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The heat pipe reactor represents a promising high-temperature microreactor design comprising heat pipes, fuel rods, and monoliths. Prolonged operation at elevated temperatures leads to an obvious thermal creep and thermal stress within the monolith. The monolith may have structural failure due to creep damage and fatigue damage caused by temperature fatigue load. This paper presents an analysis of the creep fatigue damage in the monolith of the MegaPower heat pipe reactor using the American Society of Mechanical Engineers (ASME), Boiler and Pressure Vessel Code Section III, Division 5 (BPVC Sec. III, Div. 5) inelastic design-by-analysis rules. The research findings demonstrate pronounced stress relaxation in the monolith caused by thermal creep, resulting in a redistribution of thermal stress. The region experiencing peak thermal stress within the monolith transitions from the thinnest web between the fuel rods to the edge of the monolith after 50 000 h of operation at full power. Thermal creep results in a 40.5% decrease in peak thermal stress and a 0.023% increase in the displacement amplitude of the monolith. The creep fatigue damage in the monolith at full power for 50 cycles, each lasting 1000 h, adheres to the design rule limitation of the ASME BPVC. The damage is primarily concentrated in the thinnest web region at the edge of the monolith, predominantly attributed to creep damage. The creep fatigue damage check in the monolith should carefully consider the effect of stress relaxation. [ABSTRACT FROM AUTHOR]

Exploring fire resistance of high strength steel end-plate connections is one of the key technical bases for fire resistance design and fire safety assessment of high strength steel structures. Numerical simulation and parameter analyses of high strength Q690 steel flush end-plates will contribute to a reasonable design of high strength flush end-plate connections. In this paper, a finite element model of high strength steel flush end-plate connection considering thermal creep effect was established and validated by experimental results. The thermal creep finite element model can simulate accurately heat performance, failure modes and deformations of high strength flush end-plate connections at elevated temperatures. The increase of axial compression ratio has little effect on rotation, deflection, failure time, critical temperature, and critical rotation with elevated temperatures. The increase of bending moment ratio has little effect on the development trend of rotation and deflection. Proper increase of thickness of the end-plate can improve failure time and critical rotation, and the increase range is within 7% and 36% respectively. Proper increase of diameter of bolt can improve failure time, critical temperature, and critical rotation to a certain extent, and the increase range is within 10%, 6% and 27% respectively. Proper increases of thickness of the end-plate and diameter of bolt will improve the fire resistance of flush end-plate connection. [ABSTRACT FROM AUTHOR]

In northern regions, the long-term serviceability of infrastructure founded on frozen ground is adversely affected by climate warming. Rates of change in temperature, which are not identical spatially and temporally, can remarkably dictate the magnitude and evolution of permafrost degradation. Investigating such impacts requires a non-isothermal rate-dependent geomechanical constitutive model for ice-contained geomaterials. In this paper, a critical-state thermo-elasto-viscoplastic (TEVP) constitutive model is developed for modeling time- and temperature-dependent behavior of frozen soils using the concepts of thermo-elasticity and thermo-viscoplasticity. Solid phase stress (defined as the excess of total stress over fluid pressure), in addition to the cryogenic suction, are considered as the two independent stress state variables to establish the model. The proposed model is able to satisfactorily capture the rate-dependent behavior of frozen soils observed in the experimental tests reported in the literature. [ABSTRACT FROM AUTHOR]

This paper presents assessments performed on a large database of virgin material and irradiated material thermal creep data from uniaxial and pressurised DIN 1.4970 Ti-stabilised austenitic stainless steel i.e. EN X10NiCrMoTiB15–15 or '15-15Ti' cladding tubes. The data base incorporates multi-heat data from uniaxial and bi-axial (internal pressure) creep tests conducted during the fast reactor R&D program of the DeBeNe (Deutschland-Belgium-Netherlands) consortium between the 1960s to the late 1980s together with more recent data, for example, from the European projects MATTER and PATRICIA and the EERA JPNM pilot project TASTE. The data comprises of a virgin material data base and a data base with irradiated creep data. The virgin material data comprises of time-to-rupture and time-to-0.2% creep strain in a temperature range of 600–800°C and covers a large range of stresses. The irradiated data base has less data and does not cover all material heats tested for the virgin material properties. The irradiation conditions are also different depending on the test reactor specifics and the irradiation campaign targets. The attained 'irradiation damage' in displacements per atom (DPA) for the irradiated materials range from 0.1 up to 38.1. Un-irradiated 'reference material' models for tensile strength and creep strength are constructed from the current-state of the art literature data and more recent results from the Sandvik 24% cold worked 1515–Ti cladding tube currently used as the main material batch studied at SCK CEN. The tensile strength model is used throughout the paper for normalizing creep strengths by the tensile properties, needed when applying the Wilshire (WE) creep model. Time factors (TF), stress factors (SF) and temperature ratio factors (TRF) are calculated for different material states for describing the impact of heat treatments, (virgin and irradiated) and irradiation conditions. This initial study targets to give estimates for the thermal creep properties of cladding tubes with a cold work range of 16–24% in a non-annealed state, as it is the preferred option for future designs according to the state-of-the art knowledge base. However, the main bulk of the available data on irradiated material is on claddings with a cold work range of 0-16% with and without annealing and at various levels of irradiation damage, thus leading to the need for estimation by interpolation and extrapolation assuming that irradiation damage levels and trends, e.g. time reduction factors found on a lower cold worked material can also be applied on materials with higher cold work levels. The results of the assessments clearly show relative strength differences between chosen material heats and heat treatments and has enabled constructing simple multilinear models for estimating the life of irradiated material. The models show that low creep test temperatures (and irradiation temperatures), low stress levels, low levels of cold work and high irradiation doses are increasing the detrimental difference between the specific material condition and the reference material. Also, it was found that the general level of life reduction (time factor TF and stress factor SF) for irradiation damaged material failure, in relation to the reference material failure, roughly coincides the corresponding values of time to 0.2% creep strain for virgin material. The estimated 'failure creep temperature limits' for 30 000 hours of service are studied and compared for both virgin and irradiated materials at a reference stress level of 1/3 of the tensile strength (at temperature). [ABSTRACT FROM AUTHOR]

