Heat Flow Model Research Articles

Evaluation and comparison of thermal performance of latent heat storage units with shell-and-tube, rectangular, and cylindrical configurations

• Criteria are proposed to evaluate heat transfer, flow resistance, and irreversibility. • Shell-and-tube LHSU has the shortest phase change time and highest heat transfer rate. • Shell-and-tube LSHU achieves the highest j / f factor and the best thermal performance. • Cylindrical LHSU achieves the highest melting exergy efficiency of 81.5% • Shell-and-tube LHSU has the highest exergy efficiency of thermal store-release cycle. Latent heat storage plays a significant role in tackling the mismatch between energy supply and demand. Evaluation and comparison of the thermal performance of different shaped latent heat storage units (LHSU) are essential in the optimal design of LHSU. This study developed the numerical model of heat transfer and flow in shell-and-tube, rectangular, and cylindrical LHSU with the same PCM mass and heat transfer area. Two kinds of criteria, namely the j / f factor considering both heat transfer and flow resistance and the exergy coefficient evaluating irreversibility, are proposed to assess the performance of LHSU comprehensively. It shows that natural convection intensity in cylindrical LHSU is the highest, while the shell-and-tube LHSU exhibits the lowest phase change time and the highest heat transfer rate. The average j / f factor of shell-and-tube LHSU is 21.4% larger than that of rectangular LHSU and is twice as much as that of cylindrical LHSU. Therefore, the shell-and-tube LSHU achieves the best overall thermal performance. Moreover, cylindrical LHSU achieves the highest melting exergy efficiency of 81.5%, while shell-and-tube LHSU has the highest solidification exergy efficiency of 62.8%. Regarding the thermal storage and release cycle, the exergy efficiencies of shell-and-tube, rectangular, and cylindrical LHSU are 50.1%, 49.2%, and 48.5%, respectively. Therefore, shell-and-tube is the optimal shape to achieve the largest exergy efficiency of the thermal storage-release cycle. This study deepens the understanding of the thermal behavior of PCM packed in different shaped containers, obtains the optimal geometric configuration, and gives guidance to the optimal design of geometric parameters of the LHSU.

"THREE-DIMENSIONAL MODELLING OF RAILS / WHEELS MANUFACTURED BY ER6 AND ER7 IN TRAMWAY APPLICATIONS "

The wheels and rails of the train, tram etc. are often verified from their microstructure and plastic deformation, which usually appear in the outer layer of a wheel and rail, to analyse the causes of geometrical defects by monitoring the applied loads and variation of the temperature as suggested in the literature. This paper studies the effect of thermal stress applied with variations of the loads in contact on wheel/rail for the tramway, tracking through the state of the rail to discover the causes of geometric defects started by temperature variations and loads, and applying these variations of temperature and loads to know its resistance to these climatic conditions. 3D model of temperature distrubtion and heat flow in the wheel and the rail ER6 and ER7 has been developped using the finite element method based on the COMSOL Multiphisics.

Fuel and material utilization of a waste shiitake (Lentinula edodes) mushroom bed derived from hardwood chips II thermal conductivity of compression-dried waste mushroom beds

This study investigated using waste mushroom beds as insulation material and applied compression-drying to eliminate the coarse voids that are unfavorable for insulation. The relationship between air-dry density and thermal conductivity was investigated. The mean air-dry density of the uncompacted waste mushroom beds was approximately 200 kg/m3, which increased to approximately 550 kg/m3 after four levels of compression-drying. A linear relationship was recognized between the air-dry density and thermal conductivity of the waste mushroom bed. Comparing this relationship to those of wood and mat-formed wood-based panels showed that the thermal conductivity of a waste mushroom bed is similar to that of wood at low densities and mat-formed wood-based panels at high densities. Furthermore, the thermal conductivity of heat bridges was estimated using a series–parallel heat-flow model for voids and substances, and the substance thermal conductivity was in the order of wood (0.218 W/mK), waste mushroom bed (0.288 W/mK), and mycelium (0.368 W/mK). The same heat-flow model was applied to a waste mushroom bed to obtain the substance thermal conductivity along the heat-flow direction. The results show that the waste mushroom bed substance also has an anisotropic thermal conductivity.

