Horizontal Rod Bundles Research Articles

Numerical investigation of flow stratification and heat transfer characteristics in a strongly heated horizontal rod bundles with helium-Xenon gas mixture

The helium-xenon(He-Xe) mixture has good heat transfer characteristics with a low Prandtl number(Pr ≈ 0.2 ∼ 0.3) and is suitable for the advanced compact gas-cooled reactor. The characteristics of horizontal flow stratification in the fuel rod bundles with He-Xe mixture under gravity and buoyancy were numerically simulated. Different wall heat flux q+ suitable for the reactor operating conditions were selected for calculation and analysis. Flow stratification is observed as strongly inhomogeneous in the radial direction. With the semi-local wall coordinates and Van-Driest transformation, laminarization is manifested by velocity and temperature profile and turbulent kinetic energy changes. In the horizontal flow, where the buoyancy is orthogonal to the direction of flow development, unique characteristics appear in the flow and heat transfer, and the growing longitudinal momentum and energy transport are considered the main reasons. The influence of flow acceleration and buoyancy under different bundle structures were compared. The buoyancy effect is evident in the loosely spaced rod bundles. In contrast, the thermally-induced flow acceleration effect in the tight-lattice scheme is noticeable.

Optimal cascade phase change regenerator for waste heat recovery in a batch industrial dryer

Waste heat recovery is one of the main strategies to reduce the use of primary resources. This work develops a cascade phase change regenerator to recover energy from exhaust air to fresh air of a natural gas-fired batch dryer in an existing industrial laundry, taken as a case study, where an experimental campaign is conducted. The regenerator comprises two vertical stoves made of horizontal rod bundles in an aligned configuration to mitigate the fouling. The rods are hollow smooth cylinders that are grouped into sectors, each of which is filled with a phase change material properly selected among paraffins. The number of cylinders per row and the cylinder diameter are investigated by a parametric analysis; the number of sectors and the materials filling each sector are optimized by two alternative algorithms, one based on the process physics and the other on a statistical method. At last, an economic analysis is applied to the optimal configuration. This optimal configuration turns to be an 8-sector regenerator that attains an energy recovered of 61.5% and a net annual saving of 3340 €/year and that requires a total cost of about 9500 €, yielding a payback time lower than 3 years.

Natural Convection Heat Transfer of the Horizontal Rod-Bundle in a Semi-closed Rectangular Cavity

During the refueling of PWR, the spent fuel assemblies are transferred from the reactor building to the fuel building through an underwater tube. The heat transfer characteristics of the spent fuel assembly in the top-corner area of the carrier with poor heat transfer conditions is important for the safety design. Experiments were carried out and single phase natural convection and pool boiling heat transfer coefficients of the three fuel rods in the top-corner area of the carrier under different heat flux was measured and obtained. The new correlations were presented. The results can provide a reference for evaluating the thermal safety state and the maximum surface temperature of the fuel assembly during the transportation process in future engineering applications.

Experimental investigations on single-phase convection and two-phase flow boiling heat transfer in an inclined rod bundle

Single-phase convection and steam-water two-phase flow boiling heat transfer experiments in an electrically heated 4 × 25 staggered inclined rod bundle were carried out for the following range: 15 kg m−2 s−1 ≤ G ≤ 50 kg m−2 s−1, 20.0 kW m−2 ≤ q ≤ 55.0 kW m−2, 0.05 ≤ xout ≤ 0.79 and 117 kPa ≤ Pin ≤ 260 kPa. Single-phase convective results show that the independence principle can be applied to the case that all tubes were heated in inclined rod bundles. With respect to two-phase results, the local flow boiling heat transfer coefficient increases from the bottom row to the eleventh row and then can be considered as a constant value from the eleventh row to the top row. An increasing heat flux results in a decrease of the flow boiling heat transfer coefficient. However, no significant effects of mass velocity and quality were observed. The inclination angle has a small effect on the flow boiling heat transfer coefficient at low heat fluxes. However, when heat flux is high, the flow boiling heat transfer coefficient in the inclined rod bundle is the minimum while that in the vertical rod bundle is the maximum compared with that in the horizontal rod bundle. In a general, a Chen-type correlation was developed to predict 98.5 percent of local flow boiling heat transfer coefficient data in the inclined rod bundle with a maximum deviation of ±20%.