Thermal transpiration flow, a flow from cold to hot, driven by a temperature gradient along a wall under a high Knudsen number condition, was studied using the molecular dynamics method with a two-dimensional channel consisting of infinite parallel plates with nanoscale clearance based on our previous study. To accelerate the numerical analysis, a dense gas was employed in our previous study. In this study, the influence of the number density of gas was investigated by varying the height of the channel while keeping the number of molecules to achieve the flow ranging from dense to dilute gas while maintaining a constant Knudsen number. From the flow velocity profile compared to the number density profile, the thermal transpiration flow was observed for all number density conditions from dense to dilute gas. A similar flow structure was exhibited regardless of the number density. Thus, the numerical analysis in a dense gas condition is considered to be valid and useful for analyzing the thermal transpiration flow. [ABSTRACT FROM AUTHOR]

Monotonic and cyclic thermal loading significantly affect the axial and radial deformation, and lateral pressure coefficient of soil under true triaxial conditions, which poses new challenges to the crack resistance, settlement control and bearing capacity design of underground energy structures. Thermal creep simulation is carried out for saturated clay and silt under 1D compression and plane strain in drained situation based on the Tsinghua thermodynamical soil (TTS) constitutive model and its Matlab model driver. The effect of over consolidated ratio (OCR) and thermal cycles on thermal creep and lateral earth pressure coefficient under 1D confined condition is predicted, and the effect of true triaxial stress conditions on thermal creep of different principal stress direction is simulated. The simulated thermal creep direction and magnitude of bonny silt by TTS under true triaxial drained condition are consistent with the test results. The temperature creep amplitude of Bangkok soft clay is greater than that of Bonny silt under the same test condition. The results show that the thermal shrinkage or expansion creep in each principal stress direction and temperature shear strain under true triaxial condition can be reasonably described by TTS. The research can provide reference for the design of the side friction bearing capacity of energy piles. [ABSTRACT FROM AUTHOR]

The effect of hydrogen on the creep rates of Zircaloy-4 claddings especially in the low hydrogen concentration remains largely unexplored while creep is considered a main mechanism for spent nuclear fuel failure along with hydride-induced mechanisms. Here, we study the influence of hydrogen on the creep behavior of low burnup Zircaloy-4 cladding from a microstructural perspective. The creep tests at 450 °C and 120 MPa reveal that specimens with hydrogen contents below the terminal solid solubility for dissolution (TSSD) at the test temperature exhibited decreased creep resistance as hydrogen concentration increased, while those with hydrogen contents above the TSSD demonstrated increased creep resistance with higher hydrogen concentration. The lowest creep resistance occurs at the hydrogen concentration corresponding to the TSSD. Additional microstructural analyses using electron backscattered diffraction (EBSD) show the increasing intra-granular/inter-granular hydride ratio in the post-crept specimens as the hydrogen concentration decreases, suggesting that hydrogen atoms in solid solution aid creep progression. Also, an increased amount of twin boundaries is observed in post-crept specimens with hydrogen concentration above TSSD. These results suggest that creep assessment in spent nuclear fuels may require caution especially in low-burnup regime. [ABSTRACT FROM AUTHOR]

Thermal transpiration flow, a flow from cold to hot, driven by a temperature gradient along a wall under a high Knudsen number condition, was studied using the molecular dynamics method with a two-dimensional channel consisting of infinite parallel plates with nanoscale clearance based on our previous study. To accelerate the numerical analysis, a dense gas was employed in our previous study. In this study, the influence of the number density of gas was investigated by varying the height of the channel while keeping the number of molecules to achieve the flow ranging from dense to dilute gas while maintaining a constant Knudsen number. From the flow velocity profile compared to the number density profile, the thermal transpiration flow was observed for all number density conditions from dense to dilute gas. A similar flow structure was exhibited regardless of the number density. Thus, the numerical analysis in a dense gas condition is considered to be valid and useful for analyzing the thermal transpiration flow.

