Transverse Heat Transfer Research Articles

Transverse Transport and Heat Transfer in Smooth VVER Fuel-Rod Bundle with Cellular Spacer Grid

The results of the numerical modeling of flow and heat transfer in a bundle of smooth fuel rods spaced by a cellular grid in application to VVER are presented. The results are analyzed and compared with computational-experimental studies in the homegrown and foreign nuclear power industries. The computational region simulating a VVER FA fragment, a mathematical model, and the boundary conditions are described. The results of a calculation of the coolant flow velocity field and the temperature field for the fuel-rod cladding under conditions of stabilized heat transfer in the bundle and on a section of stabilization beyond the cellular spacer grid are presented. Flow in the bundle and the mechanisms of interchannel transfer are described. Recommendations are given for taking account of the grid effect on intensification of heat transfer in the bundle, and a comparison is made with data obtained for plate-shaped spacer grids in PWR.

Experimental Study of Steady-State Transverse Heat Transfer in a Single Channel CICC

A new configuration of the THETIS installation for thermal–hydraulic tests of forced-flow superconducting cables, such as, e.g., cable-in-conduit conductors (CICCs), has been prepared at West Pomeranian University of Technology, Szczecin. In the present form, THETIS enables pressure drop and heat transfer coefficient measurements in short samples of conductors using water in a wide range of temperature and Reynolds number. We present the new configuration of the installation and demonstrate its capabilities by reporting the results of the first thermal–hydraulic test conducted on a smooth tube with a square cross section and on a reference sample (JT-60SA TF conductor). The results obtained for a smooth tube are in good agreement with predictions of the standard Dittus–Boelter heat transfer correlation, which positively verifies correctness of the applied procedures. The experimental Nu values obtained for the CICC sample remain almost constant in the considered Re range 700–1700, and they are about three times larger than those predicted by the smooth tube correlation.

Simultaneous optimisation of residence time, heat and mass transfer in laminar duct flows

Suitably designed laminar duct flows admit chaotic advection which, in concert with diffusion, can lead to rapid heat and mass transport and sharpening of the residence time distribution (RTD). Whilst evolution of these distinct scalar fields are strongly related, the exact relationships between these distinct fields is unknown, nor to what extent they can be simultaneously optimised. In this paper we present a unified framework for the simultaneous optimisation of the three scalar fields; RTD, temperature, and mass concentration. This optimisation is performed in terms of the eigenmodes of the advection-diffusion operator, which generalize classical Taylor-Aris axial dispersion. We apply this optimisation framework to a twisted pipe flow (TPF) at Péclet number Pe=105, and find 47- and 237-fold increases in transverse heat and mass transfer respectively over straight tube flow, along with a 2,000-fold suppression of RTD variance growth. We show that generality of the eigenmode decomposition suggests this framework is universal to all duct flows.

A spiral microreactor for improved stability and performance for catalytic combustion of propane

Numerical CFD simulations are used to demonstrate the performance characteristics of a novel spiral geometry, which is used for catalytic micro-combustion in this work. This reactor has catalytic channel, with inlet at the center and spiraling outwards. The spiral microreactor geometry allows preheating of the cold inlet owing to heat exchange with the hot products flowing in co-current direction in the adjacent channel. Results indicate that the spiral is more stable than the straight channel reactor, due to the transverse heat transfer between adjacent channels in the spiral reactor. 2D simulations are performed to comprehensively analyze the role of this heat recirculation on the thermal management and stability of catalytic combustion of propane/air mixtures; and the spiral microreactor is compared with the well-studied straight channel microreactor. Axial and transverse temperature profiles at various design and operating conditions are analyzed for a better understanding of heat recirculation in this geometry. Effect of various parameters viz. equivalence ratio (ϕ), inlet velocity (u0), wall thermal conductivity (ks) and number of turns (N) on the spiral reactor’s stability and performance is presented.

