Axial Conduction Research Articles

Inducing directional heat transfer by enhancing directional thermal conductivity of asphalt mixtures for improving asphalt solar collectors

Low thermal conductivity makes heat conduction in asphalt pavements quite slow, restricting asphalt solar collectors. To enhance the directional thermal conductivity of asphalt mixtures and induce directional heat transfer, directional heat-induced channels are built in asphalt mixtures by using carbon fibers with high axial thermal conductivity. A finite element method is used to simulate and study the temperature field and heat flow distribution of asphalt mixtures. A parametric analysis of the carbon fiber parameters is performed to evaluate the variation in the performance of the effective thermal conductivity of asphalt mixtures. The temperature trend of asphalt mixtures is measured by an indoor irradiation experiment and compared with the simulation results. The results show that heat-induced channels have obvious heat convergence effects and can induce heat to be efficiently conducted along its axial direction. These channels can significantly improve the directional thermal conductivity of asphalt mixtures and are expected to play a heat transfer bridge between the pavement surface and the energy collection system when using them in asphalt solar collectors in the future.

Design of a new device for fibers strand axial thermal conductivity measurement

A device for fibers strand axial thermal conductivity measurement has been designed and modeled by finite elements and realized. The method is based on Fourier's law in steady state and is inspired by the widely spread guarded hot plate method. This device has been designed to measure thermal conductivity on a wide range from 0.1W/(m.K) to 100W/(m.K) which represent the range of thermal conductivity found in fibrous materials. The numerical study has shown that radiative heat losses can reach the same order of magnitude than the researched conductive heat flux if no care is taken but are minimized when the temperature difference applied on the sample is centered on the temperature of the wall of the vacuum chamber enclosing the device. Measurements of thermal conductivity have been realized on three samples of bulk materials representative of the measurement range and on commercially available carbon fibers strands T300. The results are in good agreement with the values found in the literature and validate this device.

Experimental and numerical analysis of the multilayer distributed Joule‐Thomson cooler with pillars

SummaryThe conventional Joule‐Thomson cooler is not compact in heat exchanger and insufficient in cooling capacity, which is limited in applying integrated electronics. However, microchannels with pillars are effective in increasing the area to volume ratio and enhancing heat transfer through the presence of vortex. Besides, multilayer microchannels are essential to enlarge the mass‐flow rate in the cooler. In this study, a multilayer distributed J‐T cooler with pillars is investigated. A new mathematical model of the cooler is developed and validated against the experimental data; the relative errors of the temperature and mass‐flow rate are both within 5%. Furthermore, the distribution of temperature and pressure in the microchannel is analyzed by the model calculation. Numerical results show that the temperature of the high‐pressure fluid decreased slightly under the influence of the heat exchange and distributed J‐T effect. When the inlet pressures are 3.00, 4.00 and 5.40 MPa, the cold‐end temperatures are 233.0, 185.8 and 161.2 K, respectively, with the cooling capacity reaching 2.25, 3.40 and 3.87 W, respectively. In addition, the influences of the axial heat conduction and heat leakage on the cooling performance are analyzed by using the models. The temperature rise of the cold end increases by 6.8 and 6.2 K at 5.40 MPa, respectively, considering the above two factors. The simulation model provides a useful tool for the J‐T cooler.

Experimental study of post-CHF heat transfer in a vertical tubular test section

In this paper, post-CHF (critical heat flux) experiments are performed in a vertical tubular test section with water as the working fluid to study the heat transfer characteristics during the process of formation, growth, and propagation of dry patches. The formation, growth, and propagation of the dry patches are indicated from the measured wall temperatures of the test section. The so-called dry temperature and quench temperature, corresponding to the formation and departure of dry patches, are obtained by analyzing the first-order derivative of the measured wall temperatures. The temperature gradient along the axial direction of the test section wall is also calculated, up to 15 °C/mm. Large temperature gradients result in axial heat conduction as well as thermal stresses in the test section. The convective heat transfer coefficient for the dry patches is calculated based on the saturation temperature of the fluid and compared with three heat transfer correlations in the literature. The computed convective heat transfer coefficient for the dry patches in our experiments is larger than the calculated results of the correlations, especially for conditions of relatively low wall superheats.

