Void Fraction Change Research Articles

Particle crushing mechanism of recycled aggregate with different component categories: applied to pervious concrete base

This paper focuses on the particle crushing mechanism of construction solid waste recycled aggregate (CWRA), particularly recycled concrete (RC) and recycled bricks (RB), using natural gravel (NG) as a control group. Compression tests, combined with the Weibull distribution, fractal dimension theory, and relative crushing rate, were conducted to investigate the crushing behavior of these materials under different loads. Results indicate that, under the same load, the crushing degree ranks from highest to lowest as RB, RC, and NG. The Weibull distribution effectively describes the crushing state, with “fragmentation” being the primary mode for most materials, while RC also exhibits “grinding.” As the load increases, the relative crushing rate rises for all materials but slows beyond a certain load threshold, indicating greater difficulty in further crushing. The study identifies a load threshold of 400 kN, beyond which changes in void fraction and relative crushing rates become less significant, particularly in stronger aggregates. Highlights Identified that mortar attached to the RC surface is the primary cause of the increased particle crushing below 0.6 mm. Introduced Weibull distribution parameters a and b to accurately characterize the crushing state of RA particles. Revealed the errors between fractal dimension-based evaluations and the actual crushing state of RC particles. Proposed a two-stage crushing mode for RC. Determined the critical threshold value for the variation of crushing degree with load level.

Spatiotemporal Evolution of Gas in Transmission Fluid under Acoustic Cavitation Conditions

The presence of gas in transmission fluid can disrupt the flow continuity, induce cavitation, and affect the transmission characteristics of the system. In this work, a gas void fraction model of gas–liquid two-phase flow in a transmission tube is established by taking ISO 4113 test oil, air, and vapor to accurately predict the occurrence, development, and end process of the cavitation zone as well as the transient change in gas void fraction. This model is based on the conservative homogeneous flow model, considering the temperature change caused by transmission fluid compression, and cavitation effects including air cavitation, vapor cavitation, and pseudo-cavitation. In this model, the pressure term is connected by the state equation of the gas–liquid mixture and can be applied to the closed hydrodynamic equations. The results show that in the pseudo-cavitation zone, the air void fraction decreases rapidly with pressure increasing, while in the transition zone from pseudo-cavitation to air cavitation, the air void fraction grows extremely faster and then increases slowly with decreasing pressure. However, in the vapor cavitation zone, the vapor void fraction rises slowly, grows rapidly, and then decreases, which is consistent with the explanation that rarefaction waves induce cavitation and compression waves reduce cavitation.

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.

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%.

Measurement and modeling of void fraction for developing two-phase flow of R134a in a horizontal tube after an expansion valve

This paper applies an image analysis technique to quantify the two-phase flow videos after an expansion valve. This method first finds the liquid-vapor interface and then estimates the void fraction and velocity of each phase. This paper also proposes a one-dimensional developing two-phase flow model based on the force-momentum balance. The void fraction of the developing flow predicted by the model shows a similar trend to the quantified experiment data from image analysis: void fraction will decrease and then stabilize as flow develops along the tube. In essence, the change in void fraction and flow velocity is a result of unbalanced force in liquid and vapor phase. This model is also capable of estimating the distance to achieve fully developed flow after an expansion valve. The ability to predict two-phase flow behavior is not only important to the theoretical study of two-phase flow, but also critical to two-phase flow system design to optimize heat transfer and pressure drop.

Numerical study of the cavitating flow over backward facing step with a polydisperse two-phase flow model

We study the cavitating flow over a backward facing step with an incompressible polydisperse cavitation model. The model can predict experimental observations for this flow reasonably well, including the shedding cloud characterized by the condensation front, cavity length, void fraction, and shedding frequency. All model variations produced shedding cavities, but the turbulence model and grid resolution are essential for better predictions, with delayed detached eddy simulation (DDES) performing better than Reynolds-averaged Navier–Stokes. Quantities, such as pressures at key points, maximum void fraction location, and shedding frequency, are mildly sensitive to those factors. Finer DDES grid resolution, crucial to resolve small vortices where cavitation occurs in their low pressure cores, improves predictions. Since a fully incompressible model produces a condensation front that follows well the experimental trends, it is concluded that compressibility is not a necessary condition for the formation of a condensation front. Consequently, the speed of sound in the mixture does not appear to play an important role in the front formation and evolution. The polydisperse nature of the model allows prediction of the bubble size distribution. Small bubbles concentrate on the downstream section of the cavity, where cavity collapse is strongest and bubble fission is most intense, while larger bubbles reside near the step where the flow is milder. The condensation front is a moving source of vorticity for the liquid phase where the “compressibility,” in the sense of mixture density changes due to void fraction changes, and baroclinic effects are significant, but the buoyancy effect is negligible.

