Increase In Void Fraction Research Articles

Numerical Analysis of Enhanced Subcooled Boiling Heat Transfer in a Twisted Tri-Lobed Tube

This study presents a detailed investigation of subcooled boiling phenomenon in a twisted tri-lobed tube, aiming to elucidate its role in tube design and application in thermal management systems. Integrating the Eulerian two-fluid model with the RPI wall boiling model, this study examines the influence of geometric parameters and boundary conditions in a twisted tri-lobed tube on heat transfer. Following validation with experimental data, the study spans a range of mass flux G=500-800 kg/(m²∙s), inlet subcooling ΔTsub,inlet=25-55 K, heat flux qw=0.3-0.6 MW/m², and outlet pressure Poutlet=5-8 MPa. Results show that increasing the minor radius (r) and straight segment length (l) and decreasing the transition radius (R), boosts secondary and helical flows but does not significantly affect the increase in wall void fraction. Furthermore, elevating the wall heat flux and outlet pressure, coupled with decreasing the inlet subcooling degree and mass flow flux, promotes subcooled boiling, thereby enhancing the average void fraction and convective heat transfer coefficient. Significantly, reducing ΔTsub,inlet to 25 K allows the performance evaluation criterion (PEC) of the twisted tri-lobed tube to reach 2.18. The methodologies and findings of this study contribute significantly to a deeper understanding of subcooled boiling process in a twisted tri-lobed tube.

Investigation of two-phase flow distribution in the vertical annular distribution header of a variable refrigerant flow heat pump system

The characteristics of two-phase flow distribution from a vertically installed annular refrigerant distribution header to multiple branch ports are investigated, with a specific focus on simulating a real variable refrigerant flow heat pump (VRF HP) system. A test section is fabricated to replicate an actual distribution header for empirical investigation, utilizing an air–water mixture as the working fluid. A series of experiments are conducted to examine distribution characteristics, involving variations in the location of the main inlet port, operational mode, total mass flow rate, and void fraction. The experimental results indicate that the two-phase mixture exhibits a relatively uniform liquid phase distribution to the branch ports when the main inlet port is positioned at the bottom side. However, maldistribution to branch ports is intensified as the inlet void fraction increases and the flow rate of the two-phase mixture decreases. Furthermore, a series of computational fluid dynamics analyses is performed to investigate the distribution characteristics of R410A, the actual working fluid utilized in VRF HP systems. These analyses are conducted under the operational conditions typical of VRF HP systems. Despite identical Reynolds numbers for the two-phase R410A flow and air–water mixture flow, the former exhibits a notably uneven distribution to branch ports owing to the distinct thermophysical properties, particularly density and viscosity. The findings from this investigation enhance the comprehension of two-phase flow distribution characteristics in a vertically installed refrigerant distribution header under real VRF HP system conditions, offering valuable insights into mitigating the maldistribution of refrigerant flow to branch ports.

Phase distribution characteristics in 5 × 5 bowing rod bundles

During operation of the nuclear power plant, fuel rods tend to bend. In order to accurately predict the thermal-hydraulic phenomena in the rod bundles with bowing rod, the profile of local parameters in two-phase flow is greatly important. In this study, experiments were conducted in 5 × 5 rod bundle channel with a bowing rod. Four different bow deformation ratios (0%, 50%, 80% and 100%) were adopted for the bowing rod. Measurements were performed using a four-sensor conductivity probe to assess local parameters in two-phase flow, including interfacial area concentration (IAC), bubble chord length, void fraction and bubble velocity. The results indicate that, after passing through the bowing section, the void fraction changes from a wall-peak distribution to a core-peak distribution. The distribution of the local parameters at L/D = 78.6 and L/D = 102.1 are almost identical, suggesting that the dissipation length of the bowing section is greater than 23.5 (102.1–78.6) L/D. The void fraction reaches highest in the center of the subchannel with a bowing rod and increases with an increase in bow deformation ratio. In the bowing rod bundle channel, both of the void fraction and bubble chord length increase with the increase of gas velocity, especially in the region of the inner subchannel. Finally, based on the measured local parameters such as bubble velocity and void fraction, a new distribution parameter model in the subchannel with a rod bow deformation ratio of 80% was proposed.

