Parallel Channel System Research Articles

Experimental Investigation of Two-Phase Subcooled Liquid Flow Structure under Preburnout Conditions

The article presents the results from experimentally investigating the occurrence conditions and evolution with time of large vapor agglomerates in a rectangular channel during subcooled water boiling. The experiments were carried out at atmospheric pressure with a mass velocity of up to 1200 kg/(m2 s) and subcooling equal to 30–75 K. Agglomerates emerge as a result of coalescence of small vapor bubbles, the merging of which becomes progressively more intense with increasing the heat flux density at the heating wall starting from approximately 0.75–0.80 of the critical flux value. Decreasing the boiling liquid subcooling value facilitates the occurrence of agglomerates. Nucleate boiling may take place, and burnout phenomena (dry spots) leading to wall burnout may emerge in the liquid film between the heating wall and the agglomerate surface. The vapor agglomerates emerging in the flow of subcooled boiling liquid in a channel are burnout onset precursors. When agglomerates emerge in a system of parallel channels with subcooled boiling liquid, the hydraulic stability of its operation is upset.

Study of flow instability in parallel channel system for water cooled blanket

Water is adopted as coolant in many blanket concepts, and the flow condition is designed based on its use in boiling water reactor (BWR) and pressure water reactor (PWR). The flow instability may occur in the water cooled blanket when water undergoes two-phase flow during steady state or under some accidents. The flow rate in one channel will change and oscillate, while the pressure is still constant. In this study, the research of flow instability in parallel channels, including flow excursion and flow oscillation, is performed using the thermal-hydraulic analysis code RELAP5. The phenomenon of flow instability is detected by the method of small disturbance. The different conditions of BWR and PWR are investigated. The effect of arrangement of parallel channels, vertically and horizontally, is also studied under 7 MPa for BWR condition and 15.5 MPa for PWR condition. Finally, the stability map of the four conditions is obtained. The analysis shows that the flow is more stable under PWR condition since the equilibrium quality of stability boundary is higher. The arrangement of coolant channels has different influences under different conditions. Under PWR condition, it will stabilize the system if the flow channels are horizontally arranged. However, the influence of the channel arrangement on the flow boundary is limited under BWR condition. The research work can provide an important reference for design and operation of water cooled blanket, especially for the consideration of high heat removal.

Interaction of density wave oscillations and flow maldistribution for two-phase flow boiling parallel channels

Density wave oscillations (DWO) and flow maldistribution (FMD) are two important instabilities among others in two-phase flow boiling systems. As both the phenomena can lead to undesirable consequences such as channel burnout, the design intent is to avoid the same. DWO is caused by the interaction between single-phase and two-phase region as perturbation propagates slowly in the later region. Whereas in case of FMD, it leads to one channel receiving higher flowrate and other channel receiving lower flowrate for a twin parallel channels system under forced flow condition. Thus, channels subjected to similar thermal hydraulic and geometric condition receive non-identical flowrate. Bifurcation analysis is used to identify DWO and FMD instabilities in this work. DWO is identified by Hopf bifurcation, thus Hopf curve which constitutes Hopf bifurcations represents the DWO boundary. FMD has been observed earlier through numerical simulation but the region which depicts the same in a parametric space has not been demarcated in previous studies. In this work, FMD is identified by a pitchfork bifurcation. This leads to straight-forward identification of FMD boundary as a pitchfork curve. A combined analysis of multiple instabilities using a single stability map can demarcate regions with the dominance of particular instability. It can thus provide a better perspective for system design and system instabilities in terms of suitable operating region selection. A stability map which can capture and demarcate both the phenomena simultaneously i.e, DWO and FMD in a parametric space is the interest of present work. The boundary of DWO and FMD splits the parametric space into three distinct regions. These are DWO region, a region of symmetrical stable solutions and FMD region which contains non-identical (asymmetric) stable solutions. This breakdown of symmetry of the system is observed by non-identical flowrate in the channels. In some cases, boundary depicting DWO and FMD intersects. Hence, a fourth region is also observed which possess the characteristics of both the instabilities. The stability map shows the transition between these regions.

