1,721,321 research outputs found
Eigenvalues and Eigenvectors in Computational Modelling of One-Dimensional Flow Dynamics
No full textA Numerical Methodology for the Analysis of Fluid-Dynamic Instabilities: Application to Boiling Channels
No full textDimensionless Parameters in Stability Analysis of Heated Channels with Supercritical Fluids at Imposed Heating Flux and Wall Temperature Conditions
No full textThe paper further explores the suitability of dimensionless numbers proposed in past work for the analysis of flow stability in heated channels containing fluids at supercritical pressure. The achievements obtained by their use in the application to heated channels with imposed heat flux are summarised and the treatment is extended to imposed wall temperature conditions.
Among the aspects considered having relevance for analyses and experiments in support to the design of Supercritical Water Reactors, the following are given particular attention: basis for the selection of the dimensionless numbers; use of dimensionless numbers in defining stability boundaries; applications in fluid-to-fluid comparisons; dimensionless form to be adopted for temperature; identification of dimensionless numbers suitable for setting up heat transfer correlations with supercritical fluids. The lines of further research in the field are finally discussed
On Some Relevant Effects in the Simulation of Flow Stability With Fluids at Supercritical Pressure
No full textThe paper collects and discusses findings emerging from the analysis of systems operating
with fluids at supercritical pressure, with reference to flow stability. In particular, the in-
fluence of heating structures and numerical diffusion on the predicted dynamic behavior is
highlighted, clarifying that results obtained paying little attention to the presence of these
effects should be reconsidered for a better realistic prediction of stability characteristics.
Examples of applications in which truncation error and the presence of heating structures
play an important role are reported, in order to warn about a tendency to underestimate
these effects on the basis of the knowledge of similar phenomena (e.g., in two-phase flow) or
system configurations in which they might play a lesser role. The use of a computational
fluid dynamics (CFD) code in the analysis of a simple single-tube stability problem shows
that models more complex than the usual one-dimensional (1D) ones also show similar
effects. The results obtained by 1D numerical tools developed for the analysis of natural
circulation with supercritical pressure fluids, equipped with the capability to simulate lin-
ear and nonlinear stability with first- and second-order explicit schemes, are then reported.
The discussion of the eigenvalues and the eigenvectors calculated for an existing natural
circulation loop and a single channel highlight interesting aspects that can be helpful in
understanding the results of stability analyses. The CFD code analysis adds additional
aspects of interest for the discussion. [DOI: 10.1115/1.4032595
Continuing Assessment of System and CFD Codes for Heat Transfer and Stability in Supercritical Fluids,
No full textBasing on previous experience in assessing system codes and CFD mod-
els in the prediction of heat transfer and flow stability with supercritical fluids, new
steps of the research are presented in this paper. The new results are related to al-
ready addressed heat transfer problems, revisited by a different computational tool, the
commercial code STAR-CCM+, in front of available experimental data, and to multiple
channel flow stability phenomena, as predicted by the RELAP5 code. After a short
summary of the achievements obtained in previous phases of the research, recent re-
sults are described, pointing out different aspects. In particular, the capability to predict
heat transfer deterioration by the k-ε low-Re turbulent models available in STAR-
CCM+ is assessed, with main emphasis on the presence of heat transfer deterioration
in trans-pseudocritical conditions strongly affected by buoyancy; numerical aspects re-
lated to the implementation of fluid thermodynamic and thermophysical properties in
the CFD code are also addressed. Moreover, parallel channel instability phenomena
are considered, as predicted by the RELAP5 code, describing the instability patterns
obtained in slightly asymmetric channels and the observed numerical stabilising effects
occurring in perfectly symmetric channels. Relevant conclusions and future perspec
tives of the research are finally summarised
On the analogies in the dynamic behaviour of heated channels with boiling and supercritical fluids
No full textIn this paper, the origin and the consequences of a new dimensionless formalism recently introduced for analysing the dynamic behaviour of
heated channels with fluids at supercritical pressures are discussed. A unified view of boiling and supercritical fluid instabilities is proposed, basing
on the argument that, despite the strong differences in their operating conditions, in both cases the relevant dynamics results from the changes in
density that the fluid undergoes along the channel.
The discussion is presently supported only by modelling, performed both by a simplified program and a system code; however, considering the
complete independence of these modelling tools, the close agreement observed between their results provides a reasonable level of confidence in
the proposed conclusions.
Interesting thermodynamic relationships, devised as a by-product of the introduction of the new dimensionless parameters, are also presented,
in the aim to stimulate further studies capable to provide greater insight into the fascinating aspects raised by recognising the intimate similarity
of these two classes of phenomena, which have such a remarkable relevance for present and future nuclear reactor technology
Discussion on the stability of heated channels with different fluids at supercritical pressure
No full textThe paper extends previous work on the stability of heated channels with fluids at supercritical pressure
as predicted by available models.
A set of dimensionless numbers proposed to predict the threshold of instabilities is further discussed
to highlight their capabilities and possible improvements. In particular, it is shown that the choice made
for the reference value of the derivative of specific volume with respect to specific enthalpy is justified as
an extension of classical formulations adopted for boiling channels. Moreover, the degree of universality
to be expected by the use of these dimensionless numbers while using different fluids is clarified; in this
aim four different fluids are considered: water, carbon dioxide, ammonia and refrigerant R23.
In order to provide a clear perspective of the usefulness of the proposed dimensionless numbers for
dealing with different fluids, linear stability maps generated by a previously developed in-house code,
making use of balance equations in dimensionless form, are then compared with the results obtained by
computations performed in dimensional terms. In this aim, both an in-house code and RELAP5 are used.
The reference considered system is a long circular channel with uniform heating and no singular pressure
drops, already addressed in previous analyses, here assumed both in vertical upward and in horizontal
flow conditions. The comparison of the predictions obtained for the different fluids allows to ascertain
the level of applicability of the dimensionless numbers and, as an interesting by-product, confirms the
possibility to encounter static instabilities also in systems at supercritical pressure
A spectral perspective about the accuracy of numerical predictions of flow instabilities
No full textThe paper discusses the usefulness of information provided by the eigenvalues, eigenfunctions and eigenvectors of flow stability problems, aiming to propose in a spectral perspective an evaluation of the accuracy of the related numerical solution schemes. The presented material further develops the results of past applications of numerical techniques in the analysis of the stability of heated systems containing single-phase, two-phase and supercritical pressure fluids. Since in all these applications both nonlinear and linear stability analyses were performed adopting very similar techniques, information on the different modes of oscillation or excursion exhibited by these systems during unstable behaviour was also made available. These spectral data, whose usefulness was up to now not completely exploited, are now considered in a new perspective, by addressing the simplest of the previously addressed problems both by analytical and numerical means. Information on the level of accuracy obtained by numerical solution techniques in capturing the intrinsic dynamics of the addressed flow systems is presented
Heat Transfer Behaviour and Thermohydraulics Code Testing for Supercritical Water Cooled Reactors (SCWRs)
No full textNon applicabil
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