Mode Of Heat Transfer Research Articles

Розпізнавання аварійного режиму погіршеної тепловіддачі в каналах перспективних легководних реакторів із надкритичними термодинамічними параметрами

From the standpoint of physical analysis of the micro-kinetics of the near-wall layer of the supercritical coolant, as well as the emergency dynamics of the heat transfer process from the surface of the fuel rods under these conditions, the features of the emergency mode development of deteriorated heat transfer in the channels of advanced light water reactors with supercritical thermodynamic parameters are analyzed. The defining physical differences in the onset of the deteriorated heat transfer mode in the reactor core with supercritical thermodynamic parameters are specified. The fundamental differences between the procedures for operational diagnostics of heat exchange crises on the surface of fuel rods, which may arise in water-cooled reactors under subcritical parameters, and the development of the emergency mode of deteriorated heat transfer in the core under supercritical parameters are physically substantiated. The fundamental impossibility of applying deterministic approaches to the implementation of existing monitoring and diagnostics systems for the current operational state of subcritical reactors of the VVER type to solve the problem of early detection of the initial phases of the deteriorated heat transfer mode is justified. Taking into account the defining features of the molecular kinetics of the onset and development of the deteriorated heat transfer mode on the surface of fuel rods in the reactor core with supercritical thermodynamic parameters, the main diagnostic principles for the automatic identification of this emergency thermal-hydraulic mode, which must be timely detected in future reactors with supercritical coolant parameters, are specified. In this context, the fundamental principles of building advanced in-core monitoring systems for light water reactors with supercritical parameters are formulated. It is justified that the onset of the emergency mode of deteriorated heat transfer on the surface of fuel rods in a supercritical light water reactor must be detected at the initial phase of this emergency mode. It is shown that the core of the mathematical software for advanced in-core monitoring systems for reactors with supercritical thermodynamic parameters of the light water coolant should consist of intelligent procedures for automatic recognition of the initial phases of the pseudoboiling mode, which precedes the pseudo-film boiling mode and the destruction of the fuel rod cladding in the deteriorated heat transfer mode. A mathematical model of preventive diagnostics of the emergency mode of deteriorated heat transfer is proposed, based on procedures for constructing a separating hyperplane in a multidimensional feature space. The adequacy of the developed model for automatic recognition of the pseudo-boiling mode in an experimental cylindrical channel under the conditions of a thermal-hydraulic stand with supercritical thermodynamic parameters was verified based on the spectral parameters of the acoustic emission process during the pseudo-phase transition process. A 100% reliability of correct identification of the pseudo-boiling mode was achieved.

Heat dissipation performance analysis and cooling module optimization of oil-immersed transformer based on multi-physical field simulation

The long-term operation of the oil-immersed transformer will cause the internal temperature to rise rapidly, which may lead to accidents such as transformer burning, which has a certain impact on the safe and reliable operation of the power grid. In this paper, the model of oil-immersed transformer is established by finite element simulation software, and the three-dimensional circuit magnetic field simulation model is established according to the structural parameters of the transformer, and the core loss and winding loss of the transformer are calculated accurately. The calculated loss is loaded into the simulation model of the flow temperature field, and the distribution characteristics of the temperature field and flow field of the transformer core and winding are obtained accurately by analyzing the heat transfer mode of the transformer and calculating the convection heat transfer coefficient formula in the heat convection theory. According to the structure of the transformer winding, the main variable parameters of the winding that affect the transformer temperature rise are selected. Through the center composite design method and range analysis, the winding geometry parameters which minimize the hot spot temperature of the transformer are obtained. In order to further reduce the temperature rise of the transformer, the principle of enhanced heat dissipation is used, and the cooling module is attached to the radiator of the transformer model, and the shape, installation position, installation method and number of the cooling module are changed respectively. Through heat flow coupling calculation, the optimal structure and installation mode of heat dissipation performance are obtained: symmetrical rectangular cooling module is installed at the upper end of the heat sink, which can effectively reduce the hot spot temperature of the transformer. The results show that the hot spot temperature of the oil-immersed transformer can be reduced from 78.1? to 60.7?, and the hot spot temperature can be reduced by 22.3%. The above research provides a new idea and reference for oil immersed transformer to reduce temperature rise and improve heat dissipation performance.