Numerical investigation of the effect of inlet subcooling on flow instabilities in a parallel channel natural circulation boiling system

Some of the modern reactor concepts are based on natural circulation boiling systems. These systems are prone to flow instabilities and parallel channel systems can exhibit peculiar types of flow oscillations. In the present work, the existence and the nature of instabilities in a parallel channel natural circulation test facility (PCNCTF) has been investigated through numerical simulations using the system code RELAP5/MOD3.2. The numerical model is validated using experimental data of PCNCTF. The effect of inlet subcooling on flow oscillations in parallel channels is studied for different system pressure and input powers. Time variations of mass flow rate and void fraction in the channels are examined and the nature of oscillations is analyzed using phase portraits and Fourier power spectra. It is found that there exists a particular range of inlet subcoolings (at a fixed system pressure and input power), above or below which instabilities occur. Mixed mode of oscillations at low degrees of inlet subcoolings and superimposed density wave oscillations are observed at high inlet subcoolings.

Investigation of rotating mode behavior in BWR out-of-phase limit cycle oscillations – Part 2: TRACE/PARCS model and physical explanation

Previous neutronic/thermal-hydraulic (TH) coupled numerical simulations using full-core TRACE/PARCS and SIMULATE-3K boiling water reactor (BWR) models have shown evidence of a specific “rotating mode” behavior (steady rotation of the symmetry line, i.e. constant phase shift of approximately 90° between the first two azimuthal modes) in BWR out-of-phase limit cycle oscillations, regardless of initial conditions and even if the first two azimuthal modes have different natural frequencies. This suggests a nonlinear coupling between these modes; otherwise, the phase shift between these modes would change at a constant rate during the limit cycle. The previous paper (“Part 1”) presented a series of results to examine this rotating behavior with a reduced-order model. The goal of the present study is to provide additional analyses of the predicted rotating mode behavior using higher-fidelity numerical modeling, as well as a physical explanation for why this mode is favored over side-to-side or other oscillatory behaviors from a TH perspective. Results are presented using TRACE and TRACE/PARCS for a small number of parallel channels, which confirmed that the conclusions developed from the reduced-order model remain applicable when applying a full two-fluid, six-equation, finite-volume modeling approach. From these results, a physical explanation has been put forth to explain why the rotating symmetry line behavior is preferred from a TH standpoint, demonstrating that predominantly out-of-phase unstable systems are most unstable when the variation in the total inlet flow rate is minimized (which minimizes the effective single-phase to two-phase pressure drop ratio) and that the rotating mode is the most successful in minimizing this total flow rate variation as compared with the side-to-side case or any other oscillation pattern. The conclusion is that the rotating mode will be favored for any out-of-phase unstable system of parallel channels with no neutronic feedback or relatively weak neutronic feedback. Previous analyses have indicated that systems with sufficiently strong neutronic coupling may favor the side-to-side oscillation mode over the rotating mode; this topic is left as a subject of future investigation.

On the stability of river bifurcations created by longitudinal training walls. Numerical investigation

To maintain a navigable channel and improve high-flow conveyance, engineers have recently proposed constructing longitudinal training walls as an alternative to the traditional transverse groynes. However, previous work has shown that the system of parallel channels created by a longitudinal training wall might be unstable in rivers with alternate bars. Many questions remain unanswered, particularly whether a stable system can be obtained by carefully designing the bifurcation point. This work analyses the stability of the bifurcating system created by a thin longitudinal wall in sand-bed rivers with alternate bars or point bars. The methodology includes performing 102 numerical tests using the Delft3D code to reproduce an idealized low-land river, either straight or meandering. The results show that the system of parallel channels separated by a training wall may indeed become unstable. An important factor is found to be the location of the bifurcation point with respect to a neighboring bar or point bar. The same trends are observed for both constant and variable discharge, in straight and meandering channels. The results suggest that cyclic growth and decline of the bifurcating channels may arise as inherent system behavior, without the need of any additional external forcing. We explain this from changes in the relationship between sediment transport ratio and discharge ratio as the bifurcation evolves. This cyclic behavior can be regarded as a form of system stability and can be obtained by carefully placing the starting point of the longitudinal training wall, and thus the bifurcation point, near the top of a bar.

