Effective Thermal Conductivity of Coal Ash Deposits at Moderate to High Temperatures

1987 ◽  
Vol 109 (2) ◽  
pp. 215-221 ◽  
Author(s):  
D. W. Anderson ◽  
R. Viskanta ◽  
F. P. Incropera
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Thermal History ◽  
Coal Ash ◽  
Physical Structure ◽  
Conductivity Measurements ◽  
Wide Range ◽  
Particulate Form ◽  
Order Of Magnitude ◽  
Deposit Structure

The effective thermal conductivity of coal ash deposits strongly influences heat transfer in pulverized coal-fired boilers. In this study thermal conductivity measurements were performed over a wide range of temperatures for fly ash, slagging deposits, and fouling deposits. The effects of ash particle size, thermal history, and physical structure of the deposit are discussed. Thermal history and deposit structure were observed to have the greatest influence on the local thermal conductivty, which increased by an order of magnitude with particle melting. Conductivities for solid-porous deposits were twice those of the same sample in particulate form.

Brownian-Motion-Based Convective-Conductive Model for the Effective Thermal Conductivity of Nanofluids

2005 ◽  
Vol 128 (6) ◽  
pp. 588-595 ◽  
Author(s):  
Ravi Prasher ◽  
Prajesh Bhattacharya ◽  
Patrick E. Phelan
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Particle Sizes ◽  
Brownian Movement ◽  
Liquid Temperature ◽  
Interfacial Resistance ◽  
Conduction Model ◽  
Order Of Magnitude ◽  
Base Liquid ◽  
Good Agreement

Here we show through an order-of-magnitude analysis that the enhancement in the effective thermal conductivity of nanofluids is due mainly to the localized convection caused by the Brownian movement of the nanoparticles. We also introduce a convective-conductive model which accurately captures the effects of particle size, choice of base liquid, thermal interfacial resistance between the particles and liquid, temperature, etc. This model is a combination of the Maxwell-Garnett (MG) conduction model and the convection caused by the Brownian movement of the nanoparticles, and reduces to the MG model for large particle sizes. The model is in good agreement with data on water, ethylene glycol, and oil-based nanofluids, and shows that the lighter the nanoparticles, the greater the convection effect in the liquid, regardless of the thermal conductivity of the nanoparticles.

Characterization of Anisotropic and Irregularly-Shaped Materials by High-Sensitive Thermal Conductivity Measurements

2007 ◽  
Vol 124-126 ◽  
pp. 1641-1644 ◽  
Author(s):  
M. Gustavsson ◽  
Hideaki Nagai ◽  
Takeshi Okutani
Keyword(s):  
Thermal Conductivity ◽  
Structural Changes ◽  
Bulk Material ◽  
Thermal Contact ◽  
Measurement Techniques ◽  
High Sensitivity ◽  
Conductivity Measurements ◽  
Destructive Testing ◽  
Analysis And Design ◽  
Wide Range

In modern thermal analysis and design involving thermal transport in solid components it is necessary to apply different modeling of the thermal heat flow in bulk material and across solid surface interfaces either in shape of a layer or a solid-solid interface. Similar differences occur when applying different measurement techniques. Some techniques have been developed specifically for the purpose of performing measurements of bulk properties by removing the influence from thermal contact resistance between the measurement probe and the sample material. Thermal conductivity measurements on metal and ceramic objects of various geometries such as thin bars, thin sheets as well as coatings or layers are here described when using the Transient Plane Source technique. A summary overview of the recent developments of this technique, including its ability to be applied in measurement situations covering a wide range of length and time scales, is also presented. Structural changes in anisotropy can be recorded with high sensitivity by comparative measurements. The technique may be applied in situations requiring non-destructive testing, e.g. samples of particular geometry used for mechanical or tensile testing.

Effective Thermal Conductivity Measurements of the Binary Pebble Beds by Hot Wire Method for the Breeding Blanket

1998 ◽  
Vol 34 (3P2) ◽  
pp. 877-881 ◽  
Author(s):  
Mikio Enoeda ◽  
Kazuyuki Furuya ◽  
Hideyuki Takatsu ◽  
Shigeto Kikuchi ◽  
Toshihisa Hatano
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Conductivity Measurements ◽  
Hot Wire ◽  
Wire Method

Influence of Contact Mechanics in the Prediction of the Effective Thermal Conductivity of Spheroid Packed Beds

2003 ◽  
Author(s):  
G. Buonanno ◽  
A. Carotenuto ◽  
G. Giovinco ◽  
L. Vanoli
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Statistical Approach ◽  
Thermal Contact ◽  
Peak Height ◽  
Thermal Contact Conductance ◽  
Curvature Radius ◽  
Packed Beds ◽  
Wide Range ◽  
Thermal Phenomena

Thermal contact conductance is an important parameter in a wide range of thermal phenomena, and consequently a large number of experimental, numerical and statistical investigations have been carried out in literature. In the present paper an analysis of thermal contact resistance is carried out to predict heat transfer between spherical rough surfaces in contact, by means of a statistical approach. The micro-geometry of the surface is described through a probabilistic model based on the peak height variability and invariant asperity curvature radius. The numerical model has been applied to evaluate the effective thermal conductivity of packed beds of steel spheroids and validated through the comparison with the experimental data obtained by means of an apparatus designed and build up for this purpose.

