Reducing Pressure Fluctuations Research Articles

Influence of Distributor Structure on Through-Sea Valve Vibration Characteristics and Improvement Design

To address the issue of excessive transient noise during the opening and closing of a sea valve, a method for reducing pressure fluctuations during the opening of the electromagnetic hydraulic distributor has been proposed by analyzing the structure and working principle of the distributor. Based on theoretical calculation and simulation analysis, the size and shape of the buffer slot of the flow hole are determined under the condition that the stable working flow rate remains unchanged. An improved electromagnetic hydraulic distributor is developed and tested. The results indicate that this method can effectively control the opening and closing transient noise of the sea valve.

Fluidization behavior of stirred gas–solid fluidized beds: A combined X-ray and CFD–DEM–IBM study

Stirred gas–solid fluidized bed reactors are commercially employed in polyolefin manufacturing, but the complex gas–solid contacting dynamics pose challenges in design, scale-up, and operation. In this study, the influence of agitation on the fluidization performance of Geldart B particles was investigated experimentally by X-ray imaging and pressure drop measurements and numerically by Computational Fluid Dynamics (CFD) - Discrete Element Method (DEM) - Immersed Boundary Method (IBM). The experimentally obtained minimum fluidization curve and time-averaged pressure drop show good qualitative agreement with the simulation results. Visual observations underscore that an increase in the angular velocity of the agitator results in reduced bubble size and improved bed homogeneity, as further evidenced by reduced pressure fluctuations. Furthermore, the simulations reveal that while the impeller enhances solids agitation, a proper design study is imperative, considering that static immersed bodies, such as the stirrer shaft, can adversely impact solids motion.

Large eddy simulation investigation of micro-vortex generator control effect on early stage sheet cavitation

The wall-adapting local eddy-viscosity large-eddy simulation method is employed for the numerical simulation of a hydrofoil, with transient calculations conducted to compare and analyze the near-wall flow characteristics and cavity morphologies of both the baseline and micro-vortex generator (mVG) hydrofoil models under conditions of high cavitation numbers. High-speed photography combined with numerical analysis revealed that mVGs generate a pair of counter-rotating vortices, boosting the transfer of momentum between the boundary layer and the main flow while reducing flow separation. These vortices induce a new mixed cavity structure at the leading edge, combining vortex cavitation with attached sheet cavitation. During cavity evolution, the mVGs prevent overall tail shedding in the baseline hydrofoil, confining shedding to the sides, while the central vortex cavitation structure remains stable. It enhances hydrofoil stability by reducing pressure fluctuations and guiding cavitation toward more predictable dynamics without causing significant pressure impacts. This research elucidates the mechanism of mVGs in guiding fluid attachment, transforming the structure and shedding cycle of attached cavities, and emphasizing its effectiveness by controlling early-stage sheet cavitation.

Unstable combustion behavior of syngas/air mixture with different components in a narrow gap disk reactor

In this paper, the combustion behavior of the syngas/air mixture in a narrow gap disk reactor were explored. Besides, the influence of the hydrogen volume fraction and the mixture equivalence ratio in syngas on the property of propagating flames were investigated in the narrow gap disk reactor under the gap thicknesses of 8 mm, 6 mm, 4 mm, 3 mm, and 2 mm, respectively. The focus was placed on the behavior of the flame propagation under the gap thickness of 2 mm. The research results demonstrated that the increase of the hydrogen volume fraction from 5 % to 30 % exerted a significant impact on the flame propagation velocity (FPV) and the pressure in the tube. Moreover, the equivalence ratio of this mixture could markedly affect both the propagation characteristics and morphology of flames. Strong flame instabilities accompanied by pressure oscillations were observed in the reactor at a 2 mm gap thickness during the later stages of flame propagation. Notably, there were significant differences in unstable combustion behavior between different syngas components. Moreover, the combined action of heat losses and compression waves leads to significant flame instability. The insights gained from this study have practical implications in various production applications. For example, optimizing hydrogen volume fraction and mixture equivalence ratios can enhance combustion efficiency and reduce pressure fluctuations in industrial reactors, leading to improved energy utilization and reduced environmental emissions.