Long-term morphological developments of river channels separated by a longitudinal training wall

Rivers have been trained for centuries by channel narrowing and straightening. This caused important damages to their ecosystems, particularly around the bank areas. We analyse here the possibility to train rivers in a new way by subdividing their channel in main and ecological channel with a longitudinal training wall. The effectiveness of longitudinal training walls in achieving this goal and their long-term effects on the river morphology have not been thoroughly investigated yet. In particular, studies that assess the stability of the two parallel channels separated by the training wall are still lacking. This work studies the long-term morphological developments of river channels subdivided by a longitudinal training wall in the presence of steady alternate bars. This type of bars, common in alluvial rivers, alters the flow field and the sediment transport direction and might affect the stability of the bifurcating system. The work comprises both laboratory experiments and numerical simulations (Delft3D). The results show that a system of parallel channels divided by a longitudinal training wall has the tendency to become unstable. An important factor is found to be the location of the upstream termination of the longitudinal wall with respect to a neighboring steady bar. The relative widths of the two parallel channels separated by the wall and variable discharge do not substantially change the final evolution of the system.

Numerical simulation of external inertia and compressibility effects on the dynamic instabilities of two-phase boiling flows in horizontal parallel channels

Density wave oscillations and pressure drop oscillations are investigated numerically in two horizontal parallel channels using lumped parameter model assuming a homogeneous two-phase flow model. In this study the effects of external parameters, such as fluid inertia and compressible gases on the stability margins and transient behaviors of parallel channels system are analyzed. The results show, that the fluid inertia and compressible gases have high impact on the stability margins of two parallel channels; in fact, increasing the inlet inertia and outlet compressibility will increase the system stability while increasing the outlet inertia and inlet compressibility decrease its stability.

Study of Inertia and Compressibility Effects on the Density Wave Oscillations of Two-Phase Boiling Flows in Parallel Channels

In this research, a theoretical model is presented to investigate the density wave oscillations (DWOs), in two horizontal parallel channels with lumped parameter model based on two phase homogeneous hypothesis. The parallel channel is composed of the entrance section, heating section and outlet section and the model consists of the boiling channel model, pressure drop model, parallel channel model, constructive model and inertia and compressibility effects, while subcooled boiling effect is neglected and the governing equations are solved by Gear method. The model is validated with experimental data of a single channel flow instability experiment. Then the flow instability in twin channel system is studied under different conditions. This model can analyze the effects of external parameters, such as fluid inertia and compressible gases on the stability margins of density wave oscillations. The results show that, the fluid inertia and compressible gases can significantly change the stability margins of two parallel channels; in fact, the stability behavior of two parallel channel system improves with increasing the inlet inertia and outlet compressibility but, increasing the outlet inertia and inlet compressibility have negative effects the system stability.

The core instability analysis under ocean condition for offshore floating nuclear power plant with neutron coupling based on multi-point model

For offshore floating nuclear power system the ocean conditions, such as rolling, must be considered for safe design. The neutron kinetics decides the power distribution and variation. Both of them will change the characteristics of the core instability. In this paper the ocean condition model and multi-point reactor model are added into the parallel channel system thermal-hydraulic model. A reactor core with 121 assemblies based on QinshanII nuclear power plant is selected as research object. The inherent instability of the system is studied firstly as the basis of comparison. Then the instability boundaries under motion condition and neutron feedback are obtained separately. The joint influence of motion and neutron feedback is studied. The results indicate that ocean condition has very limited effect for such system with so many parallel channels. Neutron feedback in different inlet subcooling regions have different influence tendency. The combined effect is very powerful and should be paid more attention. At last part the spectral analysis method is used to discuss the influence of the coupling calculation method.

