Pulsation Intensity Research Articles

Effect of Cavitator and Nozzle Parameters on the Efficiency job of the impulse Jet Generator

Studies of fluid jet flows in the presence of a ventilated cavity with a negative cavitation number, carried out at the Institute of Mechanics of Moscow State University, showed that under certain conditions, self-cavitational oscillations with a high intensity of pressure pulsations occur in such a hydraulic system. It was shown that it is possible in principle to use the self-oscillation regime to create periodic impulsed jets during outflow from a converging nozzle into the atmosphere. The influence of the parameters of the cavitator and the outlet nozzle on the intensity of the impact of the outflowing liquid on the screen located perpendicular to the outflow direction is studied. It is shown that an increase in the length of the nozzle can significantly increase the efficiency of the generator, and a smooth narrowing of the channel in front of the cavitator can increase the operating range of the generator in the direction of larger gas blows. It is shown that there is a scale effect - with increasing head pressure, the relative intensity of self-oscillations decreases, but tends to reach a horizontal asymptote.

The effects of stator-rotor interaction on unsteady characteristics of turbine tip leakage flow

The unsteady flow characteristics of tip leakage flow significantly affect the operation stability of the turbine blade. However, there is currently a lack of research on the unsteady flow characteristics of the tip leakage flow under the stator-rotor interaction. Through experiments with a rotating turbine and simulations of the entire stage and full-annulus turbine stage flow using a self-adaptive method, this study investigates both experimentally and theoretically the impacts of rotor-stator interaction on the dynamic behavior of the tip leakage flow in the turbine. The impacts of stator-rotor interaction on pressure oscillations, mixing, and vortex evolution within the tip leakage flow are investigated. Furthermore, the mechanisms underlying the influence of stator-rotor interaction on the dynamic behavior of the tip leakage flow at different rotor speeds and different tip gaps are also analyzed. The results indicate that the passage vortex (PV) of the guide vane aggravates the instability phases experienced by the tip leakage vortex (TLV), namely vortex wandering and breakdown. Besides, the pulsation intensity of the tip leakage flow decreases, while the pulsation frequency increases as the rotational speed increases. Additionally, it is also observed that there is a significant increase in high-frequency and large-amplitude oscillations in the tip leakage flow with increasing tip gap heights.

Analysis of the performance and pressure pulsation in a high-speed centrifugal pump with different hub-cutting angles

In this study, the hub cutting of the impeller was presented to suppress the rotor–stator interference between the blade and volute tongue in a high-speed centrifugal pump with double volutes. The flow fields in the centrifugal pumps with four hub-cutting angles 0, 15∘, 30∘ and 45∘ were numerically simulated to investigate the influence on the internal flow and pressure pulsation using SST [Formula: see text] turbulent model. The predicted results of numerical simulation were in good agreement with the experimental ones. The results show that the relative velocity streamline distribution is more uniform and smoother in hub-cutting impeller compared with that in the original one. Also, the head and efficiency of the pumps with hub-cutting impeller are higher than that of the prototype pump, with the maximum increase of 1.97% of the head for the hub-cutting angle 45∘ under the design flow rate. The pressure pulsation intensity at volute 1 and the unsteady radial force of the hub-cutting pumps are smaller than that of the prototype pump due to the flow improvement. In addition, the maximum and average values of the unsteady radial force decrease with the increase of the cutting angle.

Formation and steady state characteristics of flow field effect in the header of a stripping prior to cutting combine harvester with CFD

In order to analyze the formation mechanism of the flow field in the header of a stripping prior to cutting combine harvester and compare the steady characteristics of the flow field generated by flat and concave combs, this study combined bench tests and computational fluid dynamics (CFD) to analyze them. During the formation of the flow field, the airflow produced “bottoming out” and “vortex” and other phenomena. At high speed, the maximum velocity of the airflow generated by the flat comb was 10.39 m s−1 and that of the concave comb was 10.59 m s−1; the flow field generated by the concave comb was better than that generated by the flat comb. Significant pulsation effects were generated in the airflow at the rice inlet and return outlet; the pulsation intensity was positively correlated with the pulsation mean velocity. At the return outlet, the turbulence of the airflow generated by the concave comb rotation was higher than that of the airflow generated by the flat comb rotation for both low and high speed conditions. This study explores the characteristics of stripping loss in the header of a stripping prior to cutting combine harvester from the perspective of flow field, which provides ideas for the structural optimization of a stripping prior to cutting combine harvester and the study of stripping loss reduction in the later stage, and also provides a reference for the rational use of air flow in the rotating domain in engineering applications.