Experimental investigation on steam-water two-phase flow boiling heat transfer in a staggered horizontal rod bundle under cross-flow condition

Single-phase convection and steam-water two-phase flow boiling heat transfer experiments in an electrically heated 4 × 16 staggered horizontal rod bundle under cross-flow condition have been carried out for the following range: 45 kg m−2 s−1 ≤ G ≤ 383 kg m−2 s−1, 20.0 kW m−2 ≤ q ≤ 55.0 kW m−2, 0.02 ≤ xout ≤ 0.57 and 112 kPa ≤ Pin ≤ 190 kPa. The major effects on local flow boiling heat transfer coefficients in horizontal rod bundles were analyzed. Firstly, the row number has a significant effect on local flow boiling heat transfer coefficients since the vapor bubbles generated from the lower rods impinge on the surface of the upper rods and enhance the turbulence there. Secondly, an increasing heat flux contributes to an increase of the average bundle flow boiling heat transfer coefficient at all mass velocities. Thirdly, the local quality can also slightly enhance the flow boiling heat transfer. Fourthly, the flow boiling heat transfer can be enhanced with an increasing mass velocity under lower heat fluxes. In a general, a Chen-type correlation was developed to predict local flow boiling heat transfer coefficients in horizontal rod bundles with a maximum deviation of ±20%.

Hydrodynamics of two-phase flow in a rod bundle under cross-flow condition

Pressure drop experiments were conducted for a vertical two-phase air–water flow across a horizontal staggered rod bundle under conditions of air quality ranging from 0.001 to 0.3 and mass velocity ranging from 80 to 500kg/(m2s). The experimental data were thoroughly analyzed and compared with predictions from existing correlations. New developed expressions provided the most accurate predictions. The two-phase friction multipliers for G⩽200kg/(m2s), which show a strong mass velocity effect, could be fit well in terms of the Martinelli parameter and the dimensionless mass velocity. The new correlation developed for the two-phase friction multiplier was successfully tested in predicting the experimental data for G⩽200kg/(m2s) in previous experiments of other researchers. Distributions of velocity and turbulent intensity were also obtained from the numerical simulation on single-phase flow of water using the CFD code ANSYS FLUENT 14.0. The SST k–ω model was verified to be the best turbulent model to gain flow characteristics in the horizontal rod bundle under experimental conditions.

Computational fluid dynamics analysis of the Canadian deuterium uranium moderator tests at the Stern Laboratories Inc.

A numerical calculation with the commercial computational fluid dynamics code CFX-14.0 was conducted for a test facility simulating the Canadian deuterium uranium moderator thermalehydraulics. Two kinds of moderator thermalehydraulic tests at Stern Laboratories Inc. were performed in the full geometric configuration of the Canadian deuterium uranium moderator circulating vessel, which is called a calandria tank, housing a matrix of horizontal rod bundles simulating calandria tubes. The first of these tests is the pressure drop measurement of a cross flow in the horizontal rod bundles. The other is the local temperature measurement on the cross section of the horizontal cylinder vessel simulating the calandria system. In the present study, the full geometric details of the calandria tank are incorporated in the grid generation of the computational domain to which the boundary conditions for each experiment are applied. The numerical solutions are reviewed and compared with the available test data. Copyright © 2015, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society.

