Wall Conduction Research Articles

An overview of conjugate heat transfer augmentation methods for thermal management and recent advancements in microchannel heat sink overall performance

Recent technological advancement creates high-dense circuits development which creates colossal cooling demand requirements. In the future, advanced technology and scientific applications need to balance heat fluxes beyond limitation that catalyse the research opportunities further to achieve cooling requirements. Conventional cooling equipment is inappropriate for extracting heat from microchips within the minimal surface area. Microchannel technology appeared as a next-generation heat exchanger for the given heat duty. In the classical case of accounting, micro-heat exchangers both axial and transverse wall conduction effects is usually neglected. However, from the year 2005 onwards, researchers identified conduction dominance at fluid velocity is zero zones at the interface, which created balance between conduction–convection currents, finally included axial wall/heat conduction effects to a general model. This present study made a comparative evaluation fluid flow, heat transfer characteristics, thermal performance (TP) based on geometrical parameters, substrate materials and discusses previous issues, and current issues-based future research directions. Till today, based on applications of microchannels in thermal management, several experimental and numerical investigations have been reported for further improvement in TP. However, many researchers are still trying to accomplish things over a long period. Therefore, an overview of previous studies is furnished in the present study to assist researchers in this area much useful for further improvement.

Advancements in design of radiant floor heating systems with an innovative simplified calculation method

The calculation accuracy of existing simplified methods for radiant floor heating systems (RFHS) is far from meeting the growing demand for energy saving. Therefore, an innovative steady-state simplified method is proposed based on conduction shape factors, equivalent thickness, and mirror principle. The new model is validated by experimental data and compared with an equally simple simplified method. Meanwhile, the effects of the total heat-transfer coefficient of radiant surfaces and fluid flow rate in heating pipes on the calculated results are discussed. The calculation flowchart is given. Parameter influence and sensitivity analysis are then performed. The results show that the new simplified model agrees well with experimental data, and the maximum relative error is 7.9% for heat flux and 2.0% for surface temperature. Standard deviation reduces by 68.5% and 66.8% for the heat flux and surface temperature, respectively, compared to the equally simple existing simplified method. With the same absolute increment of pipe pitch, the smaller the pipe spacing, the faster is the heat flux and surface temperature change. The order of importance of influential parameters is mean water temperature > indoor air temperature > pipe pitch > thermal conductivity of filling layer > thermal conductivity of pipe wall.

Resistive switching properties in polycrystalline LiNbO3 thin films

LiNbO3 (LNO) is currently intensively studied as an important ferroelectric material. In this work, polycrystalline LNO films were prepared through a sputtering technique, and their ferroelectricity-related resistive switching property was investigated using a device structure of PtSi/SiO2/LNO/Pt. The device exhibits a volatile resistance switching property at lower positive sweeping voltages and a stable bipolar nonvolatile switching property at higher sweeping voltages. The resistive switching mechanism of the device is discussed based on the domain wall conductivity characteristics of the polycrystalline LNO thin films. The PtSi/SiO2/LNO/Pt memristor device has potential applications in memory and artificial neural synapses.

Influence of heat flux profile on two-phase flow instability in parallel channels of helical coil once-through steam generator

To enhance the heat transfer capability, the helical coil once-through steam generator (HCOTSG) has been widely utilized in the small modular reactor (SMR). There are hundreds of parallel helical channels in the HCOTSG, and the coolant undergoes the phase change from subcooled liquid to superheated vapor in the helical tube. The flow oscillation between parallel channels may be triggered during the phase change. In present study, the two-phase flow instability is investigated with different heat flux profiles under many operating conditions. The thermal–hydraulic model for parallel channels of HCOTSG is established based on our previous work, while the local heat transfer phenomenon such as subcooled boiling, critical heat flux, and wall conduction are considered in detail. The heat flux profile along the tube is ascertained by the steady-state analysis of HCOTSG under different operating conditions and applied to the outside surface of the helical channel. The heat flux value is increased step by step to trigger the out-of-phase oscillation between parallel helical channels. The thermal–hydraulic model and analysis method are validated against experimental results. The characteristics of flow instability in the helical channel are also analyzed. It is observed that the delay effect causing the flow instability is more obvious if the void fraction exceeds 0.7 in the starting stage. Then the boundary of flow instability is compared under different heat flux profiles for various operating conditions of HCOTSG. The results suggest that the distribution of heat flux in the two-phase region plays a more significant role in the occurrence of flow instability in the parallel helical channel system. The thermal–hydraulic characteristics during flow instability are governed by the combined effects of void propagation and delayed feedback in the system. In conclusion, the thermal–hydraulic condition of HCOTSG undergoes the unstable region to achieve the normal operating point, which can be avoided by adjusting the operating parameters.

