Shut-off Conditions Research Articles

Numerical Prediction of Pressure Oscillation in a Centrifugal Pump at Shut-off conditions

In order to investigate the pressure fluctuation characteristics of centrifugal pumps under shut-off conditions, a numerical calculation was carried out using Fluent software based on the RNG k-ε turbulence model. The pressure fluctuation characteristics within the volute and impeller flow fields of the centrifugal pump under shut-off conditions were analyzed, and the shut-off head was predicted. The results show that the monitoring points within the volute flow passage exhibit obvious periodic patterns, and their peak pressure fluctuations occur at frequencies that are multiples of the vane passing frequency. Additionally, the closer the monitoring point is to the volute tongue, the greater the amplitude of pressure fluctuation. Furthermore, for monitoring points located on the centerline of the impeller flow passage, the closer they are to the impeller exit, the greater the amplitude of pressure fluctuation.

Simulation study of water shut-off treatment for heterogeneous layered oil reservoirs

The purpose of this paper is to develop a hydrodynamic model of two-phase, three-component fluid flow in a heterogeneous layered reservoir, taking into account cross-flow between layers, and to predict the process of oil displacement by water under conditions of water shut-off in the vicinity of injection or production wells. Numerical calculations at different water cuts (38%, 50%, 96%) showed that in the presence of cross-flow between layers, the current oil recovery coefficient of the reservoir at water shut-off in the vicinity of the producing well increases by 0.3254, 0.3067, and 0.0429 respectively, compared to water shut-off in the vicinity of the injection well. In the absence of cross-flow between layers, there is a decrease in oil recovery coefficient by 0.0082 and 0.0072 at 38 and 50% water cut, respectively. Additionally, there is an increase in recovery coefficient by 0.007 at 96% water cut. It was also obtained that in the presence of cross-flow between layers the gel washout period at water shut-off in the vicinity of the injection well is less at different water cut in 1.24–1.81 times than in the vicinity of the production well. In the absence of cross-flow between layers, the washout time at 38 and 50% water cut is almost the same, but increases by 1.084 times at 96% water cut. Comparison of results of water shut-off prediction using proposed hydrodynamic model and real data showed that calculations describe real data with an average error of about 7%.

Research on the Performance Characteristics and Unsteady Flow Mechanism of a Centrifugal Pump under Pitch Motion

Pitch motion is the key factor affecting the performance characteristics of centrifugal pumps on board ships and exacerbates hydraulic excitation to induce the unsteady vibration of pump units. A hydraulic test platform with swing motion is established to explore the effects of pitch motion on a pump’s performance characteristics. An obvious hump zone exists in the head characteristic curve in the low-flow-rate condition due to the pitch motion. The pump head in the shut-off condition has a significant decrease due to the pitch motion, compared to the static state. The head decrease gradually increases as the maximum pitch angle increases or the pitch period shortens. Specifically, the head in the rated flow condition decreases by 6.3 % to reach a minimum at the maximum pitch angle of 20 degrees in a period of 5 s. Based on a multiple-reference coordinate system, a large eddy simulation with a shear-modified eddy viscosity model is employed to simulate inner flow characteristics under the influence of pitch motion. A distinct vortex flow appears near the blade suction surface and becomes increasingly turbulent as the pitch period shortens. The pitch motion intensifies the unsteady stretching and deformation of vortices. The periodic variations in fluid-induced pressure over time present parabolic features, and the amplitude in the frequency domain reaches its maximum value within a pitch period of 5 s.

