Small Thermal Conductivity Research Articles

A High-Performance Heat Exchanger Using Modified Polyvinylidene Fluoride-Based Hollow Fibers

Plastic heat exchangers has the shortcomings of bulky, thick pipe wall with large thermal resistance, poor heat transfer, aging of plastic and a narrow temperature range. The key to increase the heat transfer performance of heat exchanger is improving thermal performance of heat conduction.To enhance heat transfer effects and expand the temperature range of using plastic heat exchanger, PVDF with good temperature resistance is used as matrix and modification with graphite fillers to prepare composite hollow fiber which has the advantage of small diameter, thin wall and good thermal conductivity. Also, composite materials hollow fibers are used to prepare shell and tube hollow fiber heat exchanger.The testing of "water - water" system for our heat exchanger module has been done, and the results indicate that adding graphite is helpful to improve thermal conductivity of PVDF-based heat conductive hollow fiber heat exchanger to a certain extent.hen the content of graphite is 3%, the heat transfer effect is the best.

The Studies on the Thermophysical Properties of Binary Systems of Neopentylglycol and the Derives

Specific capacity and thermal conductivity are two very important physical quantities that characterize the heat performance of heat storage material. This paper measured the specific capacities and thermal conductivities of binary systems formed by NPG、TRIS and PE in the temperature range from 30oC to 70 oC and analyzed the experimental results from the point of view of crystal vibration. Hydrogen bonds exert great influence on the thermophysical properties of the binary systems. Although the components are only simply blended, substitutional impurity defects are incurred. As a result hydrogen bond will be affected in a complex way. There is no uniform trend between the value of x and specific capacity. Because of the influence of structure defect, specific capacity and phonon speed which all change with the strength of hydrogen bond, the relationship between thermal conductivity and the value of x is non-linear too. Because 40oC is near the phase transition temperature of NPGxTRIS1-x (x=0.618), its specific capacity at 40oC decreases and thermal conductivity increases. Because hydrogen bond changes with temperature, specific capacity of PExTRIS1-x (x=0.618) rises. Since 60oC is near the phase transition temperature, its specific capacity at 60oC falls. PExTRIS1-x (x=0.618) shows smaller thermal conductivities at temperatures higher than 30oC. This is because the hydrogen bonds become weaker with temperature. Blending impurities possessing longer mean free path with polyalcohol can change the thermal conductivity.

Predicted interfacial thermal conductance and thermal conductivity of diamond/Al composites with various interfacial coatings

The interfacial thermal conductance (ITC) and thermal conductivity (TC) of diamond/Al composites with various coatings were theoretically studied and discussed. A series of predictions and numerical analyses were performed to investigate the effect of thickness, sound velocity, and other parameters of coating layers on the ITC and TC. It is found that both the ITC and TC decline with increasing coating thickness, especially for the coatings with relatively low thermal conductivity. Nevertheless, if the coating thickness is close to zero, or quite a small value, the ITC and TC are mainly determined by the constants of the coating material. Under this condition, coatings such as Ni, TiC, Mo2C, SiC, and Si can significantly improve the ITC and TC of diamond/Al composites. By contrast, coatings like Ag will exert the negative effect. Taking the optimization of interfacial bonding into account, conductive carbides such as TiC or Mo2C with low thickness can be the most suitable coatings for diamond/Al composites.

Room temperature picowatt-resolution calorimetry

Picowatt-resolution calorimetry is necessary for fundamental studies of nanoscale energy transport. Here, we report a microfabricated device capable of <4 pW resolution—an order of magnitude improvement over state-of-the-art room temperature calorimeters. This is achieved by the incorporation of two important features. First, the active area of the device is thermally isolated by thin and long beams with a total thermal conductance (G) of ∼600 nW/K. Further, a bimaterial cantilever thermometer capable of a temperature resolution (ΔTres) of ∼4 μK is integrated into the microdevice. The small thermal conductance and excellent temperature resolution enable measurements of heat currents (q = G × ΔTres) with a resolution <4 pW.