Thermally-Triggered Crystal Dynamics and Permanent Porosity in the First Heptatungstate-Metalorganic Three-Dimensional Hybrid Framework.

The hybrid compound [{Cu(cyclam)}3 (W7 O24 )]⋅15.5 H2 O (1) (cyclam=1,4,8,11-tetraaza-cyclotetradecane) was synthesized by reacting the {Cu(cyclam)}2+ complex with a tungstate source in water at pH 8. Compound 1 exhibits an unprecedented three-dimensional covalent structure built of heptatungstate clusters linked through metalorganic complexes in a POMOF-like framework that displays water-filled channels. This dynamic architecture undergoes two sequential single-crystal-to-single-crystal transformations upon thermal evacuation of water molecules to result in the partially dehydrated [{Cu(cyclam)}3 (W7 O24 )]⋅12 H2 O (2) and anhydrous [Cu(cyclam)]0.5 [{Cu(cyclam)}2.5 (W7 O24 )] (3) crystalline phases. These transitions are associated with cluster rotations and modifications in the CuII coordination geometries, which reduce the dimensionality of the original lattice to layered systems but preserving the porous nature. Phase 3 reverts to 2 upon exposure to ambient moisture, whereas the transition between 1 and 2 proved to be irreversible. The permanent microporosity of 3 was confirmed by gas sorption measurements (N2 , CO2 ), which reveal a system of parallel channels made of wide cavities connected through narrow necks that limit the adsorption process. This observation is in good agreement with Grand Canonical Monte Carlo simulations.

The core instability analysis for offshore floating nuclear power plant with nuclear coupling based on diffusion model

For offshore floating nuclear power plant the complex ocean conditions must be considered for safe design and review. And the neutron kinetics decides the power distribution and variation. Both of them will change the characteristics of the core instability. In this paper the ocean condition mode and diffusion models are added into the parallel channel system thermal-hydraulic model. An imaginary 36-channels reactor core is selected as research object. The stable and unstable cases of the system are analyzed firstly. The oscillation characteristics of mass flow rate and total power are studied. Then the instability boundaries under ocean condition and neutron feedback are obtained, which is compared with the inherent boundary. The results indicate that the combined effect have powerful influence on the system instability and should be noticed.

Predicting two-phase flow distribution and stability in systems with many parallel heated channels

Two-phase heat exchangers are used in a variety of industrial processes in which the boiling fluid flows through a network of parallel channels. In some situations, the fluid may not be uniformly distributed through all the channels, causing a degradation in the thermal performance of the system. A methodology for modeling two-phase flow distributions in parallel-channel systems is developed. The methodology combines a pressure-drop model for individual parallel channels with a pump curve into a system flow network. Due to the non-monotonicity of the pressure drop as a function of flow rate for boiling channels, many steady-state solutions exist for the system flow equations. A new numerical approach is proposed to analyze the stability of these solutions, based on a generalized eigenvalue problem. The method is specifically designed for analyzing systems with hundreds of identical parallel channels.The method is first applied to analyze the flow distribution and stability behavior in two-channel and five-channel systems. The asymptotic behavior of the flow stability is then analyzed for increasing numbers of channels, and it is shown that the stability behavior of a system with a constant flow-rate pump curve simplifies to the stability behavior for a constant pressure-drop pump curve. A parametric study is conducted to assess the influence of inlet temperature, heat flux, and flow rate on the stability of the uniform flow distribution solution as well as on the severity of flow maldistribution. Below some critical inlet subcooling, uniform flow distribution is always stable and maldistribution does not occur, regardless of heat flux and flow rate. Above this critical inlet subcooling, there is a range of operating parameters for which uniform flow distribution is unstable. With increasing inlet subcooling, this range broadens and the severity of the associated maldistribution increases.