Review on the protection properties of thermal protective clothing against hot liquids and steam

Professionals engaged in fire rescue and industrial production may face the danger of hot liquids and steam. They need to be equipped with specific thermal protective clothing (TPC) to ensure their safety. To evaluate how well TPC protects against hot liquids and steam, this paper reviewed the mechanism of hot liquids and steam, the factors affecting its protection, and measurement methods. Firstly, the characteristics of hot liquids and steam are analyzed from the perspective of heat transfer mode, and the importance of hot liquids and steam protection is emphasized. Secondly, the factors affecting protection performance of hot liquids and steam is reviewed base on relevant researches, focusing on the fundamental properties of fabrics, the air layer under clothing, the moisture factor, clothing factor and the environmental factor. Finally, the existing measurement methods for hot liquid and steam protection performance are summarized, and the deficiencies in the existing research has been discussed. At last, this paper looks forward to the development of the future research direction. This review provides reference ideas for protective measures against hot liquid and steam burns, and provides new ideas for the design and optimization of hot liquid and steam protective clothing.

Effect of flow direction on heat transfer and flow characteristics of supercritical carbon dioxide

This work is devoted to investigating the difference in flow and heat transfer characteristics between vertical upward flow and horizontal flow of supercritical carbon dioxide (<inline-formula><tex-math id="Z-20240119215215">\begin{document}$\rm sCO_2$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20231142_Z-20240119215215.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20231142_Z-20240119215215.png"/></alternatives></inline-formula>) based on the pseudo-boiling theory and the experimental parameters: mass flux <i>G</i> = 496–1100 kg/m<sup>2</sup>s, heat flux <i>q</i><sub>w</sub> = 54.4–300.2 kW/m<sup>2,</sup> and pressure <i>P</i> = 7.531–20.513 MPa. The differences in flow and heat transfer characteristics between horizontal upward tube and vertical upward tube are compared at different mass fluxes, heat fluxes and pressures fully. Finally, unlike the classical treatment of flow and heat transfer for supercritical fluid, single-phase fluid assumption is abandoned, instead, the pseudo-boiling theory is introduced to deal with the flow transfer and heat transfer of <inline-formula><tex-math id="Z-20240119215113">\begin{document}$\rm sCO_2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20231142_Z-20240119215113.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20231142_Z-20240119215113.png"/></alternatives></inline-formula> in the two tubes. Supercritical fluid is regarded as a multiphase structure in this work, including a vapor-like layer near the wall and a liquid-like fluid in tube core. The results are indicated below. 1) In terms of heat transfer, the inner-wall temperature of the vertical upward tube and the bottom generatrix of horizontal tube are basically the same under normal heat transfer mode. When the heat transfer deterioration occurs in the vertical upward tube, larger supercritical boiling number (<i>SBO</i>) will cause the wall temperature peak of the vertical upward tube to be much higher than the wall temperature at top generatrix of the horizontal tube at the corresponding enthalpy. The <i>SBO</i> (<i>SBO</i> = 5.126×10<sup>–4</sup>) distinguishes between normal heat transfer deterioration and heat transfer deterioration in the vertical upward tube. In the horizontal tubes, <i>SBO</i> dominates the maximum wall temperature difference between the top generatrix and the bottom generatrix. Comparing with vertical upward tubes, higher <i>q</i><sub>w</sub>/<i>G</i> is required for the heat transfer deterioration of supercritical fluid in the horizontal tubes under the same pressure. 2) In terms of flow, the increase in slope of pressure drop in the vertical upward tube is due to the orifice contraction effect. The mechanism that dominates the variation of pressure drop in the horizontal tube is the flow stratification effect, and we show that Froude number <i>Fr</i><sub>ave</sub> can be the similarity criterion number to connect the temperature difference between the top and bottom generatrix of horizontal tube and the pressure drop. The analysis suggests that mechanisms governing horizontal flow and vertical flow of <inline-formula><tex-math id="Z-20240119215057">\begin{document}$\rm sCO_2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20231142_Z-20240119215057.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20231142_Z-20240119215057.png"/></alternatives></inline-formula> are different in heat transfer deterioration mode. For the vertical flow, the <i>SBO</i> plays a leading role, while for the horizontal flow, the <i>Fr</i> plays an indispensable role.