An Experimental Study on Engineering Properties of Rubber Particles-Improved Fly Ash Soil

In this paper, by studying the engineering properties of rubber particles-improved fly ash soil, the authors provide technical parameters for setting up cold-resistance layer in the seasonal frozen zone. In the experiment, rubber particles are mixed into the fly ash soil by mass of 0%, 1%, 2%, 3%, 5% and 7% and unconfined compressive strength testing, frost heave test, insulation test and thermal conductivity test. By analyzing the test results, the authors drew the conclusion that fly ash soil improved by rubber particles has a high compressive strength, a small amount of frost heave, good insulation performance, small thermal conductivity coefficient.Therefore, improved fly ash soil with rubber particles is one of the preferred materials to set up the cold resistance layer in seasonal frost zone.

Specific Heat and Thermal Diffusivity and Conductivity of PbWO4Single Crystal Related to Its Crystal Orientation at (300 to 673) K

Lead tungstate (PbWO4, PWO) single crystal has been chosen as a new generation scintillation material. The specific heat and thermal diffusivity of PWO single crystal grown by the vertical Bridgman technique is investigated for the first time by differential scanning calorimetry (DSC) and a laser flash technique at (300 to 673) K, respectively. The thermal conductivity is calculated, and it exhibits anisotropy with the values along the [100] axis (Kx,y) higher than that along the [001] axis (Kz), which is 2.40 W·m−1·K−1 and 2.00 W·m−1·K−1 at room temperature, respectively. Because of the heavier elements of the Pb atom, the PWO crystal has a higher degree of anharmonicity of lattice vibrations and thus has a smaller thermal conductivity than another scintillation material bismuth germanate (BGO).

Thermoelectric properties of sol–gel derived cobaltite Bi 2Ca 2.4Co 2O y

Layered misfit cobaltite Bi 2Ca 2.4Co 2O y has been synthesized by a sol–gel method. This compound exhibits large thermoelectric (TE) power ( S 300 K ∼170 μV K −1), low resistivity ( ρ 300 K ∼42 mΩ cm) and relatively small thermal conductivity ( κ 300 K ∼2.8 W K −1 m −1) at room temperature. Furthermore, the resistivity of this compound displays a metallic behavior above T ⁎∼150 K with a semiconducting behavior below this temperature. This abnormal behavior in resistivity is analogous to those observed in Sr and Ba based misfit cobaltites. The observed features of the TE have been discussed based on the narrow band model.

Heat transport in amorphous silicon: Interplay between morphology and disorder

We present a theoretical study of the thermal conductivity (κ) of amorphous silicon (a-Si) based on molecular and lattice dynamics. We find that the majority of heat carriers are quasi-stationary modes; however the small proportion (≃3%) of propagating vibrations contributes to about half of the value of κ. We show that in bulk samples the mean free path of several long-wavelength modes is on the order of microns; this value may be substantially decreased either in thin films or in systems with etched holes, resulting in a smaller thermal conductivity. Our results provide a unified explanation of several experiments and show that kinetic theory cannot be applied to describe thermal transport in a-Si at room temperature.

Heat Transfer Reduction due to a Ceiling-Mounted Barrier in an Enclosure with Natural Convection

Effects of a ceiling-mounted barrier on natural convection heat transfer in a square cavity with differentially heated wall are numerically investigated. A limit case, in which the partition has small thickness and low thermal conductivity, is studied. The study is performed for nine different locations of barrier on the ceiling, two different lengths of barrier as 15 and 50% of the side wall, and Rayleigh numbers from 103 to 106. The vorticity and streamfunction approach is used to obtain velocity distribution, and the energy equation is solved to determine temperature field in the cavity. The variations of the local Nusselt number on the hot and cold walls and the change of mean Nusselt number with the location of barrier in the cavities with different Rayleigh numbers are presented. The obtained results show that a wall-mounted barrier can be used to reduce heat transfer rate through the cavity; however, its effectiveness depends on length and location of barrier and Rayleigh number.