Numerical Simulation of Underwater Supersonic Jet of Vehicle with Shell-Shaped Flow Control Structure

When the underwater vehicle engine operates under the condition of over-expansion, the violent pulsation of the flow field pressure at the rear of the nozzle can cause violent fluctuations in engine thrust, leading to engine instability. In order to improve the engine's stability, this study drew inspiration from the wave attenuation characteristics of the shell-shaped surface texture structure and added a multi-layer shell-shaped texture structure to the rear wall to reduce pressure fluctuations in the flow field at the rear of the nozzle . Based on the numerical simulation method, the effects of different bionic shell-shaped structures on jet morphology, wall pressure and engine thrust were compared and analyzed. The results show that the multi-layer bionic shell-shaped texture structure can effectively inhibit the occurrence of periodic phenomena such as bulge, necking, and return stroke in the rear flow field, so as to effectively reduce the pressure fluctuation in the rear flow field of the nozzle. In addition, when the momentum thrust is almost unchanged, it is found through calculations that during the initial stage of the jet, the suppression of thrust is not significant. After 0.005 seconds, the oscillation amplitude of the combined force of pressure difference thrust and back pressure thrust decreased by 22%, and the oscillation amplitude of the total thrust decreased by 20%.

Francis Turbine Draft Tube Troubleshooting during Operational Conditions Using CFD Analysis

Hydropower plant vibrations due to pressure fluctuations and their troubleshooting methods are some of the most challenging issues in power plant operation and maintenance. This paper targets these fluctuations in a prototype turbine in two geometries: the initially approved design and the as-built design. Due to topographic conditions downstream, these geometries slightly differ in the draft tube height; the potential effect of such a slight geometrical change on the applicability of troubleshooting techniques is investigated. Therefore, the water flow was simulated using the CFD scheme at three operating points based on the SST k–ω turbulence model, while the injection of water/air was examined to decrease the pressure fluctuations in the draft tube, and the outputs were compared with no-injection simulations. The results show that a slight change in draft tube geometry causes the pressure fluctuations to increase 1.2 to 2.8 times after 4 s injecting at different operating points. The modification in the location of the air injection also could not reduce the increase in pressure fluctuations and caused a 3.6-fold increase in pressure fluctuations. Therefore, the results show that despite water/air injection being a common technique in the hydropower industry to reduce pressure fluctuations, it is effective only in the initially approved design geometry. At the same time, it has a reverse effect on the as-built geometry and increases the pressure fluctuations. This research highlights the importance of binding the construction phase with the design and troubleshooting stages and how slight changes in construction can affect operational issues.

Optimal condition of simultaneous water and air injection in a Francis turbine in order to reduce vortices using experimental and numerical methods

Optimal operation of hydro turbine as one of the most important sources of renewable energy is of great importance. In this article, using a combination of experimental and numerical methods, the effect of different water and air injection on the vortex reduction in a Francis turbine draft tube was studied and the most optimal condition is selected in order to minimize pressure fluctuations and maximize effective efficiency. For this purpose, pressure fluctuations of the water turbine of a real hydropower plant were experimentally measured under different loading conditions to find the worst condition need to be improved. It was found with injecting water at 1.5% and 3% of the total volume of inlet flow rate, the performance index (γ) improves by 38% and 10% respectively, compared to the case without injection. By injecting air at 1% and 2%, the performance index shows 36% and 38% improvement respectively. Optimization was done with the help of Nondominated Sorting Genetic Algorithm II. The results showed that the most optimal condition of injections are 1.63% and 0.65% air and water volume flow rates, respectively, using 0.019-meter air nozzle diameter at −2.80-meter injection height, which reduce pressure fluctuations by 33.44%.

Generation mechanism and control methods of secondary flows in the impeller of axial flow pumps

