Decrease In Void Fraction Research Articles

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.

Study on Multiphase Pressure Wave Velocity Characteristics of Automatic Kill Annulus in Chuanyu Fractured Formation

The virtual mass force, annulus pressure, gas-liquid resistance, gas slippage, void fraction and other factors were considered, the equation was combined with small perturbation theory, the mathematical model of annular multiphase pressure wave velocity for automatic killing the well in fractured formation is proposed. Taking pengzhou pz-5-3d well (vertical depth 5827m) as an example, the model is solved programmatically with the semi-display difference method, and the results show that: The gas of fractured formation is characterized by slug flow. With the increase of void fraction, the pressure wave velocity firstly decreases and then increases. When the void fraction is between 0% and 16%, The pressure wave velocity is mainly liquid projectile, and the pressure wave velocity decreases sharply. When the void fraction is between 16% and 40%, and the pressure wave velocity tends to be flat and constant. When the void fraction is between 42% and 100%, and the pressure wave velocity shows an increasing trend, and the pressure wave velocity is mainly aeroelastic. As the annulus well depth decreases, the void fraction decreases and the pressure wave velocity decreases. The pressure wave velocity increases with the increase of the back pressure of the kill circulating exhaust wellhead. When the void fraction is between 0% and 13%, the gas slippage velocity has little influence on the pressure wave velocity. In the range of 13% to 85%, the pressure wave velocity decreases with the increase of gas slippage velocity. The time interval of throttle regulating valve follows the response time of bottom hole pressure. The time interval of throttle regulating valve increases with the increase of response time.

Onset of flow instability with one-side heated swirl tube for fusion reactor safety

In this study, the onset of flow instability (OFI) heat flux of a one-side heated swirl tube is experimentally investigated. The OFI heat flux means the minimum heat flux that can cause flow instability by the vapor generated in the flow path. An analysis of the effect of system parameters on the OFI heat flux indicates that as the pressure increases, the bubble size decreases. Therefore, the void fraction decreases and, consequently, the OFI heat flux tends to increase. Similarly, the higher the flow rate and degree of subcooling, the faster the vapor can be removed; thus, the OFI heat flux increases. In addition, the prediction performances of the existing OFI correlations developed under the subcooled flow-boiling condition are evaluated. Therefore, although the Wang correlation indicates the lowest error rate, it yields a high mean absolute error rate of 87.75%. Thus, it is difficult to predict the OFI heat flux of a one-side heated swirl tube using the existing OFI correlations. Therefore, in this study, a new correlation is developed using a Python code created by employing an artificial intelligence regression method. The developed correlation incorporates the impact of one-side heating, swirl tape, mass flow rate, subcooling, and pressure (mean absolute error = 12.17%, root mean square error = 14.99%).

Atomistic Investigation on the Wetting Behavior and Interfacial Joining of Polymer-Metal Interface.

Polymer-metal hybrid structures can reduce the weight of components while ensuring the structural strength, which in turn save cost and subsequently fuel consumption. The interface strength of polymer-metal hybrid structure is mainly determined by the synergistic effects of interfacial interaction and mechanical interlocking. In this study, the wetting behavior of polypropylene (PP) melt on metal surface was studied by molecular dynamics simulation. Atomistic models with smooth surface and nano-column arrays on Al substrate were constructed. Influences of melt temperature, surface roughness and metal material on the wetting behavior and interfacial joining were analyzed. Afterwards the separation process of injection-molded PP-metal hybrid structure was simulated to analyze joining strength. Results show that the initially sphere-like PP model gradually collapses in the wetting simulation. With a higher temperature, it is easier for molecule chains to spread along the surface. For substrate with rough surface, high density is observed at the bottom or on the upper surface of the column. The contact state is transitioning from Wenzel state to Cassie–Baxter state with the decrease of void fraction. The inner force of injection-molded PP-Fe hybrid structure during the separation process is obviously higher, demonstrating a greater joining strength.

