• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.12
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• pp.1147-1152
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• 2013

In this study, the effect of an insulation coating on the start time of a linear region is theoretically investigated when an insulation-coated hot-wire is used for the transient hot-wire method (THWM). For this purpose, important parameters affecting the start time of the linear region are presented from an analytical solution of temperature-rise for an insulation-coated hot-wire. Furthermore, a critical time to ignore the influence of important parameters is studied. The theoretical results indicate that the effect of the insulation coating rapidly disappears with a decrease in the wire radius, coating thickness, thermal diffusivity of insulation material or an increase in the thermal conductivity of the insulation material. The results of this study will be helpful for selecting a proper start time of the linear region for the THWM using insulation-coated hot-wires.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.3
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• pp.279-285
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• 2011

We perform a numerical study to determine the time of onset of natural convection in a transient hot wire (THW) device for measuring the thermal conductivity of nanofluids. The samples used in this simulation are water-based $Al_2O_3$ nanofluids with volume fractions of 1%, 4%, and 10%, and the properties are calculated by theoretical models and experimental correlations. The THW apparatus using coated wire is modeled by the control-volume-based finite difference method, and the start of natural convection is determined by observing the temperature rise of the wire under a gravity field. The onset time is 11.5 s for water and 41.6 s for water-based $Al_2O_3$ nanofluids predicted by Maxwell thermal conductivity model with a 10% volume fraction. We confirm that the onset time of natural convection of nanofluids in the cylinder increases with the nanoparticle volume fraction. We suggest a correlation for predicting the onset time on the basis of the numerical results. Finally, it is shown that the measurement error due to natural convection is negligible if the measurement using the transient hot wire method is completed before the onset of natural convection in the base fluid.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.5
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• pp.510-517
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• 2004

A fine hot-wire is used both as a heating element and a temperature sensor in transient hot-wire method. The traditional sensor system is unnecessarily big so that it takes large fluid volume to measure the thermal conductivity. To dramatically reduce this fluid volume, a new sensor fabrication and a data processing method are proposed in this article. Contrast to the conventional and most popular two wire sensor, the new sensor system is made up of divided multiple long and short wires. Through validation experiments, it is found that the measured thermal conductivities of the glycerin are exactly same each other between the conventional and proposed new method. Also some technical considerations in arranging the multiple wires are briefly discussed.

• Proceedings of the SAREK Conference
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• 2008.11a
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• pp.165-169
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• 2008

Binary nanofluids(Binary mixture + nanoparticles) have been extensively paid attention for application in absorption system as a new working fluid. Thermal property evaluation of the new refrigerants is inevitable to apply them for actual system. The objectives of this paper are to measure the thermal conductivity of the binary nanofluids by the transient hot-wire method, and to assess the application possibility of the binary nanofluids for absorption system. It was found that the thermal conductivity of the binary nanofluids ($H_2O/LiBrAl_2O_3$) increased with increasing the concentration of the nanoparticles ($Al_2O_3$) and enhanced up to 27% at 0.1 vol % of the nanoparticles.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.210-215
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• 2003

A fine hot-wire is used both as a heating element and a temperature sensor in transient hot-wire method. The traditional sensor system is unnecessarily big so that it takes large fluid volume to measure the thermal conductivity. To dramatically reduce this fluid volume, a new sensor fabrication and a data processing method are proposed in this article. Contrast to the conventional and most popular two wire sensor, the new sensor system is made up of divided multiple long and short wires. Through validation experiments, it is found that the measured thermal conductivities of the glycerin are exactly same each other between the conventional and proposed new method. Also some technical considerations in arranging the multiple wires are briefly discussed.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.1
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• pp.29-36
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• 2015

Exact comparisons of the thermal conductivities of the base fluid and a nanofluid are very important in the early stages of nanofluid development. A simple procedure of measuring the thermal conductivity of the two fluids by the transient hot wire method and numerically dividing these values is used for this purpose. However, because the experiments are not performed simultaneously and the physical properties of the measurement system are sometimes not properly known, large errors are incurred during the evaluation process. This article proposes a new apparatus for thermal conductivity comparison where the working principle is mainly based on relative measurement rather than absolute measurement. The measuring circuit and data processing steps are explained in detail; a validation test was performed using the well-known glycerine and engine oil.

• Journal of the Korean Geotechnical Society
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• v.27 no.7
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• pp.17-33
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• 2011

Recently, the utilization of recycled aggregates for backfilling a power transmission pipeline trench has been considered due to the issues of eco-friendly construction and a lack of natural aggregate resource. It is important to identify the physical and thermal properties of domestic recycled aggregates that can be used as a backfill material. This paper evaluated thermal properties of concrete-based recycled aggregates with various particle size distributions. The thermal properties of the recycled aggregates and river sand provided by local vendors were measured using the transient hot wire method and the transient needle probe method after performing the standard compaction test. The needle probe method considerably overestimated the thermal resistivity of recycled aggregates especially at the dry of optimum water content because of experiencing disturbance while the needle probe is being inserted into the specimen. Similar to silica sand, the thermal resistivity of recycled aggregates decreased when the water content increased at a given dry density. Also, this paper evaluated some of the existing prediction models for the thermal resistivity of recycled aggregates with the experimental data, and developed a new prediction model for recycled aggregates. This study shows that recycled aggregates can be a promising backfill material substituting for natural aggregates when backfilling the power transmission pipeline trench.

• Transactions of the Korean Society of Mechanical Engineers
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• v.2 no.2
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• pp.42-46
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• 1978

Thermal conductivities of Freon-12 and Freon-22 were measured at room temperature up to 35 MPa with the aid of transient hot wire method. Glycerin was used to check the performance of the experimental equipment. The thermal conductivities of Freon-12 and Freon-22 at the maximum pressure, 35 MPa, were increased by 25% approximately for those at satufated state.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.4
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• pp.341-348
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• 2007

One of the controversial research issues on nanofluids is the temperature dependence of the thermal conductivity of nanofluids, that is, whether it will increase or decrease according to the temperature rise. To evaluate precisely the thermal conductivity behavior of nanofluids, a systematic way of validation experiments for the measuring instrument has been highly recommended. In this paper, procedure of the validation test for transient hot-wire method using the temperature dependence of the base fluids was explained comprehensively and the comparison of the temperature dependence of water-$Al_2O_3$ nanofluids is made between the present work and that of Das et al.

• Fire Science and Engineering
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• v.30 no.1
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• pp.49-56
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• 2016

This paper numerically examines, straight and twisted electrical heat trace installations for their anti-freezing effects on water inside a pipe. The unsteady incompressible Navier-Stokes equations coupled with an energy equation were solved to compare the two installation methods. The heat conduction of the pipe with a heat source interacts with the natural convection of the water, and the conjugate heat transfer was considered using a commercial code (ANSYS-FLUENT) based on a SIMPLE-type algorithm. Numerical experiments, were done to investigate the isotherms and the vector fields in the water region to extract the evolutions of the minimum and maximum temperatures of the water inside the pipe. There was no substantial difference in the anti-freezing effects between the straight and twisted. Therefore, the straight installation is recommended after considering the damage and short circuit behavior of the electrical heat trace.