Numerical Modeling of the Hydrodynamic Performance of Slanted Axial-Flow Urban Drainage Pumps at Shut-Off Condition

Increasing extreme rainfall events caused by global climate change have had a significant impact on urban drainage systems. As a critical component of a pumping station, a large-scale slanted axial-flow pump (SAFP) featuring high specific speed plays a critical role in mitigating urban flooding and waterlogging. In this study, to reveal the transient characteristics of a SAFP at shut-off conditions, a computational fluid dynamics (CFD) based approach with dynamic mesh was proposed. Multiple shut-off conditions with various shut-down speeds of the sluice gate (SG) were modeled. Our analysis demonstrated that both the shut-off conditions and the slanted structure have conspicuous impacts on the hydrodynamic performance of a SAFP. Reducing the shut-down speed leads to a greater reverse flow rate and higher runner speed. The water hammer effect was simulated with different shut-down speeds, increasing the water head by 5.07–10.42 m, the axial force by 163.46–297.06 kN∙m, and the axial moment by 116.05–224.01 kN∙m. Compared with the axial direction, moments in the radial directions were found with more obvious oscillation as a result of stronger rotor–stator interaction. Due to the gravitational effect of the slanted structure, the fluctuation of the runner in vertical direction presented an off-axis characteristic compared with the horizontal one. As the SG speed increased, pressure fluctuations gradually decreased at various locations across the SAFP.

Impact of Interface Model on Simulation Results of a Radial Pump at Part Load and Shut-Off Conditions

Computational Fluid Dynamics (CFD) is a well-known tool for predicting and analyzing performance in a variety of engineering branches, including turbomachinery, allowing engineers to partially replace physical experiments with their virtual analog. Nevertheless, numerical analysis should be used carefully regarding possible deviation between simulated and experimental results due to multiple reasons (including but not limited to applied simplifications in the numerical model). These deviations usually have their minima close to the Best Efficiency Point (BEP). The paper deals with analyzing the outcome of steady-state simulations for a radial pump at strong part load and shut-off conditions by switching between three simulation types (steady-state with mixing plane, steady-state with frozen rotor, transient with sliding mesh). A comparison of velocity profiles on the interface surfaces is made, showing how the chosen interface model affects the structures being formed at part load conditions. These effects show particular impact on performance parameters (first of all, head production), which is discussed in the paper. The information provided could be helpful for adjusting the simulation parameters and finding an appropriate compromise between simulation reliability and demand for computational time thereby.

Comparison of two axial fans for cooling of electromechanical actuators at variable pressure

Electro-Mechanical Actuators (EMAs) in the aerospace field will provide major benefits to reduce overall weight, cost, and easier maintenance when servicing the aircraft. In the present study, two cooling fans were tested experimentally to determine the effects of the blade count and ambient pressure on the fan performance and efficiency. Both fans were tested at the same rotational speed of 13,000 rpm and four pressure conditions, (1, 0.7, 0.5, 0.3) atm. The lowest ambient pressure tested was 0.3 atm. Typical commercial aircraft operate at this ambient pressure at cruising altitudes. Selecting a 2-bladed and 7-bladed fan accommodates for testing of the extrema in terms of solidity, fan performance, and efficiency. The 7-bladed fan yielded the best performance to the 2-bladed fan in terms of static pressure to the volumetric flow rate, but the 2-bladed fan demonstrated to operate more efficiently than the 7-bladed fan in some ambient pressure conditions. The 7-bladed fan demonstrated turbulent flow regimes with Reynolds numbers, Re ≫ 4000, for all cases except for at the high resistance when operating at 0.3 atm.; on the other hand, the 2-bladed fan-operated in the laminar and transitional flow regimes for 0.3 atm. and during shutoff conditions at 0.5 atm. The higher Reynolds numbers allow for more air mixing which will, in turn, enhance the cooling of the system. In terms of volumetric flow rate delivery, the higher blade counts can deliver higher volumetric flow rates. The smaller blade counts perform more efficiently at (0.7, 0.5) atm which is due to lower power requirement to drive the blades. The current work addresses the effects of pressure on fan efficiency and performance. Both fans demonstrated higher efficiencies at pressure lower than ambient pressure due to the density changes at the lower pressure conditions.