Humidity-Dependent Thermal Boundary Conductance Controls Heat Transport of Super-Insulating Nanofibrillar Foams

Summary Cellulose nanomaterial (CNM)-based foams and aerogels with thermal conductivities substantially below the value for air attract significant interest as super-insulating materials in energy-efficient green buildings. However, the moisture dependence of the thermal conductivity of hygroscopic CNM-based materials is poorly understood, and the importance of phonon scattering in nanofibrillar foams remains unexplored. Here, we show that the thermal conductivity perpendicular to the aligned nanofibrils in super-insulating ice-templated nanocellulose foams is lower for thinner fibrils and depends strongly on relative humidity (RH), with the lowest thermal conductivity (14 mW m−1 K−1) attained at 35% RH. Molecular simulations show that the thermal boundary conductance is reduced by the moisture-uptake-controlled increase of the fibril-fibril separation distance and increased by the replacement of air with water in the foam walls. Controlling the heat transport of hygroscopic super-insulating nanofibrillar foams by moisture uptake and release is of potential interest in packaging and building applications.

Highly thermo-conductive but electrically insulating filament via a volume-confinement self-assembled strategy for thermoelectric wearables

• A volume-confinement strategy was proposed to prepare highly anisotropic filament. • An exceedingly high axial thermal conductivity of 9.22 W/m K was achieved. • The filament enables efficient body-heat harvesting for thermoelectric wearables. Polymer-based 1D materials with excellent axial thermal conductivity and electrical resistivity are highly demanded for thermal management in wearable electronics. In the last decades, boron nitride nanosheets (BNNS) were commonly introduced into polymer-based materials for improving their thermal conductivity, while they were mainly focused on 2D papery film or 3D bulk composite materials. 1D polymer/BNNS composite filaments are far from a rapid development, and their axial thermal conductivity is still restricted to a low value (<5 W/m K), mainly because of insufficient orientation and low addition of BNNS. Herein, a highly thermo-conductive but electrically insulating regenerated cellulose (RC)/BNNS composite filament is fabricated by a volume-confinement self-assembled method in a wet-spinning procedure, in which large-sized BNNS are mandatorily confined within the limited region among adjacent RC nanofibers at BNNS loading of 60 wt%. Attributed to its highly anisotropic conformation, the as-prepared composite filament exhibits an ultra-high thermal conductivity of 9.22 W/m K, about 4.5-fold of non-confined RC/BNNS counterpart. After being weaved into a textile and integrated with a thermoelectric generator (TEG), it enables TEG to efficiently harvest energy from body heat and continuously self-power some wearable electronics, demonstrating a significant proof of concept for its extraordinary wearable thermal management applications.

GASEOUS SLIP FLOW FORCED CONVECTION IN A MICROPIPE AND PARALLEL-PLATE MICROCHANNEL WITH A PIECEWISE UNIFORM WALL HEAT FLUX AND AXIAL HEAT CONDUCTION

A forced convective heat transfer problem inside a micropipe as well as inside a microchannel with parallel walls in the laminar gaseous slip flow regime, a with finite heating region and a prescribed wall heat flux, including the axial heat conduction effect, is analytically investigated. The temperature field and Nusselt number are derived under the assumption that the flow is hydrodynamically fully developed in the finite-length heating region. The solution is found by applying the functional analysis method, by decomposing the energy equation into a pair of first-order partial differential equations. The first-order slip boundary conditions are imposed at the gas–wall interface. The analytical solution is compared with available calculations. The results show that the thermal characteristics in the heating region are significantly affected by the axial heat conduction, rarefaction effects, and finite length of the heating region.

Root metaxylem and architecture phenotypes integrate to regulate water use under drought stress.

At the genus and species level, variation in root anatomy and architecture may interact to affect strategies of drought avoidance. To investigate this idea, root anatomy and architecture of the drought-sensitive common bean (Phaseolus vulgaris) and drought-adapted tepary bean (Phaseolus acutifolius) were analyzed in relation to water use under terminal drought. Intraspecific variation for metaxylem anatomy and axial conductance was found in the roots of both species. Genotypes with high-conductance root metaxylem phenotypes acquired and transpired more water per unit leaf area, shoot mass, and root mass than genotypes with low-conductance metaxylem phenotypes. Interspecific variation in root architecture and root depth was observed where P. acutifolius has a deeper distribution of root length than P. vulgaris. In the deeper-rooted P. acutifolius, genotypes with high root conductance were better able to exploit deep soil water than genotypes with low root axial conductance. Contrastingly, in the shallower-rooted P. vulgaris, genotypes with low root axial conductance had improved water status through conservation of soil moisture for sustained water capture later in the season. These results indicate that metaxylem morphology interacts with root system depth to determine a strategy of drought avoidance and illustrate synergism among architectural and anatomical phenotypes for root function.

