Quasi-homogeneous Flow Research Articles

Experimental study of hydrodynamics and heat transfer in two-phase natural circulation loop with reference passive cooling systems of nuclear power plants

In spite of long year practical use of natural circulation systems and not less long period of study working characteristics of these systems the problem of reliable predicting calculations of natural circulation loops (NCL) with boiling coolant at low reduced pressures is not likely can be considered as being solved. At the same time the interest to solution of this problem arises today, because low pressure natural circulation loops are considered as main type of post accident passive cooling systems for nuclear power plants. The main difficulties for calculations are concerned with determining friction pressure losses, which are strongly depended on two-phase flow pattern. At low reduced pressures large difference between liquid and vapour (gas) specific volumes predetermines the strong change of flow pattern even at small change of mass flow quality. An attempts to calculate local two-phase flow parameters (void fraction, circulation velocity) in the wide range of mass flow qualities without considering the change of flow patterns often lead to large deviations of calculated values from the results of experimental measurements. Another specific feature of two-phase low pressure NCL is hydrodynamic flow instability with circulation velocity pulsations of high amplitude and the occurrence of reverse flows. This also presents problems in developing calculation method. In present paper a method for calculating a low-pressure NCL has been developed, in which local two-phase flow parameters (void fraction, the phases velocities, pressure) are calculated using a modified homogeneous model with taking into account the distribution factor and the phases slip and a model of an annular-dispersed flow with considering droplets entrainment and deposition. A modified homogeneous model was applied for description quasihomogeneous flow regimes. At high values of void fraction annular-dispersed flow model was used. Recommendations for change from one model to another in practical calculations have been formulated and verified. The proposed calculation method has been verified by the comparison the calculated thermo-hydraulic characteristics of laboratory natural circulation loop with the experimental data. The experiments have been carried out for boiling in the loop of three different liquids – water, ethanol and perfluorohexane (FC-72) at atmospheric pressure. The comparison of calculated and experimental results showed their good agreement.

Turbulent Transitions in Optical Wave Propagation.

We report the direct observation of the onset of turbulence in propagating one-dimensional optical waves. The transition occurs as the disordered hosting material passes from being linear to one with extreme nonlinearity. As the response grows, increased wave interaction causes a modulational unstable quasihomogeneous flow to be superseded by a chaotic and spatially incoherent one. Statistical analysis of high-resolution wave behavior in the turbulent regime unveils the emergence of concomitant rogue waves. The transition, observed in a photorefractive ferroelectric crystal, introduces a new and rich experimental setting for the study of optical wave turbulence and information transport in conditions dominated by large fluctuations and extreme nonlinearity.

Void Fraction, Wall and Interfacial Friction Forces for Two-Phase Flow in Microchannel

The flow characteristics of the two-phase flows in a 100 µm circular horizontal microchannel have been investigated experimentally and analytically. In the experiments, nitrogen gas and one of four liquids (distilled water and 4.8, 49, 100 wt% ethanol in water) were injected through a T-junction type mixer to the microchannel, having smaller diameter than that of the mixer. Both quasi-homogeneous flow and quasi-separated flow was observed depending on gas and liquid flow conditions. Furthermore, the data on void faction, frictional pressure drop and interfacial friction force were found to be larger in the quasi-homogeneous flow than the quasi-separated one. In the analysis, empirical correlations of the liquid-wall friction force and the gas-liquid interfacial friction force were developed respectively for the quasi-homogeneous and the quasi-separated flows. By substituting these correlations into an analytical code based on a one-dimensional two-fluid model, we calculated the void fraction. The calculations reasonably agreed with the data irrespective of the quasi-homogeneous and the quasi-separated ones.

Void Fraction, Wall and Interfacial Friction Forces for Two-Phase Flow in Microchannel

The flow characteristics of the two-phase flows in a 100μm circular horizontal microchannel have been investigated experimentally and analytically. In the experiments, nitrogen gas and one of four liquids (distilled water and 4.8, 49, 100 wt% ethanol in water) were injected through a T-junction type mixer to the microchannel, having smaller diameter than that of the mixer. Both quasi-homogeneous flow and quasi-separated flow was observed depending on gas and liquid flow conditions. Furthermore, the data on void faction, frictional pressure drop and interfacial friction force were found to be larger in the quasi-homogeneous flow than the quasi-separated one. In the analysis, empirical correlations of the liquid-wall friction force and the gas-liquid interfacial friction force were developed respectively for the quasi-homogeneous and the quasi-separated flows. By substituting these correlations into an analytical code based on a one-dimensional two-fluid model, we calculated the void fraction. The calculations reasonably agreed with the data irrespective of the quasi-homogeneous and the quasi-separated ones.

Numerical Simulation of Transient Vaporous Cavitating Flow in Horizontal Pipelines

A mathematical model which describes the transient flow with vapour cavitation is presented. Homogeneous bubbly cavitating two-phase flow will occur as a consequence of pressure reduction. By analogy with the gas release phenomenon, when the liquid pressure in the pipe drops below the saturation vapour pressure, vapour will be released from the liquid according to Henry's law. The distribution of the formed vapour bubbles is assumed to be uniform and the vapour within the bubbles acts isothermally. The two constitutive equations of continuity and momentum, written in conservation form, yield to a quasi-homogeneous flow model which have the same form inside as well as outside the cavitation region, but with different density expressions as pressure functions. A two-step finite differences numerical scheme is applied for the computation of the transient cavitating flow. Computation results are favourably compared with some numerical examples obtained from relevant literature. Consistent agreement has been obtained between numerical predictions and experimental measurements in prototype pipelines.

