Flow Rate Fluctuations Research Articles

CFD simulation of a full-scale three-dimensional flow and temperature distribution inside a reactor under ocean conditions

A series of three-dimensional full-scale fluid flow simulations are performed based on an integrated thermal marine reactor under ocean conditions. The aim is to understand the effects of ship motions including rolling, heaving, and inclination on flow field and thermal–hydraulic performance. Both forced circulation and natural circulation are taken into account since the abilities of coolant to maintain its original motion under ocean conditions are different. The temperature distribution, velocity field, and flow rate distribution of a static reactor are presented first for comparison. After additional forces or accelerations are introduced, the flow field in the marine reactors subjected to different types of ocean motions is explored. Results show that the rolling motion would have more significant impacts on marine reactors in natural circulation concerning temperature and SG (steam generator) flow rate. In natural circulation, the SG flow rate fluctuation can reach 35 kg/s when the rolling angle increases to 45°, compared to the maximum fluctuation of 8 kg/s under forced circulation. The rolling motion also greatly affects the flow field and flow rate distribution at the core inlet. Obvious vortex flows can be observed in the annulus both for forced circulation and natural circulation. The core channel with the minimum flow rate appears to be randomly located for the reactor subjected to rolling motion. For reactors in heaving motion, the fluid velocity changes periodically, and the flow rate in the SG and core regions increases significantly with the increase of the heaving period and acceleration. As for inclined reactors, the average velocity in the core becomes smaller while the temperature increases as the inclination angle of the reactor increases.

Pulsating flows of viscous fluid in flat channel for given harmonic fluctuation of flow rate

In research work, it is pointed out the issue of a pulsating flow of a viscous incompressible fluid in a flat channel for a given harmonic fluctuation of the fluid flow rate. The study of the generation of pulsating current is used in biological mechanics, in particular, in the use of microchip systems. In addition, to ensure a constant flow of liquid, pneumatic micropumps that periodically squeeze liquid from empty volumes are widely used. In such systems, the installation of pulsating flow is shown to be economically beneficial. The transfer function of the amplitude-phase rate response is determined; with the help of these functions, the ratio of the tangential shear pressure on the channel fence to the average acceleration over the channel section is determined. In Figure 1, it can be seen that the ratio of KH = 0 of in τHC/τ0KC is close to one and α02 is less than one. If α02 takes on values greater than unity, then the ratio of to KH = 0 to τHC/τ0KC is greater than unity, and it has been shown to increase with increasing frequency of dimensionless oscillation. It was shown that in an unsteady flow of liquid, even in cases where liquid acceleration is equal to zero, it was studied that the stress on the wall of the channel exceeds its quasi-stationary value. τHC/τ0KC with the increase of the ratio KH, the increase of the parameter is explained by the fact that the change of the tensile stress on the wall advances in phases concerning the average speed along the section. Calculations determine that the non-stationary shear pressure on the channel fence increases non-monotonically with the acceleration of the liquid particle at low oscillation frequencies. The shear pressure reaches its maximum value, then decreases with increasing dimensionless oscillation rate, and asymptotically approaches the values without accelerated flow.

Characteristics and suppression methods of low-frequency oscillation in Hall thruster

Low frequency oscillation in the discharge channel of Hall thruster is an important physical phenomenon that affects its performance and stability. In this paper, the characteristics of low-frequency oscillation of the discharge current of Hall thruster and its suppression method are numerically studied by using a one-dimensional fluid model. Assuming that the discharge channel satisfies the quasi-neutral condition, the effects of electron-neutral collision, electron anomalous transport and electron-wall collision on conductivity are considered. The changes of plasma parameters and the effects of magnetic field, preionization rate, and atomic velocity on the amplitude and frequency of discharge current oscillation are also studied. Research results show that the variation of electron temperature in the discharge channel is closely related to the ionization process, and the electron temperature increases as the ionization intensity increases. The fluctuations in neutral gas flow rate and atomic density in the discharge process cause the ionization region to move forward and backward and the ionization intensity to change, which are the main driving forces for the low-frequency oscillation of discharge current in the channel. The magnetic field intensity in the discharge channel affects the axial current by influencing the electron mobility. With the increase of field strength, the oscillation frequency of current decreases, and under different magnetic field strengths, the current amplitude drops as the discharge voltage decreases. When the preionization rate of the working gas increases to above 4%, the amplitude of the discharge current oscillation gradually decreases. When the preionization rate is greater than 3% and the atomic velocity is less than 160 m/s, the discharge current oscillation in the channel exhibits damping attenuation, achieving a stabilizing effect which conduces to stabilizing the discharge of the Hall thruster.