Design techniques for improving energy performance and S-shaped characteristics of a pump-turbine with splitter blades

The development of high-head pumped-storage projects presents various opportunities and challenges. One challenge is the operational stability of pumped-storage power plants, which is closely associated with the characteristics of pump-turbines. In this study, the design and experimental verification of the hydraulic performance of a pump-turbine runner with splitter blades are investigated. The effects of several main parameters on the performance curve and S-shaped characteristics of pump-turbines in turbine mode are examined via numerical simulation. Subsequently, model tests are conducted to validate the improvements in the performance curve, including a comparison of the S-shaped characteristic curve at a constant guide vane opening and under the same initial conditions. An emergency load rejection test based on the same guide vane closing scheme is performed to verify improvements in transient pressure in the spiral case, draft tube, and rotational speed. The results confirm the enhancement in the S-shaped characteristics and transient global parameters of the splitter blade design. Furthermore, a reduction in pressure pulsation intensity in the vaneless space during off-design conditions is confirmed.

Prediction of Aircraft Surface Noise in Supersonic Cruise State

The aerodynamic noise of an aircraft leads to vibration fatigue damage to structures. Herein, a prediction method for aircraft surface noise under the comprehensive effect of mixed acoustic sources during flight, primarily surface aerodynamic, air intake, and tail nozzle jet noises, was studied. In the supersonic cruising state, the internal and external flow fields of the aircraft were solved using the Reynolds-averaged Navier–Stokes equations to obtain the statistical average solution of the initial turbulence. The non-linear disturbance equation was used to obtain the surface acoustic load of the aircraft. The calculation results revealed that the main source of aircraft surface noise is aerodynamic noise. The sound pressure level on the fuselage increases gradually from front to rear along the aircraft, and the OASPL at the air intake and tail nozzle is relatively large. The jet noise has little effect on the sound pressure level at the front of the fuselage and only contributes to the OASPL at the tail nozzle of the fuselage. The intensity of pressure pulsations from the engine exhaust in the tail section is 93.3% of the total intensity of pressure pulsations.

Circumferential groove on flow field and pressure fluctuation intensity of a semi-open centrifugal pump under design condition

The tip leakage flow (TLF) generated by the tip clearance has adverse effect on the performance of the impeller and increases pressure fluctuation. To suppress the effect of TLF on the instable flow in a semi-open centrifugal pump, the circumferential groove method is proposed in this paper. The effect of the circumferential groove on the internal flow and pressure fluctuation under the design condition is studied by combining numerical simulation with experiment. The results show that the head and efficiency were reduced by 1.1% and 1.0%, respectively. However, the circumferential groove provided a circumferential channel for leading edge overflow. The fluid in the groove formed a jet near the low-pressure region, which can effectively disperse the low-energy fluid of the blade inlet. The initial position and trajectory of the tip leakage vortex (TLV) move downstream, which decreases the relative flow angle and low-velocity region. The flow field in the blade tip region was improved remarkably. The low-frequency pressure fluctuation in the pressure surface and tip clearance of the blade inlet was suppressed by the circumferential groove, and pressure pulsation intensity decreased by 50% and 66%, respectively.

Transition to unstable oscillatory flames in porous media combustion

The flames inside porous burners with low filtration velocity are prone to oscillatory behavior similar to unstable combustion in narrow channels but accompanied by complicated dynamics due to the irregularity of porous structure, flow field, and thermal gradient formed as a result of heat recuperation. This work is a numerical study within the framework of the pore-scale simulation approach devoted to investigating the flame oscillation mechanisms and characteristics in a two-dimensional pseudo-irregular packed bed of particles generated using the Voronoi tessellation algorithm with constant pore size. The results demonstrate that unstable flame oscillations like flames with repetitive extinction and ignition (FREI) lead to the velocity pulsation in adjacent pore channels, where the stable flame fragments are exposed to the hydrodynamically-driven fluctuations. In the case of the flow rate close to the stability threshold, the flow pulses can reach a magnitude enough to shift the stable flame in the upstream quenching zone, promoting its oscillations with repetitive extinction and ignition. Results of parametric simulation with a variation of the particle diameter and flow velocity were summarized in the regime diagram revealing the transition between stable normal flame, FREI, and stable weak flame with the velocity reduction. The pulsating transitional regime was detected between the stable normal flame and the FREI. The amplitude and intensity of such pulsations increase monotonically with the flow rate reduction in a narrow range. Oscillations of the unstable flame and hydrodynamically-driven flame pulsations are similar in dynamics but different in their physical mechanisms. However, in irregularly packed beds of particles, both these phenomena unavoidably coexist and interfere significantly with each other due to the connectivity of the pore channels.