A combined experimental and numerical approach for printed circuit rectangular microchannel J-T cooler using argon

Considering the small cooling capacity of a Hampson-type Joule-Thomson (J-T) cooler, a multi-layer printed circuit board (PCB) J-T cooler with parallel microchannels was fabricated and investigated in this study. The cooler has four parts: an inlet, a counter flow heat exchanger, a throttle, and an evaporator. A mathematical model is established and iteratively solved with a developed code. Using Ar as the working fluid, the cooling characteristics of the J-T cooler at 2–8 MPa were simulated. The pressure, temperature and velocity of the fluids as well as the temperature of the materials were obtained along the length of the cooler. The discrepancy between calculation and experiment of the cold end temperature is within ±2%, and the discrepancy of the outlet pressure is within ±4%, indicating the reasonability and reliability of the mathematical model. The model was also used to obtain the heat transfer rate between high- and low-pressure fluids, the enthalpy change, the cooling capacity of the cooler and the axial heat conduction of the material. The results show that the cold end temperature can reach 155.9 K with the gross cooling capacity being 3.66 W at the inlet pressure of 8.02 MPa. It was found that if the axial heat conduction is omitted, the cold end temperature could decrease further by 2.2 K. Finally, the optimisation measures of the cooler are discussed and proposed by analysing the variation law of the J-T coefficient.

Processing, structure and properties of polyacrylonitrile fibers with 15 weight percent single wall carbon nanotubes

For the first time, a polyacrylonitrile (PAN) fiber with up to 15 wt% single-wall carbon nanotubes (SWNTs) was produced by dry-jet wet-spinning while maintaining good SWNT individualization. These fibers exhibited a tensile modulus of 32.1 GPa, tensile strength of 0.8 GPa, and axial electrical conductivity of 2.2 S/m. This was possible by using SWNTs that were non-covalently wrapped with poly (methyl methacrylate) (PMMA-SWNTs). Furthermore, this non-covalent PMMA functionalization significantly reduced the amount of solvent and time needed for processing of the spinning solution, as compared to the methods used in prior studies. Rheology of the spinning dispersions and mechanical properties and structural parameters of the produced fibers were related to the filler individualization, filler content and presence of filler-matrix interaction. Potential applications of this PAN fiber with high SWNT loading are briefly discussed.

Study on the Coupled Heat Transfer Model Based on Groundwater Advection and Axial Heat Conduction for the Double U-Tube Vertical Borehole Heat Exchanger

In this paper, a dynamic heat transfer model for the vertical double U-tube borehole heat exchanger (BHE) was developed to comprehensively address the coupled heat transfer between the in-tube fluid and the soil with groundwater advection. A new concept of the heat transfer effectiveness was also proposed to evaluate the BHE heat exchange performance together with the index of the heat transfer rate. The moving finite line heat source model was selected for heat transfer outside the borehole and the steady-state model for inside the borehole. The data obtained in an on-site thermal response test were used to validate the physical model of the BHE. Then, the effects of soil type, groundwater advection velocity, inlet water flow rate, and temperature on the outlet water temperature of BHE were explored. Results show that ignoring the effects of groundwater advection in sand gravel may lead to deviation in the heat transfer rate of up to 38.9% of the ground loop design. The groundwater advection fosters the heat transfer of BHE. An increase in advection velocity may also help to shorten the time which takes the surrounding soil to reach a stable temperature. The mass flow rate of the inlet water to the BHE should be more than 0.5 kg·s−1 but should not exceed a certain upper limit under the practical engineering applications with common scale BHE. The efficiency of the heat transfer of the double U-tube BHE was determined jointly by factors such as the soil’s physical properties and the groundwater advection velocity.

Role of foam anisotropy used in the phase-change composite material for the hybrid thermal management system of lithium-ion battery

Abstract Hybrid kind of thermal management systems (TMSs) is emerging as a powerful, reliable, and efficient cooling device for controlling the temperature of Li-ion cells in the battery pack of an electric vehicle. Adding metal foams to a pure PCM (which results in a composite material) is a suitable way for the enhancement of the passive part's performance. In this paper, a novel hybrid TMS that uses cooling water channels (as the active part of TMS) and a composite of paraffin PCM and Al foam surrounding each lithium 18650 cylindrical cell (as the passive part of TMS) is introduced, and the role of foam anisotropy (according to the three main axial, radial, and tangential directions in the cylindrical coordinate system) on its performance is investigated, for the first time. The results of this study show that the hybrid operating mode of TMS consists of two periods: PCM recovery and cell cooling. In the first period, most of the water's cooling capability is paid for the solidification of PCM through a progressive process from top to bottom; and hence, less cell cooling effect is achievable. The results demonstrate that only increasing the tangential conductivity has a notable positive effect on the average cell temperature and the liquid fraction. Additionally, only increasing the axial thermal conductivity has a positive effect on the controlling of the maximum temperature difference through the cell.