Thermal conductivity of an external wall with simulated smart Aerogel insulation system

Modern technologies are more than ever adopting intelligent designs that respond to environmental variants to produce more desirable outcomes. In the building and housing industries, intelligent designs aim to conserve energy through temperature controls and energy storage and distribution systems. In this context, an intelligent Aerogel insulation system is modelled and studied for the exterior building walls. Aerogel with a thermal conductivity of about 0.012 W/m-K has a superior insulating property, and an intelligent design built around it would introduce additional energy storage and delivery enhancements to the system. In this study, a test room and a control room in an open urban environment were built, and the intended intelligence, first considered by choosing Aerogel that has the potential to be engineered with smart properties according to researchers in composite material science, and second was considered by automation which was applied manually to the test room. The calculated thermal conductivity of an external wall with intelligent Aerogel insulation is -(1.83 ± 0.06) × 10−1 W/m. The negative sign is an attribute of a system function rather than of a material. The external wall with intelligent Aerogel insulation, in comparison to an identical non-insulated wall, exhibited 2.7 times better energy conservation.

Numerical Study of Heat Transfer in a Partitioned Cavity Containing a Porous Medium

Cavities separated by multiple vertical partitions and filled with a porous medium present a remarkable thermal insulation quality, offering potential solutions in various engineering fields. The aim of this study is to analyze the impact of the presence of a porous medium on heat transfer through a partitioned cavity. We have developed a numerical model based on the Navier-Stokes and heat transfer equations, solved using Ansys Fluent software. We examine the evolution of the Nusselt number (convection and radiation) as a function of the position of the porous medium inside the cavity, as well as physical properties such as emissivity, wall conductivity and Rayleigh number. Current lines and isotherms are obtained from this numerical model. Nusselt numbers for both convection and radiation are calculated, taking into account the position of the porous medium in the system, as well as the effect of varying physical parameters on heat transfer. It has been observed that the presence of the porous medium leads to a reduction in the rate of heat transfer within the cavity. The further the porous medium is from the hot wall, the more pronounced this reduction. In addition, radiative transfer has a downward influence on convective transfer. Furthermore, the convective transfer rate decreases with increasing emissivity. As far as conductivity is concerned, transfer rates (convective and radiative) initially increase until a maximum Nusselt number is reached, after which they gradually decrease with a further increase in conductivity. Nusselt numbers (convection and radiation) increase as the Rayleigh number increases.

Economic optimization of exterior wall insulation in Chinese office buildings by coupling artificial neural network and genetic algorithm

Envelope insulation is an essential measure to reduce building energy consumption and is the focus of many building energy codes. In China, the current standard (GB 55015-2021) only specified limit values for the thermal conductivity (U) of the exterior walls, and it ignored variations in building characteristics and internal parameters. Therefore, it is difficult for architects to quickly determine the exterior wall U-value in building design and retrofit. To address this issue, the exterior wall U-values of office buildings in China were economically optimized by TRNSYS, ANN, and GA. To begin with, the effect of enhancing exterior wall insulation on the heating and cooling energy consumption of office buildings was investigated by TRNSYS software, taking into account variations in building characteristics. Moreover, the exterior wall U-values of office buildings were optimized by coupling ANN and GA. The results showed that the design values of the GB 55015-2021 did not achieve the optimal results in China, and the most economical exterior wall U-value was 0.24 W/(m2·K) in Harbin, 0.24 W/(m2·K) in Beijing, 0.42 W/(m2·K) in Changsha, 1.82 W/(m2·K) in Guangzhou, 0.66 W/(m2·K) in Kunming. Compared with the pre-optimization, the annual life cycle cost of building heating and cooling reduced by 0.96 % in Harbin, 2.07 % in Beijing, 1.44 % in Changsha, 1.68 % in Guangzhou, and 7.37 % in Kunming. In addition, variations in building parameters should be considered in the design phase of new buildings or in the renovation of old buildings, such as internal heat gain, which may lead to different optimal results.

