Low Flow Rate Conditions Research Articles

Catalytic behavior of LaFeO3 pervoskite oxide during low-pressure gas nitriding

The LaFeO3 perovskite oxide has been proved to be an effective catalyst in the gas nitriding. However, its catalytic efficiency was sensitive to the nitriding parameters. In order to clarify the catalytic conditions of the LaFeO3 oxide, the low-pressure gas nitriding was carried out in the present work. The effects of gas pressure and ammonia flow rate on catalytic behavior of the LaFeO3 oxide were discussed. The catalytic feature and conditions of the LaFeO3 oxide was identified. The microstructure and phase composition of nitrided layers were analyzed by the optical microscope, transmission electron microscopy, scanning electron microscope and X-ray diffraction, respectively. The microhardness profiles of nitrided layers were measured by micro-hardness tester to evaluate catalytic efficiency of the LaFeO3 oxide. The X-ray photoelectron spectroscopy was applied to reveal the catalytic behavior. The results show that the LaFeO3 perovskite oxide exhibited an excellent catalytic activity via accelerating dissociation of ammonia molecules and diffusion of nitrogen atoms. The dehydrogenation reaction of ammonia molecules was the rate-determining step for the rare earth nitriding, which was greatly affected by gas pressure and ammonia flow rate. The LaFeO3 catalyst shows an excellent durability of catalytic activity in low pressure and low flow rate conditions.

Investigation of variable orifice plate design for centrifugal compressor low-end performance improvement

In the present paper, a compressor with variable inlet configuration upstream of the compressor wheel was investigated to improve the compressor low-end performance. As a concept study, the variable inlet configuration was assumed to be a variable orifice plate which can reduce the compressor inlet flow area when it is activated. The compressor performance with and without the initial orifice plate was assessed quantitatively using both numerical and experimental methods. The results indicated that the initial orifice plate design can improve the compressor efficiency at low speed and flow rate condition. Different orifice plate designs with different setting angles to the compressor inlet main flow direction and different radial heights were also numerically studied. It was found that: (1) changing the orifice plate setting angle from 90° for the initial design to 67° can improve compressor efficiency about 0.56% at low flow rate condition; (2) increasing the orifice plate radial height from 16% of the compressor inlet radius for the initial design to 27% can improve the compressor efficiency by 1.78% at near surge condition and extend surge margin by 9%. The results indicated that the radial height has larger effects on compressor performance than the setting angle at near surge flow rate. While increasing the orifice plate height variation can further improve compressor low-end efficiency and surge margin, there is an increased complexity of the actuation and control system.

Inter-Blade Vortex and Vortex Rope Characteristics of a Pump-Turbine in Turbine Mode under Low Flow Rate Conditions

Pump-turbines are often used to provide a stable power supply with a constant frequency in response to intermittent renewable energy resources. However, existing pumped-storage power stations often operate under off-design conditions because of the increasing amounts of inconsistent renewable resources that have been added to the grid. Under off-design low flow rate conditions, inter-blade vortex and vortex rope phenomena usually develop in the runner and draft tube passages, respectively, in turbine mode. These vortices cause complicated flow patterns and pressure fluctuations that destabilize the operation of the pump-turbine system. Therefore, this study investigates the influence of correlation between the inter-blade vortex and vortex rope phenomena under low flow rate conditions. Three-dimensional steady- and unsteady-state Reynolds-averaged Navier–Stokes equations were calculated with a two-phase flow analysis using a shear stress transport as the turbulence model. The inter-blade vortices in the runner passages were captured well at the low flow rate conditions, and the vortex rope was found to develop within a specific range of low flow rates. These vortex regions showed a blockage effect and complicated flow characteristics with backflow in the passages. Moreover, higher unsteady pressure characteristics occurred at locations where the vortices were especially pronounced.