Study on Distribution Law of Gas Phase and Cavitation in the Pressurization Unit of Helical Axial Flow Multiphase Pump

Due to the irregular change of gas void fraction (GVF) in multiphase pumps, the pressure distribution in the pump is often uneven, which leads to the formation of low-pressure area and thus the occurrence of cavitation. In order to study the gas phase and cavitation distribution in the impeller region of a multiphase pump under different cavitation stages and GVF conditions, this study used numerical calculations as the main method and experimental verification as a secondary method to investigate the cavitation phenomenon in the pump under different stages and GVF conditions. The results showed that at different stages, both the volume fraction and the covering area of the gas phase were reduced to a certain extent with the increase in blade height, and the distribution law of the gas phase on the blade changed with the development of the cavitation stage, especially on the blade surface. At different GVFs, cavitation first occurred at the inlet of the blade SS and then extended along the blade streamline from the inlet to outlet, with the volume fraction and distribution of cavitation gradually increasing and then extending to the blade PS. The results showed that the presence of the gas phase inhibited the development of cavitation in the multiphase pump to some extent, and the cavitation performance of the multiphase pump was better in the presence of the gas phase than in pure water conditions. The results of this study provide a theoretical basis for improving the cavitation performance of multiphase pumps.

Experimental study and analysis of design parameters for analysis of fluidelastic instability for steam generator tubing

In this paper, the evaluation method of fluidelastic instability (FEI) of newly designed steam generator tubing in pressurized water reactor (PWR) nuclear power plants is discussed. To obtain the parameters for prediction of the critical velocity of FEI for steam generator tubes, experimental research is carried out, and the design parameters are determined. Using CFD numerical simulation, the tube array scale of the model experiment is determined, and the experimental device is designed. In this paper, 7 groups of experiments with void fractions of 0% (water), 10%, 20%, 50%, 75%, 85% and 95% were carried out. The critical damping ration, fundamental frequency and critical velocity of FEI of tubes in flowing water were measured. Through calculation, the total mass and instability constant of the immersed tube are obtained. The critical damping ration measured in the experiment mainly included two-phase damping and viscous damping, which changed with the change in void fraction from 1.56% to 4.34%. This value can be used in the steam generator design described in this paper and is conservative. By introducing the multiplier of frequency and square root of total mass per unit length, it is found that the difference between the experimental results and the calculated results is less than 1%, which proves the rationality and feasibility of the calculation method of frequency and total mass per unit length in engineering design. Through calculation, the instability constant is greater than 4 when the void fraction is less than 75%, less than 4 when the void fraction exceeds 75% and only 3.04 when the void fraction is 95%.

Research on bubbly to slug flow regime transition of air-water co-current upward flow in vertical rectangular channels with different gap sizes

To investigate the transitional criterion of bubbly to slug flow influenced by gap size in rectangular channels, an experimental investigation of air-water two-phase co-current upward flow has been conducted in adiabatic vertical channels under atmospheric conditions. Experiments were carried out in three channels with different cross-sections of 4 × 66 mm, 6 × 66 mm, 8 × 66 mm, respectively. The superficial velocities are 0≤jg≤20m/s and 0.25≤jf≤3m/s for gas and liquid phase. The range of liquid superficial Reynolds numbers is 1900≤Ref≤42000 which have covered purely laminar flow to turbulent flow. Flow regimes were defined according to photos captured by a high-speed camera and characteristics of void fraction measured by flat electrodes, and a random forest algorithm was used to identify the flow regimes at the transitional region. Results show that with the increase of the gap size, the transition from bubbly to slug flow is delayed, and the regime transitional void fraction changes from 0.17 to 0.25. The mechanisms of random collision, wake entrainment, and turbulence breakup were analyzed according to the experimental phenomena. A new criterion of the bubbly to slug flow regime transition applied in rectangular channels was proposed considering the impact of the gap size.