Effects of crossflow pulsation intensity on wake properties of a circular cylinder

The effects of the crossflow pulsation intensity on the wake flow properties of a circular-cylinder were experimentally studied in a wind tunnel. A rotating plate actuator technique was used to generate periodic crossflow oscillations. The Reynolds number and oscillation Strouhal number of the crossflow were fixed at 3111 and 0.074, respectively. The crossflow pulsation intensities were varied within the range between 0.035 and 0.074. The wake evolution processes were captured by laser-light-sheet-assisted smoke flow visualization method. The wake instability frequencies and instantaneous oscillating turbulent velocities were detected by a one-component hot-wire anemometer along with a high-speed data acquisition system. The triple-decomposition technique was applied to the instantaneous velocities to consider the effects of the crossflow oscillations on turbulence. The statistical turbulence flow properties such as the Lagrangian integral time and length scales were estimated. The time-averaged flow structures and vorticity contours in the wake were obtained to analyze the wake geometric parameters by averaging instantaneously captured PIV images. The results revealed that at a fixed Reynolds number, the wake instability frequency of a circular cylinder in oscillating crossflow would present a value in-between of the natural wake instability frequency and the crossflow oscillating frequency. The turbulence intensity and Lagrangian integral time and length scales in the cylinder wake of the oscillating crossflow were drastically larger than those in the natural crossflow. The time-averaged geometric parameters of the recirculation bubble in the circular-cylinder wake in oscillating crossflow were significantly smaller than those in the natural crossflow. The crossflow oscillations induced large scale flow motions and hence enhanced entrainment and mixing in the wake. These mechanisms expedited turbulent diffusion of momentum in the wake, and therefore the turbulence intensity and turbulence time and length scales were enlarged and the geometric parameters of recirculation bubble were shortened when compared with those in the natural crossflow.

Quasiperiodic ELF/VLF Emissions Associated With Corresponding Pulsations of the Geomagnetic Field

AbstractWe present a comparison between properties of quasiperiodic (QP) extra low frequency/very low frequency emissions observed by the low‐altitude DEMETER spacecraft and ultra‐low frequency (ULF) geomagnetic field pulsations measured on the ground by the Canadian Array for Realtime Investigations of Magnetic Activity system of flux‐gate magnetometers and by the Sodankylä Geophysical Observatory magnetometer. Altogether, we have analyzed 398 QP events observed at the times when DEMETER was close to the ground‐based magnetometers. The modulation periods of the analyzed QP events were larger than 10 s and their frequency bandwidths were larger than 200 Hz. For a part of QP emissions with modulation periods about 30 s, there was a good agreement between the modulation periods and peak frequencies of ULF magnetic field pulsations measured on the ground. These QP emissions appear to be closely associated with coincident geomagnetic pulsations (QP1 type), and they represent ∼18% of the total number of analyzed QP events. No corresponding geomagnetic pulsations were identified in the remaining 82% of QP events (QP2 type). The intensity of QP1 events does not seem to correlate with the intensity of geomagnetic field pulsations, while the intensity of QP2 events increases with the integral intensity of geomagnetic field pulsations. Based on the observed association between QP emissions and geomagnetic field pulsations, we estimate the radial distance of the generation region of QP1 emissions to L ∼ 7.

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.

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.

Comparison of dispersed phase holdup and slip velocity in chemical systems with different interfacial tension in an L-shaped pulsed sieve-plate extraction column

The behavior of dispersed phase holdup (DPH) and slip velocity (SV) was studied in a new type of pulsed extraction columns, named “L-shaped pulsed sieve-plate column (LPSPC)” for five chemical systems, including water-kerosene, nitric acid-5% (v/v) TBP/kerosene, nitric acid-15% (v/v) TBP/kerosene, nitric acid-30% (v/v) TBP/kerosene, and uranyl nitrate-30% (v/v) TBP/kerosene. Then the impact of pulsation intensity, interfacial tension, and flow rate of dispersed and continuous phases, on DPH and SV was investigated. The results revealed that both DPH and SV were greatly influenced by the pulsation intensity, interfacial tension and flow rate of dispersed phase though the flow rate of continuous phase had a weaker effect. Finally, using the response surface method (RSM) and dimensional analysis procedure, new semi-experimental correlations were proposed to predict DPH and SV, which corresponded satisfactorily to the experimental data. The average absolute relative error (AARE) values of the correlations obtained by dimensional analysis for DPH and SV were about (6.56, 8.68%) and (5.52, 8.40%) for the horizontal and vertical sections, respectively.

