非线性振动下水平通道内气液两相流动研究

在地震等行为产生的非线性振动下，两相流体会影响回路传热并对装置结构进行冲击，因此对气液界面行为的把握对核安全具有十分重要的意义。本文通过将振动装置与两相流实验回路相结合的方法，对非线性振动下水平通道内气液两相流问题进行了实验研究。基于FLUENT平台，结合动网格模型及UDF编程手段建立了数学模型，并对数学模型进行验证。研究结果表明：模拟结果与实验结果具有很好的一致性；振动工况下气液两相流动形式不同于稳态工况，会出现更复杂的气液界面，主要流型有泡状流、弹状流、搅拌流、波状流及环状流；瞬时摩擦压降的波动幅度随振动幅度和频率的增大而增大，且与振动幅度相比，振动频率对其影响更大。     

Nonlinear effects in two-phase flow dynamics

Thermally induced two-phase flow oscillations in uniformly heated boiling channels have been analyzed numerically using a one-dimensional model of two-phase flow. Two different approaches to modeling of subcooled boiling have been considered: a mechanistic model and a profile-fit model. The overall model has been numerically implemented as a computer code, DYNOBOSS, which has been validated against a linear stability analysis code and experimental data.The effects of both modeling assumptions and numerical methods of solution have been studied. It has been shown that the calculated transient response of the boiling channel may be very sensitive to the numerical scheme and spatial discretization, especially for operating conditions in the linearly unstable region. For the range of operating parameters studied, phasic slip has shown a significant stabilizing effect on the system, whereas subcooled boiling has indicated smaller influence. Furthermore, it has been shown that the rate of increase of limit cycle amplitude with channel exit quality is higher for low than high inlet subcoolings.     

Instabilities in Parallel Channel of Forced-Convection Boiling Upflow System, (V)

The density wave instability in a parallel boiling channel system heated electrically has been studied experimentally and analytically by the authors. In our country, the steam generator for LMFBR has been investigated with Power Reactor and Nuclear Fuel Development Corp. as the central figure for its development, and many results of this instability were reported. Their results were different from our ones as regard to the governing factor of the period of flow oscillation in the unstable region and to the effect of the slip ratio on the stability in analysis. A new linear analytical model is proposed in this paper and the analytical results are compared with ones of two-phase analyses based on the same linear method as this model. Subsequently, the effect of the slip ratio on the stability is studied analytically by this model. The parallel boiling channel system is studied experimentally and analytically, using Freon-113 as test fluid heated by hot water as simulation of the SG for LMFBR. The governing factor of the period of flow oscillation is made clear.     

Structurally compatible fluid finite element for solid fluid interaction studies

This study develops a fluid finite element compatible with existing structural finite elements with the ultimate objective of analysing solid-fluid interaction problems. This type of problem is of interest in the design of nuclear components involving geometric complexities and nonlinearities. Employing the weighted residual method, the differential equations governing the pressure distribution in a two-dimensional viscous flow subjected to small amplitude oscillations are discretized on finite element subdivisions of the fluid region. The elemental inertia, damping and volumetric fluidity matrices are computed for plane, axisymmetric, triangular and quadrilateral fluid elements. These matrices are then assembled for all the system elements and the final differential equations are integrated numerically in time to obtain the pressure and velocity distribution in the flow region. The analysis is verified analytically by solving a wave propagation flow problem consisting of fluid between two flat plates initially at rest and accelerated suddenly by applying a step pressure at one end.     

Modeling and analysis of hydrodynamic instabilities in two-phase flow using two-fluid model

Because of the practical importance of two-phase flow instabilities, especially in boiling water nuclear reactor technology, substantial efforts have been made to date to understand the physical phenomena governing such instabilities and to develop computational tools to model the dynamics of marginally-stable/unstable boiling systems. The purpose of this paper is to present an integrated methodology for the analysis of flow-induced instabilities in boiling channels and systems. The major novel aspects of the proposed approach are: (a) it is based on the combined frequency-domain and time-domain methods, the former used to quantify stability margins and to determine the onset of instability conditions, the latter to study the nonlinear system response outside the stability boundaries identified using the nearly-exact results of the frequency-domain analysis; (b) the two-fluid model of two-phase flow has been used for the first time to analytically derive the boiling channel transfer functions for the parallel-channel and channel-to-channel instability modes. In this way, the major characteristics of a boiling system, including the onset-of-instability conditions, can be readily evaluated by using the qualitative frequency-domain approach, whereas the explicit time-domain integration is performed, if necessary, only for the operating conditions that have already been identified as unstable. Both methods use the same physical two-fluid model that, in one case, is linearized and used to derive a rigorous analytical solution in the complex domain, and, in the other case, is solved numerically using an algorithm developed especially for this purpose. The results using both methods have been compared against each other and extensively tested. The testing and validation of the new model included comparisons of the predicted steady-state distributions of major parameters and of the transient channel response against experimental data.     