The experimental temperature is 613.15~763.15 K, and the strain rate is 0.01~10 s−1. The hot compression creep test of the 6082-T6 aluminum alloy sample is carried out by Gleeble-3500 hot compression simulation compressor, and its creep behavior is studied by scanning electron microscope. The results show that the DRX crystal has an irregular shape and that content of the Mg phase, Si phase, and Mn phase in the crystal are the main factors to change the color of DRX crystal. Temperature and strain rate are important factors affecting dynamic recrystallization. Reducing temperature and increasing strain rate will weaken dynamic recrystallization, and DRX critical condition and peak stress (strain) will increase. The constitutive equation of hot creep of 6082 aluminum alloy was established by introducing the work hardening rate-rheological stress curve, and the relationship between DRX critical condition, peak stress (strain) and parameter Z during creep was explored. Based on the Av rami equation, the prediction equation of the DRX volume fraction is established. With the increase of strain, DRX volume fraction is characterized by slow increase, then rapid increase and then slowly increase. In the hot -forming extrusion process of 6082 aluminum alloy, according to the volume fraction prediction equation, the DRX can be reduced, and the internal structure of the material can be optimized by changing the extrusion conditions and particle size. [ABSTRACT FROM AUTHOR]

The cladding of the fuel rod is a cylinder-shaped structure made of Zirconium alloy, which will collapse due to structural creep under the extremely service conditions such as high-temperature, high-pressure and high-irradiation. The collapse of the cladding results in losing its structure function and threatening the safety of the reactor. Based on the continuum mechanics theory, the governing equations of three-dimensional cladding structure were established and the finite element numerical calculation software BINE3D was developed. The results show that the creep deformation of the cladding increases and the creep deformation rate decreases gradually as the increasing of the irradiation in the reactor, which may lead to the collapse of the cladding eventually. The results of the software are consistent with the commercial finite element software ABAQUS. The software can be utilized to structural design, safety analysis and life evaluation of the fuel rod cladding [ABSTRACT FROM AUTHOR]

In this study, using the molecular dynamics simulation method, three systems for fluid pumping at the nanoscale have been proposed based on the thermo-osmotic mechanism. These pumps work by applying a symmetric temperature gradient along the wall of a nanopore, which is asymmetric in shape or material. The three systems are a composite nanotube, a conical nanotube, and a composite conical nanopore. The simulation results show that, in all of the proposed systems, the fluid can be pumped continuously by means of heat energy and without using any external force or moving component. The physical mechanisms of the flow in these pumps are clarified using the principles of the thermo-osmotic phenomenon. The simulations show the geometry of the pump and the fluid-solid interaction strength play an important role in determining the pumping strength in all systems. It is shown that a composite conical nanopump compared to other proposed systems has a better performance in fluid pumping. [ABSTRACT FROM AUTHOR]

Thermal creep is one of the key properties of mixed oxide (MOX) fuel for innovative fast reactors. Thermal creep of fuel affects markedly the interaction between the fuel and the cladding. A review of correlations available in the literature is presented. The effect of porosity, plutonium concentration, and stoichiometry are discussed also in the light of recent numerical results. Our analysis pointed out some inconsistencies concerning the modelling of the effect of porosity on diffusional creep and a re-evaluation of the effect of plutonium concentration. The discussion suggested that Evans's findings on the effect of stoichiometry should be better assessed as well as the level of increase in creep moving towards stoichiometry. Typical operating conditions of fast breeder reactors (FBRs) confirmed the need for an extension of porosity and temperature correlations' domains. Besides this, a new correlation based on a separate-effect approach has been proposed for fuel performance codes. • A review of correlations for thermal creep of FBR MOX fuel is presented. • Effects of porosity, plutonium concentration, and stoichiometry are discussed. • A correlation based on a separate-effect approach is proposed. [ABSTRACT FROM AUTHOR]

This paper summarizes preliminary results of an experimental investigation on the creep behavior of transverse welded lap joints at elevated temperatures. The aim of this study is to quantify the time-to-failure for transverse welded lap joints exposed to high temperatures. In this study, two series of experiments were conducted. In the first series, the specimens were heated to target temperatures and then loaded to failure to estimate the peak load P that the specimens could sustain at each target temperature. In the second series, after the specimens were heated to the test temperatures, fractions of the peak load P were applied rapidly and kept constant throughout the test. Through the combination of these two series of experiments, the effect of load and temperature on the time-dependent or creep behavior of transverse welded lap joints was studied. The experimental results show that, for temperatures greater than 450°C, transverse welded lap joints undergo excessive deformation due to creep, and that creep effect becomes more significant as load and temperature increase. [ABSTRACT FROM AUTHOR]