Flooding behavior of the standard liquid–liquid extraction systems in a pilot plant L-type pulse packed extraction column: Experimental study

In this study, the two-phase pressure drop and flooding velocity were investigated in a pilot plant of a new type of extraction column entitled “L-shaped pulsed packed extraction column” using two liquid-liquid systems of butyl acetate-water and toluene-water in the presence of acetone as the mass transfer agent. The effects of operating variables, physical properties, and the presence of mass transfer on the flooding flowrate and pressure drop were considered. It is found that the pressure drop is strongly influenced not only by the flow rates of the continuous and dispersed phases, but also by the pulsation intensity. That is, increasing the values of these parameters increases the shear stresses and the flooding phenomenon to occur sooner. Surprisingly, it is observed that increasing the pulsation intensity and dispersed phase flow rate reduces the maximum throughput of the column but increasing the continuous phase flow rate enhances that. Furthermore, it is achieved that the interfacial tension is a physical property which has significant impact on the flooding and the presence of mass transfer only affect at the low pulsation intensity. Finally, a new correlation is proposed to accurately predict the flooding velocity. Good adaptation between predictions and experiments was found for all investigated operational conditions, chemical systems, and with and without mass transfer.

Numerical study of local heat transfer in tube bundle in pulsating flow

Forced pulsating flows enhance the heat transfer of various heat exchange equipment; however, such flows remain poorly understood. A numerical simulation was used to study how the position of a cylinder in a tube bundle affects the heat transfer during pulsating flow. The tubes of the tube bundle were arranged in an in-line order with the same relative pitch of 1.4. The Reynolds number and Prandtl number had constant values of 1500 and 4.03, respectively. The pulsating flow exhibits a reciprocating asymmetrical character. The pulsation amplitude related to the cylinder diameter was in the range from 0.1 to 0.4, and the pulsation frequency was in the range from 0.2 to 0.8 Hz. The numerical simulation results indicated that the heat transfer of the cylinder in the tube bundle was affected by the row number. With pulsating flow, the heat transfer of the cylinder in the first row had a minimum value, whereas the maximum heat transfer was observed in the last row. The effect of the position of the cylinder along the flow in the tube bundle decreases with increasing pulsation intensity. The maximum heat transfer enhancement of 51% was observed in the first row at a frequency of 0.8 Hz and amplitude of 0.4.

Development of an untraditional technique to control the structure of the output flow from a vortex chamber

A technique of using coherent vortex formations of the dead-end zone of the vortex chamber of the end type as a controlling factor influencing the structure and characteristics of the output flow has been developed. The kinematic parameters of the flow relative to chambers with elongated and extremely short dead-end parts in the range of Reynolds numbers according to the parameters of the nozzle Re=47080–86530 were investigated. The reaction of the flow structure in the output sections of the vortex chambers was determined experimentally using thermoanemometry. Profiles of time-averaged transversal and axial velocity projections, as well as corresponding values of the relative intensity of velocity pulsations, were obtained. It was found that at Re=86530, the elongation of the dead-end part of the chamber leads to a decrease in the initial cross-section of the transversal component by 15 % with an increase in the axial component by 19.7 %, and, at Re=47080, to a decrease in the transversal component by 21 % with an increase in the axial component by 8.5 %. This indicates the redistribution of kinetic energy from transversal to axial energy motion, which is confirmed by the analysis of the corresponding intensity profiles of the velocity pulsations in the output section of the chamber in the near-wall and near-axis zones of the flow. The integral intensity of the velocity pulsations along the initial cross-section of comparable chamber designs increases in a chamber with an elongated dead-end part with almost no additional energy losses. The obtained results form the basis of a rational method of controlling the macro- and microstructure of flows, which determines the efficiency of mass exchange and heat exchange processes in vortex chambers of the end type. Such designs are characteristic of vortex mixers, burners of industrial furnaces, furnace devices of hot water and steam boilers, and other technological and power equipment

Pressure pulsation reduction in the draft tube of pump turbine in turbine mode based on optimization design of runner blade trailing edge profile

Reversible pump turbine plays an important role in pumped storage technology. Its pressure pulsation performance also directly affects the stability and efficiency of unit operation. In this paper, under the conditions of multiple guide vane openings and multiple heads for a pump-turbine unit, based on the reasonable runner blade geometric deconstruction, a parameter control method of runner blade trailing edge is proposed. The Genetic Algorithm combined with Fuzzy Logic (GA-FL) is used as the optimization strategy. After optimization iteration, the optimized runner blade trailing edge shape corresponding to minimum value of pressure pulsation intensity is obtained. In addition, numerical simulation calculation and test verification are carried out for the initial and optimized units, the results are all in good agreement. The results show that the unit efficiency is slightly improved and the relative pressure pulsation intensity in draft tube is significantly reduced. Especially in the case of small guide vane opening, the pressure pulsation performance of optimized unit meets the actual engineering standards.

