• Energy Research

Abstract In the present work, the effects of modifying the blade pressure side (EPS profile) on unsteady pressure pulsations and flow structures in a low specific speed centrifugal pump are carried out by experimental and numerical methods. Results are compared to the original trailing edge (OTE profile). Unsteady pressure signals are captured at twenty measuring points at flow rate of 0–1.6Qd. It is observed that the pump head of the EPS profile is improved for all the concerned working conditions. Pressure amplitudes at the blade passing frequency are compared and discussed in detail. It is found that the EPS profile contributes to pressure pulsation reduction obviously. For all the measured flow rates, pressure amplitudes are attenuated evidently at major measuring positions, especially at high flow rates. As for the mean pressure amplitude of twenty measuring points, pressure amplitude is reduced more than 20% at the nominal flow rate using the EPS profile. From relative velocity distribution, it is found that the uniformity of flow field at the blade outlet region would be improved significantly by the EPS profile. Besides, the corresponding vorticity magnitude at the blade outlet would be reduced compared to the OTE profile. The combined effects contribute to the reduction of pressure amplitude using the EPS profile.

AbstractTo solve the severe flow‐induced vibration during the pump operation, the impeller cut is a reasonable and operable approach. In the present study, effects of cutting the blade on the pump performance, especially for pressure pulsations, are conducted by experiments, and numerical results are used to discuss the relation between the internal flow and pressure pulsation. Three cutting angles are investigated, and the obtained results are compared with the original pump. Pressure transducers are mounted on the volute wall and outlet to capture pressure signals of different pumps. Results show that at the cutting angle φ = 15°, pump heads at low flow rates are decreased, which are little affected at high flow rates. With the cutting angles increasing to φ = 25° and φ = 35°, cutting the blade contributes to the reductions of pump heads at all the measured flow rates. Pressure pulsations are significantly affected by cutting the blade. For major measuring points on the volute wall, pressure amplitudes at the blade passing frequency fBPF are attenuated by cutting the blade, especially at high flow rates. Mean pressure amplitude at fBPF is decreased by 38.6% at φ = 15°, which is caused by the increasing gap between the tongue and the blade. From comparison, it is suggested that the cut angle φ = 15° can be used to reduce pressure pulsation considering its little influence on the pump head during application. The obtained results will be useful during the pump operation to reduce pressure pulsations by the blade cut approach.

AbstractThe emitted noise and vibration induced by the unsteady flow of the centrifugal pump are always focused during its running, which is also associated with the high amplitude pressure pulsations. How to reduce pressure pulsations remains a crucial problem for the researcher considering low noise design of the centrifugal pump. In the current research, a special staggered impeller is proposed to reduce intense pressure pulsations of a centrifugal pump with ns = 69 based on alleviating rotor‐stator interaction. The numerical simulation method is conducted to illustrate the influence of staggered impeller on the pump performance and pressure pulsations, and three typical flow rates (0.8ФN–1.2ФN) are simulated. Results show that the staggered impeller will lead to the pump head increasing, and at the design working condition, the increment reaches about 3% compared with the original impeller. Meanwhile, the pump efficiency is little affected by the staggered impeller, which is almost identical with the original impeller. From comparison of pressure spectra at 20 monitoring points around the impeller outlet, it is validated that the staggered impeller contributes significantly to decreasing pressure pulsations at the concerned working conditions. At the blade passing frequency, the averaged reduction of 20 points reaches 89% by using the staggered impeller at 1.0ФN. The reduction reaches to 90%, 80% at 0.8ФN, 1.2ФN, respectively. Caused by the rib within the staggered impeller, the internal flow field in the blade channel will be affected. Finally, it is concluded that the proposed staggered impeller surely has a significant effect on alleviating intense pressure pulsation of the model pump and does not obviously alter the global performance of the pump, which is very promising during the low noise pump design considering its feasibility for manufacturing.

AbstractRotating stall contributes to global oscillation vibration problems, accompanied by noise and possible turbomachinery damage. This study with special emphasis on the vaned diffuser investigates the unsteady pressure interaction with the stall within the pump. A low specific speed centrifugal pump (ns = 69), fitted with a vaned diffuser is modeled and studied. The model pump performance curve shows the characteristic positive slope at 30% of the best efficiency point flow rate; 1.0 ΦΝ which is attributed to the stall phenomena. A finite volume method is employed with unsteady computations initialized utilizing shear stress transport k‐ω before proceeding with DDES. Pressure fluctuation and velocity magnitude normalized values are used to investigate the evolution of stall cell generation. The root mean square (RMS) values and normalized pressure (Cp) values are elicited to gain insight into pressure pulsation within the flow domain. The distinguished “starfish” shape is observed for monitor points md1 to md20, with the RMS trend decreasing with increasing flow rate from the pump shut off. Although in the vaned diffuser flow channel, an increase in pressure fluctuation along the flow channel toward the trailing edge is observed, the vaned diffuser channel shows a similar trend. The stall cell propagates at a speed of ΩRS = 0.078 at 0.2 ΦΝ, while at 0.1 ΦΝ propagates at a speed of ΩRS = 0.087; the stall speed tends to increase approaching pump shut off. Three distinguishable stall channels are observed from the flow structure for a five vaned diffuser; entering the stall, stalled, and stall recovery stages, within the flow channels.