This work presents a numerical analysis of the effect of thermal creep on gas flow and heat transfer in a microchannel under gas cooling conditions. The hydrodynamic equations in vorticity-stream function form are solved together with the energy equation using a finite difference scheme. Axial conduction, pressure work, and viscous dissipation effects are included in the analysis. Solutions are obtained with and without thermal creep effects to illustrate the thermal creep effect on gas cooling. The results show that thermal creep becomes more pronounced as the wall temperature decreases relative to the inlet gas temperature. The results also show that the thermal creep effect on gas cooling becomes more significant as Reynolds number decreases. Furthermore, increasing Knudsen number has caused thermal creep effects to extend further into the channel. Additionally, the work has concluded that thermal creep causes an increase in the hydrodynamic entrance length. Moreover, the work also showed that for low enough wall temperature, Knudsen number, and Reynolds number, the slip velocity can reach negative values resulting in a significant change in the wall velocity allowing vortices to form. The work has also concluded that, at least for most practical purposes, thermal creep has no tangible effects on the thermal part of the problem, where the solution with and without thermal creep effect, is producing almost the same results for temperature field, total heat transfer, and thermal entrance length. [ABSTRACT FROM AUTHOR]

Thermal creep is a key property of zircaloy cladding. CZ developed by CGN is a new zircaloy used as PWR fuel cladding. This research is devoted to investigating the thermal creep behavior of CZ and build the thermal creep model of CZ. Twenty internal pressure creep tests were conducted, and the ranges of temperature and Tresca stress were 320–430 °C and 70–300 MPa, respectively. Real-time creep data were analyzed by separating primary creep and steady-state creep. Based on Soderberg model and creep test data, CZ thermal creep model is derived. As a whole, the mean value and the standard deviation of P/M of CZ saturated primary creep strain are very close to these from steady-state creep rate, however, the predictive effect of primary creep is less satisfactory. Four conditions, where there exists large deviation between predicted values and test data, are 320 °C and 300 MPa, 350 °C and 190 MPa, 380 °C and 160 MPa, 380 °C and 190 MPa, respectively. As primary creep was much smaller than steady-state creep in long-time operation, the thermal creep model built can be applied to predict the thermal creep behavior of CZ cladding.

The creep behavior of Ni–Cr–W alloy at 950 °C has been investigated by a novel creep testing system which is capable of in-situ measurement of strain. Tubular specimens were pressurized with argon gas for effective stresses up to 32 MPa. Experimental results show that the thermal fatigue reduces the creep life of the tubular specimens and with the introduction of thermal cycling fatigue the primary stage disappears and the creep rate higher than the pure thermal creep (without thermal fatigue). Also the creep behavior of Ni–Cr–W alloy doesn't consist in the secondary stage. A new creep equation has been derived and implemented into finite element method. The results from the finite element analyses are in good agreement with the creep experiment.

This paper presents a study of the influence of various factors on heat transfer in microchannels associated with the use of the velocity slip (including thermal creep) and temperature jump boundary conditions on the walls. These factors can affect heat transfer both individually and together, but their effect on heat transfer in the microchannel has still not been practically studied. An attempt to eliminate this lack was made in this paper. It was shown that as the slip length increases the value of the heat flux density increases by about 40% and the average heat transfer coefficient increases by about 50% compare to no-slip conditions. Wherein, the pressure drop significantly decreased, that is, two positive effects took place simultaneously. The presence of thermal creep also reduced the pressure drop in the channel and increased the average heat transfer coefficient, but this effect was significantly less than the effect of the slip length. On the other hand, the taking into account of a temperature jump on the wall led to a certain increase in the pressure drop, but the heat transfer coefficient increased.

This study examined rarefied thermally-driven flow in a square cavity (Case 1) and rectangular bend (Case 2), with various uniform wall temperatures in two dimensions. We employed the direct simulation Monte Carlo (DSMC) to solve problems with a wide range of Knudsen numbers Kn = 0.01 to 10, and the discrete unified gas kinetic scheme (DUGKS) solver was used at Kn = 0.01. The scenario was that, in case 1, the bottom side and its opposite were set hot, and the other sides were set cold. Diffuse reflector boundary conditions were set for all walls. The imposed temperature differences created four primary vortices. The results of the continuum set of equations of the slow non-isothermal flow (SNIT) solver proved that the primary vortices in the square cavity were caused by nonlinear thermal stress effects, and other smaller vortices appearing at Kn = 0.01, 0.1 were brought about by thermal creep processes. As the Kn increased, vortices generated by thermal creep disappeared, and eddies created by nonlinear thermal stress occupied the cavity. In case 2, i.e., a rectangular bend, two sides were set cold, and the others were hot. Two primary vortices were formed, which were caused by nonlinear thermal stress effects. The direction of streamlines in the two main vortices was opposite, from the warm to the cold zone, as some eddies on the left were counterclockwise, and others were clockwise.