Modeling of mass transfer coefficient using plug, axial dispersion and forward mixing models for extraction of uranium from uranyl nitrate solution by TBP/Kerosene in a horizontal-vertical pulsed sieve-plate extraction column

The behavior of hydrodynamic parameters and mass transfer of an L-shaped pulsed sieve-plate extraction column (LPSPC) in the process of extracting uranium from uranyl nitrate solution by (30% (v/v)) tri-butyl phosphate (TBP)-Kerosene was investigated. The overall volumetric coefficient of mass transfer was subjected to sensitivity analysis by changing the operating parameters such as pulsation intensity and the volumetric flow rate of dispersed and continuous phases. Experimental observations showed that the pulsation intensity had the most significant impact on the behavior of the studied parameters so that its increase led to an increase in the intensity of mass transfer while reducing the droplet size and dispersed phase holdup (DPH) to 55 and 47%, respectively. The mass transfer coefficient was determined by plug flow model (PFM), axial dispersion model (ADM), and forward mixing model (FMM). The absolute average relative error (AARE) of the prediction of the concentration distribution of the transferred component along the length of the column for ADM and FMM was found to be less than 10%, while for PFM, it was less than 30%.

Study on a new four‐fiber sensor signal processing algorithm of hydrodynamic characteristics in liquid–liquid two‐phase flow

AbstractIn the process of two‐phase countercurrent flow in the extraction column, the study of the hydrodynamic characteristics of dispersed phase droplets is crucial for the design and scaling up of the extraction column. In a pulsed extraction column with a liquid–liquid system, the four‐sensor optical fiber probe was used to assess the hydrodynamic characteristics of the dispersed phase droplets, including size, mean velocity, and holdup. 1 M HNO3 and 30% TBP/kerosene were used as continuous phase and dispersed phase, respectively. The effects of two‐phase superficial velocities and pulsation intensity were investigated on droplet size, mean velocity, and holdup. The findings show that aside from the local holdup, the pulsation intensity has the greatest impact on the hydrodynamic characteristics. A new and concise algorithm was used to deal with the light signals to determine the droplet size and mean velocity. This algorithm created two Cartesian coordinate systems, each with a separate z‐axis using the leading probe as the origin of the coordinate instead of using the vector approach. Compared to the previous algorithm, both the needed number of the time interval between the voltage peaks and that of the direction angles decrease. The relative error of the droplet size obtained by this improved algorithm is within 7%, and droplet velocity is almost the same, compared to the previous algorithm, which indicates that the new algorithm of the four‐sensor optical fiber signal processing is reliable to the measurement of the hydrodynamic characteristics.

Flame characteristics of backward-inclined pulsating combusting jet in crossflow

The flame appearances, thermal characteristics, and combustion product concentrations of a backward-inclined pulsating combusting jet in crossflow were experimentally studied. The time-averaged and streak instantaneous flame images were acquired using a still camera and a high-speed camera, respectively. The flame extinguishment processes were captured and analysed on the basis of streak instantaneous images. The flame temperatures and combustion product concentrations were measured using a homemade fine-wire thermocouple and a gas analyser, respectively. Three characteristic flame modes (crossflow dominated, transitional, and jet-dominated) were found when the backward-inclined jet flame in crossflow was subject to acoustic excitation. These characteristic flame modes were observed in different regimes of pulsation intensity and jet backward inclination angle. Discrete flame puffs with a frequency equal to the acoustic excitation frequency appeared in the jet-dominated flame. Periodic flame puffing became unstable when the pulsation intensity was increased to values near the instability limit. When the pulsation intensity was beyond the instability limit, a strong stretching effect caused the recirculation flame, which played the role of a flame ignitor, to almost vanish (misfire). The flame extinguishment then occurred from the area around the lee side of the burner tube tip. The temperature and gas concentration measurement results showed the significant improvement of combustion performance in low inclination angle even at small pulsation intensity in the regime of the crossflow dominated flame. To enhance combustion performance at a large jet backward inclination angle, the pulsation intensity must be large enough to push the flame in the regime of the jet-dominated flame.