CONDUCTIVITY MEASUREMENT OF FERROFLUID USING TRANSIENT HOT WIRE METHOD

摘要

Thermal conductivity of the ferrofluids is a difficult property to predict theoretically. Existing models can not explain the real behavior of such fluids and mismatch the results found in the experiments. Experimentation is then the most reliable way to determine the observed enhancement in thermal conductivity of ferrofluids. The transient hot wire method is an experimental technique in which the thermal conductivity is obtained by measuring a temperature change, respect to time in a thin wire, caused by a constant current passing through it. Some advantages of this method are the almost complete elimination of natural convection effects, its fast implementation and its high accuracy. In order to use the transient hot-wire technique in a ferrofluid a modification must be made because the transient hot-wire method cannot be applied to electrical conducting fluids since part of the current will be conducted by the fluid, generating uncertainties in the current passing through the wire. These uncertainties will affect the voltage measurement over predicting the thermal conductivity. To prevent the current to pass through the fluid the hot wire has to be covered with an electrical insulating coating. Then, it is necessary to calibrate the wire with a calibrating constant in order to correct the effect of the coating in the RTD. For calibration purposes, thermal conductivity measurements of known fluids have been carried out. For this purpose, substances like water, toluene acetone and heptane are used. In this study the transient hot-wire method is implemented to measure the thermal conductivity of different water-based ferrofluids and oil-based ferrofluids. Parametric Studies are carried out numerically to understand the effect of the coating in the technique.