Parallel Channel System Research Articles

On the design of manifolds for parallel channel systems

In the design of high-performance heat and mass transfer devices such as liquid-cooled heat sinks, catalytic reactors, and catalytic convertors, parallel mini/microchannels are favored owing to their special potentials. Offering low pressure drop, providing high transfer surface area to volume ratio, and being easy to manufacture and optimize have been drawing thermal and chemical engineers attention to parallel channels for past decades. When working with parallel channels, the challenge of flow maldistribution is commonly faced which decreases their efficiency significantly. System total pressure drop and flow uniformity are two parameters that determine the system performance. In the present study, a variety of practical ideas, aiming to enhance parallel channels performance, are studied numerically. Inventive manifold designs with high hydraulic performance are created through the course of this study. The results of these designs are compared with basic conventional designs which show substantial enhancement. Analyzing less successful designs lead us to deep understanding of fluid dynamics in parallel channel heat and mass transfer devices.

Cross-shore parallel tidal channel systems formed by alongshore currents

Parallel tidal channel systems, characterized by commonly cross-shore orientation and regular spacing, represent a distinct class of tidal channel networks in coastal environments worldwide. Intriguingly, these cross-shore oriented channel systems can develop in environments dominated by alongshore tidal currents, for which the mechanisms remain elusive. Here, we combine remote sensing imagery analysis and morphodynamic simulations to demonstrate that the deflection of alongshore tidal currents at transitions in bed elevation determines the characteristic orientation of the parallel tidal channels. Numerical results reveal that sharp changes in bed elevation lead to nearly 90-degree intersection angles, while smoother transitions in bed profiles result in less perpendicular channel alignments. These findings shed light on the potential manipulation of tidal channel patterns in coastal wetlands, thus equipping coastal managers with a broader range of strategies for the sustainable management of these vital ecosystems in the face of climate change and sea level rise.

Influence of heat flux profile on two-phase flow instability in parallel channels of helical coil once-through steam generator

To enhance the heat transfer capability, the helical coil once-through steam generator (HCOTSG) has been widely utilized in the small modular reactor (SMR). There are hundreds of parallel helical channels in the HCOTSG, and the coolant undergoes the phase change from subcooled liquid to superheated vapor in the helical tube. The flow oscillation between parallel channels may be triggered during the phase change. In present study, the two-phase flow instability is investigated with different heat flux profiles under many operating conditions. The thermal–hydraulic model for parallel channels of HCOTSG is established based on our previous work, while the local heat transfer phenomenon such as subcooled boiling, critical heat flux, and wall conduction are considered in detail. The heat flux profile along the tube is ascertained by the steady-state analysis of HCOTSG under different operating conditions and applied to the outside surface of the helical channel. The heat flux value is increased step by step to trigger the out-of-phase oscillation between parallel helical channels. The thermal–hydraulic model and analysis method are validated against experimental results. The characteristics of flow instability in the helical channel are also analyzed. It is observed that the delay effect causing the flow instability is more obvious if the void fraction exceeds 0.7 in the starting stage. Then the boundary of flow instability is compared under different heat flux profiles for various operating conditions of HCOTSG. The results suggest that the distribution of heat flux in the two-phase region plays a more significant role in the occurrence of flow instability in the parallel helical channel system. The thermal–hydraulic characteristics during flow instability are governed by the combined effects of void propagation and delayed feedback in the system. In conclusion, the thermal–hydraulic condition of HCOTSG undergoes the unstable region to achieve the normal operating point, which can be avoided by adjusting the operating parameters.