Nonlinear dynamic performance of the turbine inlet valves in hydroelectric power plants

In practice, vibrations in the turbine inlet valves can develop flow rate and pressure fluctuations in the penstocks of the hydropower plants that can risk the safe operation of the power plant. Accordingly, this study has two aims: the first aim is to explain and simulate the valve’s periodic vibrations while considering the power plant’s relevant physical and geometrical parameters. The second aim is to provide recommendations to improve the dynamic stability of the valve’s vibrations. The theoretical model developed to explain the phenomenon comprises the nonlinear unsteady energy equations representing the fluid flow at the pipelines, the continuity equation modelling fluid flow through different junctions, and the valve’s seal equation of motion to compute the valve’s vibrations. The system governing equations are solved nonlinearly using the MATLAB toolbox SIMULINK. The study demonstrated that the origin of the valve’s vibrations is the leakage of the valve’s service seal. The presented model results exhibited that the unstable valve vibrations (of increasing amplitudes) are more prone to occur at higher input reservoir energy levels. Also, installing a valve that can be closed to a certain degree at the pilot pipeline can enhance the dynamic stability of the valve’s vibrations.

Effect of Intake position on powder fluidization and conveying characteristics in powder supply device

In this work, a novel built-in intake channel powder supply device was developed for the piston-driven powder fuel feeding system, and a multi-physical coupling model of the gas-powder-moving wall was established using the User Defined Function. A numerical study of powder fluidization and conveying characteristics in a novel powder supply device was presented by using a two-fluid model, and the effects of different intake positions on the flow characteristics in terms of dense gas-solid two-phase in the powder supply process were studied. The results show that the gas phase region is mainly distributed around the intake channel. The powder outlet flow rate fluctuation and the powder layer (εp = 0.1) area peak decreased by 36.6% and 62.5%, respectively, when the intake position is moved forward from 153.9 mm to 172.2 mm in the axial. The area-averaged gas velocity increases and the area-averaged powder volume fraction decreases at the two-phase throat with the forward movement of the intake position. There is a stable stage of the pressure change in the powder storage tank and in the delivery pipeline when the intake channel is arranged at 153.9 mm in the axial, which is more beneficial to the stable conveying of powder.

Investigating the hydraulic resonance behaviour in pressurised in-series piping systems

This study outlines a methodology to inspect the resonant characteristics of in-series-pipes hydraulic systems. The numerical model is based on the impedance approach, assuming the sinusoidal fluctuations of flow rate and pressure parameters, in conjunction with the transfer matrix concept, being implemented for automatic calculations within a complex piping systems framework. To verify the validity of the used numerical technique, the computed results were compared with a conventional numerical solution quoted in the literature. Applications address the free and forced vibration cases occurring into an in-series-pipes system. The free vibration case, is caused by an oscillating valve downstream with a constant level tank upstream; while the forced vibration case is caused by the closure of a turbine downstream. The obtained results show the reliability of the proposed numerical procedure to determine the natural frequencies of a complex in-series hydraulic system. However, this study unveils that the ‘equivalent pipe’ concept is not accurate for analysing the resonating behaviour of a complex in-series pipe system.

