Pulsating Flow Field Research Articles

Analyses of gas pulsation characteristics in the discharge pipeline of a hyper compressor

For the analysis of gas pulsation characteristics in the pipeline systems of low density polyethylene (LDPE) hyper compressors, a time-domain calculation method for compressor pipeline gas pulsation based on real gas properties is proposed. First the thermophysical property tables are prepared with gas state parameters as independent and dependent variables. Then one-dimensional unsteady flow equations and characteristic equations for pipeline flow are established based on real gas properties. After setting pipeline parameters and boundary conditions, the flow equations are discretized using a two-step method at the inner points of the pipeline. The characteristic equations are discretized using the trapezoidal integration method at the boundary points. Thus, the time-domain solving of pulsating flow field in the pipeline is realized. A self-developed numerical code was used to conduct the time-domain calculation and analysis of gas pulsation in the discharge pipeline of a two-stage hyper compressor. The calculated pressure pulsation curves based on real gas properties are in good agreement with experimental data, with a maximum difference lower than 4% of the local average pressure, which is smaller than the difference between the results calculated based on the plane wave theory and the measured data. The analysis of gas pulsation characteristics shows that the pressure pulsation in the main pipeline is mainly contributed by the fundamental and second harmonics. The pressure pulsation in the safety valve branch is higher than that in the main pipeline, and the harmonic components falling into the acoustic resonance frequency range of this branch have significant amplitudes. The acoustic resonance frequency of the branch pipeline is not affected by the main pipeline. Pressure pulsation varies with changes in rotational speed and back pressure, and the pulsation level is more significantly affected by rotational speed compared to back pressure. The renovation scheme of connecting the safety valve branch and expansion tube in series can effectively reduce the pressure pulsation levels of the main pipeline and branch pipeline.

Experimental study on the combustion and flame propagation characteristics of CH4/O2/N2 premixed turbulent jets

The combustion and flame propagation characteristics of CH4/O2/N2 premixed turbulent jets are investigated experimentally in a constant volume chamber (CVC) under different initial temperature (Tini, 300/400/500/600K) and oxygen fractions (γ, 0.4/0.6/0.8/1.0) conditions using High-speed camera and schlieren system. Four types of jet tip velocity were found: primary vortex motion at the jet top (M-type 1), secondary vortex motion (M-type 2), weaker positive and negative feedback between the flame and the flow field (M-type 3), and stronger positive and negative feedback (N-type 4). With the decrease of γ, the jet acceleration and deceleration mechanism is changed from the top vortex motion mechanism in M-type 1 and M-type 2 to the positive and negative feedback mechanism in M-type 3 and N-type 4. The interaction between the pulsating flow field and the heat release from the pre-chamber transitions to the flow field momentum dissipation. For M-type 1 and M-type 2, there is no significant difference in overpressure dynamics. The N-type 4 exhibits lower peak pressure and pressure rise rates. Moreover, the flame mode was predicted under higher Pini (10 bar ≤ Pini) and lower γ (γ ≤ 0.4) based on empirical equations established by the self-similar analytical model.

Development of 1D Model of Constant-Volume Combustor and Numerical Analysis of the Exhaust Nozzle

Development of 1D Model of Constant-Volume Combustor and Numerical Analysis of the Exhaust Nozzle

Study of the enhanced heat transfer characteristics of wavy-walled tube heat exchangers under pulsating flow fields

