Pressure Pulse Amplitude Research Articles

Blade redesign based on secondary flow suppression to improve the dynamic performance of a centrifugal pump

Low-frequency vibration induced by the complex internal flow of centrifugal pumps has been a long-standing challenge in the design of low-vibration pumps. In this paper, a theoretical method of blade thickness calculation is proposed for the passive control of complex flow in centrifugal impellers, and the effects of blade modification based on secondary flow suppression on the dynamic characteristics of model pump are studied by both experimental tests and numerical simulations. The experimental results show that pressure pulsations are affected by flow rate, and their amplitude at the blade passing frequency (fBPF) dominates and increases at partial load conditions. The redesigned impeller improves the dynamic performance of the model pump considering the relatively lower pressure amplitude at fBPF and the vibration level at low-frequency band under almost all concerned flow rates. Based on steady-state numerical results, the visualization and quantitative analysis of impeller outflow by introducing the secondary flow coefficient and the static pressure energy coefficient can reveal the correlation between secondary flow suppression and dynamic performance improvement. The proposed blade modification can effectively inhibit the development of secondary flow in the boundary layers and improve the uniformity of static pressure energy distribution of impeller outflow, which results in the reduction of pressure pulsation amplitude at fBPF and subsequent decrease in the low-frequency band vibration level. Meanwhile, pump operation under partial load conditions will intensify the secondary flow in the boundary layers, which contributes to a more non-uniform energy distribution of impeller outflow and deteriorates the dynamic performance of the model pump.

Estimating pulsatile ocular blood volume from intraocular pressure, ocular pulse amplitude, and axial length.

The purpose of this study was to develop a method of estimating pulsatile ocular blood volume (POBV) from measurements taken during an ophthalmic exam, including axial length and using a tonometer capable of measuring intraocular pressure (IOP) and ocular pulse amplitude (OPA). Unpublished OPA data from a previous invasive study was used in the derivation, along with central corneal thickness (CCT) and axial length (AL), as well as IOP from the PASCAL dynamic contour tonometer (DCT) and intracameral (ICM) measurements of IOP for 60 cataract patients. Intracameral mean pressure was set to 15, 20, and 35 mmHg (randomized sequence) in the supine position, using a fluid-filled manometer. IOP and OPA measurements were acquired at each manometric setpoint (DCT and ICM simultaneously). In the current study, ocular rigidity (OR) was estimated using a published significant relationship of OR to the natural log of AL in which OR was invasively measured through fluid injection. Friedenwald's original pressure volume relationship was then used to derive the estimated POBV, delivered to the choroid with each heartbeat as a function of OR, systolic IOP (IOPsys), diastolic IOP (IOPdia), and OPA, according to the derived equation POBV = log (IOPsys/IOPdia) / OR. Linear regression analyses were performed comparing OPA to OR and calculated POBV at each of the three manometric setpoints. POBV was also compared to OPA/IOPdia with all data points combined. Significance threshold was p < 0.05. OR estimated from AL showed a significant positive relationship to OPA for both DCT (p < 0.011) and ICM (p < 0.006) at all three manometric pressure setpoints, with a greater slope for lower IOP. Calculated POBV also showed a significant positive relationship to OPA (p < 0.001) at all three setpoints with greater slope at lower IOP, and a significant negative relationship with IOPdia. In the combined analysis, POBV showed a significant positive relationship to OPA/ IOPdia (p < 0.001) in both ICM and DCT measurements with R2 = 0.9685, and R2 = 0.9589, respectively. POBV provides a straight-forward, clinically applicable method to estimate ocular blood supply noninvasively. Higher IOP in combination with lower OPA results in the lowest values of POBV. The simplified ratio, OPA/ IOPdia, may also provide a useful clinical tool for evaluating changes in ocular blood supply in diseases with a vascular component, such as diabetic retinopathy and normal tension glaucoma. Future studies are warranted.