Study of Pulsation Pressures in the Stages of an Electric Submersible Pump at Shut-Off Under Various Speeds of Operation

Background: Electric Submersible Pumps (ESPs) are widely used in agricultural fields, petroleum, and various other industries. These pumps are mostly driven at a constant speed since an A.C. motor is normally used as the drive. However, ESPs can also be operated at various speeds by employing a Variable Frequency Drive (VFD). Several patents have been published in ESP. Normally these pumps are started with the discharge valve at its closed position in order to control the starting current. When the ESP runs at its shut-off position, it will develop a higher head and an increase in fluctuating pressure. In order to evaluate the effects of pressure fluctuation, its characteristics must be investigated. Objective: Experimentally investigate the characteristics of pressure pulsation which are generated at each stage of a multistage ESP during shut-off condition at various speeds. Methods: An ESP with five stages was selected for conducting experiments. A VFD was used for operating the ESP at five speed settings from 80 to 120 % of its rated speed. Piezoresistive transducer was placed at the delivery side of the pump to acquire the signals of unsteady pressure. The pump was operated at closed valve condition at each speed, and unsteady pressure signals due to pressure pulsation were captured. Results: Obtained results have indicated the presence of fundamental frequency pertaining to vane passing frequency and harmonics of higher frequencies. Conclusion: Study of pressure fluctuations at shut-off will help to analyze the failures of ESP due to vibration which occur when the pump runs at closed valve position for a prolonged length of time and stability of these pumps at lower flow rates.

Experimental Investigations on the Inner Flow Behavior of Centrifugal Pumps under Inlet Air-Water Two-Phase Conditions

Centrifugal pumps are widely used and are known to be sensitive to inlet air-water two-phase flow conditions. The pump performance degradation mainly depends on the changes in the two-phase flow behavior inside the pump. In the present paper, experimental overall pump performance tests were performed for two different rotational speeds and several inlet air void fractions (αi) up to pump shut-off condition. Visualizations were also performed on the flow patterns of a whole impeller passage and the volute tongue area to physically understand pump performance degradation. The results showed that liquid flow modification does not follow head modification as described by affinity laws, which are only valid for homogeneous bubbly flow regimes. Three-dimensional effects were more pronounced when inlet void fraction increased up to 3%. Bubbly flow with low mean velocities were observed close to the volute tongue for all αi, and returned back to the impeller blade passages. The starting point of pump break down was related to a strong inward reverse flow that occurred in the vicinity of the shroud gap between the impeller and volute tongue area.

The Critical Role of Codon Composition on the Translation Efficiency Robustness of the Hepatitis A Virus Capsid.

Hepatoviruses show an intriguing deviated codon usage, suggesting an evolutionary signature. Abundant and rare codons in the cellular genome are scarce in the human hepatitis A virus (HAV) genome, while intermediately abundant host codons are abundant in the virus. Genotype–phenotype maps, or fitness landscapes, are a means of representing a genotype position in sequence space and uncovering how genotype relates to phenotype and fitness. Using genotype–phenotype maps of the translation efficiency, we have shown the critical role of the HAV capsid codon composition in regulating translation and determining its robustness. Adaptation to an environmental perturbation such as the artificial induction of cellular shutoff—not naturally occurring in HAV infection—involved movements in the sequence space and dramatic changes of the translation efficiency. Capsid rare codons, including abundant and rare codons of the cellular genome, slowed down the translation efficiency in conditions of no cellular shutoff. In contrast, rare capsid codons that are abundant in the cellular genome were efficiently translated in conditions of shutoff. Capsid regions very rich in slowly translated codons adapt to shutoff through sequence space movements from positions with highly robust translation to others with diminished translation robustness. These movements paralleled decreases of the capsid physical and biological robustness, and resulted in the diversification of capsid phenotypes. The deviated codon usage of extant hepatoviruses compared with that of their hosts may suggest the occurrence of a virus ancestor with an optimized codon usage with respect to an unknown ancient host.

