Velocity Pulsations Research Articles

Experimental study of convective heat transfer with a multi-scale roughness

The heat transfer in a turbulent Rayleigh-Bénard convection with a multi-scale roughness at the bottom is studied experimentally. Two different regimes for the heat transfer are found. The first regime has scaling exponent γI=0.4 and corresponds to the reduced values of the Nusselt number. The second regime with enhanced values of the Nusselt number has a scaling exponent γII=0.32, which is noticeably larger than in the case of smooth boundaries. Significant variation in the Prandtl number (from 6.4 to 62) does not change the scaling exponent value of the second regime but increases the values of Nusselt number. The scaling exponent for the relation Re∼Raα is insensitive to the change of the heat transfer regime and is close to 1/2 for all values of Ra. The characteristic ratio of the velocity pulsations to the mean velocity does not depend on the Rayleigh number and is characterized by close values (about 0.8). The local temperature measurements support the mechanism of the transition from the reduced Nusselt number regime to the enhanced one, which is based on the formation of flows between obstacles.

Experimental investigation on the effect of the rotor-stator matching mode on velocity pulsation in the centrifugal pump with a vaned diffuser

Centrifugal pumps play an indispensable role in nuclear power plants, where severe turbulent pulsations induced by the rotor-stator interference(RSI) can generate a substantial threat to the safe and stable operation of these facilities.In this study, in order to elucidate the effect of the match of rotor and stator blade number on the unsteady velocity pulsation characteristics inside a vaned diffuser centrifugal pump, three different schemes of rotor and stator matching modes were investigated using LDA measurement technology. The findings indicate that the match of rotor and stator blades exerts a substantial influence on the flow field structure inside the pump, leading to significant variations in the velocity maps near the tongue region. By comparing the blade passing frequency amplitudes of the three pumps, with fewer stator blades leading to a rapid increase in blade passing frequency amplitude.This study demonstrates that altering the match of rotor and stator blades can significantly change the RSI effect and alters the velocity distribution within the pump, and consequently changes the unsteady turbulence intensity in the interaction region. The research findings greatly enhance the understanding of velocity pulsation within the pump and provide effective support for mitigating RSI effect.

Tangential Mode Combustion Oscillation of a Bluff-Body-Stabilized Diffusion Flame: Experiments

Addressing the issue of tangential mode combustion oscillation prevalent in aviation engine afterburners, this study targets bluff body diffusion flames. Innovatively, acoustic cavities were installed on either side of the bluff body flame, forming a tangential mode oscillation. The study employed experimental methods to study the flame pulsation characteristics under tangential oscillation, identifying the phase distribution of the flame front as a distinct feature differentiating it from conventional longitudinal mode oscillation. Furthermore, it was noted that the multiperiodic pulsation of the flame surface significantly contributes to the complex variability of the flame transfer function/flame description function. The research then established a phase relationship among velocity pulsation, pressure pulsation, and heat release rate pulsation, uncovering the phase coupling mechanism inherent in oscillatory combustion. It also delved into the impact of equivalence ratio and bluff body structure on tangential mode oscillation. Modifications to the bluff body structure led to a significant suppression of oscillation, reducing the amplitude by up to 81.3%. Thus, the study concluded that the flame dynamic response characteristics play a crucial role in determining the thermoacoustic coupling intensity, especially when the combustion system’s acoustic properties are stable. This finding lays a theoretical foundation for future endeavors to mitigate tangential mode oscillatory combustion in afterburners.

The influence of longitudinal duct profiling on unsteady gas dynamics and the heat transfer of pulsating gas flows in the outlet system of reciprocating-engine