Heat transfer characteristics of oil-water two-phase in pipeline transportation during shutdown

Gathering and transportation pipelines often appear shutdown condition due to damage, leakage, and other reasons. The reasonable maintenance time should be controlled by closely combined with the change law of the oil-water temperature in the pipeline. This paper established a multi-field coupling 3-D heat transfer mathematical model of the oil-water pipeline environment based on the two-phase shutdown pipeline as the research object. According to the changing law of the oil-water temperature field and flow field under a typical pipe diameter, the evolution behavior of the gelation layer in the pipe and the transformation process of heat transfer mode were analyzed, and the heat transfer characteristics between oil-water phase and its interface in the pipe were defined. With the liquid phase ratio of the oil phase changing the rule, reasonable maintenance time suggestions were given for gathering and transporting pipelines with different water content.

An enhanced heat transfer method based on the electrocapillary effect of gallium-based liquid metal.

As electronic products become smaller and more powerful, there is an increasing need for effective heat dissipation. An effective heat exchange method is necessary for the equipment to function reliably in a compact space. To tackle the limitations of current microfluidic cooling technology, including difficulty in manufacturing, maintenance, and cost reduction, a heat exchange method with a simple system is proposed in this work. This method is based on the electrocapillary effect, using eutectic gallium-indium alloy droplets with high thermal conductivity, surface tension, and controllability as the basic unit. An electric field is applied to generate unevenly distributed charges in the electric double layer on the droplet surface, thereby creating a surface tension gradient that can drive the surrounding solution to flow. Simultaneously, the oscillation of the droplet can also intensify the disturbance of the solution. The violent disturbance of the solution causes the heat transfer mode to change from conduction to convective heat transfer and greatly reduces the thermal resistance, resulting in a substantial increase in heat flux. For this heat transfer method, the temperature distribution and flow characteristics of the solution in low-frequency oscillating and direct-current-biased alternating current electric fields are studied, and the effect of voltage, frequency, and the number of droplets on heat transfer enhancement is clarified. Compared with conduction without internal disturbance, the heat flux can be increased by up to 110% based on the combined effect of two droplets. This work provides a solution for enhancing the heat transfer of microfluidics.

Numerical Simulation and Experimental Research on Heat Transfer Characteristics Based on Internal Meshing Screw

The mixing and processing of high-viscosity materials play a pivotal role in composite material processing. In this context, the internal meshing screw mixer, rooted in volume extensional rheology, offers distinct advantages, including heightened mixing efficiency, exceptional material adaptability, and favorable thermomechanical properties. This research endeavors to advance our understanding of these qualities by presenting an in-depth exploration of internal meshing screw mixing. To facilitate this, an internal meshing screw mixing experimental apparatus was meticulously constructed, accompanied by extensive numerical simulations and experimental investigations into its heat transfer characteristics. Two distinct heat transfer modes are established: Mode 1 entails the transfer of the high temperature from the outer wall of the stator to the interior, while Mode 2 involves the transmission of the high temperature from the inner wall of the rotor to the exterior. The ensuing research yields several notable findings: 1. It is evident that higher rotational speeds lead to enhanced heat transfer efficiency across the board. However, among the three rotational speeds examined, 60 rpm emerges as the optimal parameter for achieving the highest heat transfer efficiency. Furthermore, within this parameter, the heat transfer efficiency is superior in Mode 1 compared to Mode 2. 2. As eccentricity increases, a corresponding decline in comprehensive heat transfer efficiency is observed. Moreover, the impact of eccentricity on heat transfer efficiency becomes increasingly pronounced over time. 3. A lower gap dimension contributes to higher heat transfer within the system. Nevertheless, this heightened heat transfer comes at the expense of reduced stability in the heat transfer process. 4. It is demonstrated that heat transfer in Mode 1 primarily follows a convection heat transfer mechanism, while Mode 2 predominantly exhibits diffusion-based heat transfer. The heat transfer efficiency of Mode 1 significantly surpasses that of Mode 2. This research substantiates its findings with the potential to enhance the heat transfer efficiency of internal meshing screw mixers, thereby making a valuable contribution to the field of polymer engineering and science.

