Wall Subchannels Research Articles

Laminar simulation of intersubchannel mixing in a triangular nuclear fuel bundle geometry

Predicting temperature distributions in fuel rod bundles is an important component of nuclear reactor safety analysis. Intersubchannel mixing acts to homogenize coolant temperatures thus reducing the likelihood of localized regions of high fuel temperature. Previous research has shown that intersubchannel mixing in nuclear fuel rod bundles is enhanced by a large-scale quasi-periodic energetic fluid motion, which transports fluid on the cross-plane between the narrow gaps connecting subchannels. This phenomenon has also been observed in laminar flows. Unsteady laminar flow simulations were performed in a simplified bundle of three rods with a pipe. Three similar geometries of varying gap width were examined, and a thermal trace was implemented on the first geometry. Thermal mixing was driven by the advection of energy between subchannels by the cross-plane flow. Flow through the rod-to-wall gaps in the wall subchannels alternated with a dominant frequency, particularly when rod-to-wall gaps were smaller than rod-to-rod gaps. Significant phase-linking between rod-to-wall gaps was also observed such that a peripheral circulation occurred through each gap simultaneously. Cross-plane flow through the rod-to-rod gaps in the triangular subchannel was irregular in each case. This was due to the fundamental irregularity of the triangular subchannel geometry. Vortices were continually broken up by cross-plane flow from other gaps due to the odd number of fluid pathways within the central subchannel. Cross-plane flow in subchannel geometries is highly interconnected between gaps. The flow structure through the gaps is more complex than has been previously reported and depends very much on the subchannel geometry.

Simulation of turbulent flow inside different subchannels in tight lattice bundle

In this paper, both steady and unsteady Reynolds Averaged Navier Stokes (RANS and URANS) methodology are applied to the prediction of turbulent flow inside different subchannels in tight lattice bundles. Two typical configurations of subchannels (i.e., wall subchannel and center subchannel) are chosen to be investigated. In this work the application of different turbulence models implemented in the commercial code CFX v12 is shown. The validity of the methodology is assessed by comparing computational results of axial velocity, wall shear stress and turbulent intensity distributions with the experimental data (Krauss, 1996; Krauss and Meyer, 1998). This study shows that RANS simulation with anisotropic turbulent model produces excellent agreement with experiment, whereas it failed to predict the flow behavior accurately in the case of tightly packed geometries ( P/ D < 1.1). On the other hand, the URANS simulation is in good agreement with the results in tightly packed geometries with flow oscillation in the gap region. The effects of the Reynolds number and the bundle geometry on the flow oscillation are investigated in details.

CFD analysis of thermal–hydraulic behavior of supercritical water in sub-channels

Investigations on the thermal–hydraulic behavior in the supercritical water-cooled reactor (SCWR) fuel assembly have obtained a significant attention in the international SCWR community. However, there is still a lack of understanding and ability to predict the heat transfer behavior of supercritical fluids. In this paper, computational fluid dynamics (CFD) analysis is carried out to study the thermal–hydraulic behavior of supercritical water flows in sub-channels of a typical SCWR fuel assembly using commercial CFD code CFX-5.6. Three types of sub-channels, e.g. regular sub-channel, wall sub-channel and corner sub-channel, are analyzed. Effects of various parameters, such as boundary conditions and pitch-to-diameter ratios, on the mixing phenomenon in sub-channels and heat transfer are investigated. The turbulent mixing in tight lattice ( P/ D = 1.1) is lower than that in wide lattice ( P/ D > 1.1), whereas, the effect of pitch-to-diameter ratio on the turbulent mixing is slight at P/ D > 1.1. The amplitude of turbulent mixing in wall sub-channel is slightly higher than that in regular sub-channel and is close to that in corner sub-channel. The mixing coefficient in the sub-channel at P/ D ≥ 1.2 is in the range from 0.022 to 0.028. The results also show unusual behavior of turbulent mixing in the vicinity of the pseudo-critical point, and further investigation is needed. The mass mixing due to cross flow in wall sub-channel is much stronger than that in regular sub-channel at a same pitch-to-diameter ratio. The mass mixing in wall and regular sub-channels, especially at small pitch-to-diameter ratio, brings an unfavorable feedback to the heat transfer and strengthens the non-uniformity of the circumferential distribution of heat transfer. The strong mass mixing in corner sub-channel should be paid attention.