Propagation characteristics of Doppler ultrasonic wave in gas–liquid two-phase flow in an offshore deepwater riser

During offshore deepwater drilling, the strata with abnormal pressure are often encountered and gas invasion and overflow occur easily. If they cannot be detected and controlled in time, blowout may happen and even get out of control, which will cause considerable economic loss and irreparable casualties and may even damage the marine ecologic environment seriously. How to monitor overflow early during deepwater drilling is a global research hot, focus and difficulty at present. In order to provide theoretical guidance for the early monitoring of overflow in the riser during offshore deepwater drilling, this paper designed an experimental device for the early monitoring of gas invasion based on the propagation principle of Doppler ultrasonic wave. Then, the installation mode and angle of Doppler probe were optimized. Finally, the propagation of Doppler ultrasonic wave in the gas–liquid two-phase flow with a void fraction of 0–46% and a liquid flow velocity of 0–0.7 m/s was experimentally studied, and the change laws of Doppler ultrasonic wave with void fraction were revealed. And the following research results are obtained. First, when the void fraction changes, the signal voltage will jump up and down at different amplitudes and frequencies on the basis of initial curve. The signal voltage amplitude increases firstly and then decreases with the increase of void fraction. Second, when the increase amplitude of mean signal voltage caused by multiple reflection is greater than the attenuation degree of ultrasonic wave, the mean signal voltage increases. Otherwise, the signal voltage decreases. Third, the fitting curve of mean signal voltage scatters and void fraction under different flow velocities and void fractions during pump stopping and starting present a change law of quadratic function. In conclusion, void fraction can be quantitatively predicted based on the measured signal voltage, so as to provide guidance for the early monitoring of riser overflow and well kill operation during offshore deepwater drilling.

Theoretical Analysis of Type-I Flow Instability of Natural Circulation

Fukuda and Kobori proposed three types of Leginegg instabilities and five types of density wave instabilities, according to different mechanisms of density wave instabilities. The instability dominated by gravity pressure drop is classified as Type-I flow instability. The instability dominated by friction pressure drop is classified as Type-II flow instability. And the instability dominated by accelerated pressure drop is classified as Type-III flow instability.In present work, the Type-I flow instability between rectangular parallel channels of natural circulation was studied theoretically. Models including two-phase flow instability, natural circulation system components were established in combination based on the homogenous model. A computational program was written. The program was validated with experiments, and the results matched well with the experiment data. The marginal stability boundary (MSB) maps under different parameters were obtained by using nondimensional numbers Nsub and Npch. The influence of different kinds of pressure drop, inlet subcooling temperature of heating channels of type-I flow instability were analyzed separately. And the phenomena were explained from mechanism and the change of void fraction. The results show that the induce of inlet subcooling temperature enhances the stability of system. With the increase in system pressure, the system stability of natural circulation is enhanced.

Experimental Study on Temperature Change by Cavitation Accompanying Self-Pressurization of Propellant for Small Rocket Engines

Abstract As a propellant for hybrid rocket engines using liquid oxidizer and solid fuel and for liquid rocket engines, the use of self-pressurized fluids such as nitrous oxide has become widespread. Since these fluids can be self-pressurized by their high saturated vapor pressure, the propulsion system becomes smaller and simpler. However, this self-pressurization generally forms a gas–liquid two-phase flow by flashing or cavitation. This flow is considered highly unsteady because the temperature and pressure greatly change with the discharge process. In this study, unsteady flow characteristics due to self-pressurization were experimentally obtained by conducting many cold flow tests with carbon dioxide as self-pressurizing fluids. As a result, it was clarified that the fluid temperature dropped about 10–15 K with the pressure drop due to feedline pressure loss during the discharge process. From these experimental results, we estimated the bubble growth and void fraction change that would satisfy the temperature drop. In this paper, the obtained test results and estimated temperature drop are reported.