Experimental analysis of Cascade CSTRs with step and pulse inputs

In industrial reactors and equipment, non-ideality is quite a common phenomenon rather than an exception. These deviations from ideality impact the process's overall efficiency and the effectiveness of the equipment. To recognize the associated non-ideality, one needs to have enough understanding of the formulation of the equations and in-depth knowledge of the residence time distribution (RTD) data of real reactors. In the current work, step input and pulse input were used to create RTD data for Cascade continuous stirred tank reactors (CSTRs). For the aforementioned configuration, experiments were run at various flow rates to validate the developed characteristic equations. To produce RTD data, distilled water was utilized as the flowing fluid, and NaOH was the tracer substance. The ideal behavior of tracer concentration exits age distribution, and cumulative fraction for each setup and each input was plotted and experimental results were compared with perfect behavior. Deviation of concentration exit age distribution and cumulative fractional distribution from ideal behavior is more in pulse input as compared to a step input. For ideal cases, the exit age distribution curve and cumulative fraction curves are independent of the type of input. But a significant difference was observed for the two cases, which may be due to non-measurable fluctuations in volumetric flow rate, non-achievement of instant injection of tracer in case of pulse input, and slight variations in the sampling period. Further, with increasing flow rate, concentration, exit age, and cumulative fractional curves shifted upward, and this behavior matches with the actual case.

Development of kinetic model for supercritical water gasification and dynamic characteristics investigation on tubular reactor

Supercritical water gasification (SCWG) technology is considered as a potential technology to realize resource utilization of biomass waste. Both experimental and modeling researches are performed to guide the development and scale-up of this technology. Among these researches, dynamic study of the reactor is an important aspect. However, related published literature is relatively limited. In this study, a one-dimensional dynamic model was developed, and a kinetic model of soybean stem containing detailed reaction pathway of biomass was established based on the lumped parameter method. It was calculated through fitting experiment data (obtained at 600, 650, and 700℃ three temperatures and pressure of around 25 MPa), after which it was applied to the tubular reactor for further analysis. Sensitivity analysis of the reactor displayed that H2 mole fraction was most sensitive to the inlet temperature, which increased or decreased by around 10 % when the temperature had a ± 5 % fluctuation. Dynamic simulation results showed that the gasifier responds quickly under a transient change of concentration. Gasification efficiency decreased from 83.8 % to 41.9 % at 500 s, kept constant for 240 s, and then increased to 83.4 % and stabilized at this value. Total response time was 900 s. Molar fraction of CO, CO2 and CH4 increased by 0.8 %, 0.66 %, 3.08 % respectively. While that of H2 decreased by 4.54 % within 660 s. As for fluctuation of flowrate, gasification efficiency increased by 3.4 % when residence time increased from 527 s to 584 s. Mole fraction of H2 and CO2 increased with time, while that of CH4 and CO decreased, and response time was 900 s.

Dynamic modeling and optimization of organic Rankine cycle in the waste heat recovery of the hydraulic system

The replacement of cooling system with the organic Rankine cycle could reduce cooling energy consumption and improve the overall energy efficiency in the hydraulic system. However, dynamic characteristics have a significant impact on stable operating and performance improvement of ORC systems in applications. Therefore, based on the physical prototype, the ORC dynamic model with whole process, including start-stop process, was established and calibration. Then the dynamic characteristics of the system were studied, and the response surface method was used to optimize the operating parameters with the dynamic model. The results show that the simulation model has sufficient accuracy under both steady and transient conditions. Compared with the oil flowrate, the connected-load affects the system performance more obvious. The evaporating pressure has increased by 3.4% as the connected-load increases from 60 W to 120 W. As to the response time of evaporating pressure and evaporator outlet temperature, they are 240 s and 290 s respectively in the starting stage, and are 210 s and 190 s respectively in the stopping stage. Through the optimization, the output power of the ORC system could reach the maximum value of 832 W, when the working medium pump and expander speed are 1500 rpm and 1400 rpm, respectively. The dynamic fluctuation of oil flowrate has relatively little influence on ORC system, it can work stably in the waste heat recovery of hydraulic system, and the operation and control should consider the long response time of ORC starting and stopping process.