Investigations on the influence of tandem blades on inner flow and performance characteristics of centrifugal pump

Although significant advances have been made in tandem-blade technology for axial and centrifugal compressors, little attention has been paid to its application in centrifugal pumps. In this study, we propose a new tandem-blade design method for improving inner flow characteristics and overall performance of a centrifugal pump. With the SST k − ω turbulence model, three-dimensional turbulent flow fields in the centrifugal pump with tandem blades are simulated and analyzed. The effects of tandem blades on the inner flow and performance characteristics of the centrifugal pump are investigated. The predicted velocity and pressure distributions and flow behavior of the tandem-blade impeller are compared with those of a conventional single row blade impeller. It is indicated that the centrifugal tandem-blade impeller exhibits a significant advantage in terms of the uniformity of the impeller discharge flow. The tandem blades improve the jet-wake structure and uniformity of velocity and pressure distributions at the impeller outlet, and thus reduce the pressure fluctuation and hydraulic loss. Moreover, the hump phenomenon is eliminated or alleviated under low flow rate conditions, and the tandem-blade impeller has better hydraulic performance within a wider operating range as well as high reliability. This study provides a basis for the further development of the centrifugal pump with tandem blades.

Two-dimensional spatial distributions of the water content of the membrane electrode assembly and the electric current generated in a polymer electrolyte fuel cell measured by 49 nuclear magnetic resonance surface coils: Dependence on gas flow rate and relative humidity of supplied gases

A PEFC (polymer electrolyte fuel cell) with a large power generation area of 140 mm × 160 mm has spatial distributions of the water content of the MEA (membrane electrode assembly) and the electric current produced that depend on the flow rate and relative humidity of the supplied gas. When 49 NMR (nuclear magnetic resonance) surface coils arranged in 7 rows × 7 columns were inserted into the PEFC, two-dimensional spatial distributions of the water content of the MEA and the electric current could be measured from the acquired NMR signals. The water content of the MEA was calculated from the intensity of the NMR signal, and the electric current distribution was determined by an inverse analysis that reproduces the spatial distribution of the resonance frequency of the NMR signals. The PEFC, which was supplied with hydrogen and oxygen at a high humidity of 80 %RH, generated electric power at 130 A over the whole area of the MEA. On the other hand, under a low humidity of 40 %RH and low flow rate conditions, the water content of the MEA became high in the midstream and downstream regions, and a high electric current was generated in these regions.

Measurement of local two-phase flow parameters of downward bubbly flow in mini pipes

In order to extend a precise database on local two-phase flow parameters in mini pipes, experiments were conducted for adiabatic gas–liquid bubbly flows flowing down in vertical mini pipes with inner diameters of 1.03, 3.00, and 5.00 mm. A stereo image-processing was applied to observe the phase distribution characteristics in pipe cross-section. The local flow parameters including profiles of void fraction, Sauter mean bubble diameter, and interfacial area concentration in pipe cross-section were obtained at three axial locations in the test pipes with various flow conditions: superficial gas velocity of 0.00508–0.0834 m/s and superficial liquid velocity of 0.208–3.00 m/s. The axial developments of the local flow parameters were discussed in detail based on the obtained data and the visual observation. It was confirmed that the core peak distributions were formed at low liquid flow rate conditions in which the buoyancy force dominated while the wall peak distributions were formed at high liquid flow rate conditions in which the body acceleration due to the frictional pressure gradient dominated. The result indicated the existence of lift force pushing the bubbles towards the pipe wall even in vertical downward flows. The database obtained through the present experiment is expected to be useful in modeling the interfacial area transport terms, the validation of the existing lift force models as well as the benchmarking of various CFD simulation codes.