Analysis of flow distribution instability in parallel thin rectangular multi-channel system

The flow distribution instability in parallel thin rectangular multi-channel system has been researched in the present study. The research model of parallel channel system is established by using RELAP5/MOD3.4 codes. The transient process of flow distribution instability is studied at imposed inlet mass flow rate and imposed pressure drop conditions. The influence of heating power, mass flow rate, system pressure and channel number on flow distribution instability are analyzed. Furthermore, the flow distribution instability of parallel two-channel system under asymmetric inlet throttling and heating power is studied. The results show that, if multi-channel system operates at the negative slope region of channel ΔP–G curve, small disturbance in pressure drop will lead to flow redistribution between parallel channels. Flow excursion may bring the operating point of heating channel into the density-wave oscillations region, this will result in out-phase or in-phase flow oscillations. Flow distribution instability is more likely to happen at low power/flow ratio conditions, the stability of parallel channel system increases with system pressure, the channel number has a little effect on system stability, but the asymmetry inlet throttling or heating power will make the system more unstable.

Experimental studies in a single-phase parallel channel natural circulation system: preliminary results

Abstract Natural circulation systems find extensive applications in industrial engineering systems. One of the applications is in nuclear reactor where the decay heat is removed by natural circulation of the fluid under off-normal conditions. The upcoming reactor designs make use of natural circulation in order to remove the heat from core under normal operating conditions also. These reactors employ multiple vertical fuel channels with provision of on-power refueling/defueling. Natural circulation systems are relatively simple, safe and reliable when compared to forced circulation systems. However, natural circulation systems are prone to encounter flow instabilities which are highly undesirable for various reasons. Presence of parallel channels under natural circulation makes the system more complicated. To examine the behavior of parallel channel system, studies were carried out for single-phase natural circulation flow in a multiple vertical channel system. The objective of the present work is to study the flow behavior of the parallel heated channel system under natural circulation for different operating conditions. Steady state and transient studies have been carried out in a parallel channel natural circulation system with three heated channels. The paper brings out the details of the system considered, different cases analyzed and preliminary results of studies carried out on a single-phase parallel channel system.

STM observation of a box-shaped graphene nanostructure appeared after mechanical cleavage of pyrolytic graphite

A description is given of a three-dimensional box-shaped graphene (BSG) nanostructure formed/uncovered by mechanical cleavage of highly oriented pyrolytic graphite (HOPG). The discovered nanostructure is a multilayer system of parallel hollow channels located along the surface and having quadrangular cross-section. The thickness of the channel walls/facets is approximately equal to 1nm. The typical width of channel facets makes about 25nm, the channel length is 390nm and more. The investigation of the found nanostructure by means of a scanning tunneling microscope (STM) allows us to draw a conclusion that it is possible to make spatial constructions of graphene similar to the discovered one by mechanical compression, bending, splitting, and shifting graphite surface layers. The distinctive features of such constructions are the following: simplicity of the preparation method, small contact area between graphene planes and a substrate, large surface area, nanometer cross-sectional sizes of the channels, large aspect ratio. Potential fields of application include: ultra-sensitive detectors, high-performance catalytic cells, nanochannels for DNA manipulation, nanomechanical resonators, electron multiplication channels, high-capacity sorbents for hydrogen storage.

Evidence of extensive carbonate mounds and sublacustrine channels in shallow waters of Lake Van, eastern Turkey, based on high-resolution chirp subbottom profiler and multibeam echosounder data