Numerical simulation of fluid flow and heat transfer in steady state laminar natural convection in horizontal circular cylindrical enclosures

Heat transfer by natural convection often occurs in a closed enclosure in various applications. Different geometries of the cross section of a two-dimensional enclosure, such as triangle, rectangle, and trapezoid have already been investigated. In this research, a regular polygon with twelve sides is considered. The study focuses on the rate of heat transfer through the enclosure, using one side of the polygon as the hot surface and another side as the cold surface. The dodecagon can serve as a model for circular cylindrical enclosures under constant temperature boundary conditions over a specified length. The Nusselt number of the enclosure is obtained for each side as the hot surface. Both the lowest and highest Nusselt numbers are evaluated based on the position of the cold side. The findings reveal that the highest Nusselt number does not necessarily correspond to configurations where the hot and cold sides are directly opposite each other. In situations where convection heat transfer through the enclosure is the predominant mode of heat transfer, the Nusselt number is a power function of the Rayleigh number. Conversely, a logarithmic relationship is observed in cases of conduction heat transfer within the enclosure.

Thermal Modeling of Photovoltaic Panel for Cell Temperature and Power Output Predictions under Outdoor Climatic Conditions of Jodhpur

The rise in the temperature severely affects photovoltaic cell efficiency and hence its power output. Moreover, it also causes the development of thermal stresses that may reduce their life span. Thus, there is a need for an accurate estimation of the cell’s working temperature. In this paper, a detailed thermal model based on various heat transfer modes involved and their governing equations has been presented to estimate the cell temperature of a PV module using MATLAB software under different climatic and solar insolation conditions. In order to validate the presented model, an experimental setup has been built and operated under actual outdoor conditions of Jodhpur, a city in the Thar Desert of Rajasthan. For the peak summer month of June, the predicted glass cover outer surface temperature has been found to be within 0.2–4.5°C of experimentally measured values and the back sheet temperature is found to be within 0.5–5.5°C. The predicted and measured power outputs have been found to be within 0.85–1.2 W while the efficiency values are within 0.17–0.38%. For the early summer month of April, the variations are 0.13–4.1°C, 0.2–4.1°C, 0.44–1.65 W, and 0.1–0.5% for glass cover temperature, back sheet temperature, power output, and efficiency, respectively. Thus, the predictions of the developed thermal model have exhibited a good agreement with the experimental results. The maximum glass cover temperatures recorded were 60°C and 65.5°C when the ambient temperatures were 35°C and 42°C near the noon for the early summer and peak summer day experiments, respectively. The presented model can be used to generate a year-round cell temperature data for the known environmental data of a location, which can help in the selection or development of appropriate cooling technology at the planning stage of the installation of a solar PV plant.

Role of applying PCMs on thermal behavior of innovative unit roof enclosure

As a technical approach, using Phase Change Materials (PCM) can be an effective method to prevent energy losses through building enclosures. In the present paper, different roof structures containing PCM have been examined to analyze their thermal behavior. The governing equations were developed and solved for a sample unit extracted from a typical roof. The control volume based finite element method has been applied for solving coupled equations governing heat transfer through the roof and flow of melted PCM. The effect of key parameters including thermal diffusivity of the PCM and the wall, Prandtl number (Pr), Rayleigh number (Ra) and solid wall thickness to total thickness has been examined. The main results indicate an enhancement of heat transfer through the increase of the thermal diffusivity of the PCM with respect to the building material. Increasing the thermal diffusivity ratio from 1 to 25 doubles the heat transfer rate and decreases the full melting time by 80% for high Rayleigh. Pr affects the mode of heat transfer but has little impact on PCM melting rate. Raising Ra improves heat transfer under the condition of high thermal diffusivity in the PCM. Increasing Rayleigh number from 104 to 106 doubles the heat transfer rate and reduces the melting by 67% for high thermal diffusivity ratio. Finally, increasing the size of the wall thickness does not affect the fraction of melted PCM but reduces the heat transfer rate. The heat transfer rate can be increased by up to 75% when the non-dimensional thickness of the solid layer is reduced from 0.5 to 0.005.

