Determination of the Thermal Conductivity by Using the Hot Wire Method: Theory, Simulation and Experiment

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Published on October 18, 2020

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1. Mediterranean Journal of Basic and Applied Sciences (MJBAS) Volume 4, Issue 3, Pages 110-135, July-September 2020 ISSN: 2581-5059 www.mjbas.com 110 Determination of the Thermal Conductivity by Using the Hot Wire Method: Theory, Simulation and Experiment Giovanni Alcocer Independent Researcher, Guayaquil, Ecuador. Email: [email protected] Master in Physics with Specialization in Astrophysics and Medical Physics, Professor of Physics, Advanced Mathematics and Science in General. DOI: 10.46382/MJBAS.2020.4309 Article Received: 29 June 2020 Article Accepted: 19 August 2020 Article Published: 30 September 2020 1. Introduction The measurement of the physical properties (density, viscosity, surface tension, thermal conductivity, etc.) is of great importance to the research, industry and physical, chemical and biomedical applications. The thermal conductivity is a measurement of the material’s ability to conduct heat. The Transient Hot Wire method is a suitable method to measure the thermal conductivity due to its very cheap cost of construction, accuracy and because it is a fast method of measurement. The implementation requires accurate temperature sensing, automatic control, data acquisition and data analysis. The basic procedure consists of measuring the temporal temperature rise in a thermoresistance (thin wire) immersed in the solution by applying an electrical current in the wire. Therefore, the wire works as a heat source and a temperature sensor. It is necessary to coat the platinum wire in order to avoid disturbances and perturbations of the signals. Besides, the convection heat transfer effects can be minimized due to the brief period of time of the measurement. For other hand, if a small convection effect appears, it can be identified and corrected by using the deviation of the linearity in the plot of ΔT versus ln(t). Then we can adjust the time control of the measurement to minimize it. If the physical properties are assumed to be constants (which is true in this case due to the short period time of measurement), the thermal conductivity can be determined from the slope of the curve ΔT versus ln(t). It is because of the linear relation between ΔT and ln(t). A basic scheme of the experiment is showed in the figure 1. It mainly consists of a testing container where is the liquid, the heat source/sensor of platinum wire and the coating of the cell (recipient is thermal isolated). Besides, we can observe the data acquisition system HP 34970 A connected to the PC computer by the serial RS232, the DC power supply and the Wheatstone bridge. We can ABSTRACT The measurement of the physical properties (density, viscosity, surface tension, thermal conductivity, etc) is of great importance to the research industry and for the physical, chemical and biomedical applications. The thermal conductivity is a measurement of the material’s ability to conduct heat. The transient hot wire method is a suitable method to measure the thermal conductivity due to its very cheap cost of construction, accuracy and because it is a fast method of measurement. The implementation requires accurate temperature sensing, automatic control, data acquisition and data analysis. The basic procedure consists of measuring the temporal temperature rise in a thermoresistance (thin wire) immersed in the solution by applying an electrical current in the wire. Therefore, the wire works as a heat source and a temperature sensor. The time of measurement is very short and therefore the convection effect could be minimized. Then, the heat transfer to the infinite medium is due only to the conduction transfer effect. The thermal conductivity can be determined from the slope of the curve ΔT versus ln(t) due to the linear relation between ΔT and ln(t). Keywords: Thermal Conductivity, Hot Wire Method, Physical Properties, Theory, Simulation, Experiment

2. Mediterranean Journal of Basic and Applied Sciences (MJBAS) Volume 4, Issue 3, Pages 110-135, July-September 2020 ISSN: 2581-5059 www.mjbas.com 111 observe three thermocouples in order to monitor the uniformity of the fluid. The computer controls the time, the signal measurements and also performs the processing data. By using the Lab-View software, it is possible to convert the voltage registered into temperature and finally it is converted to thermal conductivity. Fig.1 Basic Scheme of the Thermal Conductivity Measurement The method consists of measuring the temporal temperature rise in a thin wire immersed in the liquid (which is initially at equilibrium) by applying an electrical current to the wire. Therefore, the wire is a heat source which produces a time-dependent temperature field within the liquid and also it acts as a sensor. As we mentioned before, the time of measurement is very short and therefore the convection effect could be minimized. Then, the heat transfer to the infinite medium is due only to the conduction transfer effect. The theoretical model is derived from the analytical solution of the heat conduction equation. The supposition consists of considering an ideal condition of line heat source of radius

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