High fidelity core flow measurement experiment for an advanced research reactor using a real scale mockup

Owing to spatial effects and vortex flow, flow in research reactors that use plate-type fuels can be maldistributed to the parallel channels of the core, which significantly impacts the reactor safety. In this study, the core flow of an advanced research reactor was measured in a real-scale facility under various hydraulic conditions. For flow measurement, integrated pressure lines were embedded in the mockups of 22 fuel assemblies and six fission molybdenum assemblies. Each assembly mockup was individually calibrated to obtain the relationship between the pressure drop and flow rate. Real-scale facility, which implements the characteristics of the hydraulic conditions in research reactors, was then used to evaluate the assembly-to-assembly flow distribution under normal operating condition, a partially withdrawn condition for the follower fuel assemblies, no flow for the pool water management system, and 1:1.5 asymmetric inlet flow condition. As a parallel channel system, core flow distribution was analyzed with conventional header design approach. Taking into account the measuring uncertainty, the core flow was uniformly distributed within 5 % under all conditions. This was mainly because the core flow resistance was sufficiently high and the vortex flow was minimized by the perforated plate.

Acoustic standing wave with a frequency sweeping in a microfluidic system of parallel channels

In acoustofluidics, particles driven by fluid flows in microchannels are subject to acoustic waves, which are transmitted through the channel walls. Increasing the flow rate, one of the major challenges in microfluidic operation, can be achieved by parallelization of channels. In the present paper, inducing acoustic waves in devices with 4 and 16 parallel channels in silicon-based chip is studied. The resonance quality was evaluated by acoustic focusing of particles in the channels. First, the quality of resonance was measured in every channel separately as a function of the wave frequency. Second, a methodology with a sweeping of the frequency with time was explored to compensate for the variation of the optimal frequency between the channels.As a result, for a chip with a single channel, the sweeping is less efficient because of the lower energy transfer due to a non-optimal acoustic wave frequency. However, this developed method allowed simultaneous resonance in all the channels, which is not possible with application of a single frequency.

Small-scale superadiabatic combustors with a two-step chain-branching chemistry model: Asymptotic models and the effect of two-dimensionality on lean mixtures burning

Small-sized super-adiabatic combustion devices consisting of countercurrent channels with heat-exchange segments are investigated for their ability to burn ultra-lean mixtures. The study is carried out numerically using a two-step chain-branching kinetic model in which the flammability limit appears explicitly. Various asymptotic approximations for modeling the process are considered, together with the solution of the two-dimensional conservation equations for species and temperature in the gas and the solid walls. The influence of the width of the channels and the separating walls properties on the efficiency of the device in burning mixtures below the flammability limit is investigated. The main goal of the study is to determine the applicability of the considered asymptotic models for the effective prediction of the operation of such combustion devices. Novelty and significance statementThe study contains a demonstration of the possibility of burning ultra-lean mixtures in counterflow type combustion devices consisting of a system of parallel channels, using a two-step chain-branching kinetic model. A number of model asymptotic approximations and areas of their applicability are investigated. In particular, the present study considers finite width for the channels and the separating walls and the results are compared satisfactorily to previous papers of the same authors were the one-dimensional narrow channel approximation was used, validating in this way this approximation. The results of the study may be of importance for the design of small-scale combustion devices used for generating energy in remote conditions.

Parametric study of density wave instability in parallel channels of a water-cooled blanket in a fusion reactor

In fusion reactors, many blanket concepts are designed with water as a coolant to transfer high-density heat from the fusion reaction out of the reactor core. The coolant temperature and pressure are maintained as the validated use in water-cooled fission reactors. However, the flow channel in a water-cooled blanket is independent of each other, and there is no flow mixing between coolant channels. Therefore, flow instability may occur in the independent parallel channels in a water-cooled blanket due to its unique structure and heat distribution, especially under the high heat flux caused by plasma rupture. In this study, the parametric analysis of density wave instability is performed using a thermal-hydraulic code developed for independent parallel channels based on the homogeneous model for the two-phase flow. The parallel-channel system in a water-cooled ceramic breeder (WCCB) blanket of the China Fusion Engineering Experimental Reactor (CFETR) is established for its first wall structure. A small disturbance is introduced into the system to determine if it is stable under different conditions. It is found that the channel number has no obvious influence on the prediction of the flow instability boundary. Therefore, the two-channel system is adopted to investigate the influence of different parameters, such as the pressure, resistance, flow rate, and inclination, on the flow instability boundary of the parallel-channel system in the CFETR WCCB blanket. The results show that flow instability occurs more easily in this study compared to the traditional instability analysis, especially under high-pressure conditions. In general, conditions of high pressure, large flow rate, and no inclination can stabilize the system, while the influence of resistance is quite different under different conditions of resistance and pressure. The research work indicates that more attention should be paid to the joint influence of different parameters for the water-cooled blanket during its design and operation.