Propagation properties of broadband terahertz pulses through a bounded magnetized thermal plasma

An analysis of THz waves propagation in dense, collisional, thermal, magnetized and bounded plasma is presented. By introducing the dielectric constant of a warm magnetoplasma and using the method of impedance transformation with multiple dielectrics, the coefficients of power reflection (R) and absorption (A) are derived for a bounded plasma model by a lossless plate and a conductor plate. The effects of electron temperature, collision frequency, external magnetic field, electron density and thickness of the plasma slab on the absorption coefficient are analyzed numerically. It is found that these plasma parameters can cause significant change in the value of A. Some phenomena, for example negative power absorption, upper-hybrid resonance absorption and geometric resonances absorption, are observed and the behavior of the THz wave propagation inside the plasma model is explained numerically and physically.     

水基磁性流体池沸腾传热强化的实验研究

通过水和水基磁性流体池沸腾传热的对比实验,确定了水基磁性流体强化沸腾传热的效果,并进行了机理分析。实验结果显示,热流密度相同时,水基磁性流体的沸腾换热系数比水至少增强2倍,施加磁场可进一步强化沸腾传热,增强倍数可超过5倍。通过分析磁场对磁性流体中沸腾汽泡的影响,认为施加磁场有使汽泡脱离直径减小,生长速度加快和脱离频率增加的作用。     

Pressure wave phenomena in bubbly liquid

Propagation of shock waves in dilute bubbly liquids is investigated numerically taking into account internal phenomena inside the bubbles. Governing equations for the bubbly liquid are formulated with emphasis on the radial and transverse motions of the bubbles. A numerical method, where individual bubbles are traced to estimate the effects of transverse motions and volumetric changes on the wave phenomena, is developed. Numerical results under several conditions reveal that the radial motion of the bubbles, which is affected by the internal phenomena, such as thermal conduction through the bubble wall, have significant influences on the change of propagation velocity of the shock wave and relaxation phenomena behind the wave. Slippage between bubbles and liquid do not have so much influence on the wave phenomena as the thermal conditions inside the bubble. The nondimensional thermal diffusivity,

is one of the most effective parameters to be correlated with the wave propagation processes, such as the propagation velocity and the waveform change with the wave propagation.     

An equivalent linearization technique for two-phase flow instability analysis

An equivalent linearization technique has been developed to analyze two-phase flow instability oscillation with finite amplitude in the frequency-domain. A one dimensional slip flow model has been used to formulate equivalent linear equations, and implemented into a computer program.A simple boiling channel has been analyzed by the program. Results for very small amplitude oscillations agreed well with results by a detailed linear frequency-domain program.Analyses for larger amplitude oscillations gave more stable results. Limit cycle amplitudes have been calculated from the amplitudes at which channel characteristics change from unstable to stable as the amplitude is increased.     

Pressure wave propagation in air-water bubbly and slug flow

Related to nuclear reactor safety problems, such as the loss of coolant accident caused by some small crevasses in nuclear reactor, choked flows after postulated breaks of hot and cold legs of pressurized water reactors and the boiling flow instability in parallel channels, the characteristics of pressure wave propagation were investigated experimentally for the air-water bubbly and slug two-phase flow in a vertical pipe. Pressure wave was generated from the small pressure disturbance by the up-and-down movement of piston in the test section. Air void fraction was up to 0.7 and superficial liquid velocity was up to 1.5 m/s as experimental conditions. The experimental results show that the pressure wave propagation velocity in bubbly flow decreases acutely with the increase of air void fraction from 0 to 0.05. In slug flow, it is constant when the air void fraction is less than 0.5 but increases gradually when the void fraction increases beyond 0.5. The attenuation coefficient of pressure wave increases with the increase of air void fraction in bubbly flow. The dependency of pressure wave propagation velocity on angle frequency ω in air-water flow shows the dispersion characteristic. The propagation velocity and attenuation coefficient increases gradually with the increase of angle frequency. However, the increase vanishes slowly as the angle frequency reaches 250 Hz in bubbly flow. The propagation of pressure wave in bubbly flow is independent of the superficial velocity of fluids in the range of experiment.     

Non-linear dynamics of a two phase flow system in an evaporator: The effects of (i) a time varying pressure drop (ii) an axially varying heat flux

In this paper we study the phenomena of density wave oscillations (DWO) in a vertical heated channel. The homogeneous equilibrium model is used to simulate the flow in the two-phase region. The equations are solved numerically using a ‘shooting-method' technique. This in its turn employs an implicit backward finite difference scheme. The scheme can incorporate the movement of the interface. It is very elegant and does not involve storage of variables in large N×N matrices. This scheme is sufficiently general and can be used to simulate the dynamic behaviour when: (i) the heat flux imposed at the surface is non-constant, i.e. exhibits an axial variation; and (ii) the imposed pressure drop is varied periodically at a fixed frequency. A possible explanation for the conflicting reports of the effect of a periodic variation in heat flux is provided using a linear stability analysis and the D-partition method. The interaction of the natural frequency of the DWO and the fixed forcing frequency of the imposed pressure drop gives rise to various phenomena viz relaxation oscillations, sub-harmonic oscillations, quasi-periodic and chaotic solutions. To aid the experimentalist describe this infinite-dimensional system on the basis of his experimental results we discuss the characterisation using only the velocity time series data. This is done employing the method of delay coordinate embedding. The phase portraits, stroboscopic map and correlation dimension of the actual attractor are compared with that of the reconstructed attractor from the velocity time series.     