Effects of wall conductivities on magnetoconvection in a cube

Effects of wall conductivities on magnetoconvection in a cube

Curvature conservation and conduction modulation for symmetric charged ferroelectric domain walls

Curvature conservation is a common feature for the physical properties with topological constraints, from the macroscopic cosmic string loops to the microscopic quantized vortex rings in the superfluid. Interestingly, ferroelectric domain-wall curvature is important to determine its conduction, which is the key parameter for designing domain-wall nanoelectronic devices. Here, we demonstrate the curvature conservation of the charged domain walls confined in ferroelectric topological structures with specific domain symmetry by combining piezoelectric force microscopy, conducting atomic force microscopy, and phase-field simulations. Significantly, the intrinsic conductivity of the charged domain walls exhibits an inverse functional relationship with the local domain-wall curvature in the ferroelectric BiFeO3 nano-islands, which can also be precisely and continuously modulated by applying an electric field. This work provides us with an insightful understanding of the conduction mechanism of charged domain walls confined in ferroelectric topological structures with a high degree of spatial symmetry, which paves the way for the application of charged domain walls as flexible conducting channels with continuously changing resistance states to be used for ferroelectric domain-wall memristors.

Evaluation of equivalent conductivity of reverberation chambers wall

Evaluation of equivalent conductivity of reverberation chambers wall

A phase-field simulation of easily switchable vortex structure for multilevel low-power ferroelectric memory

Ferroelectric materials with multi-directional polarization orders can form a variety of special domain structures, which have a wide range of applications. In this work, lead-free potassium sodium niobate ferroelectric thin films were simulated by solving the Time-Dependent-Landau-Ginzburg (TDLG) equation. Based on vortex topological structure characteristic of perovskite-type ferroelectric, domain types are simplified from 26 to 9 through virtual polarization vectors. By temperature selection, we accomplished a large easily switchable vortex structure with 20% side length. Based on this, as low as 0.04V/nm electric-field pulse was applied to explore vortex domain structures change. Under different low external electric-field pulses, conductance states change in vortex have been calculated and discussed systematically. Free energy computation results reveal that the domain wall (DW) conductance states primarily originate from the competition between electrostatic energy, gradient energy, and elastic energy. Finally, we propose the multilevel data ferroelectric memory through easily switchable vortex topological structures for the first time. Under electric-field pulses, diverse storage states were presented through 0/1 measurement of 8 positions. By achieving an easy-to-distinguish and robust-to-switch data map at the vortex structure, this work might provide a significant idea of vortex structure on low-power ferroelectric memory applications.

Excitation Equations for Longitudinally-Azimutally Irregular Waveguides Taking into Account the Finite of the Walls Conductivity

Excitation equations for longitudinal-azimuthally irregular waveguides are formulated taking into account losses in the walls. The inner surface of the waveguide walls is given by an arbitrary smooth function b(ϕ, z). The coordinate transformation method replaces the original cylindrical coordinate system r, ϕ, z with a new one ρ, ϕ, z, where ρ = r/(b(ϕ, z)). The new system defines the waveguide boundary as ρ = 1 = const, i. e. the waveguide geometry transforms as a regular cylinder. Taking these functions into account, the standard procedure of the incomplete Galerkin method is used to determine the amplitudes of partial waves. The resulting general equations can be used in the calculation and optimization of both microwave and EHF electronic devices of various types, as well as passive microwave devices of various applications.

Uncoupling of stomatal conductance and photosynthesis at high temperatures: mechanistic insights from online stable isotope techniques.