An experimental study of inter-subassembly heat transfer during natural circulation decay heat removal in fast breeder reactors

The evaluation of core thermohydraulics under natural circulation conditions is significant to utilize passive safety features of fast breeder reactors (FBRs). Under low flow conditions, it is predicted that buoyancy effects and heat transfer through wrapper tubes, i.e. inter-subassembly heat transfer, will significantly influence the flow and temperature distributions in subassemblies. Thus, steady-state sodium experiments were carried out using a three-subassembly model. The transverse temperature distributions in the subassemblies were measured under conditions wherein inter-subassembly heat transfer occurred. A wall subchannel factor was introduced to estimate the sodium temperature near the wrapper wall, which characterizes the inter-subassembly heat transfer. This factor enables a one-dimensional system code to predict the inter-subassembly heat transfer accurately. The characteristics of the factor were studied experimentally. It was shown that a buoyancy parameter, Gr*/Re, and a heat flux ratio of wrapper wall to pin surface were essential to predict the wall subchannel factor and also the peaking factor. Experimental analyses were also carried out using a three-dimensional analysis code that modeled the multi-bundle system. Good agreement between experiments and calculations was obtained for temperature distributions influenced by the inter-subassembly heat transfer.

Characteristics of turbulent velocity and temperature in a wall channel of a heated rod bundle

Turbulent air flow in a wall subchannel of a heated 37-rod bundle ( P D = 1.12, W D = 1.06 ) was investigated. Measurements were performed with a hot-wire probe with x-wires and a temperature wire. The mean velocity, mean fluid temperature, wall shear stress and wall temperature, turbulent quantities such as the turbulent kinetic energy, Reynolds stresses, and turbulent heat fluxes were measured and are discussed with respect to data from isothermal flow in a wall channel and heated flow in a central channel of the same rod bundle. Also, data on the power spectral densities of the velocity and temperature fluctuations are presented. These data show the existence of large-scale periodic fluctuations of velocity and temperature in the gap region between two adjacent rods or between rods and the wall. These fluctuations are responsible for the high intersubchannel heat and momentum exchange.

Flow measurements in rod bundle subchannels with varying rod-wall proximity

Abstract Detailed measurements of fully developed, turbulent, air flow through a five-rod sector of a 37-rod bundle have been conducted for the design geometry of the bundle, as well as for several cases with the central rod displaced towards the external tube wall and/or towards a neighboring rod, including cases with rod-wall and rod-rod contact. The wall shear stress on an outer rod reached minima at rod-wall and rod-rod gaps and maxima at open flow regions. The average and the minimum wall shear stresses decreased dramatically only for very small values of the rod-wall gap. Measurements of the mean velocity, Reynolds stresses and turbulent scales in the wall and inner subchannels are presented mostly as iso-contours. Isotachs bulged towards narrow gaps and corners, with the bulging becoming more pronounced as the rod-wall gap decreased. The local friction factor not only varied appreciably around the rod as the gap decreased, but also had values much larger than the average friction factor based on the subchannel bulk velocity, due to the variability of the local flow width.

Turbulent transport in wall subchannels and in finite rod clusters

Turbulent transport through wall subchannels of circular and hexagonal rod clusters has been studied using a two-dimensional eddy diffusivity. Fully developed flow and thermal conditions are assumed, and finite-difference solutions have been generated for the uniform heat flux condition on the rod surface. Transport rate values are estimated for subchannels with curved as well as straight boundaries with the pitch-to-diameter and wall-to-diameter ratios varying over a wide range. The cluster-average transport coefficients are next generated for circular bundles by suitably combining the results for constituent subchannels. Quantitative agreement with experimental results in the literature are attained. The accuracy of turbulence modeling is attested to by the fact that the cluster-average friction factor and Nusselt number values are in reasonable agreement with the well-established internal flow correlations based on hydraulic diameter.

Prediction of natural mixing in tightly packed seven-rod bundles ( [formula omitted

In this study the predicted fine structure of flow within a seven-rod bundle at a fixed rod-to-rod gap distance ( P/D = 1.1) and at different rod-bundle-to-wall distances are presented ( N Re = 10 5 ; N Pr ≈ 1.0 ). The numerical code VANTACY-II has been used to calculate the single-phase, fully developed turbulent flow conditions in arbitrary two-dimensional solution domains. The code includes a validated model of the anisotropic turbulent exchange of momentum and heat appropriate for tight lattices. The different rod-bundle boundary structures affect the fluid flow conditions, although the influence of the hydraulics on the temperature field is partly compensated by the turbulent microconvective heat transport between the central and the wall subchannels. In other words, the turbulent mixing depends to a great extent on the size and on the wetted perimeter of the edge region. It may be concluded that a distributed parameter approach using two-dimensional turbulence models is • bb modelling the flow in detail, providing the local distributions of the fluid velocities and temperatures, the wall shear stresses and the wall heat fluxes, and integral quantities like the axial pressure gradient • bb helping to interpret experiments and to expand their results • bb and providing well defined input data for lumped-parameter analysis codes.

