Effect Of Heat Conduction Research Articles

Insights into the catalytic CO2 methanation of a boiling water cooled fixed-bed reactor: Simulation-based analysis

This article presents the simulation-based analysis of experimental results from a catalytic fixed-bed reactor for the methanation of CO2. During previously published experiments, temperature profiles inside bidisperse catalyst-silicon carbide fixed-beds were gathered and reactant conversions at the reactor outlet measured. On the basis of the temperature profiles along the fixed-bed length and outlet reactant conversions, a procedure for the numerical characterisation of these results was developed. Primary parameters like the inserted multipoint thermocouple and the heat transfer coefficient between reactor wall and surrounding boiling water cooling are systematically investigated. Concerning the binary bidisperse fixed-beds of porous catalyst cylinders and highly thermal conductive silicon carbide, a simple numerical method for the determination of the effective heat conductivity of the bed is presented. The modelling of effective reaction rates within an industrial sized catalyst and reactor is a non-trivial task, due to interlinking aspects like intrinsic reaction rate and internal and external transport limitations. Intrinsic reaction rate, internal and external transport limitations are linked, what makes a separate investigation challenging. Thus, a step by step analysis is presented and the single effects investigated in detail. From the results of the stepwise numerical characterisation, an excellent agreement between experiments and numerics was reached. Additionally, a novel partially resolved 3D fixed-bed reactor model is presented and selected results like particle overheating presented. Overall the present numerical investigations provided a deep insight into the methanation of CO2 in a cooled fixed-bed reactor.

Heat waves interference regarding dual-phase-lag, hyperbolic and Fourier heat conduction in CNT reinforced composites under a thermal shock

We present a framework to investigate the effects of transient heat conduction on heatwave interference and the overshooting phenomenon in carbon nanotube (CNT)-reinforced composites. Material properties of the nanocomposite are temperature and volume dependent. Different kinds of heat conduction models such as Fourier, hyperbolic or single-phase-lag (SPL), and dual-phase-lag (DPL) are considered while the media is subjected to a heat pulse or periodic thermal shock. This paper attempts to add three contributions to the existing literature: (1) using the differential quadrature method (DQM) to solve for the first time, the nonlinear DPL heat conduction equations while the materials and properties are geometry- and temperature-dependent; (2) showing the effects of the volume fraction of CNTs and their distribution on the transient heat conduction and heatwave interference; (3) demonstrating heatwave interference and the high-temperature gradient as the origins of the overshooting phenomenon. The accuracy of the present solution is confirmed by comparing it with available results from the literature.

Heat‐in‐leak and longitudinal wall heat conduction influence on three‐fluid cross‐flow heat exchanger performance

AbstractThe combined effect of the longitudinal wall heat conduction in the partitioning wall and ambient heat‐in‐leak (HL) from the surrounding to the heat exchanger is a serious factor affecting the performance of a multifluid heat exchanger. The performance of a three‐fluid cross‐cocurrent flow heat exchanger under such circumstances is investigated in this paper. The governing equations for three‐fluids and two separating walls are formulated on the basis of energy balance equations and solved using the finite element method (FEM). Nondimensional parameters (C1,2, C3,2, R1, R3, Ht, Hb, , , , and the number of transfer units [NTU]) are generated and used for the investigation. Effectiveness‐number of transfer plots method is used to analyze the combined effect of wall heat conduction and HL from the surrounding. The combined effect reduces the hot fluid effectiveness significantly for all the parameters considered. The effect of HL alone is predominant as compared with longitudinal wall conduction alone. However, the negligence of the above two parameters in the thermal design of the heat exchanger contributes to a significant loss in hot fluid effectiveness. For a balanced mass flow rate of hot and cold fluid, the drop in the hot fluid effectiveness is found to be 37% approximately (with C3,2 = 0.5, R1 = R3 = 1, Ht = Hb = 0.05, and = = 0.025 at NTU = 10). The results present insight and important considerations involved in the design of such heat exchangers. Also with the application of the FEM, for any number of elements, the stiffness matrix can be generated by writing a program and the exit temperatures are calculated very easily.

Computation of the conducto-radiative effective heat conductivity of porous media defined by Triply Periodic Minimal Surfaces

Triply Periodic Minimal Surfaces are attractive porous media for the design of volumetric solar energy receivers; their design involves the evaluation of their heat transfer capabilities in the mixed conductive/radiative mode. We present image-based computations of effective thermal conductivities (ETC) for different radiation/conduction ratios in twenty distinct TPMS structures, using a Hybrid Random Walk numerical method, considering Opaque/Transparent media. Results are cast in simple analytical correlations : the ETC is the sum of a purely conductive contribution proportional to the relative density and of a radiative contribution proportional to the radiation/conduction ratio; the coefficient of proportionality contains an emissivity-independent contribution and another contribution proportional to the emissivity which fades out above a critical ratio. This critical ratio is linearly proportional to the solid/void volume ratio of the media. These results are compared to similar ones obtained on open-cell foams. Application of these dimensionless laws to two realistic cases are presented as illustrative examples.