Heat machines based on reciprocating-engines remain in demand in various fields of engineering and technology. Therefore, further research is needed to improve the efficiency, reliability, and environmental friendliness of engines. The thermomechanical improvement of non-stationary processes in outlet systems is an appropriate way to improve engine performance. The purpose of this research is to obtain and analyse the gas-dynamic and heat transfer characteristics of pulsating gas flows in an outlet system with ducts of various designs, using a laboratory model to simulate the outlet process in an engine. Thermal anemometry is used to receive data on the instant velocity values and local heat transfer coefficient of unsteady flows in ducts. The article examines two designs of outlet ducts, namely cylindrical (basic) and conical (with a taper of 0.0225) ducts. Spectral analysis of velocity, pressure and heat transfer coefficient pulsations, assessment of turbulence intensity, and calculation of flow characteristics of pulsating flows were performed to obtain detailed information on gas dynamics in the outlet system. The use of a conical duct (in comparison with a cylindrical one) leads to a slight increase in the turbulence intensity by up to 12 %, a decrease in the heat transfer coefficient by 15–20 %, and a change in volumetric gas flow within ± 7.5 %. Thus, the use of a conical duct will lead to the stabilisation of the flow in the outlet system, improved cleaning of the cylinder from outlet gases, a reduction in thermal stress, and a slight growth in the specific power of engines.

Study on the inlet backflow characteristics and particle deposition of screw mixed-flow deep-sea mining pump under small flow conditions

Abstract When a screw mixed-flow deep-sea mining pump operates under small flow conditions, significant backflow occurs at the impeller inlet and in the inlet pipe, which seriously affects the pump performance. To investigate this backflow characteristic, this paper presents a numerical study of the internal flow field of the mining pump based on the CFD-DEM coupling method. The flow state at the impeller inlet, the velocity distribution law in the inlet pipe, and the particle motion state under the flow rate condition of 0.52Qd are obtained. The results show that, when the pump operates under small flow conditions, the leakage from the tip gap of the impeller head causes a screw high-speed backflow with the same rotation direction as the impeller on the wall of the inlet pipe. The backflow intensity attenuates as it propagates towards the pump inlet. The backflow mixes with the main flow in the middle of the pipe and reaches a balanced state, which causes the particles to deposit here. The deposition process can be divided into a formation stage and a stable stage. When the particles in the middle of the pipe reach the stable stage, their shape and position become stable. The velocity pulsation of the monitoring points near the wall is periodic, and its pulsation frequency is the same as the impeller rotation frequency.

Two-dimensional boundary layer receptivity to finite periodic disturbances

Receptivity is the focus and frontier of the research on boundary layer transition and flow drag reduction, but the temporal and spatial evolution of Tollmien–Schlichting waves (T-S waves) is not yet fully investigated, limiting the development of highly efficient laminar flow control techniques. In the present study, the local receptivity problem of the laminar boundary layer on a zero-pressure-gradient flat plate is investigated by using the direct numerical simulation, considering both the temporal and spatial evolution characteristics of the T-S waves. External disturbances at fixed frequencies are introduced in the form of velocity pulsations with different periods to excite T-S waves. The temporal and spatial evolution characteristics of the T-S waves excited by different forms and periods of disturbances are studied. It is found that the amplitude, frequency, and wave velocity of the T-S wave induced by the external multi-period disturbances are different from those induced by the constant disturbances. These conclusions are the same as those of T-S wave induced by wall inhalation. After a further investigation on this particular phenomenon, the influence mechanism of external disturbances on the receptivity process is revealed. This new research finding enriches the instability theory and provides a reference for more efficient applications on active laminar flow control technologies.

Numerical simulation analysis of turbulent pulsation drag reduction at different intermittent times

The shear stress generated by wall turbulence is the main cause of wall friction resistance in turbulent flow through pipes. This paper investigates the impact of inserting rest periods (regions of constant Reynolds number) within the pulsating operating cycle of velocity on the fully turbulent flow at large time-averaged Reynolds numbers, using the large eddy simulation (LES) method. The study aims to explore the effect of increasing rest periods within pulsations on drag resistance. The dimensionless shear stress and drag reduction rates during different time periods of rest were analyzed. Numerical simulation results indicate that the pulsating velocity operation mode does not necessarily lead to drag reduction; it may even result in increased resistance. Inserting rest periods within the pulsation cycle can achieve drag reduction effects, with the maximum drag reduction rate reaching 21.8%. This paper utilizes large eddy simulation (LES) to compare and validate the feasibility of LES for pipe pulsating operation modes against experimental results and direct numerical simulation (DNS) results from the literature, providing a rationale and proof of concept for further in-depth studies on other pipe pulsating flows.