Numerical Analysis of Flow Characteristics in Impeller-Guide Vane Hydraulic Coupling Zone of an Axial-Flow Pump as Turbine Device

An axial-flow pump as a turbine (PAT), as compared to the conventional Francis turbine, has the advantages of not being restricted by the terrain and having lower cost to reverse the pump as a turbine for power generation. When an axial-flow pump is reversed as a turbine, the internal flow pattern is more complicated than when in the pump mode, which can cause instability in the whole system and result in degradation of the hydraulic performance and structural vibration. The impeller and guide vane are the core of the axial-flow PAT unit. This research compares the experimental and numerical simulation results in order to verify the energy performance and pressure pulsation signal of the axial-flow PAT. The unsteady flow regime, fluid force, and pressure pulsation characteristics of the impeller-guide vane hydraulic coupling zone are analyzed in detail. The findings demonstrate that both the dominant frequency of the fluid force pulsation signal and the flow field pressure pulsation signal appear at 3 times of the rotation frequency. The blade passing frequency (BPF) of the impeller is the dominant frequency, and other frequency components are also dominated by the harmonic frequency of the BPF. The impeller and guide vane are primarily subject to radial fluid force. Under partial working conditions, the pressure pulsation intensity in the flow field greatly increases, and the pressure pulsation amplitude at the guide vane outlet and impeller outlet appears to be more sensitive to the flow rate change.

The influence of blade trailing edge modification on the pressure pulsation inside a two-stage centrifugal pump

Adjusting the clearance between the rotor and the stator in the centrifugal pump is an effective measure to reduce the pressure pulsation. In this paper, we investigated the influence of blade trailing edge modification on the internal unsteady flow and the performance of a two-stage centrifugal pump with the V-type trailing edge (VTE) and elliptic trailing edge (ETE). The pressure pulsation on the spherical casing and the performance were analyzed based on the experimental results for the pump with the modified blade trailing edges of the cutting depths 1 mm, 2 mm and 2.5 mm. Also the unsteady flow in the pump was numerically simulated, and the velocity and the axial vorticity intensity were analyzed to evaluate the influence of the flow on the pressure pulsation and the performance of the pump. The results show that cutting the blade trailing edge will reduce the head and efficiency of the two-stage pump, and different outlet shapes under the same cutting depth have little effect on the performance. The modified impeller makes the high velocity areas at the blade outlet away from the spherical casing wall and reduces the pressure pulsation amplitude at the main frequency. At the design flow rate, the heads decrease by 7.3%, 9.8% and 11.3%, the efficiencies decrease by 2.8%, 3.5% and 4.0%, and the pressure pulsation amplitudes at the main frequency are reduced by 29%, 10.1% and 28.7% for the ETE1, ETE2, ETE2.5 blade compared with that of original impeller. Also, the heads decrease by 6.3%, 8.0% and 9.1%, the efficiencies decrease by 3.5%, 3.9% and 4.1%, and the pressure pulsation amplitudes at the main frequency are reduced by 3.2%, 11.7% and 38.1% for VTE1, VTE2, VTE2.5 blade. When the cutting depth is large, the trailing edge of the blade has a large vortex intensity. This may cause the energy loss and lead to the decrease of the efficiency and head. Considering the performance and unsteady pressure pulsation of the two-stage pump, the impeller with VTE1 blade has certain reference significance for engineering application.

Experimental investigation on the cavitation modulation mechanism in submerged self-sustained oscillating jets

To investigate the cavitation evolution states in chamber and its modulation on the self-sustained oscillation, experimental research is conducted in the nozzles with three different chamber lengths using high-speed photography and high frequency sensors. The experiment results indicate that with the increase of the chamber length of Helmholtz nozzles utilized in this study, the intensity of cavitation gradually decreases and the complex congestion cavitation state will switch to the periodic shedding annular cavitation cloud and even discrete free cavitation. With the increase of inlet pressure, the self-maintenance of Helmholtz vortex ring in the jet shear layer enhances, making the random components in the shear layer and the chaotic degree of the flow field reduce. For the submerged self-sustained oscillating jets, the cavitation structure in the nozzle can be regarded as an accumulator, which shrinks and expands periodically to determine the release and aggregation of energy and modulates the pulsation frequency and intensity of the pulsed jet induced by Helmholtz nozzles. The amplitude of pressure pulsation is positively correlated with the cavitation intensity, which means that the pulsation performance of the self-sustained oscillating pulsed jet gets better with the increase of the cavitation intensity.