Natural convection heat transfer from horizontal rod bundles in liquid sodium. Part 2: Correlations for horizontal rod bundles based on theoretical results

Natural convection heat transfer from horizontal rod bundles in Nxm × Nym arrays (Nxm, Nym = 5–9) in liquid sodium was numerically analyzed for three types of the bundle geometry (in-line rows, staggered rows I and II). The unsteady laminar two-dimensional basic equations for natural convection heat transfer caused by a step heat flux were numerically solved until the solution reaches a steady state. The PHOENICS code was used for the calculation considering the temperature dependence of thermophysical properties concerned. The surface heat fluxes for each cylinder were equally given for a modified Rayleigh number, Rf, ranging from 0.0637 to 63.1 (q = 1×104 to 7×106 W/m2). Sx/D and Sy/D for the rod bundle, which are the ratios of the distance between center axes on the abscissa and the ordinate to the rod diameter, respectively, were ranged from 1.6 to 2.5 on each bundle geometry. The spatial distribution of Nusselt numbers, Nu, on horizontal rods of a bundle was clarified. The average value of Nusselt number, Nuav, for three types of bundle geometry with various values of Sx/D and Sy/D were calculated to examine the effect of the array size, S/D and Rf on heat transfer. The bundle geometry for the higher Nuav value under the condition of Sx/D×Sy/D = 4 was examined by changing the ratio of Sx/Sy. A correlation for Nuav for the three types of bundle geometry above mentioned including the effects of Sx/D and Sy/D was developed. The correlation can describe the theoretical values of Nuav for the three types of bundle geometry in Nxm × Nym arrays (Nxm, Nym = 5–9) for Sx/D and Sy/D ranging from 1.6 to 2.5 within 10% difference.

Natural convection heat transfer from horizontal rod bundles in liquid sodium. Part 1: Correlations for two parallel horizontal cylinders based on experimental and theoretical results

Natural convection heat transfer coefficients on two parallel horizontal test cylinders in liquid sodium were obtained experimentally and theoretically for various setting angles, γ, between vertical direction and the plane including both of these cylinders’ axes, over the range of 0°–90°. Both test cylinders are 7.6 mm in diameter and 50 mm in heated length with the ratio of the distance between each cylinder axis to the cylinder diameter, S/D, of 2. Theoretical equations for laminar natural convection heat transfer from the two horizontal cylinders were numerically solved for the same conditions as the experimental ones. The average Nusselt numbers Nu on the cylinders obtained experimentally were compared with the corresponding theoretical values on the Nu versus modified Rayleigh number Rf [= Gr*Pr2/(4 + 9Pr1/2 + 10Pr)] graph. The experimental values of Nu for the upper cylinder are about 20% lower than those for the lower cylinder at γ = 0° for the range of Rf tested here. The value of Nu for the upper cylinder becomes higher and approaches that for the lower cylinder with the increase in γ over the range of 0°–90°: the values for each cylinder agree with each other at γ = 90°. The values of Nu for the lower cylinder at each γ are almost in agreement with those for a single cylinder. The theoretical values of Nu on two cylinders except those for Rf < 4 at γ = 0° are in agreement with the experimental data at each γ with the deviations less than 15%. Correlations for two cylinders were obtained as functions of S/D and γ based on the theoretical solutions. A combined correlation for multi-cylinders in a vertical array based on the correlations for two cylinders was developed. The values by the correlation agree with the theoretical solution for the multi-cylinders for Rf ranging from 4.7 to 63 within 10% difference.

Numerical investigation of supercritical water-cooled nuclear reactor in horizontal rod bundles

The commercial CFD code STAR-CD v4.02 is used as a numerical simulation tool for flows in the supercritical water-cooled nuclear reactor (SCWR). The basic heat transfer element in the reactor core can be considered as round rods and rod bundles. Reactors with vertical or horizontal flow in the core can be found. In vertically oriented core, symmetric characters of flow and heat transfer can be found and two-dimensional analyses are often performed. However, in horizontally oriented core the flow and heat transfer are fully three-dimensional due to the buoyancy effect. In this paper, horizontal rods and rod bundles at SCWR conditions are studied. Special STAR-CD subroutines were developed by the authors to correctly represent the dramatic change in physical properties of the supercritical water with temperature. In the rod bundle simulations, it is found that the geometry and orientation of the rod bundle have strong effects on the wall temperature distributions and heat transfers. In one orientation the square bundle has a higher wall temperature difference than other bundles. However, when the bundles are rotated by 90° the highest wall temperature difference is found in the hexagon bundle. Similar analysis could be useful in design and safety studies to obtain optimum fuel rod arrangement in a SCWR.