An Experimental Investigation on Turbulent Flow Through Symmetric Wall Subchannels of Two Rod Bundles

Measurements of the mean velocity, wall shear stresses, and turbulent Reynolds stresses were performed in wall subchannels of two rod bundles. The rod bundle of four parallel rods was arranged symmetrically in a rectangular channel. The pitch-to-diameter ratio was 1.148, and the wall-to-diameter ratios were 1.045 and 1.074, respectively. The Reynolds numbers in these investigations were 6.11 × 104 and 7.07 × 104, respectively.The experimental results demonstrate once more that the structure of turbulence in rod bundles differs greatly from the structure in circular tubes. Especially in the narrow gaps between the rods and channel walls, there are increased levels of turbulence intensities in both the axial and azimuthal directions and, hence, of the kinetic energy of turbulence, caused by a strong turbulent momentum transport through the gaps. In comparison with the previous investigations in these geometries, however, arranged asymmetrically in the rectangular channels, the momentum transport between the subchannels across the gap between the rods is negligible.The comparison between the experimental wall shear stress distributions and those computed by the VELASCO code shows strong deviations, especially in the gap regions between the rods and channel walls.More sophisticated analytical tools than presently available are required to predict turbulent flow through rod bundles with sufficient accuracy.

Experimental observations of turbulent flow through subchannels of rod bundles

Experimental data on the structure of turbulence for longitudinal flow through rod bundles is required to validate computer codes predicting the detailed velocity and temperature distributions in rod bundles. Such codes are under active development. Experiments have been performed for a number of different geometries for turbulent flow through the wall subchannels of rod bundles. Detailed experimental results have been obtained of time-mean velocities, wall shear stresses, and the full Reynolds stress tensor by hot-wire measurements. Typical distributions of the axial turbulence intensity, the turbulence intensity parallel to the walls, and the kinetic energy of turbulence are presented. The experimental results show that the structure of turbulence in rod bundles differs greatly from that in circular tubes. The main features of this departure and the systematic trends observed are outlined. It is important for code developers to take into account the new experimental results in order to be able to predict temperature and velocity distributions in rod bundles with reasonable accuracy.

On the Development of Turbulent Flow in Wall Subchannels of a Rod Bundle

Measurements of the mean velocity, wall shear stresses, and turbulence have been performed in a rod bundle of four parallel rods arranged in a rectangular channel (P/D = W/D = 1.148) for three ratios of length-to-hydraulic diameter (L/D/sub h/). Distributions of the mean velocity and the wall shear stresses have been measured in four quadrants for L/D/sub h/ = 48.7, 73.0, and 97.4. The full Reynolds stress tensor has been determined by the hot-wire technique in only one quadrant for each L/D/sub h/ ratio. The Reynolds number of this investigation was Re = 1.17 x 10/sup 5/. The experimental results show that the structure of turbulence does not change significantly between L/D/sub h/ = 48.7 and 97.4. The flow redistribution among the subchannels caused by the inlet conditions, however, is not completed at L/D/sub h/ = 97.4. The experimental wall shear stresses at L/D/sub h/ = 97.4 agree satisfactorily with the wall shear stresses calculated by the VELASCO code.

FINITE ELEMENT ANALYSIS FOR LAMINAR FLOW AND HEAT TRANSFER IN FINITE ROD BUNDLES

This investigation deals with the application of finite element method to solve the thermohydraulic problem of laminar fully developed flow in the interior and wall sub-channels of finite fuel rod bundles. A variational principle has been used for the solution of the momentum and energy equations. Wall shear stress and temperature distributions, ƒRe and Nusselt numbers are obtained for the sub-channels of different configurations. The results are compared with solutions generated by collocation and finite difference methods.