The secondary flow in the impeller of an axial flow pump is an important factor affecting the safe and stable operation of the unit. However, there is still a lack of systematic research on the generation mechanism of secondary flow and corresponding control strategies in axial flow pumps. To better understand the secondary flow characteristics in the axial flow pump, based on the momentum equation of relative motion, the basic distribution characteristics of the potential rothalpy gradient (PRG, or the reduced static pressure gradient) in the impeller of an axial flow pump were systematically analyzed. Two typical secondary flows were found, namely, trailing-edge hub-shroud type secondary flow at the blade outlet hub side and leading-edge hub-shroud type secondary flow at the blade inlet shroud side. The generation of these secondary flows is directly related to the effect of natural adverse PRG. A new blade design method is proposed. The essential idea of this method is to give the blade loading strategy based on grasping the macro-flow characteristics and control PRG characteristics by adjusting the real blade loading δp (i.e., the static pressure difference between the blade pressure and suction surfaces) and, thereby, control the above-mentioned secondary flows. The application of an axial flow pump showed that the blades designed based on this method can effectively control these secondary flows and reduce pressure fluctuations. The average decrease in pressure fluctuation on the blade inlet shroud side and the outlet hub side is 17.79% and 20.03%, respectively.

Using Transient Modeling to Define the Effects of Heave on Wellsite Operations

Summary The drilling industry is striving to be more efficient, reducing time, costs, and emissions to provide energy solutions for a more climate-conscious world. Reducing nonproductive time (NPT) and invisible lost time (ILT) by utilizing modeling software to optimize tripping operations is one method to combat this. Additional factors such as sea heave have a significant impact on tripping operations on semisubmersible drilling vessels, which do not show a linear response to increased heave. These vessels have compensation systems, but they are often not in use or not completely accurate. Typical heave experienced in the North Sea is between 0.5 and 3-m amplitude and 10–30-second heave period. In this study, transient modeling software is used to create a digital twin of the wellbore to safely predict the tripping speed and define the effect heave has on the tripping operations, from causing additional swab and surge pressures when in or out of slips to reducing tripping speed and improving efficiency by reducing mud gelling at connections. To further understand the effect heave has on tripping operations, experimental simulations of varying heave magnitude were completed in wellbore sections of 0°, 30°, 60°, and 90° inclination. Model validation was completed before running simulations. The results indicate that in horizontal holes, low heave magnitudes result in no bit or pressure response. The same cannot be said for vertical wells, which show smooth bit movement and pressure response throughout the heave magnitude spectrum. However, in a horizontal hole when the wave height is of a larger magnitude, sufficient to break wellbore friction, bit displacement is observed to display jerky axial stick and slip movement, and resultant increased swab and surge pressures are shown even when accounting for the reduced pressure fluctuations due to string eccentricity in an inclined wellbore. It is proposed that much of the related mechanical and pressure response is due to the properties of the drilling mud, string elasticity, and the forced frequency of the waves on the natural frequency of the drilling rig and drillstring. It is clear from definitive and experimental simulations that extra care must be taken when the heave is significant to maintain wellbore stability and reduce wellbore pressure cycling.

Approach to nonlinearity parameter in liquids calculation based on the scaling theory of thermodynamic fluctuations

The nonlinearity parameter B/A is a characteristic of liquids and soft matter, which gains growing attention due to its sensibility to the composition of materials. This makes it a prospective indicator for nondestructive testing applications based on the ultrasound sounding suitable for a variety of applications from physic chemistry to biomedical studies. At the same time, the thermodynamic definition of the nonlinearity parameter requires extensive measurements at elevated pressures that are not always available; in addition, there are known certain contradiction of such data with the data obtained by methods of nonlinear acoustics. Objective. In this work, we consider a recently proposed approach to the prediction of the speed of sound at high pressures, which uses the property of invariance of the reduced pressure fluctuations and the data obtained at normal ambient pressure only. The method generalises the classic Nomoto model, which however gives only a qualitative picture, and results in the quantitative correspondence to the experimental values within their range of uncertainty. Methods. Analytical methods of the theory of thermodynamic fluctuations applied to the parameters of equations of nonlinear acoustics as well as numerical simulation in the COMSOL Multiphysics® environment. Results. Expressions for calculating the nonlinearity parameter with acceptable accuracy were obtained using thermodynamic data obtained only at atmospheric pressure. Numerical calculations were performed for toluene. In addition, the discrepancy between values of the nonlinear parameter obtained via the thermodynamic and nonlinear acoustic routes is analysed based on the numerical solution of the Westervelt equation; it is revealed that this deviation emerges when the effects of absorption of finite-amplitude waves were not properly taken into account.