Examination on Substrate Preheating Process in Cold Gas Dynamic Spraying

Substrate preheating always takes an important role in particle bonding and formation of the first layer coating in cold gas dynamic spraying (CGDS). In this study, a systemic investigation on substrate preheating process is conducted with Cu, Al, Steel, and Ti substrate by both numerical and experimental methods. The computational fluid dynamic (CFD) approach is adopted to simulate the heat exchange process between gas and solid substrate. The numerical results show that substrate can be significantly preheated by the high-temperature gas, especially by the gas at the near-wall zone behind the bow shock where the temperature is extremely high. Moreover, the comparison between different substrates implies that substrates with smaller thermal conductivity can achieve higher surface temperature and larger temperature gradient which may greatly contribute to the generation of residual stress, such as Ti substrate in this study. For the heat flux, Cu substrate obtains the largest value at the center zone of the substrate surface, followed by Al, Steel, and Ti substrate, but at the outer zone, the heat flux through the Cu substrate surface is smaller than the other three types of substrates. Besides, based on the experimental results, it is found that the substrate surface temperature amounts to the peak value only when the preheating time is sufficiently long.

Effects of porous media on thermal and salt diffusion of solar pond

Laboratory and field experiments were carried out along with numerical simulations in this paper to study the effects of porous media on thermal and salt diffusion of the solar ponds. From our laboratory experiments simulating heat transfer inside a solar pond, it is shown that the addition of porous media to the bottom of a solar pond could help enhance its heat insulation effect. The experiment on salt diffusion indicates that the upward diffusion of the salt is slowed down when the porous media are added, which helps maintain the salt gradient. Our field experiments on two small-scaled solar ponds indicate that when porous media are added, the temperature in the lower convective zone (LCZ) of the solar pond is increased. It is also found that the increase in turbidity is repressed by porous media during the replenishment of the salt to the LCZ. Thermal diffusivities and conductivities of brine layers with porous media such as pebble and slag were also respectively measured in this paper based on the unsteady heat conducting principles of a semi-infinite body. These measured thermal properties were then used in our numerical simulations on the effect of porous media on thermal performance of a solar pond. Our simulation results show that brine layer with porous media plays more positive role in heat insulation effect when thermal conductivity of the ground is big. On the other hand, when the ground has a very small thermal conductivity, the performance of solar pond might be deteriorated and total heat storage quantity of solar pond might be reduced by brine layer with porous media.

Auto-ignition of methane–air mixtures flowing along an array of thin catalytic plates

In this paper, the heterogeneous ignition of a methane–air mixture flowing along an infinite array of catalytic parallel plates has been studied by inclusion of gas expansion effects and the finite heat conduction on the plates. The system of equations considers the full compressible Navier–Stokes equations coupled with the energy equations of the plates. The gas expansion effects which arise from temperature changes have been considered. The heterogeneous kinetics considers the adsorption and desorption reactions for both reactants. The limits of large and small longitudinal thermal conductance of the plate material are analyzed and the critical conditions for ignition are obtained in closed form. The governing equations are solved numerically using finite differences. The results show that ignition is more easily produced as the longitudinal wall thermal conductance increases, and the effects of the gas expansion on the catalytic ignition process are rather small due to the large value of the activation energy of the desorption reaction of adsorbed oxygen atoms.