Influence of blade trailing edge profile on unsteady pressure pulsations in a multistage centrifugal pump

AbstractChanging the shape of the blade trailing edge profile of a centrifugal pump is one of the effective means to improve the internal unsteady flow and pressure pulsation of a centrifugal pump. In this paper, the effect of four shapes of the blade trailing edge profile on the internal nonconstant pressure pulsation and internal flow of a multistage centrifugal pump is investigated by numerical simulation. The four types of blade trailing edges include prototype impeller (original trailing edge), pressure surface cut (ellipse on pressure side [EPS]), suction surface cut (ellipse on suction side [ESS]), and two‐side cut (ellipse on both sides). The results show that reasonable cutting of the trailing edge of the blade can reduce the pressure pulsation of the centrifugal pump. By comparing the pressure pulsation at fBPF, we found that the blade trailing edge profile can change the pressure pulsation. When the speed decreases, the periodicity of pressure pulsation becomes worse and the amplitude of pressure pulsation at fBPF decreases. The static pressure distribution at the trailing edge of the blade shows that EPS and ESS can effectively reduce the area of the high‐pressure gradient region at the trailing edge of the blade, and the dynamic–static interference effect is weakened, which leads to a lower pressure pulsation amplitude. The distribution of turbulent energy in different blade trailing edge centrifugal pumps corresponds to the distribution of static pressure in different blade trailing edge centrifugal pumps, further demonstrating the reliability of the static pressure distribution. The results of this study will further provide theoretical support for the optimization of the blade trailing edge profile.

Research on pressure fluctuation induced by tip leakage vortex of axial flow circulating pump under unpowered driven conditions

Axial flow circulating pumps (AFCPs) are large marine steam turbine units for large-sized ships. One peculiar operation condition for AFCPs is when a ship cruises beyond a certain speed, the energy of pump inflow can completely overcome the frictional resisting moment of the pump itself, thereby driving the impeller to rotate. Such a condition is also known as the unpowered driven condition (UDC). At this time, the fluid is in the artesian flow state. In this paper, pressure fluctuation and inner flow of the AFCP under UDCs and different inflow conditions are analyzed using delayed detached-eddy simulation turbulence model. It is found that the intensity of the tip leakage vortex (TLV) decreases from the leading edge to the trailing edge of the blade, and the amplitude of pressure pulsation caused by TLV also decreases. Due to the jet wake structure at the blade trailing edge, the amplitude of pressure fluctuation at the trailing edge of the blade increases by 7.8% under the optimal UDC. In addition, the compression–expansion term determines the strength of the core of TLV, thus affecting the amplitude of pressure fluctuation. The viscous dissipation effect of TLV can cause high-frequency components of pressure fluctuation.

Study Regarding the Influence of Blade Rotation Angle Deviations on the Hydraulic Pulsation Characteristics of a Mixed-Flow Pump

Mixed-flow pumps with adjustable blades are widely used in municipal, agricultural, and hydropower applications. However, a limitation of adjustable blades is that the influence of the water pressure in the pump causes the rotation angle to deviate, which not only reduces the hydraulic efficiency of the pump and increases energy consumption, but it is also detrimental to the stable operation of the pump. To investigate the influence of blade rotation angle deviations (BRADs) on the hydraulic pulsation characteristics of a mixed-flow pump, in this study, a three-dimensional unsteady numerical simulation was adopted to analyze the effects of seven BRAD design schemes on the energy performance, pressure pulsation characteristics, and axial and radial forces in the impeller. When the rotation angle of a single blade deviated counterclockwise, the optimal hydraulic efficiency point of the mixed-flow pump moved toward larger flow rates, and vice versa. Unlike a situation with no BRADs, when there were BRADs, the central symmetry of the low-pressure area near the suction surface of the impeller blades was destroyed. BRADs led to increases in the pressure pulsation amplitudes at the inlet and outlet of the impeller. The dominant pressure pulsation frequencies near the shroud side at the inlet and outlet of the impeller were not affected by BRADs (both of them were equal to the blade frequency). However, the amplitude of the dominant pressure pulsation frequency at the impeller outlet and the radial force of the impeller both increased with increases in the absolute value of the deviation angle. Moreover, when the rotation angle of a single blade was only in the counterclockwise direction, the axial force of the impeller increased. This study can provide an engineering reference for the stability of mixed-flow pumps with BRADs.