Axial development of air–water annular flow with swirl in a vertical pipe

Annular flow with swirl induced by a swirler has been widely used in industry. Swirl decay which closely related to working performance and length of the flow is significantly important for its application in industry. Axial development of annular flow with and without swirl has been investigated in this work by means of experimentation and modelling. A vertical pipe system with 11 m long and 62 mm inner diameter was carried out to investigate axial development of flow pattern, void fraction and pressure drop in the annular flow with and without swirl. The experimental results show that a swirling annular flow with swirling streak and less disturbed waves is observed downstream of the swirler. Void fraction decreases, total pressure drop increases and the PDF (probability density function) of pressure drop becomes more concentrated in the annular flow with swirl, compared with these in the annular flow without swirl. However, swirling annular flow is gradually transformed to annular flow without swirl along the streamwise direction. To qualitatively predict the decay in an annular flow with swirl, a simplified theoretical model was developed here. The results calculated with the theoretical model were in agreements with experimental results. The obtained results can be used in predicting working performance and length of devices, such as separators and heat exchangers.

Numerical study on formation of an acoustic soliton in bubbly liquids based on weakly nonlinear wave equation

Oscillation of microbubbles in bubbly liquids induces a dispersion effect of waves into weakly nonlinear pressure waves, and its propagation process is described by a KdV-Burgers (KdVB) equation for a long wave case. We numerically predict an evolution of waveform by solving the KdVB equation derived by Kanagawa et al. (2010, 2011) via a finite difference method. As a result, (i) an initial Gaussian waveform is distorted due to a nonlinear effect, and a steepening of waveform is observed on about 20 000 period; (ii) the dispersion effect is observed at a steep part of waveform, and its part becomes sharp as the initial void fraction increases; (iii) the initial waveform change into an attenuated soliton on about 100 000 period due to a dissipation effect, and the attenuation of soliton is noteworthy as the initial void fraction decreases; (iv) balance of the nonlinear, dispersion, and dissipation effects thereby forms an attenuated soliton, and its amplitude strongly depends on the value of the initial void fraction. Furthermore, the period of soliton formation depends on the type of initial waveform. Although the nonlinear, dissipation, and dispersion effects were qualitatively balanced in the KdVB equation, we conclude that these effects independently appear in the sound field.

Influence of Newtonian and non-Newtonian fluid behaviour on void fraction and bubble size for a gas-liquid flow of sub-millimeter bubbles at low void fractions

This study compares the effect of Newtonian and non-Newtonian blood-mimicking test liquids on void fraction and bubble size distribution (BSD) for the case of gas–liquid bubbly flow. Along with this comparison, the novelty of the present work is extended to the examined conditions, which combine low void fractions (<10−1) with bubbles smaller than 1 mm, resembling bubbly flow in human bloodstream during Decompression Sickness. Such conditions, however, can be also found in common two-phase applications such as subcooled flow boiling in macro-channels. Experiments are performed in co-current upward bubbly flow. Void fraction and BSD are determined by a non-intrusive electrical impedance technique and by image analysis of bubbly flow images, respectively. Xanthan gum is employed (0, 150 and 1000 ppm) to impart non-Newtonian (shear-thinning) behaviour to the baseline aqueous glycerol/NaCl test liquid. The role of surface tension (by adding Triton X-100) and gas/liquid superficial velocities (Usg, Usl) on acquired data is studied as well. Results demonstrate that increase of bubble size and decrease of void fraction is primarily due to the non-Newtonian liquid character rather than the increase of dynamic viscosity caused by the addition of high Xanthan gum concentration. The addition of surfactant (Triton X-100) decreases bubble size and increases void fraction. Furthermore, bubble size and void fraction increase with Usg and decrease with Usl. Void fraction signal fluctuations show a strong dependence on bubble size. Furthermore, the performance of an empirical equation that predicts average bubble diameter from void fraction statistical quantities is tested and seems to be unaffected by the Newtonian/non-Newtonian liquid behaviour. Of particular interest are the intense peaks noticed in void fraction signals for the highest examined Xanthan gum concentration (1000 ppm) and the lowest examined liquid superficial velocity (2.89 cm/s). These peaks correspond to voluminous bubbles clusters rising among isolated bubbles, which do not appear at higher liquid velocities.