Heat exchangers have a very wide range of applications in many industrial fields, so the rational design and the performance are to improve the heat exchanger energy efficiency and reduce the production cost of an important means. In this paper, the heat transfer mechanism of pulsation is revealed by simulating and analyzing the effects of three pulsation parameters: volumetric flow rate, pulsation frequency, and pulsation amplitude on the flow and heat transfer of a wavy-walled tube heat exchanger, with the study focusing on the instability behavior that affects the heat transfer mechanism. The results reveal that the combined heat transfer performance of the wavy-walled tube heat exchanger is about 8.2% higher than that of the straight-walled tube heat exchanger. The flow field of the heat exchanger is more fully developed under the pulsating flow field, and its performance evaluation coefficients (PECs) are all greater than 1. It is also found that with the increase in the volume flow rate Qv, the heat transfer enhancement coefficient and PEC first increase and then decrease, and reach the maximum value near Qv = 2.0 m3/h; with the increase in the amplitude A, the vortex and heat transfer enhancement coefficient inside the heat exchanger increase, but the integrated heat transfer performance of the heat exchanger is gradually weakened; with the increase in the frequency fT, the heat transfer enhancement coefficient and PEC first increase gradually, but the increase gradually decreases and levels off in the later stages, reaching a maximum value near fT = 0.5 Hz. Meanwhile, the field coefficients of the wavy-walled tube heat exchanger were analyzed and found to be much smaller than 1, with an order of magnitude of 10−4, and the results also showed that the field coefficients were inversely proportional to the volume flow rate and directly proportional to the amplitude of pulsation and that the field coefficients showed a tendency of increasing and then decreasing with the change of pulsation frequency and reached a maximum value near fT = 0.5 Hz. This work provides an important reference for current manufacturing industries to optimize heat exchanger sizing and develop efficient thermal management systems.

Prediction of pulsating turbulent pipe flow by deep learning with generalization capability

The spatiotemporal development of turbulent pipe flows with various conditions of pulsation is predicted by deep learning. In the present model, convolutional autoencoder (CAE) extracts the spatial features of the input as latent space vectors and long short-term memory (LSTM) evolves them in time. Time-delay neural network (TDNN) is accompanied by LSTMs to treat general unsteady variations of the spatial mean pressure gradient of the pulsatile flows. Temporal evolution is processed in a sequence-to-sequence manner. Note that the pulsating turbulent pipe flows are statistically unsteady flows for which the spatiotemporal development has been seldom predicted by the deep learning technique. The flow field obtained by direct numerical simulation (DNS) was used to train the model. When the pulsating parameters for the training and prediction are the same, several model parameters are validated. To examine further generalization capability, the present model was trained by data from four pulsation conditions. The model was tested afterward by test data from twenty different pulsation conditions of which the range of parameters is either within or out of the range covered by the training data. The model successfully predicts the various pulsating flow fields with reasonable accuracy. The change in the period yields a larger influence on prediction accuracy than the change in the amplitude. The degradation of the prediction accuracy of the bulk velocity for the case with a long period and large amplitude is not caused by the latent space vectors themselves, but by the decoder which reconstructs the flow field from the latent space vectors.

Advancing electrochemical drilling process via coupling of flow field and electric field in pulsating state generated by a novel tube tool

This paper introduces an improvement to electrochemical drilling process by coupling flow field and electric field in pulsating state. A novel tube with half-wedged shape at the end (HW-tube) is prepared, with both sidewall and wedged part of the HW-tube insulated. Only the flat part is utilized to provide electric field for electrochemical drilling. By rotating the HW-tube, both flow field and electric field in pulsating state are generated, alternating in different positions within the inter-electrode gap (IEG). The pulsating flow field enhances the mass transfer process, while pulsating electric field disperses material dissolution process and distribution of electrolytic by-products. Both pulsating fields are coupled at the same frequency, further enhancing the electrochemical drilling process. Simulation results indicate that both flow field and electric field in pulsating state are generated. Compared to the traditional tube, the HW-tube significantly reduces the number of residual particles in IEG, and this number is further reduced by increasing the rotation speed. Experimental results reveal that the surface quality and dimensional uniformity of small hole are improved with HW-tube. With feed rate of 2.22 mm/min, a small hole with diameter of 1.52 ± 0.017 mm is drilled, resulting in a surface roughness of 0.331 μm.

Flow behavior and aerodynamic noise characteristics of ultra-high-speed elevator based on large eddy simulation

PurposeThis paper aims to guide the implementation of noise reduction measures in hoistway and reduce the aerodynamic noise generated by elevator operation, this paper aims to propose an aerodynamic noise analysis method that can solve the flow field in hoistway.Design/methodology/approachA turbulence-acoustic model solving the flow field in a hoistway and a numerical wind hoistway model of the ultra-high-speed elevator were established by using large eddy simulation (LES) and Curle acoustic theory.FindingsThe characteristics of pulsating flow field and aerodynamic noise around ultra-high-speed elevator are analyzed. The asymmetric characteristics of the flow field could be observed using the turbulent kinetic energy and the instantaneous vortexes in the wind hoistway model. Vortex shedding, air flow separation and recombination around the car were the key factors for aerodynamic noise generation. The sound pressure level was approximately linear to the logarithm of car speed. The increase of car deflection angle in a certain range would reduce the peak frequency of wake noise and increase the sound pressure level (SPL) value.Originality/valueThis paper provides important guidance for researches studying the aerodynamic noise in the hoistway and the technical personnel that look for the reduction measures, which greatly improves the shortcomings in the numerical simulation of the aerodynamic noise of the hoistway.

