Values Of Effective Thermal Conductivity Research Articles

An experimental study on the thermal performance of ground heat exchanger

A knowledge of ground thermal properties is most important for the proper design of large GHE (ground heat exchanger) systems. Thermal response tests have so far been used primarily not only for in situ determination of design data for GHE systems, but also for the evaluation of grout material, heat exchanger types and groundwater effects. The main purpose has been to determine in situ values of effective ground thermal conductivity, including the effect of groundwater flow and natural convection in boreholes.

Single-phase heat transfer in the high temperature multiple porous insulation

An experimental study of steady state flow and heat transfer has been conducted for the multiple plate porous insulation used in the reactor pressure vessels of ‘Magnox’ nuclear power stations. The insulation pack studied, consisting of seven dimpled stainless steel sheets and six plane stainless steel sheets, was of the type installed in the Sizewell A plant. A large scale experimental test facility, based on the guarded hot plate method, was used for measuring the effective thermal conductivity of Magnox reactor pressure vessel insulation, which consists of alternate layers of plain steel foil and dimpled foil. The measurements were made both with the fluid within the insulation pack nominally stationary and with an imposed flow through it, simulating leakage through the insulation pack. The experimental conditions corresponded to a heat flux of 75–1000 W/m 2, fluid pressures of atmospheric to 5 bar gauge, pack orientations in range of 0°–45° relative to the horizontal, leakage velocities ranging from 0.05 m/s to 0.20 m/s and inlet air bulk temperatures ranging from 18 °C to 290 °C. Local values of effective thermal conductivity of 0.04–0.23 W/m K were obtained for the above experimental conditions. The heat transfer modes in the insulation pack were conduction through the contacting metallic foils, thermal radiation across the gas gaps, and conduction and convection in the air. The effective thermal conductivity of the porous insulation increased with increasing air pressure, inclination angle, and air velocity. Buoyancy effects increased with increasing inclination angle and air pressure.

Near-wall effects in micro scale Couette flow and heat transfer in the Maxwell-slip regimes

The effects of modified transport characteristics within an extremely thin layer adjacent to the fluid–solid interfaces are investigated for fully developed laminar micro-scale Couette flows with slip boundary conditions. The wall-adjacent layer effects are incorporated into the continuum-based mathematical model by imposing variable viscosity and thermal conductivity values close to the channel walls, for solving the momentum and energy conservation equations. Analytical expressions for the velocity profiles are derived and are subsequently utilized to obtain the temperature variations within the parallel plate channel, as a function of the significant system parameters. It is revealed that the variations in effective viscosity and thermal conductivity values within the wall-adjacent layer have profound influences on the fluid flow and the heat transfer characteristics within the channel, with an interesting interplay with the wall slip boundary conditions. These effects cannot otherwise be accurately captured by employing classical continuum based models for microscale Couette flows that do not take into account the alterations in effective transport properties within the wall adjacent layers.

Temperature dependence of effective thermal conductivity and diffusivity of Zn xTe 100− x ( x = 5, 10, 30 and 50) chalcogenide material

Measurements of thermophysical properties of Zn x Te 100− x ( x = 5, 10, 30 and 50) chalcogenide material in pellet form have been made in the temperature range from room temperature (15 °C) to 200 °C and in cooling cycle from 200 °C to 15 °C using transient plane source (TPS) technique. In heating mode the values of effective thermal conductivity ( λ e) and effective thermal diffusivity ( χ e) increase from 15 °C to 200 °C. During the cooling mode λ e and χ e decrease slightly in the temperature range from 200 °C to 160 °C below which λ e and χ e remain almost constant at all temperatures. Such type of behaviour shows thermal hysteresis in this sample, which can be explained on the basis of the change in structure of the material. XRD studies suggest that the material is polycrystalline in nature.

Simultaneous measurement of thermal conductivity, thermal diffusivity and prediction of effective thermal conductivity of porous consolidated igneous rocks at room temperature

Thermal conductivity, thermal diffusivity and heat capacity per unit volume of porous consolidated igneous rocks have been measured, simultaneously by Gustafsson's probe at room temperature and normal pressure using air as saturant. Data are presented for eleven samples of dunite, ranging in porosity from 0.130 to 0.665% by volume, taken from Chillas near Gilgit, Pakistan. The porosity and density parameters have been measured using American Society of Testing and Materials (ASTM) standards at ambient conditions. The mineral composition of samples has been analysed from their thin sections (petrography). An empirical model to predict the thermal conductivity of porous consolidated igneous rocks is also proposed. The thermal conductivities are predicted by some of the existing models along with the proposed one. It is observed that the values of effective thermal conductivity predicted by the proposed model are in agreement with the experimental thermal conductivity data within 6%.