Prediction of flow phenomena, performance and thrust forces of three-stage pump by using URANS

This paper describes the results of CFD prediction of performance, internal flow fields and thrust force acting on the impellers of a three-stage centrifugal pump which was provided for the Workshop “Single-&Multistage Pump Flow Prediction”. URANS (Unsteady Reynolds Averaged Navier-Stokes) was applied for the CFD prediction. Parts of the results were already reported by the authors. In this paper, more details are described, especially on the relationship between the changes of the flow fields in the pump and performance of axial thrust characteristics. The effects of axial shift on the performance and axial thrust characteristic are also investigated. URANS calculations were conducted at the design flow (Qd) to a near shut-off condition (10% Qd). The predicted Q-H performance shows a good agreement with the experiments. This pump shows positive slope in the head curve near the shut-off condition. URANS predicted the positive slope in the head curve near the shut-off condition, its flow rate, however, was slightly higher than that of the experiment. The reverse flow between the impeller and the diffuser developed when the flow rate was reduced. The interaction between the reverse flows and the flow in the side-wall gaps (front, back) affects pressure distributions and then axial thrust acts on the impeller. The details of the change of the interaction by flow rate predicted by URANS are described including the effects of axial shift of the rotor. URANS predicted earlier change in the axial thrust characterized by axial shift of the rotor than occurred in the experiments. This discrepancy was caused by interaction between reverse flows and side wall gap flows starting at higher flow than in the experiments.

Experimental evaluation of temperature rise in centrifugal pumps at partial flow rates

When the centrifugal pumps operate near shut-off condition, different kinds of energy losses happen from inlet to discharge nozzle leading to lower hydraulic performance and overall efficiency. A part of dissipated energy turns into heat and increases the operating liquid temperature. This may seem to be very dangerous when talking about pumping oil or any other flammable chemical liquids. In the present investigation, several centrifugal pumps were selected to perform experimental analysis of measuring temperature rise at inlet and discharge nozzles during part load operation. Pumps were entirely insulated using glass wool to prevent heat leakage from pump casing to the environment and pure water was selected to play as the working fluid. Temperature rise of pumps was reported from start time up to assuaging steady-state condition. Obtained results demonstrated that higher rotational speed of impeller led to higher value of outlet water temperature in a shorter time rather than lower rotational speeds. Steady-state temperature values were used to establish a novel relation for temperature rise that can be implemented for wide range of centrifugal pumps.

Study of Stage-wise Pressure Pulsation in an Electric Submersible Pump under Variable Frequency Operation at Shut-off Condition

Pressure pulsation causes vibration in the Electric Submersible Pump (ESP) and affects the life and performance of its system. ESP systems are installed at depths ranging from a few meters to several hundred meters. Unlike pumps used on the surface, once they are installed they become inaccessible for maintenance or for any kind of diagnostic measurement that might be taken directly on them. Therefore a detailed knowledge of mean and fluctuating pressures is required to achieve an optimal pressure distribution inside the ESP. This paper presents the results of an experimental investigation of the stage-wise pulsating pressure in ESP at shut-off condition at different speeds. Experiments were conducted on a pump having five stages. A variable frequency drive was used to operate the pump at five different speeds. Piezoresistive transducers were mounted at each stage of ESP to capture the unsteady pressure signals. Fast Fourier Transformation was carried out on the pressure signals to convert into frequency domain and the spectra of pressure pulsation signals were analyzed. The obtained results indicated the existence of fundamental frequency corresponding to the speed of rotation times the number of impeller blades and of the whole series of harmonics of higher frequencies.

Wide-ranging and unexpected consequences of altered Pol II catalytic activity in vivo.

Here we employ a set of RNA Polymerase II (Pol II) activity mutants to determine the consequences of increased or decreased Pol II catalysis on gene expression in Saccharomyces cerevisiae. We find that alteration of Pol II catalytic rate, either fast or slow, leads to decreased Pol II occupancy and apparent reduction in elongation rate in vivo. However, we also find that determination of elongation rate in vivo by chromatin immunoprecipitation can be confounded by the kinetics and conditions of transcriptional shutoff in the assay. We identify promoter and template-specific effects on severity of gene expression defects for both fast and slow Pol II mutants. We show that mRNA half-lives for a reporter gene are increased in both fast and slow Pol II mutant strains and the magnitude of half-life changes correlate both with mutants’ growth and reporter expression defects. Finally, we tested a model that altered Pol II activity sensitizes cells to nucleotide depletion. In contrast to model predictions, mutated Pol II retains normal sensitivity to altered nucleotide levels. Our experiments establish a framework for understanding the diversity of transcription defects derived from altered Pol II activity mutants, essential for their use as probes of transcription mechanisms.