Turbulent flow generated by the intense rotor–stator interaction is detrimental to the safe running of the centrifugal pump. In order to gain more insight into unsteady velocity pulsation characteristics, this research applies the laser Doppler anemometry technique to capture velocity signals at various flow rates. Besides, pressure transducers are arranged on the model pump to sample transient pressure pulsation signals. The study paid particular attention to pulsation signals in the diffusion section of the volute. Results show that at low flow rates, a prominent hump phenomenon generated within the pump head curve, which indicates the development of rotating stall in the model pump. As noted from the spectra, the discrete blade passing frequency and impeller rotating frequency dominate the velocity and pressure spectra. Root mean square values of velocity signals increase rapidly at off-design flow rates, especially within the rotating stall region, and a similar trend is observed for pressure amplitude at the blade passing frequency. From the measuring point (P3) at the inlet of the discharge channel upstream of the volute tongue to the point (P10) at the volute outlet, pressure amplitude rises significantly. Meanwhile, the minimum point of pressure amplitude vs flow rate occurs around 0.4ΦN, and again, the findings show differences for comparisons between measuring points in the core flow region of the volute. This resulting phenomenon can be attributed to flow patterns in the diffusion section.

Abstract Rotating stall, one of the intense unsteady flow phenomena, is detrimental to the stable operation of the pump. To investigate rotating stall characteristics in a centrifugal pump, pressure signals are conducted based on the coherence analysis, meanwhile the unsteady evolution process of the stall structure is also depicted in detail. Results show that unsteady pressure signals are significantly affected by rotating stall characterized by the increased pulsation amplitude and low frequency components generated in the pressure spectrum. The point at θ = 18° shows strong coherence characteristics with the other points. According to the coherence results, it is found that at the stall conditions 0.09ФN and 0.06ФN, the same stall frequency at 0.25fn could be captured. However the typical frequency at the stall condition 0.2ФN could not be identified. By using the numerical simulation method, the alternative blocked and unblocked processes caused by the stall cell in the impeller are obtained, and the same stall frequency at 0.25fn is observed for the pump working at 0.2ФN. So, it is concluded that for the stall frequency of the current model pump, they keep unchanged at different stall conditions. Finally, we believe that the current method could capture the rotating stall characteristics, which could be extended to the other fluid machineries.

A high-speed centrifugal pump is the key facility to deliver oil in an aero-engine. The stable operation is quite important to the safety of the engine. High-speed pump stability is essentially caused by the transient pressure pulsations excited by the complex flow within the pump, which needs to be clarified, especially for the pump under a rotating stall condition. In the current research, unsteady pressure pulsation and the corresponding flow distribution of the high-speed centrifugal pump are analyzed using the delayed detached-eddy simulation (DDES) method. Pressure signals within the pump are extracted by monitoring points. Results show that the dominant components in the pressure spectrum exhibit a significant difference at various flow rates, which locates at the blade passing frequency fBPF under the rated working condition and deviates to five times the shaft frequency (5fn) at the stalled condition. Such phenomenon is not observed in the normal centrifugal pump with low speed when using numerical and experiment methods, and usually the amplitude at fBPF reaches the maximum. Under the stalled condition, the component at 0.2fn is generated and considered as the rotating stall frequency, which is the same at different stalled flow rates. From velocity distribution, it is found that several blade channels are stalled as characterized by the large-scale separation bubbles, which are induced and triggered by the volute tongue.

The match of rotor and stator blades significantly affects the flow field structure and flow-induced pressure pulsation characteristics inside the pump. In order to study the effects of the rotor and stator matching mode on the complex flow field and pressure pulsation of a centrifugal pump with a vaned diffuser, this paper designs three different vaned diffusers (DY5, DY8 and DY9) and uses the DDES (Delayed Detached Eddy Simulation) numerical method combined with structured grids to simulate the unsteady flow phenomena of the model pump under rated conditions. The results show that, under different rotor and stator matching modes, the pressure pulsation spectrum is dominated by the blade passing frequency and its harmonics. The matching mode of the rotor and stator significantly affects the time–frequency domain characteristics of the pressure pulsation inside the pump, and it is observed that the pressure pulsation energy of vaned diffusers with more blades is significantly smaller than that of fewer-blade vaned diffusers in comparison to the energy of the pressure pulsation at the blade passing frequency and within the 10–1500 Hz frequency band. Combined with the distribution characteristics of the complex flow field inside the pump, it can be found that increasing the number of vaned diffuser blades can reduce the energy of flow-induced pressure pulsation, improve the distribution of high-energy vortices in the interaction zone and stabilize the flow inside the centrifugal pump effectively.