Role of Hydrogen Flow Rate on Microstructure of a-Si:H(n) Films: Spectroscopic Ellipsometry Studies

The influence of hydrogen flow rate on the microstructure and optical properties of phosphorous doped hydrogenated amorphous silicon (a-Si:H(n)) films is studied through spectroscopic ellipsometry (SE). A series of a-Si:H(n) films were deposited using radio frequency plasma enhanced chemical vapour deposition (RFPECVD) technique by varying the hydrogen flow rate (HFR) in the range of 30–80 SCCM. The SE measurements showed a shift in peak position of pseudo dielectric function of these films from 3.6 eV (for pure a-Si:H) to 4.1 eV with a shoulder peak at 3.4 eV (nanocrystallites embedded a-Si:H) as the hydrogen flow rate was increased. A systematic increase in crystalline fraction along with a decrease in void fraction was observed as HFR was increased up to 70 SCCM during deposition. The results confirm that hydrogen dilution helps in improving the microstructure of films and favours the formation of strong Si–H bonds. Beyond 70 SCCM of HFR, a large flux of hydrogen resulted in an increase in void fraction and disorder in the films.

Unified constitutive model with consideration for effects of porosity and its application to polyurethane foam

Abstract Polyurethane foam (PUF) is a popular insulation material and has crucial mechanical characteristics under compression in the principal loading direction, including nonlinear behavior dependent on strain rate and temperature, degradation of elastic modulus, and decrease of void fraction. Additionally, its insulation performance relies on the blowing agents in voids. These material characteristics enable the evaluation of the structural integrity and performance of PUF insulation systems, if the prediction of these characteristics and the relationship between change in void fraction and insulation performance are possible. In this paper, a unified constitutive model was proposed for this purpose. This model was verified with corresponding experimental results, and the validation showed that the material characteristics can be depicted successfully. Moreover, for the sake of applicability, the model was used in the evaluation of the insulation system in an LNG Carrier.

Bubbly-to-cap bubbly flow transition in a long-26 m vertical large diameter pipe at low liquid flow rate

The concurrent upward two-phase flow of air and water in a long vertical large diameter pipe with an inner diameter (D) of 200mm and a height (z) of 26m (z/D=130) was investigated experimentally at low superficial liquid velocities from 0.05009 to 0.3121m/s and the superficial gas velocities from 0.01779 to 0.5069m/s. The resultant void fractions range from 0.03579 to 0.4059. According to the observations using a high speed video camera, the flow regimes of bubbly, developing cap bubbly and fully-developed cap bubbly flows prevailed in the flows. The developing cap bubbly flow appeared as a flow regime transition from bubbly to fully-developed cap bubble flow in the vertical large diameter pipe. The developing cap bubbly flow changes gradually and lasts for a long time period and a wide axial region in the flow direction, in contrast to a sudden transition from bubbly to slug flows in a small diameter pipe. The analysis in this study showed that the flow regime transition depends not only on the void fraction but also on the axial distance in the flow and the pipe diameter. The axial flow development brings about the transition to happen in a lower void fraction flow and the increase of pipe diameter causes the transition to happen in a higher void fraction flow. The measured void fraction showed an N-shaped axial changing manner that the void fraction increases monotonously with axial position in the bubbly flow, decreases non-monotonously with axial position in the developing cap bubbly flow, and increases monotonously again with axial position in the fully-developed cap bubbly flow. The temporary void fraction decrease phenomenon in the transition region from bubbly to cap bubbly flow can be attributed to the formation of medium to large cap bubbles and their gradual growth into the maximum size of cap bubble and/or cluster of large cap bubbles in the developing cap bubbly flow. In order to predict the N-shaped axial void fraction changing behaviors in the flow regime transition from bubbly to cap bubbly flow, the existing 12 drift flux correlation sets for large diameter pipes are reviewed and their predictabilities are studied against the present experimental data. Although some drift flux correlation sets, such as those of Clark and Flemmer (1986) and Hibiki and Ishii (2003), can predict the present experimental data with reasonable average relative deviations, no drift flux correlation set for distribution parameter and drift velocity can give a reliable prediction for the observed N-shaped axial void fraction changing behaviors in the region from bubbly to cap bubbly flow in a vertical large diameter pipe.