The dynamics of non-premixed flames subjected to a transverse acoustic mode

A non-premixed methane/air flame under a transverse acoustic mode is studied by combining theoretical analysis and numerical simulations to understand the acoustic and flow mechanisms affecting the flame response. The experimental configuration under investigation consists of a combustion chamber, a coaxial injector with coflow, and speakers inducing stationary transverse acoustic waves. Numerical simulations reveal a strong response of the flow in the injector to transverse acoustic excitation, resulting in large fluctuations in the mass flow rates of the fuel and oxidizer streams. An analytical model is developed to describe the acoustic response of the flow in the injector to transverse excitation. This model is employed to describe unsteady inflow conditions in large-eddy simulations for different excitation amplitudes. The simulations show that the injector acoustic coupling must be considered to accurately predict the flame lift-off dynamics. Comparisons of numerical results with experimental measurements reveal similar flame structures arising from the longitudinal forcing and good agreement in the lift-off behavior. Further analyses of the flow field, mixing, and flame structure show that the coflow longitudinal forcing induces vortices that corrugate the stoichiometric surface. In addition, the jet forcing enhances the mixing at greater radial locations. The combination of these two mechanisms results in a periodically increased mixing at the stoichiometric surface, leading to significant heat release rate oscillations near the inflow. This work supports the hypothesis that strong acoustic/flame coupling can be present in non-premixed systems when the injectors are longitudinally excited by transverse acoustic waves.

Gas lifted well performance evaluation based on operational parameters’ fluctuations

The paper describes the gas lifted well performance evaluation ability based on operational parameters’ fluctuations analysis. Experimental laboratory research showed that the optimum gas-liquid lift performance correlates with minimal fluctuation of flowrate and pressures. Also, trials in the «Neft Dashlary» oilfield were conducted. Gas lifted well’s flowing tubing head pressure fluctuations were evaluated at different lift gas injection rates – below, above, and at the optimum gas injection rates. The results of field trials show that at optimum gas lifter well performance, the tubing head pressure fluctuation amplitude reduction and frequency increase is observed. This is in line with theoretical prediction and lab experiments conducted. It is shown that gas lifted and naturally flowing wells’ performance quick evaluation is doable during normal operation. The introduced method allows timely detection of well performance deviation from optimal. Necessary measures can be taken to optimize the wells without conducting additional surveillance. Keywords: well; gas lift; flow rate; operational parameters; well performance.

DIFFERENCES IN pH VALUE AND SALIVARY FLOW RATE BEFORE AND AFTER CHEWING GRAPES COMPARED WITH STRAWBERRY

The pH value and salivary flow rate are important factors in maintaining the balance of the oral cavity and affect the health of the teeth and mouth. When we chew foods high in water, fibre, and vitamin C, such as grapes (Vitis vinifera) and strawberries (Fragaria x ananassa), the value of these two elements can change due to stimulation. This study aims to evaluate pH levels and salivary flow rate fluctuation before and after chewing grapes in contrast to strawberries. The research method was a quasi-experimental study; forty-two research participants from Faculty of Dentistry, Universitas Jenderal Achmad Yani students. They were divided into two groups: grape and strawberries. The experiment was divided into two parts: before and after chewing the fruit. The salivary flow rate was assessed through visual screening, and salivary pH was measured using a pH meter. Data were analyzed by t-test (p<0.05). The result shows that the mean pH value and salivary flow rate increased after chewing grapes and strawberries. Compared with before chewing fruit, this increase was statistically different (p=0.000) in the two groups. Meanwhile, when the mean difference was insignificant. Chewing fruit that contains water, fibre, vitamin C, and a sour taste can increase salivary secretion, which follows the buffer system to increase salivary pH.

Simultaneous infusion of two incompatible antibiotics: Impact of the choice of infusion device and concomitant simulated fluid volume support on the particulate load and the drug mass flow rates

Vancomycin and piperacillin/tazobactam are known to be incompatible. The objectives of the present study were to evaluate the impact of their simultaneous infusion on mass flow rates and particulate load and identify preventive strategies. We assessed both static conditions and a reproduction of an infusion line used in a hospital’s critical care unit. A high-performance liquid chromatography/UV diode array system and static and dynamic laser diffraction particle counters were used. The mass flow rates were primarily influenced by the choice of the infusion device and the presence of simulated fluid volume support. Drug incompatibility also appeared to affect vancomycin’s mass flow rate, and the dynamic particulate load increased during flow rate changes - especially in the infusion set with a large common volume line and no concomitant simulated fluid volume support. Only discontinuation of the piperacillin/tazobactam infusion was associated with a higher particulate load in the infusion set with a large common volume line and no concomitant simulated fluid volume support. A low common volume line and the use of simulated fluid volume support were associated with smaller fluctuations in the mass flow rate. The clinical risk associated with a higher particulate load must now be assessed.