수직형 지열 열교환기(BHE)의 열성능 측정에 관한 실험적 연구

Knowledge of ground thermal properties is most important for the proper design of large BHE(borehole heat exchanger) systems. Thermal response tests with mobile measurement devices were first introduced in Sweden and USA in 1995. Thermal response tests have so far been used primarily for in insitu determination of design data for BHE systems, but also for evaluation of grout material, heat exchanger types and ground water effects. The main purpose has been to determine insitu values of effective ground thermal conductivity, including the effect of ground-water flow and natural convection in the boreholes. Test rig is set up on a small trailer, and contains a circulation pump, a heater, temperature sensors and a data logger for recording the temperature data. A constant heat power is injected into the borehole through the pipe system of test rig and the resulting temperature change in the borehole is recorded. The recorded temperature data are analysed with a line-source model, which gives the effective insitu values of rock thermal conductivity and borehole thermal resistance.

Effective Thermal Conductivity of Continuous Matrix-spherical Dispersed Phase Composite with Single-point Interfacial Thermal Contact: Continuum Regime Gas Conduction in the Gap

Analytical solutions are presented for the effective thermal conductivity of a continuous matrix composite with fully debonded spherical inclusions in single-point contact with the matrix. This situation represents the lower bound on the effect of partial debonding between the dispersed and the matrix phase on composite thermal conductivity. The heat transfer across the gap in the debonded regions is assumed to occur by gaseous conduction. Solutions for the temperature gradient parallel and perpendicular to the point of contact are considered. Numerical results for a sodaborosilicate glass matrix with spherical nickel inclusions and a range of values for the thermal conductivity kgap of the gas phase in the gap are presented. The effective composite thermal conductivity values for gradients imposed parallel and perpendicular to the point of contact were found to differ significantly depending on the magnitudes of the contact conductance and the thermal conductivity of the gas. When kgap approaches zero, the solution approaches that for a matrix with spherical pores presented by Maxwell. When kgap approaches infinity, the composite thermal conductivity approaches Maxwell’s value for a composite with perfect interfacial thermal contact.

A numerical investigation on the thermal impact of foam on an oxy-fuel-fired glass furnace

Gas evolution during the melting and refining of raw materials in glass melting furnaces may result in the formation of a layer of foam on the molten glass surface. The foam layer presents an additional thermal resistance to melting and refining between the furnace crown and combustion products and the raw materials/molten glass. A simple foam thermal model has been incorporated into an advanced comprehensive model for the prediction of the primary thermal phenomena in a prototypical oxy-fuel-fired industrial glass furnace. Heat fluxes and temperatures at the interfaces between the combustion space, the batch, the glass melt tank and the foam layer are calculated in an iterative procedure in order to simulate the entire furnace in an integrated fashion. The thermal impact of the foam on furnace behaviour is studied by comparing model simulations using different values of foam thickness and effective thermal conductivity in a 150-tonne-per-day, oxy-fuel-fired glass furnace to predictions in the same furnace without the presence of foam. The numerical results obtained quantify the effect of foam on important furnace operating parameters such as glass surface temperature, crown temperature and exhaust gas temperature.

Effective thermal actions and thermal properties of timber members in natural fires

AbstractFor the thermal analysis of structural or non‐structural timber members, using conventional simplified heat transfer models, thermal conductivity values of timber are normally calibrated to test results such that they implicitly take into account influences such as mass transport that are not included in the model. Various researchers and designers have used such effective thermal conductivity values, originally determined for standard fire exposure, to evaluate other fire scenarios such as natural fires. This paper discusses in qualitative terms some parameters that govern the burning of wood and their influence on effective conductivity values. Reviewing fire tests of timber slabs under natural fire conditions, the study explains why effective conductivity values, giving correct results for the ISO 834 standard fire scenario, should not be used in other fire scenarios. For this reason, the thermal properties of timber given in EN 1995‐1‐2 are limited to standard fire exposure. As shown by heat transfer calculations, the effective thermal conductivity of the char layer is strongly dependent on the charring rate and therefore varies during a natural fire scenario. It has also been shown that char oxidation during the decay phase in a natural fire has a significant influence on the temperature development in the timber member, since char surface temperatures exceed the gas temperature in the compartment or furnace. Using increased effective gas temperature as thermal action during the decay phase, and varying conductivity values for the char layer, fairly good agreement could be obtained regarding the temperature development in the timber member and the char depth. Copyright © 2005 John Wiley & Sons, Ltd.