Assessment of Thermal Runaway propagation in lithium-ion battery modules with different separator materials

The current work aims to understand and model thermal runaway (TR) events in lithium-ion (LIB) 18650 cells within the context of aircraft battery applications. The primary goal is to comprehend the phenomenon and discuss strategies for mitigating its consequences during aircraft operation. TR is modeled using Arrhenius kinetic equations and is implemented in both lumped parameters (Matlab SimulinkTM), and 3D CFD simulations (Ansys FluentTM) using User Defined Functions. To validate the thermochemical model, cells are initially simulated in an oven test, where a cell is exposed to a temperature-controlled atmosphere, triggering exothermic reactions. With a strong correlation between lumped parameters and 3D models, the latter is simulated under battery module installation conditions. An internal short-circuit is then implemented within the cell to observe how thermal runaway is triggered by an internal heat source. The trigger cell is subsequently placed in a battery module assembly to assess the dominant heat transfer modes and the likelihood of TR induction from one cell to its neighbors. This work’s main objective and innovation are to compare different materials in which cells are immersed while observing the main heat transfer parameters for each material. Three conditions are tested: ceramic paper fiber and G7 as solid separators, and no separator material, where air fills the gaps between cells. The analysis of heat transfer modes reveals radiation’s dominance in the case of air interstice, suggesting the possibility of using a special coating to reduce the cell surface emissivity as an alternative to decrease the likelihood of TR propagation. Thus, two values of surface emissivity were tested in the case of air. Considering a cell triggered by an internal short-circuit, a thermal runaway temperature spike is not observed in any of the four cases. However, the air interstice case with regular emissivity is the most critical one, with the closest cell reaching peak temperatures as high as 136 °C in 490 s. The ceramic paper fiber is considered the best separator material, as it postpones the temperature increase in the closest cell while also being lighter than G7. The results and discussions concerning heat propagation presented herein can serve as guidelines for developing strategies to mitigate thermal runaway in battery modules.

Primitive and gravity modulation of periodical heat transfer along magnetic-driven porous cone with thermal conductivity and surface heat flux

The significant goal of present problem is to evaluate oscillating frequency and amplitude in heat and magnetic flux with thermal conductivity and heat flux effects across magnetized porous cone under lower gravitational region. To maintain heat transport efficiency, the thermal conductivity of conducting fluid is assumed as temperature dependent. The magnetic-driven cone is placed in lower gravitational region. To improve the heating rate across the magnetic-driven porous cone, the convective heating conditions are applied. The implications of thermal conductivity, surface heat flux, and reduced gravity on the periodic behavior of convective heat transfer characteristics of conducting fluid across porous gravity-driven cone is the novelty of this research. Using the appropriate non-dimensional variables, the model's nonlinear governing partial differential has been converted into a dimensionless form. The proposed model is solved later with the help of finite difference approach. The dimensionless form of the equations is reduced to the convenient form for numerical algorithm. The influence of adjusting parameters, such as thermal conductivity parameter ζ, reduced gravity parameter Rg, Biot number Bi, Prandtl number Pr, mixed convection parameter λ, porosity parameter Ω, magnetic prandtl number γ and some others stationary parameters has been highlighted. By using FORTRAN tool, the numerical outcomes are explored in graphs and tabular form. It is depicted that magnetic field enhances as Biot number decreases because magnetic field insulates the excessive heating across the magnetic-driven porous cone. It is noticed that significant amplitude in fluid velocity is noticed with prominent changes as reduced gravity increases. It is concluded that fluctuations in heat transport increases as parameter γ decreases under thermal conductivity because magnetic material insulates the extreme heat across the porous cone. The recent thermal conductivity and lowered gravity problem is significant in the fields of radioactive waste, cooling electronic components, storing nuclear, catalysts, heat exchangers, the underground storage of radioactive or nonnuclear materials, aircrafts and space vehicles.