Reconstruction of the MPD system parameters at the gas kicks into well with the measured surface data in situ by the mathematical modeling

One of the main problems of managed pressure drilling (MPD) is detection and monitoring of gas kicks from the formation to the annulus. The model is proposed to estimate the parameters of the gas kick and gas-liquid flow in a well during MPD process based on measurements performed only at the surface. A gas-liquid flow is described by the kinematic wave model. The ability of gaseous hydrocarbon to dissolve in mud, when drilling on an oil based mud (OBM), is taken into account. The hydrodynamic parameters of the flow are estimated by numerically solving the equations of fluid motion jointly with measured input data: the flowrate of the mud at inlet, the flowrate of gas-liquid mixture at outlet, and the surface back pressure (SBP). The control parameters are the change in mass gas and void fraction over the well depth and on time, as well as BHP. To verify the proposed model, the calculation of gas-liquid flow parameters is conducted on base of the detailed measured data of field test during drilling of 16,000-ft deep well using a water based mud (WBM). The pattern of the gas kick into the well from the formation is reconstructed, the change in void fraction during gas rising was traced until it was kicked to the surface, and the total mass of gas entering the well is determined. The error is estimated by comparing deviations of the measured and calculated bottomhole pressure (BHP) value.

Experimental study of submerged hydraulic jumps generated downstream of rectangular plunging jets

This experimental study analyzes submerged hydraulic jumps generated in a 1.05-m wide stilling basin that receives overflow rectangular impinging jets, with the vertical distance between the sharp-crested weir and the bottom of the plunge pool being 2.20 m. Five submerged hydraulic jumps downstream of non-developed nappe jets were analyzed, with vertical falling distances to break-up length ratios between 0.76 and 0.78. The impingement velocity range was from 6.12 to 6.33 m/s, while the water depth / impingement jet thickness ratios were between 8.45 and 15.00. The velocity field and air entrapped were measured in several cross sections, with the farthest sections being located 1.0 m from the stagnation point. The entrapped air (void fraction and phase change detection) behavior measured with a phase-detection probe was classified as a function of the distance to the stagnation point / impingement jet thickness ratio and/or the mean void fraction of the vertical profiles. The maximum void fractions were observed near the free surface. However, the higher phase change detection frequencies (up to 130-140 Hz) were obtained near the bottom for local void fractions smaller than 0.15-0.20. The non-dimensional velocity distribution seems to be similar to previous published studies, extending their validity range. Furthermore, the free hydraulic jumps formula has been adapted to estimate the air rate in the shear stress layer and the growth of the characteristic length in the plunge pool.

Two-phase flow distribution in a refrigerant distributor having four indoor-unit connections of a variable refrigerant flow system

• Multiple indoor units are connected to a refrigerant distributor of a VRF system. • Distribution of two-phase flow was experimentally and numerically investigated. • Maldistribution of two-phase flow is significantly influenced by void fraction. • CFD simulation was performed to estimate distribution of R410A. • Maldistribution of R410A appeared to be less than maldistribution of air-water. In a variable refrigerant flow (VRF) system, the refrigerant is supplied from one outdoor unit to multiple indoor units through a header or refrigerant distributor. When the two-phase refrigerant flows into the distributor, uneven distribution occurs and deteriorate system performance. An experimental apparatus simulating the refrigerant distributor of a VRF system was constructed and air-water two-phase flow distribution characteristics were investigated. The air-water two-phase flow was more mal-distributed as the void fraction increased. A change in void fraction affected the distribution more than a change in mass flow rate in the experimental range. The flow distribution in the header was little affected by variation of the downstream flow resistance. Previous empirical correlations were investigated to predict the air-water two-phase flow distribution measured in this experiment, but performance was poor. To estimate the refrigerant two-phase flow distribution, computational fluid dynamics (CFD) simulations were performed using R410A as a working fluid. The mal-distribution of R410A appeared to be mitigated compared with air-water flow due to the large vapor phase density of R410A.

Step response of turbulent structure inside a horizontal bubbly channel flow due to sudden change in void fraction

Step response of turbulent structure inside a horizontal bubbly channel flow due to sudden change in void fraction

Studies of two-phase flow through a sudden expansion using electrical capacitance tomography

In this work, electrical capacitance tomography (ECT) was used to measure the changes in void fraction immediately after a sudden expansion for horizontally flowing refrigerant R-134a. The two-phase flow behavior occurring in the sudden expansion was also analyzed using a high-speed camera for two different area ratios of 0.145 and 0.44. Void fractions determined from correlations found in the literature were compared with experimental measurements made immediately after a sudden expansion. It was found that none of the correlations examined could represent the behavior of the void fraction immediately after the expansion. A modified correlation was proposed to model the void fraction behavior occurring right after the expansion. This new correlation was able to predict the void fraction within a ± 20% error band when compared with experimental data at mass velocities of 50 to 250 kg m−2s−1 and area ratios of 0.145 and 0.44.