Particle motion characteristics on the rotational flow pneumatic conveying of horizontal-vertical pipeline

In this study, a rotational flow device (rotational blade) is developed and installed in the upstream of the particle inlet with the aim of improving the efficiency and capacity of pneumatic conveying. Firstly, this study analyzed the energy-saving effect of rotational flow based on the pressure drop and power consumption. The results shown that the optimal velocity can be reduced by a maximum of 18.7 % and the power consumption coefficient can be reduced by a maximum of 9.8 %. Furthermore, the distributions of particle concentration, velocity and pulsation velocity are analyzed by using electrical capacitance tomography (ECT) and particle image velocimetry (PIV) system. It is found that the particle velocity and velocity pulsation intensity for rotational flow are higher, and they have the ability to enhance particle suspension. Then, the power spectrum of the particle pulsation velocity shown that the rotational flow exhibited higher peak value at lower frequencies, indicating the particles are not easily deposited at pipe bottom. Finally, the auto-correlation of particle pulsation velocity indicated that the particle motion is more stable and has a longer period at low particle concentrations. The skewness factor and probability density function of particle pulsation velocity indicated that the use of rotational blades makes the particle pulsation velocity to deviate from the Gaussian distribution.

LDA-Based Experimental Investigation of Velocity Pulsations in the Vortex Tube

LDA-Based Experimental Investigation of Velocity Pulsations in the Vortex Tube

Dynamic Characteristics of Pressure-Balanced Metal Bellows in Fluid–Structure Interaction

As a flexible component in high-pressure vessels and pipeline systems, bellows experience significant fluid–structure interaction effects under high-speed internal fluids and external vibrations. Nevertheless, their dynamic response mechanisms coupled with fluid–structure interaction mentioned below have not yet been clarified so far. In this work, a novel pressure-balanced metal bellow (PBMB) for low-stiffness and high-pressure resistance is firstly proposed. Several fluid–structure interaction models were considered to study the dynamic response characteristics of the PBMB. An experimental platform associated with fluid–structure interaction was established to validate the effectiveness of its vibration attenuation performance. The results indicate that the PBMB has an obvious vibration attenuation effect in the range of 5–90[Formula: see text]Hz, and super-harmonic and sub-harmonic resonance phenomena occur in the range of 90–200[Formula: see text]Hz. Under constant fluid conditions, fluid density, viscosity, flow velocity, and pressure are positively correlated with the response amplitude of the PBMB. The response of the PBMB oscillates at the fluid entry point due to both pulsating flow velocity and pulsating pressure. After several cycles, the response caused by pulsating flow velocity gradually decays and stabilizes. Thus, the impact of pulsating frequency on the stability of the response of bellows is insignificant during the initial cycles.

Investigation on the differences in unsteady film cooling behaviors of gas turbine blades between mainstream and cooling air pulsations for a cylindrical hole

In practice, film cooling on gas turbine blade inevitably works in an unsteady environment, which is introduced by periodical rotor/stator interaction and unsteady combustion. Previous studies have introduced two methods to simulate the realistic unsteady condition: 1) the pulsation of mainstream velocity; and 2) the pulsation of coolant injection. However, up to this point, the differences in instantaneous film cooling behaviors between these two methods remain unclear. This work presents a series of large eddy simulations to exhibit the unsteady flow and film cooling behaviors under steady and the two unsteady flow conditions. The numerical strategy is validated against our time-resolved experimental data. Time-averaged results show that the difference between the two pulsations is not significant if the averaged blowing ratio remains the same. However, the pulsation type plays a dominant role on the transient mode of film coverage. Under the steady condition, film coverage instability is induced by the unsteady trajectory of near-wall vortex structure; but with pulsed environments, the unsteadiness magnitude increases, and the area with high unsteadiness level enlarges. The pulsation of the mainstream velocity induces a more severe film coverage instability compared to the pulsation of the cooling air injection, because of the higher fluctuation energy of the mainstream bulk. Under mainstream pulsation, the probability distribution of instantaneous cooling effectiveness is the most scattered, and the corresponding fluctuation range is the largest.