Electrochemical milling of deep-narrow slots with a pulsating electrolyte flow field

Deep-narrow slots (DNSs) (width<4 mm and depth/width>2 mm) are inherent features in die-casting molds as heat-dissipating slots and turbomachinery components for seal slots. Electrochemical milling (EC-milling) is a flexible method to machine variety of structures by controlling the tool path. The process employs a tubular tool with a flat end face through which the electrolyte is supplied coaxially. However, owing to the small inter-electrode gap (IEG), the quantity and velocity of electrolyte flowing into the IEG are weakened during machining of DNSs. This reduces the machining quality and efficiency. Therefore, this paper proposes a method of EC-milling of DNSs with a pulsating flow field. A tubular tool with a wedge-shaped end face (WS-tube) is designed, and both the quantity and velocity of electrolyte flowing into the IEG will be changed with the rotation of WS-tube. Thus, a pulsating flow field is generated which can enhance the mass transport in the IEG. Consequently, both the machining efficiency and quality are improved. Flow-field simulation results indicate that as compared with the traditional flat end face tube (F-tube), WS-tube can generate a periodic pulsating flow field in both front and side IEG, and the peak electrolyte flowing velocity is also increased. The experiment results reveal that compared to that of 0.24 mm/min with F-tube, the maximum feeding speed reaches to 0.42 mm/min with WS-tube, and both the surface quality and taper are improved at the same time. Increasing the rotation speed of tube tool and pulse frequency further enhances the transport of electrolytic by-products, and the machining quality is improved. With the optimized parameters, a 5 mm deep DNS with width of 1.48 mm ± 0.02 mm, taper of 1.44° ± 0.27° and the surface roughness (Ra) of 0.7 µm has been successfully fabricated. • Pulsating flow field was proposed for electrochemical milling of deep-narrow slot. • A tube tool with wedge-shaped end face was designed to generate pulsating flow field. • With the rotation of wedge-shaped tube tool, pulsating flow field was generated in both front and side inter-electrode gap. • Both the machining quality and feeding speed of DNS were improved with pulsating flow field.

Pulsating flow velocity profile measurement at an acoustically reflecting and non-reflecting open pipe end using laser doppler anemometry (LDA)

Depending on the geometry and operating condition, occasionally strong and destructive acoustic resonances occur in the pipe spool piece between the high-pressure discharge of screw compressors and the connected pulsation damper/silencer inlet. To avoid these resonances, the use of a nozzle with defined cross-section reduction is investigated. By designing the contraction ratio as a function of the nozzle inlet Mach number, a non-reflecting termination/transition can be provided. For a defined operating point, acoustic reflection from the nozzle can be avoided in this way, thus preventing the acoustic resonance in the spool piece.Within this paper the pulsating flow field directly downstream an acoustically reflecting open pipe end is investigated using laser doppler anemometry (LDA) at the air test rig at the Chair of Fluidics at TU Dortmund University. In the modified experimental setup an operating point-specific designed nozzle is installed at the open pipe end to create a non-reflecting termination. By means of LDA measurements, the velocity profile is determined experimentally at the nozzle outlet for the designed operating condition. The 2D measurement results (radius dependant velocity and phase distribution) extend established 1D decomposition of pressure measurement signals into upstream and downstream travelling waves and allow a deeper understanding of the acoustic behaviour of non-reflecting nozzles.

Electrochemical Milling of Deep-Narrow Grooves on GH4169 Alloy Using Tube Electrode with Wedged End Face.