Transient characteristics investigation of the integrated ejector-driven hydrogen recirculation by multi-component CFD simulation

The hydrogen supply of the fuel cell system is realized by the cooperation of multiple components. Transient characteristics of a single component can affect the performance of other components. In this study, a three-dimensional multi-component computational fluid dynamics (CFD) model was developed to investigate the synergistic transient characteristics of the hydrogen recirculation components such as hydrogen injector, ejector, and purge valve in an 80 kW PEMFC. The results show that the entrainment performance of the ejector is reduced under unsteady purge conditions compared with steady conditions. The pressure fluctuation of the secondary flow is significant even under purge closed durations. There are drastic changes in velocity and pressure in the ejector, especially in the mixing chamber. Moreover, an abundant hydrogen supply capacity of the injector is necessary to deal with the excessive anode pressure fluctuation. The feedforward-feedback integrated control of the injector is a more efficient strategy to reduce pressure fluctuations compared with the feedback control.

A Novel Guide Vane System Design to Mitigate Rotating Vortex Rope in High Head Francis Model Turbine

Guide vanes are a mechanical system used to direct flow in the desired direction. At lower operating conditions, implementing a guide vane system in the draft tube of a hydro-turbine can decrease the excess swirl in the flow and, thus, reduce pressure fluctuations. The present study discusses a numerical methodology to design an effective guide vane system in the draft tube of a high head Francis model turbine. The numerical method is computationally efficient and thus, saves excess computational time and data storage required for parametric analysis of the guide vane system. The aim is to mitigate the ‘rotating’ vortex rope with minimum additional losses in the turbine. The factors considered for the guide vane system design are a) number of guide vanes, b) chord length, c) span, d) inlet-outlet angles of the guide vanes, and e) their position in the draft tube. The parametric study comprises a) ideal guide vane design and b) realistic guide vane design study. The ideal guide vane design study was with the standalone draft tube domain. The realistic guide vane design study used the passage domains of the distributor, runner, and complete draft tube. From the ideal guide vane design study, a guide vane system with two or three guide vanes of chord 86% of runner radius and leading-edge span of 30% of runner radius effectively mitigates the rotating vortex rope. The system with three guide vanes is the most efficient when placed at some distance below the runner exit with mitigation above 95%

Supersonic Cavity Flow Control Using a Spanwise Array of Leading-Edge Tabs

Motivated by the successful application of active control using segmented spanwise blowing, this work investigates passive flow control for a rectangular cavity in supersonic flow using a spanwise array of leading-edge tabs to reduce unsteady pressure loads on the cavity surface. The cavity studied here has a length-to-depth ratio of 6 and width-to-depth ratio of 6 with a turbulent boundary layer at the leading edge and a freestream Mach number of 1.4. Fluctuating surface pressure measurements within the cavity show that the leading-edge tabs have similar effectiveness as leading-edge blowing of the same spanwise wavelength in reducing pressure fluctuations on the cavity surfaces. Streamwise-aligned particle image velocimetry (PIV) measurements show evidence of deflection of the shear layer at the cavity opening and modification of recirculation characteristics associated with the controlled flow. Furthermore, cross-stream stereoscopic PIV measurements reveal counter-rotating streamwise vortices, which persist throughout the cavity length. Proper orthogonal decomposition modes of cross-stream-aligned PIV measurements show that leading-edge tabs are successful in breaking up the flow into smaller-scale modes. Surface flow visualization of the cavity floor and aft wall show differences in the recirculation patterns for the baseline and controlled flows. The present analysis provides insights on how the passive tabs effectively control high-speed cavity flow and demonstrates that the control mechanisms are analogous to steady leading-edge blowing strategies.

Research on bypass pigging in offshore riser system to mitigate severe slugging

During pigging operations in offshore pipelines, severe slugging can block the pig or cause sudden velocity changes. Therefore, this paper proposed a method involving bypass pigs to mitigate the influence of severe slugging on the pigging process. The impact of different inlet mass flow rates and pig bypass fractions on the characteristic parameters, such as the pressure in the riser bottom, pig velocity, and liquid-phase distribution patterns, were analyzed by combining OLGA and CFD simulations. Furthermore, the accuracy of the simulations was verified by the comparisons of OLGA and CFD, as well as the transient indoor experiments. The results show that the bypass pig significantly reduces pressure fluctuations in the riser bottom and pig velocity in severe slugging conditions, compared with the conventional pig. Consequently, the bypass pig dissipates downstream liquid slugs, reducing the mass flow rate and liquid holdup in the riser outlet. In addition, an increase in bypass fraction reduces the pressure fluctuations in the riser bottom and pig velocity, increases the ability to dissipate the downstream liquid slug, and decreases the mass flow rate and liquid holdup in the riser outlet. Increasing the inlet mass flow rate improves the ability to dissipate the downstream liquid slug. However, the inlet mass flow rate has a lower mitigation effect on severe slugging than the bypass fraction.