Numerical study on comprehensive performance of flow and heat transfer coupled with ortho-para hydrogen conversion

The energy consumption of hydrogen liquefaction can be effectively reduced by filling the plate-fin heat exchanger with the catalyst to achieve continuous conversion of ortho-para hydrogen while cooling. In this study, a three-dimensional numerical model of heat transfer and flow coupled with ortho-para hydrogen conversion is established, and the simulation results are verified by the experimental data. Both the temperature field and the outlet parahydrogen fraction demonstrate better accuracy of the local thermal non-equilibrium model results compared to the local thermal equilibrium model results, and the thermal non-equilibrium effect is most significant at the inlet of the porous medium, where the temperature difference between the fluid and the solid exceeds 2 K. Furthermore, it is indicated that two typical stages exist in the hot channel: the heat transfer dominates in the inlet section and the ortho-para hydrogen conversion efficiency decreases significantly, even once falling below 30%. In the thermal fully developed section, the catalytic performance is effectively improved along with the decay of velocity, with a 10% improvement compared to the lowest point of performance. Finally, an integrated comprehensive performance evaluation criterion is summarized regarding the overall performance of heat transfer, flow and catalysis, with excellent comprehensive performance being achieved at Reynolds numbers of 170 to 300. This research can provide a reference for the comprehensive performance study of catalyst-filled plate-fin heat exchanger for hydrogen.

Influences on geothermal circulation in the Okataina Volcanic Centre, New Zealand

The Okataina Volcanic Centre (OVC) in the Taupo Volcanic Zone (TVZ) is New Zealand's most recently active caldera and lava dome complex, erupting 85 km3 of magma equivalent in the past 21 ka. Successive caldera collapses over the past ∼550 kyr have created a complex crustal structure through which fluids now migrate, evidenced by hot springs and several geothermal fields located predominantly around the caldera margins. While recent magnetotelluric (MT) modelling suggests that high-temperature upwelling fluids originate from mid-crustal locations at the margins of an inferred magmatic complex, it is still uncertain to what degree various factors influence these fluid pathways from source to surface. Here, we use heat and fluid flow models to explore the roles that simplified geological structures, topography, and deep, localised heat sources play in determining the locations of upflow at the OVC.To isolate and understand individual processes that modulate fluid flow, we created TOUGH2 heat and fluid flow models that encompass a 35 by 39 km2 area extending beyond the topographic boundaries of the OVC calderas. An average heat input of 700 mW/m2 was set at the base of the models at 5 km depth, either as a uniform hot plate or as discrete zones of higher heat input with locations derived from MT models. The upper boundary of the models followed water table elevation, which is a slightly muted reflection of topography. We constructed a suite of models, ranging from uniform rock properties to more complex permeability variations that correspond to basement/volcanic rocks and large-scale faults inferred from gravity data and surface mapping. Models ran for 50 kyr, the approximate time since the most recent caldera-forming event. Elevated model temperatures were compared with low-resistivity zones from shallow (<500 m) Direct Current (DC) apparent resistivity values and deeper sensing MT resistivity models.Our models suggest that topography and localised basal heat sources are the largest influences on the locations of geothermal upflows. In the western half of the study area, observed upflows beneath Waimangu and Tikitere can be modelled with localised heat sources and topography alone. Upflow under Rotoma to the north-east is not replicated by any of our suite of models, suggesting that unmapped heat sources are important in this area. In the east of the study area, modelled upflow is dominated by present-day topography which is strongly influenced by <25ky Tarawera and Haroharo Volcanic Complexes and < 1ky Tarawera River valley. Our models suggest that strong permeability contrasts associated with large-scale faults would have effects on regional-scale fluid circulation that are not observed in real life, and that the coincidence of OVC topographic boundaries and geothermal systems is due to the boundaries' also being topographic lows rather than fluid conduits.

