Experimental Study And Numerical Simulation On Transient Characteristics Of Mixed-flow Pump During Starting Period

Mixed-flow pumps show special nature distinct from the steady-state process during rapid starting period and the transient characteristics can provide the instantaneous fluid dynamic for pump system. However, the effects of hydraulic exciting vibration, impact load and cavitation damage generated by transient effects on the safe and stable operation of pump device and piping system are serious. Therefore, the study of hydraulic characteristics and internal flow mechanism during the mixed-flow pump starting transient period and the establishment of design, optimization as well as application method for transient working environment have an important theoretical significance and application value.At present, the study theory of mixed-flow pump transient flow is considerable inadequate. In this paper, based on the unsteady flow characteristics study of a mixed-flow pump under steady-state condition, the research models with good steady-state hydraulic and cavitation performance were optimized and the numerical computation method for internal flow of quasi-steady state, transient and the coupled models of impeller rotor dynamic characteristics were established, moreover the test-bed for mixed-flow pump transient starting performance was developed. The transient external characteristic and internal transient flow mechanism were investigated by combining Numerical simulation and experimental study, providing the theoretical foundation for the control and optimization of unsteady flow during transient working period.1. Two mixed-flow pumps were optimized by using two-dimensional theory and the prediction and analysis of overall characteristics as well as cavitation performance of different speeds were conducted through steady Numerical simulation. As the dimensionless overall characteristics of case a are better than case b, then the case a of good steady-state hydraulic performance was employed as the hydraulic model for the starting transient internal flow characteristics study.2. The overall characteristics and flow field distribution of the mixed-flow pump model were analyzed by using quasi steady-state Numerical simulation at the speeds of750rpm,1150rpm and1450rpm, respectively, and the overall characteristics and flow field distribution at different operating points for1450rpm steady speed were predicted. There is a big difference in the transient overall characteristics of numerical prediction and that of experiment due to neglecting the effect of blade rotational acceleration on overall characteristics when using the quasi steady-state Numerical simulation method. Compared the inner unsteady flow under quasi steady-state and steady-state conditions, the flow fields of quasi steady-state presented a certain similarity, internal flow structure was distinct from periodic unsteady flow induced by flow rate changes of steady-state.3. Numerical simulation methods of transient characteristics for mixed-flow pumps were developed aiming at the poor accurate prediction of transient hydraulic characteristics by quasi steady-state numerical method. The sliding interface for mixed-flow pump starting transient numerical simulation was introduced, in which impeller acceleration was controlled through the CEL expression and different starting effect was achieved by changing the variable assignment. The calculation results show that transient overall characteristics predicted by transient numerical simulation agreed well with the experiment data. Compared with the internal flow field of quasi steady-state numerical simulation, the internal flow analyzed by transient numerical simulation method shown a greater sense of unsteady due to considering the impeller rotating acceleration and fluid inertia force, and thus led to the transient overall characteristics differed from quasi steady-state.4. Based on the results of numerical simulation of transient flow, the two-way alternating Fluid-Structure Interaction method was used for the combined solutions of impeller flow and structure field in the starting process of the mixed-flow pump, and obtained the effects of transient effect on the stress and strain of impeller during the starting period of the mixed-flow pump. The instantaneous water impact caused by impeller rotation acceleration led to a great stress concentration on the impeller hub, and the maximum equivalent stress increased in a geometrical progression. The change trend of dynamic stress as well as deformation with speed provides a theoretical reference for the optimized design of transient hydraulic characteristics as well as reliable operation.5. The test-bed for mixed-flow pump transient starting performance was developed, and the transient overall characteristics, PIV internal flow and transient pressure pulsation were measured. The PIV shooting of inlet flow field on a vertical axis section of the mixed-flow pump was achieved for the first time through reforming the inlet pipe structure. The overall characteristics, transient velocity vector field and pressure pulsation data got from experiments can provide the real and effective data for exploring transient flow mechanism of the mixed-flow pump.6. The research results show that, PIV measurements and pressure pulsation of the mixed-flow pump transient flow agree with the experiment results of transient external characteristics well. The transient effect of the mixed-flow pump during starting period is the result of impeller acceleration, pipe resistance characteristics and internal flow structure, the evolution of transient internal flow structure reflects the hysteresis effect of head, load and transient flow rate. The transient dimensionless head curve deviating from steady-state dimensionless curve shows that the transient hydraulic characteristics during starting period do not have a similarity, and the quasi steady-state method is not suitable for revealing the internal flow characteristics of the mixed-flow pump. When obtaining the instantaneous fluid dynamics, it is necessary to carry out the effective control of hydraulic impact of impeller caused by transient effects. The contents and conclusions of this study can provide a theoretical reference and basis for the design and optimization of hydraulic machinery applied in the transient working process.