The strong covariation of temperature and vapour pressure deficit (VPD) in nature limits our understanding of the direct effects of temperature on leaf gas exchange. Stable isotopes in CO2 and H2 O vapour provide mechanistic insight into physiological and biochemical processes during leaf gas exchange. We conducted combined leaf gas exchange and online isotope discrimination measurements on four common European tree species across a leaf temperature range of 5-40°C, while maintaining a constant leaf-to-air VPD (0.8 kPa) without soil water limitation. Above the optimum temperature for photosynthesis (30°C) under the controlled environmental conditions, stomatal conductance (gs ) and net photosynthesis rate (An ) decoupled across all tested species, with gs increasing but An decreasing. During this decoupling, mesophyll conductance (cell wall, plasma membrane and chloroplast membrane conductance) consistently and significantly decreased among species; however, this reduction did not lead to reductions in CO2 concentration at the chloroplast surface and stroma. We question the conventional understanding that diffusional limitations of CO2 contribute to the reduction in photosynthesis at high temperatures. We suggest that stomata and mesophyll membranes could work strategically to facilitate transpiration cooling and CO2 supply, thus alleviating heat stress on leaf photosynthetic function, albeit at the cost of reduced water-use efficiency.

Blunt chest trauma-induced myocardial infarction: a case of sudden death by homicide

BackgroundBlunt cardiac injuries, which are typically the result of blunt chest trauma, are not particularly significant clinically. It is vital never to underestimate blunt chest injuries, especially when they occur in the anterior thoracic area. Blunt trauma to the chest is one of the rarest causative factors of myocardial infarction (MI). Blunt cardiac injury encompasses different medical emergency conditions such as gradual damage to the myocardium leading to myocardial ischemia, injuries to the great vessels of the heart, pericardial tamponade, septum or wall ruptures, pump failure, conduction abnormalities, and atrial and ventricular fibrillations. The most common cause of blunt chest trauma is a road traffic accident (RTA), followed by a sports injury. Mostly blunt trauma chest injuries occur below the age group of 45 years.Case presentationA 45-year-old male with a history of physical assault was brought to the emergency department by his neighbor. He was allegedly kicked by his relative over the chest during an altercation sustained blunt-force trauma to the chest and collapsed immediately. On admission, he was given cardiopulmonary resuscitation and declared dead. On autopsy, there were no signs suggestive of external injury or any shoeprints/footprints on the chest. On histopathological examination, a diffuse area of discoloration consistent with an extensive myocardial infarction was noted. Old healed infarcts were seen in the free wall of the left ventricle, left posterior papillary muscle, and left apex. The coronaries showed atheromatous plaque with lumen narrowing and focal calcification.ConclusionsIt is the task of the forensic pathologist to ascertain and record evidence as to whether the myocardial infarction was caused by the alleged blunt trauma to the chest during the autopsy. This is important as it will determine the type of prosecution the accused is likely to be charged with and the extent of the punishment that he will likely receive under the Indian Penal Code.

Application of fiberglass pipes in geothermal circulation systems

The results of calculating the coefficient of thermal conductivity of the developed construction of the pipeline from multilayer composite spatially oriented fiberglass are given. A method of calculating the coefficient of thermal conductivity of a pipe wall with a complex structure of reinforcement has been developed. The fiberglass pipe is made by the continuous method of oblique-layer longitudinal-transverse winding on a self-feeding frame of reinforcing pseudo-tape moistened with a binder in the form of a rarefied mesh. With each rotation of the mandrel, it shifts by the amount of hardening of the obtained two-component material under the influence of temperature. A comparison of calculated and experimental data was made. When comparing various known calculated dependencies and experimental values, the accepted physical model and the real structure of the material are determined. The known dependences obtained for a medium with infinite long parallel cylindrical inclusions for unidirectional and orthogonally reinforced materials and therefore for materials with a complex structure and scheme of reinforcements give errors and deviations. The work assumes that fiberglass consists of two components - a reinforcing filler and a binder with pores, the reinforcing fibers are endless and have a circular cross-section, the reinforcing fibers are evenly distributed in the matrix of the binder, there is an ideal thermal contact at the boundaries of the distribution between the reinforcement and the binder. Experimental data on determining the transverse thermal conductivity of unidirectional fiberglass rods with reinforcement based on aluminoborosilicate or magnesium-aluminosilicate glass and epoxy binders showed the smallest error in comparison with the calculated ones (8-10%). They investigated the fact that the coefficient of thermal conductivity of a fiberglass pipe is 100 times lower than that of a steel pipe. It was determined that it is possible to replace a steel pipe with a diameter of 219 mm with a fiberglass pipe with a diameter of 146 mm.