Research on particle retention in continuous horizontal fluidized bed based on CFD-DEM method

Horizontal fluidized beds are widely used in continuous production lines due to their efficient heat and mass transfer mode. Continuous production goals require stabilized reaction systems, which can be achieved by controlling particle retention. But there is a lack of an effective means of controlling particle retention. In this paper, Computational Fluid Dynamics - Discrete Element Method (CFD-DEM) method is applied to investigate the particle retention in different air inlet angles, which is characterized by the particle back-mixing degree, spatial and residence time distribution. The results show that at low air inlet angles, the system is effective in controlling particle retention. This is achieved through stronger particle back-mixing and more concentrated residence times, which are essential for maintaining reaction system stability. These findings provide guidance for controlling particle retention and enhancing continuous fluidization equipment efficiency.

Study on the effect of partial filling of foam metal on the solidification performance of ice storage spheres

Ice storage sphere systems are favored in the field of energy storage, however, the poor thermal conductivity of phase change materials (PCM) seriously weakens their wide application. Metal foam has been shown to enhance its heat transfer. In this paper, we investigate the impact of varying the filling radius ratio of foam metal to further improve energy storage performance and reduce costs in ice storage spheres. Computational fluid dynamics are employed to analyze the heat transfer and solidification processes. The enthalpy-porosity method is used to describe PCM solidification, while the local equilibrium thermal (LET) model characterizes heat transfer between the PCM and foam metal. The study explores temperature field variations, solid phase fraction changes, solidification times, and cold storage volumes, accounting for natural convection. By comparing solidification rates and other relevant parameters, the effect of foam metal filling on the solidification process is explored. Moreover, a new evaluation index is proposed, which can assess the overall performance of ice storage systems in real-time, considering market prices. The results show that natural convection is the primary heat transfer mode in pure PCM-ffild ice storage spheres, whereas smaller foam metal filling radii significantly inhibit natural convection, resulting in heat conduction as the dominant mode. Without specific requirements, an optimal fill radius ratio of 9/13 is determined. This paper offers a comprehensive understanding of cold storage employing various foam metal filling radios, contributing to advancements in efficient thermal energy storage systems.

Enhanced pool boiling of refrigerants R-134a, R-1336mzz(Z) and R-1336mzz(E) on micro- and nanostructured tubes

Pool boiling of refrigerants is a primary heat transfer mode in flooded evaporators, as well as a promising solution in electronics cooling applications. In this work, the pool boiling performance of R-134a, R-1336mzz(E), and R-1336mzz(Z) on the external side of round tubes are reported and compared to Cooper's heat transfer coefficient (HTC) model. Modification of Cooper's correlation is found to be necessary for R-1336mzz(Z) at low reduced pressure. Micro- and nanostructured tubes are tested using the three different refrigerants and are shown to enhance the boiling HTC. Specifically, three various structures consisting of: boehmite (AlO(OH)) nanostructures, etched aluminum microstructures, and copper oxide (CuO) micro-nanostructures are fabricated on the external sides of aluminum and copper tubes. The surface structures have characteristic length scales of 100 nm, 1 µm, and 10 µm for boehmite, CuO, and etched aluminum, respectively. While the boiling HTC is enhanced up to 250% (compared to smooth tubes) using R-134a with etched aluminum, it shows a 15% decrease in HTC when boehmite structures are used. For CuO, higher heat fluxes showed a 25% HTC enhancement, while lower heat fluxes showed negligible effects when compared to smooth copper tubing. Different refrigerants showed various relations between structure and HTC enhancements. To take account of both structure length scale and refrigerant properties, we introduce a normalized structure size (Rn) and conclude that HTC enhancement ratio increases non-linearly with Rn. Our study not only provides valuable data on refrigerant pool boiling of recently developed low global warming potential and ozone depleting potential fluids, it also provides valuable design guidelines for the development and application of micro- and nanostructured surfaces in chiller and electronics cooling applications.