Effect of heat and mass transfer on the combustion stability in catalytic micro-combustors

The combustion characteristics of methane-air and propane-air mixtures in catalytic micro-combustors was studied numerically in order to clarify the importance of transport in combustion stability. The primary focus is on heat and mass transfer as a means of understanding energy management at small scales. A two-dimensional computational fluid dynamics model including detailed gas-phase and surface chemistry and multicomponent transport was developed for obtaining design insights. The effect of typical operating variables and design parameters was studied. Different performance measures were evaluated to assess the operability of the system. Engineering maps denoting combustion stability were constructed and design recommendations were also made. It was shown that in practical engineering design, heat and mass transfer plays a critical role in the process of catalytic combustion, and transport properties are usually key factors in determining combustion stability. Heat and mass transfer between the catalytic surface and gas phase depends strongly on the flow velocity and wall thermal conductivity. The operation of the system is limited by the heterogeneous reaction at low flow velocities and by mass transfer at high flow velocities. In the range of moderate flow velocities, combustion stability is determined by heat and mass transfer at low wall thermal conductivities and by transverse heat transfer within the system at high wall thermal conductivities. Finally, the role of combustor dimension in enhancing transport rates was illustrated, and the fuel transport properties were evaluated to assess the effect of mass transfer.

Effects of viscous dissipation and transverse magnetic field heat transfer over a stretching cylinder under convective boundary condition

Effects of viscous dissipation and transverse magnetic field heat transfer over a stretching cylinder under convective boundary condition

Flame stability and heat transfer analysis of methane-air mixtures in catalytic micro-combustors

The flame stability and heat transfer characteristics of methane-air mixtures in catalytic micro-combustors were studied, using a two-dimensional computational fluid dynamics (CFD) model with detailed chemistry and transport. The effects of wall thermal conductivity, surface emissivity, fuel, flow velocity, and equivalence ratio were explored to provide guidelines for optimal design. Furthermore, the underlying mechanisms of heat and mass transfer for loss of flame stability were elucidated. Finally, stability diagrams were constructed and design recommendations were made. It was found that the heat loss strongly affects extinction, whereas the wall thermal conductivity greatly affects blowout. The presence of homogeneous chemistry extends blowout limits, especially for inlet velocities higher than 6m/s. Increasing transverse heat transfer rate reduces stability, whereas increasing transverse mass transfer rate improves stability. Surface radiation behaves similarly to the heat conduction within the walls, but opposite trends are observed. High emissivity causes the flame to shift downstream. Methane exhibits much broader blowout limits. For a combustor with gap size of 0.8mm, a residence time higher than 3ms is required to prevent breakthrough, and inlet velocities lower than 0.8m/s are the most desirable operation regime. Further increase of the wall thermal conductivity beyond 80W/(m·K) could not yield an additional increase in stability.

Development of a transverse mixing model for large scale impulsion phenomenon in tight lattice

Tight-lattice is widely adopted in the innovative reactor fuel bundles design since it can increase the conversion ratio and improve the heat transfer between fuel bundles and coolant. It has been noticed that a large scale impulsion of cross-velocity exists in the gap region, which plays an important role on the transverse mixing flow and heat transfer. Although many experiments and numerical simulation have been carried out to study the impulsion of velocity, a model to describe the wave length, amplitude and frequency of mixing coefficient is still missing. This research work takes advantage of the CFD method to simulate the experiment of Krauss and to compare experiment data and simulation result in order to demonstrate the feasibility of simulation method and turbulence model. Then, based on this verified method and model, several simulations are performed with different Reynolds number and different Pitch-to-Diameter ratio. By fitting the CFD results achieved, a mixing model to simulate the large scale impulsion phenomenon is proposed and adopted in the current subchannel code. The new mixing model is applied to some fuel assembly analysis by subchannel calculation, it can be noticed that the new developed mixing model can reduce the hot channel factor and contribute to a uniform distribution of outlet temperature.

Status and Challenges of Nuclear Thermal Hydraulics Research in China for Water-Cooled Reactors

Nuclear energy has been considered an important part of the long-term energy mixture in China. Accompanying the development of the nuclear power program, extensive research and development (R&D) activities were initiated. Nuclear thermal hydraulics has been recognized as a key subject in the development of nuclear technology because the target of nuclear thermal hydraulics is to remove heat safely from the fuel. The thermal hydraulics activities in China cover three main areas, namely, reactor core thermal hydraulics, safety-related thermal hydraulics, and fundamental thermal hydraulics.This paper gives a brief summary of the ongoing R&D activities in these three areas with a focus on a few selected topics: (1) transversal mixing and nonuniform heat transfer distribution in fuel assemblies, (2) two-phase distribution at external reactor vessel cooling conditions, and (3) heat transfer of supercritical fluids. The state-of-the-art ongoing works and challenging aspects are presented and discussed.