Experimental study of hydrodynamic stability limits in the system of parallel steam-generating channels

Experimental study of hydrodynamic stability limits in the system of parallel steam-generating channels

The Effect of Nodalization Schemes on the Stability Characteristics of a Three Heated Channels under Supercritical Flow Condition

The present analysis is aimed at conducting node sensitivity analysis on the thermal–hydraulic performance of supercritical fluid in a three parallel channel configuration system. The heated channel was divided into different nodes and is examined under wide-ranging operating conditions. Firstly, the heated channel was divided into two nodes, like the two-phase flow system. In the second case, based on the correlation between the fluid properties, the heated channel was divided into three regions: heavy, mixture, and supercritical fluids. Finally, the channel was divided into N-nodes. Post the nodalization analysis, a non-linear analysis of three parallel channels was carried out under varied heat flux conditions. The analytical approximation functions were developed to capture the fluid flow dynamics. These functions were used to capture each node’s density, enthalpy, and velocity profiles under a wide range of operating conditions. The different flow instability (density wave oscillations and static) characteristics were observed at low pseudo- and relatively high subcooling numbers. In the density wave oscillations regime, out-of-phase oscillations and limit cycles are observed. A co-dimension parametric analysis with numerical simulations was carried out to confirm the obtained non-linear characteristics. Such analysis for parallel channel systems under supercritical working fluid flow conditions is missing in the literature which is limited to only linear stability analysis. This analysis can help to improve heat and mass transfer for designing efficient heated channel systems.

A Comparative Assessment on Different Aspects of the Non-Linear Instability Dynamics of Supercritical Fluid in Parallel Channel Systems

The thermal-hydraulic behavior of supercritical water reactors with a parallel channel configuration was examined through a non-linear instability analysis. This analysis was performed under wide-ranging conditions and aspects, including different working supercritical fluids, varied heat-flux and flow-rate conditions, and channel inclinations. The supercritical fluid (SCFs) dynamics were captured using the density, enthalpy, and velocity analytical approximation functions. The major findings show that both SCFs (water and carbon dioxide) experienced density wave oscillations at a low pseudo-subcooling number. Static instability characteristics were observed for supercritical water, at a relatively high subcooling number. Further, it was found that at different heat flux, the hotter channel makes the overall system more unstable, whereas, at equal heat flux, parallel channels perform similar to a single-channel system. However, the effect of the inclination angle was found to be negligible owing to supercritical pressure conditions. Moreover, stable and unstable limit cycles along with out-of-phase oscillation characteristics were observed in dynamic stability regions. The present model was also compared with experimental and numerical data. Moreover, co-dimension and numerical simulations were performed to confirm the observed non-linear characteristics. This study helps to enhance the heat transfer characteristics during safe operation of heated channel systems, such as nuclear reactors and solar thermal systems.

Analysis of the oscillation in the system of parallel narrow channels

In this paper, the RELAP5/MOD3.4 code is used to study the instability of the parallel narrow flow channel system in detail. First, the rationality of the RELAP5 code is verified with experimental data. By extracting the pressure drop in the channel under different node numbers, the importance of reasonable node model in pressure drop calculation using RELAP5 program is illustrated. Thus, the deep reason of the influence of the node number on the instability boundary is revealed. After that, the influences of various factors on the instability were investigated. The results show that: The more channels, the more unstable the system is, and the flow instability boundary tends to a fixed value; The fuel module separation has a significant effect on the instability in multi-channel system, and the single side heating channel has a stabilizing effect on the system; The flow instability of the system mainly depends on the hottest channel.