On the mechanics of thermal collapse of a vapor bubble translating in an isothermal subcooled liquid

In this work, we numerically simulate bubble radius and velocity in a thermal collapse of a vapor bubble translating in an isothermal subcooled liquid based on solving the equations of energy and motion simultaneously. The mechanics of the collapse in the thermal regime plays a significant role in the thermal convection from the vapor-liquid interface of a translating bubble. In our mathematical model, the equation of motion considers the added mass force due to phase change and shape change, while the equation of energy includes thermal conduction and convection. The simulations include both spherical and non-spherical bubbles, the effect of initial bubble diameter and degree of subcooling. The simulations have been performed in three modes of heat transfer (conduction, convection and combined conduction-convection) to observe their effect on the bubble dynamics. The numerical results have been compared with the two classes of thermal collapse experiments (Type A and B) and CFD analysis reported in literature. They agree well with the experiments and CFD-analysis. We have seen a remarkable effect of the initial shape of the bubble in terms of aspect ratio on the time scale and modes of the heat transfer and the collapse time. An interesting concern is that the mechanism of the collapse and the modes of heat transfer depends on how the collapse is initiated in the experiments. Type A and B thermal collapses are mainly classified based on the bubble motion and the modes of thermal transport from the interface.

Numerical investigation of subcooled two-phase flow and heat transfer characteristics in a liquefied natural gas cargo containment system

The increase demand in liquefied natural gas (LNG) leads to an increase in the import and export of LNG cargo using LNG carriers with composite insulation and double-hull structures for membrane-type storage tanks. Therefore, the transient characteristics of cryogenic heat transfer in LNG cargo containment systems have become important for understanding the physics when LNG vaporizes into the gas phase to minimize the cargo loss of boil-off gas (BOG). However, public papers on fundamental research on long-term computational fluid dynamics (CFD) simulations are still lacking. In the present study, the transient flow behaviors of LNG fluids and the heat transfer characteristics were numerically studied by applying the simplified single-layer insulation in an LNG cargo containment system, based on an initial temperature of TLNG,0 = 110.5 K and saturation temperature of TLNG,sat = 111.5 K in the boundary condition of IGC code. The proposed analysis procedure can serve as a reference for relative comparison of predicted BOG difference in LNG cargo holds.Consequently, the heat transfer mode between the two-phase methane and the solid insulation could be typically classified into two different regimes: (1) a subcooled regime before reaching the saturation temperature from the initial temperature, and (2) a boiling regime after reaching the saturation temperature. In the subcooled regime, the subcooled liquid exhibited bulk flow motion induced by natural convection. The loss rate of LNG mass per time interval was extracted from the best-fit correlations as −0.0448 kg/s at 1 d (105 s) to cool down the insulation system and −0.0072 kg/s at 5 d (5 × 105 s) to reach the LNG saturation temperature. When the predicted value of the loss rate of LNG mass by the CFD simulation was regarded as the generation rate of boil-off gas, it was smaller than the designed value of the boil-off gas of 0.464 kg/s corresponded to boil-off rate (BOR) about 0.2 %/day. In the boiling regime and under sufficiently cooled conditions, the loss rate of LNG mass per time interval did not consistently exceed the BOR value. The liquid flow exhibited a turbulent-like motion despite the relatively low velocity at 10 d (106 s). Especially, the increase in the effective thermal conductivity was a good indicator for distinguishing between the boiling and subcooled regimes.