Effect of the void and crack quantity and location on ultrasound: resin experiments and carbon brick validation

ABSTRACT In industrial applications, there is a lack of methods applicable to large-scale, rapid and non-destructive detection of voids and cracks inside carbon bricks. This paper used the ultrasonic testing (UT) A-scan method to investigate the influence of quantity (void fraction:1.4%-4.6%; number of artificial cracks:1–7) and location (five layers) of voids and cracks inside the resin on ultrasonic waves. Both fast Fourier transform (FFT) and synchrosqueezed wavelet transform (SWT) are employed to analyse the amplitude, main frequency and normalised peak time of ultrasonic waves. The results indicate that increased void fraction and number of cracks lead to greater amplitude attenuation, promoting the transition from the fundamental wave to harmonics. Void and crack location symmetrically affect ultrasound amplitude, main frequency, and normalised peak time around the sample centre. Meanwhile, fitting equations are established for the main frequency of the fundamental wave (f 1) in relation to defect quantity. A high-precision quantitative defects detection standard is also developed based on normalised peak time. The validation based on carbon bricks indicates that the derived empirical relations between these parameters and defects can lay the foundation for large-scale and rapid non-destructive detection in carbon bricks.

An experimental study of dynamic flashing in a square tube using tap water

Flashing is a pressure driven phase change phenomenon. It is widely studied due to its richness in physics and applications such as in desalination, in nuclear industry to name a few. Dynamic flashing is a situation wherein the pressure drop for flashing is initiated through the flow of liquid. A recently developed novel desalination system [1] utilizes combined dynamic flashing and vapor separation through tangential injection for producing potable water. An understanding of dynamic flashing in a tube with reference to the novel desalination system could help with its optimization. In this work, dynamic flashing of tap water occurring in a clear quartz square tube was studied. The pressure and temperature measurements along the tube were made and analyzed for different inlet flowrates and initial liquid temperatures. The pressure and temperature values showed increasing gradients along the tube indicating increased vapor production or flashing for increasing flowrates and initial liquid temperatures. The vapor production from flashing and the resulting flow regimes along the tube were tracked and identified through high-speed imaging. Visual observations showed complex flow regimes with bubbles nucleating and growing throughout the tube due to flashing. Most of the bubble formation was observed in the bulk with little nucleation on the glass tube wall. Void fraction measurements along the tube based on capacitance impedance technique were performed to supplement the visual observations which showed increasing void fraction along the tube matching the observed flow regimes from vapor production.

Deep learning-based bubble detection with swin transformer

ABSTRACT We developed a deep learning-based bubble detector with a Shifted window Transformer (Swin Transformer) to detect and segment individual bubbles among overlapping bubbles. To verify the performance of the detector, we calculated its average precision (AP) with different number of training images. The mask AP increased with the increase in the number of training images when there were less than 50 images. It was observed that the AP for the Swin Transformer and ResNet were almost the same when there were more than 50 images; however, when few training images were used, the AP of the Swin Transformer were higher than that of the ResNet. Furthermore, for the increase in void fraction, the AP of the Swin Transformer showed a decrease similar to that in the case of the ResNet; however, for few training images, the AP of the Swin Transformer was higher than that of the ResNet in all void fractions. Moreover, we confirmed the detector trained with experimental and synthetic bubble images was able to segment overlapping bubbles and deformed bubbles in a bubbly flow experiment. Thus, we verified that the new bubble detector with Swin Transformer provided higher AP than the detector with ResNet for fewer training images.

Finite amplitude wave propagation through bubbly fluids

The existence of only a few bubbles could drastically reduce the acoustic wave speed in a liquid. Wood’s equation models the linear sound speed, while the speed of an ideal shock waves is derived as a function of the pressure ratio across the shock. The common finite amplitude waves lie, however, in between these limits. We show that in a bubbly medium, the high frequency components of finite amplitude waves are attenuated and dissipate quickly, but a low frequency part remains. This wave is then transmitted by the collapse of the bubbles and its speed decreases with increasing void fraction. We demonstrate that the linear and the shock wave regimes can be smoothly connected through a Mach number based on the collapse velocity of the bubbles.