Modelling of the interaction of kinetics and external transport phenomena in structured catalysts: The effect of reaction kinetics, mass transfer and channel size distribution in solid foams

The increasing use of structured reactors and catalysts, particularly solid open foams requires a new touch to the interaction of intrinsic chemical kinetics and mass transfer. In spite that the internal mass transfer resistance in the catalyst pores is efficiently suppressed because of thin washcoat layers on structured catalysts, the low flow velocities applied in many liquid-phase processes impair the mass transfer efficiency at the outer surface of the catalyst layer inside the channels of monoliths and open foams. The coupling of external mass transfer resistance and intrinsic kinetics was considered for various power-law and adsorption kinetics in tubular reactors filled with structural catalysts. The relevant mass balance equations were derived and solved numerically and the behaviors of single-channel systems were illustrated in the parameter spaces of Damköhler numbers. The single-channel model was successfully extended to a system of parallel channels with a distribution of channel sizes. The effect of the channel size distribution on the Reynolds and Sherwood numbers was illustrated by numerical simulations. The proposed approach is a potential tool for the design of structured catalysts.

Study on flow instability in natural circulation loop with parallel channels under moving condition

In this study, the mathematical models suitable for natural circulation, parallel channels, and general forms of additional force under moving conditions were established. An analysis code was developed using SIMPLE algorithm. Based on the validated code, the natural circulation and parallel flow instability under typical moving conditions such as inclination, rolling, heaving, and coupled motion were discussed, and the corresponding flow instability boundaries were obtained. The results show that under the moving conditions, the natural circulation flow change is manifested as the superposition of the two-phase instability fluctuation and the motion-induced fluctuation. The heaving condition has the most drastic effect on flow rate with fluctuation amplitude exceeding 30%, but the average flow rate remains unchanged. The boundary of natural circulation flow instability will be slightly extended affected by motions and the variation of the boundary quality is less than 0.03. The inclination and rolling tend to destabilize the parallel channels system and reduce the boundary quality by more than 0.1, whereas the effect of heaving on the parallel instability is not significant. For the parallel flow instability, the adverse effect is mainly attributed to the decrease of the mean flow rate at the entrance of the parallel channels caused by the motions, while the flow re-distribution between parallel channels and natural circulation flow fluctuation induced by the moving conditions do not contribute much.

Non-Linear Qualitative Dynamic Analysis of Supercritical Water-Heated Channels under External Vertical Accelerations

Employing the external force method to regard seismic impact and the three-region methodology to analyze the supercritical heated channel, a non-linear dynamic model was developed to investigate the transient characteristics of single channel or parallel channels under the impacts of vertical sinusoidal and seismic accelerations. The present model was validated against the experimental data, which could suitably estimate the additional pressure drop caused by the vertical vibrations. The influences of parameters on the seismic-induced oscillation conducted in a supercritical heated channel indicated that a longer heated length, uprating operation power and a larger outlet loss coefficient all exhibit unstable effects, while the increase of inlet loss coefficient, a larger tube diameter and a lower inlet fluid temperature would tend to stabilize the system. Moreover, the supercritical fluid would present a high natural frequency in the very small NP-SUB region. The parametric effects on the parallel channel system are related to the inherent stability nature of initial state and the interactions among channels. The more uneven heat flux distribution among channels would cause a larger vibration-induced oscillation. In particular, when it is combined with the resonance effect, the system may exhibit much larger oscillations than in the case of non-resonance.