Robust optimal operation of continuous catalytic reforming process under feedstock uncertainty

The continuous catalytic regenerative (CCR) reforming process is one of the most significant sources of hydrogen production in the petroleum refining process. However, the fluctuations in feedstock composition and flow rate could significantly affect both product distribution and energy consumption. In this study, a robust deviation criterion based multi-objective optimization approach is proposed to perform the optimal operation of CCR reformer under feedstock uncertainty, with simultaneous maximization of product yields and minimization of energy consumption. Minimax approach is adopted to handle these uncertain objectives, and the Latin hypercube sampling method is then used to calculate these robust deviation criteria. Multi-objective surrogate-based optimization methods are next introduced to effectively solve the robust operational problem with high computational cost. The level diagram method is finally utilized to assist in multi-criteria decision-making. Two robust operational optimization problems with different objectives are solved to demonstrate the effectiveness of the proposed method for robust optimal operation of the CCR reforming process under feedstock uncertainty.

Correlation-Aware Flow Consolidation for Load Balancing and Beyond

Existing load balancing solutions rely on direct or indirect measurement of rates (or congestion) averaged over short periods of time. Sudden fluctuations in flow rates can lead to significant undershooting/ overshooting of target link loads. In this paper, we make the case for taking variations and correlations of flows into account in load balancing. We propose correlation-aware flow consolidation, i.e. aggregating inversely correlated (or uncorrelated) flows into superflows and using them as building blocks for load balancing. Superflows are smoother than individual flows, and thus are easier to estimate with a higher confidence, and can reduce overshooting/ undershooting of link capacities. We present heuristic methods combined with predictive models to consolidate flows and show they can lead to significant reductions in rate standard deviations compared to correlation-agnostic solutions (up to 33% and 12% improvements at the 50th and 99th percentiles respectively for 20 superflows based on real traffic traces).

Time-dependent heat transfer analysis of ellipsoidal protruded microchannel with multiple pulsating jets

Comparing with continuous jets inside microchannel, the heat removal capacity is enhanced by introducing protrusions or pulsating jets individually. Therefore, the preferred heat transfer enhancement and satisfactory thermal performance are investigated after taking both advantages of unsteady properties and ellipsoid protrusions simultaneously in this work. Especially, phase difference, Δφ, as a pivotal parameter, is applied in multiple pulsating jets. The total mass flow rate in microchannel, temperature fluctuation on heated wall, thermal and flow resistance are therefore varied. The largest fluctuation of mass flow rate is achieved when the phase difference between each adjacent jet inlets at Δφ = 0∙π, while the smallest at 0.75∙π, and the temperature distribution of the target wall correspondingly fluctuates. The heat transfer enhancement is most prominent at 0.75∙π. The increment in heat transfer is close to that of flow resistance at Δφ = 0.50∙π, 0.75∙π and 1∙π. The thermal performance TP firstly increases and then decreases with Δφ, and reaches the maximum value at 0.75∙π. Furthermore, due to the phase difference, each jet becomes the one with the highest velocity among all jets periodically, weakening adverse effect of cross flow and suppressing the drift phenomenon effectively. This is exactly the mechanism for heat transfer enhancement of unsteady multiple jets with velocity phase difference in ellipsoid protrusion microchannel.

The economic study of centralised water reuse exchange system in the industrial park considering wastewater segregation

One possible solution to minimise freshwater consumption in the industrial sector is the reuse of treated wastewater from industry. In this paper, three mathematical models are presented to maximise the centralised system's profit by considering wastewater segregation with multiple contaminants. The idea of wastewater segregation was applied to classify different qualities of wastewater into multiple grades. The basic model was tested with a case study of scenarios with a different number of water headers for segregation and different desired qualities of regenerated water. The penalty models were tested with two different penalty rates when sudden flowrate fluctuations occurred on the source side. The results obtained showed that wastewater segregation can bring a high profit to the centralised system and provide significant freshwater savings to the industry. The analysis of penalties showed that penalties were necessary to ensure that the centralised system could make a profit and operate throughout the year.