Modeling and simulation of a supercritical CO2-liquid sodium compact heat exchanger for sodium-cooled fast reactors

The study focuses on modeling and simulations of sodium-sCO2 intermediary compact heat exchangers for sodium-cooled fast reactors (SFR). A simplified 1-D analytical model was developed in companion with a 3-D CFD model. Using classic heat transfer correlations for Nusselt number, some simulation results using the 1-D model have achieved reasonable match with the CFD simulation results for longer channels (i.e., 40 cm and 80 cm). However, for short channel (10 cm) when axial conduction within the sodium fluid is significant, the 1-D model significantly over-predicted the heat transfer effectiveness. By incorporating the temperature-jump model, the 1-D model can extend its predictive capability for low-Prandtl number fluid/Peclet number flows. The results can help improve the understanding of heat transfer for sodium and low-Prandtl number fluids in general and improve designs of sodium-sCO2 compact heat exchangers. The results also confirmed that the sCO2 side dominates the overall heat transfer for Na-sCO2 heat exchangers. A preliminary attempt of optimizing the channel geometry shows mixing results – while heat transfer effectiveness was significantly increased for the wavy channel, much greater pressure drop was also predicted by the simulations.

A Versatile Numerical Tool for Simulating Combustion Features at Small-Scales

A versatile numerical tool based on the open-source framework OpenFOAM has been developed in this paper for modeling time-accurate, low-Mach number reacting flows, with a particular interest in small-scale flames. This tool consists of a gas-phase Navier-Stokes solver and a solid-wall heat conduction solver which can be implemented alone, or used together in a coupled means to reveal the small-scale combustion’s characteristics of significantly enhanced flame-wall thermal coupling. Validation works has proved that the tool is capable of reproducing experimental flames at various scales (from conventional to small scales), including well-recognized micro-flame features in literature such as three modes of premixed flame dynamics (weak flames, flames with repetitive extinction and ignition, and stable flames). Then, an experimentally-already-found but rarely-simulated unique phenomenon of diffusion flame street is successfully reproduced with well-captured flame structures. Moreover, the conjugate heat transfer model with the specific formulation of solid-wall heat conduction enables an attempt to simulate a novel, thermally-orthotropic combustor with its axial thermal conductivities superior to the transverse ones. Finally, computational performance of the developed OpenFOAM solver is compared to that of the previously-used compressible flow solver Eilmer. The OpenFOAM solver is found to show better wave-damping abilities for overcoming acoustic wave effects at the initial stage of simulations, and is much more efficient in terms of the computational cost.

Helical massive fermions under rotation

The properties of a massive fermion field undergoing rigid rotation at finite temperature and chemical potential are discussed. The polarisation imbalance is taken into account by considering a helicity chemical potential, which is dual to the helicity charge operator. The advantage of the proposed approach is that, as opposed to the axial current, the helicity charge current remains conserved at finite mass. A computation of thermal expectation values of the vector, helicity and axial charge currents, as well as of the fermion condensate and stress-energy tensor is provided. In all cases, analytic constitutive equations are derived for the non-equilibrium transport terms, as well as for the quantum corrections to the equilibrium terms (which are derived using an effective relativistic kinetic theory model for fermions with helicity imbalance) in the limit of small masses. In the context of the parameters which are relevant to relativistic heavy ion collisions, the expressions derived in the massless limit are shown to remain valid for masses up to the thermal energy, except for the axial charge conductivity in the azimuthal direction, which presents strong variations with the particle mass.

An analytical solution of convective heat transfer in microchannel or nanochannel

The two-dimensional energy equation with a first-order velocity slip model and a temperature jump model is studied analytically and a solution consisting of an infinite series is obtained. Impacts of viscous dissipation, axial conduction and rarefied effect on the local Nusselt number, the asymptotic Nusselt number and the bulk temperature profile of fluid are investigated. Results show that the cooling effect of the fluid benefits from the higher rarefied effect and axial conduction effect, as well as the lower viscous dissipation. The asymptotic dimensionless bulk temperature of fluid converges to a constant value that is higher than the wall temperature at a given set of Brinkman number, Péclet number and Knudsen number regardless of the inlet conditions. When neglecting axial conduction and the rarefied effect, the asymptotic Nusselt number with or without viscous dissipation is 17.5 or 7.54, respectively. Effects of axial conduction on the asymptotic Nusselt number are negligible when the Péclet number is greater than 10, while its influence on the non-dimensional bulk temperature of fluid and local Nusselt number can be neglected only when Pe > 100.

