scholarly journals The difference in the thermal conductivity of nanofluids measured by different methods and its rationalization

2016 ◽  
Vol 7 ◽  
pp. 2037-2044 ◽  
Author(s):  
Aparna Zagabathuni ◽  
Sudipto Ghosh ◽  
Shyamal Kumar Pabi
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Laser Flash ◽  
Flash Method ◽  
Transfer Model ◽  
Measuring Methods ◽  
Order Of Magnitude ◽  
Wire Method ◽  
The Difference ◽  
First Time

A suspension of particles below 100 nm in size, usually termed as nanofluid, often shows a notable enhancement in thermal conductivity, when measured by the transient hot-wire method. In contrast, when the conductivity of the same nanofluid is measured by the laser flash method, the enhancement reported is about one order of magnitude lower. This difference has been quantitatively resolved for the first time on the basis of the collision-mediated heat transfer model for nanofluids proposed earlier by our research group. Based on the continuum simulation coupled with stochastic analysis, the present theoretical prediction agrees well with the experimental observations from different measuring methods reported in the literature, and fully accounts for the different results from the two measuring methods mentioned above. This analysis also gives an indication that the nanofluids are unlikely to be effective for heat transfer in microchannels.

scholarly journals Temperature measurements and heat transfer in near-surface snow at the South Pole

1997 ◽  
Vol 43 (144) ◽  
pp. 339-351 ◽  
Author(s):  
Richard E. Brandt ◽  
Stephen G. Warren
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Lower Boundary ◽  
Water Vapor Transport ◽  
South Pole ◽  
Near Surface ◽  
Temperature Changes ◽  
Surface Snow ◽  
Order Of Magnitude ◽  
The Difference

AbstractTo study near-surface heat flow on the Antarctíc ice sheet, snow temperatures were measured at South Pole Station to a depth of 3 m at 15 min intervals during most of 1992. Solar heating and water-vapor transport were negligible during the 6 month Winter, as was inter-grain net thermal radiation, leaving conduction as the dominant heat-transport mechanism. The rate of temperature change at depth over 15 min intervals was smaller than that at the surface, by one order of magnitude at 20 cm depth and two orders of magnitude at 1 m depth. A finite-difference model, with conduction as the only heat-transfer mechanism and measured temperatures as the upper and lower boundary conditions, was applied to foursets of three thermistors each. The thermal conductivity was estimated as that which minimized the difference between modeled and measured 15 min changes in temperatures at the center thermistor. The thermal conductivity obtained at shallow depths (above 40 cm) was lower than that given by existing parameterizations based on density, probably because the snow grains were freshly deposited, cold and poorly bonded. A model using only vertical conduction explains on average 87% ofthe observed 15 min temperature changes at less than 60 cm depth and 92% below 60 cm. The difference between modeled andmeasured temperature changes decreased with depth. The discrepancies between model and observation correlated more strongly with the air-snow temperature difference than with the product of that difference with the square of the wind speed,suggesting that the residual errors are due more to non-vertical conduction and to sub-grid-scale variabilis of the conductivity than to windpumping. The residual heating rate not explained by the model of vertical conduction exceeds 0.2 W m−3only in the top 60 cm of the near-surface snow.

scholarly journals Temperature measurements and heat transfer in near-surface snow at the South Pole

1997 ◽  
Vol 43 (144) ◽  
pp. 339-351 ◽  
Author(s):  
Richard E. Brandt ◽  
Stephen G. Warren
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Lower Boundary ◽  
Water Vapor Transport ◽  
South Pole ◽  
Near Surface ◽  
Temperature Changes ◽  
Surface Snow ◽  
Order Of Magnitude ◽  
The Difference