Numerical Calculations of Liquid-Metal Magnetohydrodynamic Flows in Rectangular Ducts with Sudden Expansions

Relating to the design of liquid-metal blankets in a fusion reactor, numerical calculations have been performed on liquid-metal magnetohydrodynamic (MHD) flows in rectangular ducts with sudden expansions. Conservation equations of fluid mass and fluid momentum, together with the Poisson equation for electrical potential, have been solved numerically. The numerical calculations have been performed for Hartmann (Ha) numbers up to the order of 10000 and expansion ratios up to 4. The pressure loss through the expansion has been estimated by the loss coefficient ζ divided by the interaction parameter N, i.e., ζ/N. The loss coefficient ζ/N through the expansion parallel to the magnetic field is much larger than that through the expansion perpendicular to the magnetic field. The loss coefficient ζ/N increases consistently with the expansion ratio. The loss coefficient ζ/N does not change very much with the interaction parameter N and the wall conductance ratio.

Unsteady conjugate heat transfer simulation of wall heat loads for rotating detonation combustor

Rotating detonation engines (RDEs) are the most promising pressure-gain combustion devices, but their long-duration operations are hindered by the harsh thermal environment inside the chamber. This study analyzes the instantaneous and average heat loads on chamber walls of RDEs based on the newly developed unsteady conjugate heat transfer (CHT) OpenFOAM solver named multiRegionBYRFoam. To enhance accuracy in predicting heat transfer characteristics compared to decoupled simulations focusing solely on fluid or solid behavior, three-dimensional CHT simulations are conducted for a premixed H2/air annular RDE coupling the detonation simulation with heat conduction in both inner and outer chamber walls. The transient time evolutions of wall heat flux in RDEs are numerically captured for the first time and agree well with experiments. The calculated peak wall heat flux is 1.98 MW/m2 at the inner wall and 2.63 MW/m2 at the outer wall when a single detonation wave propagates with a mass flow rate of 0.227 kg/s. By investigating the influence of inlet flow conditions and detonation modes, valuable guidelines on designing efficient thermal management systems for RDEs are obtained: larger mass flow rates and multi-wave modes result in higher average wall heat flux; the heat flux at the outer wall exceeds that at the inner wall while both walls require effective cooling for gas turbine engines with a rotating detonation combustor. Furthermore, a novel heat transfer model for RDEs is proposed based on a thermal operation map depicting total heat transfer rates to describe the nonlinear impact of mass flow rate and number of detonation waves, which can be employed to predict total wall heat loads under specific operation conditions of RDE.

Excitation Equations for Irregular Waveguides Taking into Account the Finite Wall Conductivity and Their Application for Ultrahigh-Power Microwave Problems. Part 1

Excitation Equations for Irregular Waveguides Taking into Account the Finite Wall Conductivity and Their Application for Ultrahigh-Power Microwave Problems. Part 1

Excitation Equations for Irregular Waveguides Taking into Account the Finite Wall Conductivity and Their Application for Ultrahigh-Power Microwave Problems. Part 2. Relativistic Klynotron

Excitation Equations for Irregular Waveguides Taking into Account the Finite Wall Conductivity and Their Application for Ultrahigh-Power Microwave Problems. Part 2. Relativistic Klynotron

Prediction of thermal conduction in a four-phase composite with lowly-conducting interfaces based on variational asymptotic method

Due to the complex physical and chemical reactions, surface contamination and debonding, the interfaces within the four-phase composite, represented by hollow-glass-bead reinforced cement based material (HGB-CBM), cannot be assumed to be perfect. In order to consider the effect of imperfect interfaces, a novel thermal conduct model is developed. An imperfect interface is used to replace the pre-existing transition layer in the composite material, and the discontinuous temperature field proportional to the normal heat flow is preserved by using Hadamard’s relation and Taylor’s expansion. Based on the principle of minimum functional and variational asymptotic method, the governing variational statement for thermal conduct model is derived, and validated through comparison with RVE-based FEM and experimental results. It can be concluded that HGB-CBM with lowly-conducting imperfect interfaces has lower effective thermal conductivity, smaller heat flux within HGB and more significant changes at imperfect interfaces, which may be due to longer and more complex heat flux paths along the imperfect interfaces. HGB-CBM with larger volume fraction of HGB and smaller thermal conductivity of bead wall can provide better thermal insulation effect, which could provide reference for using HGB-CBM as thermal-insulating material in building engineering.