Pressure Drop Oscillations With Symmetry Breakdown in Two-Phase Flow Parallel Channels

Abstract Two-phase flow in parallel heated channels is prone to symmetry breakdown resulting in mass flow maldistribution. Moreover, in the presence of compressible volume (CV), such systems also undergo pressure drop oscillations (PDOs). The performances of such systems depend on the effect of these flow instabilities. However, the simultaneous occurrence of these two-phenomena has been rarely reported in the literature. In this work, an approach is applied in a two-channel system to demarcate the parameter space of mass flow rate and inlet temperature into several areas, where these two phenomena take place. The loss in the symmetry in the flow rate is observed as the mass flow rate is varied, which leads to flow maldistribution. The PDO are also observed for specific values of mass flow rate in the system. One unique feature of the parallel channel system is the existence of the oscillatory and stable (albeit asymmetric) states at the same parameter values. For these parameter values, the final state of the system is dependent on the type of initial disturbance. The flow maldistribution due to symmetry breakdown is identified by the pitchfork bifurcation, and oscillations of mass flow rate are identified by the presence of Hopf bifurcation. Moreover, the physical interpretation of the different phenomena in the system is carried out using internal and external pressure drop characteristics curves.

Heat and mass transfer during adiabatic fluid boiling in channels of contact exchangers

Reducing mass and size characteristics of heat and mass exchangers requires intensifying the heat and mass transfer processes inside them. The authors propose improving the performance of adiabatic evaporators by placing a packing made of capillary-porous material into the working chamber behind the nozzle. This packing is designed as a system of parallel channels positioned at an angle to the axis ensuring a stable film formation.This article presents results of a physical experiment on the influence of regime and geometric characteristics of the contact exchanger and some of its elements on the heat and mass transfer intensity during film formation on the packing surface with capillary-porous coating under adiabatic boiling.The research technique used by the authors consists in determining the heat and mass transfer characteristics of the proposed exchanger design and comparing them with the characteristics of the design currently used in the industry.The analysis of the obtained results shows that using the capillary-porous packing in adiabatic evaporator units allows increasing vapor capacity by 18.5%.Analyzing the experimental data made it possible to obtain dependences that allow determining the heat transfer intensity on the channel surfaces of the packing covered with capillary-porous coating during adiabatic boiling of superheated liquid within a wide range of Reynolds numbers Re∗=0,02...4. The obtained dependences may help improve the existing techniques for calculating contact heat exchangers.

Modeling the core of a lead-cooled reactor

This paper presents the results of thermohydraulic modeling of the reactor core with a lead coolant, which is based on the calculation scheme for the ATHLET code, obtained on the basis of open information on the BREST-OD-300 reactor plant. Only the reactor core and methods of its modeling are considered. The work also presents the results on the development of the division of the in-reactor space of the BREST-OD-300 reactor into a system of parallel channels. The subdivision of the in-core space is based on the type and number of different core elements. This way of modeling the core allows you to see changes in different parts of the reactor when calculating transients. The developed model of the partitioning of the in-core space will be further used in the calculation of various transient modes (MCP shutdown, steam generator tube rupture, etc.). Neutron physics is not considered in this paper. There are plans to carry out joint neutron-physical and thermohydraulic calculations using the model from this paper.

Flow instability in parallel-channel systems with ultra-supercritical water

In this study, a numerical code that applies a time-domain method is established to analyze the flow instability of a uniformly heated parallel-channel system. Nonlinear dynamics of the ultra-supercritical flow in the water wall are discussed. Results show that the inlet and outlet mass flow rates demonstrate antiphase. The mass flow rates with heat flux perturbations imposed on either single channel decline gradually with time, suggesting that flow stability can be maintained at normal operation. In addition, the effects of heat flux perturbation on transient responses of the typical parallel channels are studied. Results illustrate that the maximum of the inlet mass flow rate in both channels are high when perturbation is imposed on a strong heated channel. Moreover, flow in the stronger heated channel reveals more remarkable dynamic characteristics than in the weaker one. Approximately 141.6 and 139.8 s are needed for perturbation.1 and perturbation.2 to restore to steady state, respectively. Furthermore, data under different inlet temperatures and mass flow rates are calculated to generate the dimensionless instability boundary. When the inlet temperature is low, the inlet mass flow rate should be increased to stabilize the system. Otherwise, the effect of the mass flow rate on the instability boundary is almost negligible.