The effect of creep on the strength capacities, ductility, and failure modes of shear-bolted lap joints at elevated temperatures is investigated. Thirteen bolted lap joints (representing shear tab connections) are tested under different loading rates: fast rate (10 mm/min) and slow rates (1 mm/min and 0.3 mm/min) for temperatures ranging from 450°C to 700°C. The temperature- and the time-dependent behavior of the lap joints are examined. The results show a change in failure mode from tear-out failure when using fast loading rate to net section fracture when using slow loading rates at temperatures 450°C and 500°C. The results show that the failure mode of the bolted lap joints changed from plate failure to bolt shear failure at temperatures larger than 500°C. The slow loading rate tests resulted in a reduction in the bolt shear capacities when compared to the fast rate tests. Moreover, finite element models are developed to understand the change in failure mode of the connections from tear-out failure to net section fracture and to bolt shear failure. This study is considered as a step towards developing design guidelines for shear tab connections at elevated temperatures while considering the effect of creep. [ABSTRACT FROM AUTHOR]

Owing to its superior performance at high temperatures, stainless steel is usually considered as a potential replacement for mild steel in a wide range of structural fire engineering applications. When it comes to the fire design of steel structures, time‐dependent deformations occurring in the adequately‐heated steel material, called "thermal creep" need to be somehow considered in the design calculations. In EUROCODE 3‐1‐2, for example, the thermal creep strains are implicitly included in the transiently‐determined stress‐strain curves for steel at elevated temperatures. Using the same approach, this paper investigates the fire‐temperature mechanical properties of Grade 304 stainless steel by conducting transient tensile tests at temperatures of up to 900°C on standard coupons taken from a 3.0 mm thick plates. To simulate structural fire conditions in the transient tests, the temperature of the sample is steadily increased when a constant tensile load is applied. From these tests, strain‐temperature curves are obtained, wherein the strain contains both mechanical and thermal components. The strain‐temperature curves with the same heating rate of 10°C/min are then converted to elevated‐temperature stress‐strain curves suitable for fire design applications. [ABSTRACT FROM AUTHOR]

The post-irradiation anneal tests for dispersion fuel plates are commonly employed to determine the blistering threshold temperature in order to evaluate their resistance to accidents. In this study, a three-dimensional simulation method for UMo/Zr dispersion fuel plates is developed to predict the normal stresses in the fuel skeleton and some related variables during in-pile irradiation and out-of-pile anneal tests. The effects of the intrusion of extra fission gas atoms into gas bubbles are investigated, and the contributions of the creep performances of fuel particles and the matrix are examined. The predictions reveal the following insights: (1) the thermo-mechanical behaviors during anneal tests are strongly dependent on the intrusion of extra fission gas atoms into gas bubbles and the additional fission-gas induced swelling during post-irradiation anneal tests; the maximum tensile stress in the fuel skeleton increases significantly, from 57.4 MPa to 346.0 MPa, as the fraction of generated fission gas atoms in gas bubbles shifts from approximately 23.4 % to 100 % in the annealing process; (2) the thermo-mechanical responses during anneal tests are greatly influenced by the creep performances of fuel particles and the matrix; the significantly weakened creep ability of fuel particles is found to be the critical factors responsible for particle cracking in the annealing process; the fuel particle fracture will hardly occur when the irradiation creep rate model is maintained during the considered annealing scheme, because the skeleton tensile stress does not appear. It is important to note that the particle cracking and the subsequent blister behaviors under in-pile neutron irradiation conditions are possible to occur if a rapid temperature increase takes place under accident conditions. Consequently, the out-of-pile annealing tests are of significance. [ABSTRACT FROM AUTHOR]

Clayey soils in the vicinity of energy geostructures may be exposed to long-term periodic thermal cycles. The creep and consolidation behaviors of the clayey soils can be both rate-dependent and temperature-dependent, and the underlying physical mechanisms are merely investigated theoretically. In this study, based on the theory of thermodynamics, a fully coupled thermo-hydro-mechanical (THM) finite element (FE) program for saturated soils is developed for this purpose. The FE formulation accounts for the combined effect of rate and temperature through the novel concept of granular temperature. Simulations of THM coupled validation cases and a series of experimental observations on the soft Bangkok clay are carried out. The obtained numerical results exhibit good agreement with analytical solutions and laboratory measurements. It is found that three fundamental physical mechanisms contribute to the irreversible thermal contraction observed for normally consolidated and lightly overconsolidated clays under drained thermal cycles: (1) the thermal creep excited by mass exchange from adsorbed water to free water; (2) the mechanical creep induced by confining stresses; and (3) the increase in granular packing caused by the thermal expansion of soil particles. The thermal contraction generally stabilizes within a few thermal cycles, as a result of the noticeable reduction in the thermal creep rate. It is further demonstrated that the transient heat transfer and the heating rate can greatly influence the deformation of clays subjected to thermal cycles. [ABSTRACT FROM AUTHOR]

The study of fluid flow and heat transfer inside micromechanical systems is the focus of many researchers due to their importance in microchannels as small-scale devices. Reducing the size of channel has lead the scientist to concentrate on microsensor. Metal oxide gas microsensors, as a micromechanical device, are used to detect gases such as O3, SO2, CO2, NO, NH3, CH4, etc. The purpose of the current study is to numerically investigate the influence of three-dimensional diverging/converging microchannel on its gas inlet temperature while the flow of gas is due to thermal creeping. The coupled governing nonlinear differential equations, mass, momentum, energy, and species, are solved using an in-house code that is based on finite element. Maxwell equation for the slip boundary condition where the Knudsen number is between 0.01 and 0.1 is utilized. The result shows as channel width increases from 500 to 1300 µm the maximum increase in velocity is about 19% and the maximum decrease in temperature is about 1%, and the velocity and temperature stay constant after l = 1300 µm for all converging channels. From 500to 50 µm, the maximum decrease in velocity is about 30%, and the maximum decrease in temperature is about 1% that the maximum velocity changes take place at y = 1000 µm. [ABSTRACT FROM AUTHOR]