Forced Convective Flow of a Nanoliquid due to a Stretching Cylinder with Free Stream

Steady, transverse boundary layer flow and heat transfer caused by an exponentially stretching cylinder of constant radius immersed in an uniform flow of an incompressible, viscous nanoliquid are considered in the present study. The paper discusses a systematic procedure of obtaining a local similarity transformation that reduces the governing partial differential equations into ordinary differential equations. Power series solution is then obtained for velocity, temperature and nanoparticle concentration distributions using the uni-variate differential transform method. Help is sought from Domb-Sykes plots in making a decision on the minimum number of terms required in the power series expansion to ensure convergence. Radius of convergence is quite naturally suggested by these plots. Pad ´ e approximants are then appropriately decided upon to increase the radius of convergence. The algorithm used succeeds in capturing boundary effects, free stream flow effects and nanoparticle effects on flow and heat transfer. An important finding of the paper is the prediction of accelerated cooling of the stretching cylinder due to the nanoparticles in the cooling liquid. In having a desirable property for the extruding cylinder nanoliquid coolant seems an attractive proposition.

Transverse heat transfer coefficient in the dual channel ITER TF CICCs. Part III: Direct method of assessment

The data resulting from the thermal-hydraulic test of the ITER TF CICC are used to determine the flow partition and the overall effective heat transfer coefficient (hBC) between bundle and central channel in a direct way, i.e. by analysis of the heat transfer between both flow channels, based on the mass and energy balance equations and the readings of thermometers located inside the cable. In cases without a local heat source in the considered cable segment the obtained hBC values were consistent with those obtained in earlier studies by analysis of experimental data using indirect methods. It was also observed that the transverse heat transfer was strongly enhanced in a cable segment heated from outside. This phenomenon results from the mass transfer from the bundle region to the central channel. The experimental hBC data obtained for the case without a heat source in the considered segment were also compared with those calculated using various heat transfer correlations.

Numerical investigation on transverse heat transfer properties in cross section of full size Nb3Sn CICC ITER conductor

The contact mechanical characteristics in the cross section of the Nb3Sn cable are sensitive to the cryogenic cooling and cyclic transverse electromagnetic loads, which may affect the cable’s performance. In this paper, based on a proposed discrete dynamic model (DEM), where the contact heat transfer among strands and the convective heat transfer in liquid helium are taken into account, the cooling process under two heat transfer mechanisms is performed. Simulation results show that the temperature variation of Poloidal Field Insert Sample (PFIS) cable with time agrees well with the existing experimental results, and the role of contact heat transfer cannot be neglected during cryogenic cooling. It is obtained from the further analysis that the effect of contact heat transfer becomes more prominent with the decrease of mass flow rate of liquid helium, which leads to the stress status within cable changed significantly. With the temperature boundary condition imposed on the cable radial direction, the effective thermal conductivity (ETC) of cable can be obtained. It can be found that the ETC increases with increasing the transverse loads and is sensitive to the low temperature environment, while it is not affected by load cycles basically. These results may provide the guide for the design and application of the future CICC conductors.

Transverse heat transfer coefficient in the dual channel ITER TF CICCs Part II. Analysis of transient temperature responses observed during a heat slug propagation experiment

A heat slug propagation experiment in the final design dual channel ITER TF CICC was performed in the SULTAN test facility at EPFL-CRPP in Villigen PSI. We analyzed the data resulting from this experiment to determine the equivalent transverse heat transfer coefficient hBC between the bundle and the central channel of this cable. In the data analysis we used methods based on the analytical solutions of a problem of transient heat transfer in a dual-channel cable, similar to Renard et al. (2006) and Bottura et al. (2006). The observed experimental and other limits related to these methods are identified and possible modifications proposed. One result from our analysis is that the hBC values obtained with different methods differ by up to a factor of 2. We have also observed that the uncertainties of hBC in both methods considered are much larger than those reported earlier.