Effect of perforation density distribution on production of perforated horizontal wellbore

To address the issue of horizontal well production affected by the distribution of perforation density in the wellbore, a numerical model for simulating two-phase flow in a horizontal well is established under two perforation density distribution conditions (i.e. increasing the perforation density at inlet and outlet sections respectively). The simulation results are compared with experimental results to verify the reliability of the numerical simulation method. The behaviors of the total pressure drop, superficial velocity of air-water two-phase flow, void fraction, liquid film thickness, air production and liquid production that occur with various flow patterns are investigated under two perforation density distribution conditions based on the numerical model. The total pressure drop, superficial velocity of the mixture and void fraction increase with the air flow rate when the water flow rate is constant. The liquid film thickness decreases when the air flow rate increases. The liquid and air productions increase when the perforation density increases at the inlet section compared with increasing the perforation density at the outlet section of the perforated horizontal wellbore. It is noted that the air production increases with the air flow rate. Liquid production increases with the bubble flow and begins to decrease at the transition point of the slug–stratified flow, then increases through the stratified wave flow. The normalized liquid flux is higher when the perforation density increases at the inlet section, and increases with the radial air flow rate.

Research on void drift between rod bundle subchannels

Void drift between subchannels in a rod bundle is a crucial phenomenon affecting the calculation accuracy of thermal-hydraulic parameters in SMRs. It holds significant importance in enhancing the precision of safety analysis for SMRs. Existing research on experiment and model of void drift between rod bundle subchannels is relatively rare, and the accuracy of model calculations requires improvement. In this study, experiments on gas-liquid two-phase non-equilibrium flow were conducted to measure the redistribution of two-phase flow induced by void drift in a 1 × 2 rod bundle. The experiment results indicated that in bubby flow regime with void fraction less than 0.3, the void diffusion coefficient showed little variation with changes in void fraction. However, in slug flow and annular flow regimes with void fraction exceeding 0.3, the void diffusion coefficient significantly increased with an increase in void fraction. Furthermore, a new void drift model was developed and validated based on a subchannel code. The overall predicted uncertainty for the outlet void fraction in the rod bundle benchmark was less than 13%.

A semi‐empirical approach for determining the number density and void fraction of acoustic cavitation bubbles in sono‐reactors

AbstractA new semi‐empirical technic has been established relying on the linkage between the chemistry in the bulk liquid and that taking place in the acoustic cavitation bubble. The open‐source COPASI software has been used for the optimization of number density according to the total yield of a single bubble and the fitting of the experimental yield of hydrogen peroxide in the sonicated solution. It was observed that the number density is increased with the rise of ultrasound frequency from 200 to 1140 kHz, independently of the saturating gas nature (O2, Ar or air). Within this range of wave frequencies, i.e. from 200 to 1140 kHz, the number of active bubbles goes up from 9.35 × 107 to 3.65 × 1015 L−1 s−1. On the other side, it has been demonstrated that the number density obtained under air atmosphere is greater than that resulting either under argon or oxygen‐saturating gas. Interestingly, with respect to the saturating gas nature (O2, Ar, air) and the range of ultrasound frequency (200–1140 kHz), it was observed that the increase of number density was not necessarily accompanied by a proportional increase of void fraction (total volume of bubbles).