Particle motion characteristics of vertical pipe on a the horizontal-vertical pneumatic conveying system installed dune models in the different curvature elbows

To minimize energy consumption in transportation, a dune model is developed and installed in elbow with varying radii of curvature in the horizontal-vertical pneumatic conveying system in this study. The experimental study focuses on the effect of dune model in elbow with different radius of curvature on the pneumatic conveying system in terms of pressure drop, additional pressure drop coefficient, and the power loss coefficient. Furthermore, the particle concentration and velocity distribution are measures by using the electrical capacitance tomography (ECT) and the high-speed particle image velocimetry (PIV) technology. Finally, the particle pulsation velocity is analyzed to reveal the particle motion mechanism of vertical pipe with dune model by using the fourier transform and wavelet transform. The results indicate that the minimum gas conveying velocity is reduced by installing the dune model, with a maximum reduction rate of 10.36 %, and the maximum reduction rate of power loss coefficient decreased is 11.56 %. At the same time, the particle concentration near the outside and inside wall of the pipe with dune is lower and higher than of without dune, and the particle axial velocities with dune are larger than the case of no dune, and the particle axial pulsation intensity with dune are larger than the case of no dune. Otherwise, the peak value of power spectrum of with dune is lower than that of no dune model in the low frequency region, and the wavelet fluctuation energy of low frequency has a great influence on the axial velocity of particles near the inside wall of pipe for the case of dune model.

Characterization of underwater manipulator based on the fluid-structure interaction under pulsating flow

Abstract Underwater manipulators are taking on an increasingly significant function in marine biological fishing operations. Under the action of the ocean flow field, vortex-induced vibration will be induced, which will cause fatigue damage or even fracture of the underwater manipulator. Based on the Ansys fluid-solid coupling software, this paper simulates the vortex-induced vibration response characterizations of underwater manipulators under pulsating flow conditions. The pulsation flow at the entrance is realized by changing the velocity amplitude and pulsation frequency. The influence laws of pulsating frequency and amplitude on the CL , CD, and wake vortex shedding are investigated. The results showed that the CL and CD of the underwater manipulator increased by 133.33 % and 39.18 %, respectively, with an increase in the pulsation parameter. Additionally, with the increase of pulsating frequency, the banded vortex behind the underwater manipulator is gradually obvious, and the energy in the vortex is also increasing.

Effect of free and compound vortex designs on the energy characteristics and operational stability of mixed flow pumps

The effectiveness of the inverse design method has been widely proven in previous studies, but research on the effect of different vortex designs on the performance of mixed flow pumps is relatively scarce. In this paper, the performances of models I1 and I2, which were designed with free and compound vortex designs, respectively, are compared to study the effect of different vortex designs on the energy characteristics and operational stability of mixed flow pumps. The results show that the efficiency of the compound vortex design at 0.8Qdes, 1.0Qdes, and 1.2Qdes is improved by 0.54%, 0.95%, and 5.91%, respectively, compared to that of the free vortex design, and the velocity and pressure pulsations under the design conditions are also significantly reduced. The internal flow analysis shows that the increased efficiency in the compound vortex design is mainly related to the reduction in the local entropy production from the hub to the mid-span and the wall entropy production from the mid-span to the shroud within the diffuser due to the improvement in the jet-wake structure near the hub. The increased operational stability is mainly related to the suppression of low-momentum fluid aggregation and H-S secondary flow caused by the increase in axial velocity near the hub and the spanwise uniformity of the total pressure.