Impeller radial force evolution in a large double-suction centrifugal pump during startup at the shut-off condition

Double-suction centrifugal pumps play an important role in the main feedwater systems of nuclear power plant. The impeller radial force in a centrifugal pump varies dramatically during startup at the shut-off condition. In this study, the startup process of a large double-suction centrifugal pump is investigated using CFD. During testing, the impeller speed is accelerated from zero to its rated speed in 1.0s (marked as t0) and is then maintained at the rated speed. The results show that the radial force increase lags behind the impeller speed increase. At 0–0.4t0, the radial force is small (approaching zero). At 0.4–1.4t0, the radial force increases rapidly. After 1.4t0, the average radial force stabilizes and reaches its maximum value of 55,619N. The observed maximum radial force value during startup is approximately nine times as high as the radial force under rated condition. During startup, the overall radial force direction is proximate to the radial line located 25° from the volute tongue along circumferential direction. A transient radial force formula is proposed to evaluate the changes in radial force during startup. The streamline distribution in impeller passages and the impeller outlet pressure profile varying over time are produced. The relationship between radial force evolution and the varying axial-to-spiral vortex structure is analyzed. The radial force change mechanism is revealed. This research provides a scientific basis for startup control in large double-suction centrifugal pumps.

A Performance Prediction Model for Low-Speed Centrifugal Fans

A generalized model for mapping the trend of the performance characteristics of a double-discharge centrifugal fan is developed based on the work by Casey and Robinson (C&R), which formulated compressor performance maps for tip-speed Mach numbers ranging from 0.4 to 2 using test data obtained from turbochargers with vaneless diffusers. The current paper focuses on low-speed applications for Mach number below 0.4. The C&R model uses four nondimensional parameters at the design condition including the flow coefficient, the work input coefficient, the tip-speed Mach number, and the polytropic efficiency, in developing a prediction model that requires limited geometrical knowledge of the centrifugal turbomachine. For the low-speed fan case, the C&R formulas are further extended to a low-speed, incompressible analysis. The effort described in this paper begins by comparing generalized results using efficiency data obtained from a series of fan measurements to that using the C&R model. For the efficiency map, the C&R model is found to heavily depend on the ratio of the flow coefficient at peak efficiency to that at the choke flow condition. Since choke flow is generally not applicable in the low-speed centrifugal fan operational environment, an alternate, but accurate estimation method based on fan free delivery derived from the fan test data is presented. Using this new estimation procedure, the modified C&R model predicts reasonably well using the double-discharge centrifugal fan data for high-flow coefficients, but fails to correlate with the data for low-flow coefficients. To address this undesirable characteristic, additional modifications to the C&R model are also presented for the fan application at low flow conditions. A Reynolds number correction is implemented in the work input prediction of the C&R model to account for low-speed test conditions. The new model provides reasonable prediction with the current fan data in both work input and pressure rise coefficients. Along with the developments for the efficiency and work input coefficient maps, the use of fan shut-off and free delivery conditions are also discussed for low-speed applications.

유체의 점성이 원심펌프 성능에 미치는 영향

원심펌프 작동유체의 점도변화에 따른 성능특성을 실험적인 방법으로 연구하였다. 비교적 비속도가 낮은 소형 원심펌프에 뉴턴유체인 설탕수용액과 글리세린수용액을 사용하여 작동유체의 점도를 변화시켜 가며 펌프성능시험을 수행한 후 전양정, 축동력 및 펌프의 효율을 청수의 경우와 비교·분석하여 얻은 결과는 다음과 같았다. (1) 전양정은 작동유체의 점도를 변화시킬 경우 청수에 비해 점도가 높아짐에 따라서 차단점에서의 양정은 극히 적은 변화를 보였으나, 유량이 증가함에 따라 양정의 감소하는 폭이 크게 나타났다. 따라서 유량-양정곡선(H-Q 곡선)의 기울기가 급해짐을 알 수 있었다. (2) 축동력은 작동유체의 점도가 청수에 비해 클수록 증가하였으며 차단점에서는 소요동력이 열로 소산되어 차이가 거의 없었으나, 유량이 증가할수록 축동력의 증가하는 폭이 커짐을 알 수 있었다.