Excitation characteristics of lobe hydrogen circulating pump in polymer electrolyte membrane fuel cell under different clearances and pressure ratios

More specifications for the suction volume, hydrogen return volume, and pressure ratio of the hydrogen circulating system are needed with the development of high-power and high-efficiency vehicular polymer electrolyte membrane fuel cells (PEMFC). The working clearance and pressure ratio are essential in parameters such as pulsation characteristics, radial excitation force, leakage flow rate, and volumetric efficiency of hydrogen circulating pumps. These parameters are closely related to the noise level, reliability, and service life of the fuel cell. Transient numerical simulations of lobe hydrogen circulating pump for fuel cell vehicles (FCVs) based on CFD and dynamic mesh method were employed. The effects of four working clearances on the pulsation characteristics, flow field distribution, and radial excitation force were studied. The relationship between different pressure ratios on internal flow characteristics and volumetric efficiency was analyzed. The results showed that the pressure pulsation in the pump chamber and the radial excitation force on the lobe rotor have distinct periodicity. The radial force pulsation is minimized when the rotor-rotor clearance ζL is 0.18 mm, and the rotor-casing clearance ζr is 0.21 mm. It was found that the increase of the working clearance helps to reduce the amplitude. Moreover, as the pressure ratio rises, the pressure pulsation amplitude at the rotor-casing clearance and the leakage flow rate increase, while the volumetric efficiency decreases. The pressure ratio is the main factor affecting exhaust temperature rise.

The Analysis of Cavitation Flow and Pressure Pulsation of Bi-Directional Pump

A bi-directional pump is designed by using S-shaped hydrofoil, is the most convenient way to achieve bi-directional operation. In this paper, high-speed photography is used to visualize the flow field characteristics of the bidirectional pump under different cavitation numbers, and the flow field changes caused by cavitation are quantitatively analyzed in combination with the pressure pulsation sensor. The results show that the operation efficiency of the bidirectional pump in reverse operation is lower than that in forward operation. Tip clearance cavitation occurs on both suction and pressure surfaces of the impeller under reverse operation and large flow. In reverse operation, the influence of guide vane on the main frequency of pressure pulsation in the impeller is obvious. The quasi-periodic vertical cavitation flow phenomenon increases the amplitude of pressure pulsation in the impeller and becomes the main component of the internal flow in the bidirectional axial flow pump.

ОЦЕНКА ГОМОГЕННОСТИ ТОПЛИВОВОЗДУШНОЙ СМЕСИ В МАЛОЭМИССИОННОЙ КАМЕРЕ СГОРАНИЯ ГАЗОТУРБИННОГО ДВИГАТЕЛЯ С ПОМОЩЬЮ КОРРЕЛЯЦИИ ПУЛЬСАЦИЙ ДАВЛЕНИЯ И РАСХОДА ПИЛОТНОГО ТОПЛИВА

Connection between pilot fuel flow and pressure pulsations in dry low emission combustor is established on base of experimental data. Determining quantitative criteria of homogeneous or diffusion flame is formulated. The criteria give chance to evaluate quality of mixing fuel and air and combustion without emission measuring. The root-mean-square deviation of the dependence of the distribution of the air-fuel mixture concentration field over the combustion chamber cross section on the correlation coefficient between the pilot fuel ratio and the double amplitude of pressure pulsations in the flame tubes for a homogeneous (technically mixed air-fuel mixture) circuit was approximated based on the data obtained. The results obtained make it possible to significantly improve the quality of emission prediction by refining the semi-empirical mathematical model of the nitrogen oxides' generation, built on the basis of the Zeldovich thermal mechanism for low-emission operating modes of dry low emission combustor of the gas turbine engines. The account of heterogeneity of the process in the flame tubes makes it possible to move from the assumption of the geometric and gas-dynamic identity of the flame tubes to their actual heterogeneity. This conclusion allows refining the model. The developed mathematical model can be used as a virtual emission sensor in advanced tracking control systems of gas turbine engines.

Research on the Vortex Rope Control Techniques in Draft Tube of Francis Turbines

Francis turbines are most widely used in hydropower due to their characteristics which include a fast response and wide time-scale operation. The vortex rope inside Francis turbines is a common flow phenomenon, which always causes strong vibration, pressure pulsations, fatigue load, and even serious failure of the components. Vortex suppression methods can effectively change the velocity and pressure distribution of the flow field in the draft tube, reduce the volume of vortex rope and the amplitude of pressure pulsation, inhibit the development of cavitation erosion, and improve the operation stability of the hydro turbine. However, the vortex suppression method is not suitable for all working conditions, and the vortex suppression effect is also different. There are still many problems with how to analyze the vortex suppression effect and practicability of the turbine from multi-dimensions. It is of great significance to analyze the vortex suppression techniques and their practicability in hydraulic turbines from various aspects. The primary focus of the present study is to analyze the hazards of vortex rope in draft tubes and summarize the methods of suppressing vortex rope and pressure pulsation. This review article provides a basis for controlling the vortex rope in the draft tube, which can help the designers choose the suitable control method to mitigate it. Future research directions are also briefly discussed.

