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1.
This paper describes the instability maps for the excursive and the density wave instabilities which may occur in an open two-phase natural circulation loop. Criteria for the excursive and the density wave instability were obtained from the steady loopwise momentum equation and the characteristic equation, respectively. A simple, one-dimensional two-phase homogeneous equilibrium flow was assumed. Heat flux, inlet subcooling, inlet- and exit-line restrictions, condenser liquid level, loop height and the heater section length were taken as parameters. The results were summarized on the instability map in the plane of Npch(Fr)1/2 vs. Nsub. The stable region appears at the lower left part of the map, bounded by two boundaries; the excursive instability region appears at the upper right part of the map. From the map, it can be readily confirmed that the dynamic stability criterion automatically satisfies the excursive stability criterion. Effects of parameters on the stability of the system were also discussed in this paper. The upper boundary of the stable region was compared with the experimental results reported previously. 相似文献
2.
Ph. Mertens H. Altmann T. Hirai M. Knaup O. Neubauer V. Philipps J. Rapp V. Riccardo S. Sadakov B. Schweer A. Terra I. Uytdenhouwen U. Samm 《Fusion Engineering and Design》2009,84(7-11):1289-1293
In the frame of the ITER-like wall project, a new row of divertor tiles has been developed which consists of 96 bulk tungsten load-bearing septum replacement plates (LB-SRP). Exposed to the outer strike point for most ITER-relevant, high triangularity configurations, they shall be subject to high power loads (locally 10 MW/m2 and above). These conditions are demanding, particularly for an inertially cooled design as prescribed. The expected erosion rates are high as well as the risk of melting, especially with transients and repetitive ELM loads. The development is also a real challenge with respect to the inevitable excursions of the tungsten material through the so-called DBTT, ductile-to-brittle transition temperature.A lamella design has been selected to fulfil the requirements with respect to the thermo-mechanical and electromagnetic loads during disruptions (∂T/∂z ≤ 5 × 104 K/m vertically, induction rate of change ∂B/∂t ≤ 100 T/s, and Ihalo ≤ 18 kA/module). Care is taken to act on refractory metals solely with compressive forces to a large extent. The dedicated clamping concept is described. Results of a test exposure to an electron beam around 70 MJ/m2 substantiate the resort to ‘high temperature’ materials like – among others – high-grade Nimonic® alloys, molybdenum or ceramic coatings. 相似文献
3.
H. Kordisch R. Bschen J. G. Blauel W. Schmitt G. Nagel 《Nuclear Engineering and Design》2000,198(1-2)
The effect of warm prestressing has been investigated representative for the core weld metal of the RPV Stade. Model experiments on CT specimens show a significant rise of effective fracture toughness Keff after warm prestressing and the conservative WPS hypothesis, ‘no failure, if ∂KI/∂t≤0’, is verified. Partial unloading and reheating show no influence on the effective fracture toughness Keff. The magnitude of the WPS effect as a function of warm prestress level and temperature, path of unloading and cooling can be predicted using a modified Beremin model with temperature dependent parameters. It is shown that the Weibull stress is an appropriate crack tip loading parameter for decreasing load paths. 相似文献
4.
This paper discusses the mathematical models used as a basis for the calculation of critical conditions in two-phase flow. In practice, solutions are obtained with the aid of discretized versions of such models. It has now been established that the portrait of solutions in the phase space of any analytic model does not stand in a one-to-one relation to the map of solutions of any discretized version of such a model. As a result, computer outputs may develop spurious solutions. Branches of such spurious solutions often depart from the correct analytic solution in a haphazard manner, and bear no relation to the correct solution. Such degenerate solutions do not differ from their correct counterparts by merely containing acceptable numerical or truncation errors, but fail to fit the correct solutions even approximately. In the case of the most common analytic models used in two-phase flows through channels of varying cross-sectional areas, spurious solutions develop when the correct solution passes through or close to the saddle point in phase space. The paper examines two modes of obtaining critical-flow solutions. In some cases, a steady-state version (∂/∂t = 0) of the more general model is used. In other cases, the full equation (∂/∂t ≠ 0) is used, and the solution contains a transient but converges to the steady-state mode. This paper shows that the implied critical cross-section locations are identical in either mode of operation. The reason for the occurrence of spurious solutions is found in a term which assumes the indeterminate singular form 0/0 in the steady-state version of the model. By a suitable transformation, it is shown that this term is also present in the factor of ∂σ/∂t in the time-dependent version. It has been shown earlier that solutions passing through the singular point can be obtained with the aid of the theory of dynamical systems. In order to exclude the possibility of the appearance of spurious solutions, the numerical code, in either version, must be so arranged as to start the integration with the singular point. 相似文献
5.