A bubbling fluidized bed solar reactor model of biomass char high temperature steam-only gasification

A two phase biomass char (biochar) steam gasification model based on the systems kinetics is developed in a bubbling fluidized bed with concentrated solar heat as source of energy. The model calculates the dynamic and steady state profiles, as well as the complex parameters of fluidized beds. This robust model is capable of predicting the temperature and concentration profiles of gases in the bubble, emulsion gas and solid phases. The Rosseland equation is used to calculate the radiative transfer within the bed. Due to the nature of the fluidized bed, the small bed thermal conductivity and bigger void between particles, there is a large temperature gradient throughout the bed, indicating that the system is highly non-isothermal. The set-up of a fluidized bed with solar irradiation in the upper side of the reactor is found to be a less efficient gasifying system in comparison with a packed bed, but could be optimized if the source of heat is changed to the bottom of the reactor. The trends and responses of the model are in good agreement with the experimental trends reported in the literature. Hydrogen is the principal product followed by carbon monoxide, the carbon dioxide production is small and the methane production is negligible.

Perturbation analysis of the heat transfer in porous media with small thermal conductivity

An approximate analytical solution for the one-dimensional problem of heat transfer between an inert gas and a porous semi-infinite medium is presented. Perturbation methods based on Laplace transforms have been applied using the solid thermal conductivity as small parameter. The leading order approximation is the solution of Nusselt (or Schumann) problem. Such solution is corrected by means of an outer approximation. The boundary condition at the origin has been taking into account using an inner approximation for a boundary layer. The gas temperature presents a discontinuous front (due to the incompatibility between initial and boundary conditions) which propagates at constant velocity. The solid temperature at the front has been smoothed out using an internal layer asymptotic approximation. The good accuracy of the resulting asymptotic expansion shows its usefulness in several engineering problems such as heat transfer in porous media, in exhausted chemical reactions, mass transfer in packed beds, or in the analysis of capillary electrochromatography techniques.

Multi-artery heat pipe spreader: Experiment

We constructed a low thermal resistance, multi-artery heat pipe spreader vapor chamber by designing a thin (monolayer) evaporator wick and distributed permeable columnar arteries supplying liquid (water) to highly concentrated heat source region. The condenser wick is layered copper screens in intimate contact with the columnar arteries. The vapor chamber is sealed and externally surface-convection cooled on the condenser side. For the evaporator wick and arteries, sintered, surface etched-oxidized copper particles are used to enhance wettability. The measured evaporator thermal resistance is less than 0.05 K/(W/cm 2) using a 1 cm 2 heat source, and the critical heat flux is about 380 W/cm 2. This is in good agreement with thermal–hydraulic network models prediction, 389 W/cm 2. The resistance is dominated by the small effective thermal conductivity of the evaporator wick and by the small conduction path through the receding meniscus within it. This resistance decreases nonlinearly with the heat flux, due to a decrease in the radius of the receding meniscus.

On reaction coupled transport phenomenon in reformer ducts

In compact steam reformer the probability of component degradation and failure depends strongly on the local temperature gradients coupled by various transport processes and chemical reactions in multi-functional materials. In this paper, the modeling and analysis of coupled mass transport and heat transfer processes are conducted for compact design steam reformer duct, which consists of a porous layer for the reforming reactions of methane, the fuel gas flow duct and solid plate. A fully three-dimensional computational fluid dynamics (CFD) approach is applied to calculate transport processes and effects of thermal conductivities of the involved multi-functional materials on reforming reaction rates and heat transfer/temperature distributions, in terms of surface temperatures/heat fluxes and Nusselt numbers. The steam reformer conditions such as mass balances associated with the chemical reactions and gas permeation to/from the porous layer are implemented in the calculation. The results reveal that a small thermal conductivity of the porous layer and solid plates promote high reforming reaction rates, and the convective heat transfer at the top interface varies more significantly along the main flow reformer duct.

Research on Temperature Field about Cutting Area in Plunge Milling Ti Alloy Based on Finite Element

Because of the small thermal conductivity coefficient of Ti alloy, the heat cannot disperse timely and accumulate seriously when plunge milling Ti alloy. If the cutting parameters can not be controlled well, the phenomenon of sticking will happen easily. According to the simulation and analysis of temperature field by using finite element software ABAQUS, the influence on cutting speed and feed for the Distributing of Temperature Field about Cutting Area in Plunge Milling Ti Alloy is acquired.