The Effects of Meridian Surface Shape on the Pressure Pulsation of a Multi-Stage Electric Submersible Pump

The influence mechanism of the internal pressure fluctuation propagation law of multi-stage submersible electric pump (ESP) is still unclear, which has been a major problem restricting the stable exploitation of deep-sea oil and gas. In order to investigate the effect of different meridian profiles on the pressure pulsation characteristics of three-stage submersible electric pumps, the unsteady Reynolds-averaged Navier–Stokes (URANS) method is used to numerically investigate it. The results show that the lower the pressure pulsation amplitude in the pump caused by the meridional shape that is more in line with the flow law, has a positive effect on the operation stability. The change of the shape of the meridian greatly affects the pressure pulsation law in the secondary and final pumps. The rotor–stator interaction causes the pressure pulsation amplitude of the monitoring point in the middle of the pump chamber to reach a peak value. By using continuous wavelet transform analysis, it is found that the regularity of 1–2 times frequency conversion is complicated due to multiple pulsation sources and low frequency propagation coupling between stages. At 3–6 times frequency, it is basically close to the pulsation rule of the blade frequency. The above research provides a basis for improving the operation stability of the ESP.

Transient simulation of the rapid guide vane adjustment under constant head of pump turbine in pump mode

In order to understand the flow mechanism of pump turbine during the rapid guide vane adjustment (RGVA), the shear stress transition (SST) k-ω turbulence model was used for transient simulation. The dynamic mesh technique was used to simulate the closing process of the guide vane. Combined with the verification of grid independence and the validation of experimental results, the variations of performance parameters (head, flow, torque), entropy production rate and pressure pulsation were calculated. And this process can be basically divided into three stages, including the preparation stage before the guide vane closing (Stage I), the guide vane closing stage (Stage II) and the completion stage of the guide vane closing (Stage III). The numerical simulation results showed that the performance parameters were varying in the form of exponential function during RGVA. The variation trend of the torque of each guide vane in the Stage II is basically the same, but the pulsation intensity in Stage III is different. In the Stage II, the guide vane is the component with the largest change in hydraulic loss, rising by about 362.5 %. The high energy dispersion sites are filling the flow passages of the guide vanes and the static vanes. The position with high pressure pulsation amplitude in the vaneless region also shifted, and the offset angle was 2–3°. The composition and evolution of the vortex structure in the vaneless region and the flow passage are determined by the Q-criterion. These structures, including horseshoe vortex and spanwise vortex, are the main reasons for the increase of pressure pulsation amplitude.

A pressure pulse recognition method based on flow-adaptive double threshold for pressure pulse telemetry

Pressure pulse telemetry as a novel and effective method of communication has received more and more attention in the field of separate layer water injection for the intelligent oilfield, and identification of the effective pulses and their positions is one of the key technologies. For the logging while drilling process, the fixed threshold method is widely used to identify the pressure pulse. However, in the process of water injection, the amplitude of pressure pulse changes with the change of flowrate controlled by process, which will lead to low recognition rate of traditional fixed threshold pressure pulse. Therefore, a pressure pulse recognition method based on flow-adaptive double threshold with fixed time window is proposed. The packet length of instructions and responses is designed to be short and fixed in order to reduce communication time and failure cost, and the fixed time window method is adopted to improve the decoding efficiency of for a frame of data. The pressure pulse recognition method based on flow-adaptive double threshold is used to identify the effective pulses and their positions to adapt to the change of pressure pulse amplitude induced by different flowrates. The first threshold which is set to fixed and lower than the pulse peak with minimum possible flowrate, is used to filter the most of noise and catch the effective pulses with little noise pulses. The second threshold which is adaptive to changes of the flowrates, is determined by average of three maximum peaks of caught pulses from the first threshold, and is used to identify the effective pulses and their positions. In the experimental well with 2000 m deep, the test results show that the error rate of pulse recognition reduce to 0.003% and communication success rate significantly increase from 50% to more than 96.5%. It means that the proposed double threshold method can be adaptive to identify effective pressure pulse under different flowrates, and then can significantly reduce the error rate of pulse recognition and improve two-way communication performance between the wellhead controller and downhole distributors.