A comparison of critical heat flux (CHF) fuel bundles data with CHF data obtained in simple flow geometries was made. The base for the comparison was primary experimental data obtained in annular, circular, rectangular, triangular, and dumb-bell shaped channels cooled with water and R-134a. The investigated range of flow parameters (pressure, mass flux, and critical quality) in R-134a was chosen to be equivalent to modern nuclear reactor water flow conditions (p=7 and 10 MPa, G=350–5000 kg (m2 s)−1, xcr=−0.1–1). The proper scaling laws were applied to convert the data from water to R-134a equivalent conditions and vise versa. The effects of flow parameters (p, G, xcr) and the effects of geometric parameters (D, L) were evaluated during comparison. The comparison showed that no one simple flow geometry can be used for accurate and reliable bundle CHF prediction in wide range of flow parameters based on local (critical) conditions approach. The comparison also showed that the limiting critical quality phenomenon is unique characteristic for each flow geometry which depends on many factors: flow conditions (pressure and mass flux), geometrical parameters (diameter or surface curvature, gap size, etc.), flow obstructions (spacers, appendages, turbulizers, etc.) and others. 相似文献
6.
W.J. Green 《Nuclear Engineering and Design》1982,72(3)
An empirical correlation has been developed for calculating critical heat flux (CHF) at low mass fluxes for vertical upflow in uniformly heated tubes. The correlation is based upon dimensionless groups. It compares favourably with experimental CHF data for both Freon-12 and pressurized water. When solved iteratively in conjunction with the heat balance equation, an overall mean ratio of predicted to experimental CHF of 0.986 was obtained with a root means square (r.m.s.) error of 7.0%, for the 233 low flow rate data sets examined.The boundary between the high flow rate correlation developed in earlier work and the proposed low flow rate correlation can be specified by a dimensionless factor δ1. For values of δ1 greater than 0.07, the low flow correlation is valid whereas for values less than 0.07 the high flow correlation applies.Development of this correlation and a means of defining its range of validity enables the prediction of CHF levels to be made over an increased range of coolant flow conditions. This is important in the analysis of postulated loss-of-coolant accidents in water-cooled nuclear reactors. 相似文献
7.
A numerical study has been performed to investigate the hydrodynamic aspects of the pool boilingon horizontal-, vertical- and downward-facing surfaces. The FlowLab code, which is based on a Lattice–Boltzmann (LB) model of two-phase flows, is employed. Macroscopic properties, such as surface tension (σ) and contact angle (β), are implemented through the fluid–fluid (Gσ) and fluid–solid (Gt) interaction potentials. The model is found to express a linear relation between the macroscopic properties (σ, β) and microscopic parameters (Gσ, Gt). The simulation results on bubble departure diameter appear to have the same parametric dependence as the empirical correlation. Hydrodynamic aspects of two-phase flow regime transition mechanism are investigated for different surface–coolant configurations. Results of the LB simulation clearly demonstrate that not only the bubble nucleation site density (related, e.g. to the heater surface condition and heat fluxes), but also the surface position have a profound effect on the flow regime (pool boiling) characteristics. The results of the LB simulation of hydrodynamics of two-phase flow on the horizontal surface provide the pictures quite similar to the experimental observation for saturated pool boiling. Two mechanisms of flow (boiling) regime transition on the vertical surface are predicted for the local bubble coalescence at bubble generation site and the downstream bubble coalescence. On the downward-facing surfaces, friction between bubbles and the surface wall is found to significantly enlarge the bubble size prior the bubble slip upwards. This behavior is responsible for the earlier bubble coalescence, and therefore, lowers the maximum heat removal rate, in a similar regime of nucleate boiling on a downward-facing surface. 相似文献
8.
Critical heat flux in a vertical tube at low and medium pressures. Part II — new data representation
An analysis of the physical processes taking place in a dispersed-annular flow which govern dry-out type CHFs has been carried out. The analysis has shown that the number of variables required to describe the critical phenomena can be reduced by the introduction of a new parameter: the length over which dispersed-annular flow takes place, Ldan. In this case only, for a given tube diameter, pressure and mass flux, the critical heat flux may be expressed in terms of a single variable: Ldan. A correlation which may be used to determine this length has also been developed. The representation of the CHF data obtained at low pressures in terms of the coordinate system (Ldan, q″cr) has shown that the dispersion of the data about the regression curves is considerably reduced as compared with the traditional presentation of the critical heat flux as a function of the thermodynamic quality at the end of the heated length. 相似文献
9.