Thermal Conductivity of Metal-Oxide Nanofluids: Particle Size Dependence and Effect of Laser Irradiation

2006 ◽  
Vol 129 (3) ◽  
pp. 298-307 ◽  
Author(s):  
Sang Hyun Kim ◽  
Sun Rock Choi ◽  
Dongsik Kim
Keyword(s):  
Thermal Conductivity ◽  
Particle Size ◽  
Effective Thermal Conductivity ◽  
Laser Irradiation ◽  
Volume Fraction ◽  
Suspended Particle ◽  
Titanium Dioxide Nanoparticles ◽  
Size Change ◽  
Conductivity Enhancement ◽  
Wide Range

The thermal conductivity of water- and ethylene glycol-based nanofluids containing alumina, zinc-oxide, and titanium-dioxide nanoparticles is measured using the transient hot-wire method. Measurements are performed by varying the particle size and volume fraction, providing a set of consistent experimental data over a wide range of colloidal conditions. Emphasis is placed on the effect of the suspended particle size on the effective thermal conductivity. Also, the effect of laser-pulse irradiation, i.e., the particle size change by laser ablation, is examined for ZnO nanofluids. The results show that the thermal-conductivity enhancement ratio relative to the base fluid increases linearly with decreasing the particle size but no existing empirical or theoretical correlation can explain the behavior. It is also demonstrated that high-power laser irradiation can lead to substantial enhancement in the effective thermal conductivity although only a small fraction of the particles are fragmented.

Qualitative Equivalence Between Electrical Percolation Threshold and Effective Thermal Conductivity in Polymer/Carbon Nanocomposites

2011 ◽  
Vol 134 (1) ◽  
Author(s):  
Majid Baniassadi ◽  
Akbar Ghazavizadeh ◽  
Yves Rémond ◽  
Said Ahzi ◽  
David Ruch ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Monte Carlo ◽  
Monte Carlo Simulations ◽  
Percolation Threshold ◽  
Effective Thermal Conductivity ◽  
Electrical Percolation ◽  
Aspect Ratios ◽  
Wide Range ◽  
Statistical Continuum ◽  
Electrical Percolation Threshold

In this study, a qualitative equivalence between the electrical percolation threshold and the effective thermal conductivity of composites filled with cylindrical nanofillers has been recognized. The two properties are qualitatively compared on a wide range of aspect ratios, from thin nanoplatelets to long nanotubes. Statistical continuum theory of strong-contrast is utilized to estimate the thermal conductivity of this type of heterogeneous medium, while the percolation threshold is simultaneously evaluated using the Monte Carlo simulations. Statistical two-point probability distribution functions are used as microstructure descriptors for implementing the statistical continuum approach. Monte Carlo simulations are carried out for calculating the two-point correlation functions of computer generated microstructures. Finally, the similarities between the effective conductivity properties and percolation threshold are discussed.

In Situ Characterization of Coal Ash Thermal Conductivity Under Oxidizing and Reducing Conditions

2011 ◽  
Author(s):  
D. Cundick ◽  
D. Maynes ◽  
T. Moore ◽  
D. R. Tree ◽  
M. R. Jones ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Flow Reactor ◽  
Coal Ash ◽  
In Situ Measurements ◽  
Powder River Basin ◽  
Reducing Conditions ◽  
Bituminous Coals ◽  
Thermal Conductivities

This work presents in situ measurements of the effective thermal conductivity in particulate coal ash deposits under both reducing and oxidizing environments. Laboratory experiments generated deposits on an instrumented deposition probe of loosely-bound particulate ash from three coals generated in a down-fired flow reactor with optical access. An approach is presented for making in situ measurements of the temperature difference across the ash deposits, the thickness of the deposits, and the total heat transfer rate through the ash deposits. Using this approach, the effective thermal conductivity was determined for coal ash deposits formed under oxidizing and reducing conditions. Three coals were tested under oxidizing conditions: two bituminous coals derived from the Illinois #6 basin and a subbituminous Powder River Basin coal. The subbituminous coal exhibited the lowest range of effective thermal conductivities (0.05–0.18 W/m· K) while the Illinois #6 coals showed higher effective thermal conductivities (0.2–0.5 W/m· K). One of the bituminous coals and the subbituminous coal were also tested under reducing conditions. A comparison of the ash deposits from these two coals showed no discernible difference in the effective thermal conductivity based on stoichiometry. All experiments indicated an increase in effective thermal conductivity with deposit thickness, probably associated with deposit sintering.

Effective thermal conductivity of silicone/phosphor composites

2011 ◽  
Vol 45 (23) ◽  
pp. 2465-2473 ◽  
Author(s):  
Qin Zhang ◽  
Zhihua Pi ◽  
Mingxiang Chen ◽  
Xiaobing Luo ◽  
Ling Xu ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Particle Size ◽  
Effective Thermal Conductivity ◽  
Volume Fraction ◽  
Numerical Study ◽  
Element Model ◽  
Interfacial Thermal Resistance ◽  
Conductivity Measurements ◽  
The Monte Carlo Method ◽  
Random Particle

The effective thermal conductivity of silicone/phosphor composites is studied experimentally and numerically. Thermal conductivity measurements are conducted from 30°C to 150°C for the composites with phosphor volume fraction up to 40%. In the numerical study, a finite element model with empirical particle size distribution and random particle position is constructed using a probability density function and the Monte Carlo method, and the interfacial thermal resistance layer between phases also introduced in the model. The results indicate that when phosphor concentration is below 25 vol.%, the conductivity of the composite increases slightly with either phosphor volume fraction or temperature, and the Kapitza radius of the composite is 0.8 µm. When phosphor concentration is above 25 vol.%, the increase of conductivity correlates positively with phosphor volume fraction significantly but negatively with the temperature, and the Kapitza radius is 0.032 µm.

Sign in / Sign up

Export Citation Format

Share Document