Investigation on Convective Drying of Mixtures of Sewage Sludge and Sawdust in a Fixed Bed

This work is part of a project aiming to develop a renewable fuel for gasification purposes, through convective drying of sludge/wood mixtures. The first step consists of characterizing the behavior of sawdust/sludge mixtures during the application of convective drying. The influence of the mixing step (no mixing versus 30 s at 40 rpm), as well as the sawdust : sludge ratio (1:9, 2:8, 3:7, and 4:6 on a dry basis) and the effect of drying temperature (50, 80, and 110°C) were investigated. In this study, X-ray tomography, a noninvasive imaging technique, is used to assess changes in the volume, void, and exchange surface at the beginning and the end of the drying process. Results first confirm the importance of the mixing step on the drying behavior: the drying rate of the mixed sludge is lower than that of the original sludge. Nevertheless, the addition of sawdust is shown to have a positive impact on the drying process from a mass ratio of 2:8, with drying rates higher than that of the original sludge. With increasing amounts of sawdust, the initial and final bed volumes, initial and final total exchange surfaces, and initial void fraction increase linearly, but the bed volume shrinkage and final void fraction decrease linearly.

Performance of a packed bed solar energy storage system having large sized elements with low void fraction

Packed beds are the commonly used storage units for air-based solar system. Stratification improves the efficiency of collector with the bed and a decrease of void fraction lead to an increase in stratification. Therefore an experimental study in a low range of void fraction is carried out in this paper so as to increase the stratification. Five different shapes of large sized storage elements of concrete were used and arranged differently to obtain four values of void fraction including the lowest possible void fractions that could be obtained for spherical elements. Correlations were developed for Nusselt number and friction factor by taking into account the effect of system parameters on them. The Nusselt number was maximum for spherical elements (ψ=1.0) and at the minimum value of void fraction (0.275) whereas the friction factor is found to be minimum for cubical elements (ψ=0.80) and at the maximum value of void fraction (0.48). The stratification coefficient has been found to be maximum for the minimum value of void fraction (0.275) and for the bed of spheres (ψ=1.0).With spherical elements and for lowest void fraction the thermo-hydraulic performance is having the maximum value.

Influence of Entrapped Air Pockets on Hydraulic Transients in Water Pipelines

The pressure variations associated with a filling undulating pipeline containing an entrapped air pocket are investigated both experimentally and numerically. The influence of entrapped air on abnormal transient pressures is often ambiguous because the compressibility of the air pocket permits the liquid flow to accelerate but also partly cushions the system, with the balance of these tendencies being associated with the initial void fraction of the air pocket. Earlier experimental research involved systems with an initial void fraction greater than 5.8%; this paper focuses on initial void fractions ranging from 0 to 10% to more completely characterize the transient response. Experimental results show that the maximum pressure increases and then decreases as the initial void fraction decreases. A simplified model is developed by neglecting the liquid inertia and energy loss of a short water column near the air-water interface. Comparisons of the calculated and observed results show that the model is able ...

Effect of Volume Fraction of Material on Separation by Density Difference in a Liquid‐Fluidized Bed of Inert Particles

AbstractA liquid‐fluidized bed was used to separate a pure material from a mixture. A quantity of relatively large sized material was immersed in an inert‐particle fluidized bed and the behavior of materials was examined for different liquid velocities. In particular, the volume fraction of the material was varied and its effect on the separation characteristics was examined.The material floats on the inert‐particle fluidized bed when the density of the material is lower than the apparent density of the bed, regardless of the volume fraction of the material. The apparent density of the bed can be adjusted by changing the liquid velocity. The materials in the upper portion of the bed affect the properties of the bed below them, i.e., the void fraction decreases and the apparent density increases in the inert‐particle suspension when materials are present in the upper portion of the bed. Therefore, the materials float on the bed although the apparent density of the inert‐particle suspension obtained from the case without material is less than the density of the material at a relatively high volume fraction of material. This phenomenon occurs more easily for lighter and smaller materials. This means that small inert particles and low liquid velocities are the optimum operating conditions for the separation.