This article presents an analysis of kinetic studies of dry methane reforming (DRM) in a reactor with a membrane catalyst (RMC) in the modes of a contactor with “diffusion” and “forced” mass transfer. Comparison of the specific rate constants of the methane dissociation reaction in membrane and traditional reactors confirmed the phenomenon of intensification of dry methane reforming in a membrane catalyst (MC). It has been experimentally established that during DRM, a temperature gradient arises in the channels of the pore structure of the membrane catalyst, characterized by a decrease in temperature towards the inner volume of the MC, and initiates the phenomenon of thermal slip. The features of this phenomenon are highlighted and must be considered in the analysis of kinetic data. The main provisions of the hypothesis explaining the effect of intensification by the occurrence of thermal slip in the channels of the pore structure of the MC are formulated. The proposed hypothesis, based on thermal slip, explains the difference in rate constants of traditional and membrane catalysts, and substantiates the phenomenological scheme of DRM stages in a reactor with a membrane catalyst.

The study of the mass and heat transfer phenomena in micro-nano devices and systems (MEMS/NEMS) very often involves polyatomic gas flows in long capillaries. In the case of polyatomic gases, the internal degrees of freedom of gas molecules may have a significant effect on the macroscopic quantities of practical interest. The scope of the present study is two-fold. First, an accurate methodology for calculating the mass and heat flow rate coefficients is proposed based on a simple kinetic model proposed by Holway. The proposed methodology is based on the available experimental data in the literature and allows overcoming the well-known limitation of the Holway model to describe all the transport coefficients simultaneously. Second, the proposed methodology is applied to investigate the rarefied polyatomic gas flows through long micro- and nanotubes under temperature and pressure differences taking into account the translational, rotational and vibrational degrees of freedom of the gas molecules on the basis of the Holway kinetic model. Results are presented for N2, CO2, CH4 and SF6 representing linear and non-linear, but always non-polar molecules in a wide range of the gas temperature. The mass flow rates of the polyatomic gases in the temperature-driven flow differ significantly from the corresponding monatomic ones, with the difference reaching a maximum of about 25% for the examined gases and temperatures varied between 300 and 1000 K. The maximum relative deviation between monatomic and polyatomic heat flow rates can reach 39%, 64%, 73% and 87% for N2, CO2, CH4 and SF6, respectively. The results are compared against the corresponding published ones based on the Rykov model highlighting the very good agreement between the two models and the accuracy of the proposed methodology for the calculation of the mass and heat flow coefficients is confirmed. [ABSTRACT FROM AUTHOR]

Knowledge of elevated-temperature mechanical properties of structural steel is essential for accurate evaluation of structural behavior in a fire. As a result, various test procedures have been adopted to obtain the stress–strain behavior and to establish elevated-temperature mechanical properties of structural steel suitable for structural-fire applications. This paper provides a critical review of steady-state and transient-state temperature material tests, the two common methods of characterizing the mechanical behavior of structural steel subjected to fire. The main focus in this review is the thermal creep of steel and how it is modeled in each material test. It is indicated that these two test methods result in rate-dependent stress–strain curves, in which creep effects are implicit in the behavior of steel at elevated temperatures. Alternatively, steady-state temperature creep tests allow for the explicit consideration of thermal creep in the form of isochronous stress–strain curves. It is shown that creep can result in significant time- and temperature-dependent reductions in the strength of structural steel in fire and that the adopted method of characterizing the stress–strain behavior of steel for structural-fire engineering applications can lead to underestimation of creep effects depending on the applied stress, temperature, and steel grade. [ABSTRACT FROM AUTHOR]

Correct assessment of the steel connection performance under fire requires including the time-dependent response of steel material in the structural analysis. Failure to do so might impose critical threats on the stability and integrity of steel structures. To this aim, the objective of this study is to investigate the effect of thermal creep on the behavior of shear endplate beam-column connections subjected to transient-state fire temperatures. First, finite element models of shear endplate assemblies are developed using ABAQUS and validated against experimental work available in the literature. Parametric studies are then carried out to study the effect of key geometrical, thermal, and material parameters on the overall response of the frame assembly in fire while explicitly including creep. This includes heating and cooling rates, initial cooling temperature, column size and height, load ratio, plate thickness, and steel grade. The results show that including thermal creep causes a reduction in the induced compressive forces and an increase in the mid-span beam deflection, for about six times higher in some cases, thus earlier development of beam catenary action. It is also concluded that lower heating and cooling rates result in larger beam tying forces on the shear end plate connections, which can reach values around ten times larger than when creep is neglected. This study shows that the current practice of neglecting creep in fire analyses, especially in slow heating, may underestimate the forces that are exerted on the shear endplate connections during fire and thus leads to unsafe structural design. [ABSTRACT FROM AUTHOR]