Centrifugal pump impeller and volute shape optimization via combined NUMECA, genetic algorithm, and back propagation neural network

This paper presents a fast, efficient, and convenient shape optimization design method for the centrifugal pump impeller and volute. A meridional curve, stream surface, blade stacking, and two-dimensional blade profile are obtained for impeller parameterized fitting by NUMECA, which substantially decreases the parameters to be optimized. A combination of genetic algorithm (GA) and back propagation neural network (BPNN) is then employed to optimize the impeller design while preventing prematurity or stagnation due to the GA. The head and efficiency of the optimized impeller under the designed flow rate condition increase by 7.69% and 4.74%, respectively, while power decreases by 2.56% post-optimization. Static pressure in the optimized impeller middle span is more uniform post-optimization, and the hydraulic performance of the centrifugal pump with the optimized impeller exceeds that of the original centrifugal pump under low and designed flow rate conditions. Head increases by 2.69 m and efficiency increases by 4.32% under the designed flow rate condition as well. The base circle diameter, volute inlet width, and volute baffle tongue can be modified to optimize the volute shape design. The head of the centrifugal pump with the optimized volute and optimized impeller increases by 4.83 m and 6.35 m and efficiency increases by 9.12% and 18.65% under 1.2 and 1.4 times the designed flow rate compared to the pump with the original volute and optimized impeller. Vortices in the optimized volute are reduced significantly and particularly relative energy losses. Under low flow rate conditions, compared with the original centrifugal pump, the head and efficiency of the experimental centrifugal pump with optimized impeller and optimized volute increase by 1.56 m and 1.12%; under the designed flow rate condition, they increase by 4.34 m and 5.23%; and under the high flow rate condition, they increase by 3.71 m and 8.54%, respectively. Compared to the traditional optimization method, as evidenced by numerous shape optimization design cases, NUMECA-GA-BPNN produces better optimized shapes with stronger hydraulic performance more quickly and efficiently.

Methodology for production logging in oil-in-water flows under low flow rate and high water-cut conditions

This study aimed to obtain the production profiles of oil-in-water flow under low flow rate and high water-cut conditions in oil wells. A combination production profile logging composed of an arc-type conductance sensor (ATCS) and a cross-correlation flow meter (CFM) with a center body is proposed and experimentally evaluated. The ATCS is designed for water holdup measurement, whereas the CFM with a center body is proposed to obtain the mixture velocity. Then, a drift-flux model based on flow patterns is established to predict the individual-phase superficial velocity of oil-in-water flows. Results show that the ATCS possesses high resolution in water holdup measurement and that flow pattern information can be deduced from its signal through nonlinear time series analysis. The CFM can enhance the correlation of upstream and downstream signals and simplify the relationship between the cross-correlation velocity and mixture velocity. On the basis of the drift-flux model, individual-phase superficial velocities can be predicted with high accuracy for different flow patterns.

Performance Evaluation and Dimensional Analysis of Multistage Helicoaxial Pump for Two-Phase Flow

A four stage helicoaxial pump was tested under varying operating conditions. A range of inlet pressures, rotational speeds (3000, 3600 rpm), and gas void fractures (GVFs) were considered for two fluid viscosities. The head developed and power input to run the pump were recorded. Head, power input and efficiency decrease as the GVF increases with best efficiency point (BEP) moving towards lower flow rate conditions. Dimensional analysis was conducted to evaluate the applicability of current affinity laws to the two-phase flow performance of the pump under consideration. Dimensionless head coefficient and power coefficients were defined for two-phase flow, considering the homogeneity in the two-phase fluid properties. Deviations in the two-phase affinity coefficients from the common law curve increases with GVF. To bridge this gap, a new correlation is proposed with a revised flow coefficient that allows all the head coefficient data to collapse on a single line with a greater degree of accuracy.