The role of interfacial friction on the shock wave propagation in stratified gas-liquid flow inside pressure tubes

The admission of the interfacial friction factor in the analysis of the shock wave propagation in the horizontal two-phase stratified flow systems is observed to weaken the shock wave travelling through the phases. A quasi-steady energy balance alludes the importance of the inclusion of the frictional loss in the shock wave phenomena in the horizontal two-phase flows. Finally the problem is dealt with a fuel bundle placed inside the pressure tube, as in a CANDU system to demonstrate experimentally the adverse effect of the pressure transients inside the fuel bundles during a LOCA (loss of coolant accident).     

Modelling and computation of cavitation and boiling bubbly flows with the NEPTUNE_CFD code

This paper focuses on the modelling and the numerical simulation with the NEPTUNE_CFD code of cavitation phenomena and boiling bubbly flows.Compressible, unsteady, turbulent 3D two-phase flow is computed by the NEPTUNE_CFD solver, developed jointly by EDF R&D and CEA. The numerical approach is based on a finite-volume co-located cell-centred approach and makes use of an original pressure-based multi-field coupling algorithm [Mechitoua, N., et al., 2003. An unstructured finite volume solver for two-phase water/vapour flows modelling based on an elliptic oriented fractional step method. In: Proceedings of the 10th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics (NURETH 10), Seoul, Korea].The cavitation nuclei come from wall nucleation or are pre-existing in the flow. Generated vapor bubbles are advected by the flow and expand in the regions where the local pressure is below the saturation with a tendency to agglomerate into slug bubbles.The model predictions compared with experimental data on enough selective local variables showed that satisfactory agreement could be obtained without any floating parameter to fit the data.The second part of the paper deals with boiling bubbly flow through a mixing device representing the effect of a fuel assembly spacer grid equipped with mixing blades (DEBORA-mixing experiment, CEA, Grenoble). Local measurements of the void fraction are provided downstream the mixing enhancer. The computations compare favourably with the experimental results; in particular, the global effect of the mixing blades was observed. A modification of the classical nucleate boiling model is proposed to overcome the strong model sensitivity with respect to near wall grid refinement.     

Water hammer phenomena in a one-component two-phase bubbly flow

Water hammer phenomena caused by a rapid valve closure in a one-component two-phase bubbly flow were investigated experimentally. The experiments were conducted in a horizontal tube of 21.4 mm in inner diameter and 16.17 m long in ranges of superficial liquid velocity j_l from 0.5 to 1.5 m/s and of pressure from 0.25 to 1.5 MPa, using a refrigerant (R113), ethanol and water as the working fluid. The profiles of transient pressure, value of potential surge, and propagation velocity of the compression waves were obtained and discussed, comparing them with those in an air-water two-component two-phase flow. In particular, the pressure transient in a one-component two-phase flow is characterized by the existence of an exponential pressure decay just after the initial pressure rise. This phenomenon is closely related to the interface mass transfer.     

垂直上升管内泡状流压力波传播

研究了垂直上升管内气液两相泡状流压力波的传播速度和衰减规律,为了提高压力波测量精度．实验中设计了不影响两相流动结构的调频式压力扰动装置．实验结果表明,随着含气率的增加,泡状流中压力波波速开始陡降,当含气率大于0．05以后波速缓慢下降;衰减系数随含气率的增加连续增加：工质的流速对压力波的传播没有影响;压力波的传播速度及其衰减与扰动频率有关．随着扰动频率的增加,波速及其衰减都增加本文实验验证了泡状流压力波色散特性的临界频率现象．即高于临界频率．压力波的色散特性消失．在本试验条件和参数范围内．临界扰动角频率为300Hz．     

Boundary-layer effect on pressure variations in expansion waves

In order to estimate the pressure load on a reactor structure during a blowdown, pressure variations in expansion waves propagating through a tube were numerically calculated and compared with experimental results. The expansion wave was divided into an inviscid main flow and a boundary-layer flow, and a coupled calculation was performed by using a finite difference method. The numerical results were in good agreement with the experimental results. The reduction of the effective cross-sectional area of the flow due to the development of the wall boundary layer was found to be important pertinent to the evaluation of the pressure variation in the expansion wave.     

细长自然循环系统流动不稳定性实验研究

以水为工质,在常压下对拥有细长回路和较长水平段的自然循环系统进行可视化实验研究,并以典型的实验现象（ P =1.46 kW）为例分析该系统的瞬态运行特性和不稳定性机理。结果表明：阻力系数较大的细长自然循环回路难以产生有效的单相自然循环,只能通过间歇性沸腾和两相流动将热量导出。这是因当回路阻力较大时,过冷沸腾产生的驱动力无法驱动回路产生有效的自然循环,而只有当加热段内流体发生饱和沸腾时才能驱动系统产生循环流动。较大的回路阻力和沸腾过程中产生的系统降压闪蒸是细长自然循环系统难以维持稳定的流动驱动压头从而产生间歇性沸腾和强烈流动不稳定性的根本原因。