The preparation and performance of a polyaniline/graphene composite coated fabric

Graphene was compounded with polyaniline to prepare a polyaniline/graphene composite coated fabric. The influence of the graphene on a series of properties of coated fabrics was discussed, and the results show that when the graphene content was 10 to 15% that of the polyaniline, the polarizing ability, loss ability and absorbing-attenuation ability of fabrics to electromagnetic waves were all stronger. As the coating thickness increases, the breaking strength and elongation at break of the fabric both showed an increase initially before decreasing; when the thickness was 0.5 mm, both fabrics were at their maximums, and the mechanical properties were at their best. The thermal decomposition temperature of the polyaniline/graphene composite coated fabric (411.3 °C) was higher than for the polyaniline coated fabric (402.9 °C) by 8.4 °C. The residue rate at 800 °C for the polyaniline/graphene composite coated fabric (18.83%) was larger than for the polyaniline coated fabric (14.17%) by 4.66%; the fabric had better thermal stability after the graphene was added. The graphene, with high thermal conductivity, was added to the polyaniline matrix as a filler, which can effectively fill the gaps between the polyaniline molecules, and an effective heat-conduction pathway forms and the heat-dissipation performance for the polyaniline/graphene composite coated fabric is improved. When the graphene content was 15% that of the polyaniline, the coefficient of thermal conductivity increases from 0.1011 W/mK to 0.1843 W/mK, an increase of nearly 82%.

Space-time adaptive ADER discontinuous Galerkin schemes for nonlinear hyperelasticity with material failure

We are concerned with the numerical solution of a unified first order hyperbolic formulation of continuum mechanics that goes back to the work of Godunov, Peshkov and Romenski [65,68,97] (GPR model) and which is an extension of nonlinear hyperelasticity that is able to describe simultaneously nonlinear elasto-plastic solids at large strain, as well as viscous and ideal fluids. The proposed governing PDE system also contains the effect of heat conduction and can be shown to be symmetric and thermodynamically compatible, as it obeys the first and second law of thermodynamics. In this paper we extend the GPR model to the simulation of nonlinear dynamic rupture processes, which can be achieved by adding an additional scalar to the governing PDE system. This extra parameter describes the material damage and is governed by an advection-reaction equation, where the stiff and highly nonlinear reaction mechanisms depend on the ratio of the local equivalent stress to the yield stress of the material. The stiff reaction mechanisms are integrated in time via an efficient exponential time integrator. Due to the multiple spatial and temporal scales involved in the problem of crack generation and propagation, the model is solved on space–time adaptive Cartesian meshes using high order accurate discontinuous Galerkin finite element schemes endowed with an a posteriori subcell finite volume limiter. A key feature of our new model is the use of a twofold diffuse interface approach that allows the cracks to form anywhere and at any time, independently of the chosen computational grid, which is simply adaptive Cartesian (AMR). This is substantially different from many fracture modeling approaches that need to resolve discontinuities explicitly, such as for example dynamic shear rupture models used in computational seismology, where the geometry of the rupture fault needs to prescribed a priori. We furthermore make use of a scalar volume fraction function α that indicates whether a given spatial point is inside the solid (α=1) or outside (α=0), thus allowing the description of solids of arbitrarily complex shape. We show extensive numerical comparisons with experimental results for stress-strain diagrams of different real materials and for the generation and propagation of fracture in rocks and pyrex glass at low and high velocities. Overall, a very good agreement between numerical simulations and experiments is obtained. The proposed model is also naturally able to describe material fatigue.

Effect of nonlocality and memory responses in the thermoelastic problem with a Mode I crack

The analysis of fracture is very important along with the miniaturization of the device and wide application of ultra-fast lasers, where size effect on heat conduction and elastic deformation increase and classical theory of thermoelastic coupling does not hold any more. Due to these, size-dependent thermoelastic model have been introduced for higher order simple material to adopt both the size effect of heat conduction and elasticity with the aids of extended irreversible thermodynamics and generalized free energy. With this motivation, the present study is now formulated to provide the nonlocal phenomena of a homogeneous, isotropic infinite space weakened by a finite linear mode I crack. The boundary of the crack is subjected to prescribed temperature and stress. The governing equations have been solved on employing the Laplace and the Fourier transforms, which reduces to four dual integral equations, the solution of which is equivalent to solving the Fredholm's integral equation of the first kind. The Bellman method has been used to compute numerical inversion of the Laplace transform. The result provides the direction how the nonlocality and size-dependence can control the fracture and what is the significant effect of various kernel functions and the effect of memory is also reported.