Deep-narrow grooves (DNGs) of nickel-based alloy GH4169 are extensively used in aerospace industry. Electrochemical milling (EC-milling) can manufacture special structures including DNGs by controlling the moving path of simple tool, showing a flexible process with the advantages of high machining efficiency, regardless of material hardness, no residual stresses, burrs, and tool wear. However, due to the inefficient removal of electrolytic by-products in the inter-electrode gap (IEG), the machining accuracy and surface quality are always unsatisfactory. In this paper, a novel tube tool with wedged end face is designed to generate pulsating flow field in IEG, which can enhance the removal of electrolytic by-products as well as improve the machining quality of DNG. The flow field simulation results show that the electrolyte velocity in the IEG is changed periodically along with the rotation of the tube tool. The pulsating amplitude of electrolyte is changed by adjusting the wedged angle in the end face of the tube tool, which could affect the EC-milling process. Experimental results suggest that the machining quality of DNG, including the average width, taper of sidewall, and surface roughness, is significantly improved by using the tube tool with wedged end face. Compared with other wedged angles, the end face with the wedged angle of 40° is more suitable for the EC-milling process. DNG with the width of 1.49 mm ± 0.04 mm, taper of 1.53° ± 0.46°, and surface roughness (Ra) of 1.04 μm is well manufactured with the milling rate of 0.42 mm/min. Moreover, increasing the spindle speed and feed rate can further improve the machining quality of DNG. Finally, a complex DNG structure with the depth of 5 mm is well manufactured with the spindle speed of 4000 rpm and feed rate of 0.48 mm/min.

Unsteady pulsating flowfield over spiked axisymmetric forebody at hypersonic flows

The paper gives experimental observations of the hypersonic flow past an axisymmetric flat-face cylinder with a protruding sharp-tip spike. Unsteady pressure measurements and high-speed schlieren images are performed in tandem on a hypersonic Ludwieg tunnel at a freestream Mach number of M∞=8.16 at two different freestream Reynolds numbers based on the base body diameter (ReD=0.76×106 and 3.05×106). The obtained high-speed images are subjected further to modal analysis to understand the flow dynamics parallel to the unsteady pressure measurements. The protruding spike of length to base body diameter ratio of [l/D]=1 creates a familiar form of an unsteady flowfield called “pulsation.” Pressure loading and fluctuation intensity at two different ReD cases are calculated. A maximum drop of 98.24% in the pressure loading and fluctuation intensity is observed between the high and low ReD cases. Due to the low-density field at low ReD case, almost all image analyses are done with the high ReD case. Based on the analysis, a difference in the pulsation characteristics is noticed, which arises from two vortical zones, each from a system of two “λ” shocks formed during the “collapse” phase ahead of the base body. The interaction of shedding vortices from the λ-shocks' triple-points, along with the rotating stationary waves, contributes to the asymmetric high-pressure loading and the observation of shock pulsation on the flat-face cylinder. The vortical interactions forming the second dominant spatial mode with a temporal mode carry a dimensionless frequency (f2D/u∞≈0.34) almost twice that of the fundamental frequency (f1D/u∞≈0.17). The observed frequencies are invariant irrespective of the ReD cases. However, for the high-frequency range, the spectral pressure decay is observed to follow an inverse and −7/3 law for the low and high ReD cases, respectively.

FRACTAL ANALYSIS AND NUMERICAL SIMULATION ON PULSATING FLOW PATTERNS IN A THREE-DIMENSIONAL BRONCHIAL TREE

The pulsating airflow through human bronchial tree is of great significance for understanding its function and morphology. Fractal theory and numerical simulation are applied in this paper to study the global and local flow characteristics in the bronchial tree under unstable conditions. First, the pulsating flow impedance of fractal bronchial tree is derived, and the structure of bronchial tree is optimized by minimizing flow impedance. It has been shown that the optimal structure depends on the physical law governing the airflow. The optimized diameter ratio between parent and daughter branches for pulsating flow is different from Murray’s law, and the fractal dimension for branch diameter lies in 2 and 3. Afterwards, the local pulsating flow field by three-dimensional (3D) numerical simulation on a symmetrical bronchial model is compared with the global flow characteristics by fractal analysis. The numerical results show that asymmetrical airflow characteristics can be found at high Reynolds number, and the velocity distribution of the main bronchus is more irregular and the turbulence phenomenon is more evident. This work can help to understand the association between function and structure of the bronchial tree, and it may shed light on the physical mechanisms and drugs targeting of pulmonary disease.