Molten pool behavior and solidification characterization in steady magnetic field assisted laser-MIG hybrid welding of aluminum alloy

Microstructure evolution is highly correlated with the melt flow characterization in welding. In this study, a 3D heat flow model with consideration of magnetic field was constructed to describe the temperature field, the flow and keyhole behaviors in the molten pool during the laser-MIG hybrid welding. The role of magnetic flux density in the molten pool dynamics and microstructure evolution was explored. A moving hybrid heat source model including Gaussian surface heat source model and Gaussian rotary body heat source model as well as double ellipsoidal heat source model was carried out to elucidate the laser and MIG arc energy distribution. Decent agreement is realized between the measured weld geometry and predicted weld pool dimension. Numerical results indicate that an expanded molten pool is clearly observed under magnetic field. As the magnetic flux density increases, the downward flow velocity of the molten pool is enhanced. Moreover, visible difference of oscillation growth period for keyhole depth is produced and this leads to a variation in the time it takes for the molten pool to stabilize. Significant grain refinement appearing in both arc-affected zone at the weld upper part and laser-affected zone at the weld lower part is documented although the refinement degree varies at different zones. This is attributed to the formation of many initial nuclei or crystallites, originated from the enhanced flow of the molten pool with the assistance of magnetic field. This study provides a perspective in understanding the physical phenomena involved in magnetic field assisted laser-MIG hybrid welding and lays the foundation for weld microstructure optimization.

Heat flow modelling of the alkaline activation of fly ash with sodium hydroxide in the presence of portlandite

Alkali-activated cement is a promising alternative to replace totally or partially Portland cement. The heat flow and the degree of reaction are widely analyzed variables on the alkali-activation process. In this paper, a phenomenological-based semi-physical model of the alkali-activation process was developed. The model predicts the heat flow and the degree of reaction, by correlating the composition of the fly ash, the NaOH concentration (alkaline activator), the concentration of portlandite and curing temperature. The model considers balanced chemical reaction and the process diffusive phenomena. For validation microcalorimetric measurements at 25 °C and 35 °C was made, and secondly Freisleben-Hansen model was fitted with the experimental data. The results indicate a reaction acceleration for concentration of NaOH in (6–10) M and a reaction deceleration due to portlandite. The fly ash particle size and diffusivity-associated factors have an incidence of more than 5 % on the heat flow and degree of reaction.

Theoretical analysis and design of fire protection and thermal insulation of parallel wire or strand cables

In this study, a transient heat flow model has been established for parallel wire or strand cables at high temperature by employing the lumped thermal mass approach, and the numerical solution of the surface temperature as a function of time in each layer of the steel wire or strand inside the cable was calculated. Accuracy of the theoretical method is verified through uniform heating test of 73Φ15.7 mm steel strand cable. The results calculated show that temperature field inhomogeneity of cable section is overestimation on the condition that heat conduction inside the cable is not considered. Considering heat conduction or not, the maximum temperature difference of the core steel stand at the same time point is 373 . Unprotected 73Φ15.7 mm cable is damaged in only 12 min in UL1709 fire, and arrangement of fire protection layer around cable can effectively retard the temperature rise of cable surface. The experimental results are in good agreement with the theoretical calculation values in early stage of fire, and the temperature difference between the two is within 10%. Besides, the numerical calculations were analyzed in accordance with the fire protection requirements limiting the surface temperature of cables. It was observed that the minimum thickness of the fire protection layer required to meet the PTI DC45.1-12 standard was linearly related to the numerical value of the section factor of the outermost layer of steel wires or steel strands in the equivalent model, and the slope of the function was approximately equal to the conduction coefficient of fire protection layer, as the cable was subjected to fire for 30 min. Further, based on this, a simple method was proposed to calculate the minimum thickness of the fire protection layer for parallel wire or strand cables.

A geothermal heat flow model of Africa based on random forest regression

Geothermal heat flow (GHF) data measured directly from boreholes are sparse. Purely physics-based models for geothermal heat flow prediction require various simplifications and are feasible only for few geophysical observables. Thus, data-driven multi-observable approaches need to be explored for continental-scale models. In this study, we generate a geothermal heat flow model over Africa using random forest regression, originally based on sixteen different geophysical and geological quantities. Due to an intrinsic importance ranking of the observables, the number of observables used for the final GHF model has been reduced to eleven (among them are Moho depth, Curie temperature depth, gravity anomalies, topography, and seismic wave velocities). The training of the random forest is based on direct heat flow measurements collected in the compilation of (Lucazeau et al., Geochem. Geophys. Geosyst. 2019, 20, 4001–4024). The final model reveals structures that are consistent with existing regional geothermal heat flow information. It is interpreted with respect to the tectonic setup of Africa, and the influence of the selection of training data and observables is discussed.