Absence of Marangoni convection at Marangoni numbers above 27,000 during water evaporation

Two mechanisms by which Marangoni convection can be produced at the interface of water with its vapor are: (1) by imposing a temperature gradient parallel to the water-vapor interface, and (2) by imposing a temperature gradient perpendicular to the interface that results in the liquid becoming unstable. A series of evaporation experiments conducted with H2O and with D2O maintained at the mouth of a stainless-steel funnel indicated the presence of Marangoni convection, but the mechanism producing the convection was unclear. We have investigated the mechanism using a funnel constructed with a polymethyl methacrylate that has a small thermal conductivity relative to that of water and repeating the evaporation experiments. Marangoni convection was eliminated with this funnel even though the Marangoni number, Ma, was in the range 8277< or =Ma< or =27 847 . A comparison of the assumptions made in the theories available to predict the onset of Marangoni convection with the observations made in this study indicates some of the assumptions are invalid: although generally neglected, energy transport through the vapor to the interface of evaporating water is significant; there is an interfacial temperature discontinuity, but it is in the opposite direction of that assumed in the existing theories: the interfacial-vapor temperature is greater than that of the liquid during evaporation; and the prediction of the critical Marangoni number is based on an arbitrarily chosen value of the heat-transfer coefficient. When the temperature gradient is perpendicular to the water-vapor interface, these invalid assumptions indicate present theories do not apply to volatile liquids.

Self-Assembled Germanium Quantum-Dot Supercrystals in Silicon with Extremely Low Thermal Conductivities for Thermoelectrics

Superlattices with one-dimensional (1D) phonon confinement were studied to obtain a low thermal conductivity for thermoelectrics. Since they are composed of materials with a lattice mismatch, they often show dislocations. Like 1D nanowires, they also decrease heat transport in only one main propagation direction. It is therefore challenging to design superlattices with a thermoelectric figure of merit ZT higher than unity. Epitaxial self-assembly is a major technology to fabricate three-dimensional (3D) Ge quantum-dot (QD) arrays in Si. They have been used for quantum and solar-energy devices. Using the atomic-scale phononic crystal model, 3D Ge QD supercrystals in Si also present an extreme reduction of the thermal conductivity to a value that can be under 0.04 W/m/K. Owing to incoherent phonon scattering, the same conclusion holds for 3D supercrystals with moderate QD disordering. As a result, they might be considered for the design of highly efficient complementary metal–oxide–semiconductor (CMOS)-compatible thermoelectric devices with ZT possibly much higher than unity. Such a small thermal conductivity was only obtained for two-dimensional layered WSe2 crystals in an experimental study. However, electronic conduction in the Si/Ge compounds is significantly enhanced. The 0.04 W/m/K value can be computed for different Ge QD filling ratios of the Si/Ge supercrystal with size parameters in the range of current fabrication technologies.

Thermoelectric figure of merit of [formula omitted]-(EDO- S,S-DMEDT-TTF) 2(AuBr 2) 1+ y, ( [formula omitted]) and (TMTSF) 2PF 6

Electrical resistivity, thermopower and thermal conductivity were measured simultaneously for τ -(EDO- S,S-DMEDT-TTF) 2(AuBr 2) 1+ y , ( y ≤ 0.875 ) and (TMTSF) 2PF 6, respectively. The thermoelectric figure of merit Z of the materials is determined by combining these transport coefficients. It was found that the dimensionless figure of merit Z T of τ -(EDO- S,S-DMEDT-TTF) 2(AuBr 2) 1+ y , ( y ≤ 0.875 ) amounts to 0.042 at 130 K. This relatively large Z T is due to large thermopower and small thermal conductivity measured by an improved method as compared with that previously reported by the present authors.