Composition dependence of effective thermal conductivity and effective thermal diffusivity of Se100−xInx (x=0, 5, 10, 15 and 20) chalcogenide glasses

Simultaneous measurement of effective thermal conductivity (λe) and effective thermal diffusivity (χe) of twin pallets of Se100−xInx (x=0,5, 10, 15, and 20) glasses, prepared under a load of 5 tons, have been made at room temperature using the Transient Plane Source (TPS) technique. The values of λe and χe were found to increase initially with the increase of concentration of In in Se–In alloy and had their maximum at 10at.wt% of indium. For indium concentration beyond 10at.wt% of the values of effective thermal conductivity and effective thermal diffusivity decrease linearly. This is suggestive of fact that 10at.wt% of indium can be considered as a critical composition at which the alloy becomes, chemically ordered and maximum thermally stable than other composition. Further addition of indium in selenium decreases the values of λe and χe. The behaviour is explained on the basis of decrease of localized states and increase in disorderness for higher composition indium.

Increasing the accuracy of structure function based thermal material parameter measurements

The Structure functions based evaluation of the thermal transient measurements is now a broadly accepted way for the characterization of the time dependent behavior of the heat flow path. The usual way of generating structure functions considers one main heat flow path. By using a large mathematical tool set it generates for this path the Rth-Cth map of the structure. This enables easy detection of partial thermal resistances in the heat flow path, with which we can determine the values of, e.g,. interface thermal resistances, local effective thermal conductivity values, etc. The accuracy that we can obtain with this material parameter measuring methodology is in the order of 20%. In this paper, we present a methodology that enhances the accuracy of the structure function based material parameter measuring methods. In this procedure, on one hand, we measure the thermal transients for the system to be characterized and on the other hand we measure the "parasitic" heat flow path, that influences our measurement. The material parameters are calculated by appropriately modifying the measured results with the data of the parasitic heat flow path. In this paper, we present this methodology with mathematical details, and prove it with measured results.

A smart model to estimate effective thermal conductivity and viscosity in the weld pool

Calculations of fluid flow and heat transfer in the weld pool are strongly influenced by the values of effective thermal conductivity and effective viscosity of the liquid metal. The values of these variables are uncertain since the welding conditions and the fluid flow characteristics within the weld pool influence them. Following an inverse modeling approach, the present work develops a smart model that embodies a multivariable optimization scheme within the framework of a phenomenological heat transfer and fluid flow model to estimate the uncertain parameters necessary for weld pool modeling. The optimization scheme considers the sensitivity of the calculated weld geometry with respect to the unknown parameters. To avoid unrealistic optimized solutions, the smart model is internally guided to look for only the physically significant solutions. The model could estimate the effective thermal conductivity and effective viscosity for conduction mode laser welding as a function of nondimensional heat input from six sets of experimental measurements of weld pool depth and width.

Experimental Investigation and Analysis of the Effective Thermal Properties of Beryllium Packed Beds

ABSTRACTBeryllium, in its pebble form, has been proposed in various blanket concepts to serve different purposes. Thermal property data for such a heterogeneous packed bed is needed, particularly data on the impact of compression forces on its magnitude and consequent temperature profile. The objectives of this work are to obtain and quantify experimental data on the effective thermal conductivity of a Be-He packed bed, on the interface heat conductance between Be and SiC, and on the effects of externally applied pressure on these effective thermal properties. The effective thermal conductivity of a Be-He pebble bed increases as the bed mean temperature increases. The values of effective thermal conductivity vary from 2.15 to 3.00 W/m.K for bed mean temperature ranges from 90 to 420 °C. Similar temperature effects are seen in the Be/SiC interface heat conductance, as the values of interface heat conductance range from 1140 to 2200 W/m2.K. In addition, effective thermal conductivity increases remarkably with the increase of applied pressure (by a factor of 2.53 at 2 MPa), while it remains higher than the initial value by ~0.3 W/m.K when external pressure is released (hysteresis effect).

Heat Transfer in High-Temperature Fibrous Insulation

The combined radiation/conduction heat transfer in high-porosity, high-temperature fibrous insulations was investigated experimentally and numerically. The effective thermal conductivity of fibrous insulation samples was measured over the temperature range of 300-1300 K and environmental pressure range of 1.33 x 10(exp -5)-101.32 kPa. The fibrous insulation samples tested had nominal densities of 24, 48, and 72 kilograms per cubic meter and thicknesses of 13.3, 26.6 and 39.9 millimeters. Seven samples were tested such that the applied heat flux vector was aligned with local gravity vector to eliminate natural convection as a mode of heat transfer. Two samples were tested with reverse orientation to investigate natural convection effects. It was determined that for the fibrous insulation densities and thicknesses investigated no heat transfer takes place through natural convection. A finite volume numerical model was developed to solve the governing combined radiation and conduction heat transfer equations. Various methods of modeling the gas/solid conduction interaction in fibrous insulations were investigated. The radiation heat transfer was modeled using the modified two-flux approximation assuming anisotropic scattering and gray medium. A genetic-algorithm based parameter estimation technique was utilized with this model to determine the relevant radiative properties of the fibrous insulation over the temperature range of 300-1300 K. The parameter estimation was performed by least square minimization of the difference between measured and predicted values of effective thermal conductivity at a density of 24 kilograms per cubic meters and at nominal pressures of 1.33 x 10(exp -4) and 99.98 kPa. The numerical model was validated by comparison with steady-state effective thermal conductivity measurements at other densities and pressures. The numerical model was also validated by comparison with a transient thermal test simulating reentry aerodynamic heating conditions.