Process Modelling Applied to Aluminium-Steel Butt Welding by Hybrid Metal Extrusion and Bonding (HYB)

In the present investigation, the numerical code WELDSIM is used to simulate butt welding of 4 mm thick plates of S355 steel and AA6082-T6 by Hybrid Metal Extrusion and Bonding (HYB). This is a new solid state joining process using continuous extrusion as a technique to enable aluminium filler metal additions. In WELDSIM, the finite element heat flow model is coupled to a frictional heating model, an isokinetic diffusion model for the interfacial intermetallic compound (IMC) formation and a nanostructure model for simulating reversion and re-precipitation of hardening phases inside the aluminium part of the joints during welding and subsequent natural ageing. The HYB process model is validated by comparison with experimental data obtained from in-situ thermocouple measurements and hardness testing carried out on three different Al-steel butt welds. Furthermore, scanning electron microscope examinations of the Al-steel interfaces have been conducted to check the predicted power of the IMC diffusion model. It is concluded that the process model is sufficiently relevant and comprehensive to be used in simulations of both the thermal, microstructure, and strength evolutions fields in these dissimilar butt welds. Some practical applications of the process model are described toward the end of the article, where particularly its potential for optimising the load-bearing capacity of the joints, is highlighted.

The Numerical Simulation and Experimental Study of Heat Flow in Seabed Sediments Based on COMSOL

In situ electrothermal conduction heating technology refers to the setting up of a heat source directly within the sediments, using the heat conductivity of the sediments and the heat radiation of the heat source for heat transfer to achieve the in situ heating of the sediments. The in situ electrothermal conduction heating of sediments has the disadvantage of the heating equipment being easily damaged and difficult to operate, and requires the equipment to be able to withstand seawater pressure and marine corrosion. In this paper, based on the combination of numerical simulations (using COMSOL Multiphysics software) and approximate in situ electrothermal conduction heating experiments, the temperature field and other factors of sediments heated by in situ conductive heating (in a specific area) were studied to determine a numerical model for sediment heat flow in a specific area under different pressures and initial temperatures, and the correctness of the numerical model was further verified by comparative experiments. The results of this study have important implications for future studies on the actual thermal properties of sediments and even heat transfer mechanisms during gas hydrate extraction.

Tempering kinetics during multilayer laser additive manufacturing of a ferritic steel

Grade 91 steel forms martensite during additive manufacturing and the extent of tempering of martensite significantly affects the mechanical properties of parts. Currently, there is a lack of quantitative understanding of the tempering kinetics for Grade 91 steel, and as a result, the effects of repeated thermal cycles on properties for different processing conditions cannot be determined. Here we evaluate the tempering kinetics by determining the constant terms in the Johnson Mehl Avrami kinetic equation from the tempering data available in the literature and the thermal cycles computed using a rigorously-tested heat and fluid flow model of multi-layer additive manufacturing. The raw tempering data are cleaned using a neural network to enhance accuracy. The lower layers experience repeating cycles of heating and cooling when the upper layers are added. As a result, the hardness is reduced owing to the tempering of martensite. In contrast, martensite formed in the upper layers is not tempered to the same extent and the hardness remains high. Therefore, the hardness of the part increases with the distance from the substrate. Variations in the heat input at different laser powers and scanning speeds significantly affect the extent of tempering. Since the method used here can provide a quantitative understanding of the tempering of martensite and the spatial variation of hardness, it can be used to tailor the microstructure and hardness of heat treatable printed metallic parts.