Experiment and numerical simulation of bubbly two-phase flow across horizontal and inclined rod bundles

Experimental and numerical analyses were carried out on vertically upward air-water bubbly two-phase flow behavior in both horizontal and inclined rod bundles with either in-line or staggered array. The inclination angle of the rod bundle varied from 0 to 60° with respect to the horizontal. The measured phase distributions indicated non-uniform characteristics, particularly in the direction of the rod axis when the rods were inclined. The mechanisms for this non-uniform phase distribution is supposed to be due to: (1) Bubble segregation phenomenon which depends on the bubble size and shape; (2) bubble entrainment by the large scale secondary flow induced by the pressure gradient in the horizontal direction which crosses the rod bundle; (3) effects of bubble entrapment by vortices generated in the wake behind the rods which travel upward along the rod axis; and (4) effect of bubble entrainment by local flows sliding up along the front surface of the rods. The liquid velocity and turbulence distributions were also measured and discussed. In these speculations, the mechanisms for bubble bouncing at the curved rod surface and turbulence production induced by a bubble were discussed, based on visual observations. Finally, the bubble behaviors in vertically upward bubbly two-phase flow across horizontal rod bundle were analyzed based on a particle tracking method (one-way coupling). The predicted bubble trajectories clearly indicated the bubble entrapment by vortices in the wake region.

A Numerical Study of Convection Heat Transfer Within Enclosed Horizontal Rod Bundles

Natural-convection heat transfer in enclosed horizontal N × N arrays of heated rods with a constant heat flux dissipation is numerically investigated. For a fixed rod diameter d, the width of the i...

Heat transfer to horizontal tubes in a fluidized bed: The role of superficial gas and local particle velocities

The simultaneous measurement of local heat transfer coefficients and local particle velocities presents a unique opportunity to examine the mechanisms of heat transfer to immersed surfaces in fluidized beds. This was carried out for several locations of a simulated tube bank as well as a single immersed cylinder. The experiments were conducted in a 184 mm (7.25 in.) I.D. air fluidized bed with soda-lime glass beads of diameters ranging from 425 to 600 μm and superficial velocities of 35.6, 54.9, 77.2, and 100.4 cm/s. The tube bank consisted of an inline horizontal rod bundle of 15.88 mm (0.625 in.) diameter, with a pitch/diameter ratio of 4. The results showed that there is a lack of consistent correlation between particle dynamics and surface-to-bed heat transfer, either local or average. This finding casts doubt on the validity of heat transfer models that are based solely on particle convection. Comparison of the overall average heat transfer coefficients with Decker and Glicksman's heat transfer model and other published correlations showed poor agreement at low fluidization velocities but reasonably good agreement at high velocities.

Two-phase flow regimes during the rewetting and refilling of hot horizontal tubes

The development of accurate mechanistic models for the refilling and rewetting of hot horizontal tubes or rod bundles requires knowledge of the relative phase distribution (ie, flow regimes) encountered during such processes. Characterization of the relative phase distribution is essential in evaluating the interfacial transport of mass, energy, and momentum and hence is important in the development of mathematical models of the refilling and rewetting phase of the hypotheticall loss of coolant accident (LOCA) in nuclear power reactors. Experiments on the refilling and rewetting of hot horizontal tubes were carried out. The relative phase distribution near the advancing quench front was obtained by analyzing the simultaneous measurements of transient void fraction and transient circumferential surface temperature distribution taken at the same cross section of the tube. The void fraction was measured by using a gamma densitometer, and subminiature thermocouples were used to measure wall temperatures. The results showed clearly that three different flow regimes can be encountered: stratified, inverted annular, and annular. For moderate refilling flow rates [up to 900 kg/(m 2·s)], the propagating quench front was preceded by a stratified refilling front. For higher flow rates, stratification effects were less pronounced, and the advancing refilling front approach an inverted annular flow pattern. However, for low tube wall heat capacity and at low inlet flow, annular flow was encountered. The effects of inlet flow rate, initial wall temperature, inlet subcooling, and the wall heat capacity on the phenomena are analyzed, and the implication of the results on the physical modeling are discussed.