Enhancing mechanical properties of natural fiber composites: a study on the effects of fiber loading and filler addition

In this research, we conducted an extensive analysis of two distinct composite materials: NWBF/EP (nonwoven banana fiber/epoxy) and NWBF/EP/WNP (nonwoven banana fiber/epoxy with walnut powder). These composites were meticulously engineered, utilizing epoxy as the matrix, nonwoven banana fiber as the primary reinforcement, and walnut powder as the secondary reinforcement. Our investigation unveiled that the NWBF/EP/WNP hybrid composite exhibits superior mechanical properties in comparison to the NWBF/EP composite. Notably, the BW4 hybrid composite demonstrated a substantial increase in tensile strength, reaching an impressive 76.7 MPa. This enhancement underscores the potential for augmenting composite stiffness by elevating the WNP ratio up to a specific threshold, though exceeding this threshold leads to a reduction in epoxy resin content. Furthermore, our study revealed substantial improvements in flexural strength as WNP was introduced, with a noteworthy 5.8% rise at a 5% weight percent WNP loading. The pinnacle of flexural strength, 43.6 MPa, was achieved at a 20% weight percent loading. Impact toughness also displayed significant improvements, with the highest impact strength (5.2 J) observed in BW3. This highlights the potential for enhancing the toughness of the hybrid composite within a defined WNP weight percent range. We also gained valuable insights into hardness, void fraction, and the influence of walnut powder. The addition of walnut powder increased void fraction, reduced density, and enhanced various mechanical properties. Our evaluation of wear performance emphasized the pivotal role of factors such as sliding velocity, fiber content, sliding distance, and normal load. In conclusion, this research not only elucidates the mechanical advantages of the NWBF/WNP/epoxy hybrid composite but also offers critical insights for potential applications. The findings underscore the potential of these hybrid composites to serve as sustainable and competitive alternatives to synthetic fiber products in a range of engineering and manufacturing contexts.

ESTIMATION OF THE OPTIMUM BED THICKNESS OF A FLOWTHROUGH POROUS ELECTRODE (FTPE) WORKING UNDER MASS TRANSFER CONTROL

In this paper, a theoretical analysis of optimum bed thickness operates under mass transfer control for realizing a high efficiency and reaction conversion of an electrochemical reactor has been made based on flowthrough porous electrode (FTPE) configuration. Many models have been used to represent the optimum bed thickness by taking a look into previous works concerned and collecting all related information, data, and models. The parameters that affect the optimum bed thickness have been visualized and reviewed, and almost all of them have been examined by experimental data from different sources and based on the various models. It has been found that the increase in electrolyte flow rate, concentration, limiting current density, and specific surface area reduce the optimum bed thickness, and the increase in electrolyte conductivity, void fraction, and overpotential range increases optimum bed thickness. The most important design parameter that has a great effect on optimum bed thickness is found to be the electrolyte flow rate for any certain operation. It has been concluded that the most appropriate two models to represent the optimum bed thickness of FTPE electrochemical reactor operating under mass transfer control based on the results are those predicted theoretically and stated by Kreysa in (1978) and Doherty et al. in (1996).

An experimental study of boiling two-phase flow in a vertical rod bundle with a spacer grid-Part 2: Effects of vane angle

We performed boiling flow experiments and measured the void fraction in a 3 × 3 rod bundle including a spacer grid with split type vanes using X-ray computed tomography, which provides high-resolution time-averaged void data without disturbing the flow. We studied the effects of mixing vanes with different vane angles, namely, 20°, 29° and 40°, for a mass flux between 535 and 1950 kg/m2 s and the central rod being heated giving a heat flux of 85.7 kW/m2. The experiments were conducted using octafluorocyclobutane (RC318) as the working fluid. The presence of vanes leads to an increase of the cross-sectional averaged void fraction up to an axial position of Z≈0.8Dh. After that, the void fraction decreases until 3Dh<Z<4Dh due to the induced swirl flow, before it increases again. It was further found that the vanes cause a high local void fraction near the spacer, which increases the possibility of DNB occurrence. From this study, it can be concluded that a vane angle of 29° is optimal for two-phase flow, which is consistent with the findings in the literature for single-phase flow.