Numerical investigation of heat transfer mechanism in an embedded PCM-TEG for better overall performance under the fluctuating heat source

Using phase change materials (PCM) can effectively ensure the stable operation of semiconductor thermoelectric generators (TEGs). Conventional PCM-TEG structures exhibit relatively poor output performance due to energy barriers caused by low thermal conductivity. The embedded PCM-TEGs proposed in this study can address this issue well by adopting variable structure composite forms of PCM and TEG, resulting in a more complex heat transfer mode than traditional systems. To evaluate embedded systems' characteristics and performance enhancement methods, this paper first establishes a multiphysics mathematical model for embedded PCM-TEGs. Then, employing the TOPSIS method, an optimal embedded structure that considers system output and stability is obtained. Finally, we investigate the effects of key parameters on system performance, including PCM volume, latent heat, melting point, and metal-added composite materials. The CCC PCM-TEG (embedded PCM-TEG with PCM closer to the cold side) exhibits superior performance with a 28.6 % increase compared to conventional structures. For a given PCM volume, embedded structures with equal heights show better performance due to weak influence from asymmetric configurations on internal heat flow direction. Unlike traditional PCM-TEGs, metal-added composite PCMs in embedded structures reduce the temperature difference range across TEGs and thus deteriorate system performance.

Thermal conductivity analysis of natural fiber-derived porous thermal insulation materials

Natural fibers, derived from plants such as wood, hemp, straw and cotton, have been explored for the fabrication of porous structures for thermal insulation applications due to their widespread availability, sustainability, and cost-effectiveness. Understanding the fundamental heat transfer mechanisms within natural fiber-derived porous structures is crucial for both optimized geometric design and real-world insulation applications. Herein, we developed a theoretical framework considering geometric parameters, including pore size, fiber diameter and porosity (i.e., density), to examine the contribution of various heat transfer modes (i.e., conduction, convection, and radiation) on the effective thermal conductivity of porous structures derived from natural fibers. Our results indicate that thermal radiation is largely responsible for the rapid increase in effective thermal conductivity of the natural fiber-derived porous insulations in low-density regions (< 50 kg/m3) and that natural convection rarely occurs within these materials. The insulation materials derived from natural fibers with diameters in the micron range (5–50 μm) can achieve their minimum thermal conductivity at an optimal density of 50–90 kg/m³. Effective strategies to lower the effective thermal conductivity of natural fiber-derived porous materials include increasing porosity to curtail solid conduction, incorporating nanoscale pores by using nanosize fibers to diminish gaseous thermal conductivity. This research offers valuable insights into the heat transfer mechanisms in natural fiber-derived materials and should guide the structural design and optimization process toward developing super-thermal insulation materials derived from natural fibers.

Modelling and simulation of thermal runaway phenomenon in lithium‐ion batteries

AbstractTo improve the operational time of electronic devices and the driving spectra of electric cars, multiple investigation missions regarding batteries are focussed on refining the energy capacity of batteries. However, this pursuit of better performance introduces potential risks, such as fire incidents, due to the use of dynamic materials in lithium‐ion batteries (LIBs) to enhance their electrochemical performance. These fire incidents have resulted in financial losses and raised safety concerns. One common type of battery failure, called thermal runaway (TR), takes place when the rate of heat discharge from internal chain reactions accelerates the level of external cooling. This study aims to model and comprehensively analyse the phenomenon of TR in LIBs by investigating its underlying physics.To conduct this research systematically, a mathematical framework of heat transfer from existing literature has been employed as a foundation to come up with a novel framework specifically for LIBs built on the ideas of Semenov's equation. The established model associates both thermal and mass balance aspects, with the consideration of response kinetics including other heat transfer modes (convective and radiation). Range–Kutta technique is used to unravel the model's equations instantaneously using MATLAB. Consequently, the impact of model factors on the thermal performance of LIBs has been evaluated. The outcomes of the study reveal that when an increase in activation temperature occurs, a decrease in peak temperature is followed whilst constant circumstances are ensured. This implies that more time is needed in order for the temperature to reach its maximum. Furthermore, the simulation demonstrated that any growth in the body capacity of the battery for absorbing heat during the reaction results in a higher maximum temperature. Heat convection is found to have a greater impact on the occurrence of TR in LIBs compared to heat radiation. For improved accuracy in calculating LIBs TR, an accurate kinetic model could be utilised in future research endeavours.