Ice accretion measurements of Jet A-1 in aircraft fuel lines

Ice accretion and blockages in aircraft fuel systems continue to be of interest as a result of their influence on system design and alternative fuel synthesis as well as their role in aviation incidents and accidents. Ice accumulation tests are performed in a recirculating fuel system with Jet A-1 fuel. Removable stainless steel test pipes and variable flow rates allow for the examination of ice accretion with variation of pipe diameter and Reynolds number in a repeatable manner. The flow loop is installed in an altitude chamber capable of −50 °C and the experiments involve full-scale flow components. Temperature of the flow loop is controlled with an initial −11 °C/h cooling rate followed by a constant temperature test time. Instantaneous measurements of pressure drop along the test pipe are used to characterize ice accretion. Additionally, end-view imaging of the test pipes is performed to characterize the size and quality of the accreted ice. Normalized pressure drop is seen to increase nearly linearly with time for the range of Reynolds numbers and temperatures tested. Approximate ice accretion rates are extracted from the time varying pressure drops assuming the ice accumulation changes the pipe surface roughness. Accreted ice layer growth rates as high as 0.91±0.12 mm/hr are seen at low flow rate conditions (Re∼4000, −10.5 °C) and decrease with increasing Reynolds number. Growth rates at −7 °C and −19 °C are ∼25% and ∼1.5% of the −10.5 °C growth rate, respectively, and follow the same Reynolds number trend.

프란시스 수차 모델의 블레이드 내 와류 특성에 대한 수치해석적 분석

In this study, a numerical analysis of the inter-blade vortex characteristics of a Francis hydro turbine model was performed with various flow rate conditions in the operating range. The model turbine demonstrated notably different internal flow characteristics for different operating conditions. Particularly, an inter-blade vortex between the runner blade passages was observed at lower flow rate conditions. An inter-blade vortex can cause reduction in performance, and vibration and instability in the operation of a turbine system. Therefore, understanding inter-blade vortex characteristics is important for the safe and stable operation of turbines. To investigate internal flow and unsteady pressure characteristics, three-dimensional steady and unsteady Reynolds-averaged Navier-Stokes calculations using a shear stress transport turbulence model were performed with two-phase flow analysis. Inter-blade vortices were captured at the leading and trailing edges of the near runner hub between blade passages. This vortex region exhibited flow separation and lower velocity and contributed to decreased hydraulic performance and higher unsteady pressure characteristics.

Study on Flow Characteristics in Volute of Centrifugal Pump Based on Dynamic Mode Decomposition

To investigate the unsteady flow characteristics and their influence mechanism in the volute of centrifugal pump, the Reynolds time-averaged N-S equation, RNG k-ε turbulence model, and structured grid technique are used to numerically analyze the transient flow-field characteristics inside the centrifugal pump volute. Based on the quantified parameters of flow field in the volute of centrifugal pump, the velocity mode contours and oscillation characteristics of the mid-span section of the volute of centrifugal pump are obtained by dynamic mode decomposition (DMD) for the nominal and low flow-rate condition. The research shows that the first-order average flow mode extracted by DMD is the dominant flow structure in the flow field of the volute. The second-order and third-order modes are the most important oscillation modes causing unsteady flow in the volute, and the characteristic frequency of the two modes is consistent with the blade passing frequency and the 2x blade passing frequency obtained by the fast Fourier transform (FFT). By reconstructing the internal flow field of the volute with the blade passing frequency for the nominal flow-rate condition, the periodic variation of the unsteady flow structure in the volute under this frequency is visually reproduced, which provides some ideas for the study of the unsteady structure in the internal flow field of centrifugal pumps.

Experimental study on onset of nucleate boiling and flow boiling heat transfer in a 5 × 5 rod bundle at low flow rate

Onset of nucleate boiling and flow boiling heat transfer are experimentally investigated in a 5 × 5 rod bundle at low flow rate under the pressure of 6 MPa. The experimental mass flux ranges from 25 to 200 kg/m2 s, heat flux varies from 25 to 75 kW/m2, the maximum vapour quality is 0.45. Effects of parameters on ONB and flow boiling are examined. The wall superheating at ONB point increases with the rise of heat flux while decreases with the increase of mass flux. The effects of mass flux and quality on flow boiling heat transfer coefficients are not distinct. For both subcooled and saturated flow boiling in the 5 × 5 rod bundle, the effect of heat flux on heat transfer is more remarkable at high flow rate than at low flow rate. Comparisons between predictions by existing empirical correlations and the experimental data are conducted. Basu’s correlation could predict the wall superheating at ONB point within a deviation less than 30% under conditions of higher mass flux (>150 kg/m2 s). As for the comparisons in flow boiling regions, predictions by Chen’s correlation agree well with the experimental data in both subcooled and saturated regions, except under conditions of low mass flux with low heat flux. Most of the correlations for boiling heat transfer and ONB are derived from the experimental data obtained at high flow rate, however, current experiments indicate that these correlations cannot be directly used under low flow rate conditions.