Exact numerical solutions to the boundary layer similarity equations of MHD flow and heat transfer past a semi-infinite flat plate of an incompressible viscous fluid have been presented. The velocity of the fluid U and the magnetic field H0 at a distance from the plate are both assumed to be uniform and parallel to the plate which is considered as isothermal. Velocity, magnetic and temperature fields have been shown graphically whereas the numerical values of ƒ′(0) and {−θ′(0)} are entered in tables. We observe that both ƒ′(0) and {−θ′(0)} decrease with increasing S (magnetic field parameter) and increase with increasing λ (ratio of magnetic diffusivity and viscous diffusivity). 相似文献
10.
Analysis of plume mixing in the annulus of a pressurized water reactor (PWR) are presented. The plume mixing analysis is based on a simple two-dimensional model that accounts for the surrounding flow and confinement. A correlation for entrainment is presented and comparison with experiment is made.Mixed convection resulting from downflow between parallel heated plates is studied experimentally. The experimental system used to obtain the data is described with the scaling rationale for choosing the working fluid. Heat transfer results are presented in terms of a Nusselt number and a correlation is given. Results show an enhancement in heat transfer with increasing GrDh/ReDh2 due to an increase in turbulence intensity associated with the buoyant wall layer. The correlation obtained for GrDh/ReDh2 < 2.29 was found to be NuDh/NuDh,0 = 1 + 2.93 (GrDh/ReDh2)0.54, where where NuDh,0 is given by the Dittus-Boelter correlation. Use of this correlation for GrDh/ReDh2 > 2.29 is not recommended due to an observed flow bifurcation in this neighborhood. 相似文献
11.
Jianhua Wang Yuliang Sun Jianjun Wang E. Laurien 《Nuclear Engineering and Design》2011,241(12):4972-4977
Based on the one-dimension two-phase drift flow model, the numerical simulation of two-phase flow stability characteristic on the test loop (HRTL-5) for 5 MW heating reactor (developed by the Institute of Nuclear and New Energy Technology of Tsinghua University, Beijing) is performed with and without coupled point neutron kinetics. The density wave oscillation instability is analyzed in the system under low pressure at 1.5 MPa and low steam quality less than 10%. The effect of inlet subcooling and heating flux on the system instability is simulated under the system pressure Psys = 1.5 MPa. The numerical results show that there exist two instability inlet subcooling boundaries at different heat flux. The numerical results show good agreement with the experimental results on HRTL-5 without consideration of point neutron kinetics. If coupled with point neutron kinetics, the system will exhibit little difference on instability boundaries from that without considering the nuclear characteristics. But the amplitude and the phase of the oscillation of the thermal hydraulic parameters of the system will be somehow affected in unstable zone if the system is coupled with point neutron kinetics. 相似文献
12.
K. Ioki M. Nagami H. Yoshida H. Yokomizo M. Shimada S. Izumi K. Shinya N. Brooks T. McMahon A. Kitsunezaki 《Nuclear Engineering and Design》1983,74(3):393-397
Heat flux on the Doublet III limiters was measured with an infrared camera and thermocouples during low-q discharges. The total heat load to the limiters increases with
in both Dee and circular plasmas. The peak heat flux on the limiters in low-q discharges of qa* ≈ 2 is ≈ 2 times higher than that in high-q discharges of qa* ≈ 4. Since a low-q discharge is essential in order to have a high-β tokamak reactor in the future, higher heat flux on the limiters may be an inevitable problem. It is proposed that the increase in peak heat flux during low-q discharges can be reduced by modification of the limiter to an asymmetric shape. 相似文献
13.
Multiphase flows consist of interacting phases that are dispersed randomly in space and in time. An additional complication arises from the fact that the flow region of interest often contains irregularly shaped structures. While, in principle, the intraphase conservation equations for mass, momentum, and energy, and their initial and boundary conditions can be written, the cost of detailed fluid flow and heat transfer analysis with explicit treatment of these internal structures with complex geometry and irregular shape often is prohibitive, if not impossible. In most engineering applications, all that is required is to capture the essential features of the system and to express the flow and temperature field in terms of local volume-averaged quantities while sacrificing some of the details. The present study is an attempt to achieve this goal by applying time averaging after local volume averaging.Local volume averaging of conservation equations of mass, momentum, and energy for a multiphase system yields equations in terms of local volume-averaged products of density, velocity, energy, stresses, and field forces, together with interface transfer integrals. These averaging relations are subject to the following length scale restrictions:
dℓL,