Measurements of Wall Shear Stress in Axial Flow in a Square Lattice Rectangular Rod Bundle

Hot film probe measurements of the distribution of the wall shear stress were made for axial flow along a rectangular 3 × 6 array of rods with a pitch to diameter ratio, P/D = 4/3, and a wall to diameter ratio, W/D = 2/3. Measurements were performed on rods at several locations and on two duct side walls at a position 62 hydraulic diameters from the entrance. Local shear stress maxima occur near the largest subchannel flow areas with the lowest maximum local shear stress on rods nearest the sidewalls. Maximum to the minimum shear stress ratio on an individual rod is largest for the corner rod. Side wall maximum local shear stress occurs in the first wall subchannel. Overall friction factors calculated from the wall shear stress measurements agree with those calculated from pressure drop data.

Laminar convection in wall sub-channels and transport rates for finite rod-bundle assemblies by superposition

Finite rod-clusters in circular, square and hexagonal shells have been divided along the symmetry lines into a number of interior, wall and corner subchannels. Laminar fluid flow and heat transfer results have been generated for these subchannels with varying pitch-to-diameter and wall distance-to-diameter ratios. Wall shear stress and temperature variations for typical wall subchannels are presented. Friction factor values for finite bundles are then obtained by superposing the results of the subchannels that constitute the bundles. The values so generated are in excellent agreement with previous work in the literature obtained by the method of symmetry sectors. Considerable disagreements were, however, observed in the superposed values of Nusselt number, apparently due to conduction effects.

A Simplified Finite Element Model for Fully Developed Axial Turbulent Flow in Rod Bundles

A simplified computational scheme that can be efficiently applied to predict fully developed axial flow in rod bundle subchannels is presented and compared with experimental results for different flow conditions. Galerkin's finite element method and the physical model of turbulence proposed by Roco and Zarea (1978) are used in this approach. The flow in the corner, wall, and central subchannels for both triangular and square rod bundles is investigated. Different values of the subchannel pitch-to-diameter ratio and various Reynolds numbers are considered. In all these cases, the present computational scheme gives the correct qualitative trends for the distributions of the axial velocity and wall shear stress. The comparison performed with current measurements in the subchannels of a 3 x 6 rectangular rod bundle and with other available experimental and analytical results taken from the literature shows good agreement.

Turbulent momentum transport in rod bundles

Abstract An experimental investigation was performed with air to obtain detailed information on the velocity and turbulence distribution for parallel turbulent flows through subchannels of rod bundles. Experimental results were obtained for wall and corner subchannels of rod bundles of four parallel rods. The pitch-to-diameter ratios were varied between 1.07 and 1.4. The Reynolds numbers ranged from 6 × 10 4 to 2 × 10 5 , depending on the rod bundle arrangement. On the basis of the data measured, the eddy viscosities in the directions normal and parallel to the wall were calculated. The experimental results of the velocity and wall shear stress distributions are compared with the predictions by the VELASCO code. There are considerable differences between computed and experimental results especially for low pitch-to-diameter ratios. The reasons for the discrepancies are discussed together with the results of attempts to adjust the VELASCO code against the experimental data.

The structure of turbulent flow through a wall subchannel of a rod bundle

An experimental investigation was performed to establish reliable information on the transport properties of turbulent flow through subchannels of rod bundles. Detailed data were measured of the distributions of the time-mean velocity, the turbulence intensities in all directions and hence, the kinetic energy of turbulence, of the shear stresses in the directions normal and parallel to the walls and of the wall shear stresses for a wall subchannel of a rod bundle of four parallel rods. The pitch to diameter ratio of the rods equal to the wall to diameter ratio was 1.07, the Reynolds number of this investigation was Re = 8.7 × 10 4. On the basis of the data measured the eddy viscosities in the directions normal and parallel to the walls were calculated. Thus, detailed data of the eddy viscosities in direction parallel to the walls in rod bundles were obtained for the first time. The experimental results were compared with predictions by the VELASCO code. There are considerable differences between calculated and measured data of the time-mean velocity and the wall shear stresses. Attempts to adjust the VELASCO code against the measurements were not successful. The reasons of the discrepancies are discussed.

Velocity Measurements in the Subchannel of the Wire-Spaced Subassembly

Measurements of the flow velocity and direction in wire-spaced rod bundles were performed in air flow with a 37-rod subassembly that consisted of rods 31.6 mm in diameter, 3464 mm long, 6.0 mm in spacer wire diameter, and 1100 mm in wire wrapping pitch. The fine flow velocity and its direction were measured in both interior and wall subchannels in two sections of axial direction. Circular flow and decrease of axial flow velocity were observed in the subchannel containing the spacer wire. The variation of the transverse flow rate within one spacing wire pitch, which occurred between two adjacent subchannels, was obtained from the measured value to develop the computer program for the subchannel analysis.