Effects of Different Turbulence Models on Simulation Accuracy of the Miniature Super-low Specific Speed Centrifugal Pump

In order to study the effects of different turbulence models on simulation accuracy of the miniature super-low specific speed centrifugal pump, the RNG k-ϵ turbulence model and SST k-ω turbulence model had been used in numerical simulation by Fluent. The performance curves and the internal flow filed distribution of those two turbulence models had been analysed and compared with the experiment results. The results show that at the low flow rate conditions, the simulation accuracy of the SST k-ω model is lower than that of the RNG k-ϵ model, while at the high flow rate conditions, the result is just the opposite. In the whole operating conditions, the average simulation error of the SST k-ω model is 3.5% and that of the RNG k-ϵ model is 4.3%.

Investigation of internal flow pattern of a multiphase axial pump

Due to recent advances in computational techniques, single phase performances of pumps can be easily estimated using CFD. However, when the working fluid is a mixture of liquid and gas (multi-phase flow) computations may struggle to capture the more complex physical phenomena that occurs, and the precision of the prediction worsen as the gas volume fraction at pump inlet is increased. To provide reliable benchmark for the improvement of simulations, in this study we experimentally investigated the internal flow pattern of a multiphase pump using high speed camera and high frequency pressure measurements, so as to obtain the data of pressure distribution inside of impeller, or to obtain blade loading, for several gas concentration conditions. Unsteady phenomena were identified at low flow rate conditions, and an explanation of their nature has been attempted.

Integrated topology optimization for vibration suppression in a vertical pump

This article presents a new approach aiming to reducing pump vibration by modifying its baseplate structure. The finite element models of the vertical pump were established and validated by the experimental impact test. The natural frequencies of pump were mapped in both experimental and numerical methods. The weak stiffness of the baseplate was identified as the root cause for the pump vibration. A topology optimization was used for enhancing the stiffness of baseplate and controlling its weight. The new baseplate was designed according to the inputs from optimization results and manufactured by the casting method. Both the vibration tests and the numerical simulations were carried out to investigate the vibration behaviors of the optimized pump model. The differences of vibration characteristics between original and optimized pumps were evaluated using 1/3 octave-band filter technique. Results show that the vibration was suppressed, and the resonance at 31.5 Hz was eliminated using the optimized baseplate. In particular, the maximum vibration amplitude of the vertical pump was reduced from 4.05 to 1.75 mm/s at the low flow rate condition. It was experimentally confirmed that the vibration amplitude of the optimized model complies with the requirements of the International Organization for Standardization standard and ensures the pump can operate stable for a long time.

Bubble formation on submerged micrometer-sized nozzles in polymer solutions: An experimental investigation

This research presents detailed formation of air bubbles on different submerged micrometer-sized nozzles in Newtonian and non-Newtonian fluids. An experimental study is performed using the high-speed camera on three nozzles with diameters of 150, 450, and 600 μm under relatively low gas flow rate conditions (0.1∼1 ml/min). Distilled water and various concentrations of Carboxymethyl cellulose (CMC) aqueous solutions are utilized as the continuous phase for each type of fluid. Different characteristics of the bubble formation such as volume change, frequency, height, maximum width, and instantaneous contact angle are obtained using an image processing technique and the Young-Laplace equation. Effect of different parameters including gas flow rate, nozzle diameter, concentration, and rheological properties of the continuous phases are investigated. Also, a detailed dynamic force analysis is performed to survey the role of fluid properties on the effective forces. The force balance reveals that the surface tension force is dominant in the smallest orifice, while its influence decreases as the orifice diameter increases. Moreover, it was observed that the drag and hydrostatic forces have relatively noticeable contributions in this low gas flow rates. Furthermore, results show that under the conditions of this research, the characteristics of the bubble are weakly dependent on the gas flow rate. On the other hand, the concentration of the CMC and the rheological behavior of the non-Newtonian fluids so-obtained have a significant influence on bubble formation process. Also, observations made and predictions done as to the bubble’s curvature (say, based on Young-Laplace equation) suggest that the necking phenomenon does not exhibit itself at the top of the nozzle in non-Newtonian liquids flowing through micron-sized nozzles.