Rational Transfer Function Model for a Double-Pipe Parallel-Flow Heat Exchanger

Transfer functions of typical heat exchangers, resulting from their partial differential equations, usually contain irrational functions which quite accurately describe the spatio-temporal nature of the processes occurring therein. However, such an accurate but complex mathematical representation is often not convenient from the practical point of view, and some approximation of the original model would be more useful. This paper discusses approximate rational transfer functions for a typical thick-walled double-pipe heat exchanger working in the parallel-flow configuration. Using the method of lines with the backward difference scheme, the original symmetric hyperbolic partial differential equations describing the heat transfer phenomena are transformed into a set of ordinary differential equations and expressed in the form of N subsystems representing spatial sections of the exchanger. Each section is described by a rational transfer function matrix and their cascade interconnection results in the overall approximation model expressed by a matrix of rational transfer functions of high order. Based on the rational transfer function representation, the frequency and steady-state responses of the approximate model are evaluated and compared with those resulting from its original irrational transfer function model. The presented results show better approximation quality for the “crossover” input–output channels where the in-domain heat conduction effects prevail as compared to the “straightforward” channels, where the transport delay associated with the heat convection dominates.

Impact of Temperature on the Transient DC Field Distribution of ±1100 kV UHVDC Wall Bushing

UHV DC wall bushing is the only channel connecting the valve hall and the DC electric field in the converter station. For the epoxy - SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas composite insulation wall bushing, the main insulation material is the epoxy/crepe paper core and SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas. The transient process of establishing the DC electric field for the wall bushing is impacted by the temperature gradient. In this paper the 3-D temperature field of the wall bushing has been simulated considering the effect of heat conduction, convection and radiation. The simulation results have been confirmed using a temperature rise experiment. The transient process of the wall bushing under the actual operation condition has been simulated using the gas saturation current density method. The temperature impact on space charge density in the core, the surface charge on the interface, and the electric field distribution on the bushing is investigated. In this way a theoretical basis is developed for the optimal design of the bushing that is conductive to the safe operation of the bushing.

Hybrid 1D + 2D Modelling for the Assessment of the Heat Transfer in the EU DEMO Water-Cooled Lithium-Lead Manifolds

The European demonstration fusion power reactor (EU DEMO) tokamak will be the first European fusion device to produce electricity and to include a breeding blanket (BB). In the framework of the design of the EU DEMO BB, the analysis of the heat transfer between the inlet and outlet manifold of the coolant is needed, to assess the actual cooling capability of the water entering the cooling channels, as well as the actual coolant outlet temperature from the machine. The complex, fully three-dimensional conjugate heat transfer problem is reduced here with a novel approach to a simpler one, decoupling the longitudinal and transverse scales for the heat transport by developing correlations for a conductive heat-transfer problem. While in the longitudinal direction a standard 1D model for the heat transport by fluid advection is adopted, a set of 2D finite elements analyses are run in the transverse direction, in order to lump the 2D heat conduction effects in suitable correlations. Such correlations are implemented in a 1D finite volume model with the 1D GEneral Tokamak THErmal-hydraulic Model (GETTHEM) code (Politecnico di Torino, Torino, Italy); the proposed approach thus reduces the 3D problem to a 1D one, allowing a parametric evaluation of the heat transfer in the entire blanket with a reduced computational cost. The deviation from nominal inlet and outlet temperature values, for the case of the Water-Cooled Lithium-Lead BB concept, is found to be always below 1.4 K and, in some cases, even to be beneficial. Consequently, the heat transfer among the manifolds at different temperatures can be safely (and conservatively) neglected.

Fluid model for the piezothermal effect

When a gas in an externally imposed potential field is compressed, temperature gradients appear. This has been called the piezothermal effect. It is possible to analytically calculate the time-dependent behavior of the piezothermal effect using a linearized fluid model. Quantitative differences between the fluid-model results and previous numerical calculations can be explained by the effects of viscosity and heat conductivity. The fluid model casts the piezothermal effect as a spectrum of buoyancy oscillations, which yields new physical insights into the effect.