AbstractTo study near-surface heat flow on the Antarctíc ice sheet, snow temperatures were measured at South Pole Station to a depth of 3 m at 15 min intervals during most of 1992. Solar heating and water-vapor transport were negligible during the 6 month Winter, as was inter-grain net thermal radiation, leaving conduction as the dominant heat-transport mechanism. The rate of temperature change at depth over 15 min intervals was smaller than that at the surface, by one order of magnitude at 20 cm depth and two orders of magnitude at 1 m depth. A finite-difference model, with conduction as the only heat-transfer mechanism and measured temperatures as the upper and lower boundary conditions, was applied to foursets of three thermistors each. The thermal conductivity was estimated as that which minimized the difference between modeled and measured 15 min changes in temperatures at the center thermistor. The thermal conductivity obtained at shallow depths (above 40 cm) was lower than that given by existing parameterizations based on density, probably because the snow grains were freshly deposited, cold and poorly bonded. A model using only vertical conduction explains on average 87% ofthe observed 15 min temperature changes at less than 60 cm depth and 92% below 60 cm. The difference between modeled andmeasured temperature changes decreased with depth. The discrepancies between model and observation correlated more strongly with the air-snow temperature difference than with the product of that difference with the square of the wind speed,suggesting that the residual errors are due more to non-vertical conduction and to sub-grid-scale variabilis of the conductivity than to windpumping. The residual heating rate not explained by the model of vertical conduction exceeds 0.2 W m−3 only in the top 60 cm of the near-surface snow.

scholarly journals Computational Assessment of Thermal Conductivity of Compacted Graphite Cast Iron

2019 ◽  
Vol 2019 ◽  
pp. 1-7
Author(s):  
Yue Wu ◽  
Jianping Li ◽  
Zhong Yang ◽  
Zhijun Ma ◽  
Yongchun Guo ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Thermal Conductance ◽  
Laser Flash ◽  
Flash Method ◽  
Compacted Graphite ◽  
Graphite Cast Iron ◽  
Calculation Results ◽  
The Difference ◽  
Metallographic Images ◽  
Experimental Values

Two-dimensional FE models of CGI with different pearlite contents for thermal conductivity analysis were established according to the real metallographic images obtained by Pro/E and ANSYS. Meanwhile, thermal conductivity of CGI with different pearlite contents was tested through the laser flash method. It is indicated that the thermal conductivity of CGI declines with the increase of pearlite. When pearlite is increased from 10% to 80%, the experimental values decline from 46.63 W/m·K to 36.86 W/m·K, reducing by 21%. However, this declining tendency becomes gentle and slight when pearlite is more than 40%. In addition, the calculation results with the consideration of interfacial contact thermal conductance (ICTC) and pearlite are much close to experimental values; especially when pearlite is 80%, the difference between them is only about 2%. It can be concluded that the FE models are convenient and reasonable to analyze thermal conductivity of CGI.

Recent Development in the Investigation on Thermal Conductivity of Silicate Melts

2012 ◽  
Vol 31 (4-5) ◽  
pp. 491-499 ◽  
Author(s):  
Hiroki Hasegawa ◽  
Hiromichi Ohta ◽  
Hiroyuki Shibata ◽  
Yoshio Waseda
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Thermal Diffusivity ◽  
Chemical Composition ◽  
Laser Flash ◽  
Laser Flash Method ◽  
Silicate Melts ◽  
Flash Method ◽  
Bottom Surface ◽  
High Time Resolution

AbstractAccurate values of thermal conductivity of the silicate melts systematically measured as a function of chemical composition are necessary to understand a mechanism of heat transfer in the silicate melts. Hot wire method and laser flash methods have been used to measure thermal conductivity or thermal diffusivity of oxides melts at high temperatures. Laser flash method has been improved to measure thermal diffusivity of oxides melts with high accuracy. However the effects of radiative heat transfer and low electrical resistivity of samples have been made it difficult to derive precise values. To overcome these difficulties, a front-heating front-detection laser flash method with use of high time resolution detector has been proposed. The temperature response at the bottom surface of thin platinum cell containing sample irradiated by pulse laser is measured. The measurement techniques used for measurement oxide melts are compared. Then, thermal conductivity of Al2O3-Na2O-CaO-SiO2 silicate melts was measured at temperature up to 1830 K. Thermal conductivity of the molten silicate shows insignificant temperature dependence for all investigated melts. A fairly good correlation has been found between the thermal conductivity and the value of NBO/T (Non-Bridging Oxygen ions/Tetrahedrally coordinated cation) calculated from the chemical composition. The thermal conductivity increases with decrease of NBO/T for small NBO/T value and becomes constant for larger NBO/T value.

scholarly journals Preparation and Enhancement of Thermal Conductivity of Heat Transfer Oil-Based MoS2Nanofluids