High viscosity gas fluidization of fine particles: An extended window of quasihomogeneous flow

We explore the role of gas viscosity in the behavior of gas-fluidized beds of fine powders by means of experimental measurements using nitrogen and neon as fluidizing gases, and theoretical considerations. The existence of a nonbubbling fluidlike regime has been recently observed in beds of fine powders fluidized with nitrogen. Our experiments with neon reveal a discontinuous transition from heterogeneous fluidization to a highly expanded homogeneous fluidization state. We point out that increasing gas viscosity enhances the coherence of agglomerate swarms, which promotes a local void-splitting mechanism, thus improving the uniformity of fluidization. Our theoretical analysis predicts that further increase of gas viscosity would produce a full suppression of the bubbling regime, i.e., the uniformly fluidized bed would undergo a direct transition to a turbulent regime as seen in beds of nanoparticles fluidized by nitrogen and in liquid-fluidized beds of moderate-density beads.

Effects of Gas-Liquid Inlet/Mixing Conditions on Two-Phase Flow in Microchannels

In order to know the effects of gas-liquid inlet and mixing conditions on two-phase flows in microchannels, adiabatic experiments were conducted with changing a combination of inner diameters for the mixer and the microchannels. In the experiments, horizontal circular microchannels of 100, 176 and 251 μm diameter were used as the test channel, and water and nitrogen gas as the working fluids. It was observed that both “quasi-homogenous flow” and “quasi-separated flow” occurred if the area ratio of the microchannel to the mixer was below a certain value. Furthermore, void fraction and two-phase frictional multiplier were found to be smaller in the quasi-homogeneous flow than the quasi-separated one.

Experiments on Deposition Behavior of Fine Sediment in a Reservoir

The deposition behavior of fine sediment is an important phenomenon, and yet it is unclear to engineers concerned about reservoir sedimentation. Laboratory experiments were conducted to produce both quasi-homogeneous flow and a turbidity current region divided by a plunge section. Silica powder (a noncohesive sediment) and kaolin (a cohesive sediment) were used as the suspended material. Because the effective gravitational force is the primary driving force for velocity in turbidity currents, the velocity profile was closely related to the concentration profile. The deposition rate of noncohesive coarser particles exponentially decays along the flow path. Most of the coarser particles were deposited in the quasi-homogeneous flow region or within a small distance downstream of the plunge section. The plunge did not carry those coarser particles further downstream. Deposition in the region of the turbidity current was found mainly by cohesive particles. Hydraulic sorting exists in the quasi-homogeneous flow...

Plastic deformation in glassy polymers by atomistic and mesoscopic simulations

We present here a short summary of two rather different, but complementary, simulations of plastic deformation in a flexible chain glassy polymer. In the first, atomistic simulation, a molecular structure model is used in which well‐established force fields between atoms and atom groups on a typical chain polymer are introduced to account for the most relevant molecular degrees of freedom that govern both structural and shear relaxations. The principal result of this simulation which has been described in great detail elsewhere [Mott et al., Philos. Mag. 67, 931–978 (1993a)] is that shear relaxations are in the form of abrupt shear transformations occurring in volume elements of ∼10 nm size and resulting in transformation shear strains of ∼2%. This simulation has established that the chemically specific conformational constraints are internal to the volume elements and that the interaction of the elements with their surroundings is elastic, suggesting that the phenomenon can be meaningfully simulated by a two‐dimensional mesoscopic model. In the second part of the present communication we summarize the most important findings of the extensive two‐dimensional mesoscopic simulation which we [Bulatov and Argon, Model. Sim. Mater. Sci. Eng. 2, 167–184 (1994a); 185–202 (1994b); 203–222 (1994c)] have performed. These findings include the important effects of disorder related misfit stresses in governing both quasihomogeneous flow at elevated temperatures and localized shear flow at lower temperatures. In addition, we demonstrate that these misfit stresses are the key ingredient that governs the initial deformation transients, as well as the well‐known distributed kinetics of the deformation process in amorphous media in general.

Similarity of Two-Fluid Flow Heated by a Fuel Rod

General scaling laws for transient two-fluid flow heated by a rod are presented. The similarity of these processes can be ensured only by applying the same volume and time scale with identical model parameters. In practice, the requirement of similar interfacial friction terms cannot be fulfilled because of volume (diameter) reducing scale. Numerical examples show remarkable deviations between the state variables (the values of slip) of the prototype and those of the scaled model, especially with unsteady flow rates. The deviation becomes significant when the slip of phase velocities exceeds the range of 1.6 to 1.8.Volume and time scaling can be carried out only if the phase velocities are similar (slip equal to ∼1 in the quasi-homogeneous flow model).Maintenance of the similarity of heat transfer processes of a heated fuel rod may necessitate time scaling. Furthermore, numerical examples are presented for a scale model of a prototype pressurized water reactor, employing the time-scaled homogeneous flow model.