Experimental modeling of three-dimensional modes of hypersonic flow past cylindrically-blunted bodies

Several sets of experimental studies of the structure of transverse hypersonic flow past blunt bodies (cylinder and truncated wedge) and heat transfer on them are performed in the UT-1M shock tube of the Central Aerohydrodynamics Institute. The purpose of the investigation was to obtain three-dimensional modes of hypersonic flow past the nose surfaces of blunt bodies in an artificially disturbed and nominally uniform flows. The controlled disturbances in the freestream were produced by thin threads pulled over the nozzle exit. In the experiment the flow was visualized using the Topler method and the heat flux distribution over the cylinder was measured using luminescent temperature transformers. The experiments show that both the flow and the heat transfer in the vicinity of the cylinder nose are very sensitive to vortex disturbances in the oncoming hypersonic flow. In a nominally uniform flow (M∞ = 8 and Re∞ = 3160–11670) a steady three-dimensional mode of flow past the nose surface of a blunt wedge could be obtained in the form of a single vortex pair.

Numerical study of the turbulent flow and vortex structures in a new lattice

Quasi-periodic large scale vortex structures are the major reason for the enhancing of transverse fluid mixing and heat transfer in rod bundles. In this work, the turbulent flow, heat transfer and vortex structures in a new lattice with enhanced heat transfer and fluid mixing are investigated. The numerical simulations are executed by Unsteady Reynolds-Averaged Navier–Stokes (URANS) method with the Reynolds Stress Model (RSM). It is revealed that the transverse fluid mixing and the vortex structures are enhanced as the local wall thickness increases, whereas the Nusselt number shows a peak value. The streamwise velocity in gap regions and the global frictional resistance coefficient will monotonically decrease as the local wall thickness increases. This new lattice is superior to the traditional rod bundles and an optimized value of the local wall thickness with ΔR/δ=20% is recommended.

Understanding the Convection Heat-Transfer Mechanism in the Steam-Assisted-Gravity-Drainage Process

Summary Steam-assisted gravity drainage (SAGD) is the preferred method to extract bitumen from Athabasca oil-sand reservoirs in western Canada. In SAGD, steam, injected outward from a horizontal injection well, loses its latent heat when it contacts the cold bitumen at the edge of a steam chamber. Consequently, the viscosity of the bitumen falls several orders of magnitude, enabling it to flow under gravity toward a horizontal production well directly below to the injection well. It is commonly believed that conduction is the dominant heat-transfer mechanism at the edge of the chamber. Heat transfer by convection is not considered in classic SAGD mathematical models such as the one derived by Butler. Researchers such as Butler and Stephens (1981), Reis (1992), Akin (2005), Liang (2005), Nukhaev et al. (2006), and Azad and Chalaturnyk (2010) considered the conduction from steam to cold reservoir to be the only heat-transfer component. Farouq-Ali (1997), Edmunds (1999a, b), Ito and Suzuki (1996, 1999), Ito et al. (1998), Sharma and Gates (2011), and Irani and Ghannadi (2013) questioned the assumption that thermal conduction dominates heat transfer at the edge of a SAGD chamber. Sharma and Gates (2011) and Irani and Ghannadi (2013) studied convective flux from condensate flow at the edge of an SAGD steam chamber. Irani and Ghannadi (2013) derived a new formulation that solves the energy balance and pressure-driven condensate flow normal to the steam-chamber interface into the cold bitumen reservoir and concluded that the assumption of conduction-dominated heat transfer is valid; however, all previous analyses do not include convective heat transfer arising from draining bitumen and condensate. Although a few researchers have studied convective flux from condensate flow at the edge of an SAGD steam chamber (e.g., Sharma and Gates 2011; Irani and Ghannadi 2013), there is a lack of understanding of bitumen and condensate drainage parallel to the edge of the chamber and of its effect on transverse heat transfer into the oil sand beyond the chamber. In this study, the relative roles of convective heat flux both parallel and normal to the edge of a steam chamber are examined. The results suggest that the convective heat flux associated with flow parallel to the chamber edge is minor compared with that normal to the edge.