2013 ◽  
Vol 2013 ◽  
pp. 1-6 ◽  
Author(s):  
Yuan-Xian Zeng ◽  
Xiu-Wen Zhong ◽  
Zhao-Qing Liu ◽  
Shuang Chen ◽  
Nan Li
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Surface Modification ◽  
Stearic Acid ◽  
Mass Fraction ◽  
Laser Flash ◽  
Thermal Conductivity Enhancement ◽  
Flash Method ◽  
Conductivity Enhancement ◽  
Increasing Temperature

The lipophilic MoS2nanoparticles are synthesized by surface modification with stearic acid (SA). The heat transfer oil-based nanofluids, with the mass fraction of lipophilic nanoparticles varying from 0.25% up to 1.0%, are prepared and their thermal conductivity is determined at temperatures ranging from 40 to 200°C using an apparatus based on the laser flash method. It has been found that the nanofluids have higher thermal conductivity and the thermal conductivity enhancement increased not only with increasing mass fraction of nanoparticles, but also with increasing temperature in the range 40–180°C The results show a 38.7% enhancement of the thermal conductivity of MoS2nanofluid with only 1.0% mass fraction at 180°C.

Method for Obtaining Thermal Conductivity From Modified Laser Flash Measurement

2005 ◽  
Author(s):  
Bochuan Lin ◽  
Heng Ban ◽  
Chao Li ◽  
Rosalia N. Scripa ◽  
Ching-Hua Su ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Thermal Diffusivity ◽  
Rear Surface ◽  
Temperature Response ◽  
Laser Flash ◽  
Fused Silica ◽  
Laser Flash Method ◽  
Flash Method ◽  
Silica Cell

Laser flash method is commonly used to measure the thermal diffusivity of solids. In the original thermal analysis, adiabatic boundary conditions were used and the time for sample rear surface temperature to reach 50% of maximum value was used to calculate the thermal diffusivity. Later other boundary conditions were included in the analysis to compensate for the heat loss. The laser flash method can be modified to determine the thermal conductivity by comparing the temperature rise of the sample with a standard sample, both of which are coated to ensure identical surface emissivity. In our previous studies of applying the laser flash method to semiconductor melts, we have shown that it is possible to obtain thermal conductivity, specific heat capacity and thermal diffusivity from the experimental data. In these studies, the melt sample was sealed in a specially-designed fused silica cell. The heat transfer between melt sample and the fused silica cell allows the thermal conductivity to be included in the analysis. Therefore, the temperature response of the melt sample was controlled not only by the thermal diffusivity and conductivity of sample, but also by the thermal properties of fused silica cell. Using a computational fitting process, we obtained both thermal diffusivity and thermal conductivity of the sample. In this paper, an analytic solution for the transient heat transfer inside the sample and fused silica cell was developed. The influence of fused silica cell was included and the heat transfer to fused silica cell had a significant effect on the time-temperature response of the sample. Therefore, the rear surface temperature of the sample, described by an analytical solution, could be used to obtain both thermal diffusivity and thermal conductivity of the sample with known properties of the fused silica cell. The results indicated that this method was applicable for a wide range of sample and cell properties. The original solution for laser flash method became an extreme case in the current theory

scholarly journals Conjugate heat transfer performance of stepped lid-driven cavity with Al2O3/water nanofluid under forced and mixed convection

2021 ◽  
Vol 3 (6) ◽  
Author(s):  
Naveen Janjanam ◽  
Rajesh Nimmagadda ◽  
Lazarus Godson Asirvatham ◽  
R. Harish ◽  
Somchai Wongwises
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Mixed Convection ◽  
Conjugate Heat Transfer ◽  
Heat Transfer Performance ◽  
Pure Water ◽  
Interface Temperature ◽  
Transfer Model ◽  
Transfer Performance ◽  
Driven Cavity