Optimization of blade high-pressure edge to reduce pressure fluctuations in pump-turbine hump region

Hump characteristic is one of unique hydraulic instabilities of pump-turbines, which restricts the stable and safe operating range of the units. High-amplitude pressure fluctuations could be observed in the hump region, leading to hydraulic vibration of pumped storage power plants. To reduce the high-amplitude pressure fluctuation in the hump region, two optimization strategies for high-pressure edge shape of runner blades have been proposed. One is to increase the outlet angle near the shroud and the other increases the radius near the shroud. A large eddy simulation, which has been validated using performance and pressure fluctuation experiments, was performed to study the optimization effect. The valley point (0.65QBEP) in the hump region was selected for conducting time and frequency analyses under different optimization strategies. Analyses show that the pressure fluctuations were primarily caused by the rotation of three low-pressure regions at the circumference of the guide vanes. Both optimization strategies reduced the stall vortices formed in the three low-pressure regions and mitigated the rotating stall phenomenon. The influence of the runner-outlet geometries on pressure fluctuations is primarily reflected in the amplitude reduction of the dominant low frequency 0.2fn (where fn denotes the rotational speed) in the vaneless region and guide/stay vanes. Compared with strategies involving larger outlet angles near the shroud, this strategy of increasing the radius can more effectively reduce the amplitude of the dominant low frequency, and it makes the circumferential distribution of pressure fluctuations more uniform. These findings can help guide efforts in pump-turbine design optimization, which have been applied in a 700 m head pump-turbine design.

Noise control of a backward-curved centrifugal fan by reducing pressure fluctuations between the impeller and cut-off surface

The noise emitted by the impeller is mainly produced through the pressure fluctuations of impeller and cut-off surface. In this study, it is attempted to minimize the high tonal sound generated by impeller– surface interaction of a backward-curved centrifugal fan. Firstly, the aero-acoustic performance of the two impellers with flat and curved shroud plates is compared inside the fan with the original cut-off. Secondly, the effects of using an open-cell metal foam (OCMF) on the cut-off for both mentioned impellers is examined. From the experimental investigations, it is shown that the impeller with curved shroud plate reduces RMS pressure around 11.7 dB compared to the flat shroud plate. Moreover, employing the OCMF layer (20ppi) on the surface of the cut-off can reduce the RMS pressure around 10.5 dB without significantly reducing outlet flow rate. In general, the modifications are reduced the fan noise by up to 13.7 dB at maximum RPM.

Influence of plasma-chemical products on process stability in a low-emission gas turbine combustion chamber

Abstract The article describes the stability of gaseous fuel combustion in gas turbine low-emission combustion chambers with the plasma-chemical assistance. The mathematical model of unsteady processes in a low-emission combustion chamber with a plasma-chemical stabilizer that takes into consideration the impact of low-temperature plasma on aerodynamics flow in a combustion chamber and the characteristics of heat release is developed. A methodology of a numerical experiment concerning the stability of gaseous fuel combustion in a combustion chamber with plasma assistance using computational fluid dynamics, which enhances the efficiency of designing and adjustment, is proposed. Practical recommendations for improvement of stability of a gas turbine combustion chamber with partially premixed lean fuel–air mixtures, working on gaseous fuels, are developed. They allow to reduce pressure fluctuations inside the flame tube by 10–35%, to decrease spectral power of static pressure in the flame tube in 1.5–2.0 times, to reduce nitrogen oxide emission up to 33.6 ppm in the exit section while retaining a carbon monoxide emission level, that corresponds modern international ecological standards.