Abstract At the low atmospheric pressure on Mars, thermal creep within its soil is unavoidable. Soil is a complex mixture of particles and pores and it is not clear a priori how to quantify such subsoil thermal creep gas flow. A capillary model of flow through granular beds was set up recently based on laboratory experiments. Here, we extend the earlier experiment to probe different parameter ranges of the model. For a different granular bed we directly traced the resulting gas flow in a pressure regime of 2–300 Pa. We specifically analyze the dependence of thermal creep on different gas species - we used He, CO 2 , N 2 and air. These data agree well to the model calculations. This latest evidence strongly supports the simple but analytical model of granular thermal creep. Including CO 2 in the measurements the model now allows quantitative calculations of gas circulation and pressure distribution within Martian soil as a network of Knudsen pumps. Highlights • We experimentally verified the analytical model by Koester et al. (2017) for a thermal creep gas flow through a granular bed. • The gas circulation and pressure distribution within Martian soil can be approximated as a network of Knudsen pumps. [ABSTRACT FROM AUTHOR]

In order to help comprehend the underlying deformation mechanisms related to irradiation creep of metallic polycrystalline materials, the crystal plasticity theory and viscoplastic self-consistent method are combined in this work. In the developed constitutive laws for single crystals, the influence of irradiation-induced defects on the creep strain rate has been effectively addressed by considering dislocation climb and glide. For the former, dislocation climb with irradiation effect can be enhanced through the absorption of point defects by dislocations, where the steady-state concentration of point defects is affected by the defect sinks including dislocations and defect clusters. As a comparison, dislocation glide may be retarded through the impediment of mobile dislocations by irradiation-induced defects. More importantly, the effect of dislocation static recovery has been incorporated into the dislocation evolution law, which is noticed to play a deterministic role for the annihilation of dislocations during the long-term creep process. In addition, the viscoplastic self-consistent method is taken as a cross-scale way to predict the irradiation creep properties of polycrystals. To validate the developed creep model, experimental data of both single crystalline and polycrystalline nickel has been considered under both thermal and irradiation creep. A good agreement between the theoretical results and experimental data is achieved, which offers a solid basis to further analyze the macroscopic irradiation creep deformation from the aspect of microstructure evolution. • A theoretical framework is proposed for thermal and irradiation creep of metallic polycrystals. • Crystal plasticity theory and viscoplastic self-consistent method are combined in the developed model. • Dislocation static recovery is incorporated into the dislocation evolution law. • Theoretical results can match well with corresponding experimental data. [ABSTRACT FROM AUTHOR]

The transient temperature gradient driven flow through a microporous ceramic medium is analyzed experimentally. The steady state characteristic of this flow, the Thermomolecular Pressure Difference (TPD), is measured for five gases; helium, neon, nitrogen, argon, and krypton. The TPD is reached when the temperature driven flow is counterbalanced by the pressure driven flow generated by it. Transient measurements of the pressure evolution at hot and cold side of the porous sample allow the extraction of the pressure relaxation time, then the mass flow rate through the medium. Only for helium the mass flow rate is measured in large range of the rarefaction parameter, and it has the Gaussian type shape. The quasi analytical expression for the relaxation time is proposed and compared with the measured data. Experimental characterization of the temperature driven flow could be helpful for further development of the Knudsen pumps. • Setup is built to characterize temperature driven flow through microporous medium. • Thermomolecular Pressure Difference (TPD) is measured for five gases. • The pressure relaxation time is extracted and compared with the proposed expression. • The mass flow rate through the porous sample is measured. • Experimental characterization could be helpful for development of the Knudsen pumps. [ABSTRACT FROM AUTHOR]

Steel connections play a crucial role in maintaining the integrity and stability of steel building frames especially when exposed to fire temperatures. The behavior of flush endplate connections in fire is shown to be governed by tension bolt failure as bolts lose their strength and stiffness more rapidly at higher temperatures. As a result, the ability to predict the development of stresses in tension bolts of flush endplate connections at different stages of fire is of special importance. One of the factors influencing bolt stresses in fire is the thermal creep or time-dependent inelastic response of steel to elevated temperatures. Therefore, time- and temperature-dependent behavior of tension bolts of flush endplate connections in fire is the focus of this study. Stress-time histories in tension bolts are obtained by explicit consideration of thermal creep of steel in FE models of flush endplate connections at elevated temperatures. To better understand the effect of thermal creep on tension bolt behavior, the correlation between time-dependent rotational deformation of flush endplate connections and bolt stresses is also investigated. Further, the isochronous representation is utilized to study the rotational deformation and the tension bolt stresses under various applied moments ranging from 50% to 95% of the moment capacity and fire temperatures ranging from 450°C to 600°C with 25°C increment. Through such representation, it is indicated that the connection behavior is not only dependent on bolt strength degradation and applied moment, but also affected by the time duration of applied moments and temperatures. Also, with the inclusion of thermal creep of steel, the connection experiences higher rotation and excessive endplate deformation with stress relaxation leading to top tension bolt failure at earlier stages of fire. More specifically, for time exposure greater than or equal to 60 min, the failure temperature of the connection decreases from 600°C to around 550°C. Therefore, neglecting thermal creep of structural steel may result in an unsafe prediction of the overall response of flush endplate connections in fire. [ABSTRACT FROM AUTHOR]