Ідентифікатор передпомпажних станів компресора авіаційного ГТД

Surging is a dangerous operating mode of the compressor; in this mode, destabilization and flow stall occur, causing intense pressure fluctuations of the working fluid in the flow path or gas-dynamic shocks. Surging phenomena in gas turbine engines lead to a loss of thrust, are accompanied by an increase in the temperature of gases in front of the turbine and an increase in the level of vibration because of large amplitudes of pressure pulsations and mass flow rates along the engine path. The possibility of occurrence of these phenomena is a serious obstacle to increasing the reliability of gas turbine engines, including flight safety in general. The study of non-stationary processes in compressors shows that surge is preceded by such phenomena as pre-surge and rotating stall, which are called the pre-surge state of an aircraft GTE compressor. The study aims to develop informative criteria for the timely detection of the pre-surge state of the compressor of an aircraft gas turbine engine and to build an anti-surge algorithm for the functioning of the proposed identifier based on the developed criteria. To assess the stability of pressure pulsation periodicities, it is proposed to process and compare the signals received from fast-response sensors located in the compressor. In this article, informative criteria are formed for identifying the pre-surge state of an aircraft GTE compressor. The first criterion is the level of stability of the periodic component of pressure fluctuations. The second criterion is the limit of the relative range of pressure pulsations in the compressor. With the help of the obtained criteria, the pre-surge area is divided into pre-surge, weak stall, and intense stall. Such detailing makes it possible to prevent erroneous wear (in high-cost modes) and premature wear (in case of non-intensive disruption) of the anti-surge control system. Based on the criteria obtained, an algorithm for anti-surge adjustment was developed, according to which the anti-surge system is prepared for testing when a pre-surge or weak stall occurs in the compressor flow path, and is triggered when an intense stall occurs. Based on the anti-surge algorithm, a scheme for identifying the pre-surge states of an aircraft GTE compressor is proposed in the form of a three-level alarm and protection system. Recommendations for the practical implementation of the identifier are described.

Pressure fluctuation characteristics of a pump turbine in a draft tube: New insight into water column separation

The pumped-storage hydropower station is the most reliable, economic, long-term, large capacity, and mature energy storage technology in the power system, and it is an important component of renewable energy. Cavitation and water column separation of a pumped storage unit are important and widely researched factors in the safe and stable operation of a unit. This study focused on the evolution of water column separation of a pump turbine and its relationship with the pressure distribution of the cross section of a draft tube as well as the pressure pulsation characteristics of different measuring points in the cross section of the draft tube. A pumped storage experimental platform that can realize water column separation is established, and experiments with different opening angles are carried out. The results show that there are three factors that impact water column separation and cavitation: gas nucleus, vaporization pressure, and duration of vaporization pressure. Water column separation is the development and continuation of cavitation. The difference between the center pressure of the vortex rope and the wall pressure is large, reaching 2.23 m at a large opening. The pressure fluctuation amplitude of the wall measuring point is greater than that of the other measuring points in the same cross section, but the frequency characteristics are the same. In the transition process, the pressure pulsation amplitude of the liquid column bridging is the largest, and the largest pressure pulsation amplitude can reach 4.18 m at a small opening.

Simulation of pressure pulsations for pumped-storage units with wide operating range and comparison to experiments

The diversity of energy conversion sources in the current energy market increases the demand for stabilizing the electrical grid through ancillary services, frequency and power regulation from the facilities. Pumped-storage units with reversible pump-turbines or ternary sets constitute the currently most advanced solution for providing these services. This implies that pumped-storage units are required to operate in larger head and power ranges than before. In order to increase their flexibility in pump mode, variable speed units have been applied more frequently with additional challenges for the operating range in pump mode.The extension of the operating flexibility in pump mode makes use of a large portion of the model pump characteristic curve, especially in the case of variable speed units. Important challenges to the hydraulic development are among others the cavitation behaviour, hydraulic stability and pressure pulsation level. This study concentrates on the pressure pulsations in pump mode, which are of great relevance for the machine smooth operation.The pressure pulsations in a regulated radial reversible pump-turbine with low specific speed are numerically simulated with computational fluid dynamics (CFD) for several points along the complete operating range of the pump characteristic curve. The finite volume model includes the complete hydraulic machine from draft tube to spiral case and hybrid turbulence models were used, in this case scale adaptive simulation (SAS). The numerical simulation offers the possibility to assess different flow quantities at any point of the finite volume model, providing additional data to the experimental model test results.The integral quantities, e.g. head, flow and efficiency, were compared to the model test results to validate the numerical model. The simulated pressure pulsation amplitude and frequency were also compared to the measured pressure pulsations at the model at the available measuring locations in the spiral case, vaneless space and draft tube cone. After the validation, the computed flow fields were used to derive the pressure pulsation amplitude at other machine locations and components, e.g. at the runner.