Influence of the Amplitude of Inlet Axial Pulsating Flow on Turbine Working Characteristics

The flow field at the outlet of the rotating detonation combustor has strong pulsation characteristics, and this strong impact will bring fatal damage to the downstream turbine. In this paper, the simplified sine wave model was used to simulate the rotating detonation wave to study the characteristics of the turbine working in a pulsating flow field. The results show that the greater the amplitude of the incoming flow pulsation, the stronger the non-uniformity of the aerodynamic parameters and heat transfer parameters of the flow field inside the turbine. The increase in the amplitude of the incoming flow pulsation will aggravate the separation of the end wall secondary flow and the cascade flow, resulting in greater flow loss and reducing the working efficiency of the turbine.

Analysis of Unsteady Flow Field in Rotating Detonation Turbine Engine

The rotating detonation turbine engine is a research hotspot in recent years, but the research is mainly at the phase of theoretical analysis. In this paper, the influence of the circumferentially rotating pulsating flow field on the performance of the turbine is studied by numerical simulation. The results show that the uneven turbine inlet flow will increase the working load of the stator vane, but will produce greater flow loss. At the same time, the strength of the channel vortex and the tip leakage vortex inside the rotor is also increasing. The mass flow rate and work efficiency of the turbine decrease with the increase of the unevenness of the flow field.

Experimental study on unsteady flow field in a centrifugal compressor at pulsating backpressure conditions

Centrifugal compressor is confronted by pulsating backpressure when coupled with turbocharged internal combustion engine. Few studies have been reported on the understanding of pulsating flow field in the compressor although its performance is notably influenced by the strongly pulsating condition. This paper experimentally studies unsteady flow field in a centrifugal compressor at pulsating backpressure conditions. Experimental facilities and methodology are introduced firstly, followed by detailed discussions on experimental results of unsteady flow at different pulsating conditions. Results show that the pulses of pressure and mass flow rate are alleviated as the pulses propagate from compressor exit to the impeller, but then are enhanced remarkably in the impeller inducer. Flow field at compressor inlet manifests that the axial velocity and the incidence angle are evidently distorted in both circumferential and radial directions induced by the volute at both steady and pulsating conditions. Specifically, the incidence angle is higher in the region upstream the volute tongue than the values downstream the configuration. The patterns of distortions vary periodically with the pulsating backpressure. In particular, comparing with the variation near hub region, the incidence angle varies with the magnitude as about 10 degrees near 85% blade height, implying that the flow in spanwise direction responses to the pulsating backpressure with different sensitivity. The study experimentally demonstrates the clear evidence of the unsteadiness of the flow field in the compressor at pulsating backpressure conditions.

Effects of the shape of tube and flow field on fluid flow and heat transfer

The arc tube is an innovative heat exchange device that has the advantages of high heat exchange efficiency, which slows down the damage thermal stress to the pipeline and high mass transfer rate, in comparison to the traditional straight wall tubes. Therefore, it is widely applied in the field of heat and mass transfer, particularly in heat exchanger. To study the influence of the ratio of the circumscribed arc radius of the arc tube on the heat transfer and flow resistance characteristics of the fluid inside the tube, geometric models of sine wave wall tubes with various radius ratio i, and the same size parameters have been established. The flow field and temperature field in the tube have been analyzed by numerical simulation and experiment, and the comprehensive heat transfer performance of different wave-wall tubes has been evaluated using the performance evaluation factor PEC. The results show that the heat transfer capacity is stronger for the smaller i. Further, Nu decreases with increasing i, and the reduction decreases with increasing Re. The frictional resistance coefficient at 1 is proportional to i at radius ratio between 0.1 and 1, and the frictional resistance coefficient is inversely proportional to i at 1 < i < 10. When i = 1, the frictional resistance coefficient appears to be the maximum, PEC increases as Re increases and the PEC of external tangential arc wave wall tube is higher than the sine wave wall tube. To improve the heat transfer efficiency of the sine wave wall tube, a pulsating flow field has been applied to the flowing fluid in the wave wall tube. It has been found that the application of the pulsating flow field can improve the heat transfer coefficient of the fluid in the wave wall tube, and the pulsation parameter has a greater impact on the heat transfer coefficient.