Numerical study of transient flow characteristics of gas-liquid two-phase flow in inclined upward tube under periodic vibration

The transient flow characteristics of gas-liquid two-phase flow in inclined upward tubes exhibit a high degree of complexity. The tube vibration caused by multiple uncertain factors, such as seawater impacts or subsea earthquakes, poses more significant challenges for flow safety assurance. Thus, it becomes increasingly crucial to explore the transient flow characteristics of gas-liquid two-phase flow in inclined upward tubes under various vibration conditions. In this paper, the transient flow characteristics of gas-liquid two-phase flow in an inclined upward tube under different vibration conditions are numerically studied based on CFD dynamic mesh technique. In the current study, the studied parameters include vibration direction (heaving, transverse and coupling), vibration amplitudes (0.5D, 1.0D and 1.5D), and inlet velocity (jsg=0.5∼1.0m/s; jsl=0.1∼1.0m/s). The transient flow structure, cross-sectional void fraction and pressure drop are analyzed. The findings indicate that the transient flow structure of the slug flow demonstrates periodic variation characteristics under transverse and coupling vibrations, and the cross-sectional void fraction increase. For transition flow, the cross-sectional void fraction increases under transverse and coupling vibrations, while displays an increase initially and subsequently a decrease significant with an increase in the heaving vibration amplitude. Additionally, all considered vibration conditions intensify the pressure drop of the transition flow and decrease the cross-sectional void fraction of the turbulent flow.

Study on acceleration and suppression properties of coal gangue with different void fractions on gas explosion propagation

Gas explosion is one of the main disasters in coal mining. Plenty of coal gangue are generally distributed in the disaster areas in gob. Experiments were carried out to explore the propagation law of the gas explosion distributed by coal gangue. The variation characteristics of the overpressure, pressure rise rate, and flame shape with void fractions were analyzed. The results showed that the effect of the coal gangue on the explosion intensity changed from suppression to acceleration with the increase of void fraction, the flame front upstream blockage area changed from laminar state to turbulent divergent state, and a reverse flame was formed. When the void fraction of the coal gangue was 0.50–0.65, the maximum overpressure downstream of the blocked area were positively correlated with the void fraction and the critical suppression range was between 0.50 and 0.55. When the void fraction was lower than 0.50, the flame was quenched in the coal gangue, neither the flame nor the pressure could pass through the blocked area. It is helpful to guide the improvement of coal recovery process to avoid the expansion of the explosion impact in coalmine gob.

Relationship between two-phase flow in bottom outlet and air-core vortices at intake

Model experiments were conducted on the hydraulic characteristics of air-water two-phase flow inside the bottom outlet of shaft spillways as a result of natural air suction from air-core vortices at the pipe intake. Experiments were performed in a cylindrical reservoir 2 m in diameter and 1 m high, under a swirling flow around the intake, investigating the most severe two-phase flow regimes. Three different vertical pipe intakes, attached to the floor of the reservoir at its centre, were examined for different flow discharges, establishing different vortex types. Results indicate that the void fraction decreases by changing the type of swirling flow. For a constant submergence depth, reducing the intake pipe diameter or increasing its height led to an increase in void fraction and two-phase slug flow wave velocity, length and frequency. However, by the increase in Kolf number, the frequency of slug waves reduced and the two-phase flow pressure inside the bottom outlet was increased. The relationships between two-phase flow hydraulic characteristics and regimes inside the bottom outlet and types of vortices at the intakes will be of interest to both scientific research and design engineers, to increase the efficiency and safety of pressurised waterway systems.

An experimental study of boiling two-phase flow in a vertical rod bundle with a spacer grid. Part 1: Effects of mass flux and heat flux

We conducted boiling flow experiments in a 3 × 3 rod bundle containing a spacer grid with split type mixing vanes where the central rod was heated. Void fraction around the spacer grid was measured with high-resolution X-ray computed tomography. The study focusses on the effects of mass and heat flux on the void fraction downstream of the spacer for mass flux and heat flux ranging from 535 to 1950 kg/m2 s and 42.8 to 107.2 kW/m2, respectively. As the working fluid, refrigerant Octafluorocyclobutane (RC 318) was used and fluid scaling was applied. We found that the void fraction increases as the flow passes through the vanes and then decreases downstream until Z≈4Dh, and then increases again. In addition, we found that the mixing vanes cause a local increase in void fraction even at low heat flux or high mass flux, and that the arrangement of the vanes influences the size and location of the high and low void content regions. We also found that the effect of heat flux on the relative void fraction is more noticeable at high mass flux than at low mass flux. In the second part of the study, which will be presented in a separate paper, we investigated the effect of different vane angles on the void fraction.