Modeling and analysis of different control strategies for fogging system in a subtropical greenhouse

A dynamic environment model was established on the effects of greenhouse cooling on different control strategies in a fogging system, which simulations were then conducted by Matlab. Simulation results showed that by applying a suitable set of control strategy, the indoor temperature dropped and was less than the outdoor temperature of 34.4°C The temperature reduction degree changed within limits and delayed time. Inversely, when relative humidity and time delay were used as the control factors, the indoor temperature rose under high solar irradiation. The above phenomenon can be avoided when temperature control was applied. When operations of the fogging system used the dead zone as the setting control, variations of the indoor temperature was mainly relevant to the upper and lower limits of the zone. In situations where the spraying can reach the shutoff condition, the temperature variance increased as the range was larger, and the actuation frequency fogging system also decreased. When the time delay factor was employed in the fogging system, the indoor temperature varied in terms of fogging time and stop time. In order to narrow down the variation of indoor temperature, the operation and stop time in a cycle must be shortened. In this study, simulations with the cooling effects in a greenhouse were performed by using different control strategies for a fogging system. The results can serve as reference for planning future cooling control strategies for a fogging system. Key words : Greenhouse, fogging system, control strategy.

Selective translation of the measles virus nucleocapsid mRNA by La protein

Measles, caused by measles virus (MeV) infection, is the leading cause of death in children because of secondary infections attributable to MeV-induced immune suppression. Recently, we have shown that wild-type MeVs induce the suppression of protein synthesis in host cells (referred to as “shutoff”) and that viral mRNAs are preferentially translated under shutoff conditions in infected cells. To determine the mechanism behind the preferential translation of viral mRNA, we focused on the 5′ untranslated region (UTR) of nucleocapsid (N) mRNA. The La/SSB autoantigen (La) was found to specifically bind to an N-5′UTR probe. Recombinant La enhanced the translation of luciferase mRNA containing the N-5′UTR (N-fLuc), and RNA interference of La suppressed N-fLuc translation. Furthermore, recombinant MeV lacking the La-binding motif in the N-5′UTR displayed delayed viral protein synthesis and growth kinetics at an early phase of infection. These results suggest that La induced predominant translation of N mRNA via binding to its 5′UTR under shutoff conditions. This is the first report on a cellular factor that specifically regulates paramyxovirus mRNA translation.

Effects of Compact Radial-Vaned Air Separators on Stalling Characteristics of an Axial-Flow Fan

The stall-prevention effect of air separators incorporating radial vanes in place of the existing axial vanes was investigated on a low-speed, single-stage, lightly loaded axial-flow fan for effective and compact air separators of a simplified structure. From the survey, paying attention to several geometrical dimensions of the device, the following conclusions are obtained: (1) Simplified radial vanes made of flat plates could show strong stall-prevention effect comparable to those of the curved-vane type one. The most favorable ones showed no stall up to the fan shut-off conditions. (2) Radial heights of the recirculation passage within the air separator showed significant influences on the stall improvement. It should be larger than some critical size experimentally given in the study. (3) The axial length of the device should be larger than some critical size given experimentally in the study. Too much reduced axial length could give rise to an abrupt loss in the effect. (4) The optimum axial locations of the rotor-tip blade leading edge within the device inlet opening were found to lie near the center of the width of the inlet opening from both aspects of stall improvement and fan efficiency.

224 給油停止時の玉軸受の挙動に関する研究

224 給油停止時の玉軸受の挙動に関する研究