Properties and biases of the global heat flow compilation

Geothermal heat flow is inferred from the gradient of temperature values in boreholes or short-penetration probe measurements. Such measurements are expensive and logistically challenging in remote locations and, therefore, often targeted to regions of economic interest. As a result, measurements are not distributed evenly. Some tectonic, geologic and even topographic settings are overrepresented in global heat flow compilations; other settings are underrepresented or completely missing. These limitations in representation have implications for empirical heat flow models that use catalogue data to assign heat flow by the similarity of observables. In this contribution, we analyse the sampling bias in the Global Heat Flow database of the International Heat Flow Commission; the most recent and extensive heat flow catalogue, and discuss the implications for accurate prediction and global appraisals. We also suggest correction weights to reduce the bias when the catalogue is used for empirical modelling. From comparison with auxiliary variables, we find that each of the following settings is highly overrepresented for heat flow measurements; continental crust, sedimentary rocks, volcanic rocks, and Phanerozoic regions with hydrocarbon exploration. Oceanic crust, cratons, and metamorphic rocks are underrepresented. The findings also suggest a general tendency to measure heat flow in areas where the values are elevated; however, this conclusion depends on which auxiliary variable is under consideration to determine the settings. We anticipate that using our correction weights to balance disproportional representation will improve empirical heat flow models for remote regions and assist in the ongoing assessment of the Global Heat Flow database.

Coupled Model of Heat and Power Flow in Unventilated PV/PCM Wall-Validation in a Component Scale

The paper concerns the problem of the modeling of the thermal interaction between the phase change material (PCM) and photovoltaic (PV) panel, in the case of unventilated construction. The study aims to develop the numerical model of such a building element to support its proper future design under variable boundary conditions. The need for such a study comes from the realization of the research project which aims at developing a novel energy-activated thermal insulation composite system. Two different methods of PCM simulation using ESP-r software were compared. The model that was finally proposed was validated against experimental data, which proved its robustness. The MBE for the simulated and measured temperatures on the back of the PV panel did not exceed 2.0 °C and the maximum observed energy production difference was 4 Wh/m2.

Noninvasive Methods of Active Thermographic Investigation: Short Overview of Theoretical Foundations with an Example of Application

The field of non-destructive testing using thermography (NDT—Non-Destructive Testing and NDE—Non Destructive Evaluation) was established in the 1960s as a competitive and complementary method to ultrasonic or X-ray testing. Non-destructive quality control of objects is essential in modern industry, especially in mass production. It is an integral part of the quality control process. The reason for the introduction of non-destructive testing into diagnostic techniques were disasters and failures of various equipment and structures. Over the years, a division has been drawn between passive and active thermography. The aim of this publication is to present the theoretical basis of non-destructive testing with the use of active infrared thermography methods. It describes selected models of transient heat flow (thermal wave) in objects of various shapes and physical properties. They are the basis for understanding the changes in the surface temperature of the studied objects in time, which are the response to thermal excitation of different nature. An example of their effective application in engineering practice is the author’s software, in which the recorded time response is matched to the analytical solution, and on this basis detected anomalies are parameterized, detected by the fusion of various techniques of thermal image processing and analysis methods.

Solidification cracking of a nickel alloy during high-power keyhole mode laser welding

Nickel alloy Inconel 740H, a candidate material for use in ultra-supercritical power plants, is susceptible to solidification cracking during high power deep penetration laser welding. Here we examine how cracking is affected by welding variables and determine the locations where the cracks occur experimentally and theoretically. We use a solidification cracking model to calculate the effects of welding variables on cracking and the locations where the cracks form during high power laser keyhole mode welding of IN 740H. The parameters needed for the cracking model are obtained from a well-tested numerical heat transfer and fluid flow model for keyhole-mode welding. Model predictions of cracking and their locations for different welding conditions are verified by experiments. • Solidification cracking of nickel alloys during laser welding is poorly understood. • We propose a novel methodology to understand why, when, and where cracking occurs. • A heat and fluid flow and solidification model, and experimental data are used. • Model predictions of cracking locations and conditions are verified by experiments. • The work can be used to prevent weld failures in thick section laser keyhole welds.

A novel model for macroscopic simulation of oscillating heat and fluid flow in porous media

In thermoacoustics, stacks and regenerators are porous media where energy conversion takes place. Modelling full thermoacoustic devices with a CFD approach, in order to capture some nonlinearities, can be extremely expensive from a computational perspective compared to a standard linear approach used in the frequency domain. At the same time, macroscopic models for porous media developed for steady-state flows cannot be directly applied in oscillating flow conditions. Moreover, macroscopic models in the available literature for oscillating flows are inaccurate at high frequencies or require a closure coefficient to be determined numerically (with Direct Numerical Simulations) or experimentally. In this article, a time domain macroscopic model for heat and fluid flow is proposed based on the concepts of complex Darcy and Nusselt numbers in the linear regime. Such coefficients, introduced in the past to describe the oscillatory phenomena, have been used for the first time to build a CFD macroscopic model in terms of their real and imaginary parts. For two different porous media, a parallel plate and a transversal pin array, the developed macroscopic model is verified with the microscopic solution. Furthermore, for a transversal pin array stack, the proposed model is validated against experimental data from the available literature, showing a very good agreement. The findings of this paper can help to strongly reduce the computational costs of oscillatory flow simulations without prior direct numerical simulations of the porous core.