Heat transfer and fluid dynamics of air–water two-phase flow in micro-channels

Heat transfer, pressure drop, and void fraction were simultaneously measured for upward heated air–water non-boiling two-phase flow in 0.51 mm ID tube to investigate thermo–hydro dynamic characteristics of two-phase flow in micro-channels. At low liquid superficial velocity j l frictional pressure drop agreed with Mishima–Hibiki’s correlation, whereas agreed with Chisholm–Laird’s correlation at relatively high j l . Void fraction was lower than the homogeneous model and conventional empirical correlations. To interpret the decrease of void fraction with decrease of tube diameter, a relation among the void fraction, pressure gradient and tube diameter was derived. Heat transfer coefficient fairly agreed with the data for 1.03 and 2.01 mm ID tubes when j l was relatively high. But it became lower than that for larger diameter tubes when j l was low. Analogy between heat transfer and frictional pressure drop was proved to hold roughly for the two-phase flow in micro-channel. But satisfactory relation was not obtained under the condition of low liquid superficial velocity.

In situ 타원법을 사용한 광기록매체용 Ge-Sb-Te 박막의 최적성장조건 연구

타원법(ellipsometry)을 사용하여 광기록 매체용 Ge<TEX>$_2$</TEX> Sb<TEX>$_2$</TEX> Te<TEX>$^{5}$</TEX> (GST) 박막의 성장과정에 따른 타원상수 Ψ와 <TEX>$\Delta$</TEX>를 측정하여. GST 박막의 최적성장조건을 연구하였다. 아르곤기체압력과 DC 출력 그리고 기판의 온도를 변화시키면서 GST 박막을 성장시켰다. 제작된 시료들의 분광타원 데이터를 모델링 분석하여 GST박막의 밀도분포를 구하고 한편으로는 GST 박막이 성장하는 동안 측정한 in situ 타원 성장곡선을 분석하여 박막의 밀도분포의 변화를 추적하였다. 아르곤기체압력이 7 mTorr일 때 박막의 상대적인 밀도분포가 고르게 되었고 DC출력이 증가함에 따라 그리고 기판의 온도가 증가함에 따라 GST 박막의 밀도 균일성은 크게 향상되었다. 주사형전자현미경(SEM)을 사용하여 최적 밀도 균일성을 가지는 성장조건(7 mTorr, 45 W, 15<TEX>$0^{\circ}C$</TEX>)에서 제작된 GST 시료가 가장 균일한 구조를 보여줌을 확인하였다. 균일한 밀도 분포를 가지는 GST 박막의 성장조건 확립을 통하여 여러번 기록/재생할 때 광기록 박막의 안정성을 유지하는데 크게 기여할 것이다. The spectroe-ellipsometric constant <TEX>$\Delta$</TEX>, Ψ and the ellipsometric growth curves at the wavelength of 632.8 nm are collected. These are critically examined to find out the optimum growth condition of phase change <TEX>$Ge_2Sb_2Te_5(GST)$</TEX> thin films as an optical recording medium. GST films are prepared using DC magnetron sputtering technique, under the selected experimental conditions of Ar gas pressure (5 mTorr, 7 mTorr and 10 mTorr), DC power of sputtering gun (15 W, 30 W and 45 W), and substrate temperature (from room temperature to 18<TEX>$0^{\circ}C$</TEX>). Based on the three film model, the density distribution of deposited GST films are obtained versus Ar gas pressure and DC power by analyzing spectro-ellipsometric data. The calculated evolution curves at the wavelength of 632.8 nm, are fit into the in situ observed ones to get information about the evolution of density distribution during film growth. The density distribution showed different evolution curves depending on deposition conditions. The GST films fabricated at DC power of 30 W or 45 W, and at Ar gas pressure of 7 mTorr turned out to be the most homogeneous one out of those prepared at room temperature, even though the maximum density difference between the dense region and the dilute region of the GST film was still significant (~50%). Finally, in order to find the optimum growth condition of homogeneous GST thin films, the substrate temperature is varied while Ar gas pressure is fixed at 7 mTorr and DC power at 30 W and 45 W respectively. A monotonic decrease of void fraction except for a slight increase at 18<TEX>$0^{\circ}C$</TEX> is observed as the substrate temperature increases. Decrease of void fraction indicates an increase of film density and hence an improvement of film homogeneity. The optimum condition of the most homogeneous GST film growth turned out to be 7 mTorr of Ar gas pressure, 15<TEX>$0^{\circ}C$</TEX> of substrate temperature. and 45 W of DC power. The microscopic images obtained using scanning electron microscope, of the samples prepared at the optimum growth condition, confirmed this conclusion. It is believed that the fabrication of homogeneous GST films will be quite beneficial to provide a reliable optical recording medium compatible with repeated write/erase cycles.