The Effect of Jet Flow Impingement on the Corrosion Products Formed on a Pipeline Steel in Naturally Aerated Sour Brine

Corrosion was generated by the action of a jet impingement flow of sour brine on pipeline steel samples of X70. Flow-assisted corrosion affected nature, number and peak intensity of the chemical species formed as corrosion products. Iron sulfides predominated in static and low flow rate conditions (1.1 m/s), whereas at 2.4 m/s iron oxides were mainly formed, which led to higher corrosion rates and suggested that oxides are less protective than sulfides. On inhibition, imidazoline seems to mitigate oxide formation and support sulfide formation balancing both species on steel surface. Ferrite phase in laminar pearlite was preferentially dissolved with/without inhibitor, and mackinawite (FeS2) was formed at every flow rate, angle with and without inhibitor. Theoretical stresses determined by computational flow dynamics for corrosion product removal showed a fair approximation to those proposed in the literature.

Water quality variability in the middle and down streams of Han River under the influence of the Middle Route of South-North Water diversion project, China

The middle and down streams of Han River are complex river systems influenced by hydrologic variations, population distributions, and the engineering projects. The Middle Route of China’s South-to-North Water Transfer (MSNW) project planned to transfer 95 billion m3 annually from Han River to north China. The operation of the MSNW project may alter the flow rate and further influence the water quality of Han River. This study used min/max autocorrelation factor analysis (MAFA) and dynamic factor analysis (DFA) to analyze spatio-temporal variations of the water quality variables in three typical tributary-mainstream intersection zones in Han River from June 2014 to April 2017. MAFA results showed that chlorophyll-a (Chl-a), chemical oxygen demand (COD), suspend solid (SS) and phosphate (PO43−) (represented as trophic dynamics) are main concerned water quality variables in densely populated zones (Zones 1 and 3), and total nitrogen (TN), nitrate nitrogen (NO3−), COD, and PO43− (regarded as nutrient formations dynamics) represent the underlying water quality variations in agricultural cultivation zone (Zone 2). DFA results indicated that domestic and municipal effluent pollutants influence the organic concentrations and nutrient formations in the mainstream in Zones 1 and 2. The non-point source nitrogen and phosphorus discharged from the tributaries Tangbai and Hanbei Rivers elevate the nutrient concentrations and increase Chl-a concentration (i.e. promote the algal growth) in densely populated zones. In addition, controlling the flow rates in low and middle flow rate conditions can avoid degrading water quality. The flow rate should be elevated to more than 700 cm (cubic meters per second) in the middle stream and to more than 800 cm in the downstream of Han River for preventing water quality deterioration from high loadings of organic pollutants and nutrients. The integrated MAFA and DFA method establishes an efficient analysis distinguishing spatio-temporal variation of water quality variables and provides useful site-specific management to control water quality in various flow conditions.

Effect of rotor–stator axial spacing on the pressure-rise performance and flow field in an axial pump

In this paper, the effect of the rotor–stator axial spacing is investigated in an axial pump with the working fluid of water. The pressure-rise performance was tested at a range of flow rates. Results indicate that decreased axial spacing generates improved hydraulic head, especially when the flow rate is low. Particle image velocimetry measurement was performed and flow fields for five rotor phases were obtained in a low flow rate condition. Particle image velocimetry results demonstrate that the stator inlet flow is both affected by the wake of the rotor and the existence of the stator. As the axial spacing gets close, the incidence angle of the stator decreases and the flow separation on the suction side is restrained, and therefore the pressure rise ability is improved.