Assessment of Cerebral Autoregulation Using Invasive and Noninvasive Methods of Intracranial Pressure Monitoring.

Pulse amplitude index (PAx), a descriptor of cerebrovascular reactivity, correlates the changes of the pulse amplitude of theintracranial pressure (ICP) waveform (AMP) with changes in mean arterial pressure (MAP). AMP relies on cerebrovascular compliance, which is modulated by the state of the cerebrovascular reactivity. PAx can aid in prognostication after acute brain injuries as a tool for the assessment of cerebral autoregulation and could potentially tailor individual management; however, invasive measurements are required for its calculation. Our aim was to evaluate the relationship between noninvasive PAx (nPAx) derived from a novel noninvasive device for ICP monitoring and PAx derived from gold standard invasive methods. We retrospectively analyzed invasiveICP (external ventricular drain) and non-invasiveICP (nICP), viamechanical extensometer (Brain4Care Corp.). Invasive and non-invasive ICP waveform morphology data wascollected in adult patients with brain injury with arterial blood pressure monitoring. The time series from all signals were first treated to remove movement artifacts. PAx and nPAx were calculated as the moving correlation coefficients of 10-s averages of AMP or non-invasiveAMP(nAMP) andMAP. AMP/nAMP was determined by calculating the fundamental frequency amplitude of the ICP/nICP signal over a 10-s window, updated every 10-s. We then evaluated the relationship between invasive PAx and noninvasive nPAx using the methods of repeated-measures analysis to generate an estimate of the correlation coefficient and its 95% confidence interval (CI). The agreement between the two methods was assessed using the Bland-Altman test. Twenty-four patients were identified. The median age was 53.5years (interquartile range 40-70), and intracranial hemorrhage (84%) was the most common etiology. Twenty-one (87.5%) patients underwent mechanical ventilation, and 60% were sedated with a median Glasgow Coma Scale score of 8 (7-15). Mean PAx was 0.0296 ± 0.331, and nPAx was 0.0171 ± 0.332. The correlation between PAx and nPAx was strong (R = 0.70, p < 0.0005, 95% CI 0.687-0.717). Bland-Altman analysis showed excellent agreement, with a bias of - 0.018 (95% CI - 0.026 to - 0.01) and a localized regression trend line that did not deviate from 0. PAx can be calculated by conventionaland noninvasive ICP monitoring in a statistically significant evaluation with strong agreement. Further study of the applications of this clinical tool is warranted, with the goal of early therapeutic intervention to improve neurologic outcomes following acute brain injuries.

Variation law of pressure pulsation during variable speed operation of pumped storage units

Variable speed operation technology has become an emerging development direction in hydropower industry. Variable-speed pumped storage plants (VSPSPs) constantly undergo various transient processes, and ensuring the amplitude of pressure pulsation within the control range is critical to the safe operation of variable-speed pumped storage units (VSPSUs). However, its extreme value is usually determined by rule of thumb in design stage, and few researches focus on the relationship between pressure pulsation and variable speed operation of VSPSUs. In this paper, characteristic curves of pressure pulsation are plotted by processing model test results with Gridfit function, taking a real VSPSP as an example. The total pressure during transient processes is predicted by simulation of a numerical model. Load rejection conditions (change initial speed) and large load regulation conditions (change speed command and power command) are simulated, and the influence of operating trajectory on pressure pulsation during variable speed operation is analyzed. The results show that with the increase of initial speed or speed command, the value of pressure pulsation increases; the trajectory passes through more high-amplitude pressure pulsation region and gradually shifts to the S-shaped region, causing the pressure oscillation. This study could provide theoretical reference for the operation of VSPSUs.