scholarly journals Correction to “New Measurements of the Apparent Thermal Conductivity of Nanofluids and Investigation of Their Heat Transfer Capabilities”

2018 ◽  
Vol 63 (11) ◽  
pp. 4277-4279 ◽  
Author(s):  
Georgia J. Tertsinidou ◽  
Chrysi M. Tsolakidou ◽  
Maria Pantzali ◽  
Marc J. Assael ◽  
Laura Colla ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Apparent Thermal Conductivity

A Micro-Insulation Concept for MEMS Applications

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
Rui Yao ◽  
James Blanchard
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Radiation Heat Transfer ◽  
Thermal Conversion ◽  
Power Source ◽  
Radioactive Isotopes ◽  
Small Scale ◽  
Power Sources ◽  
Planar Surfaces ◽  
Apparent Thermal Conductivity

Small scale, thermally driven power sources will require appropriate insulation to achieve sufficiently high thermal conversion efficiencies. This paper presents a micro-insulation design, which was developed for a thermionic microbattery, which converts the decay heat from radioactive isotopes directly to electricity using a vacuum thermionic diode. The insulation concept, which is suitable for any small scale application, separates two planar surfaces with thin, semicircular posts, thus reducing conduction heat transfer and increasing the relative radiation heat transfer. In this case, the surfaces are silicon wafers and the columns are SU-8, a photoresist material. The experimental results indicate that this design is adequate for a practical power source concept, and they are supported by a numerical model for the effective thermal conductivity of the structure. The results show that a typical design of 20 columns/cm2 with a 200 μm diameter and a 10 μm wall thickness has an apparent thermal conductivity on the order of 10−4 W/m K at a pressure of 1 Pa. System models of a thermionic power source indicate that this is sufficiently low to provide practical efficiency.

Effects of Fin Configuration on Heat Transfer Rate in Packed Bed Reactors for Improvement of Their Thermal Characteristics

2010 ◽  
Author(s):  
Koichi Nakaso ◽  
Takuro Aoki ◽  
Jun Fukai
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Thermal Response ◽  
Packed Bed ◽  
Packed Bed Reactors ◽  
Apparent Thermal Conductivity ◽  
Spiral Type ◽  
Thermal Conductivities

Packed bed reactors are utilized for catalysts, chemical heat pumps, etc. Because the effective thermal conductivities of the packed beds of particles are generally low (≈10−1 W/mK), this matter often results in low performance and degradation of catalyst. Many heat transfer tubes with fins and/or much filler with high thermal conductivities are inserted in the packed bed reactors to improve heat transfer rate. In return to this, the volume of reactive particles packed into the reactors, or stored energy, decreases. In this study, the effect of fin configurations on the heat transfer rate in the reactors is numerically investigated. Three configurations of fins are studied. (1) “Sheet type” is a longitudinal fin attached on the heat transfer tubes. It is placed to connect between heat transfer tubes. (2) “Straight type” is several longitudinal fins in the half length of the tube pitch attached on the tube with radial structure. (3) “Spiral type” is many narrow rectangular fins attached on the tube with spiral structure. To discuss the effect of fin configuration on the heat transfer generally, the heat conduction equation in the packed bed around the tube is converted to the dimensionless form. The transient temperature responses in the packed bed and fins at a uniform temperature are calculated when the temperature of the tube surface is stepwise changes. In another analytical system, a homogeneous body around the tube is assumed. To evaluate the thermal performance of the fin, apparent thermal conductivity is defined as the thermal conductivity which gives the same thermal response as that calculated in the heterogeneous system. As a result, the spiral type rather than the straight and sheet types effectively increases apparent thermal conductivity. The apparent thermal conductivity of the spiral type is two-three times larger than the straight type, and ten times as large as the sheet type. This result indicates dispersion of fins in packed bed is essential to improve the thermal response in the reactors.

scholarly journals New Measurements of the Apparent Thermal Conductivity of Nanofluids and Investigation of Their Heat Transfer Capabilities

2016 ◽  
Vol 62 (1) ◽  
pp. 491-507 ◽  
Author(s):  
Georgia J. Tertsinidou ◽  
Chrysi M. Tsolakidou ◽  
Maria Pantzali ◽  
Marc J. Assael ◽  
Laura Colla ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Apparent Thermal Conductivity

scholarly journals Effects of Microscopic Properties on Macroscopic Thermal Conductivity for Convective Heat Transfer in Porous Materials

Micromachines ◽  
2021 ◽  
Vol 12 (11) ◽  
pp. 1369
Author(s):  
Mayssaa Jbeili ◽  
Junfeng Zhang
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Porous Materials ◽  
Thermal Conductance ◽  
Complex Flow ◽  
Material Development ◽  
Interfacial Thermal Conductance ◽  
Apparent Thermal Conductivity

