Study Of Pressure Fluctuation In A Mixed-flow Pump With Vaned Diffuser

The structure and performance of mixed flow pump lie between centrifugal pump and axial flow pump, axial flow pump has been widely applied to many fields of national economy. In practical operation, pressure fluctuation is caused by impeller-diffuser interaction in a mixed flow pump is one of the main hydraulic factors that lead to the unstable operation and structural vibration, and may result in further local cavitation or resonant in some case, which is a seriously threat to the high efficient and stable operation of mixed flow pump.This paper aims at guide vane mixed flow pump (250HW-32) of specific speed ns=500.After finishing the verification of grid and time step independence, the main works are as follows:1) Based on three different turbulence models (standard k-ε RNG k-ε and SST k-ω model),a steady numerical investigation is carried out to predict the external characteristic and the internal flow field is analyzed by means of multiple reference frame (MRF) in a mixed-flow pump;2) RNG k-ε turbulence model and moving mesh technique were applied to simulate the3-D unsteady flow, and compared with external characteristic of the unsteady and steady calculation;3) Setting monitoring points of pressure fluctuation in the guide vane mixed flow pump, the study on pressure fluctuation characteristics was developed on the different monitoring locations (suction chamber, excessive channel, space diffuser and discharge pipe) and operation condition respectively. Time domains, frequency domains and structure of internal flow field have been analyzed;4) According to the different impeller blade numbers, gap of impeller-diffuser and guide vane blade numbers,3D calculation models of guide vane mixed flow pump were built respectively. The unsteady numerical calculation is done setting monitoring points of pressure fluctuation in the flow field, and time domains, frequency domains were analyzed on these points.According to the research and analysis, the result showed:1) RNG k-ε and SST k-ω turbulence model were more accurate to predict the positive slope of head-flow curve of the mixed-flow pump, while standard k-ε model is not suitable. Near the design point, the distribution and trend of internal flow field are consistent by three kinds of turbulence models. Near the off-design points, the real flow field can be reflected by used of SST k-co model, thus the model’s calculation have good applicability and reliability.2) Because the relative error is smaller to predict external characteristic using steady and unsteady numerical model, steady numerical can be used to performance prediction and unsteady one can be used to the study of pressure fluctuations;3) From inlet to outlet of pump:the dominant frequency of pressure fluctuation is always approximate to the blade passing frequency, and some high frequency and low frequency fluctuations gradually appear; Amplitude of pressure fluctuation increases at first then decreases, and reaches the maximum in the middle flow channel; Waveform and flow field distribution of pressure pulsation have a good periodicity along the circumferential direction; the analysis of time domain and frequency domain is consistent with the flow field distribution, and the area which the blade leading edge passed is lower pressure area, while the one that the blade trailing edge passed is higher pressure area; In the suction chamber, the waveform of pressure fluctuation is more stable, but the amplitude attenuates rapidly and increases gradually with the increase of radius; the secondary peaks and troughs are found in the excessive channel, pressure fluctuation mixed fully and amplitude attenuates more slowly, and the secondary peaks and troughs weakens within the space diffuser and discharge pipe;4) No matter at the design or off-design points, the dominant frequency of pressure fluctuation is always approximate to the blade passing frequency; The amplitude of pressure fluctuation decreases with the increase of flow rate in suction chamber; there exists three kinds of pressure fluctuation components, the blade passing frequency, shaft passing frequency and higher frequency, in excessive channel, space diffuser and discharge pipe, and the amplitude of the three kinds of pressure pulsation are different at the different operating conditions;5) With the increasing of the number of impeller blades, the head of mixed-flow pump would gradually increase. The efficiency would increase along with the increase of blade number under design condition and mass flow. The efficiency reached highest along with four blades of impeller. Gap between impeller and guide vane has less impact on the head of mixed-flow pump, but in order to ensure the high efficiency a moderate clearance (d=0.45b2) is needed. The number of guide vane of mixed-flow pump has less impact on the head and efficiency. The head and efficiency decreased gradually along with the number of guide vane increased. The head of mixed-flow pump gradually decreased along with the number of guide vane increased under small flow working condition, but the efficiency had increased before it increased6) With the increase of the impeller blade numbers, the dominant frequency of pressure fluctuation is always approximate to the blade passing frequency and amplitude decreases, but the changes of amplitude of second frequency are different in the inlet and outlet of impeller; With the increase of the gap on impeller-diffuser, the waveforms of pressure fluctuation have little difference, and the amplitude increases at first and then decreases; with the increase of guide vane numbers, the frequency of pressure fluctuation is constant and the amplitude increases step by step and then reduced rapidly.