The effects of the parameters of a refrigeration system working with R600a on the non-equilibrium subcooled two-phase flow of the refrigerant

Abstract Household refrigerators built with vapor compression refrigeration systems use capillary tubes as expansion devices because of their low cost and simple structure. To prevent decreasing mass flow of the refrigerant and ensure the stability of the system, the refrigeration system is designed so that the refrigerant enters the capillary tube in a subcooled liquid state rather than a saturated two-phase state. However, previous researchers have reported that a two-phase flow entered the capillary in a refrigeration system running with R600a even though the refrigerant was under the subcooled temperature condition. The observed two-phase flow was in a thermodynamic non-equilibrium state, which is a mixture of subcooled liquid and subcooled vapor. Experiments were performed to investigate the effects of the refrigeration system's operating conditions on the contents of the non-equilibrium vapor phase of R600a. Experimental results demonstrated that the void fraction was changed by the system parameters in addition to the pressure and temperature. The void fraction of the non-equilibrium R600a two-phase mixture at the condenser outlet was reduced with increases in degree of subcooling, refrigerant mass flow rate, flow resistance of capillary tube, and decrease in thermal resistance on the secondary side of the condenser. The parameter that caused the biggest change in void fraction was the flow resistance of capillary tube (which was changed by a metering valve in front of capillary tube) while the effects of changes in pressure and refrigerant charge were small.

A Concave Electrode Based Capacitive Sensor for Cavitation Monitoring—Experimental Evaluation

Monitoring cavitation in the flow channel of industries like hydro-power plants and industries with large liquid cooling systems, is an important and crucial task. Cavitation is responsible majorly for the metal loss of turbine blades, erosion of the internal walls of pipes/penstock, and when the bubble explodes it creates impulse waves which create disturbance in measurement of flow. Also, the cavitation is the major source of vibration in the pipes while handling a high-speed fluid flow. Measuring cavitation in the pipes can help to schedule the regular maintenance and repairs of the affected system. In present work, the design of a simple non-invasive concave capacitive sensor is presented to measure the cavitation in terms of void fraction. The sensor is designed with the grounded guard electrodes and grounded shield, so it is immune to the stray electrical fields. Electrical models of the capacitive sensor are presented in different cases of void fractions. The proposed sensor is designed and experimentally validated by another concave capacitive sensor and the standard wire mesh sensor. The wire mesh sensor is designed along with its interface circuit. All three sensors are mounted on the same hardware setup, and the experiment is performed multiple times. The capacitance value changes from 16.58pF to 43.43pF for void fraction change of 100% to 0%. The uncertainty analysis is also done and the uncertainty of measurement budget is presented. The standard uncertainty of the proposed capacitive sensor is less than 0.372 pF for the whole range of measurement.

A multi-dimensional dataset for two-phase instability in low pressure natural circulation based on direct transient local measurement

Thermal hydraulic instability can challenge the control and the safety of nuclear reactors. This experimental research captures and investigates the instability in a low-pressure (1–10 atmospheres) natural circulation loop with an adiabatic chimney installed above the heated section. Direct transient local measurement of two-phase flow is performed at multiple axial locations along the flow channel with thermocouples and conductivity probes traversed by customized high-precision mechanisms. Based on the periodicity of oscillations, a post-processing method is proposed to align the data captured non-simultaneously to reconstruct the instability behavior in one representative period. The averaging operation over multiple oscillation cycles is performed to reduce the random errors. Cyclic phenomena are clearly demonstrated by the post-processed data, and rapid void fraction change induced by the flashing is observed to be significant in the driving mechanism for low pressure oscillations. Parametric effects of inlet subcooling, heat flux, inlet flow restriction and system pressure are studied within the experiment range, and qualitative results are explained and compared with existing literature. The ratio between the oscillation period and the travelling time of the fluid particle through the test section is compared with the theoretical value for the density wave oscillation. The new dataset contains directly measured transient multi-dimensional two-phase data along the flow channel for natural circulation under density wave oscillations. The generated dataset is valuable for the validation of system analysis codes and CFD codes in the simulation of natural circulation phenomena and dynamic two-phase flow.