Influence of operating parameters on the vortex structure in the main flow passage of the helico-axial multiphase pump

The flow is extremely complex within the main flow channel during the operation of the multiphase pump, resulting in constant changes in the vortex structure, disrupting the orderly flow of the existing flow field, and reducing the performance of the pump. Numerical calculations, supplemented by experimental verification, are used as the main method for investigating influencing factors that affect the vortex structure in the impeller passage of the pump, and vortex structure present on isosurfaces is selected based on Q criterion to study the evolution of vortex structures under different working conditions. Results indicate that the flow rate affects vortex structure generation on the suction side and trailing edge of the blade. With an increase in flow rate, the vortex becomes complete from fine broken structures. Speed has a greater influence on vortex structures in inlet and outlet areas: the higher the speed, the larger the vortex structure attached to the impeller leading edge. The vortex structure is separated from the blade surface at two-thirds of the impeller under gas–liquid two-phase working condition, with an increase in inlet gas void fraction, and the vortex structure expands to the center of impeller passage. Therefore, exploring evolutions of the vortex structure can provide a certain theoretical basis for improving the stability of multiphase pump internal flow.

Experiment on bubble characteristics of turbulent bubbly jets in pipe crossflow

Air-water bubbly jets in crossflow are commonly encountered in various practical applications such as artificial aeration in oceans and submarine pipelines. This study reveals the bubble characteristics for bubbly jets in pipe crossflow by conducting a series of physical experiments. It was found that the horizontal distance from nozzle exit to where bubble centerline touches the top pipe wall can be expressed as a function of the Reynolds and Weber numbers of the ambient pipe flow, and the Reynolds numbers of both the air and water phases of bubbly jets. The centerline gas void fraction and bubble size increase with distances after bubbles touch the top wall, mainly because bubbles are prone to gathering to the centerline to form larger bubbles. Afterwards, the (horizontal) bubble velocity decreases mainly due to the resistance force from the top wall. Good agreement has been achieved between horizontal bubble velocity and the 1/7th power law for water velocity in the pipe, and a correlation was proposed for predicting bubble rise velocity. Furthermore, turbulence characteristics of bubbles (e.g., root-mean-square of bubble fluctuating velocity, bubble turbulence intensity) were investigated for the bubbly jets.

Experimental and Numerical Study on Flow-Induced Vibration of PWR Steam Generator U-Tubes

The flow-induced vibration (FIV) of steam generators (SGs) in pressurized water reactors (PWRs) is a significant problem in the design process. This problem contains issues such as two-phase flow, nonlinear dynamics and fretting wear. In this paper, an experimental setup was established to study the FIV of a newly designed U-tube SG. Numerical simulations, including computational fluid dynamics (CFD) and vibration studies were performed to reveal the detailed mechanisms of FIV. To study the U-bend region of the tube bundles, the experiments were carried out under 23 experimental conditions containing void fractions of 70%, 80%, 90 and 95%. The flow fields of the tested model in each condition were calculated to reveal the causes of the vibration phenomenon, and the vibration results were calculated through structure dynamics. The experimental results showed that the vibration amplitudes of the U-tube increased with increasing inlet velocity, flow rate and bending radius. It decreased with increasing void fraction under the same inlet velocity, while it increased with increasing void fraction under the same flow rate. Along with the out-of-plane vibration, the in-plane vibration also featured high amplitude vibration of the U-tubes due to lack of support by the anti-vibration bars (AVBs). The simulation results show the same variation trends of the vibration responses as the above experiments. The prototype SG can be fluid-elastic stable in given working conditions, and the main vibration mechanism of this model is turbulence buffeting. The experiment and simulation methods and data can be used in the design process of new SGs similar to this prototype.