut-off condition. Vibration acceleration amplitude in the direction of Y is the largest while the smallest in the direction of Z. With the increase of start acceleration, vibration acceleration amplitude is also increased at the main frequency.(4)Compared with the instantaneous depressurization process and stable non-cavitation process, there is a broadband vibration which is range from 4000 Hz to 6000 Hz. Rapid decompression process at inlet of super-low specific speed centrifugal pump will accelerate the process of cavitation, which may lead to the critical point of cavitation occurs in advance.4、Three-dimensional unsteady numerical calculation of the super-low specific speed centrifugal pump during transient and steady processes under shut-off condition were conducted. The results show that:(1)There was a significant higher impact head than the stable working condition at the last of the three different start acceleration startup processes, and also, the impact head is increasing with the increase of start acceleration. The maximum error are all less than 5%.(2)At the same time, static pressure distribution, absolute speed streamline, relative speed streamline in the middle section and speed streamline shaft section of inlet pipe during stable process are different from the distribution during transient startup progress. The differences are narrowing with the increase of rotational speed. At the same time, the development of inside transient flow field during transient startup progress generally lags behind stable condition.