Influence of Impeller Speed Patterns on Hemodynamic Characteristics and Hemolysis of the Blood Pump

A continuous-flow output mode of a rotary blood pump reduces the fluctuation range of arterial blood pressure and easily causes complications. For a centrifugal rotary blood pump, sinusoidal and pulsatile speed patterns are designed using the impeller speed modulation. This study aimed to analyze the hemodynamic characteristics and hemolysis of different speed patterns of a blood pump in patients with heart failure using computational fluid dynamics (CFD) and the lumped parameter model (LPM). The results showed that the impeller with three speed patterns (including the constant speed pattern) met the normal blood demand of the human body. The pulsating flow generated by the impeller speed modulation effectively increased the maximum pulse pressure (PP) to 12.7 mm Hg, but the hemolysis index (HI) in the sinusoidal and pulsatile speed patterns was higher than that in the constant speed pattern, which was about 2.1 × 10−5. The flow path of the pulsating flow field in the spiral groove of the hydrodynamic suspension bearing was uniform, but the alternating high shear stress (0~157 Pa) was caused by the impeller speed modulation, causing blood damage. Therefore, the rational modulation of the impeller speed and the structural optimization of a blood pump are important for improving hydrodynamic characteristics and hemolysis.

Vibrational convection in a heterogeneous binary mixture. Part 1. Time-averaged equations

High-frequency vibrations of a container filled with a fluid generate pulsation flows that however are barely visible with the naked eye, and induce the slow but large-amplitude averaged flows that are important for various practical applications. In this work we derive a theoretical model that gives the averaged description of the influence of uniform high-frequency vibrations on an isothermal mixture of two slowly miscible liquids. The miscible multiphase system is described within the framework of the phase-field approach. The full Cahn–Hillard–Navier–Stokes equations are split into the separate systems for the quasi-acoustic, pulsating and averaged flow fields, eliminating the need for the resolution of the short time scale pulsation motion and thus making the analysis of the long-term evolution much more efficient. The resultant averaged model includes the effects of concentration diffusion and barodiffusion, the dynamic interfacial stresses and the generation of the hydrodynamic flows by non-homogeneities of the concentration field (when they are combined with the effects of gravity and vibrations). The resultant model for the vibrational convection in a heterogeneous mixture of two fluids separated by diffusive boundaries could be used for the description of processes of mixing/de-mixing, solidification/melting, polymerisation, etc. in the presence of vibrations.

High-speed unsteady flows past two-body configurations

This paper presents a detailed investigation of unsteady supersonic flows around a typical two-body configuration, which consists of a capsule and a canopy. The cases with different trailing distances between the capsule and canopy are simulated. The objective of this study is to examine the detailed effects of trailing distance on the flow fields and analyze the flow physics of the different flow modes around the parachute-like two-body model. The computational results show unsteady pulsating flow fields in the small trailing distance cases and are in reasonable agreement with the experimental data. As the trailing distance increases, this unsteady flow mode takes different forms along with the wake/shock and shock/shock interactions, and then gradually fades away and transits to oscillate mode, which is very different from the former. As the trailing distance keeps increasing, only the capsule wake/canopy shock interaction is present in the flow field around the two-body model, which reveals that the unsteady capsule shock/canopy shock interaction is a key mechanism for the pulsation mode.

COMPARISON OF FLOW FIELD BETWEEN STEADY AND UNSTEADY FLOW OF AN AUTOMOTIVE MIXED FLOW TURBOCHARGER TURBINE

Global decarbonizing efforts in transportation industry have forced the automotive manufacturers to opt for highly downsized high power-to-weight ratio engines. Since its invention, turbocharger remains as integral element in order to achieve this target. However, although it has been proven that a turbocharger turbine works in highly pulsatile environment, it is still designed under steady state assumption. This is due to the lack of understanding on the nature of pulsating flow field within the turbocharger turbine stage. This paper presents an effort to visualize the pulsating flow feature using experimentally validated Computational Fluid Dynamics (CFD) simulations. For this purpose, a lean-vaned mixed-flow turbine with rotational speed of 30000 rpm at 20 Hz flow frequency, which represent turbine operation for 3-cylinder 4-stroke engine operating at 800 rpm has been used. Results indicated that the introduction of pulsating flow has resulted in more irregular pattern of flow field as compared to steady flow operation. It has also been indicated that the flow behaves very differently between pressure increment and decrement instances. During the pressure decrement instance, flow blockage in terms of low pressure region occupies most of the turbine passage as the flow exit the turbine.