Thermal properties of soft clayey soils from the former Lake Texcoco in Mexico

Thermal properties of soils are essential for the design of many engineering projects, such as underground cable systems, thermal ground improvement techniques, and energy geostructures. Although the thermal properties of typical soils have been widely studied, little research has focused on the thermal characterization of soils with unusual properties, such as the soft clays from the former Lake Texcoco in Mexico. These sediments are known for their exceptionally high water content, plasticity index, and porosities. The present study describes a systematic evaluation of the thermal conductivity, specific heat capacity, and thermal diffusivity of Texcoco clays and their inherent variability. Forty laboratory samples from eleven different locations were studied using the dual-probe heat pulse method, X-ray diffractometry, Scanning Electron Microscopy, and standard soil classification tests. The results showed that the studied samples have lower thermal conductivities and higher specific heat capacities than those previously reported for saturated clays. These differences were mainly attributed to the high porosities of the peculiar clays from the former Lake Texcoco. The thermal conductivity did not show a specific trend with depth. From statistical analyses, it was established that the shifted lognormal distribution and the normal distribution accurately describe the natural variability of the thermal conductivity and specific heat capacity, respectively. Finally, the efficacy of eight selected prediction models for the thermal conductivity of saturated clays was assessed. The geometric mean and the weighted series-parallel heat flow (WSP) models delivered the best fit to the experimental data. This study deepens our understanding of ground thermal properties by providing information about soil with unusual characteristics. Furthermore, it gives an initial database for the thermal properties of Mexican soils that can serve as a reference for the development and design of some of the geotechnical applications previously described.

Effect of magnetic field orientation on suppressing porosity in steady-magnetic-field-assisted aluminum alloy deep-penetration laser welding

The steady magnetic fields were utilized to decrease the porosity of aluminum alloy laser welded joints, and the effect is related to the magnetic field orientation. The influence of the magnetic field orientation on the suppression of porosity was investigated by establishing a three-dimensional heat flow model coupled with magnetohydrodynamics (MHD) and free surface of a keyhole. The Lorentz force decreased the backflow velocity toward the keyhole wall when the magnetic fields were horizontally parallel to the welding direction (magnetic field X-axis, MFX) and perpendicular to the welding direction (magnetic field Y-axis, MFY), thereby clearly improving the keyhole stability. Additionally, the melt flows in the rear region of the molten pool were suppressed downward and forward under the MFX and MFY, respectively, leading to additional heat accumulation in the rear periphery of the molten pool. As a result, a long molten pool in the longitudinal section and a small penetration depth were observed under the MFX and MFY. Therefore, the porosity defects decreased from 9.4% to 3.5% and 3.2% under the MFX and MFY, respectively. However, many pores were observed when the magnetic field orientation was vertical (magnetic field Z-axis, MFZ), owing to the high penetration depth and poor keyhole stability. This study provides a deeper understanding of the effect of magnetic fields on the laser welding of aluminum alloys and potentially lays a foundation for the adjustment of suitable magnetic field parameters toward obtaining the desired effect.

Discrete element thermal conductance model for sintered particles

A discrete element thermal conductance model suitable for the modelling of heat flow between sintered particles has been proposed. The model is formulated using the sintering geometry consisting of two spheres connected with a cylindrical neck. The calculation of the neck size is based on the criterion of volume conservation. Therefore the neck obtained is more accurate than that of the popular Coble's model. The thermal conductance is determined for different neck sizes by the finite element simulations of the heat flow in half of the sintering geometry. The numerical results are fitted with a linear relationship which is the basis to determine the equivalent conductance between two sintered particles. The model can be used in the pipenetwork formulation of the discrete element method for simulation of heat conduction problems in powder sintering or in sintered porous materials.