In Lake Van of eastern Turkey, the fourth largest soda lake in the world, high-resolution subbottom profiles and bathymetric data acquired in 2004 and 2012 revealed several hundreds of topographic mounds in shallow waters (<130 m) off the historical town of Adilcevaz in the northern lake sector. These structures are characterized by strong top reflections of transparent internal character, and are 10–300 m wide and 0.5–20 m high. Consistent with previous work, they are interpreted as carbonate mounds formed by precipitation from CO2-rich groundwater discharge into the highly alkaline lake. Their age remains to be determined but their alignment along faults suggests tectonic control on their growth. Several sublacustrine channel networks were observed on the eastern shelf of the lake, which connects with onshore rivers. The channels are up to 500 m wide and 20 m deep, and plausibly were formed by fluvial processes during the major lake level drop reported to have occurred by 14 ka in earlier publications. Erosion is common on the channel walls flanked by levees. The channels are presently inactive or abandoned. At a water depth of 100 m, they all merge into a single larger channel; this channel has a sinuous course initially trending southwestward and then northwestward at a water depth of 130 m. Numerous closely spaced small channels (~10–200 m wide, 1–10 m deep) are also seen on the eastern lacustrine shelf, interpreted as denditric and parallel channel systems formed during lake level fall terminating at ~14 ka. Bathymetric data provide evidence of numerous sublacustrine canyons on the western slope of the lake’s northern basin, most likely remnants of relict rivers formed during this lowstand.

TRANSIENT ANALYSIS OF HEAT TRANSFER IN PARALLEL SQUARED CHANNELS FOR HIGH TEMPERATURE THERMAL STORAGE

An investigation on honeycomb solid matrix systems employed for high temperature thermal storage is provided numerically considering two models in the transient regime. The two models are related to a direct model with multiple channels and a porous medium model. The system with parallel squared section channels is described by a conjugated convective-conductive model by coupling the governing equations for fluid and solid matrix. The porous medium is modeled by assuming a Brinkman-Forchheimer-extended Darcy model and the LTNE is assumed. The models for different number of parallel squared channels or pores per unit of length (PPU) are considered. The honeycomb system is considered as an anisotropic porous medium, and assuming that the thermal storage system is adiabatic in order to estimate fluid dynamic and thermal characteristics for different PPU values. The Ansys-Fluent code is used to solve numerically the governing equations for both models. Results in terms of solid and fluid temperature profiles are given for different PPU values and for both models; they show that the two models are in very good agreement. The main consequence is that a honeycomb system can be simulated as a porous medium allowing a simpler numerical simulation also for parallel channel systems with high PPU. It is found that for high PPU systems the charging time decreases and for assigned partial charging time an increase in stored thermal energy is detected increasing the PPU value.

Experimental study of pressure drop oscillations in parallel horizontal channels

Two-phase flow instabilities are an undesirable phenomenon present in many fields and scales, ranging from large heat exchangers and boilers in industrial applications to micro-scale heat exchangers for high density power electronics. In the present study pressure drop oscillations in a two parallel horizontal channels system have been experimentally investigated, focusing in the individual behavior of each channel. The balanced (same characteristic pressure drop curve) and unbalanced cases have been analyzed finding different limit cycles than the typical single channel case. No pressure drop oscillations with both channels following the typical limit cycle were found. The oscillation mode detected consisted in one channel performing the usual limit cycle, while the other was always oscillating in the superheated vapor region.

Prediction of correlation between two-phase natural circulation flows in heated and unheated channels of a parallel channel system – based on electrical analogy

Abstract A common characteristic of many industrial two-phase natural circulation systems is the presence of a large number of parallel boiling channels. Sensitivity of the steady state behavior of such a two-phase natural circulation system to different system parameters has many implications vis-à-vis performance of the system as per the design intent under various operating conditions. Experimental studies were carried out, to study the characteristics of a low pressure two-phase natural circulation system having ten transparent parallel channels with their individual heat sources. In view of its particular significance in nuclear industry, a special system condition with zero power in one of the parallel channels was also studied. The flow in the unheated channel and that in the heated channels were observed to be strongly interdependent in their sensitivity to downcomer resistance. A simple theory based on electrical analogy has been developed to predict the hydraulic behavior in such a system. Agreement between the theoretical predictions and the experimental observations has been found to be good. This article describes the experimentally observed phenomenon, the electrical analogy and the comparison between the experimental data and the analogical predictions.