Behavior of hydrofoil cavitation in a slit channel

The paper presents the results of a cavitation in a slit channel study and offers an analytical description of cavity development. Special emphasis was placed on examining partial cavitation near a NACA 0012 hydrofoil (NACA – the National Advisory Committee for Aeronautics) inside slit channels of different geometries. Experimental investigation was carried out via high-speed imaging and the laser Doppler anemometry (LDA) method. The experimental data showed that the local flow velocity in hydrofoil leading edge area increased abruptly under arising cavitation. In addition, occurrence of cavitation raised flow velocity pulsation by 20%. In the case of a shorter channel, cavity growth occurred at higher cavitation numbers than for a longer channel. The cavity growth velocity was higher for a shorter channel. We showed that the tendency of partial cavitation development in the slit channel can be described as follows: L/C ∼ σ−1, where L is the cavity length; C is the hydrofoil chord; σ is the cavitation number; and parameter A changes as the slit channel length is varied. Comparison of cavitation development near hydrofoil at different attack angles α inside the slit channel with a three-dimensional (3D) cavitation tunnel was conducted. Cavitation in the slit channel occurred at lower σ/2α values compared to 3D cavitation flow around the hydrofoil. To directly compare lengths of the attached cavities arising in slit channels and 3D cavitation tunnels, an additional parameter is proposed, taking into account friction of the slit channel: K = λ·l/D. This parameter allowed us to quantitatively compare the characteristics of cavitating hydrofoils in slit channels and 3D tunnels. The paper provides the governing criteria of the cavitation in the slit channel. Our results propose the physical foundations for the development of cavities in the slit channel.

The effect of a textured surface in the form of triangular prisms on the occurrence and development of cavitation behind the cylinder in microchannels

Cavitation flow in a microchannel behind a cylinder with a smooth and textured surface is investigated using mathematical modeling methods. The textured cylinder has 72 triangular prisms on its surface. The height of the prism, normal to the surface of the bluff body, was 100 nm. Profiles of the flow velocity and volume fraction of vapor are constructed. The monitoring of the velocity and pressure at five points behind both the smooth and the textured cylinder was carried out, while the inlet pressure changed up to 30 bars. It is shown that there is no restructuring of the vortex street to a symmetrical form at quite high-pressure values at the inlet to the channel for a rough cylinder. Namely, roughness prevents the flow stabilization. A pressure jump in the microchannel is observed when a cavity appears with a uniform increase in the flow rate. Two pulsation frequencies are determined for each of the flow modes. The first pulsation frequency ranges from 480 to 2200 Hz and is associated with the formation of cavitation. The second pulsation frequency is associated with the hydrodynamic flow around the cylinder and its values range from 26 to 95 kHz. An increase in surface roughness leads to a growth of the cavitation pulsation frequency and intensifies cavitation. At that, the velocity pulsations in the flow before the onset of cavitation increase, and the frequency of hydrodynamic pulsations after its onset decrease. The paper provides an analysis of the drag coefficient of a hydraulic section with rough and smooth cylindrical bluff bodies. The effect of roughness on the change in the hydrodynamic characteristics of the flow is described.

Numerical simulation of JAG-type corrugated plate structure optimization and enhanced heat transfer by pulsating flow

The paper presents a numerical investigation of the structural optimization of a JAG-type corrugated plate and enhanced heat transfer under pulsating flow based on the research results that have been achieved. Firstly, the corrugation pitch, corrugation depth, and corrugation inclination of the corrugated plate were optimized using Taguchi method to obtain the best heat transfer performance. Secondly, a detailed study is carried out on the flow and heat transfer characteristics under different pulsation velocities (0.01 m/s∼0.1 m/s), frequencies (0.4Hz–3.2Hz), and amplitudes (0.1–0.9). Additionally, the relationship between the flow variation at the contact tail of the corrugated channel and enhanced heat transfer is elucidated in combination with the field synergy theory. The results demonstrate that the application of pulsation characteristics to fluid flow enhances heat transfer efficiency under different working conditions, with significantly superior heat transfer performance observed at low flow rates compared to high flow rates. Moreover, the modification of amplitude has a greater impact on heat transfer performance compared to changes in frequency and speed conditions. Changing the amplitude can produce a better synergistic effect. The research results can provide important reference for the engineering application of enhanced heat transfer by pulsating flow.