Оценка эксплуатационных характеристик коротковолокнистой базальтовой теплоизоляции при криогенных температурах

This work shows the potential of highly porous thermal insulation made of short basalt super-thin fibers chopped using liquid technology for insulating cryogenic piping. The effective heat conductivity coefficient, maximum-permissible porosity and insulation density were determined for short basalt fibers with a mean diameter of 1.88 mcm and a length of 1.0–1.5 mm, taking into account radiation heat transfer. The results of compression tests are presented for highly porous flat insulation samples made of short basalt fibers without binding agents and those reinforced with Al2O3. It is established that the thermal insulation material containing a binding agent has the strength, elasticity modulus and elastic compression deformation two times higher than samples without a mineral binding agent.

Conjugate heat transfer analysis of liquid metal turbulent flow through a horizontal channel by LES

Conjugate heat transfer analysis of liquid metal turbulent flow through a horizontal channel has been numerically studied using a large eddy simulation (LES) technique. The effect of wall thickness and heat conduction in the wall on flow and thermal characteristics of low Prandtl number liquid metals has been investigated and reported here. Results are presented for three different liquid metal flows with different Reynolds numbers and different wall materials with wall thickness. The flow field is modeled as transient two-dimensional incompressible turbulent forced convection flow. Wall Adapting Local Eddy Viscosity (WALE) sub-grid scale (SGS) model is used to solve the turbulence closure. Two-dimensional transient heat conduction equation is solved to know the temperature distribution in the wall. A finite difference method solver is developed for solving the governing equations using sixth-order accuracy of compact schemes. Prandtl number and Reynolds number alter the heat transfer characteristics. Wall thickness shows a significant effect on the convective heat transfer rate. The new correlation of average Nusselt number has been proposed, including the effect of wall thickness for constant heat flux boundary condition. The present results are validated with DNS data and experimental results available in the literature.

Comprehensive performance prediction and power promotion for wearable thermoelectric generator with flexible encapsulation in practical application

With ability of converting the heat harvested from human body into electricity, the wearable thermoelectric generators (WTEGs) are attracting lots of interests for self-powered electronic devices. However, the output power density of the WTEG is limited for supplying the electronic devices. Therefore, the performance enhancement by optimal design of the developed WTEGs is needed. In this paper, a method of comprehensive modeling of a WTEG is proposed by considering all essential components and realistic boundary conditions. By solving the model with finite element modeling (FEM) method, the calculation performances agree well with experimental results at various temperature difference and load resistance. Through the temperature and voltage distribution of the WTEG obtained, the effects of side heat loss and heat conduction of the encapsulation are revealed. Besides, the impact of the thermal conductivity of encapsulation and the geometric dimensions of thermoelectric legs on the output performance are explored. Finally, the performance prediction results indicate that at the temperature difference of 15 K, the output power of the WTEG with optimized design can be improved about 598.1% with matched loading resistance compared to the device with original design.

Numerical Study on the Phase Sensitivity Variation in Low Frequency Primary Microphone Calibrations

The low frequency phase characteristics of microphones in a monitoring system are crucial for characterizing large-scale natural and artificial activities—e.g., earthquakes, nuclear explosions, or rocket launchings. At present, microphones are simultaneously calibrated using in-situ or calibrator methods to get their phase consistency. However, the essential primary calibration, which traces their phase sensitivity to basic physical quantities, is grossly overlooked. Recently, we speculated that the microphone phase sensitivity is acoustically controlled by the pressure leakage and heat conduction effects in its back chamber, which will vary at low frequencies. Therefore, by means of the FEA (Finite Element Analysis) technique, simulations of laser pistonphone-based primary microphone calibrations are conducted both in the frequency and time domains. The frequency domain simulation quantifies the phase variation, while the time domain analysis helps us to understand the variation mechanism. It is found that the low frequency phase sensitivity is greatly influenced by its geometries and the venting state and should be pre-calibrated before serving.

Analysis on the behavior of dispersed plate-type fuel based on fluid-solid coupling method

In this study, a three-dimensional bi-directional fluid-solid coupling method FLUENT-ABAQUS-MpCCI was used to analyze the behavior of the fuel plate. ABAQUS was used to simulate the solid domain, including the heat conduction, mechanical behavior and irradiation effects. Several subroutines were developed and validated, including CREEP, UHARD, USDFLD and UEXPAN. FLUENT was used to simulate the fluid domain, including pressure and temperature. FLUENT and ABAQUS were coupled by MpCCI to consider the interaction between fluid domain and solid domain. Many parameters were obtained, including fluid temperature distribution, Mises stress distribution and strain distribution. The influences of the fluid domain, particle volume fraction, creep behavior and burnup were analyzed. The results showed that the zero-dimensional model or one-dimensional model on fluid could not accurately simulate the mechanical behavior of the fuel plate. The results also showed that the creep behavior could reduce the Mises stress on the cladding.