AbstractTwo-dimensional conjugate heat transfer performance of stepped lid-driven cavity was numerically investigated in the present study under forced and mixed convection in laminar regime. Pure water and Aluminium oxide (Al2O3)/water nanofluid with three different nanoparticle volume concentrations were considered. All the numerical simulations were performed in ANSYS FLUENT using homogeneous heat transfer model for Reynolds number, Re = 100 to 500 and Grashof number, Gr = 5000, 13,000 and 20,000. Effective thermal conductivity of the Al2O3/water nanofluid was evaluated by considering the Brownian motion of nanoparticles which results in 20.56% higher value for 3 vol.% Al2O3/water nanofluid in comparison with the lowest thermal conductivity value obtained in the present study. A solid region made up of silicon is present underneath the fluid region of the cavity in three geometrical configurations (forward step, backward step and no step) which results in conjugate heat transfer. For higher Re values (Re = 500), no much difference in the average Nusselt number (Nuavg) is observed between forced and mixed convection. Whereas, for Re = 100 and Gr = 20,000, Nuavg value of mixed convection is 24% higher than that of forced convection. Out of all the three configurations, at Re = 100, forward step with mixed convection results in higher heat transfer performance as the obtained interface temperature is lower than all other cases. Moreover, at Re = 500, 3 vol.% Al2O3/water nanofluid enhances the heat transfer performance by 23.63% in comparison with pure water for mixed convection with Gr = 20,000 in forward step.

Study on Effective Radial Thermal Conductivity of Gas Flow through a Methanol Reactor

2015 ◽  
Vol 13 (1) ◽  
pp. 103-112 ◽  
Author(s):  
Kun Lei ◽  
Hongfang Ma ◽  
Haitao Zhang ◽  
Weiyong Ying ◽  
Dingye Fang
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Reynolds Number ◽  
Temperature Distribution ◽  
Large Scale ◽  
Gas Flow ◽  
Fixed Bed ◽  
Heat Transfer Model ◽  
Transfer Model ◽  
Particle Reynolds Number

Abstract The heat conduction performance of the methanol synthesis reactor is significant for the development of large-scale methanol production. The present work has measured the temperature distribution in the fixed bed at air volumetric flow rate 2.4–7 m3 · h−1, inlet air temperature 160–200°C and heating tube temperature 210–270°C. The effective radial thermal conductivity and effective wall heat transfer coefficient were derived based on the steady-state measurements and the two-dimensional heat transfer model. A correlation was proposed based on the experimental data, which related well the Nusselt number and the effective radial thermal conductivity to the particle Reynolds number ranging from 59.2 to 175.8. The heat transfer model combined with the correlation was used to calculate the temperature profiles. A comparison with the predicated temperature and the measurements was illustrated and the results showed that the predication agreed very well with the experimental results. All the absolute values of the relative errors were less than 10%, and the model was verified by experiments. Comparing the correlations of both this work with previously published showed that there are considerable discrepancies among them due to different experimental conditions. The influence of the particle Reynolds number on the temperature distribution inside the bed was also discussed and it was shown that improving particle Reynolds number contributed to enhance heat transfer in the fixed bed.

scholarly journals XVI Международная конференция "Термоэлектрики и их применения --- 2018" (ISCTA 2018), Санкт-Петербург, 8-12 октября 2018 г. Влияние неидеальности геометрической формы образца на неопределенность измерений теплопроводности методом лазерной вспышки

2019 ◽  
Vol 53 (6) ◽  
pp. 731
Author(s):  
А.В. Асач ◽  
Г.Н. Исаченко ◽  
А.В. Новотельнова ◽  
В.Е. Фомин ◽  
К.Л. Самусевич ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Measurement Uncertainty ◽  
Conductivity Measurement ◽  
Laser Flash ◽  
Comsol Multiphysics ◽  
Flash Method ◽  
Cylindrical Shape ◽  
Plane Parallel ◽  
Coefficient Of Thermal Conductivity ◽  
Truncated Cylinder

The influence of the geometric shape of the samples on the uncertainty of the coefficient of thermal conductivity measurement of materials by the method of a laser flash has been studied. Using a method of mathematical modeling in the Comsol Multiphysics software, a model that simulates the process of measuring the coefficient of thermal conductivity of samples made of graphite, Mg2Si0.4Sn0.6 and bismuth telluride using a laser flash method has been created. Samples of cylindrical shape with plane-parallel sides and samples in the form of a truncated cylinder, as well as samples in the form of a parallelepiped with a square base, were investigated. It is shown that the measurement uncertainty of samples with plane-parallel sides and sizes up to 12.7 mm, does not exceed 2%. For samples in the form of a truncated cylinder with a diameter of 3 mm and at an angle of ϕ= 1.5°, the measurement uncertainty does not exceed 3%. With an increase in the sample diameter and the ϕ angle, the measurement uncertainty increases significantly.

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