Unique ZnS–SiO2 Morphologies Reflecting a Laser-Induced Heat Distribution

Unique morphologies including a hemispherical dot and an inverse trapezoid line were fabricated from a mixture of zinc sulfide (ZnS) and silicon dioxide (SiO2) using thermal lithography. Depending on the laser power, three types of morphologies were formed for each of the dot and line patterns. The patterns were affected by the concentric heat transfer from the underlying light-absorption layer, and the transverse heat transfer caused by the spatial restriction. These unique morphologies are difficult to fabricate using photolithography, and they are therefore promising for new functional applications involving microscale structures.

Following the energy transfer in and out of a polyproline–peptide

The intramolecular and intermolecular vibrational energy flow in a polyproline peptide with a total number of nine amino acids in the solvent dimethyl sulfoxide is investigated using time-resolved infrared (IR) spectroscopy. Azobenzene covalently bound to a proline sequence containing nitrophenylalanine as a local sensor for vibrational excess energy serves as a heat source. Information on through-space distances in the polyproline peptides is obtained by independent Förster resonance energy transfer measurements. Photoexcitation of the azobenzene and subsequent internal conversion yield strong vibrational excitation of the molecule acting as a local heat source. The relaxation of excess heat, its transfer along the peptide and to the solvent is monitored by the response of the nitro-group in nitrophenylalanine acting as internal thermometer. After optical excitation, vibrational excess energy is observed via changes in the IR absorption spectra of the peptide. The nitrophenylalanine bands reveal that the vibrational excess energy flows in the peptide over distances of more than 20 Å and arrives delayed by up to 7 ps at the outer positions of the peptide. The vibrational excess energy is transferred to the surrounding solvent on a time scale of 10-20 ps. The experimental observations are analyzed by different heat conduction models. Isotropic heat conduction in three dimensions away from the azobenzene heat source is not able to describe the observations. One-dimensional heat dissipation along the polyproline peptide combined with a slower transversal heat transfer to the solvent surrounding well reproduces the observations.

Heat Transfer to MHD Oscillatory Viscoelastic Flow in a Channel Filled with Porous Medium

The combined effect of a transverse magnetic field and radiative heat transfer on unsteady flow of a conducting optically thin viscoelastic fluid through a channel filled with saturated porous medium and nonuniform walls temperature has been discussed. It is assumed that the fluid has small electrical conductivity and the electromagnetic force produced is very small. Closed-form analytical solutions are constructed for the problem. The effects of the radiation and the magnetic field parameters on velocity profile and shear stress for different values of the viscoelastic parameter with the combination of the other flow parameters are illustrated graphically, and physical aspects of the problem are discussed.

Well-balanced schemes using elementary solutions for linear models of the Boltzmann equation in one space dimension

In the kinetic theory of gases, a class of one-dimensional problems can be distinguished for which transverse momentum and heat transfer effects decouple. This feature is revealed by projecting the linearized Boltzmann model onto properly chosen directions (which were originally discovered by Cercignani in the sixties) in a Hilbert space. The shear flow effects follow a scalar integro-differential equation whereas the heat transfer is described by a $2 \times 2$ coupled system. This simplification allows to set up the well-balanced method, involving non-conservative products regularized by of the stationary equations, in order to produce numerical schemes which do stabilize in large times and deliver accurate approximations at numerical steady-state. Boundary-value problems for the stationary equations are solved by the technique of elementary solutions at the continuous level and by means of the analytical discrete ordinates method at the numerical one. Practically, a comparison with a standard time-splitting method is displayed for a Couette flow by inspecting the shear stress which must be a constant at steady-state. Other test-cases are treated, like heat transfer between two unequally heated walls and also the propagation of a sound disturbance in a gas at rest. Other numerical experiments deal with the behavior of these kinetic models when the Knudsen number becomes small. In particular, a test-case involving a computational domain containing both rarefied and fluid regions characterized by mean free paths of different magnitudes is presented: stabilization onto a physically correct steady-state free from spurious oscillations is observed.