Nonlocal dual-phase-lag heat conduction and the associated nonlocal thermal-viscoelastic analysis

In this paper, the extension of the nonlocal dual-phase-lag (DPL) theory is formulated to account for the heat conduction at nanoscale. Both the temporally and spatially nonlocal effects are considered in heat conduction, which are verified experimentally by the size-dependent thermal conductivity of silicon nanofilms and the transient temperature variation during the femtosecond laser heating of gold films. A generalized uncoupled nonlocal thermoviscoelasticity is hence proposed, which is applied to a one-dimensional analysis of a finite plate under sudden thermal shock. Non-dimensional numerical analyses are performed to illustrate the effects of both nonlocal heat conduction and nonlocal elasticity on thermal propagations and thermoviscoelastic responses.

Thermohydrodynamic Model of Cavitating Flow and Dynamic Characteristic Calculation for High-Speed Water-Lubricated Pump-Out Spiral Groove Bearing

A complete thermohydrodynamic lubrication model for pump-out spiral groove thrust bearings (SGTB) with cavitating flow is established based on two-phase flow and transport theory. Various effects (cavitating flow interface effect, convective heat transfer effect, heat conduction effect, inertia effect, breakage and coalescence of bubbles) of water lubricant at high speed are included when modeling. The bubble size distributions, bubble number density of the cavitating flow, and the dynamic coefficients of the SGTB are predicted using this model. The probability distribution of the bubble size, bubble number density, and axial film stiffness coefficient agree with the experimental values. The results show that the number of small-sized bubbles is much larger than the number of large-sized bubbles in cavitating flow. When the SGTB has a specified spiral angle, the direct stiffness coefficients increase and the cross-stiffness coefficients decrease due to the cavitation effect, but the influence of the cavitation effect on the damping coefficient is not obvious.

Experimental investigation of temperature field and fusion zone microstructure in dissimilar pulsed laser welding of austenitic stainless steel and copper

Laser dissimilar welding of copper/ stainless steel was experimentally conducted to investigate the process parameters influence on the temperature field and formation mechanism of melt pool microstructures. The experimental approach is to estimate the influence of the Nd:YAG pulsed laser process parameters (e.g welding speed, laser power, focal length and laser beam deviation) on the temperature gradient and the resultant metallurgical aspects of the melt pool such as Solidification cracking, copper particle diffusion and microstructure of the fusion zone. Creating the appropriate temperature gradient during welding plays a crucial role to effectively control solidification cracking at the fusion zone in order to reduce heat conduction effect of the copper. Increasing welding speed from 2 to 6 mm/s not only reduces the temperature about 50 °C but also decreased the fusion zone cracking significantly. Evidently, increasing the laser power from 240 to 260 W transmitted the fusion zone cracks toward the stainless steel base due to creating higher temperature about 230 °C and 150 °C for steel and copper respectively. Therefore, higher discrepancy of the temperature field between copper and steel clearly changed the place and pattern of cracks propagation. The EDS analysis of the fusion zone confirms diffusion of copper particles inside the weld pool. The temperature measured near the molten pool region showed that clear discrepancy at the temperature gradient of the melt pool at the copper and stainless steel side has increased the possibility of cracking the stainless steel fusion zone. Also, high temperature gradient for compensating copper high rate of cooling could lead to excessive copper ablation or even diffusion of copper elements into the stainless steel fusion zone.

Three-Dimensional Graphite Filled Poly(Vinylidene Fluoride) Composites with Enhanced Strength and Thermal Conductivity

Three-dimensional (3D) network structure has been recognized as an efficient approach to enhance the mechanical and thermal conductive properties of polymeric composites. However, it has not been applied in energetic materials. In this work, a fluoropolymer based composite with vertically oriented and interconnected 3D graphite network was fabricated for polymer bonded explosives (PBXs). Here, the graphite and graphene oxide platelets were mixed, and self-assembled via rapid freezing and using crystallized ice as the template. The 3D structure was finally obtained by freezing-dry, and infiltrating with polymer. With the increasing of filler fraction and cooling rate, the thermal conductivity of the polymer composite was significantly improved to 2.15 W m-1 K-1 by 919% than that of pure polymer. Moreover, the mechanical properties, such as tensile strength and elastic modulus, were enhanced by 117% and 563%, respectively, when the highly ordered structure was embedded in the polymer. We attribute the increased thermal and mechanical properties to this 3D network, which is beneficial to the effective heat conduction and force transfer. This study supports a desirable way to fabricate the strong and thermal conductive fluoropolymer composites used for the high-performance polymer bonded explosives (PBXs).