Influence of plasma-chemical products on process stability in a low-emission gas turbine combustion chamber

Abstract The article describes the stability of gaseous fuel combustion in gas turbine low-emission combustion chambers with the plasma-chemical assistance. The mathematical model of unsteady processes in a low-emission combustion chamber with a plasma-chemical stabilizer that takes into consideration the impact of low-temperature plasma on aerodynamics flow in a combustion chamber and the characteristics of heat release is developed. A methodology of a numerical experiment concerning the stability of gaseous fuel combustion in a combustion chamber with plasma assistance using computational fluid dynamics, which enhances the efficiency of designing and adjustment, is proposed. Practical recommendations for improvement of stability of a gas turbine combustion chamber with partially premixed lean fuel–air mixtures, working on gaseous fuels, are developed. They allow to reduce pressure fluctuations inside the flame tube by 10–35%, to decrease spectral power of static pressure in the flame tube in 1.5–2.0 times, to reduce nitrogen oxide emission up to 33.6 ppm in the exit section while retaining a carbon monoxide emission level, that corresponds modern international ecological standards.

Our experience of posterior polar cataract phacoemulsification

Purpose: to analyze posterior polar cataract (PPC) phacoemulsification results with regard to surgery technique. Materials and methods . 14 PPC patients (26 eyes) were examined; 8 of them under 50 years and 6, over 50. Before surgery, visual acuity was 0.3–0.6. The operations were performed by one and the same surgeon on an Infiniti Vision System phacoemulsifier (Alcon). In all cases, posterior chamber IOLs were implanted (in 24 cases, complete intracapsular fixation was achieved, in 2 cases, mixed fixation). The follow-up was 1 to 10 years. The main features of the proposed surgery technique: 1) to ensure a stable position of the anterior/posterior chamber, the introduction of solutions and capsulorhexis are carried out through paracentesis before the main incision is made, 2) complete rejection of hydrodissection, hydrodelineation at the stage of preparation for the emulsification of the core itself, as well as rejection of additional hydrodissection of residual epinucleus before its aspiration, 3) the choice of constant torsion ultrasound mode to reduce the oscillatory movements of nuclear fragments, 4) reduction of hydrodynamic parameters (irrigation bottle height no more than 70 mm, aspiration no more than 30 cc/min, vacuum no more than 250 mmHg on the Infiniti Vision System), 5) technique of sequential segmental stepwise fracture of the nucleus during emulsification with minimal rotation of fragments while reducing the ultrasonic power (up to 30%), 6) selection of a separate bimanual irrigation / aspiration technique for a more predictable anterior chamber depth, in particular when extracting tips that can be carried out separately, 7) rejection of polishing the posterior capsule for the prevention of mechanical contact and damage to the posterior capsule. Results. The postoperative period was uneventful. Best corrected visual acuity improved in all patients: to 0.4–0.5 (4), 0.6–0.8 (16), 0.9–1.0 (6). In the late postoperative period, 4 patients required a laser dissection of secondary opacities in the posterior lens capsule. Conclusion. The obtained functional results demonstrate the possibility of successful phacoemulsification with the minimum incidence of complications in patients with PPC. In the reported practice, the known surgical techniques aimed at reducing pressure fluctuations in the capsule bag as well as in the anterior chamber as a whole, were used.

Gas Accumulator System Design to Prevent Water Hammer Effect on Slop Pump Pipeline Network at Papa Flowstation, PT. Pertamina Hulu Energi ONWJ

Water hammer is pressure fluctuation caused by sudden increase and decrease in flow velocity. At Papa Flow Station, water hammer occurs in the slop-pump pipeline network that use level controller operating system. The pressure fluctuation caused by water hammer can damage pipeline or shorten the life of the components. This research was conducted using AFT Impulse software to simulate the modeling. The results showed the gas accumulator could reduce the pressure oscillations on check valve to 1.5 oscillations. The gas accumulator installation alone did not have a significant effect on reducing the fluctuation from the highest pressure of 70 psi. Adding a check valve on the tie-in header-downstream of the pump combined with the installation of a gas accumulator can reduce pressure fluctuations from a maximum of 70 psi to 15 psi and the pressure oscillation will stabilize after the second second when the pump trips at 11 psi. Keywords: cavitation, water hammer, pressure oscillation, pressure fluctuation, gas accumulator DOI: 10.7176/JETP/10-5-03 Publication date: September 30 th 2020