Thermal properties of neutron irradiated Se80Te10In10 glass

The thermal transport properties of Se 80 Te 10 In 10 chalcogenide glass irradiated by slow neutrons for different time exposures were studied using the transient plane source method. The effective thermal conductivity ( λ ) and effective thermal diffusivity ( χ ) increase with exposure time of the material and reaches to a saturation value. Constant values of effective thermal conductivity and effective thermal diffusivity beyond a certain time exposure could be explained on the basis of decrease of concentration of Se–Te polymeric chains and Se–Te rings.

Calculating effective board thermal parameters from transient measurements

The paper presents a method to find the effective thermal conductivity and effective volumetric heat capacity values of PWBs from the results of thermal transient measurements and a series of subsequent evaluation steps. Various powering and temperature sensing methods were investigated. The experiments show the feasibility of creating a method for obtaining effective thermal parameters of printed wiring boards with the help of usual thermal transient testers.

Temperature and moisture dependence on the thermal conductivity of wood-cement-based composite: experimental and theoretical analysis

Results are presented of a study on the effect of moisture contents and temperature on the thermal conductivity of wood-aggregate-clay-cement composites, energy saving concretes. The thermal conductivity has been measured in the range of 2-60 °C, which covers the majority of thermal conditions in which the material is used. The unit cell of the parallel model, already validated for dry wood concretes, i.e. two-phase system, has been extended in order to calculate the thermal conductivity of wood concretes for various moisture contents. Experimental results show a significant dependence of thermal conductivity on moisture contents and temperature. The theoretical analysis of these phenomena has been conducted in two stages using the classical De Vries approach. The first stage consists of estimating the thermal conductivity at low temperatures for the three-phase system (i.e. unsaturated materials) using a modified unit-cell parallel model. The validity of the expression developed is then tested by comparing experimental results with computed values of effective thermal conductivity. The second part consists of determining the `f' factor that characterizes the effects of temperature on thermal conductivity. The experimental results of thermal conductivity at various moisture contents and temperatures were compared with the results obtained from De Vries's approach and a very high level of agreement was observed.

The Effective Thermal Conductivity of a Bed of 1.2-mm-diam Lithium Zirconate Spheres in Helium

The effective thermal conductivity of a bed of lithium zirconate spheres in helium has been measured over the ranges 0 < P < 200 kPa and 600 < T < 1500 K. The data are fitted to the equation k = ap + bpT3. At 100 kPa, the thermal conductivity is given by k(T) = 0.69 + 2.2 × 10-10 T3 W/m·K-1. A table of values for ap and bp is provided for other pressure settings. Values of the near-wall effective thermal conductivity are estimated from the data.

Investigation of Whey Protein Deposit Properties Using Heat Flux Sensors

Heat exchanger fouling due to whey proteins was investigated in a concentric-tube device using aqueous solutions of 3.5 wt% whey protein concentrate.The void (liquid) fraction of the deposit changed as the fouling mechanism changed from surface reaction control to mixed bulk reaction and mass transfer control. The thermal resistance of deposits and their effective thermal conductivity was measured in situ using a novel technique based on a heat flux sensor. This method gave values of (effective deposit thermal conductivity×density, ρ f λ f) around 470Wkgm−4K−1 for deposits exposed to wall temperatures less than 85° C. Deposits formed at higher wall temperatures showed the effects of ageing and gave a larger value of ρ f λ f.

Two-phase Rayleigh–Bénard instabilities

The influence of particles on the first Rayleigh–Bénard bifurcation from the motionless to the convective state has been studied in a suspension layer heated from below and cooled from above, and effective physical properties have been measured. The particle parameters are the diameter and the concentration. The experimental setup allows one to measure accurately the temperature and the heat flux and to computerize the data recording. The suspensions consisted of polystyrene particles (diameters 0.1, 0.2, 3, and 100 μm and solid volume fraction from 1% to 10%) in water with the addition of surfactants and sodium sulphate for particles greater than 1 μm. The onset of convection was detected by a change of the slope of the curves showing the heat flux versus the temperature difference. The experimental results show (i) a good agreement between the experimental values of effective thermal conductivity and the theoretical formulas, (ii) an increase of the convective threshold with a volume fraction that will be attributed to the stabilizing effects of the particles on the fluid (growth of the thermal relaxation time and decrease of the buoyancy time), and (iii) a nonmonotonous subcritical bifurcation for particles smaller than 1 μm and supercritical bifurcation otherwise.