Do flame describing functions suitably represent combustion dynamics under self-sustained oscillations?

Transfer function concepts that appear in many areas and most notably in control systems have been extensively used to represent the flame response in low-order models of combustion instability. Much of the theoretical work is based on flame transfer functions (FTFs). In recent years, its nonlinear extension, namely the flame describing function (FDF), has been used to get a more accurate representation of the flame response when the level of oscillation becomes large and the system reaches a limit cycle. Despite their wide and reasonably successful use in predicting instabilities, the direct validity of using FTF/FDFs to represent the flame response still remains to be experimentally substantiated. This article is aimed at providing a direct assessment of the capacity of the FDF to suitably describe the flame behavior under self-sustained oscillations (SSOs) for a spray-swirl flame anchored by an injector that is weakly-transparent to acoustic waves. This is accomplished by making use of an experimental combustion configuration that not only exhibits unstable oscillations but also features a set of driver units to modulate the flame (namely stable flame modulation or SFM). The flame is modulated at the frequency of SSO, and the amplitude of incident velocity modulations is then progressively varied until it coincides with that found under SSO. The injector dynamics is shown to be different between SSO and SFM for an injector that is weakly-transparent to acoustic waves and imposes a certain degree of decoupling between plenum and chamber. For such injectors, the FDF built with the upstream velocity would not suitably represent SSO, as this lumps the injector and flame dynamics together. It is then important to use velocity measurement at the injector outlet, at a point where the relative velocity fluctuation matches the relative volumetric flow rate fluctuation. The describing function with velocity reference at the injector outlet is measured for various input levels and found to approximately match those measured under SSO. The best match is obtained when the amplitude of external modulation induces a level of velocity oscillations that comes closest to that prevailing under SSO. This demonstrates that the FDF may suitably capture the nonlinearity of the flame response, at least in the configuration investigated in this research.

Noise-to-state exponentially stabilizing (state, input)-disturbed CSTRs with non-vanishing noise

In this paper, we have proposed a (state, input)-disturbed CSTR (sidCSTR) model that considers unknown but bounded fluctuations in kinetics, flow rates, and heat exchange. A feedback control law is developed that stabilizes the sidCSTR system to reach noise-to-state exponential stability (NSES). The lower bound of the distribution for the state of the NSES sidCSTR system is analyzed both from the probability domain and the time domain. The performance and the validity of the proposed method are demonstrated using a case study dealing with a second-order reversible reaction.

Mechanism of Cuttings Removing at the Bottom Hole by Pulsed Jet

Vibration drilling technology induced by hydraulic pulse can assist the bit in breaking rock at deep formation. Simultaneously, the pulsed jet generated by the hydraulic pulse promotes removal of the cuttings from the bottom hole. Nowadays, the cuttings removal mechanism of the pulsed jet is not clear, which causes cuttings to accumulate at the bottom hole and increases the risk of repeated cutting. In this paper, a pressure-flow rate fluctuation model is established to analyze the fluctuation characteristics of the pulsed jet at the bottom hole. Based on the model, the effects of displacement, well depth, drilling fluid viscosity, and flow area of the pulsed jet tool on the feature of instantaneous flow at the bottom hole are discussed. The results show that the pulsed jet causes flow rate and pressure to fluctuate at the bottom hole. When the displacement changes from 20 L/s to 40 L/s in a 2000 m well, the pulsed jet generates a flow rate fluctuation of 4–9 L/s and pressure fluctuation of 0.1–0.5 MPa at the bottom hole. With the flow area of the tool increasing from 2 cm2 to 4 cm2, the amplitude of flow rate fluctuation decreases by 72.5%, and the amplitude of pressure fluctuation decreases by more than 60%. Combined with the fluctuation feature of the flow field and the water jet attenuation law at the bottom hole, the force acting on the cuttings under the pulsed jet is derived. It is found that flow rate fluctuation improves the mechanical state of cuttings and is beneficial for cuttings tumbled off the bottom hole. This research provides theoretical guidance for pulsed jet cuttings cleaning at the bottom hole.