Flow Instability Investigation at Supercritical Pressures in Parallel Channels by Computational Fluid Dynamics Adopting Low- and High-Power Boundaries

Abstract Supercritical water-cooled reactor (SCWR), which is considered as the logical extension of existing light water reactors (LWRs) (pressurized water reactor and boiling water reactor (BWR)), has the potential of increasing the efficiency of power generation to 45% compared to 33% of that of LWRs. But without the challenges of heat transfer and hydrodynamics, and reactor core design materials due to supercritical flow instability which is associated with sharp variation in fluid properties near the vicinity of the pseudo-critical temperature. Supercritical flow instability therefore needs to be addressed ahead of the deployment and operation of SCWR in the near future. The main purpose of this study is to carry out flow instability analysis in parallel channels with supercritical water. The study also aims at examining the capability of using three-dimensional (3D) simulation of turbulent flow in arbitrary regions computational continuum mechanics C++ based code (3D STAR-CCM+ CFD code) to predict flow oscillation amplitude and periods, and instability power boundaries at low-power boundary (LPB) and at high-power boundary (HPB). Parameters considered in the investigation include mass flowrate, system pressure, and gravity. Two different threshold power instability boundaries were obtained from the study. These instability power boundaries include lower threshold where stability of the parallel channel system decreases with increasing coolant inlet temperature, and upper threshold where stability of the parallel channel system increases with increasing coolant inlet temperature. From the results of the investigation, it can be found that: (1) for LPB at 23 MPa, only lower threshold was obtained as flow instability power boundary; and for HPB at 23 MPa, both lower and upper thresholds were obtained as flow instability power boundaries. The numerical findings quite well agree with the experimental findings at 23 MPa for both LPB and HPB; (2) only lower threshold was obtained as flow instability power boundary at both 23 MPa and 25 MPa for LPB. For HPB, both lower and upper thresholds were obtained as flow instability power boundaries at both 23 MPa and 25 MPa; (3) only lower threshold was obtained as flow instability power boundary for the parallel channel system with or without gravity influence for LPB. For HPB, both lower and upper threshold flow instability power boundaries were obtained for the parallel channel system with gravity influence, but only lower threshold flow instability power boundary was obtained for system without gravity influence; (4) only lower threshold was obtained as flow instability power boundary at system mass flowrates of 125 kg/h and 145 kg/h for LPB. For HPB, both lower and upper threshold flow instability power boundaries were obtained for system mass flowrate of 125 kg/h, but only lower threshold flow instability power boundary was obtained for system mass flowrate of 145 kg/h. For both LPB and HPB, the numerical findings agree quite well with the experimental results for a system operated at 125 kg/h and 145 kg/h; (5) the investigated parameters such as mass flowrate, pressure, and gravity have significant effects on amplitude of mass flow oscillation, but have little effects on the period of mass flow oscillation for both LPB and HPB. Results from the numerical simulation were compared with the results from the experiment for both LPB and HPB. The numerical amplitude results obtained were far less than the amplitude results obtained from the experiment. But there was no significant difference between the oscillation periods obtained from both the numerical simulation and experiment. (6) Flow instability studies including predicting flow oscillation amplitude and periods, and instability power boundaries could be carried out using 3D STAR-CCM+ CFD code. The effects of heating structures on flow instability results have not been considered in this study. Previous studies have shown that including heating structures in geometrical models for numerical studies may have effects on flow instability results. More experimental studies are needed for validation of similar numerical studies carried out at supercritical pressures using various numerical tools.

Construction of new codes in term‐rank metric

This study investigates codes in the term-rank metric. These codes can be used to correct crisscross errors in a ( M × N ) matrix. These errors can be found in memory chip arrays, in magnetic tape recordings, or in the parallel-channel system with interference. In this study, the authors will build an algorithm for computing term-rank of a matrix and new construction of term-rank codes. Single error-correcting codes in the term-rank metric are considered and help us build a generator matrix and parity-check matrix for term-rank codes. Research results derived from this study show that the term-rank metric is suitable for correcting crisscross errors.