Expression of liver-specific functions by rat hepatocytes seeded in treated poly(lactic-co-glycolic) acid biodegradable foams.

Techniques of liver replacement would benefit patients awaiting donor livers and may be a substitute for transplantation in patients whose livers can regenerate. Poly(lactic-co-glycolic acid) (PLGA) copolymers are biodegradable and have been shown to be useful as scaffolds for seeding and culturing various types of cells. In this study, foam disks were prepared from PLGA (lactic-to-glycolic mole ratio of 85:15) by lyophilization of benzene (5% w/v) solutions. These disks were then used as scaffolds for rat hepatocyte culture. Foams were coated with either a type I collagen gel (0.1% w/v), coated with gelatin (5% w/v), or treated with oxygen plasma (25 W, 90 s) to modify their surface chemistry and wettability. The disks were then seeded with rat hepatocytes (10(6)/mL) and cultured for a period of 2 weeks. All surface treatments resulted in increased hydrophilicity, the greatest being obtained by collagen treatment (contact angle < 10 degrees ), and a minimal decrease in void fraction (5%). DNA content after a 2-week culture period increased proportionally with the wettability of the treated foam surface. Urea synthesis in untreated foams averaged 15.3 +/- 2.3 microg/h/microg DNA, which was significantly higher than that for controls, whereas gelatin and collagen treated foams exhibited urea synthetic rates below the control levels at all times. The DNA content decreased significantly by about 50% between days 1 and 12. PLGA foams, treated and untreated, represent a promising scaffold for scaling up hepatocyte cultures.

In situ ellipsometry studies of temperature-dependent Au thin-film growth

We used an in situ ellipsometer to measure the temperature-dependent growth curves, which are Δ versus ψ curves measured in real time, of the Au thin-films deposited under identical conditions, with the exception of the substrate temperature, for 240 s on thermally oxidized Si wafers by magnetron sputtering. The growth curves measured at substrate temperatures of 50, 100, 150, and 200°C, respectively, exhibited distinctive temperature-dependent trajectories; as we raised the substrate temperature, the radius of curvature increased. We were able to analyze the temperature dependent variation of ellipsometric constants by introducing a growth model such that the void fraction decreased as a function of thickness, and also using the results of spectroscopic ellipsometry analysis. We found that the remnant void fraction, the void fraction decrease rate, and the depolarization factor are the major parameters that govern the observed temperature dependence of growth curves.

Two-phase flow structure of sheet cavitation

An experimental study of flow within sheet cavities is performed in a cavitation tunnel equipped with a Venturi-type test section. The flow is investigated by means of a double optical probe allowing void fraction, velocity, and chord length of the vapor structures to be measured. Laser velocimetery, wall pressure measurements, and visualization techniques are also used to characterize the liquid flow around the cavity. The consistency of the experimental results was checked though mass and momentum balances. The effects of Reynolds and cavitation numbers are analyzed. Special attention is given to the dynamic behavior of the flow, and to the vapor flow rate within the cavities. The measurements show a complex two-phase flow characterized by the presence of an extended reversed flow occurring along the solid surface and a regular decrease in void fraction along the cavity. The phase transitions seem to be mainly restricted by the dynamic of the bubbles and thermodynamic effects.

Integral formulation of balance equations for two-phase flow through a sudden enlargement Part 1: Basic approach

A general expression of irreversible losses related to two-phase flows through a sudden enlargement is derived from basic balance equations (mass, momentum and energy). The need to take into account void fraction and quality changes is emphasized for some one-fluid flows. From a comparison with several experimental results, it is concluded that a void fraction decrease downstream of the enlargement must be considered in order to predict satisfactorily pressure variations on the basis of the momentum balance procedure. When expressed in the form of a fraction of reversible pressure recovery, dissipation losses appear to depend strongly on the quality. Their minimum value is observed for a larger quality value when the pressure of the system is increased.