This paper presents a methodology for explicit modeling of the time-dependent behavior or creep of structural steel in the Abaqus finite element (FE) models under transient temperature conditions representative of building fires. In this methodology, available creep models based on conventional creep tests under constant or steady-state temperature conditions were used to predict the time-dependent or creep behavior of steel under variable or transient-state temperature conditions. Specifically, user-defined CREEP subroutines were created in Abaqus to define time-dependent or viscoplastic deformation of structural steel under changing temperatures. To demonstrate the robustness of the proposed methodology, the time-dependent behavior of structural steel at elevated temperatures was modeled under three distinct conditions. These were: steady-state temperature conditions, stepwise steady-state temperature conditions, and transient-state temperature conditions. This paper further presents and discusses the application of the methodology in studying the creep behavior of a shear tab connection assembly under the transient temperature conditions of a fire. Through this application, the importance of explicit consideration of creep in predicting the response of connection assemblies at different stages in the evolution of a structural-fire is also emphasized. [ABSTRACT FROM AUTHOR]

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This article discusses the influence of irradiation in the BN-350 fast neutron reactor on the mechanical properties and the thermal creep of 0.12C18Cr10NiTi austenite steel for the purpose of estimating the lifetime of structure materials with regard to long-term storage of spent nuclear fuel. Mechanical tests have been performed at 350 and 450°C. It is established that at 450°C the strength properties increase and the relative elongation decrease up to destruction, which is attributed to inverse α' → γ transformation intensely running in the irradiated steel at 450°C. With the temperature increase from 350 to 450°C, the rate of established creep of the irradiated steel increases by ~3.7 times. The amount and sizes of carbide inclusions generated upon long-term exposure at 450°C decrease, which can shorten the time of dry storage of spent reactor fuel assemblies. [ABSTRACT FROM AUTHOR]

In the present work, we analyze asymptotically and numerically the conjugate heat transfer between a rarified gas flow and the lower wall of a thin horizontal microchannel. The laminar motion of the gas is originated only by the thermal creep or transpiration effect on the lower wall of the microchannel. It is well known the need to impose, in general, a variable temperature regime at the lower wall to induce the transpiration effect. Usually, it can be reached by setting a linear temperature profile as a boundary condition. However, in our case, we prefer to avoid this simplification taking into account that in practical applications, the temperature profile at the lower wall can be unknown. This case can occur, for instance, in a heat sink or a similar device with a well defined heat dissipation rate. Under this physical configuration, we can assume then that the bottom or external face of this heat sink with finite thermal conductivity is exposed to a uniform heat flux. On the other hand, the upper wall of the microchannel is subjected to a prescribed condition. The above conditions are sufficient to consider the simultaneous or conjugate heat transfer analysis of the heat conduction equation for the heat sink and the mass, momentum and energy equations for the gaseous-phase. Resulting governing equations are written in dimensionless form, assuming that the Reynolds number associated with the characteristic velocity of the thermal creep and the aspect ratio of the microchannel, are both very small. The velocity and temperature profiles for the gas phase and the temperature profiles for the solid wall are predicted as functions of the involved dimensionless parameters and the main results confirm that the phenomenon of conjugate thermal creep exists whenever the temperature of the lower wall varies linearly or nonlinearly. [ABSTRACT FROM AUTHOR]

We carried out two types of drop tower experiments to quantify gas and particle motion induced by temperature gradients inside a porous structure in a low pressure environment. In one setup 400 μm sized particles were traced inside heated channels at pressures of a few Pascal. Their motion is consistent with pure thermophoresis. In the second setup tracer particles were used to track the thermal creep gas flow through a porous dust bed. Here, the flow was traced outside of the dust bed and without thermophoretic motion. The results are consistent with a simple capillary model of the dust bed. [ABSTRACT FROM AUTHOR]

A mechanical-based model is developed to predict the time-dependent effect (thermal creep) and the fast loading-rate behavior of flush endplate connections at elevated temperatures during a fire. The fast loading-rate behavior model consists of multi-linear springs that predict each component stiffness, strength, and rotation. The multi-linear springs temperature expressions are based on the ambient temperature formulations proposed in Eurocode3 Part 1.8 where material properties are considered temperature-dependent. To include the effect of thermal creep in the proposed model, a modified Burgers creep model is developed to predict the time-dependent connection rotation and temperature of flush endplate connections. The modified Burgers creep model consists of linear springs and viscous dashpots that predict the time-dependent connection rotation. The proposed model is validated against experimental results available in the literature and finite element (FE) simulations developed as a part of this study. Through explicit consideration of thermal creep effect, the proposed model helps providing important insights into fire-induced thermal stresses and deformations and their implications on designing of flush endplate connections in fire. [ABSTRACT FROM AUTHOR]