Porous materials are widely used in many heat transfer applications. Modeling porous materials at the microscopic level can accurately incorporate the detailed structure and substance parameters and thus provides valuable information for the complex heat transfer processes in such media. In this study, we use the generalized periodic boundary condition for pore-scale simulations of thermal flows in porous materials. A two-dimensional porous model consisting of circular solid domains is considered, and comprehensive simulations are performed to study the influences on macroscopic thermal conductivity from several microscopic system parameters, including the porosity, Reynolds number, and periodic unit aspect ratio and the thermal conductance at the solid–fluid interface. Our results show that, even at the same porosity and Reynolds number, the aspect ratio of the periodic unit and the interfacial thermal conductance can significantly affect the macroscopic thermal behaviors of porous materials. Qualitative analysis is also provided to relate the apparent thermal conductivity to the complex flow and temperature distributions in the microscopic porous structure. The method, findings and discussions presented in this paper could be useful for fundamental studies, material development, and engineering applications of porous thermal flow systems.

Heat Transfer Across Opaque Fibers

2012 ◽  
Vol 134 (7) ◽  
Author(s):  
Krishpersad Manohar ◽  
Gurmohan S. Kochhar ◽  
David W. Yarbrough
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Thermal Conductivity ◽  
Fiber Diameter ◽  
Radiation Heat Transfer ◽  
Physical Parameters ◽  
Density Range ◽  
Radiation Heat ◽  
Apparent Thermal Conductivity ◽  
Air Conduction

Predicting the thermal conductivity of loose-fill fibrous thermal insulation is a complex problem, when considering the combined conduction, convection, and radiation heat transfer within a scattering, emitting, and absorbing medium. A piecewise model for predicting the overall apparent thermal conductivity of large diameter opaque fibrous materials was developed by considering the radiation heat transfer, solid conduction and air conduction components separately. The model utilized the physical parameters of emissivity, the density of the solid fiber material, the percentage composition and range of fiber diameter, and the mean fiber diameter to develop specific equations for piecewise contribution from radiation, solid fiber conduction, and air conduction toward the overall effective thermal conductivity. It can be used to predict the overall apparent thermal conductivity for any opaque fibrous specimen of density (ρ), known thickness (t), mean temperature (T), and temperature gradient (ΔΤ). Thermal conductivity measurements were conducted in accordance with ASTM C518 specifications on 52 mm thick, 254 mm square test specimens for coconut and sugarcane fibers. The test apparatus provided results with an accuracy of 1%, repeatability of 0.2%, and reproducibility of 0.5%. The model was applied to and compared with experimental data for coconut and sugarcane fiber specimens and predicted the apparent thermal conductivity within 7% of experimental data over the density range tested. The model also predicted the optimum density range for both coconut and sugarcane fibers.

Heat transfer in the packing of cellular pellets: microstructure and apparent thermal conductivity

1998 ◽  
Vol 30 (6) ◽  
pp. 709-715 ◽  
Author(s):  
Daniel Quenard ◽  
Daniel Giraud ◽  
François-Dominique Menneteau ◽  
Hébert Sallee
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Apparent Thermal Conductivity

An Experimental Rig for Verification of the Mechanical Properties of Welds Produced at In-Service Welding

2003 ◽  
Author(s):  
Per R. M. Lindstro¨m ◽  
Anders Ulfvarson
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Boundary Layer ◽  
Cooling Rate ◽  
Base Material ◽  
Welding Process ◽  
Forced Flow ◽  
Welding Parameters ◽  
Apparent Thermal Conductivity ◽  
Experimental Rig

The strength of a weld joint is determined by its geometry and its metallurgic structure, which is dependent on the cooling rate, its chemical composition and the original grain size of the base material. During in-service welding of structures affected by a forced flow of fluid on its reversed side the cooling rate depends on the fluid’s boundary layer, the material’s thickness and the heat input of the welding process. Currently, the calculation of the cooling rate during in-service welding is made by means of numerical methods such as the Finite Element Method, FEM. Through the introduction of an apparent thermal conductivity, kPL, it possible to determine the cooling rate for specific welding parameters by means of Rosenthal’s equation. This can be done with a standard pocket calculator. An experimental rig for measurement of the heat transfer during the in-service welding of structures affected by a forced flow of fluid on its reversed side has been designed and built. The physical principles of welding on plates affected by a forced flow of fluid on their reverse side are the same as for welding on the circumference of a pipe containing a forced flow of fluid. In the rig, the required boundary layer is built up in a pipe system by means of a pump. As the flow and the temperature of the fluid can be controlled to simulate the specific heat transfer, it is now possible to verify the values of the apparent thermal conductivity, kPL, that were calculated by means of FEM. A quantitative database will be filled with values of the apparent thermal conductivity, kPL, for various configurations. For the purpose of evaluation and qualification of in-service Welding Procedures Specifications, WPS, the sponsors of the research project use the experimental rig.

THERMAL CONDUCTIVITY AND HEAT TRANSFER OF CERAMIC NANOFLUIDS SHOW CLASSICAL BEHAVIOR

2011 ◽  
Author(s):  
Stefan Pohl ◽  
Steffen Feja ◽  
Matthias H. Buschmann
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Classical Behavior
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