Ultrasonographic diagnosis of fetal hemodynamic parameters in pregnant women with diabetes mellitus in the third trimester of pregnancy

ObjectiveIt was to investigate the diagnosis of fetal hemodynamics in pregnant women with diabetes mellitus in the third trimester of pregnancy by color Doppler ultrasonography. Methods55 women with gestational diabetes mellitus (GDM) in the third trimester of pregnancy who were clinically diagnosed and treated in Haian City People's Hospital of Jiangsu Province were selected as the observation group, and 55 pregnant women with normal prenatal examination results were selected as the controls. The hemodynamic parameters of fetal middle cerebral artery (MCA), umbilical artery (UA), and renal artery (RA) were detected, including the ratio of maximum systolic blood flow velocity to end-diastolic blood flow velocity (S/D), resistance index (RI) and arterial pulsation index (PI). Fasting serum levels of maternal patients were collected for detecting Cystain C (Cys C) and homocysteine (Hcy) to analyze the predictive value of serological indexes and target arterial hemodynamics parameters for adverse pregnancy outcome (APO). ResultsThe results showed that compared with controls, in the observation group, RI, PI, and S/D of MCA and RA increased significantly, while RI, PI and S/D of UA decreased obviously (P < 0.05), the levels of serum Cys C and Hcy were clearly increased (P < 0.05). The APO rate of controls and observation group was 10.91 % and 25.45 %, respectively. It was found that the area under the curve of serum Cys C, Hcy, and the APO predicted by the hemodynamic parameters of fetal MCA, UA, and RA were all greater than 0.75 (P < 0.05). Multiple Logistic regression analysis showed that serum Cys C and Hcy, and the hemodynamic parameters of fetal MCA, UA and RA were correlated with APO (P < 0.05). ConclusionIn summary, maternal blood glucose level can affect fetal hemodynamic parameters. In the third trimester of pregnancy, the changes of blood flow parameters of fetal MCA, UA, RA, and maternal serum Cys C and Hcy levels are helpful to understand fetal status in utero, and can be used to predict APO.

Effect of hydrogen blending on ammonia/air explosion characteristics under wide equivalence ratio

In order to evaluate the effect of mixing hydrogen on the explosive characteristics of ammonia, this work explored the evolution of explosion overpressure and flame propagation characteristics of ammonia/hydrogen/air mixture under different hydrogen blending ratios in closed pipelines through experimental research. Experimental results show that as the hydrogen blending ratio increase, the instability of flame increases, and flame surface changes from smooth to wrinkled. Flame propagation undergoes the evolution of jellyfish-shaped/near-spherical, finger-shaped, plane-shaped, irregularly protruding flame and tulip flame. Flame surface area is constantly changing, accompanied by violent flame tip velocity pulsations. The maximum explosion pressure and the maximum explosion pressure rise rate are consistent with the evolution law of adiabatic flame temperature. They first increase and then decrease with increase of the equivalence ratio, and their peak value appears near the equivalence ratio of 1.1. On the contrary, the explosion duration and rapid combustion period first decrease and then increase with increase of equivalence ratio, and decrease monotonically with increase of hydrogen blending ratio. The conclusions obtained in this work will contribute to the structural design of flame arresters and the formulation of explosion venting strategies during the preparation, transportation and utilization of ammonia/hydrogen mixtures.

Large eddy simulation of the cavity shedding characteristics of ventilated cavitation around the underwater vehicle

Ventilated cavitation is utilized for reducing drag and improving the headway of underwater vehicles. The ventilated cavitating flow around the axisymmetric body is simulated using the Large Eddy Simulation (LES) method in this study. Three cavity morphologies with different ventilation rates (Cq) are observed: Foamy Cavity (FC), Continuous Transparent and Foamy Cavity (CTFC) and Super Cavity (SC). The cavity shedding characteristic of the three different cavity morphologies is revealed. For partial cavity, the periodic cavity shedding is dominated by the local high pressure, re-entrant jet and vortex. The distributions of the turbulent kinetic energy and the pulsation enstrophy exhibit different migration mechanisms during the cavity shedding. The energy characteristic is reflected by three terms of the entropy generation. And the entropy production rate by direct dissipation occupies a dominant position, accounting for about 80% of the total entropy production, while the entropy production rate by viscous dissipation only accounts for 4%. For super cavity, gas leakage is dominated by a re-entrant jet induced by the intense adverse pressure gradient. And the results also demonstrated the velocity and vorticity pulsations around the test body wrapped in a super cavity are not intense.