Research On The Mechanism Of Energy Exchange And Pressure Fluctuation For Regenerative Flow Pumps

Based on the exergy exchange mechanism between the impeller and channel,regenerative flow pumps(RFPs)have the characteristic of the high head at relatively low flow rates,which means the low specific speed that is difficult to achieve for general centrifugal pumps.Although the structure of RFPs is simple,the intenal flow is quite complex,thus could easily lead to the lower efficiency,intenser pressure fluctuation,and larger vibration and noise.Therefore,clarifying the internal flow mechanism of RFPs is the theoretical basis to provide guidance for the design of RFP’s components with high efficiency and low fluctuation.In this paper,through investigating the mechanism of the energy exchange and unsteady pressure fluctuation in RFPs,methods of enhancing energy exchange and suppressing pressure fluctuation were proposed,which has been utilized for the performance reinforcement of RFPs and has been verified by experiments.The main contents of this paper are as follows:1.Aiming at the energy exchange pattern inside the RFP and the influence of this energy exchange pattern on RFP’s performance,an analytical method of combining the dimensionless axial distance,circulation number and entropy production was proposed to quantitatively measure the exchange flow between the impeller and channel.The intensity of the longitutional vortex,the mass exchange intensity and the amount of the entropy loss were employed to illustrate the pumping ability,the energy exchange intensity and the energy dissipation,which revealed the energy exchange mechanism between the impeller and channel.The results show that decreasing the distance could effectively enhance the intensity of the longitudinal vortex;increasing the circulation number could strength the intensity of the mass exchange;and the entropy production has a positive correlation with the energy dissipation loss of RFPs.2.The proper orthogonal decomposition(POD)was introduced to establish the relationship among the time-frequency characteristics of the pressure fluctuation and the unsteady flow and its eigenvalues in time-space domains.Firstly,investigations on the time-frequency feature of the pressure fluctuation under different operating conditions were carried out to explore the changing law of pressure fluctuation of each component in the RFP.Afterwards,the mode,the flow energy and the time coefficient were applied to illustrate the spatial properties of the flow field,the contribution of various modes to the whole flow field and the changing characteristics of the mode with time.The results show that the pressure fluctuation at inlet and outlet fluctuated periodically and the fluctuating period is equal to the blade number.Additionally,the predominant frequencies in the spectrum are the blade passing frequency(BPF)and its harmonics,which indicates that the rotating interference of the impeller is the main reason for pressure fluctuation in the RFP.The first four modes of the transient pressure fluctuation field occupy the majority of the flow energy.The first and second modes both show the tendency of a gradual change along the impeller’s rotating direction,and the distribution of these modes in the region overlapped with each blade is relatively uniform,which is attributed to the RFP’s pressure increasing pattern.In the third and fourth modes,the stripped regions characterized by local maxima and minima both appear along the blade surface at the blade’s inner and outer radii,which is related to the energy exchange flow between the impeller and channel.The main frequencies of the time coefficient are located at BPF and its harmonics,and the amplitudes are in accordance with the amplitudes in the frequency domain of the pressure fluctuation.3.Based on the principle of energy exchange in the RFP,a quantitative index of the width ratio of the rotor’s and stator’s channel was proposed to investigate the matching relation between the impeller and flow channel of RFP.By the combination of the methods of the theoretical prediction,numerical simulation and experimental verification,the internal flow inside the RFP and the matching relation between the impeller and flow channel can be quantitively evaluated,thus uncovering the influence of the matching mechanism on the exergy exchange performance.The results show that,when the ratio is equal to 1,the longitudinal vortex could well guide the circulation flow exchanging between the impeller and channel along the geometrical profiles of RFP,resulting in the formation of the steady,strong exchange flow.There is no flow separation in the impeller,and the low and high velocity ciontours in the channel are well-circumscribed.Hence,the circulation number is large,the entropy loss is small,and the highest theoretical efficiency of the impeller and channel and the best performanceof the RFP can be achieved.4.Based on the principle of pressure fluctuation in the RFP,the method of supressing the source of the rotor-stator interference and reducing the pressure fluctuation in RFP was proposed.Two kinds of RFPs with symmetrical and staggered blade-arrangements were established,and relavant comparisons and analyses with regard to the internal pressure field and the time-frequency characteristics were carried out by numerical simulation and experimental verification.The results show that the interference superposition effect appeared in the RFP with symmetrical blades,where the modulation mode characterized by local maxima and minima is intense,the BPF component is prominent,and the pressure fluctuation is strong.While in the RFP with staggered blades,the interference superposition effect is avoided,thus the uniformity of the flow field is improved.In addition,the efficacy of the modulation mode is weakened,and the BPF component is reduced significantly,therefore,the pressure fluctuation in this RFP is suppressed.In view of the analysis of the internal flow characteristic and performance for RFPs,a dimensionless axial distance,circulation number and the entropy production were proposed to quantitatively evaluate the exchange flow in RFP.Based on the POD method,the mode,the the flow energy and the time coefficient were employed to illustrated the transient flow field of RFP.Through the ablve two aspects,this paper revealed the energy exchange and pressure fluctuation mechanisms of RFP,and proposed a design method of improving the matching relation between the impeller and channel to enhance energy exchange,and a design method of staggered blade arrangement to supress pressure fluctuation.The design and improvement methods were finally verified by experiments in products,which could serve as a reference for RFP’s optimal design with high effiency and low fluctuation.