Silver Attached Graphene-Based Aerogel Composite Phase Change Material and the Enhancement of Thermal Conductivity.

Phased energy storage technologies are highly advantageous and feasible for storing and utilizing clean renewable energy resources, for instance, solar energy and waste heat, and it is an effective method to improve energy efficiency and save energy. However, phase change energy storage has some problems, for example, low thermal conductivity and phase change leakage, which lead to limited application. In this paper, anisotropic graphene aerogels were prepared by ice crystal template method with high thermal conductivity of graphene, and silver was attached to the pore wall graphene sheets and the graphene sheet boundaries of the aerogels. The results show that anisotropic graphene aerogels were successfully prepared, and SEM and EDS indicate that up to 9.14 at % silver was successfully attached to the graphene sheets and boundaries. The anisotropic thermal conductivity of the PArGO phase change composites after adsorption of the paraffin is significant, with a maximum axial thermal conductivity of PArGO of 1.20 W/(mK) and radial thermal conductivity of 0.54 W/(mK), compared to the pure paraffin (0.26 W/(mK)) increased by 362% and 108%, respectively. The enthalpy of the composite has been reduced to 149.6 J/g due to the silver particles attached, but the thermal properties have been greatly improved. In experiments simulating real temperature changes, PArGO achieves phase transitions very fast, with a 74% improvement on thermal efficiency of storage and discharge over the pure paraffin.

A review of heat and fluid flow characteristics in microchannel heat sinks

AbstractHeat transfer and flow characteristic in microchannel heat sinks (MCHS) are extensively studied in the literature due to high heat transfer rate capability by increased heat transfer surface area relative to the macroscale heat sinks. However, heat transfer and fluid flow characteristics in MCHS differ from conventional ones because of the scaling effects. This review summarizes the studies that are mainly based on heat transfer and fluid flow characteristic in MCHS. There is no consistency among the published results; however, everyone agrees on that there is no new physical phenomenon in microscale that does not exist at macroscale. Only difference between them is that the effect of some physical phenomena such as viscous dissipation, axial heat conduction, entrance effect, rarefaction, and so forth, is negligibly small at macroscale, whereas it is not at microscale. The effect of these physical phenomena on the heat transfer and flow characteristics becomes significant with respect to specified conditions such as Reynolds number, Peclet number, hydraulic diameter, and heat transfer boundary conditions. Here, the literature was reviewed to document when these physical phenomena become significant and insignificant.

Comparisons with wheat reveal root anatomical and histochemical constraints of rice under water-deficit stress

AimsTo face the challenge of decreasing freshwater availability for agriculture, it is important to explore avenues for developing rice genotypes that can be grown like dryland cereals. Roots play a key role in plant adaptation to dry environments.MethodsWe examined anatomical and histochemical root traits that affect water acquisition in rice (Oryza sativa) and wheat (Triticum aestivum). These traits and root growth were measured at two developmental stages for three rice and two wheat cultivars that were grown in pots under three water regimes.ResultsWheat roots had larger xylem sizes than rice roots, which potentially led to a higher axial conductance, especially under water-deficit conditions. Suberization, lignification and thickening of the endodermis in rice roots increased with increasing water deficit, resulting in stronger radial barriers for water flow in rice than in wheat, especially near the root apex. In addition, water deficit strongly impeded root growth and lateral root proliferation in rice, but only slightly in wheat, and cultivars within a species differed little in these responses. The stress sensitivity of rice attributes was slightly more prominent at vegetative than at flowering stages.ConclusionsRice root characteristics, which are essential for growth under inundated conditions, are not conducive to growth under water deficit. Although rice roots show considerable plasticity under different watering regimes, improving root xylem size and reducing the radial barriers would be required if rice is to grow like dryland cereals.

Pseudo spectral solution of extended Graetz problem for combined pressure-driven and electroosmotic flow in a triangular micro-duct

Thermally developing mixed electroosmotically and pressure-driven flow in a triangular micro-duct with a step change wall temperature is investigated. Both axial conduction (Graetz problem extended) and Joule heating effects are taken into consideration. A new higher order accurate method is proposed. The proposed method is based on pseudo spectral Galerkin approach and the eigenfunctions of the Helmholtz equation for a triangular geometry. Convergence of the method is investigated and the effect of the Péclet (Pe) number, Joule heating and Debye–Hückel parameter on the temperature distribution and Nusselt number (Nu) are discussed in detail.